Characterization of polymer materials based on structure analyses of vicinal water

In this paper, we review the vibrational spectroscopic analyses of the structure and hydrogen bonding of water in the vicinity of various polymers based on research performed by our group. Aqueous solutions of water-soluble polymers have been examined using the contours of O-H stretching in polarized Raman spectra. For water sorbed into polymer thin films, infrared spectroscopy was used and the structure of water at polymer-liquid water interfaces was examined using sum frequency generation spectroscopy. Zwitterionic polymers and amphoteric polymers containing comparable numbers of anionic and cationic groups were inert to the structure of vicinal water. In contrast, lopsidedly charged polymers strongly perturbed the structure of water. The inert properties of polymers with comparative contents of anionic and cationic groups with respect to the hydrogen-bonded network structure of water may be owing to the counteraction of the electrostatic hydration effect by the proximity between the cationic and anionic groups, which may endow the charge-neutralized (zwitterionic or amphoteric) polymers with biocompatibility. Similar inert properties to vicinal water were observed for a thin film of a well-known anticoagulant polymer (i.e., poly(2-methoxyethyl acrylate)). The correlation between the freezing/melting behavior of water sorbed into various nonionic polymer materials and their biocompatibility is also discussed.