Low-Frequency Vibrational Motions of Polystyrene in Carbon Tetrachloride: Comparison with Model Monomer and...

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 (M₁) 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 M₁ value of the low-frequency spectral band of PS shifts to higher frequencies when the molecular weight of PS increases up to M_w = ∼1000, which corresponds approximately to the decamer, and then remains constant upon further increasing the molecular weight.