Glass Transition and Dynamics of the Polymer and Water in the Poly(vinylpyrrolidone)–Water Mixtures Studied by Dielectric Relaxation Spectroscopy

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• 作者：Kaito Sasaki ; Yurika Matsui ; Masahiko Miyara ; Rio Kita ; Naoki Shinyashiki ; Shin Yagihara
• 刊名：Journal of Physical Chemistry B
• 出版年：2016
• 出版时间：July 14, 2016
• 年：2016
• 卷：120
• 期：27
• 页码：6882-6889
• 全文大小：434K
• 年卷期：0
• ISSN：1520-5207

文摘

In this study, broadband dielectric spectroscopy and differential scanning calorimetry (DSC) measurements are performed to study the dynamics of water and polymers in an aqueous solution of poly(vinylpyrrolidone) (PVP) with concentrations of 60, 65, and 70 wt % PVP in a temperature range of 123–298 K. Two distinct relaxation processes, l- and h-processes, which originate from the segmental chain motion of PVP and the primary relaxation process of water, respectively, are observed simultaneously. The relationship between l- and h-processes and their temperature dependences mimic those of the α-process and Johari–Goldstein β-process, which are observed in ordinal glass formers. The relaxation time of the l-process, τ_l, obeys the Vogel–Fulcher (VF)-type temperature dependence, and the glass-transition temperature of the l-process, T_g,l, which is defined by the temperature that is reached in a τ_l of 100–1000 s, shows good agreement with the calorimetric T_g obtained by DSC. The temperature dependence of the relaxation time of the h-process, τ_h, exhibits a crossover from VF to Arrhenius behavior at the so-called fragile-to-strong transition (FST) of water at T_g,l. The temperature dependence of the relaxation strength of the h-process, Δε_h, increases with a decrease in temperature from 298 K to T_g,l. Below T_g,l, Δε_h is nearly constant or slightly decreases with decreasing temperature. According to previous studies on aqueous solutions of sugars and alcohols, the Δε of the ν-process, which originates from local motion of water, decreases with decreasing temperature above the T_g of the α-process, which originates from the cooperative motion of the solute and water. Therefore, the l-process in the PVP–water mixture is not a result of the cooperative motion of PVP and water but rather a result of the polymer–polymer cooperative motion of PVP. In addition, agreement among T_g,l, the temperature of the FST of water, and calorimetric T_g suggests that the FST of water occurs at T_g.