Influence of Solvent Swelling on Ultrafast Structural Dynamics in Polydimethylsiloxane Thin Films by Two-Dimensional IR Spectroscopy

Fourier transform infrared and two-dimensional IR (2D-IR) spectroscopies were applied to polydimethylsiloxane (PDMS) cross-linked elastomer and a siloxane oligomer without solvent and swollen or dissolved in various solvents. The silicon hydride, which is covalently bound to the polymer chains, was the vibrational probe for the systems studied. There is almost an absence of vibrational solvatochromism in these systems. Frequency-frequency correlation functions obtained by 2D-IR spectroscopy show that the insensitivity of the FTIR spectra is due to overwhelming heterogeneity. However, the homogeneous contribution to the FTIR spectrum is smaller for the elastomer than the oligomer showing that the cross-linking process restricts the frequency fluctuations that are experienced by the hydride mode. The silicon hydride mode in a cross-linked, solvent-free PDMS film also exhibits spectral diffusion that must be due to polymer structural motions on the ultrafast time scale that are active above the glass transition temperature. Once solvents penetrate and swell the elastomer, the polymer likely continues to experience these polymer structural motions. However, we find that the vibrational dynamics are characteristic of the infiltrating solvents, showing that at least some fraction of the measured dynamics originate in solvent motions.