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Abstract The temperature dependence of optical birefringence, order parameter, effective geometry parameter, and normalized polarizability was determined for nematic liquid crystal mixtures by measuring the refractive indices. The effects of the adjustable parameters obtained from temperature-dependent refractive indices were analyzed appropriately for the crossover temperature and order parameter values. Besides, a four-parameter power law expression method was used for the optical birefringence measurement to address inappropriate fits in the weak first-order phase transition region. From this method, the effective second order phase transition temperature T ⁎⁎ was determined by showing the slightly higher values than phase transition temperature. The value of the order parameter critical exponent λ obtained from this method showed a good agreement with the mean field theory by demonstrating closeness to the tricritical hypothesis (β = 0.25). As a consequence, the measurement of the temperature-dependent optical birefringence was successfully verified the pre-transitional phenomena accurately, and it can facilitate an insightful description of that region. Graphical abstract Unlabelled Image Highlights • Temperature dependence of high birefringence (∆ n) liquid crystals was analyzed. • The temperature dependent order parameter was sorted based on ∆ n values. • Lower (∆ n) o and β values were valuable factors for increasing T co at the same T NI. • A four-parameter power law expression shows the high critical exponent value. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, polyimide surfaces irradiated by an ion-beam for liquid crystal alignment are investigated by using atomic force microscopy, Raman spectroscopy, and spectroscopic ellipsometry. A liquid crystal cell aligned homogeneously through the ion-beam exposure exhibits electro-optic switching behavior similar to that of a rubbing-aligned liquid crystal cell. However, we found that the surface morphology and bonding molecules of ion-beam-treated polyimide surfaces show properties very different from mechanically-rubbed ones. Experimental results show that optical anisotropy of ion-beam-treated polyimide surfaces results in the formation of hydrogenated amorphous carbon-like structure with a short main-chain, while mechanical rubbing has little effect on structural and compositional variations of polyimide layers. [Copyright &y& Elsevier]