Effect of Structural Stiffness on Fragility of Polymers: A Qualitative Picture

Glass transition in polymeric liquids is accompanied by strongly non-Arrhenius temperature variations of segmental relaxation time . The deviations of from Arrhenius behavior is described by the fragility parameter, where is the glass transition temperature. It appears that most polymers are very fragile compared to small molecular weight liquids, the reasons for which remain unclear. We have studied the segmental dynamics of various polymers using dielectric spectroscopy and dynamic mechanical analysis. Our analysis reveals that the relative stiffness of side groups relative to the backbone is an important parameter that controls fragility. This approach suggests that the high fragility of seemingly flexible-backbone polymers such as polydimethysiloxane may be a result of the high relative stiffness of the side groups compared to the backbone - a characteristic that distinguishes it from similar structures with comparably stiff side groups and backbone with extremely low fragility, such as polyisobutylene. We have also studied the fast dynamics of several polymers using Raman and Brillouin spectroscopy in order to understand how these fast motions correlate with the much slower segmental dynamics.