Study of Birefringence and Dielectric Properties and Determination of Dipole Moment of the Mesogen: 4-Cyano-4′-n-Octylbiphenyl as a Function of Temperature

Abstract

Optical and dielectric studies have been performed on one of the members of mesogenic series 4-cyano-4′-n-alkyl biphenyl. The structural formula of the mesogen 4-cyano-4′-n-octyl biphenyl (code name K24) is given below:

Texture studies have been conducted on the above mesogen to confirm the transition temperatures and the various mesophases. Optical studies using He–Ne laser beam have been performed and refractive indices have been determined as a function of temperature by the application of Chatelain Wedge principle. From this principle, polarizability and orientational order parameter have been determined. The experimentally determined orientational order parameter is compared with theoretical Maier–Saupe values. To study the dielectric property, we have determined the parallel and perpendicular components of dielectric permittivities, i.e., ϵ_|| and ϵ_⊥ in the presence of magnetic field, and from dielectric anisotropy, polarizability, and order parameter, we have calculated effective dipole moment μ_eff and angle of inclination β of the axis of molecule with the director direction as a function of temperature. Dielectric studies of the sample have also been performed in the solution at various concentrations, and by extrapolating the values at infinite dilution, we have obtained isolated molecular dipole moment μ_iso. We have compared the values of μ_eff and μ_iso and have calculated correlation factor “g” that throws a light on nature of bonding of the molecules.

Keywords:

• Birefringence
• dielectric constant
• dipole moment
• mesogen
• order parameter
• PACS No. 61.30

Acknowledgments

We are thankful to Merck Ltd. for kindly supplying the sample. Thanks are due to Professor P. Nandy, Department of Physics, Jadavpur University, for allowing us to use her laboratory for texture studies. We thankfully acknowledge a University Grants Commission (UGC) Research Project Grant. S. Chakraborty is thankful for the UGC Research Associate fellowship under the above project.