Dependence of Tg and T ll on tacticity of PMMA by differential scanning calorimetry

A differential scanning calorimeter (DSC) investigation (heating rate 10 K/min) is presented on the multiple transition (relaxation) spectra of PMMA: T_β < T_g; T_g, T_ll > T_g; and T_lρ > T_ll, as a function of tacticity. Specimens are characterized by fractional triad content: isotactic (it-), Xii; syndiotactic (st-), Xss; and atactic (at-), Xis. Values for the seven specimens are it-, 1.0;, at-, 0.495 to 0.750; st-, 0.958. Results on T_β were inconclusive. Our T_g results clarify some discrepancies in the prior literature.

Linear least squares regression analyses give: T_g (°C) = 56.6 + 76.6 Xss (our data) T_g (°C) = 49.1 + 87.3 Xss (our data plus selected literature data)

Extrapolated T_g 's for Xss = 1 are 133.2°C and 136.1°C, respectively, in contrast to Thompson's extrapolated value of 160°C. Similarly T_g(°C) = 99.5 + 71.6 (1?Xii) for our DSC data. The extrapolated T_ll for Xss = 1 is 171.1°C. The intensity of T_ll is high for st-and it-, with a broad minimum over the at-region. A second liquid state process, T_lρ > T_ll, occurs at 149°C for it-, but is above the measured range for at-and st-. T_ll (at-) from DSC compares favorably with reported literature values by a variety of techniques. T_ll and T_lρ at all tacticities agree well with those from a companion study by the thermally stimulated current (TSC) technique on the same group of specimens, as reported elsewhere. This includes the influence of tacticity on intensity. These cross-comparisons by a variety of methods indicate that neither T_ll nor T_lρ is an artifact of our DSC technique. The Frenkel segment-segment contact hypothesis is favored to explain the molecular origin of T_ll Sequences longer than triads may be needed for improved correlations of T_g and T_lρ with tacticity.