A. F. Martins

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.

Shear induced textures of thermotropic acetoxypropylcellulose

Acetoxypropylcellulose is a thermotropic cellulose derivative which is cholesteric from below room temperature up to 170 °C. Its textures during and after shear flow were studied by optical microscopy and small-angle light scattering (SALS) light scattering. The main features are similar to those occurring in hydroxypropylcellulose solutions, i.e., a polydomain texture evolving into a striated texture, as seen by optical microscopy. Elliptical or else four lobes with a streak are seen in the SALS patterns. Defects appearing as short dark lines not aligned with the flow direction are shown to be responsible for the existence of the four lobes of the SALS patterns. In addition these defects seem to be located where the band texture develops during relaxation.

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.

Cellulose-based composite films

The mechanical and optical properties of cellulose-based composite films are investigated.It is shown that the use of toluene diisocyanate as a coupling agent and Avicel fibers as reinforcing elements give films with the highest mechanical characteristics. Using differential scanning calorimetry, it is also found that the glass transition temperature Tg of all the materials studied is below the room temperature and that the Tg increased with cross-linking and introduction of Avicel.

Comparison of thermal and cholesteric mesophase properties among the three kind of hydroxypropylcellulose (HPC) derivatives

SummaryThree kind series of (2-hydroxypropyl)cellulose (HPC) derivatives, the esters (ES), the ethers (ET) and the esters containing fluorocarbon components (FES) were prepared, and the thermal and cholesteric mesophase properties were compared among them. The optical pitch of ES increased with decreasing the degree of substitution (DS). Contrary to ES, the optical pitch of ET decreased with decreasing DS. The cholesteric-isotropic transition temperature of FES exhibited the even-odd oscillation according to the number of carbons in substituent. The reverse optical pitches of ET and FES were larger than that of ES, respectively, at the same temperature and at the same number of carbons in substituent.

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.

Properties and processing of cork powder filled cellulose derivatives composites

This paper deals with the use of cork powder, a by-product from cork industry, as a filler to reinforce hydroxypropyl cellulose (HPC) matrix. Several films were prepared using HPC, as a matrix, filled with different amounts of cork powder (average diameter < 50 μm) (0.0; 0.5; 1.0 and 10.0% w/w) and in the presence or not of 1,4-diisocyanatobutane (BDI) (7.0% w/w). Before the elaboration of these films, the surface properties of cork powder as well as that of suberin (main component of cork) were determined by Inverse Gas Chromatography (IGC). The tensile properties of the solid films obtained were studied and, as expected, for the films with BDI but without cork powder, the Youngs modulus and the tensile strength increased, while the elongation decreased. However with the filled films it seemed that the Youngs modulus decreased and the elongation increased. The Scanning Electron Microscopy showed that the fractured plane of samples with cross-linking agent and cork powder displayed some nucleation points (0.3 μm) which indicates a strongly bonded interface and which could be considered as a responsible for the high mechanical properties observed.

New bio-composites based on short fibre reinforced hydroxypropylcellulose films

The present work deals with the preparation and the characterisation of solid films, having thickness between 15 and 40 μm, prepared with hydroxypropyl cellulose (HPC) and different amounts of commercial cellulose fibres (0; 0.5; 10 and 15% w/w) (system A) and with 1,4-butyl diisocyanate (BDI), as a cross-linking agent (system B). Before the preparation of these films, the surface energy of cellulose fibres, as such and after purification with different solvents, was determined by Inverse Gas Chromatography (IGC), which gave the values of dispersive energy and acid-base properties of their surfaces. The tensile and photo-elastic properties of the solid films obtained were studied. There were no significant changes in Youngs modulus between the two systems. However, as expected, the values of the elongation and those of the fracture stress were systematically higher for system B in comparison with system A. The same trend was found for the birefringence, measured at the same elongation values for the films obtained from both systems.

Rheological Properties of Acetoxypropylcellulose in the Thermotropic Chiral Nematic Phase

Abstract Experimental data for the rheological behavior of two thermotropic liquid crystalline (LC) samples of acetoxypropylcellulose (APC) with different molecular weights, at 120°C, and in shear rates between 0.01 and 10s −1, are presented and analyzed in the framework of the continuum theory for LC polymers recently proposed by Martins1. The viscosity η(γ.)shows a strong shear thinning in the range of shear rates γ. studied, with an hesitation at shear rates of about 0.1-0.2 s−1, depending on the molecular weight, and the first normal stress difference Nl(γ.) shows only positive values, increasing with shear rate γ., with an hesitation at shear rates of an order of magnitude higher, i.e. about 1-2 s−l, also depending on the molecular weight. The hesitation points of the flow functions are displaced towards lower values of the shear rate, with increasing molecular weight. For small and intermediate γ. the shear viscosity of the higher molecular weight sample is greater than the corresponding viscosity f...

Theory and numerical simulation of field-induced director dynamics in confined nematics investigated by nuclear magnetic resonance

We investigate the director reorientation in a nematic liquid crystal confined between two parallel plates and subjected to both a magnetic and an electric field. The permanent magnetic field is used first to align the director and, subsequently, to allow nuclear magnetic resonance (NMR) observation of the director response following the application of the electric field. For sufficiently strong electric fields, the director reorients and aligns parallel to the electric field. In this work we focus on the geometry where the electric and magnetic fields are orthogonal to each other. In this configuration the state of the system, immediately after applying the orthogonal electric field, is steady and unstable, at least in theory, since the director is assumed to be everywhere parallel to the magnetic field. In practice the real state of the system always deviates slightly from the perfect unstable steady state, which induces the start of the director reorientation toward the electric field. The nature and the characteristics of the initial deviation partially determine the reorientation process. In the classical approach, the small deviations from the ideal state are assumed to be due to thermal fluctuations, but this approach fails to account for some recent experimental results. For this reason we were led to investigate slightly non-uniform initial director configurations that are stable under the sole effect of the magnetic field but are sufficient to break the ideal unstable steady state created with the application of the orthogonal electric field. Such non-uniformities must be local or distributed over very small sample volumes, since their effects on the equilibrium NMR spectrum (before the application of the electric field) are not usually observed. In other words, we consider the presence of inversion walls in the bulk of the sample and local misalignments of the director on the boundary plates and investigate the effects of such non-uniformities on the response of the nematic to the application of the electric field, as observed by NMR. The model we propose, including such effects in parallel with thermal fluctuations, is able to account for the recently observed features of the field-induced director dynamics.