Growth of a single-domain smectic phase in a thin liquid-crystalline polymer film

The ordering process and kinetics in thin films (200-800-nm thick) of a thermotropic side-chain liquid-crystalline polymer have been investigated vertically and laterally, respectively, by x-ray reflectivity and atomic-force microscopy. The original smooth and amorphous spin-coated films initially become corrugated upon annealing in the smectic mesophase. The roughening of the surface results from the formation of randomly oriented microcrystalline domains in the film. At the same time, however, a laterally macroscopic crystal starts to grow from the substrate surface in the direction of the polymer-air interface at the expense of these domain structures. Finally, a nicely ordered single crystal with parallel-ordered bilayers is formed in the film as well as at the polymer-air interface. This one-dimensional crystallization, actually recrystallization, depends strongly on the temperature due to viscosity effects. At low temperatures, just above the glass-transition temperature, the ordering is very slow, but with increasing temperature the crystal growth is faster. An Arrhenius-type plot gives an activation energy of 122 kJ/mol, which we ascribe to the expected reorientations of the mesogenic groups during the recrystallization process.