Vibrational Spectroscopy on the Silicon Hydride Mode: Probing Ultrafast Dynamics in Small Molecules to Macromolecular Polymer Systems

Abstract

This thesis describes Fourier transform infrared (FTIR) and two-dimensional infrared (2D-IR) spectroscopy applied to small molecule silanes (trimethoxysilane and triphenylsilane) and polydimethylsiloxane (PDMS). 2D-IR spectroscopy gives information about the dynamics that the vibrational probe is sensitive to and the heterogeneous and homogeneous contributions to the linear FTIR lineshape. The vibrational probe used for all the studies in this thesis is the silicon hydride stretch due to being present in the small molecule silanes and in PDMS. The studies presented show how the silicon hydride mode was first characterized in small molecules to understand the probe more. Then, the probe was utilized in polymer systems to study more complex motions to make the connection between the ultrafast dynamics of polymers to the macroscopic properties. The first study involved studying the solvation dynamics of two small molecule silanes in three neat solvents using FTIR and 2D-IR spectroscopies along with molecular dynamics simulations. The two different molecules exhibited different degrees of vibrational solvatochromism, and the differences was found to be a result of higher mode polarization with more electron withdrawing ligands using density functional theory calculations. The solvent dynamics were found to be dominated by their interactions with neighboring solvent molecules rather than with the solute. Next, FTIR and 2D-IR spectroscopies were used to study PDMS cross-linked films and siloxane oligomers without solvent and swollen or dissolved in various solvents. There is an absence of vibrational solvatochromism in these systems, which was shown by 2D-IR spectroscopy to be due to the heterogeneity. The silicon hydride mode in the cross-linked, solvent-free PDMS film exhibited spectral diffusion, which must be due to the polymer structural motions. However, once the solvent penetrates the network, the dynamics become a convolution of the solvent and polymer motions due to the motions being of similar timescale. In the last study discussed, FTIR and 2D-IR spectroscopies were used to study the ultrafast structural dynamics of PDMS thin films with various physical and chemical changes done to the polymer, which included elevated curing temperature, increased cross-linker agent concentration, compression, and cooling near the glass transition temperature. The FTIR spectra were found to be relatively insensitive to all of these perturbations, which 2D-IR spectroscopy revealed was caused by the overwhelming heterogeneity. There is clearly a disconnect between the microscopic and macroscopic behavior in this polymer due to having only slight differences in the heterogeneous and homogeneous dynamics.