Influence of thermal history on the melting behavior of isotactic polystyrene

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Abstract
Multiple melting transitions of thermally crystallized isotactic polystyrene were studied. This behavior was attributed to a very rapid reorganization during the melting of the crystalline structure originally formed. From differential scanning calorimetry and optical microscopy it was shown that the rate of reorganization decreases with increasing reorganization temperature TR. This reorganization will only occur if its rate is larger or equal to the scanning rate. A TR value can be defined for each scanning rate and the resulting melting temperature TRM increases when the scanning rate decreases. This increase of TRM is also examined by annealing at different temperatures. A linear relationship between TRM and TR is observed and gives an extrapolated equilibrium melting point of 240°C; moreover the slope is smaller than for isothermal crystallization (TM versus TC). These phenomena are confirmed by optical microscopic observations.

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