Abstract
<p class="1Body">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 (<em>f</em><sub>s</sub>, <em>R</em><sub>1</sub>). A hard gel was confirmed to form by the decrease in <em>f</em><sub>s</sub> and the increase in <em>R</em><sub>1</sub> 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.</p>
Highlights
The quartz crystal microbalance (QCM) is used in biological, medical and industrial fields as a molecular probe because of its fine spatiotemporal resolution of several tens of nanometers to sub-microns and its economic advantages (Janshoff, Galla, & Steinem, 2000; Marx, 2007; Iniewski, 2012; Diethelm, 2015)
The size and/or amount of samples used in both the differential scanning calorimeter (DSC) analysis and QCM measurements were much larger than the size of the micellar crystalline aggregates of poly(vinyl alcohol) (PVA) (Hassan & Peppas, 2000) that are produced in the initial stage of the gelation of PVA sol and the pore sizes created in the PVA by the freeze–thaw cycle treatment
The gelation process of PVA gel during repeated freeze–thaw thermal cycles was evaluated based on the thermal behavior of pore water and the viscoelastic behavior of the PVA sol obtained by the cryo-QCM developed in this study
Summary
The quartz crystal microbalance (QCM) is used in biological, medical and industrial fields as a molecular probe because of its fine spatiotemporal resolution of several tens of nanometers to sub-microns and its economic advantages (Janshoff, Galla, & Steinem, 2000; Marx, 2007; Iniewski, 2012; Diethelm, 2015). Modern QCMs realize a stable resonator by using a network analyzer with a sufficient power supply. This development has inspired the application of QCMs to large loading system associated with solid phases of samples and soft materials. QCMs have been applied in measuring the phase behavior of crosslinked thermo-responsive hydrogels, which are a typical soft material (Nakano, Kawabe, & Seida, 1998; Nakano, Seida, & Nakano, 2007; Seida, 2007; Seida, 2013). The QCM allows the hydration behavior of a thermo-responsive hydrogel to be observed in its collapse phase at higher temperature than the volume phase transition temperature (i.e., 306 K). The QCM is effective at identifying the phase behavior of a hydrogel and/or the temperature dependence of the hydration behavior at polar sites of the hydrogel, which is difficult with other more conventional techniques during the collapse phase of the gel
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