The influence of water on the dynamics of alternating polymers P(C ₈ EG ₄ ) and P(C ₄ EG ₄ ) by broadband dielectric spectroscopy

Abstract The influence of water on the molecular dynamics of sequence-defined alternating polymers was investigated using broadband dielectric spectroscopy. Understanding how water couples to polymer dynamics is essential for controlling transport, mechanical properties, and functionality in hydrated soft materials. Two alternating copolymers, P(C 4 EG 4 )3.2k and P(C 8 EG 4 )2.9k, which share identical ethylene glycol segments but differ in aliphatic spacer length, are studied over a wide temperature range and at water contents up to 10 wt%. In the dry state (= 0 wt% water), both polymers exhibit well-defined α- and β-relaxation processes associated with cooperative segmental motion and localized molecular dynamics, respectively. The interpretation of the data consistently shows that, upon hydration, water acts as an efficient plasticizer, leading to a significant increase in free volume, which in turn accelerates α-relaxation and lowers the glass transition temperature. In contrast, the β-relaxation remains much less sensitive to water content. A separate water-related relaxation process emerges, attributed to the reorientational dynamics of confined water molecules. The free volume, estimated from the temperature dependence of the α-relaxation using the Williams–Landel–Ferry (WLF) approach, increases systematically with both temperature and water content. A clear sequence-dependent response is observed: the more polar P(C 4 EG 4 )3.2k shows stronger hydration-induced free volume enhancement and dynamic changes, whereas P(C 8 EG 4 )2.9k exhibits faster intrinsic dynamics but weaker sensitivity to water. These results establish molecular sequence as a key molecular design parameter for controlling hydration-dependent dynamics and transport-relevant properties in polymer systems.