U. Maschke

Electro-optical properties of polymer-dispersed liquid crystals

This paper deals with the electro-optic (EO) properties of polymer-dispersed liquid crystals (PDLCs). Several systems are considered to analyze the effects of preparation conditions and film characteristics on the EO response functions. Particular emphasis is put on systems based on mixtures of the commercial compound ASX-95, the difunctional acrylate tripropylene diacrylate and the eutectic mixture of low-molecular-weight liquid crystals E7. Other systems are considered to assess the influence of monomer functionality on EO properties using for example the trifunctional glycerylpropoxytriacrylate. Various modes of preparing PDLCs are considered based on the mechanism of polymerization-induced phase separation using either electron-beam (EB) or UV radiation curing processes. The dose is changed in both techniques to improve film strength and determine which method leads to the best samples in terms of EO response functions. Other important parameters, such as film thickness, composition and applied voltage, are also considered to evaluate the impact on these functions. The article focuses on a comparison of EO performances of films elaborated by exposure to EB and UV radiations. Under similar conditions, one definitively finds a net superiority of the former technique. In addition this technique does not require any photoinitiator and leads to a higher conversion of the monomeric compounds, i.e. higher mechanical strength and less severe aging conditions.

Grafting of cyclodextrins onto polypropylene nonwoven fabrics for the manufacture of reactive filters. I. Synthesis parameters

A novel method for the preparation of immobilized α-, β-, and γ-cyclodextrins (α-, β-, or γ-CD) on polypropylene (PP) nonwoven fabrics is presented. These new materials were prepared by graft-polymerization of glycidyl methacrylate (GMA) onto PP filters using the electron-beam technique, followed by coupling of α-, β-, and γ-CDs with the epoxide group. Optimization of various reaction parameters such as time, temperature, irradiation dose, monomer, and CD concentration was carried out.

On the phase behavior of blends of polymers and nematic liquid crystals

The phase behavior of mixtures of polymers and nematic liquid crystals (LC) is investigated. Two types of systems are examined. The first one deals with blends in which the polymer is made of linear chains. In this case, a systematic study of the effects of various parameters on the phase diagrams is performed. In particular, it is shown how increasing the polymer size and/or the LC molecule size increases the miscibility gap of the mixture. It also reduces the region where a single nematic phase is observed in the presence of a tiny amount of polymer. Likewise, the relative effects of the isotropic and the nematic interaction parameters on the phase diagrams are examined. The second part of this investigation deals with blends involving crosslinked polymers. Here, substantial differences are observed as compared to the case where the polymer components are made of linear chains. These differences are illustrated by showing the phase diagrams in similar conditions for both blends. Unlike the case of a linear polymer matrix, it is observed that the single nematic phase and the nematic-isotropic spinodal branches are absent from the phase diagram of crosslinked polymers. This results into significant distortions of the phase diagram. In order to highlight all these effects, examples representing hypothetical blends are considered. These examples are chosen for illustration of the results in which the choice of numerical parameters is made consistently with the existing values in the literature which makes comparison with published data possible.

Photopolymerization kinetics and phase behaviour of acrylate based polymer dispersed liquid crystals

Polymer dispersed liquid crystal (PDLC) materials were prepared by a polymerization induced phase separation (PIPS) process using UV radiation. The samples were obtained from the liquid crystalline component E7 and a blend of monofunctional (2-ethylhexyl acrylate) and difunctional (1,6-hexane diol diacrylate) monomers as precursors of the matrix. Polymerization energetics and thermodynamic properties of PDLC materials were studied by differential scanning calorimetry. Photopolymerization kinetics and phase behaviour of the PDLC are presented and discussed as a function of UV polymerization conditions (UV light intensity, UV time exposure and isothermal curing temperature).

Grafting of cyclodextrins onto polypropylene nonwoven fabrics for the manufacture of reactive filters. II. Characterization

A novel method for the preparation of immobilized α, β, or γ-cyclodextrins on polypropylene nonwoven supports has been previously presented. The obtained new materials were prepared by graft-polymerization of glycidyl methacrylate onto polypropylene after activation of the support by the electron beam technique, followed by the coupling of cyclodextrins with the epoxide groups. The structure of the resulting materials is characterized in detail using Fourier transform infrared spectroscopy, solid state nuclear magnetic resonance analysis, differential scanning calorimetry, thermogravimetric analysis, and optical microscopy.

PDLC films prepared by electron beam and ultraviolet curing: influence of curing conditions on the electro-optical properties

Polymer dispersed liquid crystal (PDLC) films were prepared by polymerization-induced phase separation processes using ultraviolet (UV) and electron beam (EB) radiation. A mixture of the nematic LC material E7, an aromatic polyester acrylate, and additional monomeric acrylates was exposed to the EB radiation. A photoinitiator was included in the initial mixture in the case of UV exposure. The electro-optical behaviour of the PDLC films obtained has been investigated as a function of the chosen radiation. The transmission versus voltage curves strongly depend on the curing conditions, and are highly reproducible. Threshold and saturation voltages continuously increase with increasing dose values for UV-cured films, whereas plateau values were reached for EB-cured samples. A small memory effect has been observed for UV-cured systems.

Influence of liquid crystal concentration on the electro-optical behavior of polymer dispersed liquid crystal films prepared by electron beam processing

Abstract Polymer dispersed liquid crystal (PDLC) films can be switched electrically from a light scattering off-state to a highly transparent on-state. Polymerization induced phase separation (PIPS) initiated by electron beam irradiation has been used as a powerful method to obtain defined PDLC films. Blends of an aromatic polyester acrylate in additional monomers were prepared as polymer matrix precursors. The eutectic nematic mixture E7 was used as liquid crystal material in this work. Mixtures including different amounts of these materials were exposed to the electron beam radiation. The optical transmission properties of the obtained PDLC films were investigated as a function of composition and amplitude of the applied AC voltage. The electro-optical curves strongly depend on the liquid crystal concentration and are highly reproducible.

Thermophysical and Electro-Optical Characterization of Nematic Liquid Crystal / Acrylate Systems

Abstract New Polymer Dispersed Liquid Crystal (PDLC) materials were prepared by a Polymerization Induced Phase Separation (PIPS) mechanism using U.V. radiation. The samples are obtained from the liquid crystalline mixture E7 and a blend of monofunctional (2-Ethyl Hexyl Acrylate) and difunctional (1,6-Hexane Diol DiAcrylate) monomers as precursors of the matrix. The thermodynamic properties of both cured and uncured samples were studied by Differential Scanning Calorimetry and Optical Microscopy. The optical transmission of selected PDLC films has been measured as a function of frequency and amplitude of the applied voltage for several film thicknesses.

Electro-Optical and Morphological Characterization of Electron Beam Cured Liquid Crystal-Polymer Composite Materials

Abstract Polymer dispersed liquid crystals (PDLCs) are promising new materials for electro-optic applications such as switchable windows and flexible displays. Thin films were prepared via a polymerization induced phase separation process using electron beam radiation. The PDLC material was obtained from a blend of a eutectic nematic liquid crystal mixture E7 and a polyester acrylate based polymer as a precursor of the matrix The electro-optic behavior of the obtained composite material has been examined To understand the optical response of these materials, the relationship between the electro-optic properties and the morphology of the PDLC films has been investigated. The size, size distribution, shape, volumetric number density and spatial distribution of droplets are important factors influencing the electro-optic performance. Scanning electron microscopy (SEM) was used to characterize quantitatively PDLC film morphology

Dielectric and Electro-Optical Properties of Liquid Crystals Doped with Diamond Nanoparticles

Dielectric and electro-optical properties of nematic liquid crystals (LCs) doped with diamond nanoparticles (DNPs) have been investigated. It is established that the effect of DNPs on the electric conductivity depends on the purity of LC; the nanoparticles adsorb ions from LCs with a substantial content of ionic impurities, but release their own ions in highly pure LCs. The DNPs also influence LC permittivity due to orientational disordering of LC and contribution to overall permittivity of the samples. In contrast to LC-Carbon Nanotubes counterparts, the LC-DNPs composites do not show memory type electro-optic switching that may be assigned to isotropic shape of DNPs and their relatively weak interaction with LC hosts.