J. L. Figueirinhas

Novel PDLC type display based on cellulose derivatives

Abstract A novel type of PDLC display utilizing solid films obtained from the cellulose derivative hydroxypropylcellulose (HPC) with molecular weights of 60000 (HPC60000) and 100000 (HPC100000), cross-linked with two different di-isocyanates, 1,4-di-isocyanatobutane (BDI) and 1,6-di-isocyanatohexane (HDI), is presented. This kind of display (optical cell) is composed of a porous elastomeric film (20–60 μm thick) of HPC-BDI or HPC-HDI, impregnated with a nematic liquid crystal and contained between two transparent conducting substrates. The electro-optic behaviour of several optical cells is studied as a function of film thickness, the molecular weight of the polymer and the cross-linking agent. The preliminary studies presented in this work show that these systems present good contrast, high transmissivity in the on state and switching times in the range of some milliseconds to hundreds of milliseconds.

Biaxial Nematic Mesophases from Shape-Persistent Mesogens with a Fluorenone Bending Unit

Nematic phases, applied in almost all commercial liquidcrystal (LC) displays, are LC phases with mesogens isotropically distributed and exhibiting orientational long-range order of one preferred molecular axis along a common direction defined by the so-called director. These are the phases with the lowest viscosity and besides the anisotropic properties, are most similar to isotropic liquids. Biaxial nematics should realise a long-range orientational order of all three molecular axes along three mutually perpendicular directors, but maintain the isotropic distribution of the molecular centres of gravity and their molecular mobility. Such phases are not only appealing from the viewpoint of basic research and theoretical modelling, but are also of technological interest to speed up the switching of LC displays. [1, 2, 3] After the prediction by Freiser, [4] Saupe and Yu were the first to discover such mesophases in a narrow region of the phase diagram of a lyotropic LC. [5] The much-pursued syn

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.

Nuclear magnetic resonance spectroscopic investigations of phase biaxiality in the nematic glass of a shape-persistent V-shaped mesogen

Deuterium and carbon-13 NMR spectroscopy were used to study both the high temperature uniaxial nematic and the low temperature biaxial nematic glass of a shape-persistent V-shaped mesogen. It was found that biaxial ordering determined in the domains of the latter has symmetry lower than D(2h) and is compatible with C(2h) symmetry or lower. In particular, elements of the ordering matrix including biaxial phase order parameters were determined from (2)H NMR at two temperatures, one just below the glass transition, and the other deep inside the biaxial glass, which allowed for the characterization of the dominant molecular motions at these temperatures. (13)C NMR magic angle spinning sideband patterns, collected both in the high temperature nematic phase and in the nematic glass, clearly show the difference between them in terms of the phase symmetry.

Cross-linked hydroxypropylcellulose films: mechanical behaviour and electro-optical properties of PDLC type cells

We study the effect of the amount of cross-linking agent upon the mechanical and electro-optical behaviour of several cells prepared from hydroxypropylcellulose (HPC) cross-linked with 1,4 diisocianatobutane (BDI) (0.0%, 3.1%, 7.6%, 12.3% w/w). The tensile properties and the sol/gel fractions were obtained as a function of the amount of the cross-linking agent used to prepare the solid films. The Youngs modulus appears to be constant, over the range of concentrations studied. The electro-optical cells prepared with the nematic liquid crystal (E7) were analysed by light transmission. Several trends were observed, e.g. the cross-linking agent, at the percentages used, has a strong influence on the cells contrast but not on the cells maximum transmission or turn on voltage, while the film thickness shows preponderantly its influence on the cells maximum transmission and turn on voltage. The mechanical properties of the HPC films are important for a recent application that is now being developed [Liq. Cryst., in press], where a flexible substrate is used. The results obtained point out ways for the realisation of an optimum electro-optical cell.

NMR study of the long time, defect-controlled, magnetic reorientation of a nematic polymer liquid crystal

Abstract The magnetic field reorientation of an initially aligned sample of a nematic polymer liquid crystal was followed by proton NMR. Evolution to a metastable (banded) state was considered using a Rheo-NMR technique developed previously. Late stage reorientation was studied by taking into account the dynamics of defects following the formation of splay–bend walls. NMR spectra simulation allowed us to obtain the wall density as a function of time. This result, together with a defect-controlled wall dissolution model proposed by Rey [1], was used to complement the Rheo-NMR technique of measuring the viscoelastic parameters of nematic polymer liquid crystals.

Shear-induced order effects in bi-soft segment urethane/urea elastomers

Abstract In this work we study shear-induced order effects, observed on thin films (∼20–60 μm) of urethane/urea elastomers prepared from a polypropylene oxide based isocyanate terminated triol prepolymer (PU) and polybutadiene diol (PBDO) in the amount of 25% (PU/25PBDO), 50% (PU/50PBDO), 60% (PU/60PBDO) and 75% (PU/75PBDO) by weight of polybutadiene. Optical microscopy and light scattering as a function of temperature were used in this study. The initial isotropic elastomers, PU/60PBDO and PU/75PBDO, after cessation of a uniaxial mechanical deformation relax to a band texture, consisting of equidistant bright and black lines perpendicular to the shear direction. If the uniaxial mechanical stress is applied again perpendicular to the band structure in PU/60PBDO, a new periodicity with wave vector perpendicular to the axis of the field develops, while for PU/75PBDO two new periodicities can be observed, one with wave vector perpendicular to the axis of the field and the other transverse. The process of band formation is completely reversible in cycles of increasing and decreasing strain ratios. In samples PU/25PBDO and PU/50PBDO this kind of effect is not observed. The bands periodicity is temperature independent. The temperature dependence of the scattered intensity produced by the bands is different for bands perpendicular and transverse to the axis of the stress field. We show evidence that the urethane linkage is responsible for the relaxation instabilities observed, in this kind of material.

Anisotropic cellulose‐derived matrix for dispersed liquid crystals

New cellulose‐derived dispersed liquid crystal free‐standing thin films were prepared by a shear‐casting technique from anisotropic and isotropic solutions of thermotropic (acetoxypropyl)cellulose in N,N‐dimethylacetamide with different amounts of the nematic E7. For films prepared from anisotropic solutions, dispersed nematic E7 droplets of micron and submicron size were found to coexist, with the band texture characteristic of polymer liquid crystals after shearing. Mechanical properties including Youngs modulus and stress–strain curves were determined along and perpendicular to the shear direction, and revealed the anisotropic behaviour of films prepared from lyotropic solutions. The nematic E7 component appears to promote chain mobility and rearrangement of the polymer matrix and to modify the viscoelastic properties of the matrix.

Flexible cellulose derivative PDLC type cells

In this work we perform a study of 250 ≈ μm thick flexible electro-optical PDLC type cells made from a biocompatible cellulose derivative film and several conductive substrates. The deposition of an ITO layer by reactive thermal evaporation on a polymeric substrate was referred to in the literature very recently and this type of coated substrate was used in the present work. In order to consider the influence of the substrates on the electro-optical behaviour of the cells, five cells were made using different substrates (three flexible polymers and two glass for comparison). Three of the substrates were coated under the same conditions, and the other two were commercially available substrates.

Electro-Optical Properties of Cellulose Based PDLC Type Cells: Dependence on the Type of Diisocyanate Cross-Linking Agent Used

Abstract We have studied the effect of different diisocyanate cross-linking agents on the electro-optical properties of cellulose derivative based PDLC type cells and on the dielectric properties of the films used on their preparation. Each optical cell was assembled with a cross-linked film produced with a different type of diisocyanate and with the nematic liquid crystal (E7). One was assembled with a non cross-linked film. The six different cells were then analysed by light transmission and the results were correlated with the dielectric properties of the cellulosic films used in each one of them. It was found that the cell with the higher contrast is the one prepared with the film cross-linked with 1,4-diisocyanatobutane (BDI) [6]. Based on the results obtained, we propose a mechanism to describe the electro-optical behaviour of these systems.