Phase Diagrams and Phase Separation Dynamics in Mixtures of Side-Chain Liquid Crystalline Polymers and Low Molar Mass Liquid Crystals

Abstract

Miscibility phase diagrams and phase separation dynamics in mixtures of side-chain liquid crystalline polymers (SCLCP) and monomeric liquid crystals have been investigated experimentally in order to test with theoretical predictions. The theoretical calculation on equilibrium phase diagrams of binary nematic mixtures involves a combination of the Flory-Huggins (FH) free energy for isotropic mixing in conjunction with the Maier-Saupe (MS) free energy for nematic ordering of the mesogenic units. Two orientational order parameters and two nematic-isotropic (NI) transitions of the constituent mesogens are taken into consideration in the calculation. This combined FH-MS theory predicts a variety of phase diagrams depending on the relative strength of the cross-interaction between two dissimilar mesogens as compared to that in the same species. A series of epoxy resins [ethylene glycol glycidyl ether (EGDE)] grafted with mesogenic amines 4-(ω-aminoalkoxy)-4′-cyanobiphenyl were synthesized. The temperature-composition phase diagrams of these SCLCP mixtures and the eutectic low molar mass liquid crystal (E7) have been established by a cloud point method. The calculated phase diagrams are found to accord well with the experimental phase diagrams. The dynamics of phase separation has been investigated by means of time-resolved light scattering and optical microscopy. The time evolution of the structure factor following various temperature quenches into the nematic-liquid and/or nematicnematic coexistence regions has been analyzed in terms of a power law scheme.

Phase behavior of binary mixtures of thick and thin hard rods

“…We mention, ÿnally, that phase diagrams involving I-I, I-N, and N-N coexistence have also been found recently in model calculations of thermotropic binary mixtures of side-chain liquid crystalline polymers and low molecular weight liquid crystals [14]. These calculations are, however, based on a combination of Maier-Saupe and FloryHuggins theory, in which the temperature is the driving parameter while the density or pressure is not considered explicitly.…”

“…Consequently, the mechanisms behind the phase transitions discussed in Ref. [14] are energetic rather than entropic in nature, although one could argue that some entropic e ects are hidden in model parameters such as the temperature independent part of the Flory-Huggins interaction parameter. It would therefore be interesting to consider a full theory for such binary mixtures, in which both the energetic contributions of Ref.…”

“…It would therefore be interesting to consider a full theory for such binary mixtures, in which both the energetic contributions of Ref. [14] and the entropic contributions discussed in this paper are taken into account explicitly and consistently.…”

Model phase diagrams of binary nematic mixtures. A comparative study between linear and crosslinked polymers

“…The nematic free energy of binary nematogen mixtures has been worked out by others 3,4) in the framework of the Maier-Saupe theory 15,16) …”

“…If these parameters are below s 0 then the nematic free energy does not contribute and only the isotropic free energy governs the phase behavior of the mixture. The limiting value s 0 = 0.429 was also calculated for E7 mixture and will be adopted in the present case as well 3,4) .…”