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Phase diagrams of unpolymerized and UV-polymerized 2-ethyl hexyl acrylate (EHA) mixtures with the liquid crystal E7 are established using optical microscopy and differential scanning calorimetry. Both diagrams show upper critical solution temperature behavior. From 50 to 90 wt % liquid crystal (LC), the (I+I) phase located between the (N+I) and (I) phases was clearly shown. The nematic phase inside the droplets exhibits a twisted radial structure indicating that homeotropic anchoring occurs at the polymer interface. The experimental phase diagrams were successfully analyzed using a model based on the Flory-Huggins theory of isotropic mixing supplemented with the Maier-Saupe theory of nematic order. The LC solubility limit in the polymer matrix and the fractional amount of LC contained in the droplets were deduced from the calorimetric measurements. For the specific composition EHA/E7 (50:50), the scattering and morphological properties of the films were studied as a function of time elapsed after UV exposure. Drastic changes in the size, shape, spatial distribution, and number density of nematic droplets were observed and analyzed in terms of coalescence/diffusion phenomena.

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Equilibrium phase diagrams and thermophysical properties of mixtures of poly(styrene) (PS) and 4-cyano-4′-n-octyl-biphenyl (8CB) are investigated. Three systems with widely different molecular weights of the polymer are considered in an attempt to assess the effects of the polymer size on the miscibility of PS and 8CB. The experimental phase diagrams are established using polarized optical microscopy (POM), light scattering (LS), and differential scanning calorimetry (DSC), and the results were analyzed with the predictions of the Flory-Huggins theory of isotropic mixing and the Maier-Saupe theory of nematic order generalized by McMillan to include smectic-A order. Good agreement is observed between theory and experiments. The solubility properties of mixtures with different polymer sizes are analyzed using the method suggested by Smith. The solubility limit of 8CB in PS is deduced from enthalpy changes at the nematic-isotropic transition temperature (T NI) as a function of polymer molecular weight. It is found that the decrease of the solubility limit with increasing molecular weight is not linear and reaches a plateau value for higher molecular weights. The results obtained for the systems investigated here and for three other systems reported in the literature fall on a single master curve representing the solubility limit at T NI as a function of polymer molecular weight.

The thermophysical, dielectric and electro-optic properties of polymer-dispersed liquid crystal (PDLC) films made of monodisperse polystyrene (PS) and 4-n-pentyl-4(')-cyanobiphenyl (5CB) are investigated by polarized optical microscopy, differential scanning calorimetry, ac impedance analysis, and forward transmittance measurement technique. The PS-5CB system exhibits an upper critical solution temperature (UCST) shape phase diagram with a wide isotropic+isotropic (I+I) miscibility gap between the isotropic and nematic+isotropic (N+I) regions. An absorption domain in the dielectric spectrum of PDLC films was clearly observed at low frequency, and unambiguously assigned to the confined liquid crystalline phase in both nematic and isotropic states. The correlation between the dielectric and electro-optical results for PS-5CB (30:70) samples has shown that in the vicinity of the low frequency absorption domain ( approximately 200 Hz at T=25 degrees C), a drastic decrease in the optical transmittance of the film occurs. This phenomenon can be related to an interfacial polarization process resulting from a charge accumulation at the droplet-polymer interface (Maxwell-Wagner-Sillars effect).