Influence of Substrate Surface Properties on Spin Dewetting, Texture, and Phase Transitions of 5CB Liquid-Crystal...

We report how the texture and stability of a nematic liquid-crystal (LC) thin film of 5CB vary as a function of UV-ozone (UVO) exposure of the underlying poly(methyl methacrylate) (PMMA) substrate. UVO exposure of the PMMA substrate not only increases its surface energy, making it more wettable, but also results in the generation of oxygen-containing polar functional groups on the PMMA surface due to photolysis of ester. While the stability of the 5CB films is expectedly enhanced on UVO-exposed PMMA substrates against thermally induced dewetting, the texture of the film also changes as a function of the UV exposure time (t_E). We show that the films continue to exhibit the nematic Schlieren texture for t_E ≤ 20 min, although the disclination point density (|m|) gradually reduces with an increase in t_E. However, the texture changes completely to a spherulite or fanlike texture in t_E ≥ 20 min due to enhanced anchoring of the 5CB molecules on the substrates. In addition, enhanced wettability and stronger anchoring by the UVO-exposed PMMA substrates also suppress the tendency of spin dewetting of the 5CB films due to spontaneous rupture of the dispensed solution layer during spin coating, particularly when the solute concentration (C_n) is very low. The latter observation allows possible creation of thinner LC films, which are otherwise difficult to form by spin coating due to enhanced cohesive interactions between the anisotropic LC molecules. Finally, we show that in continuous films, the nematic-to-isotropic (N → I) and I → N phase transitions with gradual heating and cooling remain completely reversible, irrespective of the texture of the film and wettability of the substrate.