The influence of network topology on time-crosslink density reduction

Abstract

The time-crosslink density reduction principle asserts that changes in crosslink density of polymer networks are reflected as multiplicative time-scale shifts of the distribution of retardation times in the terminal zone of viscoelastic response. Dramatically differing sensitivities of these shifts to crosslink density changes have been reported. We have proposed that differences in network topology associated with crosslink density changes may be responsible for such behavior. To test this hypothesis we have carried out creep experiments on polyurethane samples where the crosslink density was varied through photodimerization of anthracene molecules which were incorporated into the networks so that specific different topological changes would result. We have found that the coupling of dangling chain ends causes relatively slight shifts in the time scale of the distribution function while shifts associated with crosslink formation are much more substantial. From these results we have concluded that network topological changes accompanying crosslink density variations have strong influences on the quantitative aspects of time-crosslink density reduction.