Influence of Underlying PMMA Layer Thickness, Thermal Cycling, and Coarsening on Texture and Stability of 5CB (4-Cyano-4′-pentylbiphenyl) Thin Films

We report a host of novel observations related to texture, phase transition, and stability in spin-coated 5CB thin films over poly(methyl methacrylate) (PMMA) substrates. First, we show that the disclination point density (|m|) of the as cast 5CB thin films, which shows schlieren texture over a PMMA layer, depends on the thickness of the PMMA layer (hPMMA) and gradually reduces with increase in hPMMA. We relate this observation to variation in the roughness of the PMMA layer as a function of hPMMA. Subsequently, we show that during thermal phase transition of nematic (N) to isotropic (I) followed by I to N transition in the film, there is a drastic drop in the value of |m|. Interestingly, from the second thermal cycle onward, the value of |m| does not change and N–I and I–N phase transition exhibits well-known reversibility. We argue that both observations are related to the physics of film formation in spin coating. Finally, we show that the value of |m| gradually drops if a 5CB thin film is kept at atmospheric conditions after spin coating. The slow dynamics in the film first leads to joining of some poles followed by spontaneous rupture of the film and transformation of the texture to radial; however, no such textural transformation is observed if the film is subject to one or multiple thermal cycles. Our observations are related to defect development, molecular alignment, and texture evolution in liquid crystal (LC) films, which provides a better understanding in creating defect free LC films.