A. R. Kmetz

Smetic and nematic liquid-crystal materials can be homogeneously aligned by buffed thin films of appropriate polymers. We propose that the buffing process orients the polymer’s molecular chains in a manner similar to cold drawing of bulk polymer samples. Experimental verification of this theory is obtained by measuring buffing-induced birefringence in thin films of various polymers coated on glass. Further experiments establish that the oriented state of the polymer chains, and not scratching or grooving of the surface, is necessary to produce alignment. Alignment is found to occur when the polymer is both oriented and crystalline. A picture of alignment is presented in which the formation of a liquid-crystal phase on the crystalline,oriented polymer surface is analogous to the epitaxial growth of conventional solid crystals.

Abstract Chatelain hypothesized that orientation of liquid crystals by interaction with the substrate is due to an adsorbed layer of fatty contaminants, but substrate topography has also been postulated as a causal factor. Experiments with chemically cleaned surfaces rubbed without contamination now confirm the importance of the impurity layer, which has been investigated using Auger spectrometry. Through the use of known surfactans, intentionally deformed substrates, and electron microscopy, the roles of surface topography and surface chemistry have been distinguished: substrates with surface energy lower than the liquid crystal surface tension cause homeotropic alignment; otherwise alignment is parallel to the substrate plane, following any long-range order, e.g., grooves, present on the surface.

The analysis of Rapini and Papoular for homeotropic and homogeneous geometries is extended to predict the effects of weak boundary coupling on liquid-crystal display performance. An implicit expression for deformation as a function of applied field is derived for the case of equal elastic constants. Steeper transitions with complete saturation at low fields are found, which promise improved optical performance and multiplexing capability. A similar analysis of twisted nematic displays predicts a less pronounced improvement in electro-optic response and indicates a minimum boundary-coupling strength below which spontaneous deformation occurs.

A new general analysis of the addressing of an rms-responding matrix for uniform display of unrestricted patterns reveals the ultimate performance limit for any conceivable driving waveforms. The Alt and Pleshko limit for multiplexing can be exceeded whenever the number of matrix rows is not the square of an integer by appropriate choice of more general waveforms. A practical scheme for a two-line matrix yields a 24-percent improvement in voltage selection ratio over multiplexed addressing.