X-ray diffraction from liquid crystalline carbosilane dendrimers

We have studied the structure of two series of liquid crystalline dendrimer s based on the carbosilane dendritic matrix. Generations 1 to 5 have been i nvestigated for the series with cyanobiphenyl mesogenic groups and generati ons 1 to 4 for the series with methoxyphenyl benzoate mesogenic groups. Gen erations 1 to 4 of both series show a diffraction pattern corresponding to a simple smectic phase with disordered layers indicating that the tendency of the mesogenic units to form smectic layers overcomes the restrictions im posed by the globular dendritic core of the molecule. The variation of laye r spacing with temperature and generation number indicates that there are s ubtle differences in the packing of the mesogens in the different samples. The 5th generation of the dendrimers with cyanobiphenyl groups shows the sa me smectic structure below 40 degrees C but at higher temperatures there is some in-plane structure that suggests that columns may be formed. This can be rationalised in terms of the molecular shape becoming more spherical at higher temperatures. Above 121 degrees C there is only a broad peak implyi ng that this mesophase has short-range order (or possibly D-hd phase) until it becomes isotropic at 135 degrees C.