Correlation between the surface properties and liquid crystal anchoring on polymer films

The alignment of liquid crystals (LCs) on solid substrates involves a wide variety of interfacial phenomena such as surface ordering, surface transitions, surface wetting, etc. and has been a topic of interest since the discovery of LCs. The mechanical rubbing of polyimide (PI) film is the most widely used method for achieving anisotropic surface potential at the LC-substrate interface needed for uniform alignment.; In recent years, photo-alignment of LCs, especially using PI films exposed to linearly polarized UV (LPUV) light, has emerged as a promising non-contact technique for the alignment of LCs. We have found that a homogeneous alignment of LCs can be achieved by in-situ exposure of PI films during imidization. The alignment layers prepared by this method exhibit higher thermal stability while requiring shorter processing time compared to the conventional UV alignment method which employs LPUV exposure after the imidization of PI is complete.; The morphological structure of the commonly used LC alignment layers viz. rubbed polystyrene (PS), rubbed PI and LPUV exposed PI have been studied by high resolution x-ray reflectivity and optical retardation measurements. In all cases, the interfaces are found to possess anisotropy of the surface morphology. In the case of rubbed PS, the rubbing direction is rougher than the orthogonal direction. For LPUV exposed PI film, the direction parallel to the polarization direction of LPUV is rougher than the orthogonal direction. The optical retardation measurements on these films suggest that the polymer chain reorientation/depletion is responsible for the observed anisotropy and hence the LC alignment direction.; A Gaussian function has been used to approximate the azimuthal distribution of PI chains on the rubbed PI film. The width of the distribution function has been determined from the rotation of the easy axis of the rubbed PI film induced by exposure to LPUV using the optical retardation measurements. The azimuthal anchoring energy on the rubbed PI film is found to depend inversely upon the distribution width. (Abstract shortened by UMI.)...