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

Abstract

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, τl, obeys the Vogel-Fulcher (VF)-type temperature dependence, and the glass-transition temperature of the l-process, Tg,l, which is defined by the temperature that is reached in a τl of 100-1000 s, shows good agreement with the calorimetric Tg obtained by DSC. The temperature dependence of the relaxation time of the h-process, τh, exhibits a crossover from VF to Arrhenius behavior at the so-called fragile-to-strong transition (FST) of water at Tg,l. The temperature dependence of the relaxation strength of the h-process, Δεh, increases with a decrease in temperature from 298 K to Tg,l. Below Tg,l, Δεh 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 Tg 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 Tg,l, the temperature of the FST of water, and calorimetric Tg suggests that the FST of water occurs at Tg.

Heterogeneous Solvent Dielectric Relaxation in Polymer Solutions of Water and Alcohols

“…For the PVP-water mixture, Figure 5A clearly shows that the non-A-A crossover of the ν-process occurs at T g,PVP , which is consistent with the results reported in Sasaki et al [3] and also the results of Cerveny et al [5]. The non-A-A crossover of the ν-process is mostly observed in various systems containing water.…”

“…This crossover has been discussed based on the observations of water dynamics over an extremely wide range of relaxation times and viscosities. The non-A-A crossover of water dynamics has been observed in various types of aqueous systems, such as aqueous solutions of alcohols, ethylene glycol (EG) oligomers, sugars, polymers, and protein and porous systems those contain water [3,[5][6][7][8][9][10][11]35]. The temperature dependence of the relaxation process of water in these systems changes at a certain temperature, which is called the crossover temperature, T c , in this paper.…”

“…The loss peak at 100 Hz is partly concealed by ionic contributions, namely, EP and dc conductivity. We also studied the two dielectric relaxation processes at 298 K in PVP solutions with water, alcohol, and chloroform and examined the origins in detail [3,[46][47][48][49]. The relaxation process observed above 100 MHz is the primary relaxation of the solvents.…”

“…In this study, we discuss the role of the glass transition in the dynamic crossover of binary mixtures of a polymer and hydrogen-bonding solvents. In order to discuss the non-A-A crossover of water dynamics in a polymer matrix, in addition to the previous works, we performed broadband dielectric spectroscopy measurements of poly(vinylpyrrolidone)-water (PVP-water), PVP-propylene glycol (PG), PVP-EG, and PVP−1-propanol (PrOH) mixtures with 65 wt% of PVP in a wide frequency over a temperature range of 123-318 K. For the PVP-water mixture, three relaxation processes were observed simultaneously: the slowest process originated from the segmental chain motion of PVP (called the α-relaxation of PVP), the fastest process originated from the primary relaxation process of water (called the ν-process) [3], the relatively small relaxation process at a frequency between those of the α-relaxation of PVP and the ν-process. The temperature dependence of the relaxation time of the ν-process exhibits a non-A-A crossover at the glass transition temperature T g,PVP of the α-relaxation of PVP.…”

Effects of Solvent Crystallization in Swollen net-Poly(ethyl acrylate) α Relaxation Dynamics

“…The dielectric investigation of polymer/solvent mixtures in a wide concentration, frequency and temperature range has been widely attempted as a way to probe the influence of the presence of one component on the dynamics of the other component in terms of the specific interactions between polymer and solvent molecules and their compatibility. , Especially interesting is the case of polymer/nonpolar solvent mixtures, since the weak intermolecular interactions between the mixture’s components render such mixtures simpler compared to systems with polar solvents, for instance water, that presents complex individual dynamics and specific interactions with polymer or biopolymer host. − Moreover, the polymer/nonpolar solvent mixtures present less complicated dielectric spectrum at low frequencies due to nondipolar contributions, that masks primary segmental relaxations. ,− A lot of attention has been paid to macroscopically homogeneous glass-forming polymer/solvent mixtures. In these mixtures, the solvent can be cooled without crystallization below the mixture T g . ,− In such glass forming mixtures, the effect of solvent to polymer plasticization, polymer segmental dynamics, secondary polymer relaxations, and characteristic features as the dynamical heterogeneity are the main topics of research with experimental and theoretical methods. ,− However, it is essential to investigate and compare the polymer and solvent segmental dynamics in homogeneous and partially crystallized mixtures.…”