Understanding the glass transition in amorphous polymers and the underlying principles that govern segmental motions remains a key challenge in polymer physics. Here, we investigated random copolymers composed of methyl methacrylate (MMA) and 4-tert-butylstyrene (TBS) monomers across various compositions to elucidate the influence of monomer bulkiness on the glass transition temperature (Tg), fragility (m), and segmental dynamics. The calorimetric Tg values were observed to strongly deviate from the Fox equation, showcasing a structure-independent behavior at high to medium MMA concentrations, and a ‘super-Fox’ increase at low MMA content. We correlated this to tacticity as well as frustrated chain packing, as corroborated by changes in the m values. A closer look at the β-relaxation revealed a strong dependence on the molecular composition: below the Tg, the activation energy decreased with TBS, indicating a transition toward a side-group reorientation-dominated mechanism, while above the Tg, the TBS monomers participate in the process, despite PTBS lacking a β-relaxation. The coexistence of TBS and MMA monomers revealed a fundamental shift in relaxation dynamics manifested by the decoupling of α- and β- relaxations, which we analyzed via the double-percolation mechanism. Our findings offer new insights into polymer relaxation behavior and the relationship between the α- and β- relaxation mechanisms with implications in materials optimization.
Drakopoulos et al. (2026) studied this question.