Structural heterogeneity in poly(methyl methacrylate) glasses of different tacticity studied by thermally stimulated current thermal sampling techniques

The thermally stimulated current thermal sampling (TSC-TS) technique was used to study the cooperative glass transition like relaxations in different tacticity poly(methyl methacrylates) (PMMAs) including iso- and syndiotactic forms. We use the high sensitivity of the TSC-TS method to study these weak and sometimes overlapping relaxations at a frequency of about 10(-3) Hz where one can resolve cooperative relaxations well below the main glass transition (T-g), even if the species are a minor fraction of the overall relaxing species. The magnitudes of the values of E-a are compared with a theoretical prediction as a function of temperature, showing that the low-temperature tail of the glass transition in conventional atactic (e.g., 45% syndiotriads) extends down to 15 degrees C, while in our highest syndiocontent PMMA (80% syndiotriads), the glass-transition-like relaxations only extend down to about 80 degrees C. Results taken from the literature also substantiate the results obtained here for atactic PMMA. The results are contrasted to those for bisphenol-A polycarbonate and other polymers and suggest that compositional heterogeneity, more specifically the presence of low T-g ''pockets'' of predominantly isotactic sequences, contributes to the broad glass transition extending almost 90 degrees C below the main glass transition in atactic PMMA.