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 α retardation process in amorphous poly(methyl methacrylate) (PMMA). The α 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 Δ H and entropies Δ S of the α mechanical relaxation. Δ 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 α retardation process extends to 30 °C independent of tacticity. This result is attributed to the presence of isotriad sequences enhancing molecular mobility in this temperature range.