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

In this study, the low-frequency vibrational dynamics of polystyrene (PS) in CCl4 was investigated by femtosecond Raman-induced Kerr effect spectroscopy. Ethylbenzene (EBz) was also investigated as a model monomer of the polymer to elucidate the unique dynamical features of PS in solution. The broadened low-frequency spectrum of the PS/CCl4 in the frequency region below 150 cm(-1) is significantly different from that of the EBz/CCl4. Difference spectra between the PS or EBz solutions and neat CCl4, normalized to an internal vibrational mode of CCl4, clearly show a much lower spectral intensity for the PS/CCl4 than the EBz/CCl4 in the low-frequency region below ca. 20 cm(-1). This indicates that translational motions are suppressed in the PS/CCl4 compared to the EBz/CCl4. Moreover, the high-frequency motion at ca. 70 cm(-1), mainly due to phenyl ring librations, occurs at higher frequency in PS (78 cm(-1)) than EBz (65 cm(-1)). In addition, the results of concentration-dependent experiments show that the first moment (M-1) of the low-frequency difference spectra of both PS/CCl4 and EBz/CCl4 is almost independent of the concentration. The molecular weight dependence of the low-frequency spectrum in the PS/CCl4 shows that the M-1 value of the low-frequency spectral band of PS shifts to higher frequencies when the molecular weight of PS increases up to M-w = similar to 1000, which corresponds approximately to the decamer, and then remains constant upon further increasing the molecular weight.