A Comparative Study of Elastic Constants and Eigenvalues of Stiffness Matrices among Liquid Crystalline Compounds 4,4'-di-3-alkyl azoxybenzene and 4,4'-di-4-alkyl azoxybenzene

This study investigates the elastic stiffness constants, elastic moduli and eigen values of the ‎stiffness matrix of nematic liquid crystalline compounds 4,4'-di-3-alkyl azoxybenzene and ‎‎4,4'-di-4-alkyl azoxybenzene using two different computational processes, viz.,General ‎Utility Lattice Program (GULP) and the ELATE Program from reported experimental data ‎of cell parameters. The study also attempts to predict the liquid crystal structure in terms of ‎elastic constants. Elastic constats determine the thermal stability, stiffness and degree of ‎molecular order, which determines the range of temperature of the liquid crystalline phase. ‎This study included the comparison of different physical parameters of the above ‎compounds and also made an attempt to understand the reason why 4,4'-di-3-alkyl ‎azoxybenzene exhibits the mesophase for a wider range of temperature(42°C)than smaller ‎mesophase range of temperature(9°C)in 4,4'-di-4-alkyl azoxybenzene. The two ‎dimensional representation of the variation of elastic moduli of the above compounds, ‎which explore the stability of crystals are also discussed. The study revealed that the elastic ‎stiffness constants, elastic moduli, anisotropy of elastic moduli and eigen values of the ‎stiffness matrix of liquid crystalline compound 4,4'-di-3-alkyl azoxybenzene are more than ‎those of 4,4'-di-4-alkyl azoxybenzene. The two dimensional variation of Young’s modulus, ‎shear modulus and Poisson’s ratio in xy-plane, xz-plane and yz-plane is more in 4,4'-di-3-‎alkyl azoxybenzene compared tothat in 4,4'-di-4-alkyl azoxybenzene. Interestingly, the ‎linear compressibility of compound 4,4'-di-4-alkyl azoxybenzene in xy-plane ,xz-plane and ‎yz-plane is more compared to linear compressibility of compound 4,4'-di-3-alkyl ‎azoxybenzene. The higher value of elastic moduli and eigenvalues of the stiffness matrix in ‎‎4,4'-di-3-alkyl azoxybenzene indicates that the intermolecular force is stronger and resists ‎reorientation more, which may stabilize the nematic phase for a wider range of ‎temperature(temperature range of 42°C) compared to that in 4,4'-di-4-alkyl azoxybenzene ‎‎(temperature range of 9°C). Poisson’s ratio of 4,4'-di-3-alkyl azoxybenzene is negative, ‎which specifically for display devices, can contribute to improved flexibility, ‎responsiveness and better display.