Application of Infrared ATR Spectroscopy to Liquid Crystals. V. Orientation of 5CB Molecules in the Boundary Region of a Twisted Nematic Cell and Reorientation by a DC Electric Field

Abstract

Infrared ATR measurements have been performed on a 90[ddot] twisted nematic 4-n-pentyl-4′-cyanobiphenyl (5CB) under an externally applied DC electric field. Analysis of polarized reflection absorbances for the C=N stretching band at 2225 cm-¹ allows the characterization of the anisotropic structure of a boundary layer about 3000 Å thick at the cell electrode surface. For instance, anisotropic absorption coefficients derived from the measured reflection absorbances at zero electric field confirm that molecular alignment at the boundary is substantially uniaxial. On the other hand, it is found from changes in the absorption coefficients with applied electric field that the threshold voltage for prodwing field-induced molecular reorientation is 5 V. The average direction of the cyano dipole with respect to the field direction is also obtained as a function of applied field strength.