Segmental dynamics and local motions in disordered random copolymers

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 (T_g), fragility (m), and segmental dynamics. The calorimetric T_g 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 T_g, the activation energy decreased with TBS, indicating a transition toward a side-group reorientation-dominated mechanism, while above the T_g, 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.