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

Using brood band dielectric spectroscopy (10(-5)-10(9) Hz), combining time domain and frequency domain techniques, we study the temperature dependence of the non-Debye character of the alpha relaxation of polymer melts in the glass transition temperature T-g range. The alpha relaxation process is de scribed in terms of the Kohlrausch-Williams-Watts relaxation function which has a single parameter beta to characterize the nonexponentiality of the r elaxation, At high temperatures, beta remains nearly insensitive to tempera ture changes, whereas in the vicinity of T-g a nearly linear increasing of beta with temperature is found, The temperature range where the change of t he beta(T) behavior occurs is located for all the polymers investigated aro und 1.2T(g). Moreover, our results indicate a common value of beta approxim ate to 1/3 at the temperature where the relaxation time diverges. The beta( T) behavior near T-g is discussed in terms of a "rugged landscape" phase sp ace which allows us to rationalize both the beta(T) behavior observed as we ll as the similarities of our findings near T-g with the results reported i n simulations on Ising spin glasses and other model systems.