Polymer as an important component of blends and composites with liquid crystals

The ability to control light direction with tailored precision via facile means is long-desired in science and industry. With the advances in optics, a periodic structure called diffraction grating gains prominence and renders a more flexible control over light propagation when compared to prisms. Today, diffraction gratings are common components in wavelength division multiplexing devices, monochromators, lasers, spectrometers, media storage, beam steering, and many other applications. Next-generation optical devices, however, demand nonmechanical, full and remote control, besides generating higher than 1D diffraction patterns with as few optical elements as possible. Liquid crystals (LCs) are great candidates for light control since they can form various patterns under different stimuli, including periodic structures capable of behaving as diffraction gratings. The characteristics of such gratings depend on several physical properties of the LCs such as film thickness, periodicity, and molecular orientation, all resulting from the internal constraints of the sample, and all of these are easily controllable. In this review, the authors summarize the research and development on stimuli-controllable diffraction gratings and beam steering using LCs as the active optical materials. Dynamic gratings fabricated by applying external field forces or surface treatments and made of chiral and nonchiral LCs with and without polymer networks are described. LC gratings capable of switching under external stimuli such as light, electric and magnetic fields, heat, and chemical composition are discussed. The focus is on the materials, designs, applications, and future prospects of diffraction gratings using LC materials as active layers.

Dynamic Control of Light Direction Enabled by Stimuli-Responsive Liquid Crystal Gratings.

We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm2, the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E ∼ 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic self-interaction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.

https://www.pnas.org/content/pnas/117/25/14021.full.pdf

First-principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics.

By combining polymer-dispersed liquid crystal (PDLC) and holography, holographic PDLC (H-PDLC) has emerged as a new composite material for switchable or tunable optical devices. Generally, H-PDLC structures are created in a liquid crystal cell filled with polymer-dispersed liquid crystal materials by recording the interference pattern generated by two or more coherent laser beams which is a fast and single-step fabrication. With a relatively ideal phase separation between liquid crystals and polymers, periodic refractive index profile is formed in the cell and thus light can be diffracted. Under a suitable electric field, the light diffraction behavior disappears due to the index matching between liquid crystals and polymers. H-PDLCs show a fast switching time due to the small size of the liquid crystal droplets. So far, H-PDLCs have been applied in many promising applications in photonics, such as flat panel displays, switchable gratings, switchable lasers, switchable microlenses, and switchable photonic crystals. In this paper, we review the current state-of-the-art of H-PDLCs including the materials used to date, the grating formation dynamics and simulations, the optimization of electro-optical properties, the photonic applications, and the issues existed in H-PDLCs.

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Holographic Polymer-Dispersed Liquid Crystals: Materials, Formation, and Applications

Transmission gratings were formed holographically in polymer dispersed liquid crystal (HPDLC) composite materials using commercially available UV-curable acrylate monomers and the nematic liquid crystal TL203. Monomer mixtures with average functionality ranging from 1.3 to 3.5 were used to prepare HPDLC transmission gratings. The phase-separated morphology and surface topology of the transmission gratings as observed through scanning electron microscopy and atomic force microscopy varied profoundly with the different monomer mixtures characterized by their average functionalities. The switching properties of the HPDLC gratings were correlated with the morphological changes.

Effect of Monomer Functionality on the Morphology and Performance of the Holographic Transmission Gratings Recorded on Polymer Dispersed Liquid Crystals

Ferroelectric ordering in liquids is a fundamental question of physics. Here, we show that ferroelectric ordering of the molecules causes the formation of recently reported splay nematic liquid-crystalline phase. As shown by dielectric spectroscopy, the transition between the uniaxial and the splay nematic phase has the characteristics of a ferroelectric phase transition, which drives an orientational ferroelastic transition via flexoelectric coupling. The polarity of the splay phase was proven by second harmonic generation imaging, which additionally allowed for determination of the splay modulation period to be of the order of 5-10 microns, also confirmed by polarized optical microscopy. The observations can be quantitatively described by a Landau-de Gennes type of macroscopic theory.

Ferroelectric-Ferroelastic Phase Transition in a Nematic Liquid Crystal.

We investigate physical properties of polymer dispersed liquid crystal transmission gratings which influence the light diffraction efficiency behavior. Scanning electron microscopy is used to determine the orientation and shape of nanometer-sized liquid crystal droplets in a polymer host. Dependence of diffraction efficiency on light polarization suggests that the orientation of a liquid crystal within the ellipsoidal droplets is axial. The distribution of droplet orientations appears to vary for different grating spacings. The switching properties of the gratings are investigated and analyzed with a sinusoidal dielectric tensor modulation model. The analysis also takes into account orientational distribution of average directors and spatial distribution of the droplets. Temperature dependence of diffraction efficiency is tentatively explained by a structural transition within the droplets and the presence of impurities in droplets.

Characterization of holographic polymer dispersed liquid crystal transmission gratings

We report on the investigations of electrically switchable polymer dispersed liquid crystal gratings. Diffraction properties, grating anisotropy, and temperature dependencies were analyzed to obtain information on the liquid crystal orientation inside the ellipsoidal droplets and the distribution of the droplets in a polymer host. We compare the samples of different compositions and grating spacings.

Electro-Optical Properties of Polymer Dispersed Liquid Crystal Transmission Gratings

Surface structure of Langmuir–Blodgett films of lipophilic guanosine derivatives

The dynamical behavior of deoxyguanosine 5'-monophosphate in the pretransitional region of the I-Ch phase transition was investigated by dynamic light scattering. Measurements reveal the presence of two diffusive modes: the slower mode is due to the motion of large globular aggregates while the faster mode is related to the motion of the self-assembled d(pG) columnar stacks. The behavior of the fast DLS mode is explained by the coupled mode theory, which relates the polyion motion with the dynamics of other ion species in the solution. The addition of 0.5 M KCl strongly promotes the self-assembly process resulting in a shift of the phase diagram towards lower d(pG) concentrations, while the properties of the two diffusive modes remain practically the same.

Irena Drevenšek Olenik

Papers

Ferroelectric-Ferroelastic Phase Transition in a Nematic Liquid Crystal.

Characterization of holographic polymer dispersed liquid crystal transmission gratings

Electro-Optical Properties of Polymer Dispersed Liquid Crystal Transmission Gratings

Surface structure of Langmuir–Blodgett films of lipophilic guanosine derivatives

Effect of Added Ions on the Self-Assembly of Guanosine