Interface Strengthened Graphene Oxide Reinforced PVA Nanocomposites

Abstract

Graphene, an atomic layer of sp2 bonded carbon atoms in the hexagonal lattice, is the building block of many carbon forms, including carbon nanotubes, Buckminster Fullerenes, carbon onions, and graphite. Its exceptional properties, for example, Young's modulus of 1 TPa, breaking strength of 140 GPa, thermal conductivity of 5000 Wm-1K-1, and high specific theoretical surface area of 2650 m2g-1, have warranted use in many structural and functional applications. One of the most important uses of it lies as fillers in the fabrication of polymeric nanocomposites. Its special two-dimensional morphology featuring high available surface area with a nanometric thickness of the platelets can be exploited in load bearing, electrical, and barrier applications. However, the reinforcing agents and their types, considerably influence crystallinity, microstructure, and glass transition of the composites, which in turn affect materials properties. Therefore, underpinning the processing-microstructure-property relationship in these materials is of paramount importance.
The preferred route of graphene production is through the top-down approach, where graphene oxide (GO) is synthesized by chemical exfoliation method followed by suitable reduction of GO to graphene. GO possesses various oxygen-containing functional groups that make it easily dispersible in aprotic solvents. Subsequently, by various chemical treatments, some of those functional groups can be removed, and few others attached/created, and the filler-matrix interface can be engineered. The crux of the current work lies in the approaches to strengthen the matrix-reinforcement interface by various types of amines, resulting in unprecedented ultrastrong and ultra-tough PVA nanocomposites. Positron annihilation lifetime spectroscopy has been used to highlight the effect of interfaces. X-ray diffractometry and thermal analysis have been used to understand crystallinity in the samples. Raman and FTIR spectroscopy have been used to understand the disorder in carbon and the chemical functional groups, respectively. Microstructural analysis (using scanning and transmission electrons) of matrices and fractured surfaces have been performed to reveal the distribution of fillers, fracture process, formation of nematic crystals, and the in-situ formation of carbon nanoribbons for strengthening.
An order of magnitude increase has been found in Young's modulus and fracture strength of the composites. Such profound increase in strength can be ascribed to the nematic ordering of the functionalized GO flakes in the polymer matrix. In another variant, the formation of carbon nanoribbons from the wrinkled GO platelets and their interpenetrating distribution in the polymer matrix led to enhancement in strength and toughness.