The conformation of poly(dimethylsiloxane) in the crystalline state

Abstract

The problem of the polymer chain conformation in crystalline poly(dimethyl siloxane) has been revisited using X-ray diffraction techniques. It is shown that the crystallites belong to the centered tetragonal system. A four-fold helical configuration of the polymer chain is proposed, with four monomers per turn and a pitch of ca. 1.2 nm: it corresponds to an extended configuration of the siloxane backbone with oxygen atoms located close to the 4 1 symmetry axis and methyl groups pointing outwards. These results are in deep contrast with the previously accepted monoclinic structure and ribbonlike chain conformation.