Influence of molecular weight on fast dynamics and fragility of polymers

The molecular weight dependence of the fast dynamics, sound velocity, and fragility has been studied for polystyrene (PS) and polyisobutylene (PIB). These different physical properties vary with molecular weight (M-n) and level off near the same chain length where T-g becomes independent of M-n. The latter differs significantly between PS and PIB. The results suggest that neither Kuhn segment length nor molecular weight between entanglements is an important parameter for the molecular weight dependence of these properties. The most intriguing result is that the molecular weight dependencies of the fast dynamics and sound velocity observed in PS are opposite to the one observed in PIB, although T-g increases with M-n in both cases. Moreover, there are some indications that even variations of fragility with molecular weight in PIB might be opposite to the variations observed in PS. We speculate that difference in symmetry of the monomer structures is responsible for the opposite behavior. On the basis of this idea, predictions for dependence of the fast dynamics and fragility in various polymers on molecular weight and tacticity are formulated.