Cooperativity of dynamics in supercooled polymeric materials and its temperature dependence predicted from a surface controlled model

Abstract

Relation between cooperativity and fragility of segmental dynamics is a crucial issue in elucidating the viscous slowdown of supercooled polymer liquids. The experimental results previously reported for p-substituted polystyrenes (PS derivatives) and poly(methacrylic acid ester)s (PMAEs) are reanalyzed based on a cooperative string model that was recently proposed on the assumption that the cooperatively rearranging region (CRR) takes a string shape. The analysis shows clear positive correlations between the cooperativity and fragility for both the polymer systems. To investigate the temperature dependence of the cooperativity, we proposed a simple model where the size of the CRR is controlled by its surface area (surface-controlled CRR model: SCC model). The analysis based on this model suggests that for many of the polymers, the CRR takes a non-compact shape with high surface-to-volume ratio. The configurational entropy of the smallest CRR is also evaluated from calorimetry based on the SCC model, and it is revealed to depend on the segmental relaxation time very weakly compared with that for typical non-polymeric glass-formers.