Evolution of Ultrafast Vibrational Dynamics during Sol-Gel Aging

A longitudinal 1D- and 2D-IR spectroscopy study is presented in which the homogeneous and inhomogeneous contributions to the line shape of a surface-bound Si - H vibration are monitored in a silica sol-gel while it ages. The vibrational data are complemented by time-lapse ²⁹Si NMR and rheological measurements leading up to the gel point. The silane stretching frequency evolves continuously over the equivalent of several days, tracking the increase in tertiary and quaternary functionalized silicon atoms. The 1D-IR peak shape of the mode is static up until gelation, but then broadens continuously as the gel ages. A frequency-frequency correlation function (FFCF) extracted from 2D-IR spectra reveals that the line shape changes stem from an increase in inhomogeneity while the homogeneous dynamics remain unaffected by changes in silica cross-linking.