AN EXPLANATION OF ANOMALOUS DIELECTRIC-RELAXATION PROPERTIES OF POLY(PROPYLENE GLYCOL)

Dielectric measurements on low molecular weight poly(propylene glycol) over a wide frequency window of about 10 decades show that the relaxation times of the segmental relaxation and the normal modes have different temperature dependences. The segmental relaxation time, tau(s)*, has a much stronger temperature dependence than that of the normal-mode relaxation time, tau-2*. As temperature is decreased, the shorter tau(s)* increases much faster than tau-2*, with a tendency of tau(s)* to encroach tau-2* resulting in a reduction of the dielectric strength of the normal mode. These dielectric results are analogous to that shown in viscoelastic data obtained in low molecular weight polystyrene by Plazek and explained previously by the coupling model. The dielectric data are explained quantitatively here also by the coupling model. Moreover, the molecular weight dependence of the normal-mode relaxation time at constant temperature is found experimentally to be significantly stronger than that expected from Rouse dynamics. It is a consequence of couplings between chains caused by hydrogen bonding of the OH end groups in poly(propylene glycol) and is explained here in the framework of the coupling model.