Mechanically-induced alignment of mesophases

We report the results of a systematic orientation study on side-chain liquid crystalline polymers (SCLCPs) mechanically stretched in different mesophases. Two representative polyacrylate-based SCLCPs were used. The study demonstrates that the macroscopic orientation of the mesogens reflects a direct alignment of the liquid crystalline domains as entities under the effect of a mechanical field. One important feature is that the alignment of different mesophases results in different orientation directions of the mesogens with respect to the mechanical stretching direction. On the basis of our results, it seems to be common that stretching SCLCPs in their smectic phase gives rise to a macroscopic orientation perpendicular to the stretching direction. When SCLCPs are stretched in a nematic phase covering a wide temperature range, this orientation is achieved along the stretching direction. However, a perpendicular orientation can also be induced when SCLCPs are stretched in a nematic phase which is only over a narrow temperature range (ca. 10 degrees C or fewer) beyond a smectic phase. A sharp inversion of the orientation direction often occurs when SCLCPs are stretched in the mesophase transition regions. We show that the observed alignment behavior of the different mesophases could be explained by assuming an alignment, always along the stretching direction, of the long axes of liquid crystalline domains.