Low-Frequency Vibrational Motions of Polystyrene in Carbon Tetrachloride: Comparison with Model Monomer and Dependence on Concentration and Molecular Weight

Studies on the molecular dynamics of polymers in solution often use longer time scales due to the size of the macromolecules being studied. To fully elucidate the complete dynamics and interactions of polymers in solution, however, it is necessary to also study the ultrafast (sub-picosecond) dynamics as well. In this paper, a KMLabs Griffin^TM Ti:Saphhire oscillator is employed as the light source for femtosecond Raman induced Kerr Effect spectroscopy of polystyrene in carbon tetrachloride (PS/CCl₄). By comparing spectra from PS/CCl₄ to model monomer ethylbenzene (EBz/CCl₄), it was found that the spectra of PS/CCl₄ had a lower intensity than EBz/CCl₄, indicating suppression of the translational motion of molecules in polymer form. Additionally, concentration and molecular weight dependencies on the low frequency vibrational motion of both molecules in solution were investigated. It was found that the first moment (M1) of the frequency difference spectra in the low frequency region for both compounds was almost independent of concentration. Molecular weight dependence studies also observed M1 to increase as a function of molecular weight from EBz/CCl₄ to ~PS500/CCl₄ which corresponds to a decamer, and then remains constant. Ultrafast studies of polymers in solution shine light into the complex polymer/solvent interactions that are usually hidden in longer time scale studies.