Crossover between Dewetting and Stabilization of Ultrathin Liquid Crystalline Polymer Films

Thin spin-coated films of a smectic liquid crystalline main chain polymer with an azoxybenzene as mesogen were investigated as a function of temperature and thickness. Both lateral and vertical order of the polymer film in the various mesophases were determined by scanning force microscopy in tapping mode and by specular and off-specular X-ray scattering. For films thicker than 10 nm, we observed a stabilizing effect of the smectic organization within the film. No dewetting was detected. The bulk structure of the film consists of smectic layers with a thickness of 3.5 nm, oriented parallel to the substrate. The excess material formed smectic islands on top of the film, corresponding to a noncomplete smectic layer. For films thinner than 10 nm, corresponding to one or two smectic layers only, we observed dewetting with the formation of holes. The holes were dry without any remaining monolayer on the substrate. However, the withdrawing polymer did not accumulate in rounded rims such as for amorphous polymers but in smectic towers, due to the packing of the polymer chain.