Galpaya (2015) reported that graphene consists of a single layer of sp2 hybridized carbon atoms forming a two-dimensional (2D) hexagonal lattice. A unique combination of excellent electrical, thermal and mechanical properties has made graphene a material for multifunctional reinforcement for polymers. Graphene incorporated-polymer
nanocomposites show greater mechanical, thermal, and electrical properties compared to neat polymer which can find applications in the field of automotive, aerospace, electronics and green energy. The overall performance of filler reinforced polymer composites depends not only on the properties of the individual components but also on the interactions between the filler and the polymer matrix at the contact surfaces.
The physical, mechanical and even chemical composition of this interphase region is different from the bulk polymer matrix. Experimental as well as theoretical approaches can be employed to evaluate the micromechanical properties and the chemistry of interphase. Studies have been conducted to evaluate the interphase properties of fibre-polymer, CNT-polymer composites. However, few studies have investigated graphene-polymer interphase. Due to lack of understanding of the chemical nature and material behavior at the interface, toughening and strengthening mechanisms in graphene-polymer nanocomposites are not well understood. Therefore, the main theme of this thesis was to evaluate the microstructure and the properties of graphene-polymer interphase, thereby achieving a fundamental understanding of the reinforcing behavior of graphene in polymer nanocomposites.
In order to achieve optimal enhancement in the property of graphene-polymer nanocomposites, the homogeneous dispersion of graphene into the matrix, and the strong interfacial interactions between the graphene and the matrix are required. This can be achieved by use of graphene oxide (GO) as graphene nanofiller due to its oxygen containing functional groups which can make it more compatible and uniformly dispersed in polymer matrices due to stronger interactions with the polymer matrix compared to pristine graphene . In the paper 1 presented in chapter 3, the effects of GO on the mechanical and thermal properties of epoxy resin were investigated. Epoxy resin was chosen because the polymer matrix has high stiffness, dimensional stability and chemical resistance which make it the most important thermosetting resin in industry as structural and functional composites for applications in automobile, aerospace and electronics.
It has been demonstrated that the GOepoxy nanocomposites fabricated in this work have shown excellent mechanical properties. At loading of ~ 0.1 wt% GO was found to increase the mode I fracture toughness (K1c) of the neat epoxy by nearly 50%. The Young’s modulus of 0.1 wt% GO-epoxy was ~ 28% larger than neat epoxy and the maximum enhancement of ~36% was achieved at 0.5 wt% GO loading. There was slight improvement in ultimate tensile strength with GO incorporation and 0.1 wt% GO-epoxy showed ~ 7% increase compared to the control ie. neat epoxy matrix. Differential scanning calorimetry results showed that GO addition moderately affects the glass transition temperature (Tg) of epoxy. The maximum decrease of Tg by ~7oC was shown for the nanocomposite with 0.5 wt% GO. Fractography analysis of the tested samples revealed that the toughening mechanism of GO nanocomposite is crack deflection.
As it was found that GO can effectively improve the fracture toughness of epoxy matrix, in paper 2 presented in chapter 4, the chemistry and reaction mechanisms at the GO-epoxy interphase were evaluated in order to understand the toughening improvement by GO from a chemistry perspective. However, it was recently found that the micromechanical environment of GO appears to be much more complex than the oversimplification commonly used to describe the enhanced fracture toughness of epoxy nanocomposites due to the presence of highly oxidized low molecular weight organic fragments called oxidative debris (OD). OD greatly impacts the GO-resin interfacial chemistry. Therefore, in this kinetic study, a new approach is discovered to evaluate the influence of as-produced GO (aGO) and OD free, autoclaved GO (acGO) on cure and fracture toughness of epoxy by Fourier Transform Infrared Emission Spectroscopy (FT-IES) and Differential Scanning Calorimetry (DSC).
Results demonstrate that the presence of OD on the GO greatly influences the cure kinetics, mainly lowering the crosslinking capacity of aGO-resin interphase. Moreover, OD itself catalyses the epoxide ring opening reaction and promotes homopolymerization which would also produce lower crosslinking density. The difference in cross-link densities was reflected in DSC data that showed aGO nanocomposite exhibited lower Tg and acGO nanocomposite exhibited higher Tg compared to neat resin. Therefore, our findings demonstrate that these chemical changes in the interphase compared to bulk are consistent with the result that low addition of aGO (commonly used as GO in polymer nanocomposites) can effectively improve the toughness of epoxy matrix.Further, in this thesis, new approaches to making capacitors and gas sensors based on GO were attempted which promise a simple and easy avenue for developing such devices at a larger scale. Paper 3 in chapter 5 reports a new approach to harness these properties in fabricating supercapacitors.
Thin film capacitors were fabricated by using graphene and GO as charge storage and dielectric spacer, respectively. Results show that fabricated devices exhibited high
capacitance due to a high dielectric constant and electrical resistivity. It also suggested that graphene could be a good electrode material for supercapacitors. It has been shown that graphene can be used as sensing material to detect various gas molecules. The graphene-based gas sensors detect conductivity change upon adsorption of gas molecules which could arise from the changes of charge carrier concentration in the graphene induced by adsorbed gas molecules. Sensitivity and selectivity of graphene could be engineered by introducing dopants or defects.
The cited Paper 4 (in chapter 6) reveals that doping of GO with Cs2CO3 (GO-Cs) can significantly enhanced the sensitivity of graphene for NO2 gas at room temperature. The GO-Cs sensor exhibited a significant response to NO2, down to concentrations as low as ~91 ppb, while GO sensor did not show any response to concentrations below 3 ppm. The GO-Cs sensor exhibits a good repeatability compared to GO sensors. Both sensors show a reduction of resistivity upon exposure to the gas which could be attributed to a negative charge transferred to NO2 molecules resulting in p-type behavior. The sensitivity enhancement in GO-Cs could be attributed to defects introduced into the GO-Cs films during the doping process. The findings presented in this thesis have provided not only fundamental understanding of chemical and mechanical behavior at the interphase of GO-resin nanocomposites, but also applications of GO in other potential fields.
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