Alignment of Nematic and Smectic A Liquid Crystals on Carbon, Organic and Inorganic Fibers

Abstract

The spontaneous alignment of nematic and smectic A liquid crystals on oriented and unoriented fibers has been investigated. The alignment of two nematic liquid crystals, 4-methoxybenzylidene-4′-n-bu-tylaniline (MBBA) and 4′-pentyl-4-biphenylcarbonitrile (5CB), and a commercial smectic A liquid crystal mixture, BN/75A, on a series of carbon, organic and inorganic fibers has been characterized by X-ray diffraction. We have discovered that all of the liquid crystals aligned uniformly parallel to the axes of oriented carbon and organic fibers (coaxial alignment), but were unaligned on the unoriented inorganic fibers. We hypothesize that the uniform alignment of the liquid crystals resulted from the influence of oriented microscopic grooves and/or oriented nanoscale features such as polymer crystallites or carbon fiber basal plane edges. All of the oriented fibers except Kevlar 49 have both surface grooves visible at 5000X and a high degree of molecular orientation. The alignment of the liquid crystals on Kevlar 49 may be caused by very small oriented fissures or voids in the skin of the fiber that are not visible at 5000X.

We suggest a route to composites with oriented polymer matrices by the in situ (in the presence of fibers) polymerization of uniformly aligned liquid crystalline monomers. The resulting composites should have superior mechanical properties relative to composites with unoriented matrices, particularly if transverse (perpendicular or tangential) alignment of the polymer backbones relative to the fiber axes can be achieved.