Characterization of dimethyldiacyloxysilanes by differential scanning calorimetry, Raman scattering and X-ray diffraction

The phase behaviour of diacyloxydimethylsilanes (DMS Cn; n = 10, 12, 14, 16 , 18, 20, 22) was investigated by differential scanning calorimetry, X-ray diffraction and Raman spectroscopy. All DMS Cn melt from a crystalline phas e to an isotropic liquid with a single sharp transition. On cooling, the ho mologoues DMS C16 up to DMS C22 show a characteristic monotropic phase (L b eta(H)'). In contrast to the calorimetrical investigations, it was not poss ible to analyse the monotropic phase of DMS C16 by X-ray diffraction. This behaviour is due to a two-phase region (gel phase - crystalline phase). The Raman spectra of all DMS an very similar. Only in the low frequency range we find different bands of the longitudinal acoustic modes. The Raman measu rements demonstrate undoubtedly that in the solid state the alkyl chains ar e in all-trans conformation. The factor group splitting of the CH2 scissori ng Raman mode show that the DMS Cn are arranged in a subcell packing with t wo molecules per unit cell. The highly ordered all-trans structure of the a lkyl chains is present up to the melting transition. On melting there are c hanges in different regions of the Raman spectra: C-H stretching, CH2 sciss oring mode, C-C skeletal stretching, CH3 rocking and longitudinal acoustic modes. On cooling DRIS C18 and DMS C20 from the melt to the crystalline sta te, the gel phase is also proved by Raman scattering. Based on the results of the Raman and X-ray data the gel phase is characterized by a hexagonal s ubcell packing and by an ordered structure of the alkyl chain residues.