Phase Identification in a Series of Liquid Crystalline TPP Polyethers and Copolyethers Having Highly Ordered Mesophase Structure. 3. Thin Film Surface-Induced Ordering Structure and Morphology in TPP(n = 7)

Abstract

A series of liquid crystalline polyethers has been synthesized from 1-(4-hydroxy-4'-biphenylyl)-2-(4-hydroxyphenyl)propane and α,ω-dibromoalkanes [TPP(n)]. TPP(n)s show multiple phase transitions during cooling and heating. In TPP(n = 7) bulk and fiber samples, three liquid crystalline phases have been identified: nematic phase and highly ordered smectic F and smectic crystal G phases. The detailed structures and morphology of TPP(n = 7) thin films (with a thickness ranging from 10 to 100 nm) have been studied by electron diffraction and transmission electron microscopy experiments on three different types of substrates. These include silane-grafted, amorphous carbon-coated, and clean glass surfaces. The development of homeotropic molecular alignment in monodomains has been obtained by using substrates with silane-grafted and amorphous carbon-coated surfaces. Both surfaces can also induce structural ordering in TPP(n = 7) to form an orthorhombic lateral packing which does not exist in the bulk and fiber samples and has only appeared in TPP(n ≥ 11). This phase has been identified as a smectic crystal H phase. It has been found that the monodomain morphology of the highly ordered smectic crystal phases with the homeotropic molecular alignment depends strongly on the structural symmetry. Furthermore, the silane-grafted surface produces better homeotropic molecular alignment for TPP(n = 7) than the amorphous carbon-coated surface, with the latter demonstrating a continuous change in the chain director between −30° and +30°. This change in the orientation results in observable striations in the monodomain morphology. The origin of this variation in the chain director may be associated with the fact that the vector normal to the layer of the smectic crystal phases is tilted about 32° away from the molecular direction (58° between the molecular direction and the layer surface). The clean glass surface does not induce orthorhombic packing and only polydomain structures can be found in which an in-plane homogeneous alignment of the chain directors exists. On the other hand, mechanically sheared thin films on glass surfaces show a uniaxial homogeneous molecular alignment.

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