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Multi-functional epoxy/graphene nanoplatelet composites

Cao, Gaoxiang

[Thesis]. Manchester, UK: The University of Manchester; 2016.

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Abstract

Graphene nanoplatelets (GNP) with thickness of 6 ~ 8 nm and lateral dimension of 5 μm (M5) and 25 μm (M25) have been used to prepare epoxy composites. Epoxy composites were fabricated initially by shear mixing to investigate the effects of filler type on the structure and properties of composites. The complex viscosity of GNP-epoxy mixture was found to increase by almost three orders of magnitude going from the neat epoxy to the 8 wt.% loading, leading to difficulties in their processing. Scanning electron microscopy of the composites showed that both fillers aggregated at high loadings with the M25 buckling more easily due to its larger diameter, which compromises its aspect ratio advantage over M5, resulting in only slightly better mechanical performance. Polarized Raman spectroscopy revealed that both M5 and M25 were randomly distributed in the epoxy matrix, After adding M5 and M25 fillers, the storage modulus increase with the filler loadings, however, the glass transition temperature (Tg) drops slightly after initial incorporation, then rises with further filler addition attributed to the pin effects of filler aggregations. In terms of electrical property, M25 has lower percolation (1 wt.%) than M5 composites due to its bigger aspect ratio, which enable M25 to form a conductive network more efficiently. Furthermore, M25 composites also have slightly better thermal and mechanical properties over that of M5 composites. However, the difference is not significant considering the aspect ratio of M25 is five times of that of M5. The reason is that the aggregation and buckling of M25 compromise its advantage over M5. Due to the better performance of M25 as filler, M25/epoxy composites were prepared by shear mixing, solvent compounding and three-roll mill. Samples made by solvent compounding display the lowest percolation threshold (0.5 wt.%), related to its relatively uniform dispersion of M25 in matrix, resulting in higher thermal conductivity and better mechanical properties. Water uptake in a water bath at 50 °C took 75 days to be saturated. Higher loaded samples have lower diffusion coefficient because of the barrier effects of GNP fillers, but have higher maximum water absorbed, which is owing to filler aggregation. Properties test of aged and unaged specimens show thermal conductivity of the aged was enhanced due to water’s higher thermal conductivity than epoxy resin matrix, while electrical performance was compromised due to the swelling effects caused by absorbed water. The mechanical properties of aged samples also dropped slightly due to plasticization effects.