Phase Identification in a Series of Liquid-Crystalline TPP Polyethers and Copolyethers Having Highly Ordered Mesophase Structures. 4. Phase Structures and Order Evolution in TPP(n = 12) Thin Films

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Abstract

A main-chain liquid-crystalline polyether has been synthesized from 1-(4-hydroxy-4‘-biphenylyl)-2-(4-hydroxyphenyl)propane and 1,12-dibromododecane, TPP(n = 12). In our previous report, the phase structures and phase transitions in bulk TPP(n = 12) have been identified. In this study, thin film samples of TPP(n = 12) have been prepared with thicknesses ranging from 10 to 100 nm on amorphous carbon coated substrates. Transmission electron microscopy and electron diffraction observations show that the liquid-crystalline chain molecules possess a tilted homeotropic orientation within monodomains at a structural formation temperature of 180 °C in the smectic F (SF) phase. This is due to the surface-induced alignment effect. With increasing isothermal time, the lateral size and thickness of the monodomain increase, indicating that the formation of monodomains is initiated by a uniformly oriented molecular aggregate and is conceptually similar to the nucleation-controlled process of polymer crystallization. The growth stage reveals that large-scale molecular motion is required in order to transfer the chain molecules from an isotropic orientation to a tilted homeotropic structure. Electron diffraction results on the TPP(n = 12) thin films confirm the phase structures identified in bulk materials. When the thin film samples are quenched from the isotropic melt to lower structural formation temperatures, the samples no longer show the tilted homeotropic molecular orientation. This indicates that the amorphous carbon surface-induced alignment process is sensitive to the type of phase in which the structural formation occurs. It is not possible to access the tilted homeotropic orientation when the molecules have been in an ordered structure higher than the SF phase. In the smectic crystal G phase, molecular mobility along the chain direction can still be expected, which is evidenced by an observation of significant morphological layer thickening during annealing in this phase. The regularity of morphological layers in the different ordered phases is also investigated.

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