Ulrich Maschke

Equilibrium phase diagram of polystyrene and 8CB

The experimental equilibrium phase diagram of a mixture of linear polystyrene of molecular weight Mw = 44,000 g/mol and 4-cyano-4′-n-octyl-biphenyl (8CB) is established. The three transitions smectic A-nematic, nematic-isotropic, and isotropic-isotropic are observed. The first two are observed both by optical microscopy and differential scanning calorimetry (DSC) while the isotropic-isotropic transition could be seen only via optical microscopy. Two series of samples with the same compositions were independently prepared and yielded consistent results both by microscopy and DSC. Measurements of sample compositions with less than 50 weight % of 8CB were influenced by the vicinity of the glass transition temperature (Tg) of the polymer in the mixture. This quantity is also determined by DSC as a function of composition. A single Tg is observed, which decreases with composition of the LC. Other thermodynamic quantities such as the enthalpy variations of LC in the nematic-isotropic transition and the fraction of LC contained in the droplets are also considered.

Cyclic Polymers in Good Solvents

This paper deals with the effects of excluded volume interactions on the thermodynamic and structural properties of cyclic polymers in good solvents. Several thermodynamic properties are discussed with particular emphasis on excluded volume interactions and their effects on the radii of gyration and the form factors. An empirical model describing the mean square distance between two points along a cyclic polymer is proposed and its predictions compared with those of other models. Comparison between cyclic and linear chain properties highlights the effects of chain closure under good solvent conditions.

Phase equilibrium of poly(n-butyl acrylate) and E7

The experimental equilibrium phase diagram of mixtures of linear poly(n-butyl acrylate) of molecular mass Mw = 112000 g mol-1 and the low molecular mass LC mixture E7 has been established using polarized optical microscopy and light scattering techniques. The diagram is found to be reminiscent of an upper critical solution temperature system. Two independent series of samples with the same composition were studied, yielding consistent results. A region of nematic and isotropic coexisting phases and a region of a single isotropic phase were identified in the composition-temperature phase diagram. The results were analysed within a theoretical model combining the Flory-Huggins lattice theory for isotropic mixing and the Maier-Saupe theory for nematic ordering. Interestingly, no region of isotropic coexisting phases was observed in our experiments. This is probably due to the fact that the nematic interaction overwhelms the isotropic interaction in the region where (I + I) coexisting phases could appear. A preferential solubility of certain constituents of the LC mixture in the polymer could possibly be a reason for this behaviour.

Light scattering from acrylate-based polymer dispersed liquid crystals: theoretical considerations and experimental examples

A light scattering (LS) study made using acrylate-based polymer dispersed liquid crystals (PDLCs) is presented. The polarized component IVV is measured for blends of a polyacrylate and the liquid crystal (LC) E7 at several compositions. Only the off-state configuration of the droplets with no external fields is considered here. These composites consist commonly of micron-sized nematic LC droplets dispersed in a solid polymer matrix. Theoretical expressions for the scattered intensities in the case of isotropic and anisotropic spherical droplets are given both in the Rayleigh-Gans approximation (RGA) and in the anomalous diffraction approximation (ADA). Series of VV and VH components of the scattering intensities are calculated using the models of Meeten, Stein and coworkers. The model calculations are compared with the light scattering data. This comparison enables us to extract information on the size and the shape of droplets assuming that the size distribution is uniform and that the scattering is due ...

Phase diagrams of poly(dimethylsiloxane)/E7 mixtures

Abstract The phase behavior of blends of poly(dimethylsiloxane) and the eutectic mixture of liquid crystals E7 is investigated. Experimental phase diagrams are established by polarized optical microscopy for two systems with widely different polymer molecular weights. Surprisingly, we find a very slight loss of miscibility of the mixture as a result of an increase of the polymer molecular weight by almost an order of magnitude. This unexpected phase behavior is in contrast with the results reported so far on other systems.

Mechanical Properties of Electron Beam Cured Monomer and Monomer/Liquid Crystal Films

This article reports on mechanical properties of electron beam cured tripropylene glycol diacrylate (TPGDA) and propoxylated glycerol triacrylate (GPTA) films. This study has been motivated by the need to have direct access to those properties for analyzing the thermo-mechanical behavior and electro-optical properties of polymer dispersed liquid crystal systems in general, and systems made either of TPGDA or GPTA and low molecular weight liquid crystals in particular. Representative examples of these systems are considered in this work. The effects of the degree of crosslinking on the mechanical strength of the polymer network are analyzed by considering different doses of the electron beam irradiation. As the radiation dose increases, the mechanical strength of the film is enhanced. Addition of a small amount of liquid crystals leads to remarkable plasticizing effects.

Phase Diagrams and Thermophysical Properties of Polysiloxane and E7 Systems

Phase diagrams of various polysiloxanes and the eutectic liquid cristalline mixture of cyanoparaphenylenes E7 are established. Thermophysical properties are investigated considering the case of Poly(dimethylsiloxane) and Poly(methylphenylsiloxane) with different molecular weights. Differential Scanning Calorimetry and Polarized Optical Microscopy are used to construct the phase diagrams and obtain thermophysical data that are rationalized using a combination of the Flory-Huggins theory of isotropic mixing and the Maier-Saupe theory of nematic order. The glass transition temperature versus composition of the blends investigated yields information of the plasticising effect of the small liquid crystal molecules.

PROPERTIES OF ELECTRON BEAM CURED REVERSE MODE PDLC FILMS

In this paper, we present recent electro-optical data of Polymer Dispersed Liquid Crystal (PDLC) films operating under reverse mode conditions. We consider systems prepared by Polymerization Induced Phase Separation (PIPS) using Electron Beam (EB) curing of acrylate monomers mixed with a low molecular weight liquid crystal (LC). In the present investigation, a low molecular weight LC exhibiting a negative dielectric anisotropy is used yielding electro-optical curves characterized by a transparent initial off-state and an opaque on-state film under the influence of an external field. The results enable us to characterize the sensitivity of electro-optical responses to the functionality of monomers in the precursor mixtures as well as the conditions of film preparation. Data showing the phase behavior of a monomer/LC system are also presented to rationalize the electro-optical results of these interesting but relatively less known materials.

Light Scattering from Monomeric and Electron Beam Cured Acrylate/E7 Systems

A static light scattering study of monomeric (uncured) and Electron Beam (EB) cured Tripropyleneglycoldiacrylate (TPGDA) and E7 (a nematic Liquid Crystal) systems is reported. This study includes a detailed investigation of the structural properties in terms of temperature, composition and scattering angle. Knowledge of the phase diagrams helps us to achieve a better understanding of the scattering curves and rationalize data in terms of the LC configuration inside droplets. The depolarized component of the scattered intensity measured under cross polarizers exhibits patterns reminiscent of anisotropic objects. The results can be correlated to the electro-optical responses of the investigated systems relevant to applications in display technology and smart windows.

Thermo-optical Investigations of Polysiloxane/liquid Crystal Mixtures

Abstract Thermo-optical properties of poly(dimethylsiloxane)/liquid crystal and poly(methylphenylsiloxane)/liquid crystal blends are investigated. Two low molecular weight liquid crystals are considered: 4-cyano-4′-n-pentyl-biphenyl or 5CB and the eutectic mixture of cyanoparaphenylene derivatives known as E7. Equilibrium phase diagrams are established by polarized optical microscopy and analyzed within mean field theories of isotropic mixing and nematic order. Effects of the methyl-phenyl substitution, the nature of liquid crystal and the polymer molecular weight on the phase diagram are discussed.