Electrically Conductive Elastomeric Composites with Functionalized Graphene Sheets

Abstract

Research on electrically conductive elastomeric composites (CECs) must strive to probe the limits of both new materials and new processing conditions for production of improved CECs. This thesis aims to do both, considering the properties possible in CECs containing graphene sheets, and examining how the characteristics of those sheets and their distribution in the matrix relate to the electrical and mechanical properties of the composites. In the work described in this thesis, I prepared elastomeric composites with poly(dimethylsiloxane) (PDMS) and thermoplastic polyurethane (TPU), utilizing three types of graphene-based carbonaceous fillers: (i) functionalized graphene sheets (FGSs) from thermal exfoliation of graphite oxide, (ii) unreduced sheets of functionalized graphene with carbon-to-oxygen ratio 2 (FGS2) from direct exfoliation of graphite oxide, and (iii) graphene-carbon hybrid aerogels, broken apart (BA). The first key result is to show the existence of filler effects on cross-linking in PDMS composites with carbonaceous fillers and to subsequently demonstrate control over the cross-linking in the PDMS system by control of the composition of the curing elastomer mixture. FGS-PDMS and FGS-TPU composites are then considered for their electrical and mechanical properties. It is shown that FGS-TPU can be produced with stretchability of over 300%, with initial conductivity over 1 S/m and a decline in conductivity of about one order of magnitude over 200-300% strain, while FGS-PDMS composites showed a higher threshold for percolation (lower conductivity) and a larger decline in conductivity with strain. In both types of composites the elongation at break declined sharply as filler loading approached 4 wt.%. This decline appears similar in FGS2-TPU composites when filler loading is considered in terms of volume fraction or number density of sheets, despite the large difference in dispersion and known difference in surface chemistry for the FGS2 system. This limitation on the stretchability of the composites persisted in the final system considered, BA-TPU, appearing to decline at similar filler loadings measured in terms of number density of sheets in the composite. BA-TPU does, however, demonstrate improved combinations of conductivity and elongation (greater than 100% strain with conductivity of 100 S/m) compared to FGS-TPU as a result of the unique structure of the BA particles, which are aggregates of joined sheets. These aggregates disperse better and may have reduced contact resistance compared to FGSs.