Influence of tacticity on the α retardation mode in amorphous poly(methyl methacrylate)

Thermally stimulated creep recovery (TSCR) experiments have been used to study the influence of tacticity on the alpha retardation process in amorphous poly(methyl methacrylate) (PMMA). The alpha retardation process associated with the glass transition is strongly dependent on tacticity. The high-sensitivity TSCR-fractional loading (FL) method was used to extract the distributions of activation enthalpies Delta H and entropies Delta S of the alpha mechanical relaxation. Delta H increases with temperature and is maximum at the glass transition temperature T-g. The maximum value and the full width at half-height of the activation enthalpy distribution are tacticity-dependent. Comparing experimental activation enthalpies with Starkweather's zero entropy prediction shows that the low-temperature tail of the alpha retardation process extends to 30 degrees C independent of tacticity. This result is attributed to the presence of isotriad sequences enhancing molecular mobility in this temperature range.