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

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 phase to an isotropic liquid with a single sharp transition. On cooling, the homologous DMS C16 up to DMS C22 show a characteristic monotropic phase (L beta'H). In contrast to the calorimetrical investigations, it was not possible 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 are very similar. Only in the low frequency range we find different bands of the longitudinal acoustic modes. The Raman measurements demonstrate undoubtedly that in the solid state the alkyl chains are in all-trans conformation. The factor group splitting of the CH2 scissoring Raman mode show that the DMS Cn are arranged in a subcell packing with two molecules per unit cell. The highly ordered all-trans structure of the alkyl chains is present up to the melting transition. On melting there are changes in different regions of the Raman spectra: C-H stretching, CH2 scissoring mode, C-C skeletal stretching, CH3 rocking and longitudinal acoustic modes. On cooling DMS C18 and DMS C20 from the melt to the crystalline state, 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 subcell packing and by an ordered structure of the alkyl chain residues.