On the formation of liquid crystalline texture in epoxy resins

Three epoxy resins and two curing agents were studied in their monomeric as well as polymeric states. Two monomers were monotropic liquid crystals. Characterization of liquid crystallinity before, during and after curing reaction was carried out by DSC, optical microscopy and WAXS. It was found that LC texture develops in every epoxy/(curing agent) system studied in this work. Nucleation and growth is a physical mechanism for the formation of initial liquid crystallinity. The final texture is not well defined and likely contains nematic, poorly ordered smectic and isotropic phases. Schematic diagram of the possible states of LCTs is discussed.     

Relaxation Processes in sheared films of ethyl-cyanoethyl cellulose cholesteric liquid crystalline solutions

The relaxation processes in sheared films of ethyl-cyanoethyl cellulose [(E-CE)C]/acrylic acid (AA) cholesteric liquid crystalline (LC) solutions were studied by polarizing optical microscopy (POM) and UV-Vis spectrophotometry. Under shearing normal to the helix axis and above the critical shear rate, the planar texture arrangement of the (E-CE)C/AA cholesteric LC solution was destroyed and transformed to the nematic phase. Observed by POM, the banded texture formed quickly following the cessation of the shear, but it was unstable and disappeared after several minutes. The reflection spectrum of the sheared (E-CE)C/AA cholesteric LC solution film was recorded as a function of relaxation time. It was found that the selective reflection property was lost under the shear, but the shape of the reflection spectrum recovered quickly with cessation of the shear, and the reflection peak in the spectrum became sharper with time, returning to the original form before shearing. A proposed model of the structural transformation during the relaxation was confirmed by additional optical measurement and transmission electron microscopy.     

Shear-induced band texture of side group liquid crystalline polymers

The shear-induced band texture of conventional end-on fixed side group liquid crystalline polymers (LCPs) has been investigated by using polarizing optical microscopy (POM), small angle light scattering (SALS) and infra-red dichroism techniques. The band spacing is about 1 μm, which increases very slightly on increasing the temperature of shearing and is independent of shearing rate within the range studied. The band texture is not seen to exhibit an interchange of dark and bright bands on rotation of the sample with respect to the polarizer/analyser, but a typical periodical structure is reflected by the SALS patterns of the band texture. The relaxation behaviour of the bands indicates that the band texture formed here is the result of the orderly aligning of domains exhibiting the focal-conic texture, and this is totally different from the case of main chain LCPs where the band texture is substantially an optical effect of the periodic zigzag or sinusoidal structure of parallel aligned microfibrils. Infra-red dichroism and rotating parallel-plate shearing measurements show that the axes of the backbone of the polymer tend to orient in the shearing direction and the end-on fixed mesogenic side groups tend to align perpendicular to the shearing direction.     

The liquid crystalline state of polyimide precursors

The polyimide precursor poly(4,4′-methylenediphenylene pyromellitamic acid) (PMDPAA) in N-methyl pyrrolidone (NMP) was a lyotropic liquid crystal as evidenced by a DSC thermogram and x-ray spectrum on its gel, and optical properties by the existence of the critical concentration on the solution viscosity. The configuration of PMDPAA as shown by wide angle x-ray diffraction pattern was composed of regular meta pyromellitamic acid units. These units together with the rigid character of ? CH₂? linkage between two phenyl rings, and the hydrogen bonding of the intermesogenic units were surmised to be the driving force for liquid crystal formation. The liquid crystalline characters were dependent on molecular weight, solid content, temperature, and the extent of mechanical shearing flow. Data from rheometrics mechanical spectrometer (RMS) indicated that the elastic character of the melt of PMDPAA gel at room temperature shifted to lower frequency on standing and the complex dynamic viscosity increased with time.     

Solid state crystalline and liquid crystalline structure of semifluorinated 1-bromoalkane compounds

A series of semifluorinated 1-bromoalkane (SFBA) mesogens have been synthesized and characterized to better understand their solid state crystalline and liquid crystalline structures. In the solid state, the local conformation of the fluorocarbon segments becomes disordered once the fluorocarbon chain reaches a length above eight CF units. This is evident from the pronounced decrease of molar melting enthalpy. An increasing amount of helix and helix reverse conformations and increasingly disordered packing can also be observed with each addition of a fluorocarbon segment. X-ray diffraction peaks in the small angle region can be indexed by a tilted, two dimensional layered (herring bone) structure. The crystal structure is similar to a type of plastic crystal in which the amphiphilic character is clear, because the two segments of fluorocarbon and hydrocarbon are almost immiscible. Heating of F(CF2)12(CH2)10Br leads to a transition from plastic crystal to smectic B, as revealed by time-resolved XRD and FTIR analysis. At this solid-to-liquid transition temperature, conformational analysis confirmed an onset of the CH2 gauche conformation within the hexagonal lattice, most likely due to changes occurring in the hydrocarbon segment, and a sudden increase of helix defects along the fluorocarbon segment. The disordered helix rigid-rod structure of the fluorocarbon segment and its poor compatibility with the hydrocarbon segment play an important role in the crystalline solid and liquid crystalline structures.     

Use of noncovalent interactions to form novel liquid crystalline polymeric materials

A novel concept in polymeric liquid crystalline (LC) materials, via electrostatic grafting or complexation of functionalized mesogens and appropriate polymers, is described. It is applied to three specific systems characterized by side-chain LC architecture, two of which involve hydrogen-bonding interactions, the third ionic interactions. In these systems, complexation tends to create greater order in the mesophases present. In the hydrogen-bonded systems, the transition temperature from the isotropic state tends to decrease compared to that of the functionalized mesogen; in the ionically-bonded system, the reverse is true. At low concentrations, the mesogens act as plasticizers.     

Thermodynamics of crystalline nanonucleus formation from liquid phase

The energy of crystal nucleation from liquid phase was considered, with the following two stages taken into account: (1) the formation of metastable supercooled melt (solution), containing pre-nuclei with intermediate amorphous (quasicrystalline) structure, and (2) the transformation of amorphous clusters into solid crystalline nuclei having different structures. With growth of a nucleus the nucleation energy profile manifests 2–3 maxima corresponding to these stages, and the kinetics of the non-stationary nucleation has five characteristic variations.     

Solidification-induced and shear-induced band texture in thermotropic liquid crystalline polymer films

Two phase diagrams of a six-ring double-swallow-tailed compound are presented where the mixing components involve electron-acceptor compounds. In the mixed phase region, nematic, smectic A and C, cubic and columnar phases are induced. In this way transitions between lamellar, cubic and columnar phases can be realized by variation of the con- centration. The mesophases occurring in these systems have been characterized by X-ray investigations.     

Simulation of texture evolution for nematic liquid crystalline polymers under shear flow

The development of microstructure in nematic liquid crystalline polymers under shear flow is investigated through computational simulation. By using a tensorial expression for the elastic torque, the nemato-dynamic equation is numerically resolved. The simulation shows that elastic anisotropy has a strong influence on the evolution of the director and that the ‘log-rolling’ orientation of the directors emerges for tumbling nematics if the twist constant is smaller than the splay and the bend constants, even though one starts from a structure in which the directors are aligned within the velocity and velocity gradient plane. The interaction of wedge disclination pairs subject to a shear flow field is also simulated. The generation, multiplication and interaction of inversion wall defects during shearing have been revealed. In general the wall moves to the boundaries and is absorbed by the boundaries. When two walls of opposite orientation meet, a loop may form, then shrink, and finally collapse. Correspondingly, if they have the same orientation, commutation will occur.     

Simulation of texture evolution for nematic liquid crystalline polymers under shear flow

The development of microstructure in nematic liquid crystalline polymers under shear flow is investigated through computational simulation. By using a tensorial expression for the elastic torque, the nemato-dynamic equation is numerically resolved. The simulation shows that elastic anisotropy has a strong influence on the evolution of the director and that the 'log-rolling' orientation of the directors emerges for tumbling nematics if the twist constant is smaller than the splay and the bend constants, even though one starts from a structure in which the directors are aligned within the velocity and velocity gradient plane. The interaction of wedge disclination pairs subject to a shear flow field is also simulated. The generation, multiplication and interaction of inversion wall defects during shearing have been revealed. In general the wall moves to the boundaries and is absorbed by the boundaries. When two walls of opposite orientation meet, a loop may form, then shrink, and finally collapse. Correspondingly, if they have the same orientation, commutation will occur.     

Analysis of mesophase formation in side-chain liquid crystalline polycarbosilanes

This paper is concerned with an analysis of the thermodynamics and kinetics of mesophase formation by cooling from the isotropic state of side-chain liquid crystalline polycarbosilanes containing spacers in the range from 3 to 11 CH₂-groups. The polymers are characterized by their thermotropic behaviour as far as temperature, enthalpy and entropy of the transitions are concerned. The kinetics was followed by optical and calorimetric methods. Longer spacer length leads to more perfect ordering in the mesophase, higher isotropization temperatures, and lower glass transition temperatures. The Avrami and Ozawa formalism to describe the transition kinetics to the mesophase from the isotropic state cannot be interpreted as the nucleation and growth mechanism known from crystallization.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthday     

New liquid crystalline di- and tetra-acrylates for network formation

New liquid crystalline diacrylates and tetra-acrylates containing four to six aromatic rings were synthesized and characterized, and their mesophase behaviour was investigated. They are designed to be used in combination with chiral molecules to form cholesteric mesophases which can be crosslinked by photopolymerization. The acrylates presented exhibit broad mesophase ranges since mesogenic moieties longer than three are employed. Most diacrylates show no isotropization, due to premature thermal polymerization above 180°C. Additionally, liquid crystalline dipropionates were synthesized as reference compounds which cannot be crosslinked, and selected examples of these exhibit isotropization temperatures as high as 238°C prior to thermal degradation. Substituents at the mesogenic moiety have a great influence on the mesophase characteristics. Bulky substituents such as the tert-butyl group, induce a nematic mesophase, whereas compounds with small substituents (e.g. OCH3) or unsubstituted molecules also exhibit smectic phases. Tetra-acrylates with unsubstituted and substituted mesogenic units feature nematic mesophases only as a result of the additional spacers attached. Here isotropization was observed without polymerization at temperatures around 120-160°C.     

Dynamics in dialysis process for liquid crystalline gel formation

The processes of gelation and liquid crystalline formation in the dialysis of Curdlan solution have been observed under crossed nicols, and the calcium concentration and pH of the inner solution were traced. The results showed that the gelation and the liquid crystalline formation occurred simultaneously to form liquid crystalline gel (LCG), but the birefringence of the LCG increased even after the gelation, suggesting further ordering of the Curdlan molecules. On the basis of the calcium ion diffusion, a simple theory for the time development of the thickness of the LCG layer was developed. The experimental and theoretical results agree very well until an amorphous gel (AG) ring appears. The whole process was expressed by a master curve by reducing time and distance data for different radius dialysis tubes by those at the final state; a scaling behavior with respect to the dialysis tube radius was found. The experimental analysis for the calcium concentrations and the pH indicates that forming Curdlan LCG with high ordering of Curdlan molecules consists of two steps: the diffusion of calcium ions inducing the ordering of Curdlan molecules and yielding cross-links simultaneously, and the local relaxation of the Curdlan molecules increasing the ordering degree further.     

New liquid crystalline di- and tetra-acrylates for network formation

New liquid crystalline diacrylates and tetra-acrylates containing four to six aromatic rings were synthesized and characterized, and their mesophase behaviour was investigated. They are designed to be used in combination with chiral molecules to form cholesteric mesophases which can be crosslinked by photopolymerization. The acrylates presented exhibit broad mesophase ranges since mesogenic moieties longer than three are employed. Most diacrylates show no isotropization, due to premature thermal polymerization above 180°C. Additionally, liquid crystalline dipropionates were synthesized as reference compounds which cannot be crosslinked, and selected examples of these exhibit isotropization temperatures as high as 238°C prior to thermal degradation. Substituents at the mesogenic moiety have a great influence on the mesophase characteristics. Bulky substituents such as the tert-butyl group, induce a nematic mesophase, whereas compounds with small substituents (e.g. OCH3) or unsubstituted molecules also exhibit smectic phases. Tetra-acrylates with unsubstituted and substituted mesogenic units feature nematic mesophases only as a result of the additional spacers attached. Here isotropization was observed without polymerization at temperatures around 120-160°C.     

Permeation of hydrocarbons through liquid surfactant membranes and formation of liquid crystalline structures

The separation of benzene from its mixture with heptane can be achieved by emulsifying the feed mixture to form an oil-in-water emulsion and then dispersing this emulsion in kerosene. At high agitation intensity a gel-like structure develops which prevents phase separation and thereby interferes with the overall mass transfer process. These gel-like structures have very high viscosities, and show rheopectic behaviour; polarising microscopic examination of these gel-like structures revealed the existence of liquid crystalline structures on the surface of the emulsion globules dispersed in kerosene. A possible mode of development of such liquid crystalline structures is presented, 1-Pentanol added to kerosene prevents gel formation and assists phase separation. Its action is explained on the basis of Winsor's theory of solubilization. Typical results for selectivity and benzene yield are presented.     

Lyotropic liquid crystalline phase behavior of a polymeric amphiphile polymerized via their hydrophilic ends

The reaction of the nonionic low molecular mass surfactant 3, 6, 9, 12, 15, 18, 21, 24-octaoxaoctatriacontane-1-ol (CH₃(CH₂)₁₃(OCH₂CH₂)₈OH, C₁₄E₈) with acryloyl chloride yielded the monomeric amphiphile II (CH₃(CH₂)₁₃(OCH₂CH₂)₈O₂C-(CH:CH₂) with the polymerizable group located at the hydrophilic end of the molecule. Using radical polymerization, the polymeric surfactant III is obtained. These three surfactant water systems exhibit lyotropic liquid crystalline phases. The binary phase diagrams are compared with each other. In changing from the monomer-water to the polymer-water system a stabilization of the lyotropic mesophases is observed with amphiphiles which are connected via their hydrophilic ends; it is known for the surfactants to be connected via their hydrophobic ends. The appearance of the inverse mesophases, as expected from the molecular geometry of the polymer, is not observed.     

Origin of lyotropic liquid crystalline mesophase formation and liquid crystalline to mesostructured solid transformation in the metal nitrate salt-surfactant systems

The zinc nitrate salt acts as a solvent in the ZnX-C(12)EO(10) (ZnX is [Zn(H(2)O)(6)](NO(3))(2) and C(12)EO(10) is C(12)H(25)(OCH(2)CH(2))(10)OH) lyotropic liquid crystalline (LLC) mesophase with a drastic dropping on the melting point of ZnX. The salt-surfactant LLC mesophase is stable down to -52 °C and undergoes a phase change into a solid mesostructured salt upon cooling below -52 °C; no phase separation is observed down to -190 °C. The ZnX-C(12)EO(10) mesophase displays a usual phase behavior with an increasing concentration of the solvent (ZnX) in the media with an order of bicontinuous cubic(V(1))-2D hexagonal(H(1))--a mixture of 2D hexagonal and micelle cubic(H(1) + I)-micelle cubic(I)-micelle(L(1)) phases. The phase behaviors, specifically at low temperatures, and the first phase diagram of the ZnX-C(12)EO(10) system was investigated using polarized optical microscopy (POM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), and Raman techniques and conductivity measurements.     

Nanodisperse systems as transient state upon the formation of crystalline organometalsiloxanes

Properties of colloidal solutions formed in the course of hydrolytic condensation of phenyltrialkoxysilanes are studied upon the synthesis of organosiloxanolates of alkali metals and cage-like bimetallic organometalsiloxanes containing alkali and transition (Ni, Cu) or rare-earth (Eu) metals. Sizes and shapes of particles are determined by the dynamic and static light scattering methods. The aggregation stability of colloidal solutions is studied as a function of solvent nature and concentration of phenyltrialkoxysilane. Data obtained make it possible to suggest that disclosed aggregates are the nuclei of crystalline phase. Conditions of the existence of stable colloidal aggregates of sodium phenylsiloxanolate and Ni/Na-phenylsiloxane are determined. It is shown that, upon the dissolution of crystalline Ni/Na-phenylsiloxane in butanol, particles whose sizes are comparable with molecules of organometalsiloxanes are formed in the initial solution.     

Conditions for direct formation of glassy,liquid or crystalline condensates

Some aspects of the thermodynamics and kinetics of the condensation process are analysed in details. The conditions for the predominant formation of crystalline, liquid or glassy condensates are investigated by taking into account an effect which up to now has been omitted. This effect is the size dependence of the melting point of small crystalline clusters and its influence on the nature of the initially formed phase. The possibility for secondary crystallization of small undercooled droplets is also taken into consideration. This new model is compared with the assumption that the nucleation rate determines directly the predominant formation of a given phase.A critical discussion as well as a brief review of some experimental data confirm the advantages of the proposed treatment.