Structure of Water Sorbed into Poly(MEA-co-HEMA) Films As Examined by ATR−IR Spectroscopy

The structure of water sorbed into poly(2-methoxyethyl acrylate) (PMEA), poly(2-hydroxyethyl methacrylate) (PHEMA), and their copolymers (p(MEA/HEMA)) was investigated by attenuated total reflection infrared (ATR−IR) spectroscopy. The extinction coefficient of the OH stretching band of sorbed water (ε OH ) was calculated from the band area obtained by IR measurement and the amount of sorbed water obtained by thermogravimetric analysis. When the polymers contacted with water vapor (relative humidity = ∼55%), the ε OH values were quite similar in all polymers. On the other hand, when the polymers contacted with liquid water, the ε OH values were drastically changed by the content of 2-methoxyethyl acrylate (MEA). When the MEA content of the polymers was low (<60 mol %), the ε OH value of the water sorbed into polymers in contact with liquid water was equal to that in contact with water vapor. In the higher MEA content (70−100 mol %), on the other hand, the ε OH values of the water sorbed into polymers in contact with liquid water were 5−8 times larger than that in contact with water vapor. These results seemed to indicate that the interaction between the primary hydration water around the MEA-containing polymer chain and water molecules surrounding the primarily hydrating water is very weak. Such water with a large ε OH value seemed to correspond to “cold-crystallizable” water, which has been observed by DSC as anomalous water other than intermediate and nonfreezable waters. Taking both the experimental results obtained in this work and those thermodynamically obtained previously into consideration, it was strongly suggested that the cold crystallization of water is generated by caging water molecules in a small space by the polymer chains with a small hydration region. The correlation between the ε OH value and the blood compatibility of the copolymer was also discussed.