Anchoring and electro-optical dynamics of thin liquid crystalline films in a polyimide cell: Experiment and theory

Abstract

The surface-dependent anchoring and electro-optical (EO) dynamics of thin liquid crystalline films have been examined using Fourier transform infrared spectroscopy. A simple nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB), is confined as 40, 50, and 390 nm thick films in nanocavities defined by gold interdigitated electrode arrays (IDEAs) patterned on polyimide-coated zinc selenide (ZnSe) substrates [Noble et al., J. Am. Chem. Soc. 124, 15020 (2002)]. New strategies for controlling the anchoring interactions and EO dynamics are explored based on coating a ZnSe surface with an organic polyimide layer in order to both planarize the substrate and induce a planar alignment of the liquid crystalline film. The polyimide layer can be further treated so as to induce a strong alignment of the nematic director along a direction parallel to the electrode digits of the IDEA. Step-scan time-resolved spectroscopy measurements were made to determine the rate constants for the electric-field-induced orientation and thermal relaxation of the 5CB films. In an alternate set of experiments, uncoated ZnSe substrates were polished unidirectionally to produce a grooved surface presenting nanometer-scale corrugations. The dynamical rate constants measured for several nanoscale film thicknesses and equilibrium organizations of the director in these planar alignments show marked sensitivities. The orientation rates are found to vary strongly with both the magnitude of the applied potential and the initial anisotropy of the alignment of the director within the IDEA. The relaxation rates do not vary in this same way. The marked variations seen in EO dynamics can be accounted for by a simple coarse-grained dynamical model.

Effects of Temperature on the Alignment and Electrooptical Responses of a Nematic Nanoscale Liquid Crystalline Film

“…The softening of the elastic constants at higher temperatures leads to predictably faster electric-field-induced orientation rates; the relaxation rates, however, do not vary markedly with temperature. These striking experimental results are interpreted and accounted for semiquantitatively using a theoretical framework presented in our previous report …”

“…The liquid crystal EO cell employed and its means of fabrication have been described in detail in earlier reports. ,− The cell used in this study adopts the same design rules as described there. The ZnSe windows were polished to produce a nanometer-scale, grooved morphology that serves to align the LC director.…”

“…These long-period corrugations were generated by polishing the substrate crystals using motions restricted to within a few degrees along one primary axis. Representative atomic force microscopy (AFM) micrographs of the polished surfaces are given in the earlier report …”

“…In a previous report, we examined the surface-dependent anchoring and electrooptical (EO) dynamics of thin liquid crystalline films using Fourier transform infrared (FTIR) spectroscopy and step-scan time-resolved spectroscopy (TRS). The EO dynamics were shown to be very sensitive to variations in the surface morphologies, and an assessment of the underlying mechanisms involved in the EO dynamic variations was provided by a coarse-grained dynamical model.…”

Nitrile Vibrations as Reporters of Field-Induced Phase Transitions in 4-Cyano-4′-pentylbiphenyl (5CB)

“…These previous studies have shown the nitrile group serves as an excellent indicator of the molecular orientation within the applied field. Lee et al showed a 180° change in field direction could be probed with the nitrile peak intensity as it changed along with molecular alignment in the field. , …”