Multiple Phase Equilibria in Polydisperse Polymer/Liquid Crystal Blends

Abstract

A thermodynamic analysis of phase equilibria in polydisperse polymer−liquid crystal blends is presented. Three different systems were analyzed: (a) a high molar mass polydisperse polystyrene (PS) blended with 4-cyano-4‘-n-heptylbiphenyl (7CB), (b) a low molar mass polydisperse PS blended with 7CB, and (c) an epoxy-based thermosetting polymer blended with a mixture of small mesogenic molecules usually called E7. In the latter case the analysis was performed in both pregel and postgel stages. Macroscopic phase separation taking into account the fractionation of the polydisperse polymer among different phases, was simulated when cooling from an initially homogeneous state. Predicted isotropic−isotropic and isotropic−nematic transitions showed a good agreement with experimental results. The relative volume fraction and compositions of isotropic and nematic phases were predicted. A temperature range was found where three macroscopic phases, two isotropic and one nematic, coexisted at equilibrium. This was the result of a liquid−liquid (or gel−liquid) phase separation preceding the appearance of a nematic phase. Implications of this behavior on morphologies developed in polymer-dispersed liquid crystals (PDLC) are discussed.

Probing the Mesoscopic Chemical and Physical Properties of Polymer-Dispersed Liquid Crystals

“…[80,88±94] Important parameters include initial monomer/polymer and liquid crystal solution concentrations, [79,95±97] rate of polymerization, [79,82,95,98] polymer molecular weight, [90] temperature, [79,98,99] and even the presence of gravitational fields. [88,100] Therefore, films with spatially varying properties are often obtained since a number of these parameters may vary spatially during PDLC production.…”

Polymer‐Dispersed Liquid Crystals Based on Polystyrene and EBBA: Analysis of Phase Diagrams and Morphologies Generated

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Most of these blends exhibit a liquid-liquid (L-L) immiscibility region in their phase diagrams that precedes the formation of a nematic phase during the cooling step. Therefore, the generated morphologies are determined by the mechanism of the previous L-L phase separation.…”

“…Aims of this study were: a) to build-up the phase diagram of a polydisperse PS-EBBA blend in the whole composition range, including the stability regions of nematic and crystalline phases (previous reported phase diagrams were restricted to the region of low PS concentrations [1] or did not distinguish between nematic and crystalline phases [3] ); b) to assess the suitability of the Flory-Huggins-Maier-Saupe (FHMS) model, extended to take polydispersity of the thermoplastic polymer into account [12] to fit the experimental isotropic-nematic curve in a large composition range (the extended model was previously applied to blends exhibiting an L-L immiscibility region in the phase diagram); [12] c) to analyze the influence of the cooling rate on the distribution of the particle sizes.…”

Phase separation kinetics of an SA liquid crystal in a reactive epoxy-amine PDLC system: an X-ray diffraction study

“…In spite of the difference of compositions between E7 (N) and S A LC and in spite of the fact that polymerization induced phase separation can change the composition of the initial LC mixture 22) , the evolution of the separated mesomorphic phase is consistent with the previously modeled results on system A (i. e. in the presence of E7), which showed that a separation between the epoxide-amine i-mers and a nematic phase gives a nearly pure nematic phase. It was shown by modeling that growing i-mers are excluded progressively from the mesophase 19,20) . The diffraction intensity (integrated and expressed in arbitrary unit) as a function of time is represented in Fig.…”

“…decomposition can be found for isotropic isomolecular thermoplastic polymer blends 16) or examples of experimental studies by SAXS for reactive isotropic thermoset/ elastomer blends 17,18) . When initially miscible monomers/LC isotropic solutions polymerize isothermally, phase separation is induced by the evolution of free energy of mixing, DG m , with the extent of reaction, x. DG m depends upon enthalpic, entropic, anisotropic (nematic or smectic) and elastic energy terms after gelation [19][20][21] . The conversion at phase separation, usually named cloud point conversion x CP , depends on the dominance of each of these terms as well as on the concentration and temperature chosen 14,19) ; it also governs the subsequent development of morphology.…”