Daniela Cupelli

Light Responsive Polymer Membranes: A Review

In recent years, stimuli responsive materials have gained significant attention in membrane separation processes due to their ability to change specific properties in response to small external stimuli, such as light, pH, temperature, ionic strength, pressure, magnetic field, antigen, chemical composition, and so on. In this review, we briefly report recent progresses in light-driven materials and membranes. Photo-switching mechanisms, valved-membrane fabrication and light-driven properties are examined. Advances and perspectives of light responsive polymer membranes in biotechnology, chemistry and biology areas are discussed.

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.

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.

Rough surfaces for orientation control in reverse mode polymer dispersed liquid crystal films

Reverse mode operation shutters have been achieved with polymer dispersed liquid crystals by means of polymerization-induced phase separation of nematic mixtures consisting of a low molecular mass liquid crystal and a liquid crystalline monomer. Fluid mixtures were homeotropically aligned by rough surfaces and transparent films were obtained after polymerization. Transmittance in the OFF state can be larger than 80% and decreases to less than 1% when an electric field of about 2 V μm-1 at 1 kHz is applied (ON state). Both rise and decay times can be lower than 10 ms and the drop in the OFF state normal transmittance is drastically reduced with respect to conventional polymer dispersed liquid crystals since samples exhibit a reverse morphology. The role played by surface roughness is also discussed.

Orientation control of liquid crystal droplets dispersed in a polymer matrix

The encapsulation of monomer molecules in liquid crystal droplets dispersed in a thermoplastic matrix provides a convenient method to control the orientation of liquid crystal directors. The imprint of droplet interfaces is obtained by photopolymerization of monomer molecules by means of a polymerization induced phase separation process performed in a magnetic field. In such a way, uniform orientations of liquid crystal directors can be achieved in cells without any surface treatment. The concentration of monomer molecules is an important parameter, which influences the final electro-optical properties of films.

Alignment of single-walled carbon nanotubes in polymer dispersed liquid crystals

Reverse morphology polymer dispersed liquid crystal (PDLC) films are liquid crystal dispersions in a polymer network, whose voids and crevices are filled by liquid crystal. If they are prepared with homeotropic alignment, they can give rise to transparent electro-optical devices, which can be turned into an opaque state by application of a suitable electric field. They can be easily prepared by polymerisation of mixtures of liquid crystalline monomers and liquid crystals. In this paper we have doped reverse morphology PDLCs with increasing amounts of single-walled carbon nanotubes and investigated their effects on the morphology, electro-optical properties, and conductivity of the PDLC films. The influence of a different initial alignment of liquid crystal (homeotropic, homogeneous, and random) has also been studied.

Electrochromism in switchable nematic emulsions

Switchable nematic emulsions are composite systems formed by liquid-crystal droplets dispersed in a fluid, homogeneous, monomer matrix, which can be turned from an opaque to an optically transparent state by application of a suitable ac electric field. An electrochromic device provides a reversible and visible change in its transmittance and/or reflectance as the result of either oxidation or reduction electrochemical processes. Both devices have been proven to be useful for a variety of electro-optical applications as switchable windows, electromagnetic shutters, and displays. This letter reports preliminary results on a bifunctional device based on a switchable nematic emulsion, which hosts electrochemical reactions. The presence of a liquid-crystal dispersion ensures the switching from a scattering and opaque state to a transmissive and transparent state, while the oxidation–reduction reactions allow a contemporary and independent change in color.

Surface anchoring, polarization fields and memory states in polymer dispersed liquid crystals

We have investigated the formation and development of memory states in polymer dispersed liquid crystals induced by the application of a strong electric field. Both the optical transmittance and polarization field have been followed as functions of time. We have been able to distinguish between the contributions to the memory states arising from the surface anchoring of the liquid crystal at the droplet interface and from the electrical reorientation of the mesogenic molecules. The dependence of both residual transmittance and polarization field on temperature is reported and a simple model is proposed.

Photoswitching in polymer-dispersed liquid crystals

While dealing with photonics, we find that user-controllable electro-optical devices are of great importance. In this paper, the optical control of the light scattering properties of polymer-dispersed liquid crystals has been achieved by dissolving a photoconductive material in the polymer matrix. In the dark, an electric field, lower than the threshold one, is applied across the film, which, consequently, appears opaque. After irradiation with an adequate light beam, the change in the electrical properties of the polymer matrix enhances the effective field across the liquid crystal droplets, making the film transparent. The device can be switched off by turning off either the light source or the electric field. The effects of light intensity and photoconductor concentration on the film phototunability are also discussed.

Persistence effects and memory states in charged polymer dispersed liquid crystals

Polymer dispersed liquid crystals (PDLC) can be affected by either persistence or memory states after the application of a strong electric field (charge process). We have investigated the post-charge states of PDLCs as a function of liquid crystal loading and found that the appearance of either a persistence or memory state depends on the polymer matrix glass transition temperature.