Free volume in nanodielectrics

It is now well established that the incorporation of nanoparticulates into a polymer matrix to form a nanodielectric can bring about useful improvements in electric strength provided that the processing results in acceptable particle dispersion. One of the theories of electrical breakdown in polymers seeks to associate breakdown with the polymer free volume, and the well-documented substantial changes in electric strength occurring at glass transition provide some credence to that theory. While it is clear that interactions at the large internal interfacial area in a nanodielectric facilitate the augmentation of dielectric strength, the exact mechanism has not been clarified. Very early work using a compression methodology to estimate free volume in an epoxy nanodielectric indicated that the particles did little to affect free volume. However, this contribution seeks to use positron annihilation lifetime spectroscopy to estimate free volume in an epoxy and several of its SiO₂-based nanocomposites to estimate the changes in free volume brought about both through the incorporation of nanoparticulates and also through their functionalization. This study broadly confirms the earlier work and provides good evidence that the free volume is little affected, and thus is not an explanation for the observed change in dielectric strength.