Abdelylah Daoudi

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.

Effect of the Polymer Network Density Formed in Short Pitch Ferroelectric Liquid Crystal on the Electroclinic Effect

The influence of the polymer network density formed in short pitch ferroelectric liquid crystal on the electroclinic effect has been analyzed. The polymer network is formed from a photo-reactive achiral monomer in a thin planar S c * cell. Using an electro-optic response technique, the electric field dependance of the induced tilt angle and the polarization near to the S A − S c * transition has been measured for different polymer concentrations. The results show that at phase transition temperature the electroclinic effect and the electroclinic susceptibility are largely controlled by the polymer network. These results are discussed and applied to a simple phenomenological model developed in [20]. This model confirms our observations for all of the polymer concentration studied. The average distance between two fibers and the interaction energy between the liquid crystal molecules and the polymer network are estimated. Good correlation is found between the experimental results and the proposed model.

Phase Diagrams of Monomeric and Electron Beam Cured Propoxylated Glyceroltriacrylate/Low Molecular Weight Liquid Crystal Systems

Abstract Equilibrium phase diagrams of mixtures of propoxylated glyceroltriacrylate (GPTA) and low molecular weight liquid crystals (LMWLCs) are investigated. The experimental phase diagrams of monomeric and electron beam (EB) cured GPTA/LMWLC blends are established by polarized optical microscopy (POM). Three LMWLC are used exhibiting different nematic-isotropic transition temperatures. One is the single component LMWLC 4-cyano-4′-n-pentyl-biphenyl (5CB), another one is the eutectic mixture known as E7, and the third one is the single component 4-cyano-4′-n-octyl-biphenyl (8CB) exhibiting both nematic and smectic A orders. The experimental data are successfully analyzed within a unified theoretical treatment based on mean field approximations.

Phase Properties of Monomer/Liquid Crystal and Polymer/Liquid Crystal Mixtures

Abstract The phase diagrams of monomer and polymer/liquid crystal (LC) mixtures are investigated. Two LCs are used: one is the eutectic mixture of four cyanobiphenylene derivatives known as E7 and the other is 4-cyano-4′-n-pentyl-biphenyl or 5CB. The sensitivity of the phase behavior to the nature of monomer, the nature of LC and the UV polymerization is characterized. A single example of UV polymerized/LC system is considered for comparison. Experimental phase diagrams are established by polarized optical microscopy and analyzed with a formalism using mean field theories of Flory-Huggins and Maier-Saupe.

Equilibrium Phase Properties of Polymer/Liquid Crystal Blends: Theory and Experiments

Abstract A theoretical formalism to calculate the phase diagrams of polymer/liquid crystal mixtures is presented. Mixtures involving linear and crosslinked polymers together with low molecular weight nematic and smectic-A liquid crystals are considered. The generalization to binary mixtures of nematogens is briefly shown. The method of calculating model phase diagrams of side chain liquid crystal linear and crosslinked polymers in either isotropic or nematic solvents is presented. In nematogen mixtures, the coupling parameter induces tremendous changes in the miscibility of both linear and crosslinked polymers with low molecular weight analogous or different liquid crystals. Applications of the theoretical formalism are given.

Phase Diagrams of Cured and Uncured Propoxylated Glyceroltriacrylate/5CB Mixtures

Equilibrium phase diagrams of mixtures of propoxylated glyceroltriacrylate (GPTA) and the low molecular weight liquid crystal 4-cyano-4′-pentyl-biphenyl (5CB) are studied. The diagrams of monomeric and electron beam (EB) cured GPTA/5CB blends are established using polarized optical microscopy and differential scanning calorimetry techniques. The experimental data are successfully analyzed using mean field models and a reasonable choice of parameters.

Effect of the Polymer Network Morphology Created Under Electric Field in the Short Pitch FLC on the Dielectric Responses

Polymer-stabilized ferroelectric liquid crystals (PSFLCs) are made by photo polymerizing of 7% reactive mesogenic monomer in a highly twisted ferroelectric liquid crystal (FLC). This study reveals that both the dielectric intensity and the relaxation frequencies of the collective relaxations mechanisms namely soft and the Goldstone modes are strongly affected not only by the presence of the polymer network but also by the polymerization conditions. It is shown that, for sample polymerized with the presence of electric field, the dielectric strengths decrease and the relaxation frequencies are shifted to high frequencies compared to the sample polymerized without the presence of the electric field. This behavior was found strongly dependent of the structure of the polymer fibers. A simple phenomenological model is proposed to explain the behavior of the soft mode dielectric intensity.