MOLECULAR PACKING AND FREE-VOLUME IN CROSS-LINKED EPOXY NETWORKS

A diglycidyl ether of bisphenol-A type difunctional epoxy resin was cured with different amounts of a tetrafunctional curing agent, namely metaphenylene diamine. The macrodensities of the cured samples were determined at room temperature and their volume expansion was measured from room temperature to 180-degrees-C using a dilatometer. By combining these data, specific volume-temperature plots were constructed. The occupied volume was determined either by extrapolating the specific volume to 0 K or by estimating the van der Waals volume for the network. The empty volume, packing coefficient and free volume fraction could then be calculated. It was observed that samples with a high degree of crosslinking showed good packing around 180-degrees-C but at the glass transition temperature (T(g)) and in the glassy state, the packing was poor. It is postulated that after post-curing at 175-degrees-C, as the samples are allowed to cool, the constraints imposed by the crosslink affect the rate at which the samples contract; in samples with high crosslink density, the rate of contraction is subsequently low. Due to this, and also because of their high T(g), a larger free volume is trapped in these samples when the microbrownian motion freezes at T(g). With further cooling below T(g) the intersegmental separation is relatively higher in the highly crosslinked samples because the crosslinks do not provide a suitable environment for close packing. The likely effects of molecular packing on physical properties are briefly considered.