Structural transformations in poly(di-n-alkylsiloxane)s with alkyl side groups containing 7 to 10 carbon atoms

Temperature dependent X-ray diffraction studies of poly(di-n-alkylsiloxane)s substituted with side chains containing 7 to 10 carbon atoms have been carried out in order to elucidate the crystalline structure of these materials. In contrast to their lower substituted homologues, no evidence for the presence of a hexagonal columnar mesophase could be found for the investigated materials. At low temperatures, far below the melting temperature, the polysiloxane backbones are assumed to pack in an orthorhombic lattice, with the n-alkyl side chains in a planar all-trans conformation and oriented perpendicularly to the backbone. The paraffinic n-alkyl side chains were found to crystallize in an orthorhombic lattice. The thermal behaviour of the investigated materials has been observed to show an odd-even effect. Whereas poly(di-n-heptylsiloxane) and poly(di-n-nonylsiloxane) were found to melt in a single step, an intermediate state of order was observed for poly(di-n-octylsiloxane) and poly(di-n-decylsiloxane) upon heating from the crystalline to the isotropic state. For this intermediate state, a pseudohexagonal packing of the n-alkyl side chains is proposed