Graphene-based polymer nanocomposites

Abstract

The main reason for the rapid development of polymer composite materials is that the traditional pure polymers have largely played out its performance capabilities whereas technology requires materials with new properties and advances. There are a number of advantages polymeric composites have over traditional materials (metals, ceramics, wood, pure polymers etc.): - a unique combination of properties, not typical for other individual materials (strength, strain, thermal, rheological, adhesive, electrical, friction, heat transfer, and others); - the ability to control composites properties by simply changing the composition and preparation conditions; Typically, composite materials are not champions with respect to separately considered properties. But with respect to the combination of certain properties they have no equal. Polymer nanocomposites based on carbon black and carbon nanotubes have been used for improving electrical, mechanical, thermal and gas barrier properties of polymers. The discovery of graphene with its unique combination of properties has created a new class of polymer nanocomposites. Graphene-based nanocomposites, with unique mechanical, gas barrier, electrical and dielectrical properties, could find use as engineering plastics and coatings, and could play a role as semi-conductive sheets in transistors. This thesis presents a study on the potential of graphene nanosheets as an alternative filler for multi-functional polymeric materials. The latex technology concept consisting of mixing of a preliminary prepared polymer latex and a dispersion of exfoliated graphene followed by freeze-drying and compression molding was applied for the preparation of graphene/polystyrene nanocomposites. Graphene obtained via the following routes was used for the nanocompsites preparation: 1) oxidation and exfoliation of graphite and subsequent reduction of graphene oxide with the aid of hydrazine hydrate in water in the presence of surfactant; 2) oxidation of graphite followed by thermal reduction of graphite oxide; 3) exfoliation of graphite in water by long term bath sonication in the presence of surfactant. Optimum time and temperature were found for obtaining of graphene via the first route. The final conductivities of the nanocomposites based on this type of graphene, obtained by both four point and local current measurement techniques, reveal interestingly high values up to 15 S/m, which can be achieved for low nanofiller loadings (1.6-2 wt%). A pronounced percolation threshold exhibiting a quite low value around 0.8-0.9 wt% was observed for the produced polystyrene/graphene nanocomposites. Nanocomposites via in-situ reduction of graphene oxide were prepared. The reduction of graphene oxide occurred during the compression molding step. This way of preparation allows us to eliminate the time consuming reduction step and gives a homogeneous distribution of graphene platelets in the composite film due to the ability of graphene oxide to be readily exfoliated and dispersed in water because of its hydrophilic nature. Due to incomplete reduction the ultimate conductivity of the composites is 0.1 S/m although the percolation threshold is low, 0.6 wt%. Graphene of types 2 and 3 was used for an experimental and theoretical study of the influence of the state of dispersion of graphene on the percolation threshold of conductive graphene/polystyrene nanocomposites. It was shown that graphene/polystyrene nanocomposites prepared from polystyrene latex and aqueous graphene dispersions with relatively low stability and relatively low degrees of exfoliation exhibit a lower percolation threshold than the composites based on dispersions with larger degree of graphene exfoliation and higher dispersion stability. Conductive polymeric surfactant, PEDOT:PSS, was utilized to disperse graphene in water. Both highly and low conductive PEDOT:PSS latexes were used in the study. It was shown that the conductivity of graphene itself in such a system has a minimal influence on the final conductivity of the nanocomposites which suggests that either graphene works as a scaffold for PEDOT:PSS rather than a filler contributing to the ultimate conductivity of the composites, or we deal with a random mixture of platelets (graphene) and spheres (PEDOT:PSS) where in the case of low-conductive PEDOT:PSS utilization the low conductive spheres limit the conductivity of the system. Nanocomposites based on carbon nanotubes/PEDOT:PSS dispersions and cellulose whiskers/PEDOT:PSS dispersions were prepared and both percolation thresholds and ultimate conductivities were compared. The study revealed that a conductive nanofiller, in this case carbon nanotubes, can be replaced with a non-conductive one with the similar aspect ratio, almost without losing the conductive properties of the nanocomposites. Most probably the percolating cellulose whiskers network organizes the PEDOT:PSS into a conductive network.