Banded Texture Research Articles

Restudy of the Old Poly-2,5-di(benzoyloxy)styrene as a New Liquid Crystal Polymer

Abstract The 40 year old polymer, poly-2,5-di(benzoyloxy)styrene, has been restudied from the view point of the “mesogen-jacketed liquid crystal polymer.” This old polymer was synthesized in 1952 as a precursor of an electron exchange polymer and is now found to be thermotropically liquid crystalline, from which the characteristic banded texture of rigid and semi-rigid liquid crystal polymers may be obtained. The monomer of this polymer, 2,5-di(benzoyloxy)styrene, was then also not realized as liquid crystalline, is now found to be liquid crystalline forming too. This finding offers probably the first example of liquid crystal rigid rod-like molecules with lateral but without terminal substitutions. The synthesis and primary characterization of a series of the closely related polymers poly-2,5-bis[(4-substituted-benzoyl)oxy styrenes are also reported.

A study on the relaxation phenomena of nematic polymers after cessation of shear flow

Abstract A viscoelastic model, composed of the Ericksen and Landau-de Gennes nematic continuum theories, is used to study numerically the relaxation phenomena after cessation of simple shear flow for a model rigid rod uniaxial nematic polymer. This model predicts that under certain conditions the relaxation of stored molecular and coupling elastic free energies due to periodic fluctuations in the scalar order parameter results in a transient periodic distortion of the director field. These conditions are that: (1) the ratio of the wavelength scales of the initial periodic spatial variation in the scalar order parameter k s to the initial periodic planar director orientation fluctuation kφ (i.e. k S/kφ) and the amplitude of the initial S spatial variation exceed certain minimum values, and (2) kφ is not zero. It is shown that the wavelength selection mechanism is controlled by the director reorientation-induced backflows. The digitized optical patterns of the transient periodic director field show transient periodic optical patterns similar to the transient banded texture nematic polymers exhibit after cessation of shear flow when observed between crossed polars. The numerical results and digitized optical patterns replicate frequently reported experimental observations.

Bright field image of banded texture in lyotropic liquid crystals of a polydiacetylene

AbstractFormation of a banded texture was observed in thin films of a polydiacetylene prepared from the lyotropic nematic phase subject to flow. The banded texture was experimentally investigated by polarizing light microscopy and transmission electron microscopy. The banded structure is initially formed in a nematic phase of the polymer and could be imaged by TEM after its transformation to a lamellar phase which is the polymer analogon to a smectic phase in low‐molecular‐weight compounds. A mathematical description of the texture is given. During further relaxation of the system in the lyotropic state the initial orientation is lost by the formation of disclinations.

Preparation and liquid-crystalline properties of toluene-4-sulphonyl urethane of hydroxypropylcellulose

Abstract A novel liquid-crystalline polymer, the toluene-4-sulphonyl urethane of hydroxypropylcellulose (TSUHPC), was prepared through chemical modification of hydroxypropylcellulose (HPC) of Mw = 60000 g mol−1. The resulting polymer was characterized by infrared spectroscopy, differential scanning calorimetry (DSC) and polarizing microscopy. It was found that thermotropic liquid crystal phases are formed between about 60°C and 110°C. Concentrated solutions of TSUHPC in acetone and N,N-dimethylacetamide exhibit cholesteric behaviour, at room temperature. When approaching the lyotropic mesophase to solid transition, either by cooling or by solvent evaporation, very interesting arborescent structures of a seemingly fractal nature may be observed, depending on the kinetics of the transition. A banded texture can be observed when the polymer is sheared near the transition to the isotropic phase.

Simulation of transient banded textures of sheared nematic polymers

Abstract Nematic polymers, under certain conditions, develop a transient banded texture after cessation of simple shear flow when observing the sheared sample between crossed polars. Here we present a viscoelastic model that describes the formation mechanism of this well-characterized but yet unexplained phenomenon for a typical uniaxial rigid rod nematic polymer. It predicts that the relaxation of shear-flow enhanced scalar order parameter spatial fluctuations produces spatially periodic torques on the director, thereby producing a transient banded texture when viewing the sample between crossed polars. Our numerical results and digitized optical pattern are in good agreement with reported experimental observations.

Control of the anisotropic mechanical properties of liquid crystal polymer films by variations in their banded texture

Variations in the banded texture of sheared liquid crystal polymer films are shown to control the anisotropy of their mechanical properties. Films, prepared by shearing lyotropic liquid crystal polymer solutions of 2-hydroxypropyl cellulose, were submitted to elongation both along and normal to the shear direction. The films were found to be brittle along the shear direction and ductile in the transverse direction. Stretch along the shear direction does not significantly affect the molecular orientation for any of the films.

Experimental and theoretical analysis of band formation in polymeric liquid crystals upon cessation of flow

The evolution of the band texture, that develops in a lyotropic chiral nematic (cholesteric) PBDG solution after cessation of flow, is determined by means of a polarizing microscope equipped with a shear-flow cell. Both the real and the conoscopic SALS images are used to obtain information on the increase of the band spacing as a function of relaxation time. Also, a relation betweenthe previously applied shear rate and the lateral band size is observed. The experimental results on the band spacing are compared to a simple one-dimensional (1-D) continuum mechanical model that uses the bend Frank elastic constant and the Miesowicz rotational viscosity

Band texture formation of sheared polymeric liquid crystalline state

AbstractThe phenomenon of band texture formation of sheared main chain liquid crystalline polymers is reviewed. The bands seen in a polarizing microscope are optical effects. The macromolecular chains are aggregated into zig‐zag bent fibrils perpendicular to the bands. The band texture is formed during shear relaxation. The induction period depends on the shear rate applied, the shearing time, solution concentration (lyotropic), solution layer thickness, temperature and the nature of the polymer. There exists a critical shear deformation to bring a multi‐domain nematic or cholesteric phase into a monodomain continuous phase, from which the band texture is formed. These two phases show quite different rheological behavior. In certain cases randomly oriented regions of bands can also be formed during quenching of a thermotropic nematic polymer melt or during standing of a lyotropic nematic polymer solution, where the nematic domains in the melt or in the solution have grown to a sufficient size.

Miscible polyacetal-poly(vinyl phenol) blends: 2. Thermomechanical properties and morphology

Blends containing polyacetal and poly(vinyl phenol) were prepared by melt compounding and investigated with respect to dynamic mechanical behaviour, melting behaviour, density and morphology. Evidence of complete miscibility was provided by a single, composition-dependent glass transition temperature intermediate between those of the pure components and a substantial depression of the melting point of the crystalline phase. Analysis of the melting point depression yielded an approximate interaction parameter, which was in excellent agreement with that obtained from the calorimetric mixing of chemical analogs. Also, the observation of a significant negative volume of mixing supported miscibility. The emergence of a banded spherulitic texture upon blending was also noted. The overall thermal behaviour of this miscible system was shown to conform to the theoretical model of Kumar and Yoon.

Band textures of liquid crystalline polymers in elongational flows

A rheooptical device, where an obstacle generates an elongational flow, was used to study the behavior of an anisotropic (hydroxypropyl)cellulose solution. Good agreement was found between calculated and measured elongational rates. After cessation of flow, a band texture appears if the elongation rate is larger than a critical value. The conditions for the formation of the bands, their optical features, and their spacing are similar to those arising after cessation of shear.

Microstructure of a thermotropic random copolymer of hydroxybenzoic acid, isophthalic acid and hydroquinone (HBA-IA-HQ)

Transmission electron microscopy has been used to study microstructural features in a thermotropic copolyester composed of 4-hydroxybenzoic acid, isophthalic acid and hydroquinone residues. Selected-area electron diffraction patterns indicate that thin sheared samples of the pure copolymer, and of an analogue containing glass filler, exhibit a dual molecular orientation in which the meridional maxima are closely periodic in scattering angle. Dark-field (DF) imaging in one set of the diffuse equatorial reflections reveals “bands” which have an average period of 200 nm. Annealing the material in the solid state at 250 °C leads eventually to a change in the structure in which the banded texture is replaced by regions of uniform orientation within which the molecules are aligned with the shear axis. Needle-like diffracting crystalline entities, measuring approximately 80 nm long, in the direction of the molecular chain axis, and 5–8 nm thick, have been imaged in DF using the first meridional reflection as the source of diffraction contrast.

Photoinitiated crosslinking of hydroxypropyl cellulose in the isotropic and liquid crystal states

AbstractThe influence of temperature, concentration, and shear rate on the formation of liquid crystal solutions of hydroxypropyl cellulose (HPC) in DMAc and the preservation of the structure by photoinitiated crosslinking were investigated. The rate of changes in the relaxation pattern for the characteristic band texture of HPC liquid crystals after cessation of shear is inversely proportional to the extent of the crosslinking. This shows that the liquid crystalline structure is preserved upon crosslinking. The molecular orientation of the crosslinked films as a function of mesophase organization and concentration during crosslinking was studied.

Effect of Processing Variables on Banded Textures in Hydroxypropyl Cellulose Solutions

Abstract Our research focused on banded textures in three lyotropic systems of hydroxypropyl cellulose (HPC): HPC-water, HPC-methanol, and HPC-acetone. Banded textures were induced by shearing solutions with a motorized parallel plate apparatus. The evolution of the banded texture was investigated by transmitted polarized light microscopy. Measurements of band spacing were used to develop a quantitative basis for comparing relaxation rates. Our study showed that the molecular orientation introduced by shear is preserved longest by using thermodynamically poor solvents and working with high concentrations. Alternatively, if the solvent is thermodynamically good, increasing dilution promotes slower relaxation rates.

“Tweed” structure of Fe-doped YBa 2Cu 3O 7−δ

Fe-doped YBa 2Cu 3O 7−δ single crystals have been prepared by a flux method from an Y-Ba-Cu-O precursor with copper partially substituted by iron. Electron diffraction and microscopy were applied to study the domain structure of the material. Superposed on a broad twin band texture, as also observed in undoped material, all samples reveal a finer “tweed” structure of overlapping lenticular domains oriented along [120] and [1 1 0]. This suggests a two-step formation process of the domain structure. The displacement field, corresponding with the tweed structure, resembles that of microtwinning in undoped, quenched YBa 2Cu7 3O 7−δ samples. The randomly dispersed Fe-ions act as pinning centers or fragment the structure by the induction of twin interfaces. Monte Carlo simulations support this idea.

Models for the Origin of the Pumice Associated with the 1989 Submarine Eruption Off-Ito, Central Honshu, Japan.

Three types of pumice were erupted at the 1989 submarine eruption off-Ito, central Honshu, Japan. The first one is called choco-ball type, which consists of white spherical core with rhyolitic composition and the black mantle with basaltic composition. The second is one with the banded texture of mafic and felsic compositions. The third is gray-colored type. Major compositions of 24 samples were analyzed with EPMA. The models for the origin of the pumices are discussed in terms of their texture and chemical composition.

A study of band texture formation of nematic solutions of poly(1,4‐phenyleneterephthalamide) in sulfuric acid

AbstractThe band texture formation of nematic poly(1,4‐phenyleneterephthalamide) (PPTA)/H2SO4 solutions after shearing between rotating plates was studied. For a thickness of the solution layer larger than 20 μm the band texture appears immediately after the cessation of shearing, but a critical shear rate is needed which was demonstrated directly using polarizing microscopy. When the solution‐layer thickness was less than 10 μm, an induction time tb needed for the development of the band texture after the cessation of shearing was observed, which was of the order of 10 to 102 s. It is thus shown without any ambiguity that the band texture is formed during relaxation of shear orientation. The effects of solution concentration, solution‐layer thickness, shear rate, and shearing time on tb were studied. Once the band texture is formed from a sheared solution, it will take hours to relax completely to a randomly oriented domain texture. The factors influencing this relaxation time were also studied.

Morphological studies of oriented films obtained from aromatic polyester fractions

The banded texture and its thermal stability in oriented films of aromatic polyester with different molecular weights have been investigated. Banded texture has been observed for all the fractions used and its perfection improved with increasing molecular weight of the fractions. It was found that the bandwidth and the angle between the optical axis of the bands and the shear direction both depend on the molecular weight. The banded texture was found to be stable up to temperatures very close to T i . A ‘memory effect’ of banded texture has been observed: the banded texture in oriented specimens disappears due to isotropization at temperatures above T i and may reappear during cooling and liquid crystallization. The formation of the banded texture is discussed as the result of forced relaxation due to elasticity in the sheared specimens.

Simultaneous Observation of Disclination and Texture by Optical Microscopy of a Thermotropic Aromatic Polyester

Abstract An aromatic copolyester (m.p.148°C) film quenched from 180°C melt showed a schlieren texture. The birefringent domains increased in size with increasing temperature of the melt before it was quenched. The polymer film quenched from the melt after being heating above 240°C for 10 min showed both disclinations and a banded texture of random orientation on the same picture, although the banded texture can not be directly observed in the heated melt. However, the banded texture of the quenched film persisted on heating to a melt up to 200°C and began to vanish above this temperature, but the banded texture reappeared on being requenched into a solid film. The banded texture could also be seen in the localities displaying brushes. Consequently schlieren brushes are optical effects but not isotropic material regions.

A new banded texture in sheared thermotropic liquid crystalline polymers

Liquid crystalline polymers (LCP) often show after a shearing procedure a so-called banded texture as reported in the literature. The appearance of the banded texture is mainly influenced by the chemical structure of the LCP, the shear rate and the thickness of the layer. In addition to the well known banded texture a particular tractor texture (or herring-bone texture) can be obtained. By subsequent annealing of the sheared specimens at elevated temperatures the banded texture can be changed, especially in dependence on the thickness of the sheared layer. In some cases the banded texture remains unchanged, whereas in other cases it is transformed into a domain structure or into spherulites.

Crosslinked liquid crystalline film cast from hydroxypropyl cellulose/methanol liquid crystalline solution.

Crosslinked hydroxypropyl cellulose (HPC) films were cast from liquid crystalline HPC/methanol solution with p-formaldehyde (p-FA) in the presence of hydrochloric acid. Whether or not crosslinked liquid crystalline structures were retained in the cast films was examined by polarized microscopy and circular dichroic (CD) spectroscopy. The dependence of tensile and dynamic mechanical properties of solid films on crosslinking agent concentration was determined. The films were not soluble in both water and methanol. and hence it was proved that the films had been crosslinked. From polarized microscopic observation and CD spectrum, it was found that the HPC solid films retained right-handed helicoidal cholesteric structures. Small-angle light scattering (SALS) patterns of the insoluble films were not affected by addition of crosslinking agent, but were affected by the band textures. With respect to tensile properties. Young's modulus increased with crosslinking agent concentration, and tensile strength and elongation had opposite tendencies to Young's modulus. In dynamic mechanical properties. E' increased with crosslinking agent concentration and exhibited general behaviors of crosslinked materials.