Fine structure and thermorheological complexity of the softening dispersion in styrene-based copolymers

The segmental and terminal relaxation processes of polystyrene, styrene-acr ylonitrile, and cr-methylstyrene-acrylonitrile copolymers have been investi gated by means of both dynamic-mechanical and dielectric spectroscopy in th e linear response region. The temperature dependence of the average relaxat ion time tau of the two processes follows a Vogel-Tamman-Fulcher (VTF) equa tion: tau proportional to exp[B/(T- T infinity)]. Nevertheless, the segment al and terminal relaxations exhibit appreciably different VTF parameters. T his vitiates time-temperature superpositioning in the segmental relaxation temperature region, giving rise to complex thermorheological behavior. As f irst shown by Plazek et al., this finding further confirms the Donth and Ng ai models. Peculiar relationships between the VTF parameters of the segment al and terminal relaxation of the same polymer and of the same relaxation p rocess of different polymers are pointed out. These relationships reveal ge neral features of the VTF equation. A comparison between dynamic-mechanical and dielectric segmental relaxation times (tau (mech) and tau (diel)) high lights a profound difference in the time scales explored by the two techniq ues. More precisely, segmental motions contributing to the dielectric relax ation are faster than those observed mechanically. The relative magnitude o f tau (mech) and tau (diel) was discussed using the DiMarzio-Bishop model. In addition, the ratio tau (mech)/ tau (diel) is found to be temperature-in dependent. This suggests a sealing law for the decay function phi (t) of th e segmental relaxation leading to the same temperature shift factors for di fferent material properties.