Liquid-crystalline Composite Materials Research Articles

In situ polarized micro-Raman investigation of periodic structures realized in liquid-crystalline composite materials

In situ polarized micro-Raman Spectroscopy has been utilized to determine the liquid crystal configuration inside a periodic liquid crystalline composite structure made of polymer slices alternated to films of liquid crystal. Liquid crystal, Norland Optical Adhesive (NOA-61) monomer and its polymerized form have been investigated separately. The main Raman features, used as markers for the molecular orientation estimation, have been identified. In situ polarized Raman spectra indicate that the orientation of the liquid crystal director inside the structure is perpendicular to its polymeric slices. Results show the usefulness of in situ polarized micro-Raman spectroscopy to investigate liquid crystalline composite structures.

Dual-mode control of light by two-dimensional periodic structures realized in liquid-crystalline composite materials

We report on the realization of a 2D refractive structure consisting of a polymer-liquid-crystal polymer slice grid. Nematic liquid crystal microdomains are confined inside well-sculptured elliptical cavities; experimental investigation shows that the liquid crystal director lies in the plane of the structure and its orientation follows a preferred direction. The sample exhibits both an electro-optical and an all-optical response owing to a small percentage of photosensitive azo dye included in the structure. A double external control of the two-dimensional grating efficiency can indeed be operated either by an optical pump beam or by the standard technique of applying a suitable external voltage to the sample.

POLICRYPS: a liquid crystal composed nano/microstructure with a wide range of optical and electro-optical applications

POLICRYPS (an acronym of polymer liquid crystal polymer slices) is a nano/microcomposite structure made of slices of almost pure polymer alternated with films of well aligned nematic liquid crystal (NLC). The structure is obtained by irradiating a homogeneous syrup of NLC, monomer and curing agent molecules with an interference pattern of UV/visible light under suitable experimental and geometrical conditions; the spatial periodicity can be easily varied from an almost nanometric (200 nm) to a micrometric (15 µm) scale. Where the effect on an impinging reading light beam is concerned, the POLICRYPS can be utilized either in a transmission or a reflection configuration (depending on the geometry and substrate used) with negligible scattering losses, while the effect of spatial modulation of the refractive index (from polymer to NLC values) can be switched on and off by applying an external electric field of the order of few V µm−1. In this paper, we start by reviewing the general features of the POLICRYPS structure, that is the 'recipe' to fabricate it, along with a chemical–diffusive model that indicates the right physical and chemical conditions to make samples exhibiting good morphological, optical and electro-optical properties. We then show some possible utilizations of POLICRYPS with a light beam impinging almost perpendicularly to the structure: a switchable diffraction grating and a switchable optical phase modulator. Furthermore, we put into evidence that POLICRYPS channels can become an array of mirrorless optical micro-resonators for lasing effects. Finally, we report about utilization of POLICRYPS with a light beam impinging parallel to the structure and perpendicular to the channels, demonstrating that, in this case, the structure becomes a tuneable Bragg filter. Performances exhibited in all above applications put the POLICRYPS structure at the top level of the state of art of application oriented research in optics of liquid crystalline composite materials.

Characterization of an active control system for holographic setup stabilization

We report on the realization and characterization of an active control system of the optical holographic setup used for fabrication of holographic gratings in liquid-crystalline composite materials. The system exploits a reference diffraction grating and a piezomirror in closed-loop feedback. The piezoelectric mirror exhibits a hysteresis that depends not only on the applied voltage, but also on the history of the mirror motion. In an open-loop configuration, the hysteresis can be reduced by adjusting the delay time between the application of two different control voltage values; in a closed-loop operation, it is possible to eliminate the residual hysteresis. By testing the system in different conditions, it has been shown that residual fluctuations are comparable to the resolution of the piezomirror operation.

Kinetic structures in multi-phase liquid-crystalline composite materials

A mesoscopic kinetic model for phase separation in the presence of liquid crystalline order has been formulated and solved using high performance numerical methods. The thermodynamic phase diagram on temperature–polymer concentration plane indicates the presence of coexistence regions between isotropic and liquid crystalline phases. These regions are partitioned by the phase-separation spinodal and the phase-ordering spinodal. We characterize the morphologies following temperature quenches in the phase diagram. The scenario is completely different from isotropic mixing since the continuous phase exhibits liquid crystalline ordering. Microdomains of the dispersed phase induce long- and short-range forces affecting the kinetics of the phase separation and the emerging structures. Presence of topological defects and elastic distortions around the microdomains formed during the phase separation dominate the morphology. The free energy of the system establishes dynamics and correlations of the morphological structures.

Photorefractivity in polymer dissolved liquid crystal composites composed of low-molecular-weight nematic liquid crystals and copolymer comprising mesogenic side groups

This paper describes the photorefractive effect of the liquid crystalline composite materials comprising low-molecular-weight nematic liquid crystals (L-LC), copolymer with mesogenic side groups and a small amount of photosensitizer. Copolymers with four kinds of mesogenic side groups were investigated with respect to the compatibility between the L-LC and the copolymer and the photorefractivity of the composites. Two of them could be miscible with L-LC and the composite exhibited LC phase even though the copolymer itself did not show a LC phase. High gain coefficients (> cmK1) under the low applied dc electric field (<1V/mm) was observed when the composites showed a mesophase without any macroscopic phase separation.

Characterization of the diffraction efficiency of new holographic gratings with a nematic film–polymer-slice sequence structure

We have performed a first characterization of the diffraction efficiency of gratings written in liquid-crystalline composite materials by the interference pattern of two curing beams. The grating fringes consist of polymer slices separated by films of continuous nematic phase. The dependence of the diffraction efficiency on temperature reveals a nonmonotonic behavior, with several maxima and minima. The shapes of curves are dependent on slight changes in the initial concentration of the nematic component of the mixture; the number of extrema increases with an increase of this concentration. The dependence of the diffraction efficiency on an applied external voltage also appears to be nonmonotonic: The shape depends on the sample’s temperature. Both switch-on and switch-off responses have been observed. The behavior of our gratings can be explained in the framework of the conventional Kogelnik theory for the diffraction efficiency of Bragg gratings.

Model for the photoinduced formation of diffraction gratings in liquid-crystalline composite materials

We modelize the photoinduced formation of switchable diffraction gratings in polymer-based liquid-crystalline composite materials. The model assumes that redistribution of molecules is due to mass diffusion and incorporates a realistic kinetic description of polymerization processes. Numerical simulations predict two different kinds of structure that have already been experimentally observed. Two parameters governing the phenomenon are pointed out, which determine the structure that will be formed after the curing process has been completed.

Efficiency dynamics of diffraction gratings recorded in liquid crystalline composite materials by a UV interference pattern

We have carried out an experimental and theoretical study of the efficiency of diffraction gratings recorded in samples of polymer dispersed liquid crystal pre-syrups by an interference pattern of curing UV light. The theoretical model takes into account the diffusion of excited monomers during the UV curing and the circumstance that the diffusion coefficient is strongly affected by the polymerisation process. The theoretical curves perfectly fit the experimental results and confirm the validity of our model.

High Photorefractive Gain in Nematic Liquid Crystals Doped with Electron Donor and Acceptor Molecules

Liquid crystalline composite materials have been prepared that are strongly photorefractive. Nematic liquid crystals were doped with both electron donor and electron acceptor molecules that undergo facile, photoinduced, electron transfer reactions to yield mobile ions. A photorefractive gain ratio of 1.88 was observed. This gain ratio was achieved with low applied electric fields (0.4 kilovolts per centimeter) requiring only a 1.5-volt battery and low light intensities (100 milliwatts per square centimeter) in samples 37 to 88 micrometers thick that showed no loss in gain over a 6-month period.