Mesophase Layers Research Articles

Growth kinetics of polymer crystals in bulk

Temperature-dependent measurements of spherulite growth rates carried out for i-polystyrene, poly(epsilon -caprolactone) and linear polyethylene show that the controlling activation barrier diverges at a temperature which is 14K, 22K and 12K, respectively, below the equilibrium melting points. We discuss the existence of such a "zero growth temperature" T(zg) in the framework of a recently introduced thermodynamic multiphase scheme and identify T(zg) with the temperature of a (hidden) transition between the melt and a mesomorphic phase which mediates the crystal growth. The rate-determining step in our model of crystal growth is the attachment of chain sequences from the melt onto the lateral face of a mesomorphic layer at the growth front. The necessary straightening of the sequence prior to an attachment is the cause of the activation barrier. A theory based on this view describes correctly the observations. With a knowledge of T(zg) it is possible to fully establish the nanophase diagram describing the stability ranges of crystalline and mesomorphic layers in a melt. An evaluation of data from small-angle X-ray scattering, calorimetry and optical growth rate measurements yields heats of transition and surface free energies of crystals and mesophase layers, as well as the activation barrier per monomer associated with the chain stretching. According to the theory, the temperature dependence of the crystallization rate is determined by both the activation energy per monomer and the surface free energy of the preceding mesomorphic layer. Data indicate that the easiness of crystallization in polyethylene is first of all due to a particularly low surface free energy of the mesomorphic layer.

Identification criterion for the mechanisms of orientation instability of cholesteric liquid crystals in ultrasonic fields

An identification criterion is proposed and experimentally substantiated for the two independent acoustic mechanisms (vortex and relaxation ones) governing the formation of a system of two-dimensional domains in a planar layer of a cholesteric liquid crystal in the frequency range where the sound wavelength is greater than the step of the cholesteric circuit. The results of measuring the critical compression amplitude in mesophase layers with thicknesses from 40 to 240 μm are presented for frequencies lower and higher than the frequency equal to the inverse relaxation time of the orientation order parameter in a cholesteric liquid crystal. The data are obtained for diluted solutions of cholesterilchloride in a nematic liquid crystal with a circuit step varied from 2 to 30 μm. The concept of the binary nature of the acoustic mechanism that causes destabilization of the planar texture of a cholesteric liquid crystal in the frequency range under consideration is experimentally verified.

Rheostructural studies on a synthetic mesophase pitch during transient shear flow

The steady, shear viscosities of a synthetic mesophase pitch (Mitsubishi AR-HP) obtained from rate-sweep experiments at 0.1–10 s −1 exhibited shear-thinning (region I) and plateau responses (region II), but displayed a hysteresis during the decreasing rate sweep. Transient tests revealed that the shear stress (and consequently the shear viscosity) displayed a local maximum and a minimum before approaching a steady state. Following steady flow at 1 s −1, a reversal of flow direction or a very short interruption in flow did not lead to the maxima or minima in the transient shear stress, but the maxima and the minima reappeared in the transient stress after a rest time of ∼1000 s. An experimental protocol was developed to preserve the rheological samples, and their microstructure was characterized in three orthogonal planes for the initial and final states. The initial microstructure was found to have a weak, but preferred, orientation of mesophase layers in the radial direction of the rheometer cone-plate (due to the initial squeezing flow). The initial microstructure changed to a flow-aligned fibrous structure after shearing in the viscometric flow.

On the nature of the acoustic mechanism of domain formation in a cholesteric liquid crystal

The dual nature of the acoustic mechanism of destabilizing the planar texture of a cholesteric liquid crystal is confirmed experimentally. This destabilization manifests itself in the formation of a system of two-dimensional domains in the frequency range where the wavelength exceeds the cholesteric-helix pitch. A criterion for identifying two independent acoustic mechanisms (vortex and relaxation ones) is suggested and experimentally verified by analyzing the measured dependence of the critical compression amplitude on the cholesteric-helix pitch in the 40-, 120-, and 240-μm-thick mesophase layers for frequencies of 2.95 and 14.5 MHz, which are respectively lower and higher than the reciprocal relaxation time of the orientational order parameter of the cholesteric liquid crystal.

In situ study of nanostructure and morphological development during the crystal–mesophase transition of poly(di- n-hexylsilane) and poly(di- n-butylsilane) by X-ray and hot-stage AFM

Nanostructure and morphology and their development of poly(di- n-hexylsilane) (PDHS) and poly(di- n-butylsilane) (PDBS) during the crystal–mesophase transition are investigated using small angle X-ray scattering (SAXS), wide angle X-ray diffraction and hot-stage atomic force microscopy. At room temperature, PDHS consists of stacks of lamellae separated by mesophase layers, which can be well accounted using an ideal two-phase model. During the crystal–mesophase transition, obvious morphological changes are observed due to the marked changes in main chain conformation and intermolecular distances between crystalline phase and mesophase. In contrast to PDHS, the lamellae in PDBS barely show anisotropy in dimensions at room temperature. The nonperiodic structure and rather small electronic density fluctuation in PDBS lead to the much weak SAXS. The nonperiodic structure is preserved during the crystal–mesophase transition because of the similarity of main chain conformation and intermolecular distances between crystalline phase and mesophase.

The Carbonaceous Mesophase Formed in the Pyrolysis of Graphitizable Organic Materials

Observations by polarized light of the microstructures formed during the pyrolysis of materials used to fabricate graphitic artifacts have directed attention to the narrow range of temperature during which the carbonizing liquid exists as a liquid crystal called the carbonaceous mesophase. The poor polishing characteristics of the unhardened mesophase required modifications to the preparation methods used for carbons and graphites. A temperature-gradient heat-treatment technique was employed to obtain specimens representative of the various mesophase morphologies developed in the short plastic range. Three mechanismsμthe mesophase transformation itself, the coalescence of the initial spherules to bulk mesophase, and the plastic flow before hardening—act to establish the principal features of the lamelliform morphology of a graphitizable coke. Fine insoluble particles are segregated by the mesophase transformation, and the effect of graphitic substrate particles is to align the mesophase layers parallel to the layer planes of the substrate. On heat-treatment of the hardened mesophase to graphitizing temperatures, shrinkage-cracking and fine-scale mechanisms of fold-sharpening and polygonization take place to yield the final microstructure of the graphitic product.Die Gefügebeobachtung der in der Pyrolyse entstandenen Kohlenstoff-Mesophase von graphitbildenden organischen MaterialienDie Gefüge von Materialien, die während der Pyrolyse bei der Herstellung von künstlichem Graphit entstehen, wurden im polarisiertem Licht untersucht. Hierdurch wurde man auf den engen Temperaturbereich aufmerksam, in dem die kohlenstoffbildende Schmelze als ein flüssiger “Kristall,” genannt “Kohlenstoff-Mesophase,” existiert. Die schlechten Poliereigenschaften der weichen Mesophase erforderten andere Präparations-methoden als die für Kohlenstoff und Graphit bekannten. Eine Wärmebehandlung bei verschiedenen Temperaturen wurde verwendet, um repräsentative Proben für die verschiedenartigen Morphologien der Mesophase zu erhalten. Drei Mechanismen — die Mesophasenumwandlung selbst, die Zusammenballung der Ausgangskügelchen zur kompakten Mesophase und das plastiche Fliessen von dem Hartwerden — wirken bei der Ausbildung des grundlegenden lamellaren Gefüges eines graphitbildenden Kokses zusammen. Feine unlösliche Teilchen werden bei der Mesophasenumwandlung ausgeschieden. Diese Graphitsubstratteilchen bewirken, dass die Mesophase parallel zu den Substratschichten ausgerichtet ist. Von der Wärmebehandlung der hartgewordenen Mesophase bis zur graphitisierenden Temperatur bilden sich Schrumpfrisse und eng begrenzte, scharfe Auffaltungen. Ausserdem findet Polygonasition statt. Diese Prozesse formen das endgültige Gefüge des graphitischen Produktes.Micrographie de la mésophase riche en carbone formée lors de la pyrolyse de matériaux organiques graphitiquesDes observations en lumière polarisée de microstructures formées lors de la pyrolyse des matériaux entrant dans la fabrication des produits graphiteux ont attiré l'attention sur l'étroit domaine de température où le liquide carbonisant existe en tant que cristal liquide appelé “la mésophase riche en carbone”. Les pauvres caractéristiques de polissage de ce matériel mou exigent des modifications dans la préparation des carbones et des graphites. On utilise la technique de maintenir un gradient de température durant les traitements thermiques afin d'obtenir des échantillons représentatifs des différentes morphologies de la mésophase qui se développe dans le domaine plastique. Trois mécanismes, la transformation de la mésophase elle-même, la coalescence des petites particules initiales à cette dernière et l'écoulement plastique avant le durcissement, servent à établir les principales caractéristiques de la morphologie de graphitisation du coke lamellaire. Lors de la transformation de la mésophase, de fines particules insolubles se divisent et l'effet de ces particules graphitiques est d'aligner les couches de la mésophase parallèlement à celles des substratums. La polygonisation et la formation de fissures causées par le frittage prennent naissance lors de la cuisson de la mésophase durcie aux températures de graphitisation pour finalement donner la microstructure du produit graphitique.

The internal friction of unidirectional [formula omitted] composites fabricated in a magnetic field

Previous studies have shown that lamellae of mesophase pitch prefer to be oriented parallel to a magnetic field. In the present work the matrix mesophase layers of the C/C composites were oriented perpendicular to the carbon fibers by a 8000 G magnetic field. This study is concerned with the internal friction of unidirectional C/C composites produced by the method of the present research. It is found that the internal friction of unidirectional C/C composites fabricated with mesophase pitch precursor in a 8000 G magnetic field is larger than that of the composites in the absence of a magnetic field; the higher the degree of graphitization of the composites, the larger the internal friction of the composites; and the higher the density of the composites, the larger the internal friction of the composites. The internal friction of the C/C composites decrease with the oscillation frequency from 0.01 to 1 Hz, and then slightly increases up to 5 Hz.

On the structure and the chain conformation of side-chain liquid crystal polymers

Abstract Backbone anisotropy and the structure of the mesophases of a series of side-chain liquid crystal polymers have been studied in the bulk by neutron scattering. The backbone conformation is obtained by small-angle neutron scattering on mixtures of hydrogenous polymers with deuteriated backbones. The components of the radius of gyration parallel, R ⊥ and perpendicular, R ∥ to the magnetic field are determined as a function of temperature for both the nematic phase and the smectic phase. It is shown that the polymer backbone is deformed in both phases. When the polymer exhibits only a nematic phase, it adopts a prolate conformation, where the average backbone direction is more or less parallel to the aligned mesogenic groups. Upon transition from the smectic phase to a nematic phase, the backbone in the nematic phase assumes a slightly oblate shape. This tendency towards oblate shape is due to the smectic fluctuations which are always present in such nematic phases. The exentricity of the oblate backbone conformation in the smectic phase is always larger than in the nematic phase. This is attributed to a periodic distribution of the backbone between the mesophase layers. Then, the backbone anisotropy depends not only on the smectic structure (SA1, SAd), but also on the temperature dependence of the density of aligned mesogenic groups in the layers. On the other hand, it is shown that the isotopic mixtures are no longer ideal when polymers deuteriated in the mesogenic moieties are mixed with the corresponding hydrogenous polymers.

Polarized‐light observations of flow‐induced microstructures in mesophase pitch

A capillary rheometer with quenching capability was used to study the formation of microstructure in mesophase pitch—a discotic nematic liquid crystal—under the flow conditions of fiber spinning. We report here some observations by reflected polarized‐light micrography on polished sections of specimens quenched during shear and elongational flow, or quenched during relaxation after cessation of flow. A prominent feature of mesophase flow in a capillary is the formation of ‘‘rippled’’ structures that appear to be due to the instability of shear flow in an anisotropic liquid. The ripples display a regular pattern of misorientation from the flow direction. The fully developed microstructures near the capillary exit consist of three microconstituents distributed in a concentric pattern: a fibrous core, surrounded by fine concentric bands with the mesophase layers in zig‐zag radial arrays, and a fine‐textured rim. Upon relaxation, the fine microstructures rapidly decay to disclination arrays, and coarsening continues by disclination‐annihilation reactions. The draw‐down cone, from which a filament is spun, showed no evidence of rippled structures, but instead a tendency to reorient mesophase layers to the axial direction. The rheological memory of mesophase pitch was observed in spinning experiments in which an orthogonal gridlike pattern is imposed by a screen placed above the spinneret. The ripples formed upon passage through the screen dissipate within seconds after flowing past the shear field. In contrast, the orthogonal pattern is much longer lived; its persistence upon reduction to extruded rod or drawn filament is dependent upon the viscosity determined by the spinning temperature.

Refraction Caustics Measuring the Extent of the Mesophase Layers in Fibrous Composites

Detection and measurement of sub-micron boundary layers developed between phases in composites was achieved by using the method of caustics. These singular curves were detected on a reference screen receiving the refracted rays of a parallel-light beam at these boundary layers, which were recognized as possessing slightly different values of their re fractive indices due to constraints between phases. Utilizing the theory of formation of caustics for substances with constant refractive in dices, but arbitrary and smooth boundary surfaces, the direct and inverse affinities be tween caustics and the refracting surfaces were established. This theory was applied to the inverse problem of defining from the caustic the zone of variation of the refractive surface at the interfaces between main phases of composites. In this way the extent of the mesophase in a fiber composite may be accurately established. Experimental evidence is in agreement with existing theories of the existence and extent of mesophases.

Crack propagation modes in fiber and particulate composites in the microstructure

AbstractThe influence of the hard or soft inclusions and the mesophase layers in either a soft‐hard‐soft or hard‐soft‐hard combination of biphase plates submitted to dynamic tensile loads on the fracture mode and bifurcation process in both phases was investigated in this paper. It was assumed that the soft or hard matrix is infolding the hard or soft inclusion of the plate, so that the plate constitutes a meridional section of the representative volume element of a unidirectional fiber composite, or a principal section of a particulate. The influence of the mechanical properties of either phase on the crack propagation velocity and the initiation of crack bifurcation was studied by using high‐speed photography and dynamic caustics. The results showed that the propagating crack tended to bifurcate either in the brittle or in the mesophase layer under certain conditions of propagation velocity. It was shown that bifurcation of a propagating crack depends on the elastic moduli and Poisson's ratios of the phases, as well as on the extent of the mesophase layer, which depended on the adhesion quality of phases.

Influence of the mesophases on the mechanical properties of three-phase composites

The coating of fibres of soft median layers, in order to reduce stress concentrations, and/or singularities at their common interface with the surrounding matrix material, is one of the most efficient methods of improving strength characteristics of fibrous composites. In this paper, the mechanical properties of such a composite system were investigated by means of an improved model taking into account the mesophase layers, i.e. the zones of physico-chemical and mechanical disturbance of an active phase by the presence of another. The mechanical properties, as well as the extent of these mesophases, were also determined for a given three-phase composite. The characterization of the mechanical properties of the two mesophases may prove useful in estimations concerning fracture toughness of such systems.

Crack Propagation and Bifurcation in Fiber-Composite Models: I. Soft-Hard-Soft Sequence of Phases

The influence of the hard fiber and the mesophase layers in a soft-hard-soft com bination of a biphase plate subjected to a dynamic tensile load, on the fracture mode and bifurcation process in both phases was investigated. It was assumed that the soft matrix envelops the hard-fiber layer of the plate, so that this plate constitutes a merid ianal section of a representative volume element of a unidirectional fiber com posite.

Formation of thin mesophase layers at the interface between filler and binder in prebaked anodes. Effect of mixing on mesophase

Prebaked anodes used for aluminum electrolysis are prepared by mixing petroleum coke as a filler with coal-tar pitch as a binder. Pitches which contain too large amounts of mesophase spheres are not suitable for preparing high quality anodes since mesophase microstructure is destroyed by mixing. Both the well extended parallel preferred orientation and the spherical shape disappear. It is observed that disorganized mesophase forms a shell around the coke particles. Such shells reduce the wettability of the coke by the pitch. This transformation was reproduced experimentally by strong grinding. When a disorganized mesophase is heat-treated at 700°C, it recovers a local molecular orientation, the extension of which widely varies from one area to the next one. The smaller the orientation extent, the lower the graphitizability. A mixture of non graphitizing microporous carbon and of partially graphitizing macroporous carbon is thus obtained. Such porous carbons are also found in anodes after baking as shells around the filler.

The carbonaceous mesophase formed in the pyrolysis of graphitizable organic materials

Observations by polarized light of the microstructures formed during the pyrolysis of materials used to fabricate graphitic artifacts have directed attention to the narrow range of temperature during which the carbonizing liquid exists as a liquid crystal called the carbonaceous mesophase. The poor polishing characteristics of the unhardened mesophase required modifications to the preparation methods used for carbons and graphites. A temperature-gradient heat-treatment technique was employed to obtain specimens representative of the various mesophase morphologies developed in the short plastic range. Three mechanisms—the mesophase transformation itself, the coalescence of the initial spherules to bulk mesophase, and the plastic flow before hardening—act to establish the principal features of the lamelliform morphology of a graphitizable coke. Fine insoluble particles are segregated by the mesophase transformation, and the effect of graphitic substrate particles is to align the mesophase layers parallel to the layer planes of the substrate. On heat-treatment of the hardened mesophase to graphitizing temperatures, shrinkage-cracking and fine-scale mechanisms of fold-sharpening and polygonization take place to yield the final microstructure of the graphitic product.