Effects of addition of nano-sized and micro-sized fillers into epoxy resin on its mechanical and dielectric relaxation phenomena were examined. The glass transition temperature (T g) decreases when a small content of nanoclay modified by octadecylamine was added, while the decrease in T g is suppressed when the nanoclay was modified by dimethyldodecylamine. On the other hand, T g increases when microsilica was added abundantly. At temperatures above T g, both mechanical and dielectric relaxations are accelerated in samples with octadecylamine-modified nanoclay, while the acceleration does not occur in samples with nanoclay modified by dimethyldodecylamine. Both relaxations are restricted in composites with abundant microsilica. Therefore, co-addition of dimethyldodecylamine-modified nanoclay and abundant microsilica is adequate in order to make an epoxy resin composite with a high T g and low dielectric loss.
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