Abstract
A substantial internal friction peak associated with the liquid-liquid transition (Tll) has been observed in polystyrene (PS) melts with different molecular weights Mw. The peak is of the relaxation type and suggested to be caused by the cooperative rearrangement of PS chains. The relaxation time follows the Vogel–Fulcher–Tamman equation. With increasing Mw, the PS melt exhibits a higher energy barrier, a smaller concentration of mobile species, and a stronger coupling between mobile species at Tll. In addition, to quantify the strength of the temperature dependence of the relaxation time, a parameter mll is defined for PS melt according to Angell’s fragility concept. The value of mll decreases with increasing Mw, indicating a slower cooperative rearrangement of PS chains toward Tll. Moreover, at Mw⩽52.5kg∕mol, mll rapidly drops with Mw, while it more slowly decreases at Mw>52.5kg∕mol. The fact suggests more topological constraints due to the intrachain interactions in very long chains.
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