Cooperative relaxations in amorphous polymers studied by thermally stimulated current depolarization

Thermally stimulated current depolarization (t.s.c.) was used to study the relaxations in amorphous polymers including poly (ethyl methacrylate) (PEMA), poly (methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and polyarylate (PAR) over temperature ranges covering the β and (glass transition) regions. A.c. dielectric was used to obtain activation energies (E_a) for PS and PC to verify the accuracy of those values determined by the t.s.c. thermal sampling method. At temperatures below the glass transition (T_g) the values of E_a were found to agree with those predicted using an activated states equation with a zero activation entropy. This is evidence of the localized, non-cooperative nature of the low temperature secondary β relaxations which are found to be characterized by a continuous variation of activation energies as a function of temperature. The measured values of E_a depart from the zero activation entropy curve and exhibit a prominent maximum at T_g. This behaviour is known to be due to an enhanced degree of cooperativity of segmental relaxations near T_g. The results indicate that the main advantage of the thermal sampling method is the high sensitivity and high temperature resolution for cooperative relaxations. For the polymers studied here, only PEMA and PMMA show a substantial population of cooperative relaxations more than 60°C below T_g. This is tentatively explained in terms of structural heterogeneity due to variable tacticity in the methacrylates. Compensation of the t.s.c. relaxation spectra plotted in Arrhenius or Eyring plots was found for all polymers to differing degrees. Some discussion of compensation is made in terms of independently measured values of the coefficient of thermal expansion.