Particular Water Organization in Homogeneous Water Mixtures of Amphiphilic Polystyrene-block-poly(methoxydiethylene glycol acrylate) Diblock Copolymer with Thermoresponsive Behavior

In this work, thermal transitions and dielectric behavior of mixtures of water with thermoresponsive polystyrene-block-poly(methoxydiethylene glycol acrylate) (PS-b-PMDEGA) copolymers are studied in the low water concentration regime (0 ≤ h_w ≤ 0.53). By gradually increasing the water content, the plasticization of the polymer matrix can be followed up to water contents around 0.30. At this critical water content, water crystallizes during cooling, and surprisingly, the glass transition temperature shifts abruptly to higher values becoming equal to that of a dry copolymer. This finding indicates that the water phase separation during the formation of ice crystals is accompanied by a water detaching process that leads to a complete rearrangement of PMDEGA macromolecular chains, similar to what happens during the water detaching process in the thermoresponsive coil-to-globule transition of the copolymer in aqueous solutions. Differential scanning calorimetry (DSC) measurements reveal a transition that resembles the thermoresponsive transition in aqueous solutions that takes place only for water content h_w > 0.30. For lower water contents, broadband dielectric spectroscopy (BDS) measurements indicate that uncrystallized water, weakly interacting with the PMDEGA block, may adopt an open hydrogen bond (HB) network structure of a solid-like structure with a characteristic time scale slower than hexagonal ice. Moreover, the observed crossover of the dynamics of secondary (or fast) water relaxation at about -90 °C (in the form of fragile-to-strong transition) may reflect an inherent property of HB network of hydration (interfacial/confined) water molecules that undergo changes in the structure and/or dynamics that may trigger the micro-Brownian mobility of the surrounding polymeric segments.