Simulation of transient banded textures of sheared nematic polymers

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.     

Banded texture decoration of disclinations in a main chain thermotropic aromatic copolyester observed by optical microscopy

A new method for the study of texture and director patterns in a main chain thermotropic aromatic copolyester with a flexible spacer is described, whereby the molecular chain or director orientation of the nematic mesophase becomes decorated by the formation of a banded texture during quenching, without being subjected to a shear. The pattern of the decorated banded texture may be observed directly by polarizing optical microscopy, revealing the complete texture of molecular chain orientation across the whole specimen. The molecular director orientation lies perpendicular to the long axis of the bands. Various types of disclination, including an inversion wall, in the nematic mesophase of a thermotropic aromatic copolyester have been observed. This decorating technique is particularly suited for non-crystallizable main chain liquid crystalline polymers, where the lamellar decoration technique fails.     

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.     

Simultaneous optical observation of anisotropic texture for a concentrated solution of poly(γ-benzyl-l-glutamate) under transient stress response in shear flow

Simultaneous optical observation of anisotropic texture was performed under transient stress response in shear flow for a concentrated solution of poly(γ-benzyl-l-glutamate) (PBLG). Transient stress of the PBLG solution immediately after the onset of the shear flow showed a remarkable stress overshoot and damping oscillation behavior, followed by a steady state. On the other hand, simultaneous observation of the polarized optical microscopy showed the remarkable distortion of the banded texture immediately after the onset of the shear flow and the periodical changes in the retardation followed by a steady texture of the PBLG solution. The remarkable stress overshoot and the damping oscillation are closely related to the distortion of the banded texture and to the changes in the orientation of the rod-like molecules of the PBLG, respectively. Received: 15 December 1999/Accepted: 12 July 2000     

Recent advances in theoretical liquid crystal rheology

Recent significant advances in theoretical liquid crystalline rheology are presented. Dynamic simulations are performed using a complete theory which include the three major effects of liquid crystalline materials: (1) short range order elasticity, (2) long range order elasticity, and (3) viscous flow effects. The results and discussions include rectilinear simple shear flow, complex non-linear phenomena such as defect texture generation and coarsening processes under quiescent and shear conditions, and pattern formation such as banded texture during and after cessation of flow. The complete theory predicts four in-plane (1-D orientation) flow modes and five out-of-plane (2-D orientation) flow modes in one-dimensional shear flow, depending on the magnitudes of R (ratio of short to long range order elasticity) and Er (Ericksen number: ratio of viscous to elastic force). The multistability of these flow modes is clearly explained in terms of degrees of freedoms in the nematic orientation. The number of degrees of freedom increases with increasing the spatial dimension of the system, and thus more complex orientation patterns arise in the higher dimension. Well-known defect structures arise and coarsen during simulations of the isotropic to nematic phase transition. The effect of shear flow on the defect generation process is to suppress the defect nucleation, and the simulations suggest a method of how to create defect-free nematic samples. The banded textures during and after cessation of flow are also captured by the complete theory.     

Banded texture in sheared a semiflexible thermotropic liquid-crystalline polyester and its graft copolymers with polyolefins

The formation of banded texture both in a semiflexible thermotropic liquid-crystalline polyester (SBH 112, synthesized from sebacic acid, 4,4′-dihydroxybiphenyl and 4-hydroxybenzoic acid, from Eniricerche, Milan) and in a blend of SBH with graft copolymers, consisting of a polyolefin (polyethylene or polypropylene) backbone and SBH grafts (COPPE, COPPP) has been studied by polarization microscopy and by scanning electron microscopy. The influence of the preparation conditions upon the perfection, characteristics and thermal stability of the banded texture has been investigated. The results show that the banded texture formed in SBH sheared films possesses greater perfection, smaller band width and larger angle of deviation, and higher thermal stability than the banded texture formed in COPPE and COPPP prepared under similar conditions. It has been assumed that the chain rigidity of the macromolecules of the system is mainly responsible for the formation and perfection of the banded texture. Received: 17 March 2000 Accepted: 26 July 2000     

EVIDENCE FOR THERMOTROPIC LIQUID CRYSTALLINITY OF A FUSIBLE POLYDIACETYLENE

The thermotropic liquid crystalline behavior of a fusible polydiacetylene has beenstudied by using polarizing optical microscopy and wide angle X-ray diffraction. It wasfound that the typical threaded texture can exhibit in the melt and the banded texture canbe observed when applied a gentle shear to the melt. The results indicate that thispolydiacetylene is really a thermotropic liquid crystalline polymer.     

BANDED TEXTURE FORMATION IN NEMATIC SOLUTION OF POLY (1,4-BENZAMIDE) IN SULFURIC ACID

The banded texture in films prepared from nematic poly (1,4-benzamide) (PBA)/H_2SO_4 solution by shearing and without shearing has been studied by using microscopy techniques. The kinetic parameters of banded texture formation γ_c and τ_b were measured for nematic solution of PBA by using parallel-plate shearing apparatus. The banded texture was also observed in randomly packed domains for nematic solution of PBA on standing without shearing. The properties of banded texture show no difference between the two samples with or without shearing.     

Observations of the break-up of liquid crystalline polymer threads imbedded in an isotropic fluid

A droplet, a few hundreds of microns, of liquid crystalline hydroxypropylcellulose (HPC) in water is imbedded into a polydimethylsiloxane (PDMS) matrix and subjected to a shear flow. The droplet is deformed into a thread of high aspect ratio. The textures associated with the break-up of the thread once the flow is stopped are observed by optical microscopy. After stopping the flow a banded texture appears. The droplets resulting from the break-up have a bipolar texture with the two opposite poles being along the former thread direction.     

用条带织构装饰技术研究液晶全芳共聚酯B-N的取向和非取向膜

用条带织构装饰技术研究液晶全芳共聚酯Ｂ—Ｎ的取向和非取向膜董炎明（厦门大学化学系,厦门,３６１００５）关键词共聚芳酯,条带织构,退火,取向膜,微纤全芳共聚酯Ｂ—Ｎ在分子量较高时能出现黑白相间的条带织构［１～５］,但分子量低时由于极易松驰,不能用普通剪...     

Formation of a Banded Texture in Solutions of Liquid-Crystalline Polymers 11. Poly(n-Hexylisocyanate) in Toluene

The kinetics of band formation has been followed during the relaxation of anisotropic solutions of poly(n-hexylisocyanate) in toluene previously subjected to a shear rate, for a time t_s. There is a threshold at low, or t_s, below which bands do not appear (formation time t_b→ ∞). Past the threshold, t_s decreases with either or t_s, and eventually bands are seen immediately after stopping the flow (t_b = 0). However, for still larger particularly for the more viscous solutions and when the flowing solution is well oriented, t_b may increase again. The results are similar to those already reported for hydroxypropylcellulose in water and support a previous suggestion for the origin of the banded texture.     

手征性侧链液晶高分子取向结构的研究

用偏光显微镜,红外二色性和Ｘ 射线衍射研究了一种手征性侧链液晶高分子的相态织构和弛豫行为．偏光显微镜观察这种侧链液晶高分子冻结取向液晶态薄膜时,可观察到与剪切方向垂直的明暗相间的条带织构．红外二向色性的结果表明,取向态中侧链上的介晶基元倾向于与剪切方向垂直排列．取向和非取向膜的Ｘ射线衍射揭示了该侧链液晶高分子具有反铁电性液晶的两套反相螺旋结构．取向薄膜在液晶态的弛豫行为表明,取向作用能促进侧链高分子近晶相层状结构的生长,而且介晶基元的取向在弛豫过程中能保持下来．     

乙基醋酸纤维素液晶态条带织构的形成机理

本文应用平行板移动式和转动式二种剪切装置研究了乙基醋酸纤维素的二氯乙酸体系液晶态受剪过程形成条带织构的临界剪切速率,条带织构的形成机理以及所形成的条带织构在升降温过程中的变化。结果表明,高分子液晶态受剪过程条带织构不是在受剪时产生,而是在受剪停止后的弛豫过程中形成的。可以观察到条带织构出现的诱导时间(t_b),其值的大小与溶液的浓度、剪切速率等因素有关。最后提出一模型来解释高分子液晶态受剪切过程条带织构的形成机理。     

Long‐range orientation correlations and molecular alignment in sheared thermotropic copolyester. In situ light and X‐ray scattering

Orientation correlations induced by shear flow and their relaxation were investigated using in situ small‐angle light scattering (SALS) in the thermotropic random copolyester of 60 mol% hydroxybenzoic acid (B) and 40 mol% ethylene terephthalate (ET). B‐ET displays a nematic polydomain texture, the SALS and wide‐angle X‐ray scattering (WAXS) patterns are amorphous and isotropic. Shear flow produced optical defect multiplication with the consequent reduction of the micro–domains size. However, SALS detected long‐range spatial correlations within the optically chaotic texture, the SALS patterns showed bimodal orientation of defects. After cessation of shear the orientation correlation rapidly relaxed back to a polydomain and the SALS pattern became again isotropic. Above a threshold shear rate of about the SALS pattern showed unimodal orientation arising from line defects oriented nearly orthogonal to the velocity axis. Strikingly, the texture relaxation now showed the well known “banded texture”. The threshold shear rate coincided with a significant increase in the degree of molecular alignment as determined from in situ X‐ray scattering. This technique also showed that shear flow always oriented the molecular chains along the flow direction regardless of the shear rate. Copyright © 2007 John Wiley & Sons, Ltd.     

Analysis of transient periodic textures in nematic polymers

A numerical solution of the Leslie-Ericksen equations for nematic liquid crystals is obtained for in-plane rotation of a strong magnetic field. A transient periodic orientation develops as a result of in-plane director motion and the induced shear flow. At long times the in-plane director orientation results in steady splay-bend inversion walls. A linear stability analysis shows that the inversion walls are unstable to perturbations out of the plane for elastic coefficients characteristic of nematic polymers. Calculations of transmitted light intensity through crossed polarizers for the computed orientation development predict the evolution of a banded texture, as observed experimentally.     

UV-initiated solidification of liquid crystalline ethylcellulose/acrylic acid films and bands formed in this process

Solidification of liquid crystalline ethylcellulose/acrylic acid films was realized by UV-initiated polymerization of the solvent acrylic acid. It has been found that the original textures were immobilized to a great extent, and a novel “banded texture” formed at the same time. The optical properties of this texture were different from the banded texture formed by shearing lyotropic or thermotropic polymer liquid crystals. Formation of such bands is assumed to be related to an uncompleted phase separation.     

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.     

Band texture formation of sheared polymeric liquid crystalline state

The 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.     

Effect of the thermal history on the phase transition and the liquid crystalline structure of thermotropic polyesters

A series of triad type thermotropic polyesters based on phenylsulfonyl hydroquinone and α,ω-bis(4-carboxyphenoxy)alkane was prepared, and the effect of the thermal history on the phase transition and the liquid crystalline structure of the polyesters was studied. The phase transition was significantly affected by the thermal history. When annealed at various temperatures, multiple endotherms were observed for all polyesters in DSC thermograms. A phase stability concept was employed to explain the complicated phase transitions. The time evolution of the liquid crystalline structure was investigated by optical microscopy and transmitted light intensity measurement under crossed polars during annealing. The steady decrease in the density of disclinations with accompanying increase of transmitted light intensity was observed. Depending on the surface conditions of the liquid crystal polymer film, two markedly different domain growth patterns and different textures were found on isothermal annealing; the continuous wormlike texture and the discontinuous droplet texture. The structural relaxation of the deformed texture during annealing was also studied. When shear was applied, the characteristic banded texture was formed in all polyesters.     

Morphological studies of liquid-crystalline cellulose derivatives. II. Hydroxypropyl cellulose films prepared from liquid-crystalline aqueous solutions

A morphological study of hydroxypropyl cellulose (HPC) was performed on solid films prepared by casting from a liquid-crystalline aqueous solution at rest or under shear. Electron microscopic observations reveal that many round particles composed of stacked disks are densely packed in the interior of a quiescently cast HPC film, while on the film surface formation of fibrous textures is also noted. Shear-deformed HPC films exhibit some interesting morphological features according to the shearing conditions. It is found by electron microscopy that the originally round particles become more and more elongated as shear stress increases. The resulting rodlike fibrillar entities are considerably aligned in the shear direction (SD), but form a banded structure with periodic discontinuities of molecular orientation distribution along the SD. A new mechanism of structural transformation is proposed in order to interpret these results.