Mid-Infrared Spectroscopic Investigation of the Perfect Vitrification of Poly(ethylene glycol) Aqueous Solutions

Crystallization/recrystallization behaviors of poly(ethylene glycol) (PEG) aqueous solutions with water contents (W-C's) of similar to 36-51 wt % were investigated by temperature-variable mid-infrared spectroscopy. At a W-C of 43.2 wt %, crystallization and recrystallization of water and PEG were not observed. At this specific W-C value (W-CPV), perfect vitrification occurred. Below and above the W-CPV value, crystallization/recrystallization behaviors changed drastically. The crystallization temperature below W-CPV (237 K) was similar to 10 K greater than that above W-CPV (226 K). Recrystallization above and below W-CPV occurred in one (213 K) and two (198 and 210 K) steps, respectively. These findings resulted from the difference in the (re)crystallization behaviors of water molecules associated with PEG chains with helical and random-coil conformations. These two types of water molecules might have limiting concentrations for their (re)crystallization, indicating that perfect vitrification might have occurred when the concentrations of the two types of water molecules were less than the limiting concentrations of their (re)crystallization.