Recent progress in graphene based polymer nanocomposites

Abstract

This paper reviews recent progress in the fabrication of graphene-based polymer nanocomposites and their applications. The modification of graphene, graphene oxide, reduced graphene oxide and the utilization of these materials in the fabrication of nanocomposites with different polymer matrixes have been explored. The methods for surface modification of Graphene with polymers, including various covalent and non-covalent techniques, are discussed in detail. Series of effective processing routes for producing high-quality G–polymer nanocomposites, such as melt compounding, solution blending, in situ polymerization, latex mixing, and electropolymerization, are introduced and discussed. Various organic polymers have been used to fabricate graphene-filled polymer nanocomposites using different methods. Different characterisation methods and applications of these polymer composites were reviewed.

Keywords:

• graphene
• nanocomposite
• sensor
• corrosion
• power
• graphene oxide
• reduced graphene oxide

PUBLIC INTEREST STATEMENT

Material science is currently revolutionalising the world. New materials are now fabricated for various applications such as power, corrosion protection and medical equipments. This paper reviews:

• Recent progress in the fabrication of graphene-based polymer nanocomposites and their applications is reviewed.

• The modification of graphene, graphene oxide, reduced graphene oxide and the utilization of these materials in the fabrication of nanocomposites with different polymer matrixes have been explored.

• The methods for surface modification of Graphene with polymers, including various covalent and non-covalent techniques, are discussed in detail.

• Series of effective processing routes for producing high-quality G–polymer nanocomposites, such as melt compounding, solution blending, in situ polymerization, latex mixing, and electropolymerization are introduced and discussed.

• Various organic polymers have been used to fabricate graphene-filled polymer nanocomposites using different methods.

• Properties of polymer/graphene nanocomposites and their applications are discussed in general along with detailed examples. Different characterisation methods and applications of these polymer composites were reviewed.

1. Introduction

Polymer nanocomposites are one of the most important applications of carbon nanofillers including graphene. A polymer nanocomposite is as a multiphase solid material, where one of the phases has one, two or three dimensions<100 nanometers (nm), in different polymer matrices. Nanocomposites are suitable for applications as high-performance composites, where good dispersion of the filler can be achieved and the properties of the nanoscale filler are substantially different or better than those of the matrix. Graphene with exceptionally good mechanical, thermal and electrical properties is quite suitable as a nanofiller in polymer matrixes for the development of high-performance nanocomposites.

Following the discovery of polymeric nanocomposites (Okada et al., 1990), various fillers have been tried for producing nanocomposites with enhanced properties. These include inorganic fillers, such as natural montmorillonite-type layered-silicate compounds or synthetic clay, metal nanofllers, various nanoparticles, and carbon-based fillers such as carbon black, expanded graphite (EG), CNT, and carbon nanofiber. Recently, carbon nanomaterials have shown great potential in improving the mechanical properties of polymer composites for their low density, excellent mechanical properties and modifiability. Among carbon nanomaterials, as one of the thinnest two dimensional layered materials, graphene shows excellent properties in specific surface area, conductivity and mechanic. It is a two-dimensional nanomaterial combining one atomic layer (Shioyama & Akita, 2003; Viculis et al., 2003), has high specific area in theory (2600 m2/g), excellent mechanical strength (1060 GPa) and high speed of electron mobility at room temperature (15,000 cm²/(V·s) (Chae et al., 2004; Y. Zhang et al., 2005). But, it also suffers from the tendency of aggregation. Efforts have been devoted on this issue by many researchers and many methods have been employed. Among these methods, covalently grafting polymer chains onto graphene has been adopted widely for its diversity of grafted functional group. Barua and Kumar (Baruah & Kumar, 2018) covalently grafted polystyrene (PS) chains onto the surface of graphene sheet, obtaining a uniform dispersion of graphene in PS matrix.

1.1. Graphene

Graphene and its derivatives have attracted increasing attention and have been developed for various environmental applications since graphene was first mechanically exfoliated in 2004 by (Novoselov et al., 2004). Graphene is a two-dimensional (2D) sheet composed of a monolayer of sp2 hybridized carbon atoms and possesses large specific surface area (2630 m2/g)(L. Ji et al., 2013).Graphene’s inherent crystalline quality, such as crystal structure and band structures, could result in a uniquely low noise level. Graphene is considered to have great potential to adsorb aromatic organic compounds via π-π electron coupling or van der Waals interactions (Zhuang et al., 2015). Recently, there has been continuous and tremendous progress in the study of carbon-based materials as fillers for polymer composites (H.-B. Zhang et al., 2010; H. Hu et al., 2010; Lawal, 2019; J. Liang et al., 2009; Mehdinia et al., 2020). Graphene is produced mostly by the micromechanical cleavage of graphite, epitaxial growth on silicon carbide, chemical vapour deposition (CVD) of hydrocarbons on transition metal surfaces, as well as the dispersion of graphite in organic solvents Figure 1.

Owing to the unique properties of graphene, numerous researches have focussed on developing composites with graphene as a dispersed nanofiller in various matrices, including ceramics, metals and polymers. A large majority of these efforts focus on how to homogeneously disperse graphene nanofillers in these matrices. In the case of polymer composites the exceptional properties of graphene do not readily transfer into exceptional composite properties (e.g., high strength, thermal and electrical conductivity) due to issues related to filler dispersion. Similarly, also the development of graphene-modified metals or ceramics is hindered by inhomogeneous dispersion of graphene into these matrices and the necessity of high temperature and pressure processing.

Graphene, as good nanofiller, is preferred over some conventional nanofillers such as metal oxides, CNTs, carbon black (CB), layered silicates and CNF because of its high surface area, aspect ratio, tensile strength (TS), thermal conductivity and electrical conductivity, EMI shielding ability (Abbasi et al., 2019), flexibility and transparency. Graphene with exceptionally good mechanical, thermal and electrical properties is quite suitable as a nanofiller in polymer matrixes for the development of high-performance nanocomposites. Graphene with extraordinary high elastic modulus and excellent electrical conductivity has been used as the filler material for fabricating novel polymer composites designed for electrostatic discharge and EMI shielding protection, field emission, gas sensor, and fuel cell applications (Tjong, 2014). Graphene or CNTs within the polymer matrix act as fillers which improves the engineering capabilities of a composite. The filler loading determines the flexibility chemical affinity, stability and functionality of the composite for various applications (Mittal et al., 2015). Graphene main shortcomings are surface hydrophobicity and easy agglomeration in aqueous solution, which greatly decrease the adsorption capacity of graphene in practical application (Bai et al., 2015). Therefore, functionalized graphene is necessary to be developed to overcome these shortcomings (C. Wang et al., 2013; B. Wang et al., 2014). Graphene oxide (GO) and reduced grapheme oxide (RGO) are oxidizing form of graphene and they are generally produced by exfoliation of graphite oxide. They are more popular than pristine graphene because they have better dispersion in water and they both have reactive oxygen group.

Functionalised graphene, GO, and rGO can be easily homogeneously dispersed in different polymer matrices, both thermoplastics and thermosets (Mukhopadhyay & Gupta, 2011; Potts et al., 2011) .

1.2. Graphene oxide (GO)

Graphene oxide (GO) consists of single layer of graphite oxide and is usually produced by chemical treatment of graphite through oxidation. GO can also be produced by a modified hunmer’s method.

Improved Hummers’ method can conveniently prepare GO containing many polar groups, which is convenient for mixing with other substances (Cho et al., 2017; Z. Li et al., 2014; Tjong, 2014; Wang et al., 2018; Wei et al., 2013; K. Zhang et al., 2010). GO can be easily dispersed in the polymer matrix due to the existence of functional groups such as hydroxyls, epoxides and carboxyl. These chemical functional groups alter the Van der Waals interaction and improve the interfacial bonding between GO and polymer matrix, leading to an exfoliated and uniform dispersion (Othman et al., 2019). The oxygen functional groups have been identified as mostly in the form of hydroxyl and epoxy groups on the basal plane, with smaller amounts of carboxyl, carbonyl, phenol, lactone, and quinone at the sheet edges. The wide range of oxygen functional groups on both basal planes and edges of GO makes it to be readily exfoliated and functionalized to yield well-dispersed solutions of individual graphene oxide sheets in both water and organic solvents, thus its applications in nanocomposites (Singu & Yoon, 2019; Yang et al., 2019)

Generally, GO instead of graphene is preferred for combining with other materials because of its abundant oxygen-containing groups, promoting connections with other functional groups through covalent interactions (B. Wang et al., 2015; Wei et al., 2013). Many polymeric chains can also be easily grafted to GO, e.g., poly(ethylene glycol), polylysine, polyallylamine, poly(vinyl alcohol), etc., for preparation of graphene-polymer composites (K. Zhang et al., 2010).

GO and rGO are intrinsically negatively charged due to their oxygen-containing groups, which could easily be assembled with positively charged materials via electrostatic interactions (Cho et al., 2017).

GO sheets are heavily oxygenated graphene (bearing hydroxyl, epoxide, diols, ketones and carboxyls functional groups) that can alter the van der Waals interactions significantly and be more compatible with organic polymers (Barzegar et al., 2015; Castaldo et al., 2019; Y. Liang et al., 2020; Maráková et al., 2019). GO has attracted considerable attention as a nanofiller for polymer nanocomposites because of additional carbonyl and carboxyl groups located at the edge of the sheets, which makes graphene oxide sheets strongly hydrophilic, allowing them to readily swell and disperse in water.

Covalent modifications are commonly used in GO. GO has a layered material consisting of hydrophilic oxygenated graphene sheets carrying oxygen functional groups (Dikin et al., 2007) of hydroxyl, epoxy, carbonyl and carboxyl on their basal planes and edges, which allows the attachment of other functional groups through typical organic reactions, such as amidation, silanization, esterification, substitution and cycloaddition (Z. Fan et al., 2013; S. Hou et al., 2010; Rus et al., 2019; Sayyar et al., 2013). Modification via amidation provides reaction of GO to functional molecules such as amino acids (Mallakpour et al., 2014), casein phosphopeptides (Z. Fan et al., 2013), polyethylene glycol (PEG) (Jin et al., 2012; W. Li et al., 2014), chitosan (Depan et al., 2014), polyethyleneimine (Kim et al., 2011; H. Liu et al., 2013; X.-Z. Tang et al., 2016), acid pectinase (Y. Liu et al., 2014), poly(L-lysine), polyurethane (H. Liu et al., 2013) and others (Karousis et al., 2011; Long et al., 2014; Zhu et al., 2011). Amidation, estérification (N. A. Kumar et al., 2012; DEVI et al., 2014; W. Li et al., 2014; Sayyar et al., 2013; J. Zhou et al., 2014) and silanization (L. Chen et al., 2014; S. Hou et al., 2010; Lin et al., 2011; H. Yang et al., 2009; W. Zhang et al., 2013) are other approches to modify GO with numerous functionalities. Research groups are now carrying out exciting work using functionalised graphene, GO and its composites in applications such as super capacitors (Velmurugan et al., 2016) (Veeramani et al., 2016) (Amin, 2017) ; (Ahirrao et al., 2018), (Qu et al., 2019), fuel cell (Marinoiu et al., 2018) (Perveen et al., 2018); (Arukula et al., 2019)) (Baruah & Kumar, 2018)(, (Liao & Wu, 2019); (A.-Y. Wang et al., 2019), batteries (Du et al., 2018), (N. Kumar et al., 2019); (Gao et al., 2019); (X. Hu et al., 2019); (Wu, 2019); (A.-Y. Wang et al., 2019), photocatalysis; (R. Sharma et al., 2019; B. Sharma et al., 2018) (Ton et al., 2018); (M. Wang et al., 2018)., photovoltaics (W. C. Dong, 2014), biosensors (Feng et al., 2015).; (Arduini et al., 2016); (Raicopol et al., 2016), (Chen et al., 2018) (Gupta & Meek, 2018); (Puiu & Bala, 2018); (M. Wang et al., 2018), sensors (Tian et al., 2019), gas sensors (Achary et al., 2018); (Chen et al., 2018), photonics, solar cells (S. Das et al., 2018); (Giuri et al., 2018); (Mehmood et al., 2018); (Timoumi et al., 2018); (Gao et al., 2019), lightemitting diodes, laser, optoelectronics, photocatalyst (Hafeez et al., 2018); (Jose et al., 2018); (Labhane et al., 2018); (Mahvelati-Shamsabadi et al., 2018), (Sephra et al., 2018); (Xu et al., 2018); (Jia et al., 2019) ; (Qu et al., 2019), thin-film transistors (M. Zhang et al., 2014), memristive device (Aziz et al., 2019), tissue engineering and field effect transistor (FET) ;(Mukherjee et al., 2015), (Pachauri & Ingebrandt, 2016; Siddique et al., 2017).

1.3. Reduced graphene oxide (rGO)

rGO can be produced by reducing GO by thermal reduction and electrochemical reduction of GO Figure 1. A majority of graphene/polymer composites investigated are fabricated using GO, chemically reduced graphene oxide (CRGO), or thermally reduced graphene oxide (TRGO) as fillers Tables 1 and 2.

Table 1. Covalent modification of graphene and its derivatives

Polymer	Form of graphene	Type of reaction	Notable findings	References
PEG	GO	Esterification	Drug delivery ability	(Pal et al., 2019)
PVA	GO, r‐GO	Esterification	Stereoselectivity	(B. Wang et al., 2014)
PVA	GO	Esterification
PEI	r‐GO	Amidation	Formation of hybrid carbon films for supercaps.	(Zhu et al., 2011, Z. Fan et al., 2013)
Polycaprolactone	GO, r‐GO	Esterification	Better mechanical properties	(Sayyar et al., 2013)
Casein phosphopeptides	GO	Amidation	Nonvolatile rewritable memory effect	(Z. Fan et al., 2013)
P3HT	GO	Esterification	Enhanced power conversion efficiency
Perfluorophenyl azides	r‐GO	Nitrene cycloaddition	Improved dispersability, electrical conductivity	(G. H. Yang et al., 2017)
PAc	Graphene	Nitrene cycloaddition	Better solubility
Epoxy	GO	Cross‐linking by amine‐induced ring opening	Improved mechanical properties	(Chang et al., 2014)
Epoxy	Graphene	Cross‐linking by amine‐induced ring opening		(L. Chen et al., 2014)
PNIPAM	GO	Cross‐linking by esterification/nucleophilic substitution	Thermal and pH responsive hydrogels
MA‐g‐PE	GO	Maleic ring‐opening	Improved mechanical properties
Amino acid	GO	Nucleophile epoxy ring‐opening	Amplified biosensor	(Mallakpour et al., 2014)
PNIPAM	GO	ATNRC	Improve dispersability in water and organic solvents	(Salavagione et al., 2011)
PMMA	r‐GO	Radical grafting	Electrical conductivity	(Rajabi et al., 2019)
PS	GO	Click	Solubility in organic media	(Yu et al., 2014)
PNIPAM	GO	Click	Drugs delivery
PEG	rGO	Click	Tissue engineering	(Pal et al., 2019)
Poly(fluorene)	GO	Click	Charge separation in the solid state

Table 2. Methods for grafting polymers from graphene and its derivatives

Polymers	Graphene type	Type of Polymerisation	Findings	Reference
PS, PtBA, PMMA	GO	ATRP	Enhanced solubility <?br?>Mechanically flexible macroporous carbon films	,(Rajabi et al., 2019, R. Sharma et al., 2019)
PMMA	GO	ATRP	Better solubility, use as filler of pure PMMA, improved mechanical properties	(Rajabi et al., 2019)
PDMAEMA	GO	ATRP	Improved solubility
PS	r‐GO	ATRP	Enhanced mechanical properties <?br?>Control of grafting density	(Ding et al., 2015) (Yu et al., 2014)
PEMA	r‐GO	ATRP	Enhanced diffraction efficiency
PtBA	GO	ATRP	Integration into an electroactive polymer matrix
PMMA	GO	radical	Better thermal and mechanical properties	(R. Sharma et al., 2019),(Rajabi et al., 2019)39
PU	r‐GO	Condensation	remarkable thermal and mechanical properties	(Adak et al., 2019)
PCL	GO	Ring‐opening	Improved mechanical properties	(Babaie et al., 2019)
PS	GO	RAFT	Better mechanical and thermal properties	(Yu et al., 2014),(Ding et al., 2015)
p‐PEK	Graphite	electrophilic	substitution
PP	GO	Ziegler—Natta	Enhanced electrical conductivity	(Ajorloo et al., 2019)

Several researchers have used rGO for fabrication of graphene-polymer composite. A majority of graphene/polymer composites investigated by various researchers are fabricated using GO, chemically reduced graphene oxide (CRGO), or thermally reduced graphene oxide (TRGO) as fillers.

This review aims at summarizing the incorporation of graphene in conducting polymer, biopolymer, thermoplastic and thermosetting polymer matrices, highlighting the present limitations, discussing the existing challenges and discussing its possible applications.

2. Surface modification of graphene materials

Graphene as a bulk material agglomerate in a polymer matrix (Stankovich, Dikin et al., 2006) and oxidation followed by chemical functionalization will facilitate the dispersion and stabilize graphene to prevent agglomeration (Wang et al., 2018; T. Wei et al., 2009). Small functional groups are attached to graphene (Bourlinos et al., 2009) or polymer chains [(Bai et al., 2011; Punethaa et al., 2017) (Salavagione et al., 2011) (Stankovich, Piner et al., 2006). Solubility and processability are improved by chemical functionalization of graphene. Functionalisation also enhances the interactions with organic polymers (Worsley et al., 2007). A lot of works have been done by researchers on the amination, esterification (E. Eskandari et al., 2020; Park et al., 2019; Sayyar et al., 2013), isocyanate modification (Stankovich et al., 2007), salination (Li et al., 2020) and polymer wrapping (Salavagione et al., 2011) as routes for the functionalization of graphene. Soluble Organo-modified graphene has been produced by the reduction of GO in a stabilization medium (Park & Ruoff, 2009), covalent modification by the amidation of the carboxylic groups (Z. Fan et al., 2013; Zhu et al., 2011), non-covalent functionalization of reduced graphene oxide (Wang et al., 2018), nucleophilic substitution to epoxy groups (Marinoiu et al., 2018) and diazonium salt coupling (L. Wei et al., 2019).

2.1. Covalent modification of graphene

Covalent bonding: typically, a surface functional group is grafted to the graphene wall and is subsequently exploited for creating covalent bonds with the polymer matrix.Different organic compounds such as amines, alkyl lithium reagents, isocyanates, and diisocyanate compounds have been used for covalent modification (G. H. Yang et al., 2017; Salavagione et al., 2011) Table 1 & Figure 2. These compounds reduce the hydrophilic character of graphene oxide sheets by forming amide and carbamate ester bonds with the carboxyl and hydroxyl groups, respectively. Recently, hydroxylated aryl groups were covalently attached to graphene through a diazonium addition reaction (Lonkar et al., 2015). Xiang et al. (Xiang et al., 2019) fabricated Cellulose-rGO-TDI PA6 molecular chains were covalently bonded with conductive cellulose skeleton by reaction between imino groups of PA6 and isocyanate groups of Cellulose-rGO-TDI. Karousis et al (Karousis et al., 2011) reported successful covalent functionalization of graphene oxide (GO) with 5-(4-aminophenyl)-10,15,20-triphenyl-21,23 H-porphyrin (H2P). Zhang et al. (P. Zhang et al., 2019) demonstrated the fabrication of WPU composite by covalently conjugating with hydroxyl-functionalized graphene oxide (fGO), which was wrapped with P-N flame retardants, using an in situ polymerization strategy. Minar et al. (Minář et al., 2019) investigated the covalent functionalization of graphene oxide (GO) with poly(ε-caprolactone) and its application for the in situ synthesis of polyamide 6 nanocomposites. Amrollahi et al. (Amrollahi et al., 2019) successfully polymerized emeraldine base form of polyaniline on the graphene oxide nanosheets in two forms of non-covalent bonding through π-π interactions between quinoid ring of PANI and basal plane of graphene oxide and covalent bonding through reaction with epoxide group. Jin et al. (J.-U. Jin et al., 2019) synthesized methylpiperidine-functionalized graphene oxide (MP-GO) by introducing 4-amino-1-methylpiperidine into reactive epoxy and/or carboxylic acid groups on pristine GO. Then, they applied the MP-GO as a curing catalyst for polyimide (PI) nanocomposites. The MP-GO was found to be an effective base-catalysts for the thermal conversion of polyamic acid (PAA) precursor to PI. Interestingly, when 3 wt % of MP-GO was added to the PI matrix, the complete imidization of nanocomposites was achieved at a temperature lower than 200 °C. Yao et al. (Yao et al., 2015) reported a Suzuki coupling reaction between thiophene, polythiophene (PTH) and brominated graphene (Br-Gra) in the same polymeric method. The obtained conjugated system of modified graphene was characterized by various techniques, including Fourier transform-infrared (FT-IR), ultraviolet-vis (UV-vis), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), fluorescence emission spectra, 1 H-NMR spectra, and Raman spectroscopy. All the results revealed that thiophen and PTH were successfully and covalently grafted to the graphene.

Xiang et al. (Xiang et al., 2019) fabricated conductive cellulose skeleton (Cellulose-RGO-TDI) and PA6 molecular chains were covalently bonded with conductive cellulose skeleton by reaction between amino groups of PA6 and isocyanate groups of Cellulose-RGO-TDI. As a result, PA6/Cellulose-RGO-TDI nanocomposite with high electrical and thermal conductivity was fabricated via reactive melt processing.

2.2. Non-covalent functionalization of graphene

The noncovalent modification of graphene is mostly based on van der Waals forces (G. H. Yang et al., 2017; Pang et al., 2019), electrostatic interactions, or π–π interactions (Ansari et al., 2014; G. H. Yang et al., 2017; Wang et al., 2018) with organic molecules or polymers. Compared with covalent functionalization, noncovalent functionalization would not disrupt the extended π system of graphene nanostructures, and thus not affect important properties of graphene, such as electrical conductivity and mechanical strength (G. H. Yang et al., 2017). Wang et al. (Wang et al., 2018) non-covalently functionalized reduced graphene oxide (rGO) and reinforced poly(vinyl alcohol) (PVA) nanocomposites were prepared by solution mixing. The agglomeration of graphene sheets was prevented by using surface modifying agent poly(sodium 4-styrenesulfonate) (PSS). The surface modifiers are attached to the graphene sheets via van der Waals forces, pep interactions, ionic interactions, hydrogen bonding, and so on.

The improved mechanical properties, including the Young’s modulus and tensile strength of the PVA/rGO nanocomposites compared to neat PVA were attributed to the strong interactions between PVA and rGO such as π–π, hydrogen bonding, and CH–π. A 55% maximum increase in the modulus was obtained by adding only 0.1 wt% rGO, and an increase of 48% in tensile strength was achieved by adding 0.3 wt% rGO. In addition, the thermal properties of the nanocomposites were also improved, which was attributed to the restriction of graphene oxide (GO)/rGO sheets on the chain mobility of polymers on the GO/rGO sheets surface.

2.2.1. π–π interactions

π–π interactions occur between two relatively large non-polar aromatic rings that have overlapping π orbitals. Aromatic π–π interaction is regarded as one of the most interesting noncovalent interactions. The non-covalent π–π interaction has been widely used for the synthesis of G–polymer nanocomposites. Recently Wei et al. (L. Wei et al., 2019) design s π–π taking interface for improving the strength and electromagnetic interference shielding of ultrathin and flexible water-borne polymer/sulfonated graphene composites.

Ansari et al. (Ansari et al., 2014) studied the π–π interactions between Pani and GN while Yang et al. (G. H. Yang et al., 2017) reviewed π–π interactions in functionalisation of graphene and GO.

Fu et al. (Fu et al., 2019) synthesized polymeric ionic liquid (PIL) and used it in the wet spinning of calcium alginate (CaAlg) fiber. PIL was used as an intermediate to coat graphene with outer layer of CaAlg fiber to obtain conductive core-sheath CaAlg/Graphene (CaAlg/G-PIL) fibers. They proposed blend that enhanced the properties of the fiber due to the π-π and cation-π interactions between PIL and graphene, and the electrostatic interactions between PIL and CaAlg. The composition and chemical structure of the composite fibers were characterized by infrared spectroscopy, Raman spectroscopy, scanning electron microscopy and thermogravimetric analysis.

Wang et al. (Wang et al., 2018) non-covalently functionalized rGO and prepared reinforced poly(vinyl alcohol) (PVA) nanocomposites by solution mixing. The agglomeration of graphene sheets was prevented by using surface modifying agent poly(sodium 4-styrenesulfonate) (PSS). The improved mechanical properties, including the Young’s modulus and tensile strength of the PVA/rGO nanocomposites compared to neat PVA were attributed to the strong interactions between PVA and rGO such as π–π, hydrogen bonding, and CH–π. A 55% maximum increase in the modulus was obtained by adding only 0.1 wt % rGO, and an increase of 48% in tensile strength was achieved by adding 0.3 wt % rGO.

2.2.2. Hydrogen Bonding

Hydrogen bonding interaction occurs between the residual oxygen-containing groups of G sheets and polar groups on other polymers.

Gupta et al. (Gupta et al., 2016) used poly(ethylene glycol) 200 (PEG 200) to functionalize reduced graphene oxide by γ-radiolysis. Hydrogen bonding occurred between the hydroxyl groups of rGO and the oxygen atoms of PEG 200 molecules, resulting in an increase in the spacing of the graphene sheets and a decrease in the defect density of the carbon network in rGO.

2.2.3. Ionic interactions

Ionic interactions can also occur between rGO and functionalized polymers, such as amine-terminated polystyrene, by which hydrophilic rGO might be transformed into a lipophilic rGO/polymer composite dispersible in organic solvents (Choi et al., 2010)

Wang et al. (Wang et al., 2018) non-covalently functionalized reduced graphene oxide (rGO) reinforced poly(vinyl alcohol) (PVA) nanocomposites were prepared by solution mixing. The agglomeration of graphene sheets was prevented by using surface modifying agent poly(sodium 4-styrenesulfonate) (PSS).

Biswas et al. (Biswas et al., 2018) prepared waterborne graphene/polymer nanocomposite of high uniformity and stability is developed via in situ incorporation of cationic graphene (CGR) in anionic poly(urethane-acrylate) (WPUA) prepolymer. Compared to WPUA and GR-WPUA, the water resistance, tensile strength and anticorrosive properties of CGR-WPUA are significantly increased. With respect to WPUA and GR-WPUA coatings with 0.5% GR addition, the impedance modulus at lower frequency decreased by one order of magnitude and the coating resistance decreased by two orders of magnitude after 25 days of immersion.

2.3. Nucleophilic substitution

Nucleophilic ring-opening reaction of the epoxy group can also be used for the covalent modification of GO. The nucleophilic addition of organic molecules to the graphene surface is the best way to achieve the bulk production of surface-modified graphene. This method is very advantageous in many aspects. For example, water can be used as a solvent, lower cost amine compounds can be used as surface modifying agents, the reaction can be carried in air, and surface-modified graphene can be dispersed easily in different organic media.

Marinoiu et al. (Marinoiu et al., 2018) prepared Iodine-doped graphene by nucleophilic substitution of GO by reduction with HI. The as-synthesized graphene with incorporation of iodine possesses unique structure revealing high surface area, mesopores and vacancies.

Mallakapour et al. (Mallakpour et al., 2014) covalently functionalised graphene sheets with aromatic–aliphatic amino acids (phenylalanine and tyrosine) and aliphatic amino acids (alanine, isoleucine, leucine, methionine and valine) by simple and green procedure. Natural graphite was converted into graphene oxide (GO) through strong oxidation procedure; then, based on the surface-exposed epoxy and carboxylic acid groups in GO solid, its surface modification with naturally occurring amino acids, occurred easily throughout the corresponding nucleophilic substitution and condensation reactions.

Wang et al. (Wang et al., 2019) obtained a new type of polyamine-modified rGO nanocomposite through a facile synthesis process, wherein rGO was chemically anchored by triethylenetetramine (TETA) molecules via nucleophilic reaction, forming rGO-TETA intermediate compounds that binds to Fe₃O₄ nanoparticles by forming stable chelating bonds, finally resulting in rGO-TETA-Fe₃O₄ nanocomposite.

2.4. Diazonium salt coupling

Wei et al. (L. Wei et al., 2019) prepared ultrathin and flexible water-borne polymer/sulfonated graphene nanosheets (S-GNS) composite by a facile self-assembly method in latex. The water-dispersible S-GNS was made by modifying the GNS with aryl diazonium salt.

π-π stacking interface design strategy is proposed to prepare ultrathin and flexible water-borne polymer/ sulfonated graphene nanosheets (S-GNS) composite by a facile self-assembly method in latex. The waterdispersible S-GNS was made by modifying the GNS with aryl diazonium salt. Styrene was introduced into water-borne polymer to form π-π stacking interaction with S-GNS. The composite shows good mechanical properties and high electromagnetic interference (EMI) shielding effectiveness (SE) in the X band (8.2e12.4 GHz). By Raman spectra, tensile tests, and simulation calculation, π-π stacking interaction is proved to be an effective interface bonding to enhance the mechanical properties. With a filler loading of 20 wt %, the tensile strength of composites is enhanced by 578%. The incorporation of25 wt% SGNS leads to a high EMI SE of 21.5 dB at 0.05 mm thickness, which remains unchanged after 1000 times bending. The specific SE/thickness (SSE/t) of the composite is as high as 2663 dB cm2/g, outperforming the ever reported materials with similar filler loading (25 wt %). The composite can be used as coating on the surfaces of solids, indicating the great potential for wide application.

2.5. Polymer modification of graphene and it derivatives polymerisation

Covalent method is a direct way to form covalent bonding between graphene layers and polymer chains. In this method, hybridization of some carbon atoms may change from sp2 into sp3, which disrupts the conjugated structure of graphene layers. Covalent polymer-grafting was commonly carried out by three methods of “grafting from”, “grafting through”, and “grafting to”, In the “grafting from” method, polymerization proceeds from initiator moieties attached to the graphene layers. By the “grafting through” method, a growing chain is anchored to the layers via the graphene-attached monomer moieties. In grafting to method, a preformed polymer chain can be attached to graphene layers by various coupling reactions.

Table 2 shows various polymers used in modification of GO and pristine graphene by grafting to method. Polymer-functionalized graphene is an efficient nanofiller in polymer composites to improve its mechanical, electrical, and thermal properties (Ahmad et al., 2019).

2.6. Electrochemical modification of graphene

Raicopol et al. (Raicopol et al., 2016) created glucose amperometric biosensors with improved analytical characteristics using graphene as support for glucose oxidase. First, a reduced graphene oxide film was obtained on the glassy carbon electrode surface by direct electrochemical reduction of graphene oxide from a suspension in water. For facilitating the oxidation of the enzymatically generated H₂O₂, in a second step, Pt nanoparticles were electrodeposited on the graphene-modified electrodes. The obtained biosensors showed good analytical performances in terms of high sensitivity and wide linear range

3. Preparation of graphene-based polymer composites

Majority of graphene/polymer nanocomposites are produced using GO, RGO, or thermally reduced graphene oxide (TRGO) as fillers. Dispersion of graphene particles in a polymer matrix has opened a new and interesting area in materials science. The extent of the improvement in graphene-based polymer nanocomposite is related directly to the degree of dispersion of the nanofillers in the polymer matrix. The most important aspect of these nanocomposites is that all these improvements are obtained at a very low filler loadings in the polymer matrix. There are many studies on graphene composites based on a range of polymers, including epoxy (Amrollahi et al., 2019; Anwar et al., 2016; Bayat et al., 2019), PMMA (Rajabi et al., 2019; R. Sharma et al., 2019), polyamide6/graphene (Dixon et al., 2015), nafion (D. C. Lee et al., 2014), polyimide12/graphene (Dorigato & Pegoretti, 2019), polypropylene (Ajorloo et al., 2019; Mistretta et al., 2019; You et al., 2017), LLDPE, HDPE, polystyrene (Ding et al., 2015; Giuri et al., 2018; Yu et al., 2014), PPS (Z. Fan et al., 2013), Nylon, polyaniline (Alipour et al., 2019; Amrollahi et al., 2019; Ansari et al., 2014; Chethan et al., 2019), phenylethynyl-terminated polyimide (Loeblein et al., 2015; Ma et al., 2017; Yang et al., 2015), nafion (D. C. Lee et al., 2014) and silicone rubber (Barua et al., 2019). Graphene-filled polymer composites are commonly fabricated by coating, casting (Babaie et al., 2019; Bahrami et al., 2019; Chang et al., 2014; Giuri et al., 2018; Thomas et al., 2019), solution mixing (Anwar et al., 2016; Cui et al., 2016; Jun et al., 2015; Othman et al., 2019), melt blending (Adak et al., 2019; Cui et al., 2016; Nordin et al., 2019) and in situ polymerization (Adrian et al., 2019; P. Eskandari et al., 2019). Graphene can be added during the mixing process in three different ways: (i) solvent mixing, (ii) melt processing, and (iii) in-situ polymerization. Some researchers also tried to coat graphene onto the fiber surface after spinning. There are also different spinning methods to prepare graphene/polymer fibers: wet-spinning (Li et al., 2014), (Fu et al., 2019), melt (W. Hou et al., 2014), and electrical-spinning (Bahrami et al., 2019), (C. Wang et al., 2013) Table 3.

Table 3. Processing techniques of graphene-polymer nanocomposites

Polymer	Nanofiller	Processing technique	References
Poly(vinyl chloride) (PVC)	Reduced graphene oxide	Solution(Insitu Polymerisation)	(Rafiee & Eskandariyun, 2019)
Poly(methyl methacrylate) (PMMA)	Reduced graphene oxide	Solution(Insitu Polymerisation)	(R. Sharma et al., 2019)
Polyurithane	Graphene/Reduced graphene oxide	Melt/ Solution(Insitu Polymerisation)	(Adak et al., 2019)
Polystyrene (PS)	Graphene/Reduced graphene oxide	solution	(M. Z. Tang et al., 2014)
Polyacrylonitrile (PAN)	Graphene/graphite/GO	Electrospinning/Solvent Method	(Wu et al., 2012, Rahimi-Aghdam et al., 2020)
Polycarbonate	Graphene/Reduced graphene oxide	Melt	(Lago et al., 2016)
Rubber/natural rubber	Graphene/Reduced graphene oxide	Melt Casting	(Thomas et al., 2019)
Chitosan	GO	Solution mixing	(Suneetha et al., 2019)
Poly(vinylidine fluride) (PVDF)	Reduced graphene oxide	Solvent /Melt Method	(Xiao et al., 2016)
Nafion	GO	Solution mixing/Insitu	(D. C. Lee et al., 2014)

3.1. Dip coating

Dip coating technique is used when liquid polymer is the matrix material for the fabrication of the composite. It involves the immersion of pristine graphene foam in the polymer solution and varying the dipping time, GrF content that would determine the quality and formation of the coating and composites. Curing of the polymer-GrF system takes place under specific time and temperature conditions after the completion of the dip coating Figure 3. Recently, a lot of work have been done using dip coating by following researchers:

Neito et al. (Nieto et al., 2015) synthesised Graphene foam (GrF)/polylactic acid–poly-ε-caprolactone copolymer (PLC) hybrid (GrF-PLC) scaffold in order to utilize both the desirable properties of graphene and that of foams such as excellent structural characteristics and a networked 3-D structure for cells to proliferate in. The hybrid scaffold is synthesized by a dip-coating method that enables retention of the porous 3D structure. The excellent wettability of PLC with graphene foam along with the formation of PLC bridges leads to a ≈ 3700% enhancement in strength and a ≈ 3100% increase in ductility in the GrF-PLC scaffold.

Samad et al. (Samad et al., 2015) developed free-standing graphene foam (GF) by a three-step method: (1) vacuum-assisted dip-coating of nickel foam (Ni-F) with graphene oxide (GO), (2) reduction of GO to reduced graphene oxide (rGO), and then (3) etching out the nickel scaffold. Pure GF samples were tested for their morphology, chemistry, and mechanical integrity. GF mimics the microstructure of Ni-F while individual bones of GF were hollow, because of the complete removal of nickel. The GF-PDMS composites were tested for their ability to sense both compressive and bending strains in the form of change in electrical resistance.

3.2. Casting

Casting is fabrication method that enhances complete infiltration of graphene foam, with the polymer solution. The polymer is poured into a mold containing GrF. The polymer goes deep through the pore and coats the nodes and branches of the GrF. The polymer containing the embedded GrF polymerizes when subjected to heat and forms a resultant GrF-based polymer composite. A lot of work have been done using casting by following researchers:

Thomas et al.(Thomas et al., 2019) recently scrutinized the effect of pristine few layers graphene incorporation into NBR matrix via a green approach in terms of mechanical properties and two different fabrication routes viz. latex casting and latex casting followed by dry rubber mixing. Large scale production of pristine few layers graphene was accomplished by means of planetary ball milling and was characterized using X-ray diffraction (XRD), Raman spectroscopy and Transmission electron microscopy (TEM) analysis. The produced graphene and procured MWCNT was then successfully incorporated (separately) into NBR latex.

Bahrami et al. (Bahrami et al., 2019) fabricated PU/G as membranes using electrospinning and solvent casting. The membranes were characterized for their electrical, mechanical, physiochemical, and biological properties. The electroconductivity of the electrospun mats was significantly higher than related casting films and G acted as electrical bridges leading to the improved electroconductivity of the composites. The mechanical properties of the mats were higher than those of the films and improved by increasing G concentration up to 5 wt% and then reduced.

3.3. Spray deposition

Spray deposition is a method adopted for the fabrication of carbon-based composites. The technique sometimes uses electrostatic spray to deposit polymer matrix in the powder form on the graphene foam for the formation of graphene foam-based polymer composite Figure 4. Recently, a lot of work have been done using spray deposition by following researchers:

Mohammed et al. (Mohammed et al., 2019) spray-coated etched-tapered single-mode fiber (SMF) sensors with polyaniline (PANI)/graphite nanofiber (GNF) nanocomposite at room temperature. SMF was etched with hydrofluoric acid and subsequently tapered in a glass processing workstation. The PANI/GNF ratio in the nanocomposite polymer film is 2:1. The etched–tapered SMF spray-coated with PANI/GNF nanocomposite by varying the waist SMF diameter, a comprehensive investigation of the optical response of PANI/GNF nanocomposite-coated SMF and etched–tapered SMF sensors at room temperature were performed.

Soltani et al. (Soltani-kordshuli et al., 2016) fabricated highly conductive transparent graphene-doped PEDOT: PSS composite thin film using conventional and substrate vibration-assisted ultrasonic spray coating (SVASC). To suppress the challenges associated with spraying of the precursor solution containing graphene, graphene sheets were broken by sonication and were uniformly dispersed and stabilized in PEDOT: PSS aqueous solution using isopropyl alcohol (IPA).

3.4. Vacuum infiltration

This method involves merely infusion of the polymer into GrF inside a vacuum chamber. It has been used in the development of GrF-based polydimethylsiloxane (PDMS) composite (Cao et al., 2019) (Y.-H. Zhao et al., 2016), investigated the integration of PDMS with GrF takes place under vacuum, where the PDMS infiltrates through the macroporous architecture of the GrF, as air pulled out from the vacuum chamber (Bustillos et al., 2018).

Tang et al. (L.-S. Tang et al., 2017) prepared polyethylene glycol (PEG)/graphene oxide aerogel (GA) composite phase change materials (PCMs) by introducing PEG into GAs from graphene oxide (GO) with different oxidation degree via vacuum impregnation. The structures of GAs were tuned by the oxidation levels of GO. A series of characterizations were used to analyse the chemical structure of GOs, including XRD, Fourier transform infrared spectrum (FTIR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analysis. Structural analyses confirmed that the oxygenated functional groups increased and the hydroxyl groups were transformed into carboxyl and epoxy groups with increasing oxidation level. In addition, the graphitic nature of GO decreased while the sp3 domains of GOs increased owing to the disruption of the graphitic stacking order. Morphology analysis showed that the breakage of graphene sheet became more serious with the oxidation level increasing. When GAs prepared with GOs of higher oxidation levels were used, the composite PCMs showed excellent shape-stability during phase change and excellent thermal repeatability. The change of dimension for PGA6-40 heated from 35 °C to 150 °C was negligible under the load of a constant force (7 N). Efficient photo-to-thermal energy conversion and storage was realized in the composite PCMs.

Zhang et al.(M. Zhang et al., 2015) reported the fabrication and characterization of three-dimensional (3D) graphene aerogel (GA)–polydimethylsiloxane (PDMS) composites (GAPC) with outstanding mechanical, electrical and thermal properties. GAPC was fabricated by impregnating 3D GA frameworks with PDMS via ice-bath-assisted infiltration and vacuum curing processes. Because of the well-interconnected 3D GA frameworks, GAPC exhibits extremely large deformability (compressive strain = 80% and tensile strain = 90%), high electrical and thermal conductivities (1S/cm and 0.68 W/ (mK), respectively), a stable piezo-resistance effect, rapid electric Joule heating performance ((dT/dt) max>3°C/s under a heating power of 12 W/cm³), and high hydrophobicity (contact angle = 135°).

3.5. Solution mixing

Solution mixing is considered as the simplest and the most effective way of processing polymer/graphene nanocomposites. The method involves a pre-dispersion of the nanofiller in an appropriate solvent, usually through ultrasonication, the subsequent incorporation of the polymer and the removal of the solvent by distillation or evaporation. Solution mixing is an efficient technique only if the polymer and the graphene sheets are compatible with the used solvent.

A variety of polymer nanocomposites have been prepared using this method; Polyurethane-graphene nanocomposite (Adak et al., 2019) Epoxy/graphene (Anwar et al., 2016)

Recently, a lot of work have been done on using solution mixing for fabrication of graphene polymer composite by following researchers:

Othman et al. (Othman et al., 2019) used commonly synthesis methods of graphene-based polymer composites (GPC) such as solution and melt mixing, and in-situ polymerization are compiled. An emphasis is given to the mechanism of barrier properties provided by GPC’s in terms of their state of dispersion and the intrinsic properties. The advantages and limitations are also discussed to address the challenges for future research and potential applications of graphene-based polymer composites.

Babaie et al. (Babaie et al., 2019) synthesized polycaprolactone (PCL)/ graphene composite via solution casting method. Hydrogen nuclear magnetic resonance (1 H-NMR) was used to confirm the chemical structure of PCLs and calculate their actual molecular weights. The chemical structure and hydrogen bonding content of PUs and their nanocomposites were investigated by FTIR. According to the results, the hydrogen bonding contents of nanocomposites were reduced by graphene nanosheets inhibition from the formation of hydrogen bonds between polyurethane chains. Thermal properties and crystalline morphology of samples were studied using differential scanning calorimetry (DSC) and XRD.

3.6. Melt mixing

Melt mixing is a simple processing method to fabricate G-polymer composites. The main advantages of this method are its versatility, cost-effectiveness, and environmentally friendliness. Melt mixing is considered as the most economical method to produce graphene-based polymer nanocomposites. In this technique, nanofiller is physically mixed with polymer melt. It is widely used for thermoplastic polymers at elevated temperatures. No solvent is required in this method, and graphene or its derivatives are mixed with the polymer matrix in the molten state. A thermoplastic polymer is mixed mechanically with graphene or its derivatives at elevated temperatures using conventional methods, such as extrusion and injection molding. The process has the advantages of cost-effectiveness of the process and to the possibility of processing large quantities of materials. The technique does not require the use of solvents and does not involve the presence or removal of any other additive/compound/chemical within the mixing unit. Recently, a lot of work have been done using melt mixing by following researchers:

Adak et al. (Adak et al., 2019) produced PU/functionalized-graphene nanocomposite (PGN) films by solution master-batching and subsequent melt mixing, followed by compression molding, with varying concentrations (0–3 wt %) of graphene, which resulted in uniform dispersion and partial exfoliation of graphene-sheets in PU matrix. The helium gas barrier of nanocomposite films improved gradually with increasing graphene concentration, showing about 30% reduction in gas-permeability at 3 wt% graphene-loading. The tensile strength and stiffness of the nanocomposite films also increased significantly with increasing concentration of graphene. The prepared PGN films were exposed to accelerated artificial weathering conditions up to 300 h.

Nordin et al. (Nordin et al., 2019) prepared poly(lactic acid) (PLA)/Thermoplastic polyurethane (TPU) graphene composite, by melt mixing process. The electrical conductivity was tested using resistance meter and showing that the resistivity of the composite started to percolate in the presence of GnP and the percolation threshold change when blend composition change, by showing that at PLA90/TPU10 show the lowest percolation threshold. The localization of GnP in PLA/TPU blend was predicted by calculation of wetting coefficient along with Owen and Wendt equation and it is predicted that GnP preferentially in TPU phase. Elongation at break of the composite increased as the TPU content increased and when GnP were added in PLA50/TPU50 blend, the elongation at break of the blend.

Noorunnisa et al. (Noorunnisa Khanam et al., 2016) fabricated a composites of linear low-density polyethylene (LLDPE) and graphene nanoplatelets (GNPs) using a twin screw extruder under different extrusion conditions. The effects of screw speed, feeder speed and GNP content on the electrical, thermal and mechanical properties of composites were investigated. The inclusion of GNPs in the matrix improved the thermal stability and conductivity by 2.7% and 43%, respectively. The electrical conductivity improved from 10⁻¹¹ to 10⁻⁵ S/m at 150 rpm due to the high thermal stability of the GNPs and the formation of phonon and charge carrier networks in the polymer matrix.

A variety of polymer nanocomposites have been prepared using this method, i.e.

Polymer nanocomposites based on polyethylene-grafted maleic anhydride (PE-g-MA)/graphite (Torğut, 2019), epoxy/LDH, PS, polypropylene (PP) (Ajorloo et al., 2019; Mistretta et al., 2019; You et al., 2017; S. Zhao et al., 2014), poly(vinyl alcohol) (PVA)/graphene (Pegoretti & Traina, 2013; Rattanakot & Potiyaraj, 2018; C. Wang et al., 2013; Wang et al., 2018) and poly(vinyl chloride) (PVC)/graphene (Akhina et al., 2019)

3.7. In situ polymerization

In situ polymerization is a highly effective method to fabricate nanocomposites and its dispersion ability is better than other mixing methods. (M. Zhang et al., 2015)

In situ polymerization ensures a homogeneous dispersion of graphene or its derivatives without prior exfoliation step (Cui et al., 2016). The process involves the mixing of uniformly dispersed graphene nanosheets in solvent with a monomer (and/or oligomer) together with an initiator (e.g., thermal initiators and photo initiators) (Othman et al., 2019). The filler is first mixed with neat monomers (or multiple monomers), or a solution of monomers, followed by in situ polymerization. This process helps to increase the interlayer spacing (Sadasivuni et al., 2014; Chethan et al., 2019; Khan et al., 2019; R. Sharma et al., 2019)

Graphene nanosheets are added to the polymerizable master solution and physically mixed or sonicated; then the polymerization of the matrix occurs. By this route, it is possible to achieve very good dispersion of graphene sheets within the polymer matrix. A variety of polymer nanocomposites have been prepared using this method, i.e.polyamide6/graphneoxide (Dixon et al., 2015), polystyrene (PS)/graphene (Yu et al., 2014), poly(methyl methacrylate) (PMMA)/EG (Feizi et al., 2019), polystyrene sulfonate (PSSI graphene (Baruah et al., 2019; Giuri et al., 2018), Polybenzimidazol/GO (Ahmad et al., 2019), SnO2/rGO/PANI ternary nanocomposite (Zhang, Wu et al. 2019), polybenzenimidasole/graphene (Ahmad et al., 2019), polyamide6/gO (O’Neill et al., 2017) and polyimide (PI)/graphene (Yang et al., 2015). Others include polystyrene (PS)/ graphene (Baruah et al., 2019; Ding et al., 2015), poly(methyl methacrylate) (PMMA)/expanded graphite (EG)(Feizi et al., 2019; Mallik et al., 2019; Rajabi et al., 2019; R. Sharma et al., 2019; You et al., 2017), nylon-6 (PA6)/graphene and poly(vinylidene fluoride) (PVDF)/PMMA/graphene(Zhang, J. Chen et al., 2019) etc. However, in-situ polymerization is not an economically attractive or scalable method for dispersing nanoparticles into polymers compared to solution or melt compounding method. Recently, a lot of work have been done using in situ polymerisation by following researchers:

Ahmad et al. (Ahmad et al., 2019) fabricated polybenzimidazole (PBI)/xGnP nanocomposite films via in-situ polymerization of terephthalic acid and 3,3′,4, 4′-Tetraaminobiphenyl, using above xGnP-MSA solution as a reaction medium. The DC conductivity of PBI was raised by 12 orders of magnitude by reinforcement with xGnP nanosheets. The maximum tensile strength of 2076  MPa and tensile modulus of 3.41  GPa were achieved for the PBI/xGnP nanocomposite containing only 5  wt% xGnP.

Amrolahi et.al. (Amrollahi et al., 2019) synthesized PAN-graphene oxide nanocomposite via an in-situ polymerization process. The modified hybrid nanosheets were characterized by FTIR, Field Emission Scanning Electron Microscopy (FE-SEM), XRD, Ultraviolet–visible spectrophotometry (UV–Vis) and Raman spectroscopy. It was approved that the emeraldine base form of polyaniline has been successfully polymerized on the graphene oxide nanosheets in two forms of non-covalent bonding through π-π interactions between quinoid ring of PANI and basal plane of graphene oxide, and covalent bonding through reaction with epoxide group.

Baruah et al. (Baruah et al., 2019) synthesized reduced graphene oxide (rGO)/poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) nanocomposites via in situ polymerization technique. The synthesized nanocomposite films were implanted with Xe+ ions at different fluences of 3.3 × 1014, 3.3 × 1015 and 3.3 × 1016ionscm−2. The morphological, structural and elemental analyses of the implanted samples have been studied by FESEM, EDX, TEM, XRD, FTIR, Raman, RBS and XPS measurements. The electrochemical behavior of the irradiated electrodes has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in presence of 0.5 mM ZoBell’s solution containing 0.5% KCl.

Carrasco-Valenzuela et al. (Carrasco–Valenzuela et al., 2017) synthesized Graphene oxide/poly(3,4eethylenedioxythiophene) composites through Fenton’s reaction. The synthesis was performed using graphene oxide intercalated with iron (III) chloride and hydrogen peroxide. Then, in situ polymerization of 3,4eethylenedioxythiophene monomer via Fenton’s reaction on graphene oxide was accomplished. The composites exhibit a matrix growth of poly(3,4 eethylenedioxythiophene) chains on and around the graphene oxide layers presenting pp interactions between both materials. The electroactivity and optical properties of the composite were increased. The intercalation of iron (III) chloride is demonstrated through X ray diffraction as the interlayer distance of the graphene oxide sheets increased with the addition of the compound. Transmission electron microscopy analysis was carried out to establish that the polymer matrix was growing around the layers. The Fourier transform infrared spectroscopy measurements were implemented as a complementary technique for Raman to confirm the possibility of a pep interaction. The materials show a maximum fluorescence emission at 480 and 530 nm. Properties were measured to establish the presumable application in hybrid solar cells, due to the absorption of the composite in the UV and near IR region in a range of 210e350 nm and 700e800 nm. The composites exhibited a matrix growth of conducting polymer chains on and around the graphene oxide layers presenting redeox states with a potential application as a counter electrode.

3.8. Electrospinning

Electrospinning is a versatile technique for the preparation of polymeric nanofibers with diameters in the range of nanometers to micrometers. Many electrospun functional polymer nanofibers can be reinforced with nanocarbons, including CNTs, graphenes, nanodiamonds, nanodots etc. Introducing nanocarbons, especially carbon nanotubes and graphene, to polymer matrices can significantly enhance the mechanical, electrical, and thermal properties, leading to promising applications in biology and sensors (J. K. Y. Lee et al., 2018). Recently, a lot of work have been done using electrospinning by following researchers:

Correa et al. (Correa et al., 2019) built polycaprolactone (PCL) and rGO by electrospinning technique. The ratios of rGO/PCL employed were 0.25, 0.5, 0.75 and 1 wt%. Two different voltage setup (10 and 15 kV) and distance of 10 cm were used for electrospinning. Thermal, mechanical, morphological, electrical, porosity and absorption water tests were made to the scaffolds. Samples electrospun at 10 kV with rGO showed improvement in mechanical properties with an increase of 190% of Young’s Modulus in comparison with sample without rGO. Furthermore, samples electrospun at 15 kV showed an important deterioration with the addition of rGO but had an increase in the electrical conductivity and porosity. Overall, the addition of 0.75 and 1 wt% of rGO led to a detriment on properties due to formation of aggregates. The voltage on the electrospinning process plays a very important role in the final properties of the nanocomposites scaffolds of PCL-rGO.

Guo et al. (Y. Guo et al., 2018) fabricated thermally conductive CMG/polyimide (CMG/PI) nanocomposites via a sequential in situ polymerization and electrospinning-hot press technology. NH₂-POSS molecules were grafted on the GO surface, and CMG was obtained by the reaction between NH₂-POSS and GO. The thermal conductivity coefficient (l), glass transition temperature (Tg) and heat resistance index (THRI) of the prepared CMG/PI nanocomposites were all increased with increasing the CMG loading. The l value of the CMG/PI nanocomposites with 5 wt% CMG was significantly improved to 1.05 W m1 K1, about 4 times higher than that of the pristine PI matrix (0.28 W m1 K1). The corresponding Tg and THRI values were also increased to 213.0 and 282.3 1 C, respectively. Moreover, an improved thermal conductivity model was proposed and predicted the l values of the nanocomposites more precisely than those obtained from the typical Maxwell, Russell and Bruggemen classical models.

Hou et al. (W. Hou et al., 2014) fabricated polyamide 6 (PA6)/ graphene homogeneous nanocomposite in which graphene was well-distributed, leading to increasing physico-mechanical properties of the composite. Graphene was oxidized to form graphene oxide (GO), which was then reacted with amine compounds to obtain the graphene bonding with amine functional groups of—NH₂ and—(CH₂)₆NH₂ and their continuous nanocomposite fibers were prepared by use of melt spinning and drawing process. The grafting PA6 chains on graphene sheets were confirmed by FTIR, TGA and AFM measurements. Replacement of the—COOH group by—NH₂ and—(CH2)₆NH₂ in the composite of PA6 and graphene changed the grafting polymerization chemistry, thereby leading to the covalent attachment of longer graft polymer chains to the graphene. Tensile strength of the nanocomposite fibers containing the—(CH₂)₆NH2 functional group with 0.1 wt% graphene loading was significantly increased, over twice as high as that of neat PA6.

3.9. Electropolymerization

Electropolymerization is a novel method to fabricate G–polymer composites. This method has many advantages, such as short process, easy to control, and ecofriendly. In this method, two-electrode and three electrode electrolyte systems have been widely used. The G–polymer nanocomposites synthesized with this method can be used as electrochemical biosensors and in energy storage devices like electrochemical supercapacitors and batteries (M. Zhang et al., 2015). Recently many researchers have use electropoymerisation to fabricate G-polymer composite;

Rus et al. (Rus et al., 2019) synthesized Pyrrole/graphene nanocomposite in two steps by first incorporating pyridine-pyridazine functions on graphene surface through cycloaddition followed by electropolymerization of pyrrole in acetonitrile. The specific capacitance of the material was measured by galvanic charge-discharge cycles and the stability upon cycling investigated in various electrolytic media (acetonitrile, ionic liquid, acidic and neutral water) in comparison with non-functionalized graphene with or without PPy.

Liu et al. (Y. Liu et al., 2014) prepared RGO–PPy composites as an efficient Pt-free counter electrode for plastic dye-sensitized solar cells (DSCs) by electrochemical synthesis. The RGO–PPy composites were obtained by cyclic voltammetry (CV) in N2-saturated Na2SO4 aqueous solution (0.5 M) at a scan rate of 50 mV s − 1 between 0 and −1 V for 10 cycles. The electrochemical polymerization of pyrrole was conducted with a rigid fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) as the working electrode, a Pt wire as the counter electrode and an Ag/AgCl electrode as the reference electrode. All the electrochemical experiments were performed at room temperature in GO aqueous solution which was saturated with nitrogen gas prior to polymerization. As a result, a high-performance RGO–PPy composite for DSCs was fabricated. CV and electrochemical impedance spectroscopy (EIS) results demonstrated that the created RGO–PPy composite had a superior catalytic activity.

Bairagi and Vermon (Bairagi & Verma, 2019) developed a non-enzymatic electrochemical biosensor for glucose, which shows fast response, reproducibility, high selectivity and sensitivity. rGO and Ni-nanoparticles were in situ dispersed in melamine-novolac polymer. The prepared novel Ni-rGO/P composite was used as the working electrode. Polyacrylamide (PAA) nano film was grown for the first time on the Ni-rGO/P substrate via electro-polymerization and used for the selective recognition of glucose. Melamine served as the source of the electro-conductive N heteroatom in the material. Tested using differential pulse voltammetry, PAA-Ni-rGO/P showed remarkable linearity (R2 = 0.99; S/N ratio = 3) over 0.03–250 mg dL−1 of glucose concentrations, with high sensitivity (1724.58 μAmM−1 cm-2) and low detection limit (0.0126 mg dL−1). The sensor also exhibited high reproducibility (up to ˜500 consecutive tests, RSD < 5%) and a good recovery. With negligible effects of the interfering bio-molecules (uric acid, ascorbic acid, cholesterol, etc.) on the sensor response, applicability of the present sensor was successfully tested (RSD < 6%) in a clinical setting. The approach described in this study can be used to electrochemically grow similar macromolecules as the recognition elements for different biomarkers.

4. Different graphene-polymer composites

Graphene and it derivatives can be easily homogeneously dispersed in different polymer matrices including thermoplastics, thermosets, and conducting polymers. They have been used as a nanofiller in various polymeric matrices, such as epoxy, Polystyrene, polyurethane (PU), poly(vinylidene fluoride) (PVDF), rubber, chitosan, nafion, polycarbonate (PC) and polyaniline (PANI), PET to form graphene-polymer composites. .

4.1. Thermoplastics matrices

The most recent frequently used thermoplastic polymers are polyamide (PA), PMMA, as well as some biocompatible polymers such as PLA, PCL and PVA. Numerous studies seek to introduce graphene to address the low stiffness and strength of the thermoplastic resin. Below are some of the most recent summarised achievements concerning the design and development of graphene nanocomposites based on thermoplastic matrices, as well as the main features of the obtained materials:

4.1.1. Polystyrene/graphene nanocomposites

Several researchers have used polystyrene for fabrication of graphene-polymer composite:

Ni et al. (Ni et al., 2019) reported a facile two-step synthetic strategy for fabricating the ternary polystyrene/reduced graphene oxide/ceria (PS/RGO@CeO2) nanocomposite particles through the self-assembly of the RGO on the surface of PS microsphere (PS/RGO) and then the attachment of CeO2 nanoparticles (NPs) onto the as prepared PS/RGO nanocomposite particles. SEM, EDX, XRD, XPEX, and thermogravimetric analysis have been employed to characterize the morphology and composition of the as-prepared samples

Yu et al. (Yu et al., 2014) prepared polystyrene (PS)/modified-GO nanocomposites using in situ miniemulsion polymerisation. The as-prepared nanocomposites exhibit superior anti-corrosion properties compared with pure PS, in which the corrosion protection efficiency increased from 37.90% to 99.53% with the incorporation of 2 wt% modified GO in the PS polymer matrix. The thermal stability and mechanical properties also greatly improved compared with pure PS.

4.1.2. Polyvinylidene fluoride (PVDF)/graphene nanocomposites

Several researchers have used PVDF for fabrication of graphene-polymer composite:

Zhang et al. (D. Zhang et al., 2019), prepared a high dielectric flexible material based on PVDF compositing with graphene and ionic liquid (IL, 1-butyl-3-methylimidazolium hexafluorophosphate). The introduction of IL improves the dispersion of graphene in nanocomposite matrix, and the conjugated synergistic effect of IL and graphene enhances the dielectric property of PVDF nanocomposite. IL can help to improve the interfacial polarization effect between graphene and PVDF matrix, and increase the content of β phase due to the ion-dipolar interaction between the -CF2 of the polymer backbone and imidazolium cation. Meanwhile, the uniform dispersion of graphene in PVDF can promote the formation of tiny graphene microcapacitor structure in matrix, which is significant in improvement of dielectric and electrical properties of PVDF nanocomposite. The nanocomposite with the concentration of 2.0 wt% graphene and 2.0 wt% ionic liquid has the best dielectric property in our work, the dielectric constant of nanocomposite is nearly 6 times higher than that of pure PVDF. In addition, the dielectric loss is less than 0.2 when the frequency between 104 and 106 Hz, which is acceptable in applications.

Li et al. (Achary et al., 2018), prepared poly (vinylidene-fluoride) (PVDF) particles wrapped by reduced graphene oxide (RGO) in a solution. Characterization techniques combining SEM, TGA, FT-IR, XRD, and XPS revealed reduction of PVDF diameters and successful integration between PVDF particles and RGO sheets.

Li et al. (Li et al., 2020), presented hydrophobic nanofibrous composites for vacuum membrane distillation (VMD) consisting of a hydrophobic polyvinylidene fluoride (PVDF) nanofibrous layer and a hydrophobic polypropylene (PP) nonwoven fabric (NWF) substrate. PVDF nanofibrous layer was directly fabricated on the surface of PP NWF substrate by electrospinning technique. 1 H,1 H,2 H,2 H -perfluorooctyltriethoxysilane (FTES) functionalized GO nanosheets were incorporated in the PVDF nanofiber layer during the electrospinning process to enhance the hydrophobicity and permeation water flux for VMD. Membrane surface morphology and composition were characterized through SEM and FTIR. The effects of FTESGO nanosheets content on the VMD desalination properties were investigated.

4.1.3. Polyethylene terephthalate/graphene nanocomposites

Polyethylene terephthalate (PET)/graphene nanocomposites are usually prepared using melt compounding method. Morphological analysis of the nanocomposites by TEM revealed the network of graphene to be composed of abundant thin stacks of a few sheets of monolayer graphene.

Bayat et al. (Bayat et al., 2019), designed epoxy adhesives based on recycled poly(ethylene terephthalate) (PET), ground rubber tire (GTR), and graphene oxide (GO) nanoflakes and their thermal and mechanical properties were discussed. By changing the amounts of the aforementioned components (recycled polymers and GO nanosheets), adhesive formulations were tested for tensile and single-lap shear strength applied at the interface between epoxy/carbon fiber and stainless steel. The best and the worst samples in regard to mechanical strength were specified and selected for thermal degradation analyses base on thermogravimetric analysis (TGA).

Zhang et al. (H.-B. Zhang et al., 2010) prepared polyethylene terephthalate (PET)/graphene nanocomposites by melt compounding. TEM observation indicated that graphene nanosheets exhibited a uniform dispersion in PET matrix. The incorporation of graphene greatly improved the electrical conductivity of PET, resulting in a sharp transition from electrical insulator to semiconductor with a low percolation threshold of 0.47 vol. %. A high electrical conductivity of 2.11 S/m was achieved with only 3.0 vol. % of graphene. The low percolation threshold and superior electrical conductivity are attributed to the high aspect ratio, large specific surface area and uniform dispersion of the graphene nanosheets in PET matrix.

4.1.4. Polyamide 6/graphene

Hou et al. (W. Hou et al., 2014), fabricated nanocomposite of functionalized graphene grafted by PA6 by in situ polycondensation of caprolactam (CPL) and connection of the PA6 to the functionalized graphene, and their continuous nanocomposite fibers were prepared by use of melt spinning and drawing process. The grafting PA6 chains on graphene sheets were confirmed by FTIR, TGA and AFM measurements. Replacement of the—COOH group by—NH₂ and—(CH₂)₆NH₂ in the composite of PA6 and graphene changed the grafting polymerization chemistry, thereby leading to the covalent attachment of longer graft polymer chains to the graphene. Tensile strength of the nanocomposite fibers containing the—(CH₂)₆NH₂ functional group with 0.1 wt % graphene loading was significantly increased, over twice as high as that of neat PA6

Xiang et al. (Xiang et al., 2019), fabricated PA6/Cellulose-RGO-TDI nanocomposite with high electrical and thermal conductivity via reactive melt processing. Both RGO outside the fibre and RGO inside the 3D-bundled micro-channel work together to assemble Omni bearing transscale conductive networks in the whole hierarchical porous structure of the cellulose skeleton. Finally, more integral conductive pathways are established by RGO along the cellulose fiber, which is more beneficial to transmission of charge carrier and phonons in the whole cellulose skeleton. Exfoliated RGO nano-sheets homogeneously dispersed between matrix and cellulose skeleton and a good interface forms, which promoted formation of electrically and thermally conductive network, resulting in the remarkably improved electrically conductivity of 5.8 × 10⁻¹ S/m and thermally conductivity of 0.419Wm⁻¹ K⁻¹ of the composite with 1.2 vol % RGO loading.

4.1.5. Thermoplastics polyuritane

Khtoon et al. (Khatoon & Ahmad, 2017), reviewed conducting polymer reinforced in polyurethane (CP/PU) composites. Conducting polymers as a reinforcement, offers exceptional properties when it combines with the thermoplastic polyurethane matrix.

Kausar et al. (Kausar, 2017) reviewed essential aspects of graphene reinforced polyurethane nanocomposite. Various strategies for fabricating polyurethane/graphene nanocomposite have been conversed. Recent developments in the field of polyurethane/graphene nanocomposite (as shape memory, adsorbent, electromagnetic interference shielding, and gas barrier materials), associated challenges, and future potential have been reviewed.

4.1.6. Polysulfide/graphene

Amangah et al. (Amangah et al., 2018) investigated nanoconfinement effect of graphene nanoplatelets on properties of matrix-grafted graphene/ polysulfide polymer nanocomposites. (GO) was modified with (3wqchloropropyl)triethoxysilane (CPTES) to be able to participate in interfacial polymerization of 1,2-dichloroethane (DCE), 1,2,3-trichloropropane (TCP) as crosslinker and disodium tetrasulfide (Na2S4). Successful modification of GO is proved by FT-IR and Raman spectroscopies, TGA, XRD, and TEM. Different amounts of modified GO (mGO) (0.0, 0.1, 0.3, 0.5, 0.7, and 1.0 wt %) were used to prepare nanocomposites. Nanoconfinement effect of graphene nanoplatelets on the crystallinity of synthesized nanocomposites was investigated by XRD and DSC. Results showed that crystallinity of nanocomposites increases by adding graphene up to 0.7 wt % while after that crystallinity decreases. Also, DSC results showed that glass transition temperature (Tg) and melting point (Tm) of nanocomposites depend strongly on the amount of used mGO and pass through a maximum by increasing graphene content.

4.2. Thermosets matrices

Thermosetting polymers possess advantages such as high stiffness, strength and thermostability while they are too fragile. The introduction of graphene sheets may alleviate the brittleness of thermosetting polymers. Development of graphene-based thermosetting resins via additive manufacturing (AM) method is believed to have great potential for creating new materials that enable AM of multifunctional components.

The followings are summaries of some of the most recent achievements concerning the design and development of graphene nanocomposites based on thermosetting matrices, as well as the main features of the obtained materials.

4.2.1. Epoxy/graphene nanocomposites

Epoxies are polymers with good mechanical and thermal properties, high chemical and corrosion resistance, low shrinkage upon cure, outstanding adhesion to various substrates, and good electrical insulating properties. Following are researches that haven been done recently on Epoxy/graphene nanocomposite:

Shokrieh and Shokrieh (Shokrieh & Shokrieh, 2019) stimulated the creep behaviour of graphene/epoxy nanocomposites with a random distribution of nanographene. The modified micromechanical models showed good accuracy in the low graphene content (< 0.25 wt %), but by increasing the graphene content (> 0.25 wt %) the error of the modified micromechanical models increased.

Sun et al. (Sun et al., 2019) fabricated graphene/epoxy nanocomposite coating by dispersing the graphene nanosheets into epoxy matrix with the help of ball milling. Localized electrochemical technologies are employed to investigate the corrosion behaviour of metal substrate at artificial defects of the graphene/epoxy nanocomposite coating.

Zhang et.al. (D. D. Zhang et al., 2014) fabricated GNS/epoxy nanocomposites by ultra sonication and cast molding method. The mechanical properties of GNS/epoxy nanocomposites were influenced by the specific surface area, layer stacking and oxygen-containing functional group contents of GNSs.

Liu et al. (C. Liu et al., 2019) constructed a novel graphene-epoxy nanocomposite with intrinsic self-healing underwater and superior anticorrosion properties via host-guest chemistry. Typically, the β-cyclodextrin/graphene (CD-G) was firstly synthesized and then incorporated into epoxy network to serve as “macro-crosslinker.” The artificial damage can be easily healed through the dynamic host-guest recognition between cyclodextrin in graphene surface and adamantane at polymer chains. Meanwhile, the impermeable graphene nanosheets largely prevented the electrolyte penetration and improved the healing efficiency for composite coatings. This novel graphene-epoxy nanocomposite possessed efficient self-healing and superior anti-penetration functions, simultaneously, endowing the composite coatings with smart anticorrosion performance.

Shokerin and shokerin (Shokrieh & Shokrieh, 2019) simulated the creep behaviour of graphene/epoxy nanocomposites with a random distribution of nanographene. The modified micromechanical models showed good accuracy in the low graphene content (< 0.25 wt %), but by increasing the graphene content (> 0.25 wt %) the error of the modified micromechanical models increased. In this paper, the combined finite element-micromechanics (FE-M) model previously developed by the present authors for the prediction of the elastic properties of nanocomposites was modified and extended to predict the time-dependent creep properties of nanocomposites.

Hayatgheib et al. (Hayatgheib et al., 2018) fabricated GO/epoxy and GO-PANIs/epoxy nanocomposites. They functionalized GO nanosheets with polyaniline (PANI) nanofibers through three methods. In method II the polymerization of aniline was done in the presence of sodium dodecyl sulfate as a surfactant and ammonium persulfate as an initiator. In method I the surfactant and in method III the initiator were eliminated during polymerization procedure. The morphology and chemistry of the nanosheets were characterized and their corrosion protection performance was studied on steel substrate by electrochemical impedance spectroscopy. Results revealed that GO-PANI remarkably improved the barrier performance and provided active inhibition for epoxy coating.

4.2.2. PU/graphene nanocomposites

Nanocomposites of waterborne polyurethane (WPU) with functionalized graphene sheets (FGS) are often prepared using an in situ polymerisation method (Biswas et al., 2018).

Zhang et al. (P. Zhang et al., 2019) fabricated WPU composite by covalently conjugating with hydroxyl-functionalized graphene oxide (fGO), which was wrapped with P-N flame retardants, using an in situ polymerization strategy. Results indicated that a nanostructured WPU/graphene composite (WPU/fGO) with molecular-level uniformity was prepared. Compared with pure WPU, the peak heat release rate and total heat release were significantly reduced by 39.2% and 18.6%. The well dispersion, lamellae-blocking effect of graphene nanosheets and the catalytic charring performance of P-N flame retardant were conductive to the improved fire safety. Moreover, WPU/fGO composite exhibited a 139% enhancement in tensile strength, while maintaining the superior elongation at break (high as 857.5%). Overall, this effective yet promising paradigm may provide an achievable solution toward developing graphene-based polymer nanocomposites simultaneously with high fire safety and well-balanced comprehensive performances.

Kale et al. (Kale et al., 2019) fabricated polyuritane/graphene nanocomposite. The nanocomposite surface structures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The polymer nanocomposite films show the excellent improvement in the mechanical and thermal properties. In the fractured surface study of the composite films by SEM, it was observed that the agglomeration of GO appears while the GO is uniformly distributed in the PU matrix.

4.2.3. Polycarbonate/graphene nanocomposites

Lago et al. (Lago et al., 2016) reported the preparation of polycarbonate-based graphene (PC/G) composites, by using a simple and scalable solution blending method to disperse single- (SLG) and few-layer (FLG) graphene flakes, prepared by liquid phase exfoliation (LPE), in the polymer matrix. A solvent-exchange process is carried out to re-disperse the exfoliated SLG/FLG flakes in an environmentally friendly solvent, i.e. 1,3-dioxolane, which is also used to dissolve the polycarbonate pellets, thus facilitating the mixing of the polymer dispersion with the SLG/FLG flakes. The loading of SLG/FLG flakes improves the mechanical and thermal properties, as well as the electrical conductivity of the polymer, reaching a + 26% improvement of the elastic modulus at 1 wt % loading, and an electrical conductivity 103 Sm1 at 10 wt% with a percolation threshold achieved at 0.55 vol %.

Kim and Macosco (Kim & Macosko, 2009) prepared and fabricated Polycarbonate and graphene using 0.5–3 wt. % graphene nanoribbon (GNR). Poly(methyl methacrylate-co-methacrylic amide)-polyethylene glycol (PMMA-co-MA-PEG) copolymer was prepared via condensation and blended with polycarbonate to form PMMA-co-MA-PEG/PC. The PMMA-co-MA-PEG/PC and GNR-based nanocomposite possess seamless micro-branched morphological pattern. Tensile strength and Young’s Modulus of PMMA-co-MA-PEG/PC/GNR0.5–3 increased from 64.3–74.7MPa and 76.7–99.9MPa, respectively. GNR loading increased the permselectivity aCO2/N2 (25.4–41.6) of nanocomposite membrane relative to blend membrane (20.1). However, permeability PCO2 was decreased from 163.9 to 139.7 Barrer than blend (174.3 Barrer). PMMA-co-MA-PEG/PC/ GNR revealed 51.6% increase and 24.7% decrease in permselectivity and permeability owing to molecular sieving and barricade characteristics of graphene nanoribbon.

4.2.4. Polymethyl metacrylate PMMA-graphene composite

Sharma et al. (V. Dutta et al., 2019) synthesised polymer nanocomposite Poly(methyl methacrylate)-Graphene Oxide (PMMA-GO) using in-situ free radical polymerization with varying concentrations of GO (0.5%, 1% PMMA), where GO was synthesized using Improved Hummers method and the effect of the nanocomposite on the pour point depression and rheological properties of a Indian waxy crude oil sample is observed. The synthesized nanocomposite is evaluated as pour point depressant considering the improvement in the flow properties of the crude oil. Analysis of the synthesized nanocomposites using techniques such as FTIR, Raman and UV-visible spectroscopy, Field Emission Scanning Electron Microscopy (FESEM), Gel Permeation Chromatography (GPC) revealed the formation of PMMA-Graphene Oxide and the intermediate products. Thermogravimetric analysis indicated improved thermal stability of the synthesized nanocomposite than polymer PMMA. XRD analysis confirmed the formation of graphene oxide and the resultant nanocomposite.

Feizi et al. (Feizi et al., 2019) fabricated rGO nanoflakes/polymethyl methacrylate composite to measure the dose rate of gamma radiation. A detailed characterization of the prepared reduced graphene oxide using XRD, TGA, AFM, Field Emission scanning electron microscopy (FE-SEM), XPS, FTIR and High-resolution transmission electron microscopy (HRTEM) is presented. Reduced graphene oxide–polymethyl methacrylate composite was prepared using methylene chloride solvent-assisted dispersion of nano flakes of reduced graphene oxide in the polymer matrix. The gamma sensor mainly consists of polymethyl methacrylate/reduced graphene oxide (rGO/PMMA) nanocomposite as the sensing material and two silver coated glass electrodes to make a conductive cell.

Mallik et al. (Mallik et al., 2019) prepared highly conductive poly(methyl methacrylate)–reduced graphene oxide (PMMA/rGO) composites by in situ polymerisations of methyl methacrylate (MMA) monomer in the presence of iron metal-induced deoxygenated rGO is reported. The obtained PMMA/rGO exhibited excellent electrical properties with a percolation threshold as low as 0.1 wt. % and electrical conductivity of 14 S/m at only 0.8 wt. %

4.3. Conducting polymer matrices

Some members of conducting polymer family are PANI, PPy, polythiophene and PEDOT. Most of the nanocomposite research works have been done on PANI (Adrian et al., 2019; Alipour et al., 2019; Amrollahi et al., 2019; Ansari et al., 2014; E. Eskandari et al., 2020; Feng et al., 2015), PPy (Dhibar et al., 2019; Qin & Qiu, 2019), (Politi et al., 2019) poly(3-hexylthiophene) (p. 3HT) (Husain et al., 2019; Shen et al., 2017; Yao et al., 2015) and PEDOT (Baruah & Kumar, 2018; Baruah et al., 2019; Giuri et al., 2018; Hwang et al., 2019; Maráková et al., 2019).

The followings are summaries of some of the most recent achievements concerning the design and development of graphene nanocomposites based on conducting polymer matrices, as well as the main features of the obtained materials.

4.3.1. Polyaniline/graphene nanocomposites

Zhang et al. (Zhang, Wu et al. 2019) synthesized tin oxide/reduced graphene oxide/polyaniline (SnO2/rGO/PANI) by in-situ polymerization technique. The SnO2/rGO/PANI nanocomposite film was fabricated on a flexible polyethylene terephthalate (PET) substrate with interdigital electrodes (IDEs). The SnO₂/rGO/PANI sensor possesses excellent sensing characteristics such as high response, fast response/recovery time, stable repeatability, outstanding selectivity and long-term stability.

Ansari et al. (Ansari et al., 2014) prepared conducting graphene/polyaniline (GN@Pani) nanocomposite by the in situ oxidative polymerization of aniline in the presence of GN and the surfactant, cetyltrimethylammonium bromide (CTAB). The micellar structure of CTAB assisted both, the formation of GN@Pani tubules and the dispersion of GN. Sheet-like GN was distributed uniformly in the Pani matrix, leading to high electrical conductivity because of the p–p interactions between Pani and GN. Studies of the thermoelectrical behaviour using isothermal and cyclic aging techniques showed that GN@Pani possessed a high combination of electrical conductivity and thermal stability, even beyond 150 °C. GN@Pani was used as cathode active material in microbial fuel cells, and showed an enhanced power density and cell voltage, leading to better catalytic performance compared to plain carbon paper.

4.3.2. Poly(3,4-ethyldioxythiophene)/graphene nanocomposites

Hwang et al. (Hwang et al., 2019) reported a synthetic route to fabricate graphite-poly (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) nanocomposite (GP) by partially intercalating aqueous oxidative 3,4-ethylenedioxythiophene (EDOT) monomers into colloidal graphite layers. The colloidal graphite was dispersed in EDOT solution by ultrasonication prior to in situ polymerization process, and the resulting materials were well dispersed in EDOT even after 48 h. It is assumed that graphite was surrounded by EDOT monomers on its surface or the monomers were partially intercalated in the edge side of colloidal graphite layers. The nanocomposites (GP) represents good stability of aqueous suspension and their structural and electrical properties were evaluated. This study provides a facile and potential method to develop nanocomposites based on water-soluble conducting polymers.

Giuri et al. (Giuri et al., 2018) produced PEDOT: PSS and of GGO-PEDOT free-standing nanocomposite by drop casting the solution in a proper silicone mould, followed by peeling and thermal annealing. Specific analyses, such as thermogravimetric, colorimetric and contact angle measurements, have been performed aiming at assessing the stability of the thermal and of the surface properties, even in severe moisture and UV aging conditions. Finally, The capacitive performance of PEDOT: PSS and of GGO-PEDOT was investigated by means of cyclic voltammetry (CV), in the pristine conditions and under UV aging. The deposited GGO-PEDOT film showed a good conductive behavior and stability under UV treatment of 4 h.

4.3.3. Polypyrrole/graphene composite

Mariappan et al. (Mariappan et al., 2019) synthesized hybrid nanocomposites with different weight percentages of rGO/PPy/CoFe₂O₄ and rGO/ PPy/Fe₃O₄ through a hydrothermal approach. Structural properties were investigated by using Fourier transform infrared spectroscopy, Raman spectroscopy, XRD, and TEM with selected area electron diffraction. TEM studies show embedded spinel-type metal oxide nanoparticles, branched PPy chains and PPy particles on rGO. The results of the structural and morphological investigations clearly reveal the formation of ternary hybrid nanocomposites. The influence of the content of spinel-type metal oxides on electrochemical properties of nanocomposites was investigated via cyclic-voltammetry, electrochemical impedance spectroscopy and galvanostatic charging/discharging measurements in 1 M LiNO₃ electrolyte. The specific capacitance is found to be 261, 141, 108 and 68.3 F g − 1 at 1 A g − 1 for 37 wt% rGO/58Ppy/5Fe₃O₄ (FO5), 32 wt % rGO/54Ppy/14Fe₃O₄ (FO14), 37 wt %rGO/58PPy/5CoFe₂O₄ (CFO5), and 32 wt%rGO/54Ppy/14CoFe2O4 (CFO14), respectively. Charge storage mechanisms were interpreted through Power’s law and Trasatti plot. Among these samples, FO5 exhibits high specific capacitance with good rate capacitance performance (163 F g − 1 at 10 A g − 1). A hybrid supercapacitor was fabricated with FO5 composite as a positive electrode, activated carbon (AC) as a negative electrode and 1 M LiNO3 as an electrolyte. As a result, the FO5//AC cell exhibits the specific capacitance of 31.8 F g − 1 at 3.0 A g − 1 with excellent rate capability and good cycling performances. The energy and powder densities are found to be in the range of 17.74–4.17 Wh kg−1 and 0.3–10.4 kW kg−1, respectively, with an output voltage of 0–1.6 V.

Politit et al. (Politi et al., 2019) synthesized PPy-based nanocomposites by a micro-emulsion oxidative chemical polymerization of pyrrole monomer. Graphene platelets (GP), extracted from Shungite mineral powders through innovative procedures, are used as nanoinclusions to enhance the interesting properties of the pure polymer. SEM and HR-TEM analyses evidence that diverse PPy nanostructures are obtained in the presence of different dopant surfactants: globular grains with sodium dodecyl sulfate (SDS) anionic surfactant, and tubular features with hexadecyltrimethylammonium bromide (CTAB) cationic surfactant. Raman spectroscopy reveals that the polymer chains are in a semi-oxidized state. Functional characterisation evidence that the PPy-based nanocomposites doped with SDS are characterized by improved charge transport properties.

Cai et al. (H. M. Dong et al., 2017) fabricated using GO and pre-prepared one-dimensional PPy nanotube as the feedstock. GO with various oxygen-containing groups effectively promotes the dispersion of well-defined PPy nanotubes to obtain a stable and homogeneous GO/PPy complex solution. By a one-step and large-scale electrochemical reduction, the graphene/PPy nanotube composite film is successfully synthesized. Graphene nanosheets uniformly cover the surface of the electrode and PPy nanotubes act as spacers and conducting bridges to prevent the restacking of graphene sheets and connect the isolated graphene nanosheets. Electrochemical experiments indicate that the composite films have high performances due to the combination of the advantages of graphene and PPy nanotube. When the graphene/PPy nanotube composite is directly used as the supercapacitor electrode, it shows high specific capacitance and good cycling stability during 1000 charge/discharge cycles. Furthermore, an electrochemical biosensor is constructed through the entrapment of horseradish peroxidase (HRP) onto the composite film-modified glassy carbon electrode (GCE).

4.3.4. Poly-(3-hexylthiophene)/graphene

Yadav et al. (Yadav et al., 2019) fabricated composites of poly-(3-hexylthiophene) (p. 3HT) and graphene using solution mixing. The UV/Vis/NIR absorption spectra reveal that the composites exhibit absorption in the visible range. Incorporation of graphene introduces slight red shifts in the absorption spectra of P3HT indicative of the increase in the conjugation length of P3HT. The effects of various amount of graphene on electrical performance of devices are investigated. Results show that the device with 5 wt % graphene concentration demonstrates best performance, exhibiting detectivity of 1.8 × 108 Jones and responsivity of 0.25 A/W at—9 V. Intensity and voltage-dependent photocurrent studies predict that the photocurrent is not limited by the space charge effect in the device. Impedance spectroscopy analysis is carried out, and the obtained data are fitted to the suitable equivalent circuit to extract the internal device parameters, including junction resistance and capacitance, that are further used to estimate the bandwidth of photodetector.

4.4. Other polymer matrixes

4.4.1. Chitosan-graphene nanocomposite

Chen et al. (J. Chen et al., 2019) prepared cross-linked chitosan/SiO₂-loaded graphene (CS/grapheneSiO₂) composite beads. The resulting CS/graphene-SiO₂ beads were characterized by SEM and Raman spectroscopy, which clearly showed that graphene was well dispersed in the chitosan matrix. Adsorption results demonstrated that CS/graphene-SiO₂ beads possessed much better adsorption capacity for bilirubin (77.87%) compared to pure chitosan (CS) beads (22.47%), mainly due to the synergistic effect between the hydrophobic forces of graphene and the electrostatic interactions provided by the amino groups of CS. The adsorption of bilirubin was found to match a monolayer model, as well as a pseudo-first-order kinetic model. The mechanical strength of composite beads was significantly improved as compared to CS beads, due to the incorporation of graphene. The effects on haemolytic activity and the components of blood were negligible, which indicate an excellent compatibility of the obtained composite beads with blood. Overall, the proposed CS/grapheneSiO₂ beads as an efficient adsorbent for bilirubin have high potential in blood purification applications.

Suneetha et al. (Suneetha et al., 2019) prepared nanocomposite of Zinc doped Iron oxide/GO/Chitosan using optimized quantity of the components by simple solution mixing-evaporation method. The structure of the composite was studied by UV–Vis, FTIR, XRD, SEM, HRTEM and AFM. The results obtained from these analyses showed the formation of homogeneous mixture with strong interaction between the constituents and evidenced by the shift in peak position in both UV–Vis and FTIR spectra. Band gap calculated was found to be 2.28 eV which indicates the conducting nature of the composite. The formation of composite in nano dimensions of around 20 nm was proved by XRD, SEM and HRTEM studies. Thermal stability of the sample was investigated by TG/DTA and DSC techniques and showed improved thermal stability with the increase in glass transition temperature of chitosan by 78°c. Electrochemical characteristics of the composite was studied by cyclic voltammetry and capacitative behaviour was studied by impedance studies (EIS). These electrochemical investigations revealed good adherent nature of the composite on electrode surface at pH1 and greater electrochemical stability with well-defined redox peaks. EIS showed that the nanocomposite modified electrode exhibited good capacitance behaviour with the bode phase angle of 87° which proves it to be a very good candidate for supercapacitor applications.

4.4.2. Rubber-graphene nanocomposite

Thomas et al. (Thomas et al., 2019) fabricated rubber—graphene nanocomposite via a green approach in terms of mechanical properties and two different fabrication routes viz. latex casting and latex casting followed by dry rubber mixing. The composite was characterized using XRD, Raman spectroscopy and TEM analysis. It was found out that at a particular filler loading of 3 phr for latex casting method, 5 phr for dry rubber mixing method and only 0.4 phr for MWCNT based samples the produced nanocomposites demonstrated a significant increment in tensile properties. However, when only the supernatant from the 3 phr graphene dispersion was used, the sample showed an improved increment (363%) in mechanical properties due to the removal of other bulk particles during settling which might otherwise act as stress concentrators inside the composite. The demand for multifunctional elastomers is increasing now a day and hence these findings where pristine graphene has been successfully and uniformly incorporated into NBR matrix (an important synthetic elastomer with huge applications) would be scientifically significant.

Liu et al. (X. Liu et al., 2018) prepared graphene/rubber nanocomposites and drew conclusions about factors that influence the properties of graphene-based rubber composites. They gave the outlook and prospect about the development of graphene-based rubber nanocomposites in future.

Li et al. (Li et al., 2017) reported a novel method to introduce a large fraction of graphene into a styrene–butadiene rubber (SBR) matrix. The obtained graphene/rubber nanocomposites were mechanically enhanced, acoustically absorptive under water, and electrically and thermally conductive. The Young’s modulus of the nanocomposites was enhanced by over 30 times over that for rubber. The electrical conductivity of nanocomposites was ≤219 S m⁻¹ with 15% volume fraction of graphene content, and exhibited remarkable electromagnetic shielding efficiency of 45 dB at 8–12 GHz. The thermal conductivity of the nanocomposites was ≤2.922 W m⁻¹ k⁻¹, which was superior to the values of thermally conductive silicone rubber thermal interface materials. Moreover, the nanocomposites exhibited excellent underwater sound absorption (average absorption coefficient >0.8 at 6–30 kHz). Notably, the absorption performance of graphene/SBR nanocomposites increased with increasing water pressure.

4.4.3. Nafion-graphene nanocomposite

Lee et al. (D. C. Lee et al., 2014) fabricated Nafion/GO composite membrane and Nafion/Pt-G composite membrane. The MEAs fabricated with the Nafion/GO composite membrane show significant enhancement in cell performance: that is, 0.802 A, 1.27 A, 0.827 A at 0.6 V under 100% relative humidity (RH) for 0.5 wt %, 3.0 wt % and 4.5 wt % of GO content in the composite membrane, respectively, compared to 0.435 A for casting Nafion membrane. The Nafion/Pt-G composite membrane, however, does not show sufficient enhancement under various RHs. It is attributed to poor water retention ability of hydrophobic graphene and electron loss due to the formation of electrical network by too much Pt within the membrane. Constant open circuit voltage (OCV) down to low RH indicates that GO and graphene could be prospective as filler in low humidifying polymer electrolyte fuel cell.

5. Characterisation

Polymer/ graphenes nanocomposites are usually characterised by microscopy (TEM(Thomas et al., 2019), AFM), spectroscopic (NMR (Maimaiti et al., 2019), FT-IR, Raman(Carrasco–Valenzuela et al., 2017; Thomas et al., 2019), XPS) and thermal (TGA (Adrian et al., 2019), DSC (Ajorloo et al., 2019) techniques. The host polymer and modified graphene are characterized by direct methods such as TEM, SEM, FTIR, XPS and TGA. Investigation of the level of dispersion of fillers in a polymer matrix is a difficult task while manufacturing composites or after sample preparation. SEM, TEM (Thomas et al., 2019), XRD (Thomas et al., 2019), Raman spectroscopy and AFM are generally used to assess the mode of dispersion. This is completed by image analyzes that explore composite structure, number of particles in a specific area, aggregate size, etc Short accounts of these characterization techniques are discussed here:

5.1. Transmission electron microscopy (TEM)

TEM is an essential technique to characterize the polymer functionalized graphene.

Alipour and Namazi (Alipour Ghorbani & Namazi, 2019) prepared nanocomposite (Ag–Pd-PDA/rGO) and was characterized using several analytical techniques such as FT-IR, XRD, TEM, SEM, EDX map, and BET. The effect of PDA and molar ratio of bimetal were investigated in morphology, size and catalytic activity of nanocomposite. The SEM and TEM showed that surface of PDA/rGO was decorated with Ag–Pd nanoparticles with cluster form in average diameter of 60 nm. The prepared nanocomposite was used as a new catalyst for reduction of nitro aromatic compounds such as 4-nitrophenol (4-NP) to 4-aminophenil (4-AP) and reduction of anionic and cationic dyes .

Minar et al. (Minář et al., 2019) investigated the covalent functionalization of graphene oxide (GO) with poly(ε-caprolactone) and its application for the in situ synthesis of polyamide 6 nanocomposites. For the functionalization, GO moieties were used to initiate the polymerization of ε-caprolactone, which was followed by the separation of ungrafted polymer chains. TGA and FTIR confirmed covalent bonding between the GO and poly(ε-caprolactone). The synthesis of the nanocomposites was carried out by the in situ polymerization of ε-caprolactam containing either untreated or functionalized GO. TEM and AFM images of the nanocomposites containing the functionalized GO revealed the presence of single exfoliated nanofiller layers. DMA showed that the functionalized GO had a higher reinforcing effect than the untreated one.

5.2. Atomic force microscopy (AFM)

AFM image profile graph is an important technique to characterize the polymer/functionalized graphene nanocomposite. AFM image gives knowledge about the length and thickness of GO and polymer-functionalized GO along with morphology. The thickness of the graphene sheets is measured from AFM height profile graph and similarly the thickness of initiator or polymer-functionalized samples are measured from corresponding height profile.

Rajitha et al. (Rajitha & Shetty Mohana, 2019), fabricated polycaprolactone (PCL)/graphene naocomposite, In this study graphene-based nanoparticles were incorporated into PCL matrix in order to enhance its anti-corrosion performance. The influence of graphene oxide (GO) and functionalized graphene oxide (FGO) nanosheets on the corrosion protection performance of PCL coating have been investigated. GO was functionalized with 8-bromo[1,2,4]triazolo[1,5-a]pyridin-2-amine (BTP), and characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy and Energy Dispersive X-ray spectroscopy (EDX). The composite coating mixture consisted of GO/FGO- PCL matrix applied on mild steel and surface of the coated samples were examined using Contact Angle technique, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Barrier and corrosion protection performance of the coated samples were studied.

Devi et al. (DEVI et al., 2014) fabricated azo/graphene nanocomposite. Chemical structure of the azo-GO hybrids was confirmed by FTIR and UV-visible spectroscopy. The GO functionalized with 5-((4methoxyphenyl)azo)-salicylaldehyde was further characterized by SEM, TEM, and AFM. The SEM studies demonstrated that the morphology of the azo-GO hybrid was found to be similar to the GO sheets but slightly more wrinkled. Further, TEM image of azo-GO indicates some dark spots on the GO sheets due to azo functionalization. AFM results also reveal that the azo functionalization increases the thickness of GO sheet to 4–5 nm from 1.2 to 1.8 nm. Both the azo-hybrids show absorption band around 379 nm due to the π–π* transition of the, trans azo units.

5.3. Fourier transformed infrared spectroscopy (FTIR)

FTIR spectra are an important tool to characterize the both covalent and non-covalent functionalization of GO.

Chethan et al. (Chethan et al., 2019) fabricated Polyaniline/Water soluble graphene oxide [PWGO] composites. Various mass ratios of Water-soluble graphene oxide [WGO] were mechanically mixed with PANI prepared by in-situ polymerization process to form PANI/WGO composites. For the purpose of humidity sensing studies, the samples were structurally characterized by FTIR, Raman, XRD, SEM and TEM techniques and comparatively analyzed. The film of the samples prepared by deposition on ordinary glass substrate using cost effective spin coating technique were tested for their humidity sensing performance in the relative humidity (RH) range of 11–97%. Of the four composites studied, the PWGO-4 composite recorded a good response time of 8 s and a recovery time of 9 s and a very low humidity hysteresis. The mechanism for sensing has been explained on the basis of three sequential steps: chemisorption, physisorption and condensation process.

Das et al. (G. Das et al., 2019) synthsised conducting nanocomposite membranes by crosslinking quaternized poly (phenylene oxide) with 1,4-diazabicyclo[2.2.2]octane modified graphene oxide (QGO). FTIR, SEM, AFM and TEM were used to study the functionalization, structure, and morphology of the membranes. The effect of QGO on ionic conductivity, water uptake, and swelling ratio were studied in detail. The nanocomposite membrane exhibited an ionic conductivity of 90 mS cm1 at 25 C and reached 151 mS cm1 at 80 C owing to good microphase separation. A power density of 5.2 mW cm² at C for urea/O₂ fuel cell was obtained using the nanocomposite membrane.

Li et al. (Li et al., 2020) fabricated hydrophobic nanofibrous composites for vacuum membrane distillation (VMD) consisting of a hydrophobic polyvinylidene fluoride (PVDF) nanofibrous layer and a hydrophobic polypropylene (PP) nonwoven fabric (NWF) substrate were presented. PVDF nanofibrous layer was directly fabricated on the surface of PP NWF substrate by electrospinning technique. 1 H,1 H,2 H,2 H -perfluorooctyltriethoxysilane (FTES) functionalized GO nanosheets were incorporated in the PVDF nanofiber layer during the electrospinning process to enhance the hydrophobicity and permeation water flux for VMD. Membrane surface morphology and composition were characterized through SEM and FTIR. The effects of FTESGO nanosheets content on the VMD desalination properties were investigated. And the transfer mechanism of water vapors through the FTES-GO incorporated nanofibrous layers was also proposed. Compared with the original PVDF nanofibrous layer, the surface hydrophobicity, liquid entry pressure (LEP) and water permeability of the FTES-GO incorporated nanofibrous layers had all been improved. When FTES-GO content was 4 wt%, the WCA increased from 104.0° for the neat PVDF nanofibrous layer to 140.5° for the modified nanofibrous layer. The permeation water flux reached a maximum value of 36.4 kgm−2 h − 1. It was two times the water flux of the original membrane and meanwhile the salt rejection remained above 99.9% (50 °C, 3.5 wt% NaCl aqueous solution and permeation pressure of 31.3 kPa). No obvious wetting phenomenon was observed for FTES-GO incorporated membrane during the continuous VMD experiment for 60 h.

5.4. Nuclear magnetic resonance (NMR)

An NMR spectrum provides the characteristic information about the formation of polymer functionalized graphene.

Maimaiti et al. (Maimaiti et al., 2019) synthesised Graphene/ polymer (Gr/P1) hybrid and characterized by Raman spectra, XPS and XRD. The morphology of the Gr/P1 was revealed by SEM and TEM. Then, Gr/P1 is compounded with platinum (Pt) to obtain nanocomposite material graphene/polymer/platinum (Gr/P1/Pt) for fuel cells. The structural of the Gr/P1/Pt demonstrated by high magnification SEM analysis and EDS elemental mapping images, in the SEM analysis and EDS spectrum can infer the Gr/P1/Pt was successfully synthesised. The electrochemical behavior illustrated by CV, galvanostatic charge/discharge curves and Electrochemical impedance spectroscopy (EIS). The CV curves of Gr/P1/Pt showed that the ECSA (electrochemically active surface area) of Pt is 141.6 m2/g. The EIS results consistent with Cyclic voltammograms measurement. The present study shows that the hybrid Gr/P1/Pt has high ECSA and large conductivity demonstrating a new and powerful approach for the development of high-performance Pt-based electro-catalysts for fuel cells.

Zhang et al. (K. Zhang et al., 2010) prepared graphene and PANI nanofiber composites by in situ polymerisations of aniline monomer in the presence of graphene oxide under acid conditions. The obtained graphene oxide/PANI composites with different mass ratios were reduced to graphene using hydrazine followed by reoxidation and reprotonation of the reduced PANI to give the graphene/PANI nanocomposites. The morphology, composition, and electronic structure of the composites together with pure polyaniline fibers (PANI-F), GO, and graphene (GR) were characterized using XRD, solid-state 13 C NMR, FT-IR, SEM, TEM, TGA, and XPS. It was found that the chemically modified graphene and the PANI nanofibers formed a uniform nanocomposite with the PANI fibers absorbed on the graphene surface and/or filled between the graphene sheets. Such uniform structure together with the observed high conductivities afforded high specific capacitance and good cycling stability during the charge−discharge process when used as supercapacitor

5.5. Raman spectroscopy

Raman spectroscopy is a powerful tool to characterize graphenes/polymer nanocomposite. It confirms the possibility of a π-π interaction(Carrasco–Valenzuela et al., 2017).

Chen et al. (J. Chen et al., 2019) prepared cross-linked chitosan/SiO₂-loaded graphene (CS/grapheneSiO₂) composite beads. The resulting CS/graphene-SiO₂ beads were characterized by scanning electron microscopy and Raman spectroscopy, which clearly showed that graphene was well dispersed in the chitosan matrix. Adsorption results demonstrated that CS/graphene-SiO₂ beads possessed much better adsorption capacity for bilirubin (77.87%) compared to pure chitosan (CS) beads (22.47%), mainly due to the synergistic effect between the hydrophobic forces of graphene and the electrostatic interactions provided by the amino groups of CS. The adsorption of bilirubin was found to match a monolayer model, as well as a pseudo-first-order kinetic model. The mechanical strength of composite beads was significantly improved as compared to CS beads, due to the incorporation of graphene.

Carrasco-Valensuela et al. (Carrasco–Valenzuela et al., 2017) synthesised GO/poly(3,4eethylenedioxythiophene) composites through Fenton’s reaction. Transmission electron microscopy analysis was carried out to establish that the polymer matrix was growing around the layers. The Fourier transform infrared spectroscopy measurements were implemented as a complementary technique for Raman to confirm the possibility of a pep interaction. The materials show a maximum fluorescence emission at 480 and 530 nm. Properties were measured to establish the presumable application in hybrid solar cells, due to the absorption of the composite in the UV and near IR region in a range of 210e350 nm and 700e800 nm. The composites exhibited a matrix growth of conducting polymer chains on and around the graphene oxide layers presenting redeox states with a potential application as a counter electrode.

5.6. X-ray photoelectron spectroscopy (XPS)

XPS deals with the elemental composition, chemical state. empirical formula and electronic state of the elements that exists within a material.

Ramizanzadie et al. (Ramezanzadeh et al., 2018) synthsised PAni nanofibers-CeO2 grafted graphene oxide nanosheets through a Layer-by-Layer (L-b-L) assembly approach and characterized by FT-IR, XPS, XRD, TGA, UV–vis spectroscopy, and field-emission scanning electron microscopy (FE-SEM). The GO-PAniCeO₂ nanosheets were incorporated into the epoxy matrix, applied on mild steel, immersed in chloride solution (3.5 wt.%), and characterized by salt spray and electrochemical impedance spectroscopy (EIS). Results revealed that both barrier and active corrosion inhibition properties of GO nanosheets were enhanced by deposition of PAni and CeO₂ compounds.

Imran et al. (Imran et al., 2018) synthesized PANI and PPy doped with GO or graphene (GN) through an in situ emulsion polymerization (EP) technique. Dodecyl benzene sulfonic acid (DBSA) was used as a surfactant and doping agent as well during the polymerization reaction. The morphology and microstructure of the synthesized polymers and their nanocomposites were studied by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. All of these characterization techniques confirmed the superior morphology and thermal properties of the nanocomposites. The electroconductive properties of the synthesized polymers and their nanocomposite pellets containing 5 wt % of either GN or GO pressed at pressures of 2, 4, and 6 tons were investigated with a four-probe analyzer. Nanocomposites showed very high electrical conductivity compared to individual PANI and PPy samples pressed at the same pressures. The addition of GO and GN not only improved the thermal stability but also significantly enhanced the electrical conductivity of the nanocomposites.

5.7. Thermogravimetric analysis (TGA)

In polymer grafted sample, there is more weight loss due to the degradation of polymer anchored with the graphene. The weight losses is used to calculate the percent of grafting of polymer in the graphene surfaces. TGA of GO under nitrogen atmosphere looses its mass below 100 C and it is due to the degradation of oxygen-containing groups e.g., eOH, epoxide and COOH etc. present in it. The thermal stability of the reduced graphene oxide sheets, however, increases compared to GO due to the removal of a large fraction of oxygen-containing moieties.

Li et al. (Achary et al., 2018) prepared poly(vinylidene-fluoride) (PVDF) particles wrapped by reduced graphene oxide (RGO) in a solution. Meanwhile, characterization techniques combining SEM, TGA, FT-IR, XRD, and XPS revealed reduction of PVDF diameters and successful integration between PVDF particles and RGO sheets. Tribological properties of RGO/PVDF nanocomposite as lubricating additive were investigated, and control experiments were performed by, respectively, adding PVDF, RGO as well as the mixture of PVDF and RGO. It is found that RGO/PVDF nanocomposite exhibits the best lubricating performances among all the samples and that the average friction coefficient as well as wear rate decreased by 44.4% and 98.7% compared with that of paraffin oil.

Bora and Dolui (Bora & Dolui, 2012) synthesized PPy/GO nanocomposites via liquid/liquid interfacial polymerization where GO and initiator was dispersed in the aqueous phase and the monomer was dissolved in the organic phase. The synthesized samples were characterized by FTIR, SEM, TGA, ultravioletevisible absorption (UVevis), XRD, electrochemical and electrical conductivity measurements. A good dispersion of the GO sheets within the PPy matrix was observed from the morphological analysis. The composites exhibited noticeable improvement in thermal stability and electrical conductivity in comparison to pure polypyrrole. The composites showed excellent electrochemical reversibility at the scan rate of 0.1 V/s and good cyclic stability even up to 100th cycle.

Adrian et al. (Adrian et al., 2019), investigated PANI/graphene nanocomposites. The structural and morphology of the synthesized PANI were characterized by field emission scanning electron microscopy (FESEM), FTIR, and UV-VIS. The thermal stability and electrical properties of the PANI composites were characterized using TGA and a precision resistance meter, respectively. The FESEM images reveal that by increasing amount of DBSA in the synthesis process results in increased of PANI attached on the graphene. FTIR and UV-VIS characterisation shows that the conductive PANI/graphene-DBSA is successfully synthesised using interfacial polymerisation method. Increasing amount of DBSA results in better thermal stability of PANI/graphene-DBSA. The PANI/graphene-DBSA nanocomposite at 10 wt % DBSA loading exhibits the highest electrical conductivity value of 7.31 × 10–2 S/cm.

6. Potential applications

Potential application areas of polymer/graphene nanocomposite are for gas transport, filtration, anticorrosion, and biomedical in addition to energy relevance (fuel cell, solar cell, batteries, sensor, supercapacitor, etc.) Several researchers have been able to successfully implement graphene or hybrid filler-based polymer nanocomposites in electrochemical applications (Azizi et al., 2019; Y. Ji et al., 2015), lithium ion batteries (Chen et al., 2018; Du et al., 2018; X. Hu et al., 2019; N. Kumar et al., 2019; Liao & Wu, 2019; Liao et al., 2017), sensors (Al-Ammari et al., 2019; Bairagi & Verma, 2019; Feizi et al., 2019; Feng et al., 2015; Gupta & Meek, 2018), catalysis (Arukula et al., 2019), solar cell (Carrasco–Valenzuela et al., 2017; Chuang & Chen, 2015; S. Das et al., 2018), water purification (Bassyouni et al., 2019; Li et al., 2020; Saleh et al., 2019), supercapacitors (Amutha et al., 2017; Balli et al., 2019; Chabi et al., 2015; Dhibar et al., 2019; Giuri et al., 2018; X. Guo et al., 2019; Y. Jin et al., 2014; Lawal, 2019; Teimuri-Mofrad et al., 2019; Yang et al., 2015) (Z. Li et al., 2014), gamma radiation detection (Feizi et al., 2019), drug delivery (S. Lee et al., 2019; Punetha et al., 2017),bone regeneration(B. Sharma et al., 2018), tissue engineering (Sayyar et al., 2013; Depan et al., 2014; Nieto et al., 2015., Energy storage devices D. Li et al., 2017; Shen et al., 2017; J. K. Y. Lee et al., 2018; Bahrami et al., 2019; Correa et al., 2019; Maráková et al., 2019; Olad et al., 2019; Pal et al., 2019, (Surmenev et al., 2019) (Aftab et al., 2018; Akhina et al., 2019; Allahbakhsh & Arjmand, 2019; Balli et al., 2019; Gahlot & Kulshrestha, 2019; Maimaiti et al., 2019; Palsaniya et al., 2019; Shen et al., 2017; Y. Yang et al., 2016), gas sensor (Tian, X. Liu et al., 2018, Zhang, Wu et al. 2019), heavy metal removal (E. Eskandari et al., 2020; Mehdinia et al., 2020), EMI shielding (Li et al., 2019; Omar et al., 2019) and corrosion (Chang et al., 2014; Iwan et al., 2015; D. Dutta et al., 2018; Hayatgheib et al., 2018, Amrollahi et al., 2019; Hussain et al., 2019; Kim et al., 2019). The other commercial applications of graphene polymer composites are: medical implants, sports equipment, lightweight gasoline tanks, plastic containers, more fuel efficient aircraft and car parts, antimicrobial agent (Inurria et al., 2019) and stronger wind turbines.

6.1. Electrochemical sensors

Covalently and noncovalently polymer-functionalized graphene composites generate various sensor applications (Zhang, Wu et al. 2019) (Ekabutr et al., 2018); (Bairagi & Verma, 2019; P. Eskandari et al., 2019; Wang & Peng, 2013).

AL-Ammari et al. (Al-Ammari et al., 2019), investigated an electrochemical sensor on a modified glassy carbon electrode (GCE) for the detection of 4-chlorophenol (4CP). A GCE was coated by a zinc oxide nanoparticle/graphene nanoplatelet/poly(o-phenylenediamine) polymer nanocomposite (GCE/ZnO/GNPs/MIP). The nanocomposite improved the electrochemical response and sensitivity of the sensor for the detection of 4-CP. The surface morphology and crystal structure of the prepared nanomaterial were characterized using different chemical and physical techniques, showing homogenous distributions of ZnO and GNPs over the electrode. The electrochemical behavior of the GCE/ZnO/ GNPs/MIP nanocomposite-based sensor was evaluated using CV and electrochemical impedance spectroscopy. Under optimized experimental conditions, the electrochemical sensor exhibited excellent analytical performance with a very low detection limit. The electrochemical sensor was utilized to determine 4-CP levels in real water samples; the sensor exhibited high selectivity, sensitivity, and repeatability.

Gong et al. (Gong et al., 2019) prepared a nanocomposite of polyaniline/graphene (PAN/GN) using reverse-phase polymerization. The nanocomposite material was dropcast onto a glassy carbon electrode (GCE). Then, a single-stranded DNA (ssDNA) probe for HIV-1 gene detection was immobilized on the modified electrode, and the negative-charged phosphate backbone of the HIV-1 was bound to the modified electrode surface via π-π* stacking interactions. The hybridization between the ssDNA probe and the target HIV-1 formed double-stranded DNA (dsDNA), and the electron transfer resistance of the electrode was measured using impedimetric studies with a [Fe(CN)6]3-/4- redox couple. Under the optimized experimental conditions, the change of the impedance value was linearly related to the logarithm of the concentration of HIV genes in the range from 5.0 × 10¹⁶ M to1.0 × 10¹⁰ M(R¼ 0.9930), and the HIV sensor exhibited a lower detection limit of 1.0 × 10¹⁶ M(S/N ¼ 3). The results show that this biosensor presented wonderful selectivity, sensitivity and specificity for HIV-1 gene detection. Thus, this biosensor provides a new method for the detection of HIV gene fragments.

Kumar et al. (S. Kumar et al., 2015) reported results of the studies relating to the fabrication of a paper-based sensor consisting of poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and reduced graphene oxide (RGO) composite. The effect of various solvents like methanol, ethylene glycol and H₂SO₄ on the electrical conductivity of PEDOT:PSS coated Whatman paper has been investigated. The conductivity of this solution processed conducting paper significantly increases from 1.16 × 10⁴ Scm⁻¹ up to 3.57 × 10² Scm⁻¹ (300 times) on treatment with ethylene glycol. The observed significant increase in electrical conductivity is due to conformational rearrangement in the polymer and is due to strong non-covalent cooperative interaction between PEDOT and the cellulose molecules. Further, incorporation of RGO into the conducting paper results in improved electrochemical performance and signal stability. This paper electrode is a promising alternative over the expensive conventional electrodes (ITO, gold and glassy carbon), that are known to have limited application in smart point-of-care (POC) devices. This low cost, flexible and environment friendly conducting paper-based biosensor utilized for cancer biomarker (carcinoembryonic antigen, CEA) detection reveals high sensitivity of 25.8 mAng1 mL cm2 in the physiological range, 1^–10 ng mL⁻¹.

6.2. Energy-related application

Energy is a matter of concern of present era cause the focile sources of energy are continually depleting and ever-growing consumption of energy (domestic and industrial) has increase the demand of efficient, environmental friendly, cost effective source of energy. Ever increasing population gives rise to the demand of cost effective and efficient energy conversion and storage devices. Graphen/polymer-based materials have advantages, such as high specific active surface areas, excellent electron transport capability and good capacitance. They are widely utilized as electrode materials, fuel cells, lithium ion batteries and so on. New energy storage devices include lithium-ion batteries, sodium ion batteries, potassium-ion batteries, lithium-air batteries, supercapacitors and other devices. Batteries or supercapacitors store energy chemically in the electrochemical cells.

6.2.1. Solar cells

Gao et al. (Gao, Gao et al., 2019) investigated the feasibility of the hybrid nanocomposites of the graphene quantum dot (GQD) and the phenoxazine-based dyes as the efficient sensitizer of the dye-sensitized solar cell (DSSC). Based on the first principles density functional theory (DFT), the geometrical structures of the separate GQDs, the phenoxazine-based dyes, and their hybridized nanocomposites are fully optimized.

Akhina et al. (Akhina et al., 2019), rGO was compounded with plasticized poly (vinyl chloride) to obtain flexible composites that show both high dielectric permittivity and low dielectric loss. An enhancement of 57% in dielectric constant was obtained after the addition of RGO to poly (vinyl chloride). This enhancement can be attributed to the increased number of interfaces of polymer and filler that was observed by transmission electron microscopy. The interfacial polarization in the heterogeneous interface plays an important role in imparting dielectric performance. Effective dielectric permittivity values of the composites calculated using theoretical equations were found to be lower than the experimental values. The nanocomposites were also tested for the permeability of O₂ which is very important for fabrication of energy storage devices. The permeability was decreased by 53% when compared to pristine poly (vinyl chloride). Hence, the enhanced dielectric permittivity as well as the impermeability to oxygen makes this material a suitable candidate for energy storage applications.

6.2.2. Supercapacitors

Graphene-based systems, are very important materials for energy applications, owing to their physical, morphological, and structural properties. Supercapacitor are new energy storage devices that exhibit such unique features as high capacitance, high-power density, and a long cycle. Among new energy storage device, supercapacitor has become a research hotspot due to its unique characteristics of rapid charge-discharge rate, high-power density, longterm cycling stability, and it is environmental friendly (H. Fan et al., 2017; Shen et al., 2016; Shi et al., 2018). The supercapacitor devices based on graphene/PANI composite film produced by vacuum filtration of GO and PANI dispersions exhibit large electrochemical capacitance (210 F 6.5 Balli et al.(Balli et al., 2019) discussed different polymers and polymer-based carbon derivatives as potential electrode materials for supercapacitor applications. They also discussed graphene and polymer-based materials developed recently for flexible super capacitor applications.

Suneetha et al. (Suneetha et al., 2019) prepared nanocomposite of Zinc doped Iron oxide/Graphene Oxide/Chitosan using optimized quantity of the components by simple solution mixing-evaporation method. The structure of the composite was studied by UV–Vis, FTIR, XRD, SEM, HRTEM and AFM. The results obtained from these analyses showed the formation of homogeneous mixture with strong interaction between the constituents and evidenced by the shift in peak position in both UV–Vis and FTIR spectra. Band gap calculated was found to be 2.28 eV which indicates the conducting nature of the composite. The formation of composite in nano dimensions of around 20 nm was proved by XRD, SEM and HRTEM studies. Thermal stability of the sample was investigated by TG/DTA and DSC techniques and showed improved thermal stability with the increase in glass transition temperature of chitosan by 78°c. Electrochemical characteristics of the composite was studied by cyclic voltammetry and capacitative behavior was studied by impedance studies (EIS). These electrochemical investigations revealed good adherent nature of the composite on electrode surface at pH1 and greater electrochemical stability with well-defined redox peaks. EIS showed that the nanocomposite modified electrode exhibited good capacitance behavior with the bode phase angle of 87° which proves it to be a very good candidate for supercapacitor applications.

Chabi et al. (Chabi et al., 2015), synthesized PPy functionalized 3 dimensional (3D) graphene foam (GF) with remarkable electrochemical performance. The resulting 3DPPY GF electrode is free standing and hence was used directly as a working electrode without using any binder or carbon additives. The unique features of the PPY–GF composites such as their hierarchically flexible 3D network, and high conductivity of p-doped PPY, afforded PPY–GF electrodes with enhanced pseudo capacitive properties. Azizi et al. (Azizi et al., 2019) synthesized a novel reduced graphene oxide/poly (1,5- dihydroxynaphthalene)/TiO2 (RGO/PDHN/TiO₂) ternary nanocomposite conducting polymer on gold electrodes for supercapacitor applications. The RGO/PDHN/TiO₂ nanocomposite polymer film is characterized by field emission scanning electron microscopy (FESEM), Fourier transform infrared spectra (FT-IR), and energy-dispersive X-ray spectrometry (EDX) and X-ray diffraction (XRD). The electrochemical performance of the nanocomposite polymer-modified electrode in 1.0 M HClO₄ is investigated by various electrochemical methods such as cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS). The RGO/PDHN/TiO2 nanocomposite polymer film in a three-electrode system exhibits a large specific capacitance of 556 F g1 in comparison with those obtained using RGO/PDHN (432 F g1) and PDHN (223 F g1) at a current density of 2.4 A g1. Simultaneous usage of the electrical double layer capacitance (EDLC) of RGO with the pseudocapacitive behavior of PDHN and TiO₂ results in the large specific capacitance in RGO/PDHN/TiO2. The electrochemical self-stabilities of RGO/PDHN/TiO2, RGO/PDHN, and PDHN polymer films are investigated by continuous cycling between 0.20 e0.45 V. The RGO/PDHN/TiO2 nanocomposite yields longer self-stability than that of other polymers after 1700 cycles and maintains about 74% of the initial capacitance values.

6.2.3. Energy storage batteries

Graphene, a material with exceptional properties, has dragged an attention worldwide due to its applicability in wide range of applications particularly in energy sector. With the growing human population, an intense need has aroused to explore alternate ways to meet upsurge demand of energy, where the sources of non-renewable energy are limited. Energy conversion and storage devices e.g., fuel cell, electrolyzer and batteries use polymer electrolyte membranes (PEM) as electrolyte/separator, as an important component. PEM plays a vital role in such devices, which can be prepared by functional polymers. Various PEMs consisting of various fillers have been developed to fulfill the needs of energy devices.

Gahlot et al. (Gahlot & Kulshrestha, 2019) reviewed GO-based polymer electrolyte membranes and their applications in energy devices. Advancements in the development of GO membranes, interaction with polymer matrix and their electrochemical properties have been summarized.

Their review also provides a profound insight about graphene-based polymer electrolyte membranes for energy-related applications including polymer electrolyte membranes fuel cell (PEMFC), vanadium redox flow battery (VRB) and Li-ion battery.

Pan et al. (Pan et al., 2019)prepared A 3D framework Si@N-doped C/reduced graphene oxide (Si@NC/rGO) composite by a novel polymer network method by using GO, Si nanoparticles, polymer monomers (acrylic amide), and networking agents (N,N′-methylene bisacrylamide) as the raw materials, followed by carbonization under an Ar atmosphere. After the crosslinking reaction, approximately 30 nm Si nanoparticles are embedded in the 3D NC/rGO framework. The Si@NC/rGO composite has a large reversible specific capacity, good cycle stability, and excellent rate performance as an anode electrode material for LIBs. The specific capacity is maintained at 867.4 and 479.1 mAh g − 1 at 0.1 and 2 A g − 1 after the 200 cycles, respectively. The capacity is three to fourfold higher than that of commercial graphite anode materials. The excellent electrical properties are mainly ascribed to the collaboration of the C framework and the high electrical conductivity of graphene and N doping. The Si@NC/ rGO composite shows growth potential as anode electrode materials for LIBs.

6.2.4. Fuel cell

Fuel cell is an energy conversion device, which uses chemical energy to convert into electrical energy. Fuel cell generates electricity via reactions between a fuel (anode) and an oxidant (cathode), which are continuously supplied from external source. Various kinds of fuel cells use electrolyte for ion movement and the following are fuel cells that use graphene-polymer composite as part of their components:

Arukula et al. (Arukula et al., 2019) reported a wet reflux strategy for the synthesis of rGO/polyaniline (PANI)/Pt-Pd composite, which was exploited as a potential anode catalyst with enhanced methanol oxidation capacity for direct methanol fuel cells (DMFCs). The construction of rGO/PANI/Pt-Pd involves two steps such as synthesis of PANI on GO and in-situ reduction of GO and metal precursors.

Baruha et al. (Baruah et al., 2019) synthesised-reduced graphene oxide (rGO)/poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) nanocomposites have been synthesized via in situ polymerization technique. The synthesized nanocomposite films were implanted with Xe+ ions at different fluences of 3.3 × 1014, 3.3 × 1015 and 3.3 × 1016 ions cm⁻². The morphological, structural and elemental analyses of the implanted samples have been studied by FESEM, EDX, TEM, XRD, FTIR, Raman, RBS and XPS measurements. The electrochemical behavior of the irradiated electrodes has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in presence of 0.5 mM ZoBell’s solution containing 0.5% KCl. The electrocatalytic activities of pristine and the irradiated electrodes towards methanol oxidation have been studied by cyclic voltammetry, chronoamperometry and cyclic stability. The electrocatalytic behavior reveals that the electrode irradiated with fluence 3.3 × 10¹⁶ ions cm⁻² exhibits different CV pattern than that of the other irradiated electrodes with slightly lower oxidation potential of 0.54 V and higher anodic current density of 48 mA cm⁻² with enhanced cyclic stability of 93% at 800th cycles. The enhanced electrocatalytic activity can be attributed to the transformation of carbonaceous layer to the graphite-like clusters and change in morphology of the nanocomposite at the fluence 3.3 × 10¹⁶ ions cm⁻² as observed from FESEM and TEM images.

Baruha et al. (Baruah & Kumar, 2018) synthesized, a non-precious anode catalyst material PEDOT:PSS/MnO2/rGO ternary nanocomposite by hydrothermal route followed by in situ oxidative polymerization. The morphology and structure of the synthesized samples were investigated by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). From the morphological investigations of the ternary nanocomposite, it is confirmed that PEDOT:PSS coated MnO2 nanorods are wrapped by rGO nanosheets. Brunauer-Emmett-Teller (BET) measurements confirm the porous structure and high surface area (190 m²/g) of the ternary nanocomposites. Electrochemical and electrocatalytic activities of PEDOT:PSS/MnO2/rGO coated ITO electrodes towards methanol oxidation were investigated by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) in 0.5 M NaOH as supporting electrolyte. Anodic and cathodic electron transfer coefficient (α and β) and heterogeneous rate constant (ks) of the ternary nanocomposite-coated electrode were found to be 0.51, 0.45 and 0.055 s⁻¹, respectively. The higher electrocatalytic activity i.e. higher oxidation current density (56.38 mA/cm2) and lower onset potential (0.32 V) of the ternary nanocomposite towards methanol oxidation may be due to synergistic effects of excellent conductivity of rGO nanosheets and porous nanostructure of PEDOT: PSS coated MnO₂ nanorods. Long-term stability holding of current density 50 mA/cm2 upto 1 h and higher cyclic stability (current retention factor 83%) up to 700th cycles imply that PEDOT:PSS/MnO₂/rGO ternary nanocomposite can be the potential alternative of platinum-based anode catalyst in direct methanol fuel cell.

6.3. Water purification

Water pollution is one of the greatest challenges around the world. Nanocomposite membranes are a promising-modified version of traditional polymeric membranes for water treatment, with three main characteristics of enhanced permeation, improved rejection and reduced fouling. For novel nanocomposite membranes, there is a strong connection between the membrane fabrication method, the properties of fabricated membranes, and membrane performance. This article, first, reviews the different nanocomposite membrane fabrication and modification techniques for mixed matrix membranes and thin film membranes for both pressure-driven and non-pressure driven membranes using different types of nanoparticles, carbon-based materials, and polymers. In addition, the advanced techniques for surface locating nanomaterials on different types of membranes are discussed in detail. The effects of nanoparticle physicochemical properties, type, size, and concentration on membranes intrinsic properties such as pore morphology, porosity, pore size, hydrophilicity/hydrophobicity, membrane surface charge, and roughness are discussed and the performance of nanocomposite membranes in terms of flux permeation, contaminant rejection, and anti-fouling capability are compared. Secondly, the wide range of nanocomposite membrane applications, such as desalination and removal of various contaminants in water treatment processes, are discussed. Extensive background and examples are provided to help the reader understand the fundamental connections between the fabrication methods, membrane functionality, and membrane efficiency for different water treatment processes (Esfahani et al., 2019).

Nanocomposites are now commonly employed to augment the standard polymeric membrane materials that are used in water treatment processes. A number of different materials and methods have been put forward; amongst those that show the greatest promise so far are thin-film nanocomposite (TFN), electrospun polymeric nanofibrous membranes, carbon nanotubes, metal and metal oxides, graphene and graphene oxide, and zwitterionic materials. This paper presents a detailed review of the current developments in the use of polymeric nanocomposite membranes for purifying water. Various nanocomposite membranes have been reported to evaluate their effectiveness in terms of resistance to fouling and the performance of the membranes. A specific focus has been placed on better understanding how nanomaterials can be used in a number of different ways, such as nanofiltration, micro-filtration, reverse osmosis and membrane distillation. This review aims to offer inspiration for further progress in the field of water treatment and desalination employing polymeric nanocomposite membranes (Bassyouni et al., 2019).

6.4. Electromagnetic interference (EMI) shielding

EMI protective elements have been increasingly used for isolating electrical and electronic devices, many types of cables, guaranteeing radio frequency shielding protection against possible interferences in medical and laboratory equipment, among many other applications. EMI protect electronic components from electromagnetic radiation emitted by other devices.

Wie et al. (L. Wei et al., 2019) prepared ultrathin and flexible water-borne polymer/sulfonated graphene nanosheets (S-GNS) composite by a facile self-assembly method in latex. The water-dispersible S-GNS was made by modifying the GNS with aryl diazonium salt. Styrene was introduced into water-borne polymer to form p-p stacking interaction with S-GNS. The composite shows good mechanical properties and high electromagnetic interference (EMI) shielding effectiveness (SE) in the X band (8.2e12.4 GHz). By Raman spectra, tensile tests, and simulation calculation, p-p stacking interaction is proved to be an effective interface bonding to enhance the mechanical properties. With a filler loading of 20 wt%, the tensile strength of composites is enhanced by 578%. The incorporation of25 wt% SGNS leads to a high EMI SE of 21.5 dB at 0.05 mm thickness, which remains unchanged after 1000 times bending. The specific SE/thickness (SSE/t) of the composite is as high as 2663 dB cm2/g, outperforming the ever reported materials with similar filler loading (25 wt%). The composite can be used as coating on the surfaces of solids, indicating the great potential for wide application.

Abbasi et al. (Abbasi et al., 2019), reviewed recent advances in carbon-based polymer nanocomposites for EMI shielding. After a revision of the types of carbon-based nanoparticles and respective polymer nanocomposites and preparation methods, the review considered the theoretical models for predicting the EMI shielding, divided in those based on electrical conductivity, models based on the EMI shielding efficiency, on the so-called parallel resistor-capacitor model and those based on multiscale hybrids. Recent advances in the EMI shielding of carbon-based polymer nanocomposites are presented and related to structure and processing, focusing on the effects of nanoparticle’s aspect ratio and possible functionalization, dispersion and alignment during processing, as well as the use of nanohybrids and 3D reinforcements. Examples of these effects are presented for nanocomposites with carbon nanotubes/nanofibres and graphene-based materials.

6.5. Drug delivery

Kim et al. (Kim et al., 2011), demonstrated the development of a GO-based efficient gene delivery carrier through installation of polyethylenimine, a cationic polymer, which has been widely used as a nonviral gene delivery vector. While Moustafa et al. (Moustafa et al., 2019) reviewed the advances in preparation, methods and technical applications of renewable graphene—biocomposites. At present, renewable and biodegradable biocomposites materials have drawn much attention as promising green materials in different domains of application such as intelligent food packaging, biomedical and drug delivery, bio-membranes, automotive, as well as in industrial composting applications.

Lee et al. (S. Lee et al., 2019), successfully constructed poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/graphene oxides (GOs) hybrid composites by electrochemical deposition on a gold micro-electrode. By changing the compositions and the redox states of GOs, the composites showed varied electrochemical performances as implantable neural electrodes, which have been further analyzed by Raman spectroscopy and SEM), and PC12 neural cellular attachment tests. Experimental results indicated that both PEDOT:PSS/GOs and PEDOT:PSS/reduced rGOs were significantly better than PEDOT:PSS in electrochemical performances, mechanical softness, as well as favorable protein expressions of modulating PC12 neural cells. Therefore, our PEDOT:PSS/rGO composites can be used to further improve the PEDOT in the applications of an implantable electrode, biosensors, drug delivery carriers, and neural interfaces.

6.6. Tissue engineering

Tissue engineering, such as implant materials, cell culture scaffolds, and regenerative medicine is the trend of focus by many present day material scientist.

Marakova et al. (Maráková et al., 2019) reported (PPy-GO) and (PEDOT-GO) composites. The GO incorporated in the composite matrix contributes to the patterning of the composite surface, while the electrically conducting PPy and PEDOT serve as ion-to electron transducers facilitating electrical stimulation/sensing. The films were fabricated by a simple one-step electropolymerization procedure on electrically conducting indium tin oxide (ITO) and graphene paper (GP) substrates. Factors affecting the cell behaviour, i.e. the surface topography, wettability, and electrical surface conductivity, were studied. The PPy-GO and PEDOT-GO prepared on ITO exhibited high surface conductivity, especially in the case of the ITO/PPy-GO composite. We found that for cardiomyocytes, the PPy-GO and PEDOTGO composites counteracted the negative effect of the GP substrate that inhibited their growth. Both the PPy-GO and PEDOT-GO composites prepared on ITO and GP significantly decreased the cytocompatibility of neural progenitors.

Li et al. (D. Li et al., 2017), demonstrated the recent advances in the preparation methods of G–BM hybrid materials and their applications in the field of tissue engineering, such as implant materials, cell culture scaffolds, and regenerative medicine. The perspectives and key challenges of G–BM hybrid materials in biomedical fields were also discussed.

6.7. Corrosion

Corrosion is a global challenge with the annual global cost put at USD 2.5 USD trillion which is equivalent to 3.4% of the world’s gross domestic product (B. Hou et al., 2017).

Both academic and industrial societies have recognised the importance of corrosion resistance coatings. Corrosion inhibitors form a layer over the metallic substrate during coatings against corrosion. Polymers composite and polymers have been tested for metal corrosion protection as replacement for the toxic inorganic and organic corrosion inhibitors. Their availability, cost effectiveness, and eco-friendliness (especially for natural polymers) in addition to the inherent stability arosed the interest of both academic and industrial developers. This lead to the developments of coatings with intelligent and efficient anti-corrosion properties that can be used in large aircraft and ship internal warehouse and some pharmaceutical and medical factories where long-term performance and safety is very important.

Chang et al. (Chang et al., 2014) used nanocasting to develop epoxy/graphene composites as corrosion inhibitors with hydrophobic surfaces (HEGC). The contact angle of water droplets on a sample surface can be increased from ∼82° (epoxy surface) to ∼127° (hydrophobic epoxy and EGC). It was noted that EGC coating was found to provide an excellent corrosion protection effect on cold-rolled steel (CRS) electrode. Enhancement of corrosion protection using EGC coatings could be attributed to the following three reasons: (1) epoxy could act as a physical barrier coating, (2) the hydrophobicity repelled the moisture and further reduced the water/corrosive media adsorption on the epoxy surface, preventing the underlying metals from corrosion attack, and (3) the well-dispersed graphene nanosheets (GNSs) embedded in HEGC matrix could prevent corrosion owing to a relatively higher aspect ratio than clay platelets, which enhances the oxygen barrier property of HEGC.

6.8. Gas barrier/gas sensor

Gas sensors are the crucial components to detect the type and concentration of gas. The components can transform gas composition, gas concentration and other information from non-electricity to electricity to achieve the measurement of gas. Graphene has excellent electron mobility and large specific surface area, and exhibits good gas sensing properties (Tian et al., 2018). Graphene material as a p-type semiconductor contains many holes, and has pull electron effect in gas atmosphere. After gas molecules are adsorbed by graphene, the gas molecules will undergo weak hybridization and coupling with the electron on the surface, and the fermi level to move up and down in small increments. The state of electron doping or hole doping will change the fermi level, thus lead to changes in graphene conductivity. Thus, graphene is particularly sensitive to the detection of adsorbed small molecule gases. The donor and acceptor depend on the relative position of the electron energy level orbit of the system. If the valence band of the adsorbed gas is higher than the fermi surface of graphene, the gas molecules act as donor for the electrons; on the contrary, if the valence band is lower than the fermi surface of graphene, the gas molecules act as acceptor.

Cui et al. (Cui et al., 2016) explored recent research and development of the utilization of graphene and its derivatives in the fabrication of nanocomposites with different polymer matrices for barrier application and most synthesis methods of graphene-based PNCs such as solution and melt mixing, in situ polymerization, and layer-by-layer process were covered. Graphene layers in the polymer matrix are capable of producing a tortuous path, which acts as a barrier for gases. A high tortuosity leads to superior barrier properties and lower permeability of PNCs. The influence of the intrinsic properties of these fillers (graphene and its derivatives) and their state of dispersion in polymer matrix on the gas barrier properties of graphene/PNCs were discussed.

Yanyan Wang et al. (Z. Fan et al., 2013) put forward a highly sensitive gas sensor based on graphene/polyaniline hybrid materials. PANI is an organic semiconductor molecule with excellent performance. Its preparation cost is low; the film-making process is simple; it is easily compatible with other technologies; and it can work at normal temperature. It has become a hot spot in the research of gas sensors. The combination of graphene and polyaniline can exert the advantages of two performance materials and is of great significance for improving the performance of the sensor. Researchers dispersed graphene/polyaniline hybrid materials in organic solvents; the dispersion was added dropwise Pt electrode surface and dried to obtain the gas sensor. It can be seen that the response of this graphene/polyaniline composite to NH₃ is greatly increased compared to pure graphene, and the resistance change rate can reach 30%.

7. Conclusions

In this review, preparation, characterization and properties of different graphene-polymer-based nanocomposites for a number of polymers are discussed. This review has highlighted some of the enhanced properties of polymer/graphene nanocomposites such as:

(i) Improvement in the mechanical, thermal and electrical properties of graphene-filled polymer nanocomposites which varies for different nanocomposites.

(ii) Nanocomposites with homogeneously dispersed graphene in the polymer matrix also exhibited good barrier properties because of the formation of a “tortuous path” in the presence of graphene in the nanocomposites.

(iii) The electrochemical stability of the PANI/graphene nanocomposites is much higher than the pure PANI or PANI/ITO composite electrode.

(iv) The graphene-based polymer nanocomposites showed good EMI shielding efficiency. The EMI shielding efficiency of 15 wt. % graphene-filled epoxy composites was 21 dB, which is higher than the target value (20 dB) for commercial applications.

(v) Graphene-based polymer nanocomposites exhibit superior mechanical properties compared to the neat polymer or conventional graphite-based composites.

(vi) Graphene-filled polymer nanocomposites showed superior thermal stability compared to the neat polymer. In some cases, it showed 100 ◦C improvements in thermal stability. The thermal conductivity of GO filled (5 wt. %) epoxy composites were four times higher than that of the neat epoxy resin.

(vii) Graphene-based polymer nanocomposites exhibited a several-fold increase in electrical conductivity. The appreciable improvement in electrical conductivity is due to the formation of a conducting network by graphene sheets in the polymer matrix.

This review has also highlighted potentials applications of polymer/graphene nanocomposites in biomedical application, low-cost sensors for the analysis of blood and urine, conducting electrode materials in a range of electrochromic devices, LEDs, transparent conducting coatings for solar cells and displays and high-performance composite materials for other range of applications.

8. Challenges

Uniformly dispersing nanofillers in polymer matrices is still a challenging technical difficulty.

The cytotoxic effects of graphene-polymer composites represent one of the major challenges that may have a great impact on future exploitation of graphene/polymer nanocomposite materials, especially in the field of technology and medicine.

Figure 1. Scheme depicting various conventional synthesis methods of graphene along with their important features, and their current and prospective applications

Adapted and reprinted with permission from Sambasivudu and Mahajan, Nanotech Insights,

Figure 2. Functionalization of graphene. (a) Graphene; (b), (c), and (d) are GO, RGO and graphene gel; (e) and (f) are organic small molecules and polymers modified graphene materials; (g), (h), and (i) are nanoparticles functionalized graphene materials

Figure 3. Schematic showing dip-coating process involving polymer-graphene foam coating process

Figure 4. Schematic of electrostatic spray coating process

Additional information

Funding

The author received no direct funding for this research.

Notes on contributors

Abdulazeez Tunbosun Lawal

Professor Lawaal is an environmental and material chemist. Abdulazeez T. Lawal had a BSc Hon Chemistry from University of Ibadan Nigeria. He left for Loughborough University of Technology United Kingdom where he bagged MSc Analytical Chemistry and Instrumentation. He worked for a while as research chemist with Metallurgical development centre Jos Nigeria and later migrated to Australia and gained admission to Monash University where he bagged a PhD in Analytical chemistry. He was a lecturer B and taught for a while in Charles Sturt University, Wagga Australia. He is currently a Professor of Chemistry in Fountain University Oshogbo, Nigeria. He has published many papers on material nanocomposite and among his papers is Polymer/graphene nanocomposite where, properties of polymer/graphene nanocomposites and their applications are discussed in general along with detailed examples. Different characterization methods and applications of these polymer composites were also reviewed.