Thermally stimulated creep recovery (TSCR) experiments have been used to study the influence of tacticity on the R retardation process in amorphous poly(methyl methacrylate) (PMMA). The R retardation process associated with the glass transition is strongly dependent on tacticity. The highsensitivity TSCR-fractional loading (FL) method was used to extract the distributions of activation enthalpies ∆H and entropies ∆S of the R mechanical relaxation. ∆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 R 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.
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