Polymer-liquid Crystal Interface Research Articles

Tuning of polymer-wall surface components and its effect on the optoelectronic performance of liquid crystal devices with polymer walls

In this study, surface initiation was used to form dense and flat films in liquid crystal media. As a result, low-surface-energy components are thermodynamically forced to segregate on the liquid crystal-polymer interface already during the polymerization process. This finding is confirmed by ATR and XPS. In addition, more low-surface-energy components accumulate on the wall surface of the polymer upon decreasing the UV light intensity during polymerization. These components reduce both the anchoring energy of the polymer wall as well as the threshold voltage of any liquid crystal device that uses this polymer wall.

Electrohydrodynamic instability: effect of rheological characteristics on the morphological evolution of liquid crystal–polymer interface

Electrohydrodynamic instability, in a polymer–air or polymer–polymer bilayer settings, gives rise to the formation of the ordered micropillars or microwells at the initial planar interface. It is well known that the complex interplay among the controlling parameters, such as the intensity of the electrostatic field, film thickness, interfacial tension and dielectric constants of the layers determine the morphology of the interface. In this report, for the first time, experimentally it is shown that rheological property of the lower polymer layer [here, polydimethylsiloxane (PDMS)] has a significant influence on the morphological evolution. We probe the kinetic time scale of the evolution by inducing fast destabilization of the interface due to the high dielectric contrast between two layers (liquid crystal–PDMS) and reduced interfacial tension. At this time scale, it was demonstrated that micropillars are formed for thin viscoelastic ‘soft’ PDMS film, whereas microwells were observed for viscoelastic ‘hard’ film in similar settings. A transition from micropillar to microwell was observed for viscoelastic ‘soft’ film when the thickness of the film increased from $${\sim } 4$$ to $$11\,\upmu \hbox {m}$$ . Based on this observation, by controlling the rheological properties, different patterns were developed from similar templated PDMS substrates.

Photoalignment at the nematic liquid crystal–polymer interface: The importance of the liquid crystalline molecular structure

The influence of the molecular structure of the nematic liquid crystal (NLC) on the photoalignment process at the NLC–polymer interface has been investigated experimentally. NLCs having exclusively phenyl-, or cyclohexane rings in the rigid core, as well as NLCs containing both phenyl and cyclohexane groups have been considered. Substantial differences have been found in the photoalignment process depending on the molecular structure of the NLC, supporting the assumption that the polymer–NLC interface should be regarded as a coupled system, where the two components mutually influence each other. A phenomenological explanation is given for the observed differences.

Photoalignment at the nematic liquid crystal-polymer interface: Experimental evidence of three-dimensional reorientation

We provide experimental evidence that photoalignment at the nematic liquid crystal (NLC)-polymer interface cannot be simply considered as a two-dimensional process. Moreover, our experiments clearly indicate that the photoaligning process does not depend on the individual properties of the NLC material and those of the interfacing polymer exclusively. The polymer and the NLC layer interact, i.e., the polymer-liquid crystal interface should be regarded as a coupled system, where the two components mutually influence each other. Furthermore, we show that the temperature induced anchoring transition also has to be taken into account for the complete description of the photoalignment mechanism.

Small-angle light scattering symmetry breaking in polymer-dispersed liquid crystal films with inhomogeneous electrically controlled interface anchoring

We have described the method of analyzing and reporting on the results of calculation of the small-angle structure of radiation scattered by a polymer-dispersed liquid crystal film with electrically controlled interfacial anchoring. The method is based on the interference approximation of the wave scattering theory and the hard disk model. Scattering from an individual liquid crystal droplet has been described using the anomalous diffraction approximation extended to the case of droplets with uniform and nonuniform interface anchoring at the droplet–polymer boundary. The director field structure in an individual droplet is determined from the solution of the problem of minimizing the volume density of the free energy. The electrooptical effect of symmetry breaking in the angular distribution of scattered radiation has been analyzed. This effect means that the intensities of radiation scattered within angles +θ s and–θ s relative to the direction of illumination in the scattering plane can be different. The effect is of the interference origin and is associated with asymmetry of the phase shift of the wavefront of an incident wave from individual parts of the droplet, which appears due to asymmetry of the director field structure in the droplet, caused by nonuniform anchoring of liquid crystal molecules with the polymer on its surface. This effect is analyzed in the case of normal illumination of the film depending on the interfacial anchoring at the liquid crystal–polymer interface, the orientation of the optical axes of droplets, their concentration, sizes, anisometry, and polydispersity.

A method for estimating interfacial tension of liquid crystal embedded in polymer matrix forming PDLC

ABSTRACTA simple and convenient method based on sessile drop technique for measuring surface tensions of polymer and nematic liquid crystal (LC) is described. Contact angles formed by drops of probe liquids and a nematic LC on a photocurable polymer were measured. The surface energies were evaluated using the Fowkes method, Neumann's equation, and new equations developed based on Neumann's approach. The values of surface tensions were used to evaluate the interfacial interaction in term of work of adhesion between the LC and polymer. Further, the effect of dichroic dye on the extent of interaction and work of adhesion was examined by measuring contact angle in consequence of dye addition. A difference in work of adhesion between the lower and higher dye‐doped LC droplets gave an indication of affinity relationship between polymer and LC molecules. A change in work of adhesion resulted in variability of nematic director configurations inside phase separated LC droplets embedded in polymer matrix; when viewed under polarizing optical microscope. Thus, our approach of estimating surface energy of polymer and LC has found to be useful in determining interaction at polymer–LC interface. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41137.

Mechanisms leading to fast relaxation of liquid crystal cells aligned with conductive polymers

Using nematic liquid crystal cells aligned by conductive polymers, like polyaniline or doped polypyrrole, a very fast electro-optic response is observed. We show that when a switch that interrupts the voltage across the cell is placed between the cell and the ground, the largest voltage drop is on the switch in its open position. The voltage distribution among the nematic cell and the switch is evaluated. The role played in this very fast electro-optic response of the ionic charges built on the interfaces together with the redox processes among the free charges in the polymer-liquid crystal interface is also described. For an asymmetric cell (only one side covered with a conductive polymer) a rectifying effect appears. In this case a circuital model is used to mimic the steplike behavior of the transmitted light during the relaxation of the system.

Computer Simulations and Experimental Dynamical Characterization of a Composite Liquid Crystal-Polymer System

We present a Monte Carlo study of a liquid crystal-polymer interface, focusing on a single cell with boundary conditions tailored to mimic the main features of the system and we examine the effect of an external applied field. A simple lattice spin model, based on the Lebwohl-Lasher hamiltonian, has been employed to represent the nematic and the polymer molecules. The orientations of the spins representing the low molar mass nematic molecules are updated during the simulations while the polymer chain units are kept frozen. The model allows us to investigate the molecular organization and the ordering expected across the cell. Moreover the response to the change in external field intensity and duration pulse is also studied.

Kinetics of director gliding on a polymer–liquid-crystal interface

Magnetic-field-induced surface reorientation of nematic liquid crystals on polyethyl-methacrylate films is studied. The experiments indicate that the time scale of director gliding expands from a fraction of a second to several hours. A power-law distribution function of relaxation times provides very good agreement between measurements and calculated gliding curves. From the model an upper limit of the extrapolation length can be extracted, which points to a surprisingly strong equilibrium azimuthal anchoring of the liquid crystal on the polymer layer.

Electro-optical light scattering shutter using electrospun cellulose-based nano- and microfibers

Electrospun cellulose-based nano and microfibers and a nematic liquid crystal are used to assemble an electro-optical (EO) light-scattering device that shows enhanced characteristics when compared to similar devices. Based on the controlled scattering of light in the composite system, the device can achieve light transmission coefficients tunable from 1% up to around 89%. Simulation of the EO behavior indicates that the roughness of the polymer-liquid crystal interface is crucial for the optical performance of the device.

Orientational structure transformations caused by the electric-field-induced ionic modification of the interface in nematic droplets

The films of a polymer-dispersed nematic liquid crystal doped with an ionic surfactant were studied. The surface-anchoring modification effect caused by the local increase in the concentration of surface-active ions was observed at the polymer-liquid crystal interface under the action of an electric field. The modification of the boundary conditions leads to the transformation of the orientational structure of the nematic droplets and, as a result, to an appreciable change in their texture patterns and light-scattering efficiency at the interfaces. The monopolar director configurations (normal and curved) arising in the process of orientational structure transformations are considered and their typical textures are demonstrated. The possibility that the monopolar structure can be formed is theoretically substantiated by the computer simulation of the director field in a nematic droplet with the boundary conditions corresponding to the experiment.

Polymer-Based Microelectrode Arrays

ABSTRACTWe have developed flexible, polymer-based electrodes for potential medical applications including neural recording and stimulation. Using various combinations of liquid crystal polymer (LCP) substrates, implantable grade silicone and polyimide, we have developed and tested several prototype multi-layer polymer electrodes. We report here on two specific electrodes. In the first case, a multilayer electrode consisting of high-melt temperature liquid crystal polymer (LCP) material with patterned electrodes of sputter deposited and plated gold, laminated together with a lower-melt temperature LCP was produced. Iridium oxide was deposited on the exposed electrode sites to facilitate effective charge transfer for neural stimulation. The electrode was designed for acute implantation in a cat cochlea and contained 12 contacts, with a pitch of 200 microns. The small contact spacing allowed testing of electric field focusing techniques both in vitro and in vivo. We subjected the electrodes to electrical and mechanical tests to assess potential suitability as a long-term biomedical implant. Chronic electrical leakage testing indicated higher than desired ionic permeability of the low and high temperature LCP interface. In a second case we produced a mock circuit using high-melt LCP and medical grade low durometer silicone in place of the low-melt LCP as the interlayer adhesive. Mechanical and electrical testing of the hybrid design indicated the potential to fabricate cochlear electrodes containing up to 72 contacts with a footprint and mechanical performance similar or better than current commercially available cochlear implant arrays (containing up to 24 elements). Multi-layer polymer electrode technology offers the opportunity to create new electrodes with higher numbers of channels, offering improved performance in neural stimulation applications including cochlear implants, retinal arrays, deep brain stimulators and paraplegic remobilization devices.

Texture formation under phase ordering and phase separation in polymer-liquid crystal mixtures

Computational modeling of texture formation in coupled phase separation-phase ordering processes in polymer/liquid crystal mixtures is performed using a unified model based on the nematic tensor order parameter and gradient orientation elasticity. The computational methods are able to resolve defect nucleation, defect-defect interactions, and defect-particle interactions, as well as global and local morphological features in the concentration and order parameter spatiotemporal behavior. Biphasic structures corresponding to polymer dispersed liquid crystals (PDLCs), crystalline filled nematic (CFNs), and random filled nematics (RFNs) are captured and analyzed using liquid crystal defect physics and structure factors. Under spinodal decomposition due to concentration fluctuations, the PDLC structure emerges, and the nucleation and repulsive interaction of defects within nematic droplets leads to bipolar nematic droplets. Under spinodal decomposition due to ordering fluctuations, the CFNs structure emerges, and the stable polymer droplet crystal is pinned by a lattice of topological defects. For intermediate cases, where the mixture is unstable to both concentration and nematic order fluctuations, the RFN structure emerges, and polymer droplets and fibrils are pinned by a defect network, whose density increases with the curvature of the polymer-liquid crystal interface. The simulations provide an information of the role of topological defects on phase separation-phase ordering processes in polymer-liquid crystal mixtures.

The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers

In liquid crystal cells with photoconductive poly(N-vinlylcarbazole) polymer layers, an external dc field can be completely screened by surface charge layers that develop at the liquid crystal–polymer interface. Under spatially modulated illumination, surface charge layers can be discharged in bright areas and lead to reorientation and spatially modulated Freedericksz transition. As a result, an asymmetric energy exchange in the photorefractive two-beam coupling process can take place. We propose a model to explain the origin of reorientation and phase shift in the two-beam coupling process, based on the profile and tilt of the refractive index grating. We also show that cells with just one photoconducting layer are more efficient than a typical design with two layers.

Dynamic grating features for the surface-induced photorefractive effect in undoped nematics

We investigate the highly sensitive surface-induced photorefractive gratings, observed in undoped nematic planar cells for low dc voltage (few volts) and laser intensity (few milliwatts per square centimeter). Forced-light-scattering and two-beam coupling measurements on E7 nematic samples, aligned with rubbed polyvinyl alcohol layers, verify the orientational effect of a space-charge field and the crucial role of the polymer–liquid-crystal interface in the photoinduced processes, through a wavelength-dependent photoelectric activation. The experimental behavior supports the surface-induced photorefractive effect model, according to which light with the proper wavelength locally reduces the interfacial charge density, thus producing a space-charge electric field in the cell.

Anchoring behavior, orientational order, and reorientation dynamics of nematic liquid crystal droplets dispersed in cross-linked polymer networks

The orientational ordering and the electro-optical properties of nematic liquid crystal (LC) droplets confined to cross-linked polymer networks are investigated as a function of the anchoring conditions at the polymer-liquid crystal interface. Normal alignment (homeotropic) or parallel alignment (planar) inside LC droplets was controlled by using acrylate polymers with appropriate side chains. Drastic changes in the reorientation dynamics of the confined nematic liquid crystal phase are observed, as well as in the orientational ordering of the phase-separated LC which was investigated by 13C-NMR (nuclear magnetic resonance) spectroscopy. The cross-link density of the polymer network also affects the orientational ordering and the electro-optical properties of the confined LC phase. Faster switching times and higher-order parameters were found for samples with LC droplets exhibiting planar anchoring.

Charge transport due to photoelectric interface activation in pure nematic liquid-crystal cells

We report a study of the crucial role of liquid-crystal–polymer interface on photoinduced transport and redistribution of charges in pure nematic liquid-crystal cells that exhibit a photorefractivelike effect. A stationary photocurrent that is 30% of the dark current has been measured for very low power illumination (few mW) and low applied dc electric field (about 0.1 V/μm). The experimental results indicate a clear dependence of the effect on the light wavelength. The absence of photocurrent in cells with only one component, liquid-crystal, or polymer, suggests that both are not intrinsically photoconductive, rules out light-induced charge injection by the electrodes, and indicates the polymer–liquid-crystal interface as the photoactive element in the effect. The photocurrent dynamics indicate the presence of various mechanisms. We suppose that the effect is due to photoinduced carriers injection through the liquid-crystal–polymer interface and recombination process with the counterions present on the opposite side. Different hypotheses are made and discussed.

Photoinduced Surface Alignment: The Role of Liquid-Crystalline Order

The optical control of surface alignment was investigated, using azo-dye doped polyimide films as photosensitive material. The kinetics of the realignment of the easy axis was measured in isotropic, nematic and smectic A phases. It is demonstrated that the light-induced realignment of the dye molecules at the polymer-liquid crystal interface is strongly influenced by the presence of the liquid-crystalline order. A simple model is presented, which accounts qualitatively for the experimental observations.

Nuclear magnetic resonance field-cycling proton relaxation study of polymer dispersed liquid crystals

The frequency and temperature dependence of the longitudinal proton relaxation time T1 has been studied in liquid crystal droplets embedded in a solid polymer matrix in the nematic and isotropic phase over a broad Larmor frequency range (500 Hz≤νL≤84 MHz) employing the fast-field-cycling technique. The comparison of the droplet data, bulk 5CB data, and the pure polymer data show that T1 is dominated by the cross relaxation at the liquid crystal–polymer interface in the entire frequency range. In the low frequency range (νL≤1 MHz), an additional relaxation process determines T1 in both phases, namely reorientations mediated by translational replacements in the nematic phase and the exchange relaxation in the isotropic phase. The analysis of the cross relaxation rate k reveals that the simplified model of Vilfan is only applicable in the nematic phase and leads to an anchoring time τAS of the molecules at the surface which is ≊1.3×10−4 s. This model, however, cannot be applied in the isotropic phase, but must be extended by the spin diffusion time τP, which in this case determines the cross relaxation rate. The anchoring time τAS, which in the isotropic phase corresponds to the correlation time of the additional process, is much shorter than in the nematic phase, namely τAS≊10−6 s.

Theory of domain boundary effects in a phase-separated mixture of polymer and liquid crystal

We calculate the end results of a phase-separation process in which a nematic liquid crystal is expelled from solution in a polymerizing solvent. Our model combines the essential features of the Flory–Huggins, Maier–Saupe, and Hillert models, and contains terms related to the nature of the anchoring process at the polymer–liquid–crystal interface. We find that there is a minimum size of droplet below which no nematic ordering can occur, and that in the absence of the effects due to curvature of the interface, homeotropic anchoring gives rise to a lower surface free energy than does planar anchoring.