Effect of ZnO nanoparticles dispersed in liquid crystalline p-n-propoxy/propyl benzoic acids and mixtures – optical studies

Abstract

In the present work, characterization studies are carried out on liquid crystalline (LC) p-n-propoxy benzoic acid (3oba), p-n-propyl benzoic acid (3ba) and the mixture of these two benzoic of acids dispersed with low weight concentrations of ZnO nanoparticles (nps). The existence of ZnO nps in above LC compounds are studied by scanning electron microscope and X-ray diffraction techniques. The LC phase transition temperatures of the compounds with the dispersion of ZnO nps are measured using polarizing microscope (POM) and differential scanning calorimeter (DSC). The nematic phase transition temperatures of 3ba, 3oba and mixture of 3ba and 3oba obtained from POM and DSC found to decrease with increase in the dispersion of various weight concentrations of ZnO nps. The birefringence studies are carried out for these compounds at different wavelengths (400, 500, 570, and 635 nm) with the dispersion of ZnO nps with increasing in weight concentrations (1, 1.5, 2 and 2.5 wt%) using modified spectrometer and the orientational order parameter ‘S’ of corresponding LC nanocomposites is measured from different optical methods viz., Kuczynski internal field model, Vuks isotropic model and effective geometry model . Among the three models the order parameter found to increase due to the strong alignment of LC molecules with the ZnO nps. The refractive indices measured at different wavelengths are fitted with two and three coefficient Cauchy’s model. For high birefringence LCs three coefficient Cauchy’s model is favorable than two coefficient model and vice versa. The results obtained are compared and discussed.