Dependence of Tg and T_llon tacticity of PMMA by differential scanning calorimetry

Abstract

Abstract A differential scanning calorimeter (DSC) investigation (heating rate 10 K/min) is presented on the multiple transition (relaxation) spectra of PMMA: Tβ < Tg; Tg, Tll > Tg; and Tlρ > Tll, 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 Tβ were inconclusive. Our Tg results clarify some discrepancies in the prior literature. Linear least squares regression analyses give: Tg (°C) = 56.6 + 76.6 Xss (our data) Tg (°C) = 49.1 + 87.3 Xss (our data plus selected literature data) Extrapolated Tg 's for Xss = 1 are 133.2°C and 136.1°C, respectively, in contrast to Thompson's extrapolated value of 160°C. Similarly Tg(°C) = 99.5 + 71.6 (1−Xii) for our DSC data. The extrapolated Tll for Xss = 1 is 171.1°C. The intensity of Tll is high for st-and it-, with a broad minimum over the at-region. A second liquid state process, Tlρ > Tll, occurs at 149°C for it-, but is above the measured range for at-and st-. Tll (at-) from DSC compares favorably with reported literature values by a variety of techniques. Tll and Tlρ 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 Tll nor Tlρ is an artifact of our DSC technique. The Frenkel segment-segment contact hypothesis is favored to explain the molecular origin of Tll Sequences longer than triads may be needed for improved correlations of Tg and Tlρ with tacticity.