Thein Kyu

Spinodals in a polymer dispersed liquid crystal

Thermodynamic phase equilibria of a polymer dispersed liquid crystal (PDLC) consisting of monomeric 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 were calculated by taking into consideration the Flory–Huggins (FH) theory for the free energy of mixing of isotropic phases in conjunction with the Maier–Saupe (MS) theory for phase transition of a nematic liquid crystal. The correspondence between the Landau–de Gennes expansion and the Maier–Saupe theory was found and the coefficients were evaluated. The calculation based on the combined FH‐MS theory predicted a spinodal line within the coexistence of the nematic–isotropic region in addition to the conventional liquid–liquid spinodals. The cloud point phase diagram was determined by means of polarized optical microscopy and light scattering for a polybenzyl methacrylate/E7 (PBMA/E7) PDLC system. The calculated phase diagrams were tes...

Effect of carbon nanotubes on phase transitions of nematic liquid crystals

Phase diagrams of multi‐wall carbon nanotube (MWNT)/nematic liquid crystal (E7) and buckminsterfullerene (C60‐I h)/nematic liquid crystal (E7) binary systems have been investigated by means of polarizing optical microscopy and differential scanning calorimetry. It was found that the isotropic–nematic phase transition temperature (T NI) of the liquid crystal component was enhanced by the incorporation of MWNT within a small composition gap. A chimney‐type phase diagram can be identified in the MWNT/E7 mixture over a narrow range of ∼0.1–0.2% MWNT concentration. Upon substituting the nanotubes with isotropic fillers such as fullerene, the (C60‐I h)/E7 blend showed no discernible change of T NI in the same concentration range of the chimney of the MWNT/E7 mixture, suggesting a significant contribution of anisotropy (or the aspect ratio) of the nanotubes to the entropy of the system containing liquid crystal molecules. This enhanced T NI phenomenon may be attributed to anisotropic alignment of liquid crystal molecules along the carbon nanotube bundles.

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’s law [R. B. Bird et al., Transport Phenomena (J. Wiley, New York, 1960), p. 780]. The simulated morphologies include gradient type, h...

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, s1 and s2, of the two components in the mixtures having two different clearing temperatures are taken into consideration in the calculation. The Flory–Huggins interaction parameter, χ, and the nematic interaction parameter of the Maier–Saupe theory, ν11 and ν22, 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., ν12=c (ν11⋅ν22)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 diss...

Morphology development during polymerization-induced phase separation in a polymer dispersed liquid crystal

Abstract To elucidate the emergence of liquid crystal (LC) domains during polymerization induced phase separation in a polymer dispersed liquid crystal (PDLC), numerical simulation has been performed by incorporating the kinetics of crosslinking reaction into the time-dependent Ginzburg–Landau (TDGL-Model C) equations in conjunction with the combined Flory–Huggins (FH)/Maier–Saupe (MS) free energies. The TDGL-Model C basically consists of two coupled equations in which a conserved compositional order parameter (i.e. the volume fraction) is coupled with a non-conserved orientational order parameter of the LCs. Of particular interest is the influence of nematic ordering on the emergence of domain morphology that shows a strong dependence on curing temperatures and compositions, displaying a rich variety of patterns.

Phase separation in poly(p-phenylene benzobisthiazole)/nylon 66 molecular composite

Abstract Poly (p- phenylene benzobisthiazole ) nylon 66 rigid-rod molecular composite undergoes thermally induced phase separation when heated to the nylon melting temperature. The phase-separated domains imaged by back-scattering scanning electron microscopy are 2–7 μm in size. The cloud-point curve is determined by using small-angle light scattering. Above the cloud point, phase separation proceeds with the development of a scattering ring which increases in intensity and moves towards the main beam with time.

Phase separation behaviour in blends of isotactic polypropylene and ethylene-propylene diene terpolymer

Miscibility and phase separation behaviour in blends of isotactic polypropylene (iPP) and ethylene-propylene diene terpolymer (EPDM, ethylene content 70%) have been examined by means of depolarized light scattering, polarizing optical microscopy, differential scanning calorimetry and dynamic mechanical methods. A cloud point phase diagram was established by means of light scattering after eliminating the influence of iPP crystal melting on liquid-liquid phase separation of iPP/EPDM blends. A lower critical solution temperature (LCST) was observed above the crystallization temperature, but below the melting temperature of iPP. This LCST is thermally reversible, so long as crystallization does not occur in its vicinity. The coupling between the crystal melting and liquid-liquid phase separation has been examined. Crystallization-induced phase separation and morphology development during crystallization of iPP have been investigated.

Toughening of thermoset/thermoplastic composites via reaction-induced phase separation: Epoxy/phenoxy blends

Phase morphology and phase separation behavior of amine-cured bisphenol-A diglycidyl ether epoxy and phenoxy mixtures have been investigated by means of time-resolved small angle light scattering, optical microscopy, and scanning electron microscopy. The starting reactant mixtures composed of epoxy, phenoxy, and curing agents such as diaminodiphenyl sulfone (DDS) and methylene dianiline (MDA) were found to be completely miscible. Upon curing with DDS at 180°C, phase separation took place in various epoxy/phenoxy blends (compositions ranging from 10–40% phenoxy), whereas the MDA curing showed no indication of phase separation. The mechanical and physical properties of single-phase and two-phase networks were examined, in that the DDS-cured epoxy/phenoxy blends having a two-phase morphology showed improved ductility and toughness without significantly losing other mechanical and thermal properties such as modulus, tensile strength, glass transition and heat deflection temperatures. The energy absorbed to failure during the drop weight impact event was also found to improve relative to those of the single-phase MDA-cured blend as well as of the neat epoxy. Such property enhancement of the DDS-cured blends has been discussed in relation to the two-phase morphology obtained via scanning electron microscopy micrographs of fractured surfaces.

Theoretical investigation on dynamics of photopolymerization-induced phase separation and morphology development in nematic liquid crystal/polymer mixtures

A theoretical investigation of the dynamics of photopolymerization-induced phase separation (PIPS) and morphology development in a nematic liquid crystal (LC) polymer network mixture has been undertaken by incorporating photopolymerization kinetics into the coupled time-dependent Ginzburg-Landau (TDGL–Model C) equations. The simulation on the spatio-temporal evolution of the coupled LC concentration and orientation order parameters reveals that both morphological and scattering patterns for the orientation order parameter initially lag behind those of the concentration order parameter. However, the two fields evolve to the same spatial topologies with the progression of time. The PIPS dynamics is characterized only by the late stage of phase separation. We also observed a subtle change in the curvature of the growth curve associated with the onset of nematic ordering. The growth behavior and the simulated morphology consisting of LC droplets dispersed in a matrix of polymer appears the same for all compos...

Novel filled polymer composites prepared from in situ polymerization via a colloidal approach. I. Kaolin/Nylon-6 in situ composites

A mineral-filled in situ composite was prepared by a colloidal approach by first suspending kaolin filler particles in aqueous caprolactam, and then polymerizing caprolactam in situ at high pressure and temperature. The purpose of this colloidal in situ polymerization is to improve particle dispersion and to enhance interaction of the filler to the polymer matrix. X-ray diffraction studies of the in situ kaolin/Nylon-6 composites revealed that the x-ray peak corresponding to the α-crystal form of Nylon-6 diminished with increasing kaolin loading, while the γ-crystal structure became more pronounced. The degree of crystallinity of Nylon-6 remained fairly unchanged with the kaolin loading level in the in situ composites. Calorimetric and dynamic mechanical studies exhibited that the glass transition temperature of the resulting composite increased significantly with increase in kaolin concentration, suggesting strong filler-matrix interaction at the kaolin/Nylon-6 interface. Scanning electron microscopic (SEM) results showed uniform filler dispersion in the in situ composites relative to the conventional melt-mixed composites. Modulus and tensile strength of these in situ composites were found to be distinctively higher than that of the conventional melt-mixed kaolin/Nylon-6 composites. However, as typical for composite materials, drawability and fracture toughness decreased with increasing kaolin loading.