Physical nature of complex structural relaxation in polysiloxane - PMpTS: α and α′ relaxations

Structural relaxation processes in poly(methyl-para-tolyl-siloxane) (PMpTS) polymers of three molecular weights were studied using dynamic light scattering. Two relaxation processes: the usual α and an additional slow one α′ were observed and studied as function of temperature and molecular weight. Contrary to the structural relaxation, we find that in a plot T-Tg the relaxation times for the α′ process for all molecular weights do not collapse to a single curve. For one of the samples the light scattering correlation functions were compared with the corresponding functions obtained by means of mechanical relaxation, dielectric spectroscopy and computer simulations. The simulations show that the bimodal distribution, i.e. the α relaxation and the slow (α′) process are contained in the correlation functions of most of the probes (optical anisotropy, dipole moment, chain bond, density) in agreement with experimental observations.