The glass transition of partially crystallized gelatin-water mixtures was investigated for gelatin concentrations of 40 and 20 wt. % by broadband dielectric spectroscopy (BDS) in wide frequency (10 mHz-50 GHz) and temperature (113-298 K) ranges. Three dielectric relaxation processes were clearly observed. The origin of each relaxation process was the same as that observed for partially crystallized bovine serum albumin (BSA)-water mixtures [N. Shinyashiki et al., J. Phys. Chem. B 113, 14448 (2009)]. The relaxation process at the highest frequency is originated from uncrystallized water (UCW) in the hydration shell of gelatin. Its relaxation time is almost the same as that of water in uncrystallized system; water in various binary aqueous mixtures and confined water in nanoscale region. The relaxation process at the intermediate frequency is originated from ice, and its relaxation time and strength were similar to those for the relaxation of pure ice, particularly above 240 K. The glass transition temperature Tg, is defined by BDS measurement as the temperature at which dielectric relaxation time τ, is 100-1000 s. The relaxation process at the lowest frequency, Tg is approximately 200 K, is originated from the cooperative motion of water and gelatin. This relaxation is strong and has a similar relaxation strength to that of hydrated BSA. At Tg for the relaxation process involving the cooperative motion of gelatin and water, the temperature dependence of the relaxation process of UCW crosses over from Vogel-Fulcher behavior to Arrhenius behavior with decreasing temperature. A characteristic property of the gelatin-water mixture is a change in the temperature dependence of the relaxation time of the relaxation processes of hydrated gelatin at approximately 260 K.

Dielectric relaxation process of ice-Ih has been investigated by many researchers. Pioneering studies focused on the temperature dependence of the dielectric relaxation time, τice, were reported by Auty in 1952 [ Auty, R. P.; Cole, R. H. J. Chem. Phys . 1952 , 120 , 1309 ] and Johari in 1981 [ Johari, G. P.; Whalley, E. J. Chem. Phys. 1981 , 75 , 1333 ]. However, the temperature dependences of τice found in these studies are not in agreement. While Auty et al. reported a single Arrhenius temperature dependence of τice for the entire 207-273 K temperature range, Johari et al. reported changes in the temperature dependence of τice at 230 and 140 K. In this study, the temperature dependence of τice is investigated by broadband dielectric spectroscopy for ice prepared by three different procedures. For all investigated ices, a dielectric relaxation process is observed, and τice decreases with increasing temperature. Temperature dependence of τice with rapid crystallization shows the same properties at temperatures down to 140 K as that reported by Johari et al. On the other hand, ice obtained by slow crystallization exhibits the same temperature dependence of τice as those reported by Auty et al. We suggest that the difference between the temperature dependences of τice found by Auty et al. and Johari et al. can be controlled by preparation conditions. That is, the growth rate of the ice crystal can affect τice because a slow growth speed of the ice crystal induces a smaller impurity content of ice, giving rise to an Arrhenius temperature dependence of τice.

The glass transition of partially crystallized gelatin-water mixtures was investigated using broadband dielectric spectroscopy (BDS) over a wide range of frequencies (10 mHz to 10 MHz), temperatures (113-298 K), and concentrations (10-45 wt %). Three dielectric relaxation processes (processes I, II, and III) were clearly observed. Processes I, II, and III originate from uncrystallized water (UCW) in the hydration shells of gelatin, ice, and hydrated gelatin, respectively. A dynamic crossover, called the Arrhenius to non-Arrhenius transition of UCW, was observed at the glass transition temperature of the relaxation process of hydrated gelatin for all mixtures. The amount of UCW increases with increasing gelatin content. However, above 35 wt % gelatin, the amount of UCW became more dependent on the gelatin concentration. This increase in UCW causes a decrease in the glass transition temperature of the cooperative motion of gelatin and UCW, which appears to result from a change in the aggregation structure of gelatin in the mixture at a gelatin concentration of approximately 35 wt %. The temperature dependence of the relaxation time of process II has nearly the same activation energy as pure ice made by slow crystallization of ice Ih. This implies that process II originates from the dynamics of slowly crystallized ice Ih.

Broadband dielectric spectroscopy measurements were performed on partially crystallized gelatin-water mixtures with gelatin concentrations of 1-5 wt % for temperatures between 123 and 298 K to study the dynamics of ice. These systems contain only hexagonal ice. Nevertheless, four dielectric relaxation processes of ice were observed. At temperatures below the crystallization temperature, a loss peak was observed, and it separated into four loss peaks at around 225 K. Using the temperature and concentration dependencies of these relaxation processes, we confirmed that these four processes originated from ice. For the relaxation time of ice, τ, the deviation of the temperature dependence of τ from the Arrhenius type is larger for the relaxation process at the higher-frequency side. For the temperature dependence of τ for the dominant process, three temperature ranges with different activation energies, E, were investigated. The intermediate-temperature range of τ with the smallest E decreased as the gelatin concentration increased; therefore, τ of the dominant process changed from the relaxation process with the smaller τ to that with the larger τ as the gelatin concentration increased. In addition, the relaxation process of ice with larger τ values was found to have larger values of E. These results suggest that a higher gel network density affects the temperature dependence of τ.

In this study, we investigated the cooperative molecular dynamics of poly­(vinylpyrrolidone) (PVP), ice, and uncrystallized water (UCW) in partially crystallized PVP–water mixtures by means of broadband dielectric spectroscopy. Three relaxation processes, denoted I, II, and III, were observed at temperatures ranging from immediately below the crystallization temperature (T c) to approximately 200 K. At temperatures of 173–193 K, processes I and II cannot be distinguished. Below 168 K, process II separates into two processes: process IV at higher frequencies and process V at lower frequencies. Process I contributes to process V. In partially crystallized mixtures, process I originates from UCW in an uncrystallized phase with PVP. Process II is attributed to ice in the mixture, with a relaxation time that is 2 orders of magnitude smaller than that of pure ice. The concentration dependence of the strength of process II and the relaxation time relative to that of ice in bovine serum albumin (BSA)–water and gelatin–water mixtures strongly support this conclusion. Observation of processes IV and V indicates the presence of multiple ice relaxation processes. Process III is attributed to the α process of PVP in the uncrystallized phase in 40 and 50 wt % PVP mixtures. For mixtures with 30 wt % PVP or less, process III is attributed not only to the α process of PVP but also to interfacial polarization.