Abstract

A quartz crystal microbalance (QCM) working under cryo-conditions was applied to analyzing the gelation and/or phase behavior of poly(vinyl alcohol) (PVA) sol during repeated freeze–thaw processes. The development of a porous structure with the gelation of PVA sol during the freeze–thaw cycle was examined in terms of the thermal behavior of the water in the sol and the viscoelastic behavior of the sol through thermal and QCM analyses. Water was liberated from the hydrophilic PVA during the freeze–thaw process through the aggregation of PVA. The water decreased the freezing temperature and increased the melting temperature because of the development of the porous structure with gelation by the thermal treatment. The state of the water during the gelation was estimated from the phase transition temperature and enthalpy change of the water during the thermal scan by using water-saturated silica gels with a series of pore size distributions. The viscoelasticity of the PVA sol during the freeze–thaw process was measured by cryo-QCM using admittance analysis (QCM-A). The free water and/or porous structure in the PVA sol was found to increase from the viscoelastic point of view by QCM measurements showing a shift in the resonance parameters (f_s, R₁). A hard gel was confirmed to form by the decrease in f_s and the increase in R₁ with the thermal scan treatment. The cryo-QCM was found to be an effective probe for clarifying the gelation and phase behavior of PVA sol in detail with high resolution.

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