The Morphology of Liquid-Crystalline Polymers and the Possible Consequences for their Rheological Behaviour

Abstract

Polymer liquid crystals can occur as polydomain materials where the domain size may be tens of microns. While the material within each domain may be characterized by a common order parameter, the directors of the domains can be more or less randomly distributed. Since the transition from polydomain to monodomain material only involves the removal of grain boundaries and the alignment of directors, the free energy change must necessarily be small. Such a transition can readily be achieved, therefore, by the action of any external field: electrical, magnetic, stress or surface. In this work optical photomicrographs of polymeric liquid crystals with widely varying and in some cases well controlled morphologies are presented. Probable dependence of rheological behaviour on morphology is also discussed. Such dependence is expected to be considerable under certain conditions. Due to experimental and sample limitations, however, direct correlations of rheology and morphology are sparse. Morphological consequences for the rheology of liquid-crystalline materials can be exemplified by the following possibilities. In contrast to the case of isotropic melts, wall effects can be non-negligible. Zero shear rate rheological parameters are not expected to be uniquely defined quantities since the domain sizes are large and the director may not be effectively averaged over typical sample dimensions. Non-zero shear-rate measurements of rheological parameters is effected by the propensity of: (1) individual domain directors to align under the influence of a stress field and (2) flow alignment to dominate surface-induced alignment above some critical shear rate. The effects might be manifested by a non-newtonian regime as well as yield stress behaviour and thixotropy. The kinetics of relaxation from mono- to poly-domain material has implications for the dynamic response and rheological hysterises of the material.     

Rheology of a thermotropic polyester in the nematic and isotropic states

The rheological properties of a thermotropic polyester were determined in the nematic and isotropic states. In the isotropic state, the viscosity is almost constant and the polymer is only slightly elastic. The nematic phase has a lower viscosity than the isotropic, except at low frequencies or shear rates, where the viscosity increases as though the polymer had a yield stress. There is a marked dependence of the rheology on shear history. The effects of shearing can be erased by returning the material first to the isotropic state and then back to the nematic state. The results are discussed with reference to analogous observations in small-molecule liquid crystals and in thermotropic aromatic co-polyesters.     

A rheological,optical, and x-ray study of the relaxation of orientation of nematic PBZT

The ultimate properties of liquid crystalline polymers depend upon the achievement of high orientation, usually by means of flow fields. The properties are limited by disorientation which can occur before the product is solidified. Such cooperative orientation and disorientation phenomena also underlie the complex fluid rheology and product microstructure of these materials. The orientation and subsequent disorientation can be followed dynamically by optical and x-ray techniques. Normally, monitoring of orientation is possible only by “fast” techniqes, such as birefringence, these are not applicable to opaque and strongly scattering liquid-crystalline systems. To enable examination of the full dynamic response of concentrated nematic solutions of poly(1,4-phenylene-2,6 benzobisthiazole) (PBZT) the Daresbury Synchrotron Radiation Source was used. PBZT is among the most rigid macromolecules and serves as a good model for other materials of its class. The orientation process determined optically and from x-rays is correlated with fluid rheology and availabel theoretical approaches. During relaxation from near perfect mesogen alignment three principal stages of the disorientation process were identified corresponding to solvent disorientation (first stage of stress relaxation), banding (slow stress relaxation and mesogen disorientation), and finally a very slow banding to polydomain transition. © 1993 John Wiley & Sons, Inc.     

表面活性剂溶致液晶的流变学性质

系统阐述了三种溶致液晶(六角状、立方状和层状液晶)的流变性质,概括了各自的流变性特点并给出了其理论模型,特别对立方相的流变学模型和层状相的剪切诱导转变作用进行了较详细的说明.讨论了因为这种转变而导致的囊泡的形成,并且在表面活性剂和嵌段共聚物中均可观察到剪切诱导的结构转变.     

Phenomenological theory of textured mesophase polymers in weak flows

A macroscopic viscoelastic model for incompressible, isothermal, homogeneous lyotropic mesophases exhibiting the nematic polydomain texture is presented. Under equilibrium static conditions the model describes a three dimensional tessellation, where each region or nematic domain has a characteristic size and orientation, and where the polydomain texture has a random orientation. Close form expressions that define the characteristic texture size and the number of randomly oriented domains are given. When subjecting the model lyotropic liquid crystalline polymer displaying the polydomain texture to a steady rectilinear shear flow, the predicted characteristic texture size decreases with increasing shear rates. The power law scaling relations of texture size with shear rate are in excellent agreement with the experimental measurements. The steady shear flow orientation predictions, characterized by a positive shear dependent alignment angle and a low orientation, are in agreement with experimental data.     

Milk protein interfacial layers and the relationship to emulsion stability and rheology

The properties of milk protein-stabilised, oil-in-water emulsions are determined by the structure and surface rheology of the adsorbed layer at the oil-water interface. Analysis of the segment density profiles normal to the surface show differences in the structure between adsorbed layers of disordered casein and globular whey protein. Systematic studies of stability and rheology of model oil-in-water emulsion systems made with milk proteins as sole emulsifiers give insight into the relation between adsorbed layer properties and bulk emulsion stability. Of particular importance are effects of pH, temperature, calcium ions and protein content. Colloidal interactions between adsorbed layers on different surfaces can be inferred from an analysis of dynamic collisions of protein-coated emulsion droplets in shear flow using the colloidal particle scattering technique. The role of competitive adsorption on emulsion properties can be derived from experiments on systems containing mixtures of milk proteins and small-molecule surfactants. Shear-induced destabilisation is especially influenced by the presence of fat crystals in the emulsion droplets. Aggregated gel network properties are dependent on the balance of weak and strong interparticle interactions. In heat-set whey protein emulsion gels, the rheological behaviour is especially sensitive to surfactant type and concentration. Rearrangements of transient caseinate-based emulsion gels can have a profound influence on the quiesent stability behaviour. Computer simulation provides a general link between particle interactions, microstructure and rheological properties.     

Sorbitan tristearate layers at the air/water interface studied by shear and dilatational interfacial rheology

The shear and dilatational rheology of condensed interfacial layers of the water-insoluble surfactant sorbitan tristearate at the air/water interface is investigated. A new interfacial shear rheometer allows measurements in both stress- and strain-controlled modes, providing comprehensive interfacial rheological information such as the interfacial dynamic shear moduli, the creep response to a stress pulse, the stress relaxation response to a strain step, or steady shear curves. Our experiments show that the interfacial films are both viscoelastic and brittle in nature and subject to fracture at small deformations, as was supported by in-situ Brewster angle microscopy performed during the rheological experiments. Although any large-deformation test is destructive to the sample, it is still possible to study the linear viscoelastic regime if the deformations involved are controlled carefully. Complementary results for the dilatational rheology in area step compression/expansion experiments are reported. The dilatational behavior is predominantly elastic throughout the frequency spectrum measured, whereas the layers exhibit generalized Maxwell behavior in shear mode within a deformation frequency regime as narrow as two decades, indicating the presence of additional relaxation mechanisms in shear as opposed to expansion/compression. If the transient rheological response from stress relaxation experiments is considered, then the data can be described well with a stretched exponential model both in the shear and dilatational deformations.     

Rheological behavior of cellulose/silk fibroin blend solutions with ionic liquid as solvent

The rheological behavior of cellulose and silk fibroin blend in 1-butyl-3-methylimidazolium chloride was studied. The data from the rheological results was analyzed to understand the microstructure of the blend solutions. The viscosity and dynamic modulus of the blend solution decreased with increasing ratio of silk fibroin. While comparing the experimental results with the calculated data from the log-additivity rule, it is revealed that zero-shear viscosity, dynamic modulus show positive–negative deviations and a typical continuous–discrete type of morphology could be imaged. At lower shear rate, the change of phase morphology took place at the ratio of about 0.5 volume fraction of cellulose. However, the blend solution showed positive deviations for all cellulose/silk fibroin blend ratios at high shear rate, which indicates that the dispersion of cellulose and silk fibroin was improved under shear stress. The properties of cellulose/silk fibroin blends observed by Fourier transform infrared spectroscopy and scanning electron microcopy agreed with the result from rheology.     

Peculiarities of rheological properties and flow of highly concentrated emulsions: The role of concentration and droplet size

Results of a complete study of the rheological properties of highly concentrated emulsions of the w/o type with the content of the dispersed phase up to 96% are reported. The aqueous phase is a supersaturated solution of nitrates, where the water content does not exceed 20%. Dispersed droplets are characterized by a polyhedral shape and a broad size distribution. Highly concentrated emulsions exhibit the properties of rheopectic media. In steady-state regimes of shearing, these emulsions behave as viscoplastic materials with a clearly expressed yield stress. Highly concentrated emulsions are characterized by elasticity due to the compressed state of droplets. Shear storage modulus is constant in a wide range of frequencies that reflect solid-like behavior of such emulsions at small deformations. The storage (dynamic) modulus coincides with the elastic modulus measured in terms of the reversible deformations after the cessation of creep. Normal stresses appear in the shearing. In the low shear rate domain, normal stresses do not depend on shear rate, so that it can be assumed that they have nothing in common with normal stresses arising owing to the Weissenberg effect. These normal stresses can be attributed to Reynolds’ dilatancy (elastic dilatancy). Normal stresses sharply decrease beyond some threshold value of the shear rate and slightly increase only in a high shear rate domain. Observed anomalous flow curves and unusual changes of normal stresses with shear rate are explained by the two-step model of emulsion flow. Direct optical observations show that emulsions move by the mechanism of the rolling of larger droplets over smaller ones without noticeable changes of their shape at low shear rates, while strong distortions of the droplet shape is evident at high shear rates. The transition from one mechanism to the other is attributed to a certain critical value of the capillary number. The concentration dependence of the elastic modulus (as well as the yield stress) can be described by the Princen-Kiss model, but this model fails to predict the droplet size dependence of the elastic modulus. Numerous experiments demonstrated that the modulus and yield stress are proportional to the squared reciprocal size, while the Princen-Kiss model predicts their linear dependence on the reciprocal size. A new model based on dimensional arguments is proposed. This model correctly describes the influence of the main structural parameters on the rheological properties of highly concentrated emulsions. The boundaries of the domain of highly concentrated emulsions are estimated on the basis of the measurement of their elasticity and yield stress.     

Interfacial rheology of stable and weakly aggregated two-dimensional suspensions

The interfacial rheological properties of stable and weakly aggregated two-dimensional suspensions are studied experimentally using a magnetic rod interfacial rheometer. Particle monolayers with well controlled structures were prepared. Charged polystyrene particles create two-dimensional colloidal crystals at the water-decane interface over a wide range of concentrations. Under similar conditions a predominantly liquid structure is obtained at the water-air interface for the same particles. The addition of appropriate combinations of the anionic surfactant sodiumdodecylsulfate (SDS) and sodium chloride (NaCl) to the aqueous subphase leads to a destabilization of these monolayers with the formation of fractal aggregates at low concentrations and a heterogeneous gel forming as the surface coverage is increased. After the structures have been built up a reproducible structure can be obtained, of which the interfacial rheological properties can be investigated using a magnetic rod stress rheometer. In all cases, numerical calculations were used to assess the importance of instrumental artifacts and the effect of the coupling between surface and subphase flows. The rheology of aggregated suspensions was compared to the reference case of a colloidal crystal. The two-dimensional aggregated suspensions display rheological features which are similar to their three-dimensional counterparts. These include an elastic response with small linearity limits, a power law dependence on surface coverage and a dependence on the strength of attraction. The results shed some light on the possible role of interfacial rheology on the stability of particle laden high interface systems. Additionally, the 2D suspensions could present fundamental insights in the rheological properties of dense colloidal suspensions.     

芳香族共聚酯液晶熔体流变学

较全面综述了热致液晶性芳香族共聚酯(TLCP)熔体流变学的最新研究成就，指出TLCP溶体具有许多特征流变行为，存在屈服应力、低粘虞、负第一法向应力差、孔口收缩、长松弛时间及它们显著的剪切速率(振动频率)、温度、时间和热历史的依赖性。     

The rheology of concentrated suspensions of deformable particles

The rheology of slightly deformable particles, in particular their dilatant behaviour, has been investigated using waxy maize starch as a ‘model’ suspension. Suspensions of such large particles present considerable experimental problems in rheological terms in that they exhibit phenomena such as wall slip and sedimentation. These are discussed in some detail, along with the methodologies developed to overcome them. Comparisons of the results with hard sphere data in the literature indicate that whilst the deformability of the starch granules does not play a significant role in the high shear limiting viscosity, the dilatant transition does appear to be strongly affected.     

Phenomenological transient shear behavior of liquid crystalline polymers after the start-up of shear flow

Transient stresses of liquid crystalline polymers appear as damped oscillatory patterns after the start-up of shear flow. They are examined by using the constitutive equations which were modified to include the idea of an initial domain state in the Larson-Doi polydomain equation set. In order to be consistent with the phenomenological transient shear stress after the start-up in the region of low shear rates, the Hinch-Leal closure approximations were adopted for the domain-averaged (mesoscopic) fourth-order tensors of the director field. It is predicted as an important result that the amplitude of the first overshoot in the transient stress increases with an increase in the shear rate imposed at the start-up, which was found to be largely in agreement with phenomenology. In addition, the effects of parameters such as molecular concentration, initial domain size, and intensities of viscous and elastic contributions on the transient stress under the start-up process were also evaluated.     

Rheological characterization of polysaccharide-surfactant matrices for cosmetic O/W emulsions

Rheometrical techniques can be profitably used for polysaccharide matrices in order to evaluate their suitability for the preparation of stable cosmetic O/W emulsions. In particular, the rheological properties of aqueous scleroglucan systems were investigated under continuous and oscillatory shear conditions in a polymer concentration range (0.2-1.2% w/w) embracing the sol/gel transition. The effects due to the addition of two different surfactants (up to 10% w/w) were examined at constant polymer concentration (0.4% w/w). The selected additives are a nonionic polymeric siliconic surfactant (dimethicone copolyol) and a cationic surfactant (tetradecyltrimethylammonium bromide), respectively. Polysaccharide-surfactant interactions leading to complex formation were detected also through rheology. The combined action of both nonionic and cationic surfactants in the polymer solution was examined at two different surfactant concentration levels (5 and 10% w/w), demonstrating the beneficial effects produced on the mechanical properties of the polymer matrix by the coexistence of both surfactants. Such beneficial effects are confirmed by the stability and rheology shown by the emulsions prepared. In this way, the results point out the good agreement between the rheology of the continuous phase and the final characteristics of the emulsion obtained.     

The rheological properties of concentrated cetyltrimethylammonium bromide-salicylic acid solutions in water

Data on the rheological properties of the hexadecyl-trimethylammonium salicylate system (CTAB-SA) in water are reported. Three concentrations were used (0.1, 0.01, and 0.001 M). For the highest concentration, the effect of temperature on the rheology was studied in detail.The rheology of the 0.1 M CTAB-SA solution indicates a very uniform micellar size. By contrast with concentrated polymethyl methycrylate dispersions studied by the author, there was a strong divergence between the viscosity-shear rate and viscosity-frequency data, although the plateau low shear rate and frequency values agreed over a wide range of temperature. This effect could be explained by a shear rate dependent diffusion constant. The large temperature variation of the plateau viscosity and elasticity modulus values could be explained by a combination of micellar number concentration and flexibility changes as the temperature varies.At lower concentrations, the rheological data shows evidence of polydispersity in micellar size. Strong shear thickening and extensional viscosity effects are also evident, probably due to micellar overlap and cluster formation in strong shear fields and the alignment of the very long micelles in elongational flow. The shear thickening effects take some 200 s to relax (0.01 M solution). Recovery of the elasticity after shearing the 0.1 M solution is rapid (a few hundred milliseconds).     

Lattice Boltzmann simulation of particles agglomeration and rheology in a particulate flow

The dynamics and rheology of particles in a Newtonian fluid subjected to shear are simulated using Lattice Boltzmann Method. A computationally-efficient Smoothed Profile Method is used to resolve fluid-solid interactions, and the Lennard-Jones inter-particle potential is implemented to account for inter-particle forces. The use of a bi-periodic computational domain with Lees-Edward boundary conditions allows simulation for systems consisting of a large number of particles under shear. The method is validated for single and dual particle problems and an analysis is performed for multi-particle problems under a range of shear rates and particle fractions. The introduction of particle-particle interactions, which are physically important in many engineering processes, is found to have a considerable impact on the dynamics, agglomeration and rheology. The total stress exhibits high unsteadiness primarily due to the solid component contribution, at higher particle fractions. The simulations underscore the complex interplay between shear, interparticle forces and agglomeration and the complex dependencies of the rheological properties.     

剪切均匀性对流动诱导等规聚丙烯结晶的影响

固定应变和最终应变速率,采用瞬时和缓慢2种电机加速方式对样品施加剪切,研究了流场加载模式对样品流变和结晶行为的影响.实验结果显示缓慢加速能够消除剪切过程中流场的非均匀性,使样品取向度增加,提高流场对聚合物熔体的作用效果.同时,流动诱导结晶对于加速时间有依赖性.对于速率为17.7 s-1的剪切,加速时间为1 s时,熔体流动均匀且流动诱导的晶体取向最强,短加速时间(0.5 s)和长加速时间(1.5 s)样品的流动诱导结晶效果都弱于加速时间为1 s的样品.但是,对于不同剪切速率,其对应的最优加速时间不同.对于流动诱导结晶来说,加速时间应当作为一个重要参数来考虑,其背后的真实物理含义还需要进一步研究来说明.     

Rheology of semi-dilute emulsions: viscoelastic effects caused by the interfacial tension

The steady state rheological properties of viscous emulsions are discussed in the dilute and semi-dilute concentration regions. In these systems the first normal stress differences can be measured as well. Such data have been collected over a wide range of ratios of droplet over matrix viscosity. In this manner data became available to evaluate the Choi-Schowalter model. Application of the latter to the normal stresses requires that the droplet diameter be known. At high shear rates the droplet diameter changes nearly inversely proportional to the shear rate. This results in a first normal stress difference proportional to shear rate and hence a ‘normal viscosity’ can be defined. This is used to compare the data with the available theoretical predictions. At low shear rates deviations from a constant normal viscosity can be observed. They are associated with a hysteresis region, where no single steady state droplet size can be defined anymore. Slightly viscoelastic components have been used as well to investigate whether this would result in deviations from the behaviour observed for mixtures of Newtonian fluids.     

Dispersive mixing in immiscible polymer blends

Dispersive mixing of immiscible polymer blends as well as polymer systems containing solids is achieved in compounding equipment at two stages of the system's processing experience: first, while one or more of the polymer components are melting, and second, after all polymer components have melted. That is, the first mode of dispersive mixing occurs during the melting mechanism of “dissipative mix melting” (Ref. 1), while the second is melt-melt mixing. During the compounding of a given blend system, there are a number of processing parameters that can be changed in order to improve mixing. These range from machine operating variables to the addition of processing aids. If such processing changes fail to produce the desired morphology, the most common change to consider is the screw geometry. This, in practice involves a trial and error procedure, or the use of an existing database built from prior experience. The role which the thermomechanical and rheological properties of the blend component play in dissipative mix melting and melt-melt mixing has not yet been well understood. The reason for this is that although most blend systems have components which are strongly non-Newtonian and strongly viscoelastic, the thinking and rules of thumb for mixing such materials has been heavily influenced by the analysis of G. I. Taylor (Ref. 2), who in 1932 addressed the phenomenon of the dispersion of a single Newtonian droplet by a Newtonian matrix flowing in laminar shear flow. This paper addresses the strong role that the rheology of blend components, under processing conditions, play in laminar dispersive mixing of polymer blends. From a practical point of view, if the dispersion mechanisms and rates of dispersion depend on the component rheology, then such knowledge can lead us to the selection of advantageous mixing element designs and processing conditions. The experimental results were obtained in dispersive mixing carried out in devices developed in the Polymer Mixing Study (Ref. 3). Such model devices include the Couette Flow Intensive Mixer (CIM) (Ref. 4), where a constant shear stress is applied on the blend components and the Twin Screw Mixing Element Evaluator (TSMEE) (Ref. 5), where the mixing flows are those encountered in actual mixing/compounding operations. The TSMEE will be described in the body of this paper together with its on- and off-line morphology determination capabilities and its in-line rheology sensor. The low-density polyethylene (LDPE) and polystyrene (PS) polymers studied were selected because they cover a wide spectrum of rheological properties.