Intramolecular chain dynamics by forward depolarized scattering

Abstract

Internal relaxation processes of two linear polymers are investigated by the quasielastic Rayleigh scattering in the forward depolarized configuration. The technique is shown to extract only the depolarized component of scattered light which arises solely from the intrachain dynamics without usual encumbrances due to the center of mass translational mode. The power spectrum obtained by direct Fourier transform of photocurrent which is an exact replica of the optical spectrum because of heterodyne‐beating between the leakage component normal to the polarization direction of the incident light and the scattered light depolarized by the internal modes, is given by Lorentzian shape in which each mode contributes to the total spectrum with uniform weighting factor. The first polymer, poly(n ‐hexyl isocyanate), is chosen for its rodlike configuration in low molecular weight and the forward depolarized scattering (FDS) spectrum has been shown to be a single Lorentzian consistent with a unique rotatory relaxation process about the minor axes of the rod. The half‐widths of FDS spectra are obtained at different concentrations and extrapolated to infinite dilution. The rod length calculated from the extrapolated half‐width value is found to be in good accord with that from light scattered intensity measurement. For the second polymer, which is a linear flexible chain with optical anisotropy, isotactic polystyrene, M = 3.5×106, is selected. The FDS spectra at various concentrations are shown to be multiple Lorentzian, with the spacing of half‐widths consistent with the Zimm model. The spectral half‐width extrapolated to infinite dilution is in agreement with the prediction of Zimm's theory, but the precision of the terminal relaxation time is only within 30%. At higher molecular weight, the solutions of poly(n ‐hexyl isocyanate) are found to give multiple Lorentzian spectra and their half‐width spacings are determined to be closer than that of the Zimm model. A qualitative rationale for the behavior is offered.