The Optical Response of Polymer Dispersed Liquid Crystals

Nanostructure of Unconventional Liquid Crystals Investigated by Synchrotron Radiation

The macroscopic properties of novel liquid crystal (LC) systems-LCs with unconventional molecular structure as well as conventional LCs in unconventional geometries-directly descend from their mesoscopic structural organization. While X-ray diffraction (XRD) is an obvious choice to investigate their nanoscale structure, conventional diffractometry is often hampered by experimental difficulties: the low scattering power and short-range positional order of the materials, resulting in weak and diffuse diffraction features; the need to perform measurements in challenging conditions, e.g., under magnetic and/or electric fields, on thin films, or at high temperatures; and the necessity to probe micron-sized volumes to tell the local structural properties from their macroscopic average. Synchrotron XRD allows these problems to be circumvented thanks to the superior diffraction capabilities (brilliance, q-range, energy and space resolution) and advanced sample environment available at synchrotron beamlines. Here, we highlight the potentiality of synchrotron XRD in the field of LCs by reviewing a selection of experiments on three unconventional LC systems: the potentially biaxial and polar nematic phase of bent-core mesogens; the very high-temperature nematic phase of all-aromatic LCs; and polymer-dispersed liquid crystals. In all these cases, synchrotron XRD unveils subtle nanostructural features that are reflected into macroscopic properties of great interest from both fundamental and technological points of view.

Electro-optical performance of polymer-stabilized sphere phase liquid crystal displays

Electro-optical properties of polymer-stabilized sphere phase liquid crystal (PS-SPLC) switching devices are analyzed and validated experimentally. The experimental results show the voltage-dependent transmittance curves of PS-SPLC devices. A diffraction approach, called extended anomalous diffraction approach, is proposed to fit the experimental data. Good agreement between experiment and model is obtained. The scattering model provides practical guidance for the improvement of PS-SPLC displays performance and optimization.

Field-driven dynamics of nematic microcapillaries

Polymer-dispersed liquid-crystal (PDLC) composites long have been a focus of study for their unique electro-optical properties which have resulted in various applications such as switchable (transparent or translucent) windows. These composites are manufactured using desirable "bottom-up" techniques, such as phase separation of a liquid-crystal-polymer mixture, which enable production of PDLC films at very large scales. LC domains within PDLCs are typically spheroidal, as opposed to rectangular for an LCD panel, and thus exhibit substantially different behavior in the presence of an external field. The fundamental difference between spheroidal and rectangular nematic domains is that the former results in the presence of nanoscale orientational defects in LC order while the latter does not. Progress in the development and optimization of PDLC electro-optical properties has progressed at a relatively slow pace due to this increased complexity. In this work, continuum simulations are performed in order to capture the complex formation and electric field-driven switching dynamics of approximations of PDLC domains. Using a simplified elliptic cylinder (microcapillary) geometry as an approximation of spheroidal PDLC domains, the effects of geometry (aspect ratio), surface anchoring, and external field strength are studied through the use of the Landau-de Gennes model of the nematic LC phase.

Maxwell-Wagner-Sillars effects on the thermal-transport properties of polymer-dispersed liquid crystals

We present the depolarization field effects (Maxwell-Wagner-Sillars effect) for the thermal transport properties of polymer dispersed liquid crystal composites under a frequency-dependent electric field. The experiments were conducted on polystyrene/4-Cyano-4'-pentylbiphenyl (PS/5CB) PDLCs of 73 vol.% and 85 vol.% liquid crystal (LC) concentrations. A self-consistent field approximation model is used to deduce the electrical properties of polymer and LC materials as well as the threshold electric field. Electric field-varying (at constant frequency) experiments were also conducted to calculate the interfacial thermal resistance between the LC droplets and polymer matrix as well as to find the elastic constant of LCs in droplet form.

Polarization properties of polymer-dispersed liquid-crystal film with small nematic droplets

The polarization state of light transmitted through a polymer-dispersed liquid-crystal film with small, spherical, nonabsorbing, partially oriented nematic droplets is theoretically investigated. The model used is based on the effective medium approach. Scattering properties of a single droplet are described by the Rayleigh-Gans approximation. Propagation of coherent light is described within the framework of the Twersky theory. To describe the orientation of liquid-crystal molecules inside droplets and liquid-crystal droplets in a sample, the concept of multilevel order parameters is employed. Conditions for circular and linear polarization of the transmitted light are determined and investigated.

Order parameter of elongated liquid crystal droplets: the method of retrieval by the coherent transmittance data

The inverse light-scattering problem for polymer dispersed liquid crystal film is considered. An optical method to retrieve the tensor order parameter of liquid crystal droplets arranged in a monolayer has been developed. The method is based on the measurement of coherent transmission coefficients of film at normal illumination and comparison with the results of the direct light-scattering problem solution. To find the coherent transmission coefficient, the anomalous diffraction approach, and the approximation of effective refractive indices are used. The dependence of the tensor order parameter on the applied voltage for elongated liquid crystal droplets with the rigidly fixed poles is retrieved by the available experimental data. The obtained results are in correlation with the results of known visual observations of liquid crystal droplet structure and calculations of the director field in droplets with rigidly-fixed poles.

Thermophysical, dielectric, and electro-optic properties of nematic liquid crystal droplets confined to a thermoplastic polymer matrix

The thermophysical, dielectric and electro-optic properties of polymer-dispersed liquid crystal (PDLC) films made of monodisperse polystyrene (PS) and 4-n-pentyl-4(')-cyanobiphenyl (5CB) are investigated by polarized optical microscopy, differential scanning calorimetry, ac impedance analysis, and forward transmittance measurement technique. The PS-5CB system exhibits an upper critical solution temperature (UCST) shape phase diagram with a wide isotropic+isotropic (I+I) miscibility gap between the isotropic and nematic+isotropic (N+I) regions. An absorption domain in the dielectric spectrum of PDLC films was clearly observed at low frequency, and unambiguously assigned to the confined liquid crystalline phase in both nematic and isotropic states. The correlation between the dielectric and electro-optical results for PS-5CB (30:70) samples has shown that in the vicinity of the low frequency absorption domain ( approximately 200 Hz at T=25 degrees C), a drastic decrease in the optical transmittance of the film occurs. This phenomenon can be related to an interfacial polarization process resulting from a charge accumulation at the droplet-polymer interface (Maxwell-Wagner-Sillars effect).

Electrically controllable in-line-type polarizer using polymer-dispersed liquid-crystal spliced optical fibers

The polarization-dependent transmission of light through an electrically controllable in-line-type polarizer that is made from polymer-dispersed liquid-crystal spliced optical fibers is discussed experimentally and theoretically. This in-line-type optical splicing method has the advantage of low transmission loss when it is applied in optical fiber communication systems. An anomalous diffraction approach is used to compute the scattering cross section of polymer-dispersed liquid-crystal droplets. The experimental results are supported by a theoretical analysis. This device can be employed in electrically controllable in-line-type polarizers and has the potential to yield electrically controllable polarization-dependent loss compensators.

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.

Evidence for droplet reorientation and interfacial charges in a polymer-dispersed liquid-crystal cell

We observe a new feature ( plateau), sandwiched between two exponentially decaying components, in the switching response of a polymer-dispersed liquid-crystal (PDLC) cell. Upon application of square-wave electrical pulses, the cell response exhibits a complex decay whose characteristics are significantly different within and outside the applied field. We interpret these results in terms of reorientation dynamics of nonspherical droplets in the PDLC. Irradiation of the PDLC with gamma rays induces significant changes in the cell response in a manner that is consistent with redistribution of charges at the liquid-crystal-polymer interface in the PDLC.

Fréedericksz transition in confined liquid crystals: concentration and microgeometry effects

The Fréedericksz transition in dispersions of liquid-crystal droplets is studied analytically by balancing the electrostatic energy of the droplets with a strong anchoring elastic energy. Explicit dependence of the transition threshold field on the liquid-crystal volume fraction and the spatial arrangement of the droplets is obtained for the first time. As a result of the confined geometry, this threshold field does not depend on the thickness of the sample and splitting of the transition occurs in some situations.

Local field effects and the field-dependent dielectric response of polymer dispersed liquid crystals

An anisotropic version of the Maxwell Garnett approximation is applied for studying the dielectric properties of polymer dispersed liquid crystals containing bipolar liquid crystal droplets. This approach provides an explicit link between the droplet orientation distribution and the macroscopic response of the material. The electrostatic energy of the droplets is balanced with a strong anchoring elastic energy term for different initial orientation distributions. For aligned droplets we find a switching process whose sharpness depends on the initial orientation and a concentration dependent threshold field. For a planar distribution we find sharp transitions with a hysteresis loop whose width depends on the droplet concentration. For a random distribution the droplet reorientation is more gradual. The theory is also applied to the negligible elastic energy limit, recently observed at temperatures near the nematic-isotropic phase transition, where the droplets consist of bipolar nematic cores coated by isotropic liquid shells. This structural change within the droplets causes a considerable modification of the electro-optical properties. The Maxwell Garnett approach is used to calculate the dielectric response of this structure and reproduces all the main features of the experimental results.

Influence of the director field structure on extinction and scattering by a nematic liquid-crystal droplet

On the basis of calculations under the discrete dipole approximation and a proposed extended model of nested ellipsoids for bipolar droplets, the changes in optical properties of a spherical liquid-crystal droplet, caused by a change in director configuration under the influence of an external field, have been considered.

Angular dependence of light transmittance through a polymer-dispersed liquid crystal above threshold

The transmittance of polymer-dispersed liquid crystals (PDLC's) can be easily controlled by a low-frequency electric field. However, unlike glass, even in its transparent state a PDLC is an anisotropic material, so its transmittance has a characteristic angular dependence. We introduce a mathematical model to describe the angular dependence of the light transmittance through a PDLC sample above threshold. The accuracy of the model is tested and predictions compared with experimental measurements.