Tuning the Dynamic Fragility of Aromatic Acrylic Polymers: Effects of Associating Strength, Ether Linkages, and Alkyl Substituents.

Dynamic fragility is a critical parameter for understanding material functionality, processability, and the fundamental nature of the glass transition. However, achieving low fragility remains particularly challenging for aromatic polyacrylates due to their high chain rigidity and weak cohesive energy. In this study, we aimed to reduce the fragility of poly(benzyl acrylate) by tuning associative interactions, introducing ether linkages, and incorporating alkyl substituents. It was found that hindered phenols exhibiting weak intermolecular hydrogen bonding with the host polymer, along with high rigidity or significant steric hindrance, failed to achieve a substantial reduction in fragility. This result is attributed to the significantly enhanced rigidity, which increases fragility and thereby counterbalances the reduction in fragility induced by the moderately strengthened cohesive energy. In contrast, introducing either flexible ether linkages or bulky methyl substituents into the host polymer led to a significant reduction in fragility, even in the presence of hindered phenols with high rigidity. We propose that both modification strategies enhance the flexibility balance between side groups and the polymer backbone and thus lead to an increase in configurational entropy, as confirmed by the generalized entropy theory. These design principles offer a synergistic pathway to tame the fragility of aromatic polyacrylates.