On the νSiO infrared absorption of polysiloxane films

The infrared absorption of polysiloxanes involves a strong band at around 1050 cm− 1, attributed to the antisymmetric vibration of siloxane bridges. The splitting of this band into two components is generally attributed to coupling between next-neighbor siloxane groups along the polysiloxane chain. From a quantitative analysis of the spectra of these materials, we find that this splitting is larger when the material is in thin-film form, and that the relative intensity of the two components is polarization dependent. We show that these effects are fully understandable in the theoretical framework of infrared absorption by thin films, and are related to long-range dipolar interactions responsible for the longitudinal–transverse splitting effect in crystalline materials. As a consequence, the polarization dependence of the infrared absorption observed for thin films does not appear to be associated with an orientational ordering in the film.

► We analyze the infrared absorption of polysiloxane films quantitatively. ► The complex dielectric function is deduced from a Kramers–Kronig analysis. ► s- and p-polarized spectra differ, due to the large value of the Si–O dynamic dipole. ► The observed polarization effects are no proof for a preferential chain orientation. ► These effects are related to the longitudinal–transverse splitting in crystals.