Dielectric and Mechanical Relaxations in Polymer Brushes: The Role of Architecture and Pressure

Understanding molecular dynamics in complex polymers is crucial for advanced materials, yet the influence of topology and side-chain chemistry on relaxations remains poorly understood. In this presentation, we explore the dielectric and mechanical responses of poly(mercaptopropyl)methylsiloxane (PMMS)-based polymer brushes. By grafting the backbone with flexible acrylate and rigid methacrylate isomers, we investigated how side-chain structure governs dynamics at ambient and elevated pressures. Our results reveal a surprisingly rich picture. The unmodified backbone exhibits an unusually low glass transition temperature, defying expectations based on its hydrogen-bonding ability. Grafting changes the rules: some systems show a single relaxation, while others display two distinct motions. High-pressure experiments untangle these processes, identifying a slower hidden mode invisible to mechanical tests. This elusive motion, attributed to sub-Rouse chain dynamics, becomes detectable only due to the strong polarity of side groups—challenging conventional assumptions. Most intriguingly, compression response does not follow simple expectations. Pressure sensitivity of the glass transition does not scale with side-group size or rigidity, suggesting overall molecular organization, rather than individual monomer properties, dictates behavior. The backbone shows one of the lowest pressure sensitivities recorded for a polymer, pointing to the unique role of weak interactions. This work deepens our fundamental understanding of polymer physics and demonstrates high-pressure techniques as a tool for uncovering hidden molecular motions. By bridging chemistry, architecture, and dynamics, our findings open new avenues for designing smarter, more responsive polymeric materials.