[Thesis]. Manchester, UK: The University of Manchester; 2016.
Graphene nanoplatelets (GNP) with thickness of 6 ~ 8 nm and lateral dimension of 5
Î¼m (M5) and 25 Î¼m (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
Â°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 waterâ€™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.