Recent advances in graphene/polyamide 6 composites: a review

Abstract

This paper reviews recent advances in polyamide 6 (PA6) nanocomposites with graphene-based fillers including current works using graphite nanoplatelet fillers. Almost all of the latest important publications relating to the preparation, morphology and properties (such as thermal stability, thermal conductivity, electrical conductivity, mechanical properties, and flame retardant and gas barrier properties) of graphene/PA6 nanocomposites are summarized. An outlook of the current challenges in this field is provided for a potential guide to progress in the development of graphene/polymer nanocomposites too.

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Xubing Fu

Xubing Fu received his MSc in 2013 from Sichuan University (China). Currently he is pursuing his PhD degree at Hefei University of Technology under the supervision of Prof. Guisheng Yang. His focus of research is the preparation of graphene-based materials, particularly graphene/polymer nanocomposites.

Chenguang Yao

Chenguang Yao obtained his PhD from the Institute of Chemistry, Chinese Academy of Sciences, China, and now is the director of R&D in Hefei Genius Advanced Material Co., Ltd. His current research interests include the design, development, characterization and applications of polypropylene composites, PC/ABS alloy and flame retardant products.

Guisheng Yang

Guisheng Yang is Chairman of the Shanghai Genius Advanced Material Co., Ltd. and Professor of both Hefei University of Technology and Zhejiang University. He received his PhD degree from the Institute of Chemistry, Chinese Academy of Science in 1990. He has advised nearly 35 PhD students with whom he has published over 150 papers in polymer composites. He is also one of the academic leaders in Engineering Plastics and Composite Materials of China.

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1. Introduction

The 2010 Nobel Prize in Physics was awarded to Geim and Novoselov of Manchester University for their work on isolating graphene.^1–6 Graphene is a two-dimensional, single-atom-thick carbon sheet with a 0.142 nm carbon–carbon (C–C) bond length.^7,8 Due to the unique spatial and bonding arrangement of atoms through sp² hybridization of all of the C–C bonds across the sheet, graphene has many interesting characteristics, such as ∼5300 W mK⁻¹ thermal conductivity, ∼1.1 TPa Young’s modulus, ∼130 GPa strength, ∼200 000 cm² V⁻¹ s⁻¹ electron mobility, ∼6000 s cm⁻¹ electrical conductivity, ∼2630 m² g⁻¹ specific surface area, ∼98% optical transmittance, superior gas impermeability and unique functional properties.^1,8–19 These extraordinary properties make graphene a potential candidate for practical applications including in electronics, energy storage and conversion, batteries, supercapacitors, sensing platforms, photocatalysis, field emission devices, photovoltaic devices, single molecule gas detection, electrochemical resonators, fuel cells, biological labeling, Raman enhancement and so on.^20–54 One of the significant materials used for these applications are graphene-based polymer composites, which are an important addition in the area of science and are already recognized as playing a key role in modern nanoscience and nanotechnology.^55–57 As the graphene/polymer composites have shown superior mechanical, thermal, electrical, gas barrier, flame retardant and other properties compared to the neat polymer,^58–69 lots of researchers have been studying graphene/polymer composites around the world.⁷⁰ This was obviously proved by the number of research publications of graphene/polymer composites when a search was run with “graphene” and “polymer” as the two key words using one of the most common databases – Web of Science. Fig. 1A shows the number of publications from 2009 to 2015, indicating the rapid growth in research works of this area clearly.

Fig. 1 The numbers of publications from Web of Science about: (A) polymer/graphene, and (B) PA6/graphene (2015.6 means the first six months of the year 2015).

As an important matrix for the graphene/polymer composites, polyamide 6 (PA6) has already obtained great attention.⁷¹ It is an appealing thermoplastic with good processability, chemical resistance, and is widely used in engineering plastic films and fibers.^72–75 However, it also has some weaknesses such as poor dimensional stability, low thermal stability, low electrical conductivity, and its mechanical performance is not very good for use in some areas, which limits its applications.^76,77 Therefore many works have been undertaken to improve its relative properties through the introduction of nanofillers.^78,79 PA6 composites reinforced with graphene have attracted much attention because of substantial improvements achieved in the mechanical and other physical properties recently.^80–83 Fig. 1B shows the number of research publications on graphene/PA6 composites in recent years. There have been several reviews on graphene/polymers, but no review about the research on graphene/PA6 specifically.

In this paper, we review the literature of graphene/PA6 composites. The aims of this review are to provide a comprehensive understanding of recent researches on this topic, and the main content including preparation, dispersion, morphology and various properties. All of these issues prompted us to organize this review as a summary of the recent progress on graphene/PA6 composites.

2. Preparation of graphene/polyamide 6 composites

The dispersion of graphene and its derivatives in a polymer is a crucial step in the synthesis of polymer composites.⁸ A good dispersion of fillers ensured a maximum reinforced surface area, which will affect the polymer chains, and consequently, the properties of the whole matrix.⁷ The nature of the interaction between the fillers and matrix at the interface is an important factor influencing dispersion in all preparation techniques. With this consideration in mind, several methods are commonly used to fabricate graphene/polymer composites such as in situ polymerization, solvent blending, melt blending, etc.⁵⁴ which are useful for graphene/PA6 composites preparation as well.

2.1 In situ polymerization

In situ polymerization is the most often used method for the preparation of graphene/PA6 and other graphene-filled polymer composites including epoxy,^84–88 PS,⁸⁹ PMMA,^90,91 PU,^92,93 PE,⁹⁴ PANI,^95–97 PI,^98–101 etc.

This method consists of two main steps:^7,8,55,57

(1) Graphene or its derivative is mixed with pure monomers (or multiple monomers, a solution of monomers).

(2) A suitable initiator is then diffused in and polymerization is initiated by adjusting parameters such as temperature and time.

Researchers have used GO,^72,80,102 FG,^81,83,103 RGO^83,104,105 and EG^106–108 to prepare PA6 composites by in situ polymerization. There are two routes for this method; one is anionic ring-opening polymerization, and the other is hydrolytic polymerization. Zhang et al.¹⁰⁹ developed a facile route to fabricate GO reinforced PA6 by in situ anionic ring-opening polymerization, and found that the PA6 grafted GO sheets (g-GO) contain about 74 wt% polymers, which make the GO sheets homogenously disperse in the matrix and gain good interfacial adhesion. The g-GO sheets promoted the formation of the α-crystalline form and crystallization of PA6 in the non-isothermal crystallization. Fig. 2 shows the synthetic scheme for this method. Xu et al.⁸³ reported an efficient method to prepare RGO/PA6 composites via in situ polymerization. During the polycondensation, GO was first dried and then thermally reduced to graphene, and an obvious improvement of the mechanical properties was found. However, it has been confirmed that drying and reducing GO thermally tends to result in restacking of the sheets and leads to a bad dispersion.⁵⁶ So, in order to avoid drying GO sheets, Dixon et al.¹¹⁰ produced GO/PA6 composites by hydrolytic polymerization. GO was dispersed in a monomer/water solution, and it was not necessary to dry it. Therefore, it is possible to form a good dispersion of GO sheets in the PA6 matrix. Liu et al.¹⁰³ prepared an FG/PA6 composite fiber by in situ polymerization. PA6 chains were grafted on FG, and a significant mechanical property improvement was obtained. Ding et al.¹⁰⁵ synthesized RGO/PA6 nanocomposites. During the polymerization, GO was reduced to graphene at about 250 °C, and a clearly improved thermal conductivity of the nanocomposites was found.

Fig. 2 Synthetic scheme of the modification of graphene oxide (GO) with caprolactam and subsequent grafting of PA6 by in situ anionic ring-opening polymerization. Reproduced with the permission of the RSC (ref. 109: J. Mater. Chem., 2012, 22, 24081–24091).

2.2 Solvent blending

Solvent blending is the simplest method for the preparation of graphene/polymer composites. During the solvent blending process, graphene sheets are covered by polymer and they reassemble to form the composite, sandwiching the polymer when the solvent is evaporated.^7,54,55,57 Several polymer composites have been synthesized using this approach, such as PU,¹¹¹ PS,^112,113 PVA,^114–119 PMMA,¹²⁰ epoxy^121,122 and so on.

This method consists of three main steps:^54,57

(1) Graphene or other nanofillers dispersed in a suitable solvent.

(2) Incorporation of the polymer.

(3) Removal of the solvent by distillation or evaporation.

Scully et al.¹²³ synthesized GT/PA6 and GTO/PA6 composites by solvent blending and found that both GT and GTO fillers can significantly improve the thermal stability of PA6. Compared to other methods, by utilizing solvent chemistry methods to synthesize the composites, the preparation process is simpler, but it is unhealthy and environmentally unfriendly by using more organic solvent. Therefore, using an environmentally friendly solution is more desirable in solvent blending. Gong et al.⁷¹ prepared GO/PA6 composites by solvent blending with an effective and environmentally friendly protocol. The efficient polymer-chain grafting drastically improved the dispersion of GO sheets in the PA6 matrix and the interface adhesion between them, which resulted in a good improvement of the mechanical properties. Liu et al.⁷⁶ prepared GTO/PA6 composites also by solvent blending, and found GTO can enhance the thermal stability of PA6; the temperature of the maximum decomposition rate was increased obviously.

2.3 Melt blending

Melt blending is a more practical, versatile and economical technique for the fabrication of graphene/polymer composites. It utilizes both high-shear forces and high temperature melting to blend the nanofiller and polymer matrix, and is believed to be suitable for mass production.^7,54,57 Many practical examples have been reported by mixing graphene with the following polymers: PP,^124,125 PU,¹²⁶ PLA,^127–132 PET,^133–135 PE,^136–138 PMMA¹³⁹ and PPS.¹⁴⁰

This method consists of two main steps:

(1) Graphene is mixed mechanically with the polymer.

(2) Use of extrusion, injection moulding or melt spinning to prepare graphene/polymer composites; adjustment of parameters such as screw speed, temperature and time.

Tran et al.⁷⁸ fabricated GTPS/PA6 composites by the injection moulding method, and found that the strength, stiffness and toughness of the composites were all enhanced. Though mechanical noncovalent blending is a simple way to prepare composites of graphene/polymer, the interactions between graphene and the polymer are poor. Therefore, it is necessary to modify the graphene sheets in order to obtain strong interactions between the polymer matrix and graphene. Nguyen et al.¹⁴¹ prepared GO/PA6 composites using a twin screw extruder. GO was modified by a chemical method before melt blending with PA6. GO sheets were grafted on PA6 chains. Thus, a good interaction between the GO and PA6 matrix formed, which resulted in a clear modulus improvement of the composites. As this method is simple and suitable for industrial production, many methods have been developed to observe properties such as the dielectric,¹⁴² flame retardant,⁷³ thermal¹⁴¹ and mechanical properties^78,143 of the composites.

2.4 Other methods

In addition to the methods described above, other methods have been reported for fabricating graphene/PA6 composites. Table 1 lists the advantages and disadvantages of the three main production approaches. As each approach has its own advantages and disadvantages, combining two kinds of approaches may afford the optimal result. For example, Nguyen et al.¹⁴¹ prepared GO/PA6 composites using MB chips containing PA6-grafted GO. MB was prepared by an in situ polymerization method with GO dispersed in an ε-caprolactam monomer. Then GO/PA6 composites were prepared with the MB chips and commercial virgin PA6 chips using a twin screw extruder. This process combined typical in situ polymerization and melt blending methods. Thus, GO sheets may be obtained with a good dispersion in the PA6 matrix by in situ polymerization, and GO/PA6 composites can be produced on a large scale by melt blending. This approach was also adopted by Liu et al.¹⁰³ They produced FG/PA6 composite fibers using in situ polymerization and melt spinning methods, and found an obvious improvement of tensile strength and Young’s modulus. Pant et al.¹⁴⁴ fabricated TiO₂–GO/PA6 composites using solvent blending and electrospinning methods. The spider-wave-like nano-nets that comprise interlinked thin and thick fibers are widely distributed throughout the mat when a suitable amount of GO is blended with the PA6 solution. The simple blending of GO can sufficiently decrease the pore size of the electrospun PA6 mat and thus the formed nanoporous mat may have great potential for nanoparticle filtration. They also used this approach to prepare RGO/PA6 (ref. 77) and GO/PA6 (ref. 145) composite fibers. Other methods have also been reported, such as higher shear mixing (calendering),⁵⁷ which have been used for polymers like epoxy resins, but have been not reported in the preparation of graphene/PA6 composites. Table 2 summarizes the important publications on graphene or graphite/PA6 composites in recent years, from which we can see an obvious increase of reported works in this field.

Table 1 The advantages and disadvantages of different preparation methods for graphene/polymer composites

Preparation methods	Advantages	Disadvantages
In situ polymerization	1. Good dispersion of graphene in the polymer matrix	1. Increase of viscosity that hinders manipulation and the loading fraction
	2. Strong interaction between graphene and the polymer matrix	2. Environmentally unfriendly
Solvent blending	1. Simple route to disperse graphene into the polymer matrix	1. The use of surfactants may affect the polymer properties
	2. Good dispersion of graphene in the polymer matrix	2. Solvent removed may lead to aggregation of the graphene sheets
		3. Environmentally unfriendly
Melt blending	1. More economical	1. Poorer dispersion of graphene in the polymer matrix compared to the other two methods
	2. More compatible with many industrial practices	2. It may cause graphene bucking and rolling or shortening of the strong shear forces
	3. Environmentally friendly	3. Need large special machines
	4. Suitable for mass production

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Table 2 PA6/graphene or graphite composites reported in recent years

Composites	Preparation methods	Year of publication	Ref.
PA6/FG	Melt blending	2009	143
PA6/G	Solvent blending	2009	123
PA6/GO	Solvent blending	2009	123
PA6/GT	In situ (hydrolytic)	2010	107
PA6/RGO	In situ (hydrolytic)	2010	83
PA6/GO	Solvent blending + electrospinning	2011	145
PA6/GTO	Solvent blending	2011	76
PA6/GO	In situ (anionic ring-opening)	2012	109
PA6/RGO	In situ (hydrolytic)	2012	102
PA6/EG	In situ (hydrolytic)	2012	146
PA6/FGT	In situ (hydrolytic)	2012	146
PA6/GIC	In situ (hydrolytic)	2012	146
PA6/FG	In situ + melt spinning	2012	81
PA6/GNS–Co3O4	Melt blending	2013	180
PA6/GNS–NiO	Melt blending	2013	180
PA6/RGO	Solvent blending + electrospinning	2013	77
PA6/TiO2–RGO	Solvent blending + electrospinning	2013	144
PA6/G	Solvent blending + electrospinning	2013	147
PA6/FG	In situ + melt spinning	2013	103
PA6/RGO	In situ (hydrolytic)	2014	105
PA6/RGO	In situ (hydrolytic)	2014	148
PA6/HNTs–RGO	Melt blending	2014	73
PA6/GO + RGO	In situ (hydrolytic)	2014	72
PA6/GO	In situ + melt blending	2014	141
PA6/GTPS	Melt blending	2014	78
PA6/EGTPS	Melt blending	2014	149
PA6/LTEGT	In situ exfoliation melting	2014	108
PA6/FG	In situ + melt blending	2014	104
PA6/SG + CF	Melt blending	2015	150
PA6/GO-es-PVA	Solvent blending	2015	71
PA6/MWNT–GO	Melt blending	2015	142
PA6/GO	In situ (hydrolytic)	2015	110
PA6/EG	Melt blending	2015	151
PA6/FG	Melt blending	2015	152
PA6/SMA–FG	In situ + melt blending	2015	153

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3. Morphology and crystallization behavior

It is a challenging job to get a uniform dispersion of graphene sheets in polymer matrices, because of the high aspect ratio, inter-planer π–π interactions and strong van der Waals forces of graphene, which lead to it being prone to aggregation and reduce the load carrying capacity between the reinforcing phase and the matrix.⁵⁷

As property improvements correlate strongly with changes in the nanocomposite microstructure, effective characterization of the morphology is important to establish structure–property relationships for these materials.¹⁵⁴ SEM and TEM studies are possibly the main two means to observe the state of dispersion. As can be seen from Fig. 3A, no g-GO sheets could be observed on the surface of GO/PA6 composites, indicating a strong interfacial interaction between the g-GO sheets and the PA6 matrix. It is clearly observed that the g-GO sheets are homogeneously dispersed in GO/PA6 composites without agglomeration (Fig. 3B). From Fig. 3C and D, the typical stretched and wrinkled patterns of the g-GO sheets are randomly dispersed in the PA6 regions. All of these phenomena suggested a good dispersion of g-GO sheets in the PA6 matrix.¹⁰⁹

Fig. 3 SEM images obtained from the fracture surface of PA6–GO 1.0 nanocomposites: (A) unetched; (B) etched with 30 wt% trichloroacetic acid/ethanol solution at 60 °C for 1 h; (C) enlarged image from B1; (D) enlarged image from B2. Reproduced with the permission of the RSC (ref. 109: J. Mater. Chem., 2012, 22, 24081–24091).

It has been reported that incorporation of a graphene-based filler can cause an increase, decrease, or no change in the degree of crystallinity of a crystalline or semicrystalline polymer matrix.^155–158 The rate of crystallization may also be affected as nanofillers serve as a heterogeneous nucleation site for crystal growth.¹⁵⁹ Moreover, the polymer melting temperature may be changed with the presence of graphene-based nanofillers.¹⁰⁷

It is clearly confirmed that PA6 has three crystal forms: the more thermodynamically stable α-form, the unstable β-form crystal, and the γ-form crystal.¹⁰⁵ It can be seen from Fig. 4 that neat PA6 shows one main peak at about 221 °C (Tm1), and a shoulder peak at about 215 °C (Tm2), indicating the co-existence of α and γ crystal structures of PA6 or the process related melting–recrystallization during the second heating scan. There are also two apparent melting peaks in NG composites (Fig. 4a). With increasing GO content, the crystallinity of the NG composites decreased, indicating poor dispersion of GO sheets in the PA6 matrix, while with the increasing content of GO–PVA sheets, the crystallinity of NGP composites changed little, (Fig. 4b) implying that the well dispersed GO sheets were improved by the PVA chains joined to the GO sheets. In addition, the incorporation of GO sheets leads to a considerable increase in crystallization temperature (T_c), indicating that the GO sheets can act as nucleation agents during crystallization of PA6.⁷¹

Fig. 4 DSC curves of neat PA6, PA6/GO (NG) and PA6/GO-e-PVA (NGP 4 : 1) nanocomposites: (a) and (b) are the melting scans; (c) and (d) are the crystallization scans. Reproduced with permission from Elsevier (ref. 71: Composites: Part B, 2015, 73, 46–65).

4. Properties of graphene/polyamide 6 nanocomposites

4.1 Thermal stability

Thermal stability is an important property of functional composites.⁵⁸ Polymer degradation at low temperature will limit its applications in processes which need a high temperature. There are three parameters to evaluate the degradation behavior of polymers:⁷ one is the onset temperature, at which the polymer begins to degrade; another is the degradation temperature, at which the maximum degradation rate occurs; another is the degradation rate, from which the total time of degradation may be found. It has been widely confirmed that nanofillers can improve the thermal stability of polymers, because nanofillers can act as barriers to prevent the propagation of heat generated from the external environment in polymer matrices.¹⁶⁰ Thermally enhancing the effects of nanofillers depends on three factors:¹⁶¹ the thermal stability of the nanofillers, the aspect ratio of the nanofillers, and the dispersion of the nanofillers. A significant number of reports have reported an increased thermal stability of PA6 using GO, FG, RGO and GT as fillers.^{76,102,103,123} The work on the thermal stability of GTO/PA6 composites by thermal gravimetric analysis (TGA) showed a 53 °C improvement of the maximum decomposition temperature for the composite containing 5 wt% graphite oxide,⁷⁶ indicating that graphite oxide could enhance the thermal stability of the PA6 matrix, because of the good dispersion and the barrier effect of the graphite oxide sheets in the composites.

It is clear that GO has poor thermal stability, while GO/PA6 nanocomposites are thermally stable.¹⁰² As shown in Fig. 5, the onset and maximum decomposition temperatures of GO/PA6 nanocomposites are similar to those of the neat PA6. The PA6 chain appears to be effective in enhancing the thermal stability of GO sheets.¹⁰⁹ This result confirmed that GO had been reduced during the in situ polymerization process.¹⁰⁵ It can be concluded from these studies that the improved thermal stability of graphene/PA6 composites can be attributed to the high surface area and good dispersion of the nanofillers, and strong interactions between the nanofillers and the PA6 matrix.^7,162

Fig. 5 TGA curves of neat PA6, GO and g-GO at a heating rate of 20 °C min⁻¹ in nitrogen. Reproduced with the permission of the RSC (ref. 109: J. Mater. Chem., 2012, 22, 24081–24091).

4.2 Thermal conductivity

It has been reported that 2D, platelet-like graphene can improve thermal conductivity more effectively than 1D, rod-like CNTs.^70,163 As the single-layer graphene has a thermal conductivity of up to ∼5300 W mK⁻¹ at room temperature, it is expected that a small amount of graphene can significantly improve the thermal conductivity of polymer matrices,⁵⁷ which was confirmed by several works. It is the lattice vibrations (phonons) that transfer the thermal energy through the matrix and poor coupling at the filler–polymer and filler–filler interfaces cause significant thermal resistance. This is really important as it can hinder the transfer of phonons.^8,164,165 Therefore, good thermal conductivity requires a strong filler/polymer interface, so it is more effective in the thermal conductivity enhancement of composites which were produced by in situ polymerization than by melt blending.⁷ Besides the factors based on the interactions between the fillers and polymers, the characteristics of the polymer chains are also important factors affecting the thermal conductivity. The conformations of the chains, the grafting structures of the chains, and the macromolecular length will influence the alterations in the thermal resistance.¹⁴⁸

Recently, researchers have done lots of work to achieve high thermal conductivity of graphene-based PA6 composites. Ding et al.¹⁰⁵ reported a clear increase in the thermal conductivity of the PA6 matrix filled with 10 wt% RGO. As can be seen from Fig. 6, the thermal conductivity of RGO/PA6 composites was increased by 112% compared to the neat PA6 (from ∼0.196 to ∼0.416 W mK⁻¹). This may be attributed to the uniform dispersion of RGO sheets in the PA6 matrix and the RGO sheets grafted on the PA6 chains, which formed the thermal conductive paths or networks. In consideration of the factor of the polymer chain characteristics, Ding et al. used an in situ approach to realize length-controllable PA6 chains covalently grafted onto RGO sheets.¹⁴⁸ The structure–property relationship between the macromolecular length and thermal conductivity of the composites was observed, and it was found that the thermal conductivity of the composites decreased as the length of the grafted PA6 chains increased. The key parameters to obtain more efficient thermal conductivity of composites include denser packing, thinner thermal interface layers, more ordered molecular conformations and more effective interfacial bonding of the PA6 chains.¹⁴⁸

Fig. 6 The thermal conductivity of PA6/GO composites with varied GO content. Reproduced with permission from Elsevier (ref. 105: Carbon, 2014, 66, 576–584).

4.3 Electrical percolation and conductivity

One of the most fascinating aspects of graphene and graphite is the potential for their use in electronic applications due to their very high electrical conductivity.^57,154 Addition of a small amount of graphene or graphite to an insulating polymer matrix can change them into electrical conductors with a high level of electron delocalization owing to the larger surface area of the graphene or graphite sheets.^8,143 At a certain concentration level, known as the percolation threshold, the fillers can form a network, allowing charge transport within the composite and leading to a rapid rise in the electrical conductivity of the composite.^8,57 Many studies have been reported about the electrical conductivity improvement of polymer composites with graphene currently, such as PE,¹⁶⁶ PU,^92,93,167 PET^168,169 and epoxy composites.^170,171

Similar benefits to the electrical conductivity can be achieved for PA6 with graphene or graphite. Zheng et al.¹⁰² prepared GO/PA6 nanocomposites through a one-step in situ polymerization, and found that the introduction of GO significantly improved the electrical conductivity of PA6 with a sharp transition from electrically insulating to conducting. During the polymerization, the exfoliated and dispersed GO nanosheets were thermally reduced, resulting in an enhanced conductivity; with only ∼1.64 vol% of GO, the conductivity approaches ∼0.028 S m⁻¹. Pant et al.⁷⁷ fabricated GO/PA6 and RGO/PA6 composite fibers using the combined process of electrospinning and hydrothermal treatment. It was found that the electrical conductivity of the PA6 composites was higher compared to that of the GO/PA6 composite. Besides RGO or GO, graphite was also used to improve the electrical conductivity of PA6. Du et al.¹⁰⁷ synthesized the GT/PA6 composite by in situ polymerization. The results showed that as the concentration of GTO increased from 0.25 to 3.00 wt%, the electrical conductivity increased ∼26 billion times, as Fig. 7 shows.

Fig. 7 The electrical conductivities of PA6/graphite composites (NG is the abbreviation for natural graphite and EG is the abbreviation for exfoliated graphite). Reproduced with permission from Elsevier (ref. 107: Mater. Chem. Phys., 2010, 120, 167–171).

The effective property enhancement in these reports showed that the electrical conductivity of PA6 composites depends on the nanofiller types, synthesis methods, aspect ratio of fillers, dispersion of fillers in the PA6 matrix and the matrix–filler interactions induced by the surface functional groups.^54,57

4.4 Gas barrier and flame retardant

Defect-free graphene sheets are impermeable to all gas molecules.¹⁷² The two-dimensional structure forms a network of platelets inside the polymer matrix, and provides a convoluted path which can inhibit molecular diffusion through the matrix and consequently reduces permeability.⁸ Because of this, incorporation of graphene can clearly enhance the gas barrier properties of polymers. The barrier properties may be further enhanced by alignment perpendicular to the platelet orientation, while higher platelet aspect ratios correlate with an increased barrier resistance.¹⁷³ Both graphene and GO have been investigated in various permeation studies with polymers like PP,^174,175 PC,¹⁷⁶ PS¹⁷⁷ etc. However, no works have been reported about the gas barrier properties of graphene/PA6 composites till now.

The flame retardant properties of polymer composites have become a critical issue in industrial applications.¹⁷⁸ Due to its endothermic and stable layered structure, graphene could act as a physical barrier to reduce the diffusion of gases and degradation products, thus preventing the supply of oxygen and playing a role as a flame retardant.^179,180 Many works have been reported showing that graphene can improve the flame retardant properties of PA6 (ref. 73 and 180) and other polymers such as PS,¹⁸¹ PP,¹⁷⁹ epoxy¹⁸² etc. Li et al.⁷³ reported an improvement of the flame retardant properties of PA6 by using an RGO decorated with halloysite nanotubes (HNTs-d-RGO) hybrid composite as the additive in the PA6 matrix. They found that with incorporation of HNTs-d-RGO, compared to the neat PA6, the PHRR and the THR of HNTs-d-RGO/PA6 composites decreased, while the TSP increased. Hong et al.¹⁸⁰ synthesized GNS–Co₃O₄/PA6 and GNS–NiO/PA6 composites, and found that GNS–Co₃O₄ and GNS–NiO can improve fire safety by reducing the PHRR, HRR and TSR (seen from Fig. 8). Table 3 lists the cone test results of these two reported works, from which we can see the changes of PHRR and HRR are similar, indicating that graphene-based fillers can improve the flame retardant properties of PA6 composites. The degree of dispersion and interfacial interactions between graphene and the polymers are the main reasons for the improvement in the flame retardancy of composites.^179,180

Fig. 8 Heat release rate (a), CO production rate (b), total smoke release curve (c), and fire hazards evaluation (d) of PA6 and PA6 composites. Reproduced with permission from Elsevier (ref. 180: Mater. Chem. Phys., 2013, 142, 531–538).

Table 3 Cone test results for PA6 and PA6 composites (the data come from ref. 73 and 180)

	Samples	PHRR (kW m^−2)	THR (MJ m^−2)	TSP/TSR (m^2 m^−2)
Ref. 73	PA6	663	142.6	4.26 (TSP)
	PA6/GO	617	128.2	4.98 (TSP)
	PA6/HNTs-d-RGO	588	113.6	4.72 (TSP)
Ref. 180	PA6	1434	148.1	785.7 (TSR)
	PA6/G	1257	133.1	963.5 (TSR)
	PA6/GNS–Co3O4	1282	141.4	738.6 (TSR)
	PA6/GNS–NiO	1105	130.0	622.9 (TSR)

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4.5 Mechanical properties

It has been discovered that graphene is one of the strongest materials ever measured.¹⁸³ The Young’s modulus, intrinsic strength and breaking strength reach ∼1.1 TPa, ∼130 GPa and ∼42 N m⁻¹, respectively.¹⁶¹ The promising mechanical properties of all graphene-based fillers offer potential for the development of affordable high quality polymer composites.⁸ The successful preparation of graphene/polymer composites such as PE,^94,184,185 PU,^92,186,187 PVA,^114,188,189 PA6,^{71,83,103,111,150} epoxy^190–192 etc. has been reported recently.

Several factors affected the extent of the enhancement of the polymers’ mechanical properties upon addition to graphene-based nanofillers, including the dispersion of graphene in the polymer matrix, the reinforcement phase aspect ratio, the reinforcement phase concentration, interface bonding, etc.⁵⁷ The wrinkled surface or the thin platelets of FG are capable of mechanically interlocking with polymer chains, and hydrogen bonds form between the oxygen functionalities of FG and the polymeric matrix. Therefore, it is believed that FG is better in the enhancement of polymers than pristine graphene.¹⁶¹

Zhang et al.¹⁰⁹ observed the reinforcement of PA6 by GO. They found that the tensile strength and the Young’s modulus of PA6–GO nanocomposites increased with an increase of GO loading, and have been enhanced by about 88.0% and 66.5% with 1.0 wt% GO, respectively. This improvement of the mechanical properties was determined by well dispersed GO sheets and the strong interactions between the GO sheets and the PA6 matrix. However, an obvious decrease was observed in the elongation at break with an increase of GO content, indicating that the nanocomposites became more brittle compared with neat PA6. The large number of oligomers in the samples may decrease the intermolecular interactions between the polymers, and lead to the reduction of the elongation at break of PA6.

It is believed that the use of graphene in mechanical reinforcement rather than existing carbon fillers such as CB and EG is advantageous.⁷⁰ Steurer et al.¹⁴³ studied the influence on the Young’s modulus of PA6 composites upon addition to CB and RGO, and found that the Young’s modulus increased with an increase of filler loading, while the modulus of the RGO/PA6 composite increased more than that of the CB/PA6 composite, indicating that RGO is more efficient in improving the mechanical properties than CB. Monticelli et al.¹⁴⁶ prepared an EG/PA6 composite, and found that the modulus of the composite decreased in comparison to the neat PA6. It is concluded that with the highest concentration of EG, the composite behaves as a more plastic material. This may weaken interactions between the nanofiller and the polymer. A summary of the mechanical properties of PA6 composites is given in Table 4, from which we can conclude that the mechanical properties of the PA6 composites were significantly related to the preparation methods for the composites, the nature of fillers, the size, content and dispersion extent of the fillers.

Table 4 The mechanical properties of graphene and graphite/PA6 composites

Composites	Reinforcement content (wt%)	Strength increase (%)	Young’s modulus increase (%)	Preparation method	Ref.
PA6/SG + CF	13	∼67.3	∼42.3	Melt blending	150
PA6/RGO	10	—	∼47.3	Melt blending	143
PA6/CB	10	—	∼20.6	Melt blending	143
PA6/GTPS	3	∼14.1	∼17.2	Melt blending	78
PA6/GO-es-PVA	2	∼34.0	∼41.0	Solvent blending	71
PA6/G	1	∼107.6	∼177.9	Solvent blending	147
PA6/EG	1	—	∼−36.3	In situ	146
PA6/GO	1	∼88.0	∼66.5	In situ	109
PA6/GO	0.65	∼7.1		In situ	110
PA6/FG	0.1	∼29.0	∼300.0	In situ and melt spinning	81
PA6/RGO	0.1	∼210.0	∼240.0	In situ	83
PA6/FG	0.1	∼65.0	∼290.0	In situ and melt spinning	103
PA6/FG	0.1	∼29.0	∼33.0	In situ and melt spinning	104
PA6/GO	0.015	—	∼139.0	In situ and melt blending	141

---

5. Conclusions and outlook

We have reviewed the recent progress on the preparation, morphology and properties of graphene/PA6 nanocomposites. Three main methods for preparing graphene/PA6 nanocomposites were discussed in detail, such as in situ polymerization, solvent blending and melt blending. As each method has its advantages and disadvantages, combining two or more than two different methods to fabricate this nanocomposite may provide more reasonable results. SEM and TEM are the common ways to observe the morphology of nanofiller/PA6 composites. From the present study, it has been found that many properties of graphene/PA6 nanocomposites were made superior to pure PA6 by dispersing low contents of graphene in the PA6 matrix. Among these properties, thermal conductivity, electrical conductivity and the mechanical properties of the composites were enhanced obviously. Other properties like thermal stability and flame retardancy were also found to be improved in the composites, which confirmed that graphene is a multifunctional reinforcement material for improving the properties of polymers with an extremely small loading.

Though current results have indicated that graphene is very promising for improving the properties of polymers, there are still many challenges that must be addressed for these composites to reach their full potential. (1) Till now, many efforts have been made to develop synthetic methods for graphene, while it is still difficult to obtain more than 90% or even 100% single-layer graphene (SLG) in mass production. Most of the commercial graphenes from the market are multi-layer graphene (MLG) or few layer graphene (FLG). The exfoliation method for the preparation graphene can’t provide SLG on a large scale, because of the easy re-aggregation of the graphene sheets and the lack of effective technology to exfoliate the sheets completely. The reduction methods from GO to fabricate graphene are also unable to provide SLG on a large scale, for it is difficult to reduce GO to graphene entirely. So, though tremendous works have been done to try to synthesize graphene since it was found ten years ago, it is still a challenge to produce pure SLG in commercial production. As the extremely excellent properties of graphene are from the pure SLG, not MLG, FLG and GO, the remarkable properties can’t be reflected in graphene/PA6 and other graphene/polymer composites, because the graphene used to prepare the composites is not pure SLG, and this is one of the reasons that the improvements in the properties of graphene/polymer composites are not as high as theorized. (2) In order to acquire a strong interaction between graphene and the polymer matrix, almost all of the reported works have used modified graphene (such as GO, FG, RGO etc.) or a modified process to produce the graphene/polymer composites. However, modification of graphene may change the inherent properties of pure graphene dramatically, which consequently influences the improvements of the graphene/polymer composites. Moreover the organic solvent used in the modification process is not environmentally friendly, and is unsuitable for industrial production. Few methods currently exist for the large scale production of chemically unmodified or pure graphene sheets. Therefore, it is also a challenge to produce graphene/polymer composites and both obtain a good interaction between graphene and the polymer matrix through an environmentally friendly way, and retain the inherent properties of pure graphene as much as possible. (3) As mentioned above, there are three main methods to prepare graphene/PA6 and other graphene/polymer composites. In situ polymerization and solvent blending can achieve a fine dispersion of graphene in the polymer matrix, while they may involve potential risks for pollution due to the use of some harmful reagents. Melt blending is environmentally friendly and suitable for mass production, but the dispersion of graphene in the polymer matrix is poor. Till now, it is still a challenge to produce graphene/polymer composites by an effective method which is both environmentally friendly, suitable for industrial production and results in graphene dispersed well in the polymer matrix. Owing to this, the application of graphene in PA6 and other polymers is limited. (4) The cost of graphene is also a challenge. Currently, the pricing of commercial graphene production ranges from ∼70 to ∼200 USD g⁻¹ and it will not see practical applications until a large scale production of graphene is available at low cost.

Therefore, the core issues such as the preparation of pure SLG, the achievement of a good interaction between graphene and the polymer matrix without a modified process, an appropriate method to produce graphene/polymer nanocomposites in industrial production and the cost of graphene still deserve further research. The discovery of graphene has opened a new dimension for the production of high performance composite materials with a wide range of applications. The future of graphene/polymer nanocomposites will ultimately advance by improved methods and technologies.

Abbreviations

G	Graphene
GO	Graphene oxide
RGO	Reduced graphene oxide
GTO	Graphite oxide
GIC	Graphene intercalation compounds
EGTPS	Exfoliated graphite nanoplatelets
SG	Solar graphene
GNS	Graphene nanosheets
SMA	Styrene-maleic-anhydride
THR	Total heat release
TSP	Total smoke production
FLG	Few-layer graphene
GT	Graphite
FG	Functional graphene
EG	Exfoliated graphite
GTPS	Graphite nanoplatelets
FGT	Flake graphite
LTEGT	Low-temperature expandable graphite
CF	Carbon fiber
MWNT	Multi walled carbon nanotubes
MB	Master batch
PHRR	Peak heat release rate
MLG	Multi-layer graphene
SLG	Single-layer graphene.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (2013BAE02B03). The authors thank Hefei Genius Advanced Material Co., Ltd for help with the data processing.

References

1. M. N. Shannon and R. E. Drew, Adv. Colloid Interface Sci., 2014, 209, 196–203.
2. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang and S. V. Dubonos, Science, 2004, 36, 666.
3. K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevitch and S. V. Morozov, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 10451.
4. L. Chem, Y. Hernandez, X. L. Feng and K. Mullen, Angew. Chem., Int. Ed., 2012, 51, 2–17.
5. A. K. Geim, Angew. Chem., 2011, 123, 7100–7122.
6. K. S. Novoselov, Angew. Chem., Int. Ed., 2011, 50, 6986–7002.
7. R. Verdejo, M. M. Bernal, L. J. Romasanta and M. A. Lopez-Manchado, J. Mater. Chem., 2011, 21, 3301–3310.
8. K. K. Sadasivuni, D. Ponnamma, S. Thomas and Y. Grohens, Prog. Polym. Sci., 2014, 39, 749–780.
9. S. Sato, N. Harada, D. Kondo and M. Ohfuchi, Fujitsu Sci. Tech. J., 2010, 46, 103–110.
10. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.
11. C. Lee, X. Wei, J. W. Kysar and J. Hone, Science, 2008, 321, 385–388.
12. C. N. R. Rao, K. Biswas, K. S. Subrahmanyam and A. Govindaraj, J. Mater. Chem., 2009, 19, 2457–2469.
13. D. D. L. Chung, J. Mater. Sci., 2002, 37, 1475–1489.
14. K. I. Bolotin, K. J. Sikes, Z. Jiang, M. Klima, G. Fudenberg and J. Hone, Solid State Commun., 2008, 146, 351–355.
15. Y. Geng, S. J. Wang and J. Kim, J. Colloid Interface Sci., 2009, 336, 592–598.
16. X. Du, I. Skachko, A. Barker and E. Y. Andrei, Nat. Nanotechnol., 2008, 3, 491–495.
17. C. Soldano, A. Mahmood and E. Dujardin, Carbon, 2010, 48, 2127–2150.
18. F. R. Al-Solamy, A. A. Al-Ghamdi and W. E. Mahmoud, Polym. Adv. Technol., 2012, 23, 478–482.
19. J. S. Bunch, S. S. Verbridge, J. S. Alden, A. M. Vander Zande, J. M. Parpia, H. G. Craighead and P. L. Mceuen, Nano Lett., 2008, 8, 2458–2462.
20. V. Dua, S. P. Surwade, S. Ammu, S. R. Agnihotra, S. Jain, K. E. Roberts, S. Park, R. S. Ruoff and S. K. Manohar, Angew. Chem., Int. Ed., 2010, 49, 2154.
21. J. S. Bunch, A. M. Vander Zande, S. S. Verbridge, I. W. Frank, D. M. Tanenbaum, J. M. Parpia, H. G. Craighead and P. L. Mceuen, Science., 2007, 325, 490.
22. D. R. Dreyer and C. W. Bielawski, Chem. Sci., 2011, 2, 1233–1240.
23. M. Pumera, Energy Environ. Sci., 2011, 4, 668–674.
24. S. Gilje, S. Han, M. Wang, K. L. Wang and R. B. Kaner, Nano Lett., 2007, 7, 3394.
25. F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov, Nature, 2007, 6, 652.
26. V. Dua, S. P. Surwade, S. Ammu, S. R. Agnihotra, S. Jain, K. E. Roberts, S. Park, R. S. Ruoff and S. K. Manohar, Angew. Chem., 2010, 122, 2200–2203.
27. W. R. Yang, K. R. Ratinac, S. P. Ringer, P. Thordarson, J. J. Gooding and F. Braet, Angew. Chem., 2010, 122, 2160–2185.
28. S. G. Wang, J. J. Wang, P. Miraldo, M. Y. Zhu, R. Outaw, K. Hou, X. Zhao, B. C. Holloway, D. Manos, T. Tyler, O. Shenderova, M. Ray, J. Dalton and G. Mcguire, Appl. Phys. Lett., 2006, 89, 183103.
29. R. M. Westervelt, Science, 2008, 320, 324–325.
30. Z. Yin, S. Wu, X. Zhou, X. Huang, Q. Zhang, F. Boey and H. Zhang, Small, 2010, 6, 307–312.
31. Z. Yin, S. Y. Sun, T. Salim, S. Wu, X. Huang, Q. He, Y. M. Lam and H. Zhang, ACS Nano, 2010, 4, 5263–5268.
32. W. Shi, J. Zhu, D. H. Sim, Y. Y. Tay, Z. Y. Lu, X. J. Zhang, H. Zhang, H. H. Hung and Q. Yan, J. Mater. Chem., 2011, 21, 3422–3427.
33. J. Zhu, T. Zhu, X. Zhou, Y. Zhang, X. W. Lou, X. Chen, H. Chen, H. Zhang, H. H. Hung, J. Ma and Q. Yan, Nanoscale, 2011, 3, 1084–1089.
34. C. H. Lu, H. H. Yang, C. L. Zhu, X. Chen and G. N. Chen, Angew. Chem., Int. Ed., 2009, 121, 4879–4881.
35. H. Chang, L. Tang, Y. Wang, J. Jiang and J. Li, Anal. Chem., 2010, 82, 2341–2346.
36. Z. S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li and H. M. Cheng, ACS Nano, 2010, 4, 3186–3194.
37. X. Ling, L. Xie, Y. Fang, H. Xu, H. Zhang, J. Kong, M. S. Dresselhaus, J. Zhang and Z. Liu, Nano Lett., 2009, 10, 553–561.
38. R. I. Jafri, N. Rajalakshmi and S. Ramaprabhu, J. Mater. Chem., 2010, 20, 7114–7117.
39. J. Du, X. Lai, N. Yang, J. Zhai, D. Kisailus, F. Su, D. Wang and L. Jiang, ACS Nano, 2010, 5, 590–596.
40. Y. Zhang, H. Li, L. Pan, T. Lu and Z. Sun, J. Electroanal. Chem., 2009, 634, 68–71.
41. Y. Li, L. Tang and J. Li, Electrochem. Commun., 2009, 11, 846–849.
42. H. Liu, S. Ryu, Z. Chen, M. L. Steigerwald, C. Nuckolls and L. E. Brus, J. Am. Chem. Soc., 2009, 131, 17099–17101.
43. H. Wang, L. F. Cui, Y. Yang, H. Sanchez Casalongue, J. T. Robinson, Y. Liang, Y. Cui and H. Dai, J. Am. Chem. Soc., 2010, 132, 13978–13980.
44. S. Yang, X. Feng, S. Ivanovici and K. Mullen, Angew. Chem., Int. Ed., 2010, 49, 8408–8411.
45. H. Zhang, X. J. Lv, Y. M. Li, Y. Wang and J. H. Li, ACS Nano, 2010, 4, 380–386.
46. X. Ling and J. Zhang, Small, 2010, 6, 2020–2025.
47. J. Song, Z. Yin, Z. Yang, P. Amaladass, S. Wu, J. Ye, Y. Zhao, W. Deng, H. Zhang and X. Liu, Chem.–Eur. J., 2011, 01263.
48. L. Xie, X. Ling, Y. Fang, J. Zhang and Z. Liu, J. Am. Chem. Soc., 2009, 131, 9890–9891.
49. X. Li, H. Zhu, K. Wang, A. Cao, J. Wei, C. Li, Y. Jia, Z. Li, X. Li and D. Wu, Adv. Mater., 2010, 22, 2743–2748.
50. L. S. Zhang, X. Q. Liang, W. G. Song and Z. Y. Wu, Phys. Chem., 2010, 12, 12055–12059.
51. L. Dong, R. R. S. Gari, Z. Li, M. M. Craig and S. Hou, Carbon, 2010, 48, 781–787.
52. F. Kim, J. Luo, R. Cruz-Silva, L. J. Cote, K. Sohn and J. Huang, Adv. Funct. Mater., 2010, 20, 2867–2873.
53. F. Li, J. Song, H. Yang, S. Gan, Q. Zhang, D. Han, A. Ivaska and L. Niu, Nanotechnology, 2009, 20, 455602.
54. X. Huang, X. Y. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012, 41, 666–686.
55. T. Kuilla, S. Bhadra, D. Yao, N. H. Kim, S. Bose and J. H. Lee, Prog. Polym. Sci., 2010, 35, 1350–1375.
56. S. Stankovich, D. A. Dikin, G. H. B. Domment, K. M. Kohlhaas, E. J. Zimney and E. A. Stach, Nature, 2006, 442, 282–286.
57. D. Verma, P. C. Gope, A. Shandilya and A. Gupta, Trans. Indian Inst. Met., 2014, 67, 803–816.
58. S. Ansari and E. P. Giannelis, J. Polym. Sci., Part B: Polym. Phys., 2009, 47, 888–897.
59. Y. Xu, Y. Wang, J. J. Li, Y. Huang, Y. Ma and X. Wan, Nano Res., 2009, 2, 343–348.
60. H. Quan, B. Zhang, Q. Zhao, R. K. K. Yuen and R. K. Y. Li, Composites, Part A, 2009, 40, 1506–1513.
61. J. Liang, Y. Xu, Y. Huang, L. Zhang, Y. Wang and Y. Ma, J. Phys. Chem., 2009, 113, 9921–9927.
62. G. Eda and M. Chhowalla, Nano Lett., 2009, 9, 814–818.
63. Y. R. Lee, A. V. Raghu, H. M. Jeong and B. K. Kim, Macromol. Chem. Phys., 2009, 210, 1247–1254.
64. T. Ramanathan, A. A. Abdala, S. Stankovich, D. A. Dikin, M. H. Alonso and R. D. Piner, Nat. Nanotechnol., 2008, 3, 327–331.
65. H. Kim and C. W. Macosko, Polymer, 2009, 50, 3797–3809.
66. K. L. Xu, G. M. Chen and D. Qiu, Chem.–Asian J., 2015, 10, 1225–1231.
67. K. L. Xu, G. M. Chen and D. Qiu, J. Mater. Chem. A., 2013, 1, 12395–12399.
68. S. B. Han, W. T. Zhai, G. M. Chen and X. Wang, RSC Adv., 2014, 4, 29281–29285.
69. Z. Zhang, G. M. Chen, H. F. Wang and W. T. Zhai, J. Mater. Chem. C, 2015, 3, 1649–1654.
70. H. Kim, A. A. Abdala and C. W. Macosko, Macromolecules, 2010, 43, 6515–6530.
71. L. Gong, B. Yin, L. P. Li and M. B. Yang, Composites, Part B, 2015, 73, 49–56.
72. A. O’Neill, D. Bakirtzis and D. Dixon, Eur. Polym. J., 2014, 59, 353–362.
73. L. L. Li, S. H. Chen, W. J. Ma, Y. H. Cheng, Y. P. Tao, T. Z. Wu, W. P. Chen, Z. Zhou and M. F. Zhu, eXPRESS Polym. Lett., 2014, 6, 450–457.
74. N. Abacha, M. Kubouchi and T. Sakai, eXPRESS Polym. Lett., 2009, 3, 245–255.
75. Y. Cai, F. Huang, Q. Wei, L. Song, Y. Hu, Y. Ye, Y. Xu and W. Gao, Polym. Degrad. Stab., 2008, 93, 2180–2185.
76. Y. Liu, Z. M. Chen and G. S. Yang, J. Mater. Sci., 2011, 46, 882–888.
77. H. R. Pant, B. W. W. Pant, C. H. Park, H. J. Kim, D. S. Lee, L. D. Tijing, B. S. Hwang, H. Y. Kim and C. S. Kim, Fibers Polym., 2013, 14, 970–975.
78. T. D. Thanh, L. Kapralkova, J. Hromadkova and I. Kelnar, Eur. Polym. J., 2014, 50, 39–45.
79. J. Li, L. Tian, N. Pan and Z. J. Pan, Polym. Eng. Sci., 2014, 54, 1618–1624.
80. X. Q. Zhang, X. Y. Fan, H. Z. Li and C. Yan, J. Mater. Chem., 2012, 22, 24081–24091.
81. H. H. Liu, L. C. Hou, W. W. Peng, Q. Zhang and X. X. Zhang, J. Mater. Sci., 2012, 47, 8052–8060.
82. H. Kim, A. A. Abdala and C. W. Macosko, Macromolecules, 2010, 43, 6515–6530.
83. Z. Xu and C. Gao, Macromolecules, 2010, 43, 6716–6723.
84. S. Sahila and L. S. Jayakumari, Polym. Compos., 2015, 36, 1–7.
85. P. Ming, Y. Y. Zhang, J. W. Bao, G. Liu, Z. Li, L. Jianga and Q. F. Cheng, RSC Adv., 2015, 5, 22283–22288.
86. Y. C. Li, J. G. Tang, L. J. Huang, Y. Wang, J. X. Liu, X. C. Ge, S. C. Tjong, R. K. Y. Li and L. A. Belfiore, Composites, Part A, 2015, 68, 1–9.
87. H. Gui, P. Xu, Y. Hu, J. Wang, X. Yang, A. Bahader and Y. Ding, RSC Adv., 2015, 5, 27814–27822.
88. Y. T. Park, Y. Q. Qian, C. Chan, T. Suh, M. G. Nejhad, C. W. Macosko and A. Stein, Adv. Funct. Mater., 2015, 25, 575–585.
89. K. Q. Zhou, J. J. Liu, W. K. Zeng, Y. Hu and Z. Gui, Compos. Sci. Technol., 2015, 107, 120–128.
90. Z. Fan, F. Gong, S. T. Nguyen and H. M. Duong, Carbon, 2015, 81, 396–404.
91. A. Anson-Cusaos, F. J. Pascual, C. Ruano, N. Fernandez-Huerta, I. Fernandez-Pato, J. C. Qtero, J. A. Puertolas and M. T. Martinez, J. Appl. Polym. Sci., 2015, 132, 41547.
92. P. Pokharel, S. H. Lee and D. S. Lee, J. Nanosci. Nanotechnol., 2015, 15, 211–214.
93. D. Spasevska, G. P. Leal, M. Fernandez, J. B. Gilev, M. Paulis and R. Tomovska, RSC Adv., 2015, 5, 16414–16421.
94. A. Kheradmand, S. A. A. Ramazani, F. Khorasheh, M. Baghalha and H. Bahrami, Polym. Adv. Technol., 2015, 26, 315–321.
95. W. Zhao, D. W. He, Y. S. Wang, X. Du and H. Xin, Chin. Phys. B, 2015, 24, 047204.
96. J. Li, H. Q. Xie and Y. Li, J. Nanosci. Nanotechnol., 2015, 15, 3280–3283.
97. D. K. Mahla, M. Rahaman and D. Khastgir, Polym. Compos., 2015, 36, 445–453.
98. L. Cao, Q. Q. Sun, H. X. Wang, X. X. Zhang and H. F. Shi, Composites, Part A, 2015, 68, 140–148.
99. Y. Li, X. L. Pei, B. Shen, W. T. Zhai, L. H. Zhang and W. G. Zheng, RSC Adv., 2015, 5, 24342–24351.
100. L. Zhao, C. Cheng, Y. F. Chen, T. Wang, C. H. Du and L. G. Wu, Polym. Adv. Technol., 2015, 26, 330–337.
101. M. K. Liu, Y. F. Du, Y. E. Miao, Q. W. Ding, S. X. He, W. W. Tjiu, J. S. Pan and T. X. Liu, Nanoscale, 2015, 7, 1037–1046.
102. D. Zheng, G. S. Tang, H. B. Zhang, Z. Z. Yu, F. Yavari, N. Koratkar, S. H. Lim and M. W. Lee, Compos. Sci. Technol., 2012, 72, 284–289.
103. H. H. Liu, W. W. Peng, L. C. Hou, X. C. Wang and X. X. Zhang, Compos. Sci. Technol., 2013, 81, 61–68.
104. W. J. Hou, B. Q. Tang, L. L. Lu, J. Sun, J. J. Wang, C. X. Qin and L. X. Dai, RSC Adv., 2014, 4, 4848–4855.
105. P. Ding, S. S. Su, N. Song, S. F. Tang, Y. M. Liu and L. Y. Shi, Carbon, 2014, 66, 576–584.
106. J. H. Li, M. Li, H. F. Da, Q. Liu and M. H. Liu, Composites, Part A, 2012, 43, 1038–1043.
107. N. Du, C. Y. Zhao, Q. Chen, G. Wu and R. Lu, Mater. Chem. Phys., 2010, 120, 167–171.
108. S. T. Zhou, Y. Li, X. Song, J. Chang, H. W. Zou and M. Liang, J. Appl. Polym. Sci., 2014, 131, 39596.
109. X. Q. Zhang, X. Y. Fan, H. Z. Li and C. Yan, J. Mater. Chem., 2012, 22, 24081–24091.
110. D. Dixon, P. Lemonine, J. Hamilton, G. Lubarsky and E. Archer, J. Thermoplast. Compos. Mater., 2015, 28, 372–389.
111. S. K. Khanna and H. T. T. Phan, J. Eng. Mater. Technol., 2015, 137, 4029291.
112. P. Ding, J. Zhang, N. Song, S. F. Tang, Y. M. Liu and L. Y. Shi, Compos. Sci. Technol., 2015, 109, 25–31.
113. L. Y. Zhang, J. J. Shi, D. X. Tan, H. O. Zhou and X. Zhou, Fullerenes, Nanotubes, Carbon Nanostruct., 2014, 22, 726–737.
114. J. Jose, M. A. Al-Harthi, M. A. Aima’adeed, J. B. Dakua and S. K. De, J. Appl. Polym. Sci., 2015, 132, 41827.
115. K. J. Lin, S. C. Lee and K. F. Lin, J. Polym. Res., 2014, 21, 06114.
116. S. Choi, D. K. Hwang and H. S. Lee, Carbon Lett., 2014, 15, 282–289.
117. D. S. Yu, T. Kuila, N. H. Kim and J. H. Lee, Chem. Eng. J., 2014, 245, 311–322.
118. C. P. Li, M. She and X. K. Ling, Appl. Mech. Mater., 2014, 447, 206–209.
119. R. K. Layek, A. K. Das, M. U. Park, N. H. Kim and J. H. Lee, J. Mater. Chem. A, 2014, 2, 12158–12161.
120. N. N. Hong, L. Song, B. B. Wang, A. A. Stec, T. K. Hull, J. Zhan and Y. Hu, Mater. Res. Bull., 2014, 49, 657–664.
121. B. Ahmadi-Moghadam and F. Taheri, J. Mater. Sci., 2014, 49, 6180–6190.
122. D. Galpaya, M. Wang, G. George, N. Motta, E. Waclawik and C. Yan, J. Appl. Phys., 2014, 116, 4892089.
123. K. Scully and R. Bissessur, Thermochim. Acta, 2009, 490, 32–36.
124. F. You, D. R. Wang, X. X. Li, M. J. Liu, G. H. Hu and Z. M. Dang, RSC Adv., 2014, 4, 8799–8807.
125. S. H. Ryu and A. M. Shanmugharaj, Mater. Chem. Phys., 2014, 146, 478–486.
126. M. Raja, S. H. Ryu and A. M. Shanmugharaj, Colloids Surf., A, 2014, 450, 59–66.
127. H. Norazlina and Y. Kamal, Polym. Bull., 2015, 72, 931–961.
128. B. W. Chieng, N. A. Ibrahim, W. M. Z. W. Yunus, M. Z. Hussein, Y. Y. Then and Y. Y. Loo, J. Appl. Polym. Sci., 2015, 132, 41652.
129. C. Sriprachuabwong, S. Duangsripat, K. Sajjaanantakul, A. Wisitsoraat and A. Tuantranont, J. Appl. Polym. Sci., 2015, 132, 41439.
130. B. W. Chieng, N. A. Ibrahim, W. M. Z. W. Yuns, M. Z. Hussein and Y. Y. Loo, J. Therm. Anal. Calorim., 2014, 118, 1551–1559.
131. V. Mittal, A. U. Chaudhry and G. E. Luckachan, Mater. Chem. Phys., 2014, 147, 319–332.
132. B. W. Chieng, N. A. Ibrahim, W. M. Z. W. Yuns and M. Z. Hussein, Polymers, 2014, 6, 93–104.
133. O. M. Istrate, K. R. Paton, U. Khan, A. O’Neill, A. P. Bell and J. N. Coleman, Carbon, 2014, 78, 243–249.
134. I. M. Inuwa, A. Hassan, S. A. Samsudin, M. H. M. Kassim and M. Jawaid, Polym. Compos., 2014, 35, 2029–2035.
135. I. M. Inuwa, A. Hassan, D. Y. Wang, S. A. Samsudin, M. K. M. Haafiz, S. L. Wong and M. Jawaid, Polym. Degrad. Stab., 2014, 110, 137–148.
136. Z. D. Han, Y. L. Wang, W. Z. Dong and P. Wang, Mater. Lett., 2014, 128, 275–278.
137. V. Mittal, G. E. Luckachan and N. B. Matsko, Macromol. Chem. Phys., 2014, 215, 255–268.
138. V. Mittal, G. E. Luckachan and N. B. Matsko, Macromol. Symp., 2014, 340, 37–43.
139. X. X. Sheng, D. L. Xie, W. X. Cai, X. Y. Zhang, L. Zhong and H. P. Zhang, Ind. Eng. Chem. Res., 2015, 54, 649–658.
140. S. L. Deng, Z. D. Lin, B. F. Xu, W. P. Qiu, K. Y. Liang and W. Li, J. Therm. Anal. Calorim., 2014, 118, 197–203.
141. L. Nguyen, S. M. Choi, D. H. Kim, N. K. Kong, P. T. Jung and S. Y. Park, Macromol. Res., 2014, 22, 257–263.
142. S. Biswas, G. P. Kar, D. Arora and S. Bose, RSC Adv., 2015, 5, 24132–24138.
143. P. Steurer, R. Wissert, R. Thomann and R. Mulhaupt, Macromol. Rapid Commun., 2009, 30, 316–327.
144. H. R. Pant, B. W. Pant, P. Pokharel, H. J. Kim, L. D. Tijing, C. H. Park, D. S. Lee, H. Y. Kim and C. S. Kim, J. Membr. Sci., 2013, 429, 225–234.
145. H. R. Pant, C. H. Park, L. D. Tijing, A. Amarjargal, D. H. Lee and C. S. Kim, Colloids Surf., A, 2012, 407, 121–125.
146. O. Monticelli, S. Bocchini, A. Frache, E. S. Cozza, O. Cavalleri and L. Prati, J. Nanomater., 2012, 938962.
147. B. Y. Li, H. H. Yuan and Y. Z. Zhang, Compos. Sci. Technol., 2013, 89, 134–141.
148. P. Ding, S. S. Su, N. Song, S. F. Tang, Y. M. Liu and L. Y. Shi, RSC Adv., 2014, 4, 18782–18791.
149. M. Kim, S. H. Hwang, B. J. Kim, J. B. Baek, H. S. Shin, H. W. Park, Y. B. Park, I. J. Bae and S. Y. Lee, Composites, Part B, 2014, 66, 511–517.
150. C. G. Zang, X. D. Zhu and Q. J. Jiao, J. Appl. Polym. Sci., 2015, 132, 41968.
151. M. S. Sreekanth, A. S. Panwar, P. Potschke and A. R. Bhattacharyya, Phys. Chem. Chem. Phys., 2015, 17, 9410–9419.
152. M. H. Liu, M. Li, H. Y. Hou and R. Li, J. Therm. Anal. Calorim., 2015, 120, 1303–1310.
153. L. F. Zhou, H. H. Liu and X. X. Zhang, J. Appl. Polym. Sci., 2015, 132, 41987.
154. J. R. Potts, D. R. Dreyer, C. W. Bielawski and R. S. Ruoff, Polymer, 2011, 52, 5–25.
155. X. Yang, L. Li, S. Shang and X. Tao, Polymer, 2010, 51, 3431–3435.
156. S. Ansari, A. Kelarakis, Z. L. Esteve and E. P. Giannelis, Small, 2010, 6, 205–209.
157. H. J. Salavagione, G. Martinez and M. A. Gomez, J. Mater. Chem., 2009, 19, 5027–5032.
158. D. Y. Cai and M. Song, Nanotechnology, 2009, 20, 315708.
159. L. S. Schadler, L. C. Brinson and W. G. Sawyer, JOM, 2007, 59, 53–60.
160. T. Kashiwagi, F. Du, J. F. Douglas, K. I. Winey, R. H. Harris and J. R. Shields, Nat. Mater., 2005, 4, 928–933.
161. D. Y. Cai and M. Song, J. Mater. Chem., 2010, 20, 7906–7915.
162. R. Verdejo, F. J. Tapiador, L. Helfen, M. M. Bernal, N. Bitinis and M. A. Lopez-Manchado, Phys. Chem. Chem. Phys., 2009, 11, 10860–10866.
163. S. H. Xie, Y. Y. Liu and J. Y. Li, Appl. Phys. Lett., 2008, 92, 243121.
164. H. Zhong and J. R. Lukes, Phys. Rev. B: Condens. Matter Mater. Phys., 2006, 74, 125403.
165. J. H. Seol, I. Jo, A. L. Moore, L. Lindsay, Z. H. Aitken and M. T. Peter, Science, 2010, 328, 213–216.
166. K. S. Liu, S. Ronca, E. Andablo-Reyes, G. Forte and S. Rastogi, Macromolecules, 2015, 48, 131–139.
167. S. P. Rwei and I. T. Huang, Colloid Polym. Sci., 2015, 293, 841–850.
168. H. Park, K. H. Kim, J. Yoon, K. K. Kim, S. M. Park and J. S. Ha, Appl. Surf. Sci., 2015, 328, 235–240.
169. D. I. Son, H. Y. Yang, T. W. Kim and W. I. Park, Composites, Part B, 2015, 69, 154–158.
170. P. Mancinelli, D. Fabiani, A. Saccani, M. Toselli and M. F. Frechette, J. Appl. Polym. Sci., 2015, 132, 41923.
171. J. Alam, S. H. Ryu and A. M. Shanmugharaj, Sci. Adv. Mater., 2015, 7, 993–1001.
172. J. S. Bunch, S. S. Verbridge, J. S. Alden, A. M. Vander Zande, J. M. Parpia, H. G. Craighead and P. L. Mceuen, Nano Lett., 2008, 8, 2458–2462.
173. D. R. Paul and L. M. Robeson, Polymer, 2008, 49, 3187–3204.
174. P. A. Song, Y. M. Yu, T. Zhang, S. Y. Fu, Z. P. Fang and Q. Wu, Ind. Eng. Chem. Res., 2012, 51, 7255–7263.
175. K. Kalaitzidou, H. Fukushima and L. T. Drzal, Carbon, 2007, 45, 1446–1452.
176. H. Kim and C. W. Macosko, Polymer, 2009, 50, 3797–3809.
177. O. C. Compton, S. Kim, C. Pierre, J. M. Torkelson and S. B. T. Nguyen, Adv. Mater., 2010, 22, 4759–4763.
178. C. I. Idumah, A. Hassan and A. C. Affam, Rev. Chem. Eng., 2015, 31, 149–177.
179. J. Y. Xu, J. Liu, K. D. Li, L. Miao and S. Tanemura, Sci. Technol. Adv. Mater., 2015, 16, 025006.
180. N. N. Hong, L. Song, T. R. Hull, A. A. Stec, B. B. Wang, Y. Pan and Y. Hu, Mater. Chem. Phys., 2013, 142, 531–538.
181. S. L. Qiu, W. Z. Hu, B. Yu, B. H. Yuan, Y. L. Zhu, S. H. Jiang, B. B. Wang, L. Song and Y. Hu, Ind. Eng. Chem. Res., 2015, 54, 3309–3319.
182. K. Q. Zhou, J. J. Liu, Y. Q. Shi, S. H. Jiang, D. Wang, Y. Hu and Z. Gui, ACS Appl. Mater. Interface, 2015, 7, 6070–6081.
183. C. Lee, X. Wei, J. W. Kysar and J. Hone, Science, 2008, 321, 385–388.
184. R. Ansari, S. Ajori and S. Rouhi, Appl. Surf. Sci., 2015, 332, 640–647.
185. P. K. S. Mural, A. Banerjee, M. S. Rana, A. Shukla, B. Padmanabhan, S. Bhadra, G. Madras and S. Bose, J. Mater. Chem. A, 2014, 2, 17635–17648.
186. T. K. Gupta, B. P. Singh, R. K. Tripathi, S. R. Dhakate, V. N. Singh, O. S. Panwar and R. B. Mathur, RSC Adv., 2015, 5, 16921–16930.
187. P. Pokharel, S. Choi and D. S. Lee, Composites, Part A, 2015, 69, 168–177.
188. J. Chen, Y. D. Li, Y. Zhang and Y. G. Zhu, J. Appl. Polym. Sci., 2015, 132, 42000.
189. N. Thayumanavan, P. Tambe and G. Joshi, Cellul. Chem. Technol., 2015, 49, 69–80.
190. J. A. King, D. R. Klimek, I. Miskioglu and G. M. Odegard, J. Compos. Mater., 2015, 49, 659–668.
191. Z. Y. Wang, X. Shen, M. A. Garakani, X. Y. Lin, Y. Wu, X. Liu, X. Y. Sun and J. K. Kim, ACS Appl. Mater. Interface, 2015, 7, 5538–5549.
192. B. Ahmadi-Moghadam, M. Sharafimasooleh, S. Shadlou and F. Taheri, Mater. Des., 2015, 66, 142–149.

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