LC-rich Phase Research Articles

Characterisation of polymerisation-induced phase separation process in polymer-dispersed liquid crystals based on hydroxyalkyl-methacrylate matrices

ABSTRACTHydroxyalkyl-methacrylates are very well-known hydrogel-forming biocompatible polymers. In this work, polymer-dispersed liquid crystals (LCs) based on hydroxyethyl- and hydroxypropyl-methacrylate (HEMA and HPMA) matrices are synthesised and characterised. Two different LCs from the cyano-biphenyl family (4ʹ-pentyl-4-cyanobiphenyl [5CB] and 4ʹ-octyl-4-cyanobiphenyl [8CB]) are used. Polymerisation kinetics, phase transitions, and morphologies generated during the polymerisation-induced phase separation process are analysed. Phase separation is observed at very low conversions, where a polymer-rich phase is initially segregated and this produces an increase in the low-conversion polymerisation rate (ascribed to the auto-acceleration effect of free-radical polymerisations in the polymer-rich phase). The liquid crystalline transition temperature decreases as the concentration of LC decreases, indicating that this phase is impurified. The liquid crystal 8CB is found to be less miscible than 5CB with the polymer matrices, so the LC-rich phase for 5CB is more impurified and its total volume in the material is smaller.

Nematic–nematic phase separation from uniform nematic domain in polymer LC and low-molecular-weight LC mixture showing network lattice formation in pinning stage

According to the well-defined phase diagram of a mixture of low-molecular-weight liquid crystal (LC) material and LC polymer, we investigated the LC/LC phase separation dynamics from the uniformly aligned nematic phase that was prepared by sandwiching it between rubbed cells with a thickness of 3 μm. Polymer compositions in the mixtures and decomposition temperatures varied from 28.3% to 34.4% and 14 to 21 °C, respectively. The phase separation proceeded in three stages in the following order; the early spinodal decomposition (SD) stage, later SD stage and pinning stage. In the final pinning stage, an ordered honeycomb network structure was formed wherein the low-molecular-weight LC-rich phases were accommodated within hollow microcells of the polymer LC-rich phase. The size of the network lattice was varied from 2 to 8 μm by varying the weight fraction of the mixtures and the decomposition temperature. Pinning lattice formation was discussed with respect to an interplay between orientational elasticity and anchoring effect on segregated domain boundaries, thus providing insights into nanoscopic self-organization in LC/LC segregation systems.

Confocal Raman Microscopy of Liquid-Crystal-Filled Polymer Capsules Made by Photo-Enforced Stratification

The applicability of confocal Raman microscopy for characterizing thin liquid-crystal (LC) filled polymer capsules obtained by photo-enforced stratification is demonstrated. The investigated structure consists of an array of polymer capsules (typical size 500 x 500 x 20 microm) filled with LC material and is made by photopolymerization of a mixture of monomers and LC. Such an array can be used as the electro-optical component in liquid crystal displays. Confocal Raman microscopy does not require complex sample preparation, is non-invasive, and is shown to have adequate spatial and depth resolution. Although Raman spectroscopy is inherently insensitive, the use of data preprocessing and computational modeling makes it possible to quantify both the conversion of monomer to polymer and the compositions of both the polymer-rich and the LC-rich phase.

Phase Separation and Morphology of PDLC Based on Poly(Ethyl 2-Cyanoacrylate)

The paper deals with polymer/liquid crystal composites prepared by polymerization induced phase separation of homogeneous mixtures of ethyl α-cyanoacrylate (ECA) and nematic E7. DSC analysis of the polymerization reaction indicates that the monomer conversion is unaffected by the presence of E7 which acts only as a diluent. Thermal characterization of the composite films PECA/E7 reveals a partial solubility of E7 in the polymer matrix (about 20%), a degradation of the LC purity, an anomalous variation of the specific heat increment of E7 at Tg and of the isotropization enthalpy with the E7 content in phase separated systems. SEM analysis offers adequate support to justify the DSC behavior; indeed, whereas the 20% E7 mixture has a fracture surface similar to that of the unblended matrix, the 40% E7 composite shows a LC separated droplet morphology with a low connection grade. The morphology changes considerably upon increasing the E7 concentration in the blend or lowering the curing temperature; heterogeneous LC-droplet and polymer-ball morphologies appear, arising from differences in the rate of polymerization of ECA between two early separated phases: a LC-poor one. rich in ECA/PECA and a LC-rich phase, still containing substantial amount of monomer.

Spinodal decomposition kinetics of a mixture of liquid crystals and polymers

SUMMARY: The spinodal decomposition (SD) kinetics of the mixture of polystyrene (PS) and a low molar mass liquid crystal (LC) (E7) has been studied by means of time-resolved small angle light scattering (SALS) and polarized optical microscopy (POM). It is found that, in the isotropic spinodal region, the mixture first undergoes an isotropic concentration fluctuation, and then the ordering process that occurs in pure LC will appear in the LC-rich phase after phase separation. In contrast to that, however, the concentration and orientation fluctuations take place simultaneously in an anisotropic spinodal region. A rapid phase separation via SD was observed in the anisotropic spinodal region due to the fact that the ordered LC phase excludes the polymer coils more strongly than the isotropic region.

P-V-T relations in a series of longitudinal polymer liquid crystals with varying mesogen concentration

Pressure-volume-temperature (P-V-T) relationships were determined for polymer liquid crystal (PLC) solids and melts up to 400°C and 240 J cm −3. We have studied a series of longitudinal PLC copolymers with the formula PET/ xPHB, where PET = poly(ethylene terephthalate), PHB = p-hydroxyben-zoic acid (the LC component), with the mole fractions of the LC component 0 ≥ x > 0.8. The P-V-T results are represented by the Hartmann equation of state and its characteristic parameters ν ∗, T ∗ and P ∗ evaluated. In both solid and liquid phases the ν ∗(x) and T ∗(x) curves show minima near the concentration θ LC limit above which islands of the LC-rich phase are formed. Surface tensions of melts are calculated as a function of x from the P-V-T data for the liquid state using the theory of Prigogine and Patterson. Increasing x first causes lowering of the cohesion of the single-phase structure dominated by the flexible PET. Above θ LC limit the islands appear to impart their orientation to both phases; this is the channeling effect predicted earlier from the Flory statistical-mechanical theory of PLCs (Blonski et al., Macromolecules, 1993, 26, 84)

Polymer Dispersed Liquid Crystals Based on Epoxy Networks: Transformations and Phase Separation Phenomena During Polycondensation

A diepoxy-diamine/liquid crystal mixture (50/50 wt%) is studied in the course of the crosslinking step-polymerization at different isothermal reaction temperatures or with a two temperature cure cycle. Depending on the reaction temperature, the systems undergoe phase separation either isothermally, when the reaction temperature is far below the nematic-isotropic (N-I) temperature of the neat LC, or on cooling partially reacted systems when the reaction temperature is above the N-I temperature of the neat LC. Phase separation is characterized simultaneously by two glass transition temperatures (Tg ), one nematic-isotropic transition (T N-1) and the appearance of nematic droplets as determined by differential scanning calorimetry (DSC) or polarized optical microscopy (POM). Thus, temperature-polymerization conversion phase diagrams are presented in the form of the evolution of Tg 's and of T N-1 of the LC-rich phase. Furthermore, we attempted to use the calorimetric data to calculate the mass fraction of the LC-rich phase and the mass fraction of separated LC. The gel point conversions of the epoxy-amine network are determined. For example, T N-1 of the LC phase markedly increases near the gel point during an isothermal crosslinking at 30°C. The observation of the PDLC samples confined between two glass plates showed that final morphology (at full conversion) depends strongly on the thermal cycles of crosslinking. Finally, for a cure at 100°C, in bulk samples, evidence is given for a macrosyneresis of the LC in the fully crosslinked network. However, this phenomenon occurs very slowly with time. This means that depending on the cure cycles, on the LC content and on the confinement of the LC, the resulting PDLC may be prepared in off equilibrium conditions.

Phase behaviour and electro-optic characteristics of a polymer stabilized ferroelectric liquid crystal

Abstract The effects of adding a diacrylate monomer or its polymerized network to a ferroelectric liquid crystal have been characterized. The monomer lowers the temperatures of transition to the more ordered phases, whereas the polymer network phase separates into polymer rich and LC rich phases and has little effect on the LC phase behaviour. Ferroelectric polarization decreases comparably in both monomer and networked systems. As the network concentration increases, the size of LC domains decreases considerably. With low concentrations of polymer and, thus large LC domains, optical response and tilt angle remain fairly independent of polymer concentration, but as the polymer concentration increases, switching speed and tilt angle decrease dramatically. Polymerization rate maxima increase with monomer concentration until saturation of monomer in the liquid crystal is reached. The rate maxima then decrease as monomer must diffuse from monomer rich droplets. Double bond conversion during the polymerization is comparable for all monomer concentrations below 50 per cent.

A Calorimetric Study of Phase Separation in Liquid Crystal/Matrix Systems: Determination of the Excess Specific Heat of Mixing

Abstract The effects of concentration and state of cure on mixing and phase separation of binary systems of a low molecular weight liquid crystal (LC) and an organic matrix have been determined for the first time. We found that, as temperature is increased, mixing in uncured LC/matrix systems is accompanied by a step-like decrease in specific heat at T mix, the mixing temperature. This decrease is due, for the most part, to the (negative) excess specific heat of mixing. ΔC mix. The excess enthalpy of mixing. ΔH mix, may also contribute to the thermal effect. A plot of ΔC mix versus LC concentration exhibits a minimum, as expected from theory. Such a plot for a mixture of a specific LC (E7) with uncured matrix (NOA65) allows us to estimate the solubility limit of E7 in the matrix-rich phase (∼15%) and of NOA65 in the LC-rich phase (∼5%). Uncured E7/NOA65 mixtures exhibit phase diagrams with an upper critical solution temperature (UCST) slightly above 300 K. To avoid undesired phase separation prior to fina...

Statistical thermodynamics of polymer liquid crystals: variation of concentration of the mesogenic component

Polymer liquid crystal (PLC) macromolecules consisting of LC and flexible sequences with varying composition 8 of LC segments are considered using the lattice approach of Flory:p8 with the orientational distribution function developed by Flory and Ron~a.~~~ We obtain a partition function dependent on 8, on the average length of rigid sequence 4, on the order parameter s, and on temperature T. For 8 = 1 the equations reduce to the case of fully rigid rods treated before by Flory and collaborators. A drastic change in the slope of s vs fi curve is observed around 8 = 0.2, indicating a large increase in the chain alignment. A prediction of TLC-I in function of 8 for a given 4 value is shown. LC-rich phases are formed for 8 > 8Lc limit; that limit is discussed in reIation to earlier work by Flory and Matheson, experimental evidence, and the present results.