Dielectric investigation of the temperature dependence of the nonexponentiality of the dynamics of polymer melts

Abstract

Using broad band dielectric spectroscopy (10−5−109 Hz), combining time domain and frequency domain techniques, we study the temperature dependence of the non-Debye character of the α relaxation of polymer melts in the glass transition temperature Tg range. The α relaxation process is described in terms of the Kohlrausch-Williams-Watts relaxation function which has a single parameter β to characterize the nonexponentiality of the relaxation. At high temperatures, β remains nearly insensitive to temperature changes, whereas in the vicinity of Tg a nearly linear increasing of β with temperature is found. The temperature range where the change of the β(T) behavior occurs is located for all the polymers investigated around 1.2Tg. Moreover, our results indicate a common value of β≈1/3 at the temperature where the relaxation time diverges. The β(T) behavior near Tg is discussed in terms of a “rugged landscape” phase space which allows us to rationalize both the β(T) behavior observed as well as the similarities of our findings near Tg with the results reported in simulations on Ising spin glasses and other model systems.