Thermally- and solvent-induced crystallization kinetics of syndiotactic polystyrene viewed from time-resolved measurements of infrared spectra at the various temperatures (1) estimation of glass transition temperature shifted by solvent absorption

Abstract

syndiotactic Polystyrene (sPS) glass crystallizes into the α form when it is heated above the glass transition temperature ( T g, about 100 °C). sPS can be crystallized also into the δ form in the solvent atmosphere at room temperature. In order to trace the structural evolution process, the time-resolved infrared spectral measurements have been performed in the isothermal crystallization from the glass to α form and in the solvent-induced crystallization from the glass to δ form at the various temperatures. Absorbance of crystallization-sensitive infrared bands was plotted against time, from which the crystallization kinetics were analyzed on the basis of Avrami equation: X( t)=1−exp[−( kt) n ] where X is a normalized crystallinity, n is an index, k is a rate constant, and t is a time. The isothermal crystallization was investigated also by carrying out the temperature jump experiment of DSC thermograms, giving almost the same results as the infrared spectral measurements. The Avrami index n was 2–5 depending on the crystallization temperature ( T c). The k was also dependent on the T c, about 10 −1–10 −4 s −1 and could be fitted reasonably by the equation of crystallization kinetics. An extrapolation of the k vs T c plot to the negligibly small k value allowed us to predict the temperature at which no crystallization should occur, ca. 100 °C, in good agreement with the observed T g value. On the other hand, the solvent-induced crystallization was investigated for the first time at the various temperatures from 50 to 9 °C by the time-resolved measurement of infrared spectra. Compared with the experiment at room temperature, the crystallization was highly accelerated at 40–50 °C, while the crystallization rate was reduced remarkably at such a low temperature as 9 °C. The time dependence of infrared absorbance was analyzed for the crystallization-sensitive bands on the basis of Avrami equation as the first approximation, although the crystallization mechanism was more complicated than the isothermal crystallization case. The logarithm of the k value was found to change almost linearly with temperature and an extrapolation to infinitesimally small k value gave a T g of about −15 °C. That is to say, the glass transition temperature was estimated to shift remarkably from 100 to −15 °C by absorbing solvent molecules or by a plasticizing effect.