W. W. Adams

The morphology and performance of holographic transmission gratings recorded in polymer dispersed liquid crystals

Abstract Holographic transmission gratings are formed by the anisotropic visible laser radiation curing of a multifunctional acrylate monomer blended with the liquid crystal (LC) mixture E7. This results in an anisotropic spatial distribution of phase-separated LC droplets within the photochemically cured polymer matrix. The morphology of thin films (5–20 μm) containing the gratings is examined by low-voltage, high-resolution scanning electron microscopy and transmission electron microscopy. Low concentrations of E7 (16% LC) coupled with rapid curing kinetics result in the formation of narrow LC-rich Bragg lamellae without well defined boundaries. These LC-rich lamellae, with approximate widths of 100 nm, are composed of small droplets measuring 20–50 nm in diameter. Increasing the concentration of LC in the prepolymer mixture results in larger lamellae (canals) of LC-rich material. Films formed from a mixture containing the highest LC concentration (34% LC) exhibited lamellae approximately 200–250 nm wide that are separated by polymer lamellae that also possess a small fraction of phase-separated LC droplets. The other variable examined in detail, laser writing intensity, has little effect on the morphologies exhibited in these films. The morphology is related to the performance (diffraction efficiency, transmission, switching times and fields) through a simple model.

Morphology of reflection holograms formed in situ using polymer-dispersed liquid crystals

The morphology of a reflection grating formed using a polymer-dispersed liquid crystalline material system is examined using low-voltage high-resolution scanning electron and transmission electron microscopy. The grating is formed by establishing a fringe pattern in the intensity profile of an argon-ion laser line (λ = 488 nm) leading to a periodic anisotropic cure through the thickness of the film. The in situ, one-step procedure produces periodic layers of polymer- and LC-rich planes lying parallel to the film surface. Droplet diameters are very small (<100 nm) and little coalescence of individual droplets is observed. The grating spacing measured from electron micrographs (153 nm) nearly corresponds to the expected spacing from the observed reflection notch at λ = 472 nm.

On the small-angle X-ray scattering of rigid-rod polymer fibres

Small-angle X-ray scattering was carried out on as-spun and heat-treated poly( p -phenylene benzobisoxazole) (PBO) fibre. While as-spun PBO fibre exhibits only an equatorial streak, generally attributed to elongated voids or fibrils, the heat-treated fibre shows in addition a radial four-point pattern. The long period and radial spacing of this pattern are 28 and 15 nm, respectively. The spacings do not vary with heat-treatment temperature. As the angle defined by the plane normal to the fibre axis and the X-ray beam is increased from 0° to 30° to 60°, the four-point pattern gradually changes to a two-point pattern. The measured long period in the tilted fibre is a projection of that in the untilted fibre. Various possible morphological models are discussed.

Cyclic Versus Linear Siloxane Liquid Crystals: Phase Behavior and X-ray Diffraction Results

Abstract The phase behavior of cyclic and linear polysiloxanes containing biphenyl-4-allyloxybenzoate and cholesteryl-4-allyloxybenzoate mesogens is reported. The presence of the cholesterol-based mesogen was observed to induce layered packing structures for both linear and cyclic siloxane backbones. Differences in the mesophase behavior are discussed with respect to composition and type of siloxane. Small differences in the thermal transitions of cyclic and linear compounds containing the same composition of mesogens were observed. X-ray diffraction results are used to confirm the effect of composition on previously proposed packing models. Differences in the reflection spacings and intensity are discussed with regard to composition, temperature, and type of siloxane. Orientational differences between backbone type observed in aligned fibers are also discussed.

Crystal-solvate phases of poly(p-phenylene benzobisoxazole)

The crystal structure and morphology in oriented solutions of the rigid polymer poly(p-phenylene benzobisoxazole) (PBZO) in polyphosphoric acid (PPA), at different degrees of exposure to moisture, were studied by transmission electron microscopy and X-ray diffraction. Two crystal solvate forms, crystalline complexes of PBZO and PPA, were found. The form I crystal solvates exhibit several sharp equatorial reflections, whereas only a few broad reflections are observed in the form II state. All crystal solvate phases studied exhibit three-dimensional crystallinity evident in sharp off-meridional reflections. This is considered to be due to unique associations between the protonated PBZO polycations and the PPA anions. Endothermic transitions observed upon heating the crystal solvates are interpreted as dissolution of the form I crystals at about 70°C and crystal-crystal transformation of the form II crystal solvate to PBZO crystals, due to deprotonation above 300°C. A microfibrillar morphology is observed in the oriented PBZO fibre coagulated by immersion in water. Similar microfibrils, less than 10 nm in width, are already observed in the form II crystal solvate state, obtained from the oriented solution by absorption of moisture.

Switchable Bragg Gratings Formed in Situ Within a Polymer-Dispersed Liquid Crystal Composite Medium

Holographic photo-curing of a penta-acrylate monomer-liquid crystal mixture forms a unique system of liquid crystal microdomains confined to Bragg planes. We examine the physics of this structure as it relates to the formation of electrically switchable holograms, for which many potential applications exist. The results of scanning electron microscopy and laser characterization studies lead to the development of concepts and models for explaining the microscopic morphology and electro-optical properties of these holograms. We find that a model incorporating a shaped-droplet analysis of the electro-mechanical properties of LC domains combined with standard coupled-wave theory of holography offers good numerical agreement with diffraction efficiency data for ppolarized probe light.

Modelling a nematic liquid crystal

Abstract The bulk phase liquid crystalline behaviour of a cyclic siloxane with a pentamethylcyclosiloxane core and biphenyl-4-allyloxybenzoate mesogens (BCS) was studied using molecular dynamics (MD) and wide angle X-ray analysis. This material exhibits partial crystallinity at room temperature and liquid crystalline behaviour above 120° C. For the MD simulations an ensemble of 27 molecules with 135 mesogenic units was simulated and a molecular mechanics force field was used to model the structural anisotropy of the siloxane molecules. Simulations were carried out both at room temperature and at an elevated temperature (425 K). Room temperature simulations showed that, contrary to our initial assumptions, the low energy molecular conformations were not cylindrical but splayed in shape. During the simulation a smectic-like, tilted layer structure was found to evolve for the cluster when full atom potentials were used, while no such development was observed when electrostatic interactions were neglected. Th...

Calculation of the effects of protonation on rigid-rod polymers

Abstract Semiempirical molecular orbital calculations have been used to investigate the effects of protonation on the behaviour of the structural repeat units of poly(p-phenylene benzobisthiazole), PBZT, and poly(p-phenylene benzobisoxazole), PBO. Single and multiple protonations of the heterocyclic rings at the nitrogen, oxygen and/or sulfur atoms were examined. It was found that protonation at the nitrogen was more favourable than at the oxygen or sulfur atoms. Delocalization of the charge into the phenyl ring resulted in a higher bond order for the inter-ring bond. While the energy barrier for torsional rotation increased with protonation, the resistance to bending the rings in an out-of-plane fashion decreased somewhat. Results of the semiempirical calculations indicated that protonation of the oxygen in PBO might result in opening of the five-membered heterocyclic ring by breaking the carbon-oxygen bond. Ab initio calculations on relevant models, however, did not support this result.

Probing the electric field alignment of a thermotropic liquid crystalline polymer by synchrotron radiation

Abstract The orientation of a cyclic side-chain thermotropic liquid crystalline material in an AC field was monitored in real-time using synchrotron radiation. Monitoring the realignment processes in the millisecond-to-minute time-scale was made possible by the high X-ray flux. Orientation parameters and response times were calculated as a function of temperature and frequency. Response times decreased exponentially with temperature due to a decrease in the viscosity. Very little dependence of the response time on frequency was observed, except at low temperatures, where a switch from homeotropic to planar alignment of the molecules was detected. This reorientation of the director was studied in real-time and the resulting complex diffraction patterns were due to equal but opposite director rotations from an alignment parallel to the applied electric field to an alignment perpendicular to the applied electric field. The orientation parameters were highest in the central portion of the mesophase temperatur...

The effect of electric and magnetic fields on the melt organization of a nematic cyclic siloxane liquid crystal

Abstract The presence of periodic diffuse meridional reflections in the X-ray diffraction pattern of the nematic phase of a pentamethylcyclosiloxane compound with pendant mesogenic groups is examined as a function of temperature and frequency of an applied electric field. These reflections are thought to be caused by a tendency of the mesogens to pack in columns of molecules that have no side-to-side registry (uncorrelated strings). Column lengths obtained from the intercepts of plots of δs 2 versus s 2, where s is the scattering vector and δs is the width of the reflection, indicate association of 3–6 mesogens depending on temperature and frequency. The length over which the strings are correlated are compared to values obtained from a magnetically aligned sample quenched into a glassy nematic phase. The development of a pseudo-layered packing arrangement with decreasing temperature is also discussed. The appearance of a symmetric 4-spot pattern at temperatures slightly above the crystallization temperat...