N. Gogibus

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.

Preliminary communication Electron beam cured liquid crystal-polymer composite materials: electro-optical enhancement effect

Polymerization induced phase separation initiated by electron beam radiation has been used as a powerful method to obtain well defined PDLC films. Mixtures of different amounts of an aromatic polyester acrylate, an oligoether triacrylate and nematic liquid crystal material E7 were exposed to the electron beam radiation. The electro-optical properties strongly depend on the liquid crystal concentration and on the amount of the oligoether triacrylate in the prepolymer mixture, and are highly reproducible. An unusual enhancement effect of the electro-optical sensitivity was found for samples containing 28 weight per cent and 34 weight per cent of the oligoether triacrylate.

Electro-optical properties of electron beam cured polymer dispersed liquid crystal films based on polysiloxanes and liquid crystals

Polymerization and cross-linking induced phase separation initiated by electron beam radiation is used to elaborate Polymer Dispersed Liquid Crystal (PDLC) films made of polydimethylsiloxane and 5CB. Electro-optical properties of the obtained films are investigated in terms of the composition of the initial mixture and amplitude of the applied AC voltage. A particular attention is given to transmission levels and operating voltages.

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.

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.

Effects of polymer architecture on the phase properties of polymethylsiloxane/liquid crystal blends

Abstract The phase properties of polydimethylsiloxane (PDMS) and low molecular weight liquid crystals (LMWLCs) with different size and architecture of the polymer are investigated. An oligomer of molecular weight 800 g/mol and two linear polymers of molecular weight 5500 and 45000 g/mol are considered. Electron beam cured PDMS/4-cyano-4′-n-pentyl-biphenyl (5CB) mixtures in which the polymer is a crosslinked network are also investigated. Experimental phase diagrams are established using polarized optical microscopy (POM) and analyzed using classical theories for the free energy of isotropic mixing, network elasticity and nematic order.

Polymer dispersed liquid crystals made of deuteriated polystyrene and 8CB blends: A small angle neutron scattering study

Structural properties of Polymer Dispersed Liquid Crystals made of mixtures of deuteriated polystyrene (dPS) and 4-cyano-4′-n-octylbiphenyl (8CB) are studied by Small Angle Neutron Scattering (SANS). Two distinct blends are considered with a polymer dPS molar mass Mu equal to 49500g/mol and liquid crystal concentration 60 and 75 weight% (wt.-%). Data covering the range of temperature from 30°C to 75°C are discussed. In this temperature domain, 8CB exhibits a variety of orderings such as smectic-A (21.5°C–33.5°C), nematic (33.5°C–40.5°C) or simply an isotropic state above 40.5°C. The results are rationalized in terms of the Random Phase Approximation (RPA) invoking the temperature dependence of the Flory-Huggins interaction parameter and the correlation length of fluctuations at the interface of polymer and liquid crystal.

Onset and End of Transitions in PDMS/5CB Blends as Revealed by Static Light Scattering

The phase diagram of poly(siloxanes) and low molecular weight liquid crystals as obtained by polarized optical microscopy have shown a peculiar behavior : Transitions from a nematic to an isotropic and from an isotropic miscibility gap to a single isotropic phase take place in two steps interpreted as onset and end of transitions. This behavior has been observed in various systems and particular in those involving poly(dimethylsiloxane) (PDMS) and 4-cyano-4′- n -pentyl-biphenyl (5CB). In order to confirm this behavior we performed static light scattering on blends of PDMS ( M w =45000g/mol) and 5CB at different compositions. These experiments covered a temperature range from 20 to 115°C and several scattering angles.