Melting behavior of isotactic polystyrene

Abstract

Abstract The melting behavior of isotactic polystyrene, crystallized from the melt and from dilute solutions in trans ‐decalin, has been studied by differential scanning calorimetry and solubility measurements. The melting curves show 1, 2, or 3 melting endotherms. At large supercooling, crystallization from the melt produces a small melting endotherm just above the crystallization temperature T c . This peak originates from secondary crystallization of melt trapped within the spherulites. The next melting endotherm is related to the normal primary crystallization process. Its peak temperature increases linearly with T c , yielding an extrapolated value for the equilibrium melting temperature T c ° of 242 ± 1°C as found before. By self‐seeding, crystallization from the melt could be performed at much higher temperature to obtain melting temperatures as high as 243°C, giving rise to doubt about the value of T c ° found by extrapolation. For normal values of T c and heating rate, an extra endotherm appears on the melting curve. Its peak temperature is the same for both melt‐crystallized and solution‐crystallized samples, and independent of T c , but rises with decreasing heating rate. From the effects of heating rate and partial scanning on the ratio of peak areas and of previous heat treatment on dissolution temperature, it is concluded that this peak arises from the second one by continuous melting and recrystallization during the scan.