Dynamically heterogeneous segmental dynamics in poly(vinyl butyral) under plasticization and antiplasticization: The mechanism behind cohesive energy and chain flexibility

Plasticizers and antiplasticizers are essential additives used to tailor the processability and microstructure of polymeric materials, while the complicated effects of chain flexibility and cohesive energy density (CED) on the heterogeneous dynamics remain poorly understood. Here, we compare the segmental dynamics of plasticized and antiplasticized poly(vinyl butyral) (PVB), which exhibits dynamic heterogeneity. It was found that the addition of either a plasticizer or an antiplasticizer leads to a deviation from the empirical inverse correlation between the fragility index m and the stretching exponent βKWW, indicating that changes in βKWW alone cannot reliably reflect variations in m. Although both "fast" and "slow" regions show remarkable reduction in glass transition temperature Tg and m, the "slow" regions with relatively restricted mobility possess a higher Tg while a lower m regardless of plasticization or antiplasticization because of their higher CED. Moreover, plasticized samples exhibit a more pronounced reduction in Tg and m for both "fast" and "slow" regions compared to antiplasticized ones, suggesting that chain flexibility acts as an additional factor reducing fragility. These findings provide strong experimental evidence for the roles of chain flexibility and CED in regulating segmental dynamics in either plasticized or antiplasticized PVB, thus providing new insights into the fabrication of strong polymeric glasses and high-performance materials.