Molecular Weight Dependence of Fragility in Polystyrene

Segmental relaxation was measured for a series of polysytrenes (PS) of varying molecular weight. The normalized Arrhenius plots of segmental relaxation times (fragility curves) become less steep for shorter chain lengths. Since the shape of the relaxation function is invariant to molecular weight, such behavior is a deviation from the usual correlation between time and temperature dependencies. Although the dependence of relaxation times on T g-normalized temperature is weaker (less fragile), lower molecular weight PS has a larger heat capacity change at T g. This result is at odds with an interpretation of temperature dependencies in terms of the degrees of freedom available to the glass upon transition to the viscous liquid.