How ​can Graphene be used in polymeric applications

Graphene is known to be an allotrope of carbon and is always arranged and put together in the form of a lattice. These are the most used and most stable form of carbon and that is why are known throughout the world.The work mechanics that they perform are known throughout the world and make it more usable. The properties and characteristics that are exhibited by graphene polymers are highly remarkable which increases their production demand and as a result, more and more applications are applicable for this product.

Most of these applications can be seen in the field of medicine, engineering, and chemicals. A wide range of such applications is explained in this article and enlist all the ways in which graphene polymers are being used and promoted. 

Introduction

Carbon has many allotropes, for instance, carbon nanotubes, fullerene, graphite, and diamond. They are all a part of the carbon family. One of the carbon family's allotropes is known as graphene, which is a planar monolayer of sp2 hybridized carbon atoms organized in a 2-D (two-dimensional) lattice. Graphene has been observed to be the building block for all other dimensional graphic materials. For instance, just by wrapping a graphene sheet's section, fullerenes (0-dimensional carbon allotrope, buckyballs) can be made.

Graphene nanoribbon should be rolled in order to make the carbon nanotubes (1-dimensional carbon allotropes, CNTs). Stack the graphene sheets on top of one another to make the graphite (3-dimensional carbon allotrope) and each of the sheets is separated by 3.37 A ̊.

Academic material

Despite being called a 3-dimensional material's integral part, strictly 2-dimensional graphene crystals are an academic material. In 2004, Novoselov used micromechanical cleavage to isolate the free-standing single-layer graphene when graphene was separated from graphite; and then in 2005, Novoselov and Zhang used ermions for the same purpose. In 2010, graphene’s revolutionary discovery has awarded Andre Geim and Konstantin Novoselov the Nobel Prize in Physics for groundbreaking experiments.

Graphene sheet and derivatives

Graphene sheet and their derivatives have attracted a huge amount of interest during the past decade in most of the engineering and scientific areas because of their remarkable structure and extraordinary chemical and physical characteristics. In graphene, there are many qualities like high specific surface area (2600 m2g-1), good optical transparency (97.7%), remarkable thermal conductivity (3000-3000 W m-1K-1), high carrier mobility under optimum condition (2,50,000 cm2V-1s-1), and quantum hall effect.

Graphene possesses superlative electronic, thermal, and mechanical characteristics and a combination of them. Numerous synthetic routes are made for making graphene and its derivatives for exploiting these remarkable characteristics in practical applications, varying from the bottom-up epitaxial growth to graphite’s top-down exfoliation through liquid exfoliation, intercalation, and/or oxidation.

Use of organic and inorganic materials

Numerous inorganic and organic materials have been used to make graphene-based composites successfully, and they, therefore, are extensively utilized in applications like batteries because usually, the polymer materials possess remarkable strength, specific modulus, and broad applications in defense, automobile, and aerospace industries, etc.

Silicate fillers

In 1950, Carter developed polymer nanocomposites with exfoliated layered silicate fillers, and a report came after approximately forty years displaying major mechanical characteristic improvement utilizing clay in nylon-6 matric as a filler, therefore attracting significant industrial and academic interest in the nanocomposites. As there have been developments in nanotechnology and nano-science, many nanofillers like carbon nanotubes, nano-silica, and carbon black have been broadly utilized and studied for enhancing polymer’s electrical, thermal, mechanical, and gas barrier characteristics.

Surface to volume ratio

Due to the inner nanotube surface's inaccessibility to the polymer molecules, graphene has a higher surface-to-volume ratio as compared to carbon nanotubes. Graphene is more favorable in enhancing the characteristics of the polymer matrices. Over some last years, there has been the incorporation of graphene and graphene’s derivatives into a broad variety of polymers, including polymethylmethacrylate (PMMA), nylon, polyaniline (PANI), polyethylene terephthalate, polypropylene (PP), polystyrene (PS), and epoxy for numerous functional applications.

Although there are some challenges and major issues in the fabrication of the advanced graphene/polymer nanocomposites. There are many factors that can affect the applications, functions, and characteristics of graphene/polymer composites. Those factors include graphene's network structures in the matrix, interfacial interaction between the matrix and the graphene, graphene's exfoliation and dispersion in the polymer, intrinsic characteristics of graphene and its derivatives, graphene's type, and its derivatives type.

Properties

130 GPa and 1.0 TPa is defect-free graphene’s fracture strength and in-plane elastic modulus. RGO sheers’ measure elastic modulus is still more than or equal to 0.25 TPa despite some structural distortion through tip-induced deformation experiments. Young modulus’s chart is shown as density’s function to compare the characteristics of graphene with other traditional materials, demonstrating that the strongest and stiffest material to be ever known in nature is the defect-free graphene.

Graphene is the primary load-bearing component of polymer composites because of graphene’s remarkable intrinsic characteristics or RGO sheets (in comparison with most of the polymeric materials), coupled with their large surface areas. Thus, a huge amount of interest has been gained by graphene-filled-polymer composites, making it an extremely researched direction in composite materials now. Graphene will be incorporated into the polymers to considerably improving the mechanical characteristics. There are many benefits of graphene’s presence in the mechanical reinforcement as compared to the presence of the existing carbon fillers like SWNT (single-walled carbon nanotubes), expanded graphite (EG), and carbon black (CB).

Aqueous solution mixing method

The aqueous solution mixing method was used by Zhao et al. for developing a completely exfoliated RGO/PVA composite. The Young’s modulus is increased by almost 10 times at RGO’s 1.8 vol% and there is an increase of 150% in the tensile strength as compared to the pure PVA polymer.

The mechanical and thermal characteristics of PMMA-based composite containing EG fillers, single-walled nanotubes, and functionalized graphene sheets (FGS) were investigated and compared by Ramanathan et al. According to the results provided by Ramanathan et al., wrinkles being present can result in the roughness of the surface of nanoscale which likely forms an improved mechanical interlocking and adhesion with the polymer chains. There were investigations on the PS composite’s creep and recovery with carbon nano additives various geometrical morphologies for instance CRGO sheets, MWCNT (multi-wall carbon nanotubes), and CB.

The CRGO sheets displayed better efficiencies at a fixed loading of fillers in lessening the creep and unrecovered response as compared to the multi-walled carbon nanotube and CB fillers.

Electrical conductivity

There have been general employments of the conductive fillers for insulating polymer matrices for realizing the electrical conductivity. Percolation theory can explain this, for instance, conductive pathways are formed by conductive fillers, i.e. percolation thresholds. This simple power-law expression σc=σf[(Ф-Фc)/(1-Фc))]t, can model the increase in conductivity as a filler loading function once you have achieved the electrical percolation. In this equation, the universal critical exponent is denoted by t, the percolation threshold (onset of the transition) is denoted by Фc, the filler volume fraction is denoted by Ф, and the filler’s conductivity is presented by σf.

Conductive fillers

Graphene is a promising conductive filler for enhancing the electrical characteristics of numerous polymers because of graphene’s high electrical conductivity and large aspect ratio. RGP/PS composites’ electrical conductivity was investigated by Stankovich et al. as a function of filler volume fraction. According to the attained results, a typical percolation behavior was displayed by the RGO/PS composites, and introducing RGO to PS can enhance the conductivity to the magnitude of more than 10 orders. Folding, wrinkling, and crumpling morphologies were shown by the composites having only 1.0 vol% RGO loading and they have 0.1 S/m electrical conductivity value, which refers to the fact that one can use low-loading of graphene to construct highly conductive graphene/polymer composites.

Thermal conductivity

Graphene is not famous for its exceptional electrical and mechanical characteristics only. Its thermal conductivity makes it famous too. High intrinsic thermal conductivity of more than 3000 W/mK was exhibited by graphene and its derivatives when suspended. In 2-dimensional crystals like graphene, phonons' physics is significantly different from 3-dimensional graphite. Lower interfacial thermal resistance is provided by the 2-dimensional geometry of graphene sheets, therefore forming highly-enhanced conductivity for the polymer composites, and imparting considerable anisotropy to polymer composite’s thermal conductivity because of the measured in-plane thermal conductivity which is 10 times more from the cross-plane conductivity.

Increase in electrical conductivity

There is nothing dramatic about the improvements in the thermal conductivity that are made by the graphene fillers unlike the exponential increase in the electrical conductivity, however, in comparison with 1-dimensional carbon nanotube, thermal conductivity can be more effectively improved by 2-dimensional graphene. The extremely small contrast in the polymer and graphene’s thermal conductivities as compared to not such a small contrast in the polymer and graphene’s electrical conductivities can explain this.

Applications

Graphene is a promising filler for enhancing the thermal, electrical, mechanical, and other significant characteristics of polymers according to the properties and features of graphene/polymer composites that are shared above. Despite many challenges in the development of graphene and its derivative’s fundamental understanding, there have already been much researches on their potential of having broad applications in various fields including functional materials and structural reinforcement. There have been so many studies on the usage of electronic memory devices, reinforcement, energy storage, photoelectric conversion, biomedicine, and photoelectric applications.

Structural reinforcement materials

At low graphene loading, one can obtain a considerable enhancement on polymer matrices’ mechanical performance, suggesting usage of such materials in the applications of transport that demands the combination of lightweight and high strength. Particularly, the utilization of graphene and graphene's derivatives gave the possibility of improving the mechanical characteristics of the traditional fiber-reinforced polymer composite systems further. Only 0.2% RGO additives were seen to improve the glass fiber/epoxy composite’s fatigue life in flexural bending according to Yavari et al. The fatigue life improves, and spray-coating the RGO directly at the fiber-matrix interface produced considerable benefit over RGO’s uniform dispersion in the bulk epoxy resin.

Situ ultrasound analysis

During the cycling fatigue test, composite’s in situ ultrasound analysis suggested that the fiberglass/epoxy-matrix interface is toughened by the RGO network, preventing glass microfibers from buckling/delamination under compressive stress. Moreover, carbon fibers surface were modified by using graphene oxide and that provided enhanced interfacial characteristics in the carbon fiber/polymer composites.

When GO in low loading is introduced on the surface of the fiber, then there comes a considerate improvement in the tensile characteristics, interlaminar shear strength (ILSS), and interfacial shear strength (IFSS) of the composites. Also, combined usage of graphene nanoplatelet and carbon fiber formed dramatic thermal and mechanical increments of thermoplastic composites. There is a potential seen from these high-performance hierarchical materials in enhancing the cost-effectiveness, reliability, and safety of the fiber-reinforced polymer composites that are turning out to be the material of choice in wind energy, biomedicine, sports, marine, automotive, and aerospace industries.

Sensors

Damage and strain sensing are two main facets of the sensing responses of graphene/polymer composite materials. Graphene/polymer composites’ overall conductivity is strongly determined by the tunneling between the local conductive networks, therefore a major change will be formed in the conductivity by a change in the local tunneling distance. Thus, a major change in the conductivity can be a result of the outer-stimuli-induced changes in the tunneling distance, and that can be then monitored and further utilized for the purposes of sensing. Strain sensors can be fabricated by incorporating graphene and graphene’s derivatives including RGO, GO, and GNP sheets into an insulating polymer matrix.

Resistance variation

Electrically conductive composite's conducting network changes when there is an application of external pressure, leading to resistance variation. This effect is known as the piezoresistive effect. Applications have been proposed in a broad range for piezoresistive materials, including movement sensors, wearable electronics, health monitoring, and smart textiles. Composites with 0.0136 vol% GNP loading and piezoresistive GNP/silicone rubber composites were made by Chen and co-workers, which is near the percolation threshold and under extremely low pressure, displays a sharp positive-pressure coefficient effect of the resistivity in the finger-pressure range.

Flexible conductor

When highly bent or stretched, high electrical conductivity can be maintained by the flexible conductor, therefore creating huge opportunities and has been interacting huge amount of interest recently. Just like the name, flexible conductors need a combination of high electrical conductivity and good mechanical flexibility which is a potential for having applications in field emission devices, Li-ion batteries, supercapacitors, stretchable solar cells, dielectric elastomer actuators, flexible electronics, wearable displays, and robot arm joints. They are being utilized for the preparation of flexible conductors because of their remarkable electrical and mechanical characteristics. Two major strategies are used generally; (i) usage of the novel formation methods. (ii) Conductive graphene filler’s compounding into a polymer matrix.

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Rubbery polymer matrix

Compounding conducting graphene into a rubbery polymer matrix is the first method to attain a flexible conductor (PDMS) for producing elastic composites. Highly elastic conductors with 100% higher strain can be produced by the graphene/polymer composites with a low filler loading, still, its conductivity is very little to be utilized as a flexible electronic device. Devices of a high level of conductivity can be produced by the composites with a high filler loading but those devices will have poor flexibility because of the agglomeration or segregation of the fillers. One can backfill a preformed graphene/MWCNT aerogel with PDMS to obtain an effective method to produce a stretchable and highly conductive composite. With the loading of only 1.3 wt% graphene/MWCNT, the electrical conductivity reached 2.8 S/cm, and the electrical conductivity stayed constant after repeatedly stretching 100 times by 20 % and bending 5000 times.

In the 2nd strategy, conductive graphene-based fillers are integrated onto a flexible substrate utilizing numerous technologies like room temperature rubbing, etching, and transfer processes, and rod coating to develop a flexible conductor.

Graphene films

For example, a simple way was developed by Kim et al. in 2009 for the growth and transportation of stretchable graphene films of high quality on a large scale on nickel scales through the usage of CVD which can then be easily moved by simple contact methods to stretchable substrates (PDMS). Flexible RGO-based film was successfully fabricated by the usage of the rod-coating technique but they were fabricated with good transparency and low resistance made directly on the PET substrates.

RGO/PET film is used to produce a fully functional 4.5-inch four-wire resistance touch screen and it displayed linearity which can be compared with that of the ITO-based touch screens but it also displayed high mechanical flexibility which is more than the mechanical flexibility of the ITO-based touch screens, which refers to an approach that is potentially appropriate for usable RGO film’s roll-to-roll production for numerous flexible electronic devices.

Flexible lithium-ion batteries

Since flexible electronics have been invented, a huge amount of attention is gained by the flexible lithium-ion batteries as a power source as right now is the time of emergence for many wearables and flexible electronic devices like implantable medical devices, wearable sensors, conformable active radio-frequency identification tags, touch screens, and roll-up displays. There have been investigations on the graphene/polymer composite materials for Li-ion batteries for the improvement of cyclic stability and discharge/charge rate capability because of graphene’s porous networks, high electrical conductivities, and electroactive characteristics of both conductive polymers and graphene.

Electronic conductivity is significantly enhanced by the highly-dispersed graphene in the polymer composite and it also enables the efficient utilization of polymer cathode's electrochemical activity, allowing the delivery of ultrafast discharging and charging (in few seconds, 100 mAh/g delivered). Electromagnetic interference shielding materials and electrostatic discharge shielding materials are the other applications of conductive graphene/polymer composites, and they provide potential usages from frequency shielding coatings for electronics and aircraft, telecommunication antenna, electronics packaging to the parts of mobile phone, and carpeting floor mats.

Biomedical applications

Biomedical applications have a focus on another application for graphene-based composites recently because of their biocompatibility, their remarkable characteristics, and their easy functioning. Functionalized graphene sheet’s range of potential applications starts from delivering the drug and multimodal imaging for exploiting graphene’s electrical characteristics towards the production of biosensing devices.

For example, RGO-filled glucose oxidase (GOx) biocomposite film was made by Chen and co-workers and they showed extremely high selectivity, reproducibility, and extremely good stability. Remarkable catalytic activity was especially displayed by the developed biosensor towards glucose, therefore opening up new options in the formation of biofuel cells and cheap (cost-effective) biosensors. There also have been investigations on other biodegradable and biocompatible polymer composites and RGO sheets and graphene oxide’s incorporation into polylactide, polyethylene glycol, and chitosan.

Composite chitosan films

Solution mixing is used to prepare graphene and chitosan’s composite films. According to the results of cell adhesion, L929 cells adhered to the composite films and developed on the pure chitosan films, demonstrating that good biocompatibility is possessed by the graphene/chitosan composites. The facile solution casting method was used to make RGO/PVA composite films with a nacre-like bricks-and-mortar microstructure and prepare a hybrid building block of PVA-coated graphene oxide sheets and post-reduction treatment followed it. HUVECs (human umbilical vein endothelial cells) were used to study composite film’s biocompatibility.

Cell growth

The growth of the cell can be supported by the PVA/RGO films as they are appropriate for it. On RGO/PVA films, the number of HUVECs’ cells was nearly similar to the number of cells on the TCPS (tissue culture polystyrene) plates when this attempt was made. There was no cytotoxicity and as compared to the HUVECs seeding efficiency on common TCPS plates, it was higher on the PVA/RGO films. When the excellent electrical conductivity combines with the excellent mechanical characteristics, their biocompatible characteristic turns these films into candidates for applications in biotechnology, for instance, electroactive substrates/scaffolds for biosensors, cell culture, drug delivery, and tissue engineering.

Conclusion

Graphene being the allotrope of carbon is used in various forms and one of those is in the form of polymer. The properties that graphene polymers exhibit are highly authentic and make them reliable for the applications that they then perform. All the researches that have been carried out in this regard prove the authenticity of these polymers which moves them a step ahead towards better working outcomes and benefits. 

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References

https://www.degruyter.com/document/doi/10.1515/9783110574432-007/html

https://www.tandfonline.com/doi/abs/10.1080/14328917.2018.1456636?journalCode=ymri20

https://www.azonano.com/article.aspx?ArticleID=5660

https://www.sciencedirect.com/science/article/pii/S0079670014000367

https://nanoscalereslett.springeropen.com/articles/10.1186/s11671-017-2150-5