ON THE COPOLYMER AND CROSS-LINKING CONTRIBUTION TO THE GLASS-TRANSITION TEMPERATURE OF MODEL NETWORKS

The effect of molecular structure on the glass transition temperature of end-linked networks has been studied by examining a series of stoic hiometrically imbalanced poly(propylene glycol)-Desmodur RF (PPG-DRF) and poly(isoprene)tristar-hexamethylene diisocyanate (PIP-HDI) network s. A range of different molecular weights of each prepolymer was used. The PPG-DRF system shows a strong influence of the copolymer contribu tion on T(g). The modified copolymer approach (MCA) gives good theoret ical correspondence with the experiments. This approach successfully p redicts the T(g) behavior of other cross-linked systems with relativel y high cross-link densities. Other existing universal theories have be en shown not to describe the T(g) well for the PPG-DRF system. Correct ion of the MCA is possible to account for a small plasticizing effect observed for highly stoichiometrically imbalanced samples. The PIB-HDI system shows no strong contribution of the copolymer effect by virtue of its structure. The difference in cross-link density can explain th e changes in T(g). The MCA also yields reasonable results. For both sy stems it is necessary to consider all cross-linkers for the purpose of determining the cross-link density.