Study of changes in water structure and interactions among water, CH, and COO groups during water absorption in acrylic acid-based super absorbent polymers using Raman spectroscopy.

The interactions of acrylic acid-based super absorbent polymers (SAPs) with water and the hydrogen bonding of water within its three-dimensional network were studied using Raman spectroscopy. The Raman spectra of SAP solutions suggested that both the COO and CH groups of SAPs interact with water. The Raman spectra of pure water and those of SAPs containing approximately 40, 50, and 60% water exhibited a broad band corresponding to the OH-stretching mode of water in the 4000-3000 cm region. This band was separated into three components using a curve-fitting method. The three components at 3200, 3400, and 3600 cm were assigned to the OH-stretching modes of strong hydrogen bonding (SHB), weak hydrogen bonding (WHB), and dangling bond (Dang) species of water, respectively. The fractional areas of the three components were calculated and compared. The changes in the hydrogen bonding of water were compared with those of the water present in SAPs, and their temperature-dependent variations were elucidated. At a water content of approximately 60%, the behavior of the fractional area versus temperature was similar to that of pure water. However, at a water content of approximately 40%, the behavior was significantly different. The fraction of SHB was smaller, and the fraction of WHB was larger than that of pure water. The difference in the CH and COO peak shifts of SAP, which is a result of the addition of a small percentage of water, was revealed by Raman spectroscopy. The position of the CH deformation peak changed linearly. However, the position of the COO rocking peak did not change significantly up to a water content of 30%, above which it exhibited a rapid shift to lower wavenumbers. This result indicates that the interactions of the CH and COO groups are different.