Generation of Light Scattering States in Cholesteric Liquid Crystals by Optically Controlled Boundary Conditions

Circularly polarized light was previously employed to stimulate the reversible and reconfigurable writing of scattering states in cholesteric liquid crystal (CLC) cells constructed with a photosensitive layer. Such dynamic photodriven responses have utility in remotely triggering changes in optical constructs responsive to optical stimulus and applications where complex spatial patterning is required. Writing of scattering regions required the handedness of incoming radiation to match the handedness of the CLC and the reflection bandwidth of the CLC to envelop the wavelength of the incoming radiation. In this paper, the mechanism of transforming the CLC into a light scattering state via the influence of light on the photosensitive alignment layer is detailed. Specifically, the effects of: (i) the polarization state of light on the photosensitive alignment layer; (ii) the exposure time; and (iii) the incidence angle of radiation on domain formation are reported. The photogenerated light-scattering domains are shown to be similar in appearance between crossed polarizers to a defect structure that occurs at a CLC/air interface (i.e., a free CLC surface). This observation provides strong indication that exposure of the photosensitive alignment layer to the circularly polarized light of appropriate wavelength and handedness generates an out-of-plane orientation leading to a periodic distortion of the original planar structure

Transport Controlled Pattern Photopolymerization in a Single-Component System

The purpose of the present paper is to extend the concept of pattern photopolymerization-induced phase separation of a binary blend to a single-component system containing pure photoreactive monomers with a minute level of photoinitiators. The patterning formation process was simulated in the framework of Cahn−Hilliard equation coupled with the photoreaction kinetic equation. Unlike binary systems undergoing photopolymerization-induced phase separation driven by thermodynamic force, the mechanism of pattern formation in the single-component system is essentially a photoreaction-induced transport phenomenon. Of particular interest is the observation of polymer concentration profiles evolving from a sinusoidal wave to various truncated ones. On the basis of two-wave interference optics, the microchannel layers have been fabricated. Resultant morphology was characterized using optical and atomic force microscopes. A two-dimensional light scattering device was utilized to study the diffraction patterns from the fabricated microchannel layers. The observed concentration profiles were compared with the theoretical predictions, and good agreement was found

Holographic photo-polymerization induced phase separation in reference to the phase diagram of a mixture of photo-curable monomer and nematic liquid crystal

Theoretical investigation on pattern photopolymerization has been undertaken in various coexistence regions of a mixture of photocurable monomer and nematic liquid crystal. The phase diagram of the mixtures is constructed in accordance with a combined Flory−Huggins (FH) free energy of isotropic mixing and the Maier−Saupe (MS) free energy of nematic ordering. The pattern formation dynamics of holographic photopolymerization has been explored in conjunction with the phase diagram of the starting mixture of monomer and nematic liquid crystal. To mimic the evolution of holographic structure, the combined FH and MS free energy is incorporated into the time-dependent Ginzburg−Landau (model C) equations coupled with the photopolymerization kinetic equation. The morphology evolution and mechanisms have been investigated in relation to the situations whether the system should remain in the single phase after photopolymerization or be thrust into the two-phase region. Of particular significance is that the diffraction efficiency (DE) of gratings, a key property in electrooptical applications, can be monitored in-situ during photopatterning. In the gratings of inhomogeneous structures, scattering occurs due to the interparticle interference, which needs to be accounted for in the evaluation of the DE. A fast Fourier transformation (FFT) technique of the emerging morphological domains of the holographic polymer dispersed liquid crystals uniquely provides the simultaneous determination of the diffraction peaks from the holographic layers as well as scattering from the internal LC domains; thereby, the DE correction is feasible. The simulated patterns and the trends of diffraction efficiency evolution are compared with experimental observations

Surface Limitations to the Electro-Mechanical Tuning Range of Negative Dielectric Anisotropy Cholesteric Liquid Crystals

Recent work on negative dielectric anisotropy cholesteric liquid crystals (CLCs) showed that externally applied dc voltages resulted in blue tuning of the reflection band position up to 20% of its original position. These results also showed that the observed shift in reflection band position was not caused by a direct interaction between the CLC and the applied voltage, but indirectly through electromechanical stresses that deformed the conductive glass substrates, in turn deforming the liquid crystal. In this work, the goal is to clarify that the major limiting factors on the tuning range limit result from the magnitude of the surface anchoring energy and surface induced hysteresis effects. An analytic solution for the tuning range limit and its dependence on the surface and bulk properties is derived that agrees well with the experimental data. Using this model, it was shown that tuning range limits in excess of 35% of the notch position should be expected with typically available alignment materials, and that with proper CLC/surface optimizations, values in the range of 75% are possible.</p

Deuterium NMR and morphology study of polymer-dispersed liquid-crystal Bragg gratings

The orientational order in polymer-dispersed liquid crystals on holographically written volume Bragg gratings is studied using deuterium NMR. Unique spectral patterns indicate that a homeotropic liquid-crystal director configuration at the polymer interface is retained despite the small pore size ($\rm < 100\, nm$). This structure coexists with liquid crystals in trapped defect structures of dimension comparable to the pore size

Kinetics of Photopolymerization-Induced Phase Separation and Morphology Development in Mixtures of a Nematic Liquid Crystal and Multi-Functional Acrylate

Photopolymerization behavior and reaction kinetics for a series of multifunctional acrylate monomer(s) and eutectic liquid crystal blends were investigated with particular emphasis on determination of the reaction rate coefficients for propagation and termination steps of photopolymerization. Reaction rate coefficients were determined via real-time infrared spectroscopy and compared with those obtained by photo-differential scanning calorimetry. Effects of various parameters such as LC concentration, light intensity, and monomer functionality on the kinetics were investigated. Phase transition temperature versus composition phase diagrams were established by means of optical microscopy and differential scanning calorimetry for mixtures of triacrylate/liquid crystal (LC) before photopolymerization and after exposing to ultra violet (UV) irradiation under various reaction times. The snapshot phase diagram of the reacting mixtures exhibited isotropic gel, isotropic liquid + nematic, and narrow pure nematic coexistence regions. These coexistence regions were further confirmed by morphological changes of the polymer dispersed liquid crystal films as functions of temperature and concentration using polarized optical microscopy