Porous Fiber Formation in Polymer-Solvent System Undergoing Solvent Evaporation

Temporal evolution of the fiber morphology during dry spinning has been investigated in the framework of Cahn-Hilliard equation [J. Chem. Phys. 28, 258 (1958)] pertaining to the concentration order parameter or volume fraction given by the Flory-Huggins free energy of mixing [P. J. Flory, Principles of Polymer Chemistry (Cornell University Press, Ithaca, NY, 1953), p. 672] in conjunction with the solvent evaporation rate. To guide the solvent evaporation induced phase separation, equilibrium phase diagram of the starting polymer solution was established on the basis of the Flory-Huggins free energy of mixing. The quasi-steady-state approximation has been adopted to account for the nonconserved nature of the concentration field caused by the solvent loss. The process of solvent evaporation across the fiber skin-air interface was treated in accordance with the classical Fick\u27s law [R. B. Bird , Transport Phenomena (J. Wiley, New York, 1960), p. 780]. The simulated morphologies include gradient type, hollow fiber type, bicontinuous type, and host-guest type. The development of these diverse fiber morphologies is explicable in terms of the phase diagram of the polymer solution in a manner dependent on the competition between the phase separation dynamics and rate of solvent evaporation. (c) 2006 American Institute of Physics

Theory and Computation of Photopolymerization-Induced Phase Transition and Morphology Development in Blends of Crystalline Polymer and Photoreactive Monomer

A hypothetical phase diagram of a crystalline polymer/photoreactive monomer mixture has been calculated on the basis of phase field (PF) free energy of crystal solidification in conjunction with Flory-Huggins (FH) free energy of liquid-liquid demixing to guide the morphology development during photopolymerization of poly(ethylene oxide)/triacrylate blend. The self-consistent solution of the combined PF-FH theory exhibits a crystalline-amorphous phase diagram showing the coexistence of solid+liquid gap bound by the liquidus and solidus lines, followed by an upper critical solution temperature at a lower temperature. When photopolymerization was triggered in the isotropic region, i.e., slightly above the crystal melting transition temperatures, the depressed melting transition line moves upward. When it surpasses the reaction temperature, both crystallization and phase separation occur. The temporal evolution of phase morphology is examined in the context of time-dependent Ginzburg-Landau equations coupled with the energy balance (heat conduction) equation using the aforementioned PF-FH free-energy densities. Of particular interest is that the emerged morphology in the crystalline blends depends on the competition between dynamics of liquid-liquid phase separation and/or liquid-solid phase transition (i.e., crystallization) and photopolymerization rates

Spinodals in a Polymer-Dispersed Liquid-Crystal

Thermodynamic phase equilibria of a polymer dispersed liquid crystal (PDLC) consisting ofmonomeric liquid crystals and a polymer have been investigated theoretically and experimentally.The equilibrium limits of phase separation as well as phase transition of a PDLC system werecalculated by taking into consideration the Flory–Huggins (FH) theory for the free energy of mixingof isotropic phases in conjunction with the Maier–Saupe ~MS! theory for phase transition of anematic liquid crystal. The correspondence between the Landau–de Gennes expansion and theMaier–Saupe theory was found and the coefficients were evaluated. The calculation based on thecombined FH-MS theory predicted a spinodal line within the coexistence of the nematic–isotropicregion in addition to the conventional liquid–liquid spinodals. The cloud point phase diagram wasdetermined by means of polarized optical microscopy and light scattering for a polybenzylmethacrylate/E7 (PBMA/E7) PDLC system. The calculated phase diagrams were tested with theexperimental cloud points, assuming the Flory–Huggins interaction parameter simply to be afunction of temperature

Equilibrium Phase-Behavior of Nematic Mixtures

A phenomenological model for predicting phase diagrams of a binary nematic mixture containing side chain liquid crystalline polymers and/or low molar mass liquid crystals has been proposed by combining Flory-Huggins free energy of isotropic mixing and Maier-Saupe free energy for nematic ordering of the nematogens. Two orientational order parameters, s(1) and S-2, Of the two components in the mixtures having two different clearing temperatures are taken into consideration in the calculation. The Flory-Huggins interaction parameter, chi, and the nematic interaction parameter of the Maier-Saupe theory, nu(1) and nu(22), are assumed to be functions of inverse absolute temperature. Further the cross-nematic interaction is assumed to be proportional to the square root of the product of the nematic interaction parameters of the two mesogens, i.e., nu(12)=c (nu(11).nu(22))(1/2). The theory predicts a variety of phase diagrams depending on a single parameter, c, which is a measure of a relative strength of interaction between two dissimilar mesogens to that in the same species. The predicted phase diagrams have been tested rigorously with experimental phase diagrams of various nematic mixtures reported by others as well as by us. (C) 1995 American Institute of Physics

Phase Diagrams of a Binary Smectic-A Mixture

A variety of smectic phase diagrams involving smectic-isotropic and smectic-nematic-isotropic transitions have been calculated based on a combination of Flory-Huggins (FH) theory for isotropic mixing and Maier-Saupe-McMillan (MSM) theory for smectic-A ordering of liquid crystals (LC). To describe the mesophase transitions, two nematic order parameters and two smectic order parameters have been coupled through the normalized orientation distribution and partition functions. Flory-Huggins interaction parameter (chi) for isotropic mixing and the coupling term involving the nematic interaction parameter (nu) and the McMillan smectic interaction parameter (alpha) for phase transitions of liquid crystals have been incorporated in the calculation. The predictive capability of the present combined FH/MSM model for determining the coexistence regions of a binary smectic-A mixture has been demonstrated by critically testing with a reported smectic phase diagram. (C) 1997 American Institute of Physics. [S0021-9606(97)51441-3]

Phase Equilibria of a Nematic and Smectic-A Mixture

A phase diagram of a mixture consisting of nematic and smectic liquid crystals has been calculated self-consistently by combining Flory-Huggins (FH) theory for isotropic mixing and Maier-Saupe-McMillan (MSM) theory for smectic-A ordering. However, the MSM theory can be deduced to the original Maier-Saupe (MS) theory for nematic ordering. To describe the phase transitions involving induced smectic phase and nematic + smectic equilibrium, two nematic and two smectic order parameters for the nematic/smectic mixtures have been coupled through the normalized partition function and the orientation distribution function. Self-consistent numerical solution has been sought in establishing nematic/smectic phase diagrams involving (i) phase separation between nematic and smectic liquid crystals and (ii) occurrence of induced smectic in a nematic/smectic mixture. The predictive capability of this combined FH/MSM theory has been tested critically with a reported phase diagram of a nematic/smectic liquid-crystal mixture and also with our experimental phase diagram of a mixture consisting of a nematic side-on side-chain liquid-crystalline polymer and a smectic low molar mass liquid crystal. (C) 1998 American Institute of Physics

Nucleation Initiated Spinodal Decomposition in a Polymerizing System

Dynamics of phase separation in a polymerizing system, consisting of carboxyl terminated polybutadiene acrylonitrile/epoxy/methylene dianiline, was investigated by means of time-resolved light scattering. The initial length scale was found to decrease for some early periods of the reaction which has been explained in the context of nucleation initiated spinodal decomposition We have combined the Cahn-Hilliard kinetic equation and polymerization kinetics, and predicted the initial reduction of the length scale triggered by nucleation

Photopolymerization-Induced Crystallization and Phase Separation in Poly(Ethylene Oxide)/Triacrylate Blends

The present article describes experimental and theoretical investigations of miscibility and crystallization behavior of blends of poly(ethylene oxide) (PEO) and triacrylate monomer (TA) using differential scanning calorimetry and optical microscopy. The PEO/TA blends manifested a single T(g) varying systematically with composition suggestive of a miscible character in their amorphous states. Moreover, there occurs melting point depression of PEO crystals with increasing TA. A phase diagram was subsequently established that exhibited a solid+liquid coexistence region bound by the liquidus and solidus lines, followed by an upper critical solution temperature (UCST) at a lower temperature. The emerging phase morphology was investigated to verify the coexistence regions. Upon photopolymerization in the isotropic melt above the melting point depression curve, both the UCST and the melting temperatures move upward and eventually surpass the reaction temperature, resulting in phase separation as well as crystallization of PEO driven by the changing supercooling, i.e., the thermodynamic driving force. Of particular interest is the interplay between photopolymerization-induced phase separation and crystallization, which eventually determines the final phase morphology of the PEO/TA blend such as crystalline lamellae, sheaf, or spherulites in isotropic liquid, phase separated domains, and viscous fingering liquids