Tailoring specific properties of polymer-based composites by using graphene and its associated compounds

ABSTRACT Graphene and its associated compounds have been identified as extraordinary structural nano-fillers that can tailor the properties of new polymer-based composites with specific functionalities. Graphene possesses perfect 2D atomic architecture and has a large surface area that enhances the bonding, with tailored functional groups on its surface with polymer chains to form high strength composites. Recently, a lot of research has focused on developing composites with high specific strength, high electrical and thermal conductivities, high impact resistance, excellent energy storage capability and ability to maintain their strength at low-temperature environments by using graphene, graphene oxide (GO), reduced graphene oxide (rGO) or carbon nanotube/graphene. Ultimate goals are to design composites that can meet specific requirements for different engineering applications. In this paper, the discussion on all aspects in relation to the use of graphene and its associated compounds for advanced composites will be given in detail. The potentiality of using these materials for high-tech applications will be explored. Up to date, it has been proved that the use of graphene-based nanofillers does improve the mechanical, interfacial bonding, electrical, thermal and electromagnetic interference shield properties of polymer-based materials. Appropriately adding a small amount of these nanofillers do make a big difference to the properties of host materials. Graphical Abstract


Introduction
Since the early 90 s, the world's advanced composites research has got through a dramatic change. Most of the studies have shifted from studying the properties from micro-scale to nano-scale. Majority of works have focused on how to use nano-structural fillers to improve their mechanical, thermal, electrical, and delamination resistant properties. In the early stage, carbon nanotubes (CNTs) were recognized as the strongest filler, in which its theoretical tensile modulus is 1 TPa, to reinforce polymer-based composites. As it possesses perfect atomic architecture on its surface, the formation of chemical bonding between the CNTs and polymeric matrix becomes difficult. Extensive research has paid much attention on enhancing the interfacial bonding properties between the CNTs and polymeric matrix, and the interlaminar shear properties in a composite. Besides, CNTs possess excellent electrical and thermal conductivities that can be used to extend the applicability of composites in different areas. For example, enhancing the electrical conductivity of carbon fiber reinforced polymer composites (CFRPs) for airframes is important to avoid damages due to lightning strike. The large aspect ratio of CNTs also brings them to be an idea nano-structural material to reinforce polymerbased composites. However, due to their price was relatively high in the past decades as well as the difficulty of generating good bonding with polymers, other types of relatively low-cost nano-fillers, like nano-diamond, carbon black, and nanoclay were selected as research materials to alter the properties and behaviors of composites [1,3]. Similarly, the control of dispersion properties of these nano-fillers is still an issue to date. Many attempts by using sonication and mechanical stirring techniques with the control of viscosity of resin through adjusting the curing temperature were done. Chemically forming functional groups on the surface of these nano-fillers do help avoid agglomeration. However, expected mechanical properties and electrical conductivity of resultant composites are affected. Lately, the development of coiled CNTs (CCNTs), nanoclay-supported CNTs and CNT-coated carbon fiber ( Figure 1) demonstrated their excellent performance of improving the bonding and dispersion properties in composites [4,8]. The attractiveness of using these materials is still limited because of high production cost of nanoparticles with high purity and subsequent composite manufacturing processes. Solvent effect is another critical factor that may affect the full cross-linking process if the solvents are not fully evaporated away prior to adding hardener. Multi-mixing and curing process was suggested to form a uniformly dispersed composite [4]. In general, nanoparticles agglomerate and spread unevenly after the dispersion process due to the low viscosity of resin before curing. Chan et al. [5] introduced the dynamic curing process to control the viscosity of resin, which is high enough to keep the nanoparticles in place. However, it is only possible for production of a small portion and it is not available for large-sized products. Leaving the solvents in the composites would cause incomplete curing of matrix, and thus it greatly affects the expected mechanical properties of resultant composites as the incomplete curing of matrix causes i) poor interfacial bonding properties between the fiber and matrix; ii) poor mechanical properties of matrix and iii) poor stress transfer from the matrix to the fiber.
Recent research has shown that graphene-based fillers could potentially alter specific properties of composites to suit multifunctional purposes. For example, these nano-fillers can act as reinforcements in composites, as well as dielectric materials for developing an energy storable structure. Most of the works focused on using graphene-based fillers to produce batteries, super-capacitors, ultra-thin films, electronic circuits, electrodes and etc. Graphene can be prepared by employing either top-down and bottom-up approaches [9]. In fact, graphene, graphene oxide (GO) and reduced graphene oxide (rGO) are the most common types of graphene-based fillers for polymer-based composites. Graphene is a single carbon layer of graphite structure, describing its nature by analogy of a polycyclic aromatic hydrocarbon of quasi-infinite size. However, rGO is prepared from reduction of graphene oxide by thermal, chemical, or electrical treatments. Therefore, rGO always has some defects in it. Different reducing agents will lead to various carbon to oxygen ratio and chemical compositions in rGO ( Figure 2). GO and rGO could provide superior interaction between GO and polar polymer matrix, which inhibits the agglomeration and clustering effect.

Mechanical properties
As mentioned by many research articles, carbon-based nanostructures are basically divided into three types. They are 0D fullerene (Bucky ball), 1D nanotubes and 3D graphite as shown in Figure 3. A perfect CNT is sealed by two half spherical structures at both ends to form a sealed tubular structure. As CNTs have a perfect atomic architecture, polymer chains are difficult to form chemical bonding on CNTs' surface, which makes them difficult to be fabricated as polymer-based composite materials. Other two critical factors that resist the industry to use these materials are their high price and difficulty of producing uniformly dispersed nanocomposites. The properties of multi-walled carbon nanotubes (MWCNTs) are also difficult to be predicted. On the other hand, many promising studies by growing CNTs onto the surface of carbon fiber have shown that CNTs help enhance the interfacial bonding properties between the fiber and polymeric matrix. Grafting CNTs on carbon fiber could generate both chemical bonding and mechanical interlocking with the matrix. Yet, the price and processing time are still challenging.
Owing to the relatively low production cost of graphene and large surface area as compared with other nanostructures, graphene has become a very popular structural filler for polymer-based composites. Graphene nanoplatelets as described by Hung et al. [10], consist of multi-sheets of graphene, which is a single atomic layer of hexagonally arrayed sp 2 hybridized carbon atom arranged in a honeycomb structure. Graphene exhibits many outstanding mechanical, electrical, and thermal properties as compared with other nano-materials. Their thermal conductivity, electrical conductivity, and mechanical strength reach 500 W/mk, 6000 S/cm, and 130 GPa, respectively. Similar to CNTs, its theoretical tensile modulus is about 1 TPa. High surface area (2675 m 2 /g) of graphene makes it ideal to form large bonding points, and thus produces strong stress transfer with surrounding matrix.
Many works were done by mixing GO or silane-functionalized GO (sGO) into polymer matrix or directly attached on the surface of fiber or fabric to form high strength fiber reinforced polymer-based composites [11]. This method could entirely enhance the interfacial bonding properties of the fiber and matrix. The tensile strength, Young's modulus, flexural strength, and flexural modulus of resultant composites were enhanced by 15%, 14%, 25%, and 31%, respectively, with the use of 0.5 wt.% of GO in carbon fiber/ polyethersulfon (PES) composites [12]. A SEM image ( Figure 4) clearly demonstrates that PES is attached completely on the surface of a fiber, which shows a secure bonding between the fiber and matrix. If the bonding is poor, a complete fiber pull-out with a smooth surface on the fiber would be observed. Simply coated the GO and sGO on the surface of fiber also substantially enhance the bonding strength of a lap-joint structure [11] ( Figure 5). With the addition of GO or sGO, the strength of the adhesive is increased as expected. Lau [13] has mentioned that the increase of strength and decrease of thickness of adhesive would also induce high shear and peel off stresses at the bond ends of an adhesive layer. To avoid the localized stress concentration, the degree of uniform dispersion of nanoparticle in the adhesive layer is crucial.
Zhao et al. [14] have summarized different modified theoretical models including the rule of mixture, Halpin-Tsai model, Mori-Tanaka model and other micro-mechanics models to predict the properties of GO reinforced polymer-based composites. It is assumed that the properties of the composites are behaved linear-elastically subject to loading. However, many uncertain issues, such as dispersion properties, alignment of nanostructural fillers, and purity of nanoparticles are not considered in all proposed models. These parameters are difficult to be quantified and included in the models. However, they are the most convenient models to date to be used for approximately predicting the properties of nanoparticle reinforced polymer composites. Similarly, Wang et al. [15] have used the laminate theory to determine the properties of a graphene reinforced composite beam under free vibration condition. Due to the free end condition, plane stress state is   [11]. assumed in the model. Again, the extended Halpin-Tsai model is used to estimate the properties of nanocomposites. Other studies [16] intended to use graphene and CNTs together as multi-scaled nano-fillers to reinforce polymer-based structures. However, the dispersion is also a challenging issue. The individual contribution from graphene or CNTs toward the strength enhancement in the composites is unknown.

Low temperature composites
Recently, more research interest has focused on developing graphene reinforced polymer-based composites for aircraft and automotive engineering applications. In the aircraft engineering industry, the use of advanced composites for primary and secondary structural components has increased substantially since the past two decades. Using composites could greatly reduce the gross weight of airplanes, and thus the fuel consumption. It also helps reduce the production of greenhouse gases, which are the major sources to cause the global climate change. Glass fiber reinforced polymer (GFRP) and CFRP are two common types of composites for aircraft structures. The strength of GFRP is relatively lower than CFRP. Therefore, they are normally used for the secondary structures, such as leading edges and vertical stabilizer. Recently, CFRP has been used to replace aluminum for fuselage and wing structures for Boeing 787, Airbus A350, and A380. Using advanced composites for aircraft structures needs to fulfill requirements that may not be strictly necessary for domestic products used at the ground attitude. According to the International Standard Atmosphere (ISA), the ambient temperature, pressure, and air density at the flying attitude (11,000 km) are −56.5°C, 22KPa, and 0.365 kg/m 3 respectively. At ground level, they are 15°C, 1013 KPa, and 1.225 kg/m 3 respectively. Therefore, the design of aircraft structures must be able to withstand the temperature and pressure variations during in flight condition. Material fatigue is another issue that impacts the structural integrity of the structures. It is relatively difficult for composite structures to have structural repair as compared with other metallic structures, like aluminum alloys. Bonding of composite patches on a damaged area requires special techniques and a perfect environmental control to ensure appropriate pressure and temperature applied during the curing process.
Many works have focused on structural integrity of composite structures subject to low temperature environments. In general, brittleness and low fracture toughness may be resulted for the structures subject to low velocity impact at low temperatures. Ma et al. [17,18] have studied the structural response of GFRP and coiled nanotube (CCNT)/GFRP at different temperature ranges. GFRP was tested subject to low velocity impact at room temperature and low temperature condition (~ −77°C) to see their mechanical response inside an MTS drop weight impact testing machine. Due to the shrinkage of polymer at low temperature, it generates an additional clamping force onto the surface of glass fiber. This result shows the enhanced interfacial bonding strength between the matrix and fiber in the composites. Ultimately, the damage area of GFRP structure subject to low velocity impact was smaller as compared with a similar sample at room temperature (i.e. 15°C) as shown in Figure 6. CCNTs were mixed with epoxy to form an adhesive to bond two GFRP strips. CCNTs were dispersed into epoxy resin under sonication for 5 minutes. Hardener was then added followed by further sonicating under the viscosity reach a level that can hold the CCNTs in place without sinking to the bottom of a breaker. Afterward, the mixture of uniformly dispersed CCNT/epoxy was applied to the bonding surface. The lap join shear strength was then measured after 24 hours to allow the mixture to be cured fully. The bonding properties at the lap joint were enhanced as CCNTs could enhance the strength of adhesive and introduced extra friction between the adhesive and GFRP strips ( Figure 7). The result truly reflects that the properties of advanced composites are affected by low temperature. Finite element modeling (FEM) was also studied to investigate the physical condition of the graphene at low temperature environment. In the FEM simulation, it is found that the negative radical movement of CCNTs was found, which reflects  that the contractive action of matrix did exist. Increase in the tensile modulus of CCNT/ epoxy was observed at low temperature.
Hung et al. [19] have coated GO onto the surface of carbon fabrics by using electrophoretic deposition method. It aims to reduce the time and cost for growing GO on the surface of each carbon fiber as suggested by other researchers [10]. Although the interfacial bonding properties between the fiber and matrix could be enhanced, it is too costly and time consuming for the industry to coat nanoparticles on the surface of each fiber. Hung et al. demonstrated that by coating GO onto the surface of fabrics, GO could attach evenly on the fibers which are located on the top layer of fabrics. Due to the large surface area of GO, it enhances the bonding strength between the matrix and fiber, and thus the interfacial bonding properties in the composites were excellent, which was reflected from the flexural strength test at both room and low temperatures (100 K). The optimal percentages of GO, in terms of achieving the highest flexural modulus and strength of GO for this study, were 0.5 wt.% and 0.25 wt.% at room temperature and low temperature, respectively. With the existence of GO, it could help bridge up micro-cracks that are formed at low temperature. As mentioned previously, brittleness and shrinkage of matrix may affect the mechanical properties of a polymer-based composite at low temperature. GO, as its excellent bonding characteristics with polymer, does help enhance the localized tensile properties in the composites to avoid crack propagation ( Figure 8). It delays the damage of the composites subject to thermal-mechanical loading, which could adversely accelerate the damage of composite laminates [20,21]. Similarly, by mixing GO solely into the matrix instead of coating onto the fabrics also gave the same result [22]. GO could  [20]. enhance the friction between the carbon fiber and matrix, and thus the fiber pull-out strength.
Shen et al. [23,24] used graphene nanosheets, as nanofillers to reinforce epoxy composites at cryogenic environment owing to their large contact surface area. It was found that increasing the content of nanosheets would increase the mechanical properties, in terms of tensile modulus and strength of the composites. However, further increasing the nanosheet content beyond 0.1 wt.% caused the decrease of its strength. This might be due to the agglomeration of nanosheets. Besides, the alignment and orientation of nanosheets cannot be controlled. In general, nanosheets' longitudinal direction gives a better mechanical performance.

Electromagnetic interference shielding properties
Electromagnetic radiation seriously affects the operation and functions of many electronic devices, such as sensors installed in aircraft structures, train structures, and automobiles to promote real-time structural health monitoring. In this regard, the development of high strength and lightweight materials which possess electromagnetic interference (EMI) shielding capability is essential. At this stage, most of the traditional EMI shielding materials are metals/alloys [25,26]. Yang et al. [25] used copper nanowire with annealed graphene aerogel to form a framework for EMI shielding. The shielding effect was improved by increasing the nanowire content. Liang et al. [26] used silver nanowire reinforced cellulose film to form a shielding layer. The electrical conductivity of the film increases with increasing the silver content. However, the temperature also increased with time when the current is applied. The role of the silver nanowires was to form 3D thermal and conductive networks. These nanowires also enhanced the mechanical properties of cellulose film due to strong hydrogen bonding between the nanowire and cellulose matrix. However, their specific strength to ratio is low and easy to be corroded. Recently, carbon-based nanofiller reinforced polymer materials, owning to their moderate electrical conductivity with high specific strength have attracted much attention to the industry.
Wang et al. [27] adopted carbon nanotube, as a conductive filler to mix with Polyimide foam to for a composite to isolate EMI transmission. The nanotubes formed a conductive path in the foam network. Dielectric losses would be produced because of induced currents, leading to a drop in the energy electromagnetic waves. However, the article has not specified the type of nanotubes were used as it would make a big difference in terms of their electrical conductivity via the amount of the nanotubes used. Huangfu et al. [28] mixed multiwalled carbon nanotube (MWNTs) and graphene aerogel into Polyaniline (PANI), an excellent conductive polymer to form a composite. Functionalized MWNTs were firstly mixed with graphene to form aerogel and then PANI/MWCNT/Thermally annealed graphene aerogel (TAGA)/epoxy nanocomposite. Increasing the content of MWNTs resulted in enhancing the electrical conductivity of samples. EMI shielding mechanism is same as other metals/alloys with the induced currents flow through. However, the porosity inside the aerogel also caused the internal wave reflection to minimize the EMI interference. Due to the use of graphene, their mechanical properties, such as reduced modulus and hardness increase with increasing the MWNT content. Similar to reduced graphene-based honeycomb structure [29], same mechanism and EMI shielding characteristics were found. Glass transition temperature (Tg) of the nanocomposites was changed with increasing the MXene content. With adding 3.3 wt.% of MXene, Tg was gone up by 10%.
Sun et al. [30,31] used graphene as a conductive filler to study the negative permittivity behavior of hydrosoluble Polyvinyl alcohol (PVA) and Polydimethylsiloxane (PDMS). As graphene sheet is an excellent conductive material, modified graphene sheets were mixed with PVA by sonication to form a uniformly dispersed solution. It was then cast to form a thin film for electrical property tests. Increasing the graphene content resulted in enhancing the electrical conductivity of a modified graphene/PVA composite film. From their experiment, it was found that the negative permittivity was observed when the graphene content increased to 20 wt.%. For the graphene/PDMS metacomposites, similar phenomenon was observed. Increasing the weight percentage of graphene sheets resulted in enhancing the electrical conductivity of the metacomposites. When the graphene content reached 3 wt.%, negative permittivity was seen.

Other applications
Huang et al. [32] have studied the use of silane grafted GO papers to improve the flameresistant properties of composite structures. It was found that the fire retardant properties were substantially improved by increasing the content of 3-methacryloxypropyltrimethoxysilane (MPMS). The comparison of combustion processes of GO and MPMS-GO papers is shown in Figure 9. It may be an excellent idea to explore further by laminating a layer of silane grafted GO film for polymer-based composites to improve their fire retardant properties as well as developing supercapacitors for unmanned vehicles. Sivashankari and Prabaharan [33] have developed agarose/chitosan/GO composites by using the freeze-drying method for tissue engineering. These composites, depending on the content of GO, could produce different sizes and amounts of porous for cell Figure 9. Combustion processes of (top) GO paper and (bottom) MPMS-GO paper. It shows that the improved flame resistance after silane functionalization is resulted [25]. proliferation and growth. They proved that these composites are biocompatible and ideal materials for bone and osteochondral tissue engineering. Figure 10 shows the composite scaffolds and their cell attachments on the surface of porous, which are pointed by arrows. By adding GO into biocompatible materials, it may help substantially increase the surface area to allow the cells to be attached and then grown. However, the use of GO, inside the human body is still a challenging topic in the nano and bioscience research world.
Du et al. [34] developed graphene-based polyurethane (PU) composites with multifunctional properties. GO was wrapped by nitrogen-, phosphorus-and silicon-containing units into PU. Under the light responses, it induces the shape memory and self-healing properties of the composites. It was formed due to the effect of photo-thermal conversion of graphene by the exposure of visible near-infrared light (NIR). Since graphene is a threedimensional layered structural material, it possesses excellent tribo-physical properties. Upadhyay and Kumar [35] have studied the tribological behavior of graphene/epoxy composites. It was found that the hardness of the epoxy increased, and the coefficient of friction decreased with increasing the graphene content in the composites. The wearing mechanism is clearly demonstrated in the article. During the wearing process, the graphene reacted with epoxide groups and formed functional graphene, which could reduce the wear rate and friction coefficient. As epoxy resin consists of amino group, which helps in functionalization process by restricting the graphene stacking. This phenomenon is same as a polymer coating mixed with nano carbon-particles. This coating layer does provide a self-lubrication mechanism. With the existence of graphene/polymer layer, it helps attain a transition from a soft coating layer to tribofilm formation to reduce the friction coefficient ( Figure 11).
In the past years, many researchers have started focusing on using GO to develop high electrically conductive materials and structural dielectric capacitors [36,39]. Chan et al. studied the use of a GO paper as dielectric material (Figure 12), inserted in CFRP to design a structural component with an ability of storing energy, which could be harvested from solar panels attached on the surface of unmanned air vehicles (UAVs) and the results were promising. The specific capacitance and energy density were better than other dielectric films, such as polyamide, polyethylene terephthalate, and polycarbonate. Besides, due to the inserted film in the middle of CFRP, the mechanical and structural properties may be altered. Their studies also show that there was no any adverse effect toward their tensile and interlaminar shear properties. Chan et al. [37] also attained to add nanogold particles into GO to form a nano-gold particle reinforced GO structural dielectric film (AnNPmodified GO/SDCs). Similarly, the energy density measured from AnNP-modified GO/ SDCs is much higher than other films due to the existence of nanogold particles, implying the enhancement in the electrical conductivity in the film. Increasing the nanogold particle content to 1 wt.% resulted in forming gold clusters which adversely affected the performance, such as specific capacitance, energy density, and power density of energy storage.
Kshetri et al. [39] developed a new hierarchical nanostructure by integrating graphene, CNTs, and carbon fibers to form a carbon aerogel structure for supercapacitor devices. The beauty of the structure is to allow the CNTs to grow uniformly on the graphene-wrapped carbon fiber, making the overall electrical conductivity increased ( Figure 13) due to the substantial increase of total surface area. By looking at the energy density and power density of these hybrid devices, the performance was very remarkable as compared with other devices. The hybrid device could retain 91.7% of its capacitance after consecutive 10,000 GCD cycles.
As aforementioned, increasing the surface area of conductive nano-fillers does help enhance the energy storage capability. Hou et al. [40] developed a new composite using amine-functionalized graphene and polyaniline for high-performance supercapacitors. It was found that the morphology control of the composite does deliver a better energy storage capability. The mesoporous structure is beneficial for improving the electromechanical cycling stability of the composite. Ke ta al [41]. studied the dielectric properties of CNT/graphene nanoplatelets at different temperature conditions. CNTs, graphene nanoplatelets, and CNT/graphene nanoplatelet were mixed with thermoplastic polyurethane (TPU) to for composites. In their study, high dielectric constant was observed on a sample with CNTs only at different temperature ranges (25-150°C). A sample with CNT/graphene Figure 11. Wear mechanism of pure epoxy and epoxy-graphene composites [28]. nanoplatelet exhibited better dielectric performance than pure TPU and graphene nanoplatelet/TPU.

Conclusion
Graphene and its associated compounds are ideal nanofillers for polymer-based composites for different specific applications. They can be tailored to serve the purposes of enhancing the strength, sensing, and energy storage capabilities of the composites at different temperature ranges. GO almost dominates graphene-based compounds in all recent research works. By appropriately adding GO into polymeric matrix could substantially enhance the mechanical strength of host polymer materials. Their electrical properties are also improved to make the composites more conductive and, further extend their applications to structural capacitors. GO-coated carbon fiber and carbon fabric exhibited strong interfacial bonding properties in composites. Low cost and high effective GOcoated carbon fabric polymer composites demonstrate an excellent alternative to develop a new generation of GO/CF polymer composites. In reality, there are few issues that should be considered in detail to fabricate GO/polymer composites in real life: • Degree of uniformity of graphene and its associated components dispersed in polymer composites; • Purity of nanoparticle received from different sources; • Interfacial bonding properties between the nanoparticle and surrounding matrix; • Completely removing all solvents in the solution for dispersing nanoparticle in the nanoparticle/resin solution; In the future, the structural behavior, reliability, and energy storage capability of graphene reinforced polymer composites at different temperature conditions should be focused to ensure they are used in safe in different engineering applications.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This project is supported by the research grant from Swinburne University of Technology.