The conformation of poly(dimethylsiloxane) in the crystalline state

The problem of the polymer chain conformation in crystalline poly(dimethyl siloxane) has been revisited using X-ray diffraction techniques. It is show n that the crystallites belong to the centered tetragonal system. A four-fo ld helical configuration of the polymer chain is preposed, with four monome rs per turn and a pitch of ca. 1.2 nm: it corresponds to an extended config uration of the siloxane backbone with oxygen atoms located close to the 4, symmetry axis and methyl groups pointing outwards. These results are in dee p contrast with the previously accepted monoclinic structure and ribbonlike chain conformation. (C) 2000 Elsevier Science Ltd. All rights reserved.