Neutron scattering studies of terminally attached side chain liquid crystal polymers

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Abstract
Three side chain liquid crystal polymers have been studied by neutron scattering methods. We have compared the three compounds, all with a polysiloxane backbone, a four-atom flexible spacer, (CH2)3O, and a benzoate ester mesogenic unit. However, the mesogenic unit was terminated by one of three different groups. The hexyloxy and cyano derivatives gave smectic A phases and the methoxy gave a nematic. Small angle neutron scattering measurements of mixtures of deuteriated and hydrogenous versions of the same polymer have been used to determine the dimensions and anisotropy of the backbone. The anisotropy was found to be similar in the smectic and nematic phases and suggested a tendency for the polymer backbone to be confined between the layers of mesogenic units. The SANS results show two different characteristic sizes. One is attributed to the size of a single polymer molecule. The other indicates density fluctuations on a scale larger than one polymer molecule. The local packing of the mesogenic units has been studied by comparison of neutron Bragg scattering from the smectic layers in materials with specific parts of the mesogenic units deuteriated. This has allowed the degree of interdigitation of mesogenic units in different layers to be deduced in the hexyloxy compound. It was found necessary to postulate a chain rich region midway between the polysiloxane backbones. This could only occur if the mesogenic units were significantly statistically tilted with respect to the smectic layer normal.