G. Chidichimo

Permeation through a heterogeneous membrane : the effect of the dispersed phase

By the incorporation of filler materials in a membrane one may change the permeability of the overall, dense membrane towards various gases, vapours and/or liquids. The influence of solid fillers dispersed in a polymeric matrix on permeability, diffusivity and solubility has been studied extensively in literature. In this paper the influence of the filler on overall membrane permeability resulting in an upper and a lower limit of the permeability given the filler content and the permeability of the continuous phase will be studied theoretically. From this analysis the change in membrane selectivity towards a gas mixture by incorporation of a filler, relative to the selectivity of the pure matrix membrane, can be obtained. The maximum change in membrane selectivity is a function of the filler content only, i.e. irrespective of matrix and filler permeabilities. The difficulties often encountered in estimating the filler permeability from experimental permeability data using membranes with varying amounts of the filler will be discussed, and possible solutions will be given using theoretical and experimental data.

Electrofluorochromism in π-conjugated ionic liquid crystals

Materials in which photoluminescence is modulated by redox processes are known as electrofluorochromic. Intrinsically switchable fluorophores, incorporating both redox and fluorescent moieties, could be ideal electrofluorochromic materials if they possess high fluorescence quantum yields in at least one of their redox states. Fluorescent liquid crystals with redox active centres could combine the above requirements with the advantage to work in bulk anisotropic phases. However, electrofluorochromic liquid crystals have not been reported yet because their synthesis is challenging due to aggregation-caused fluorescent quenching. Here we show the first examples of electrofluorochromic π-conjugated ionic liquid crystals based on thienoviologens. These ordered materials, combining ionic and electronic functions, are highly fluorescence in the bulk state (quantum yield>60%). Their direct electrochemical reduction leads to fast and reversible bulk electrofluorochromic response in both columnar and smectic phases allowing for fluorescence intensity modulation and colour tuning.

Organic Solar Cells: Problems and Perspectives

For photovoltaic cells to convert solar into electric energy is probably the most interesting research challenge nowadays. A good efficiency of these devices has been obtained by using inorganic semiconductor materials. On the other hand, manufacture processes are very expensive in terms of both materials and techniques. For this reason organic-based photovoltaic (OPV) cells are attracting the general attention because of the possible realization of more economical devices. Organic materials are abundant and easily handling. Unfortunately OPV cells efficiency is significantly lower than that of inorganic-based devices, representing a big point of weakness at the present. This is mainly due to the fact that organic semiconductors have a much higher band gap with respect to inorganic semiconductors. In addition, OPV cells are very susceptible to oxygen and water. In this paper we will describe some of the different approaches to the understanding and improving of organic photovoltaic devices.

Fine adjustment of conductivity in polymer-dispersed liquid crystals

The electrical properties of polymer-dispersed liquid crystals (PDLCs) are an important characteristic in their electro-optical performance. Conductivity effects can set up depolarization fields in the films reducing the effective field across the liquid crystal droplets. Both theoretical and experimental investigations have confirmed that the electric field across nematic droplets depends on the liquid crystal and polymer conductivities. In this letter, we have found that the doping of a PDLC with low percentages of a conductive polymer allows a fine adjustment of polymer matrix conductivity. In addition, we have found a large reduction in the re-orientation fields and relaxation times as a function of conductive polymer loading. Results are in rather good agreement with a simple phenomenological model.

A method to produce reverse-mode polymer-dispersed liquid-crystal shutters

Reverse-mode operation shutters have been achieved by combining the techniques of the traditional means of making polymer-dispersed liquid crystals and nematic curvilinear aligned phases. Nematic microemulsions, obtained by a thermally induced phase separation, have been photopolymerized in an external force field. After the polymerization, films show 85% transmittance in the OFF state, while it decreases to less than 1% when an electric field of about 2 V μm−1 at 1 kHz is applied. The rise-time values, about 2 ms, are in the same range as those obtained with normal-mode polymer-dispersed liquid-crystal films. On the contrary, decay time shows longer values.

Angular Transmission of Polymer Dispersed Liquid Crystals Films

Abstract The angular transmission of Polymer Dispersed Liquid Crystals (PDLC) films has been experimentally investigated as a function of droplets dimension, refractive indices and thickness of sample. The experimental data have been interpreted in terms of the Anomalous Diffraction Approximation slightly modified in order to take into account extra-scattering phenomena generated by some of the peculiar PDLC characters. The dimension of the droplets and refractive index difference between polymer and liquid crystal are found to be the most important factors affecting the light transmission as well as extra scattering factors.

Electrooptic properties of polymer dispersed liquid crystals

Abstract An investigation of the electrooptic properties of polymer dispersed liquid crystals (PDLC) is presented. These materials are light modulating systems. They show a reversible optical response from an opaque state to a highly transmitting state under the action of an appropriate electric field which aligns the liquid crystal director. The switching voltage required to establish such an electric field has been monitored as a function of (i) the starting materials used for the preparation of the PDLCs, (ii) the ageing (curing time) of the PDLC cells. Other physical properties, such as the electrical resistivity and the dielectric constant of the materials, have been measured. The correlations between these properties have been studied. The PDLC switching voltage appears to be strongly correlated with the resistivity. Our data suggest that ionic impurities play a dominant role with respect to the electrooptic response of PDLC films.

Optical nonlinearities induced by thermal effects in polymer dispersed liquid crystals

The first experimental observation of strong optical nonlinearities induced by a laser field in polymer dispersed liquid crystals is reported. The measured rise time of the effect agrees with the interpretation of a thermal origin of the phenomenon.

Conformation of ethylene glycol dissolved in a nematic-lyotropic solution: An N.M.R. analysis

The conformation of ethylene glycol (EG) has been investigated by means of liquid crystal nuclear magnetic resonance spectroscopy (LICRY-N.M.R.). The proton and 13C N.M.R. spectra were obtained from a quarternary solution of potassium laurate (KL), decanol, water and EG. These spectra were analysed to get the proton-proton and proton-carbon direct dipolar couplings. Spectra obtained from an aqueous solution have also been analysed. The experimental results were then used to investigate the conformational equilibrium of the molecule. It has been found that in the lyotropic liquid crystalline solution EG exists in a gauche conformation only, and that the value of the OC-CO dihedral angle is 72°.

Morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals

We have investigated the morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals as a function of liquid crystal loading. Reverse mode shutters have been obtained by a polymerization-induced phase separation of mixtures, consisting of a liquid crystalline monomer and a non-reactive nematic liquid crystal, placed between rough conductive surfaces. Such surfaces are able to keep the photopolymerizable mixtures homeotropically aligned without the use of any aligning polymer substrate. OFF state transmittances are always larger than 80% and the switching fields decrease if the non-reactive liquid crystal percentage is increased. Both rise and decay times are always lower than 10 ms. The electro-optical properties have been related to the sample morphology and a simple mode is proposed.