Ultrafast infrared spectroscopic study of microscopic structural dynamics in pH stimulus-responsive hydrogels^†

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 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 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 cannot decay to zero especially at the pH of 7.0. These results are expected to provide a molecular-level understanding of the microscopic structure of the cross-linked polymeric network in the pH stimulus-responsive hydrogels.