Hydrogels show versatile properties and are of great interest in the fields of bioelectronics and tissue engineering. Understanding the dynamics of the water molecules trapped in the three-dimensional polymeric networks of the hydrogels is crucial for us to elucidate their mechanical and swelling properties at the molecular level. In this report, the poly(DMAEMA-co-AA) hydrogels were synthesized and characterized by the macroscopic swelling measurements under different pH conditions. Furthermore, the microscopic structural dynamics of pH stimulus responsive hydrogels were studied using FTIR and ultrafast IR spectroscopies from the viewpoint of the SCN-anionic solute as the local vibrational reporter. Ultrafast IR spectroscopic measurements showed the time constants of the vibrational population decay of SCN- were increased from 14±1 ps to 20±1 ps when the pH of the hydrogels is varied from 2.0 to 12.0. Rotational anisotropy measurements further revealed that the rotation of SCN- anionic probe was restricted by the three-dimensional network formed in the hydrogels and the rotation of SCN- anionic probe can’t decay to zero especially at the pH of 7.0. These results presented in this study are expected to provide molecular level understanding of the microscopic structure of the cross-linked polymeric network in the pH stimulus-responsive hydrogels.
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