Shear rheology of lyotropic liquid crystals: a case study

We have investigated the rheological properties of lyotropic liquid crystals (LCs) formed by self-assembled neutral lipids and water, their relationship with the topology of the structure, and their dependence on temperature and water content. The phase diagram of a representative monoglyceride-water system, determined by combining cross-polarized optical microscopy and small-angle X-ray scattering (SAXS), included four structures: lamellar, hexagonal, gyroid bicontinuous cubic (Ia3d), and double diamond bicontinuous cubic (Pn3m), as well as several regions of two-phase coexistence of some of the above structures. Rheology in the linear viscoelastic regime revealed a specific signature that was characteristic of the topology of each structure considered. The order-order transitions lamellar-to-cubic and cubic-to-hexagonal, as well as the order-disorder transitions from each LC to an isotropic fluid, were easily identified by following the development of the storage and loss moduli, G' and G', respectively. The viscoelastic properties of both bicontinuous cubic phases were shown to be strongly frequency-dependent, following a pseudo-Maxwell behavior, with multiple relaxation times. Cubic-to-cubic transitions were nicely captured by scaling the longest relaxation time, tau, with either temperature or water volume fraction. Therefore, the set of the three main parameters used to establish the rheological behavior of the structure, that is, G', G', and relaxation time, tau, constitutes a consistent ensemble to identify the structures of the liquid crystal. Finally, relaxation spectra, extracted for all liquid crystalline phases, allowed an additional possible identification criterion of the various structures considered.     

Nonlinear viscoelasticity of sorbitan tristearate monolayers at liquid/gas interface

The interfacial rheology of sorbitan tristearate monolayers formed at the liquid/air interface reveal a distinct nonlinear viscoelastic behavior under oscillatory shear usually observed in many 3D metastable complex fluids with large structural relaxation times. At large strain amplitudes (gamma), the storage modulus (G') decreases monotonically whereas the loss modulus (G') exhibits a peak above a critical strain amplitude before it decreases at higher strain amplitudes. The power law decay exponents of G' and G' are in the ratio 2:1. The peak in G' is absent at high temperatures and low concentration of sorbitan tristearate. Strain-rate frequency sweep measurements on the monolayers do indicate a strain-rate dependence on the structural relaxation time. The present study on sorbitan tristearate monolayers clearly indicates that the nonlinear viscoelastic behavior in 2D Langmuir monolayers is more general and exhibits many of the features observed in 3D complex fluids.     

Microrheology of wormlike micellar fluids from the diffusion of colloidal probes

The microrheology of cationic micellar solutions has been investigated as a function of added organic salts using quasielastic light scattering (QELS). Two organic salts, sodium p-toluene sulfonate and sodium salicylate, were used to induce microstructural changes in cetyl trimethylammonium bromide (CTAB) micelles. The mean-squared displacement (MSD) of polystyrene probe particles embedded in CTAB micellar solutions was monitored by QELS in the single-scattering regime. Through the use of the generalized Stokes-Einstein relationship, the frequency-dependent complex shear moduli of each fluid were estimated from the Laplace transform of the corresponding MSD. The salt-induced transition from nearly spherical to elongated wormlike micelles and consequent changes in fluid response from viscous to viscoelastic are clearly captured by microrheology.     

Solid-phase rheology of an anisotropic polymer (vectra A)

The solid-phase rheology of a thermotropic polyester which is liquid crystalline in the melt (Vectra A) was studied for one-dimensional finite-amplitude deformations, including step-strain and recovery, step-stress and recovery, and step-strain followed by small-amplitude oscillations. The rheology is complex, and cannot be described by existing models. Below a critical strain, which is history-dependent, Vectra A deforms as a linear viscoelastic solid. Above the critical strain the deformation is both viscoplastic and viscoelastic. There appears to be a maximum recoverable strain of about 0.03, beyond which all deformation is nonrecoverable. A large number of relaxation modes is required to describe the stress, and one time scale is inadequate to describe the time-dependence of yielding and plastic flow.     

Microstructure and rheological properties of liquid crystallines formed in Brij 97/water/IPM system

The phase diagram of Brij 97/water/IPM systems was determined at 25 degrees C. Rich liquid crystalline phases including Lalpha, H1, and cubic Fd3m phases were identified by means of small angle X-ray scattering (SAXS). Microstructure transitions of liquid crystals with changes in surfactant concentration and oil content are explained qualitatively by the surfactant packing parameter (vL/aSlc). Dynamic rheological results indicate that all three kinds of liquid crystals investigated show high elasticity. The lamellar, Lalpha, phases formed in Brij 97/water with two different oils, oleic acid and geraniol, were also studied in comparison with those of Brij 97/water/IPM systems. The strength of the network of lamellar phases formed in Brij 97/water/oleic acid and Brij 97/water/geraniol systems are appreciably stronger than for Brij 97/water/IPM systems, indicated by the smaller area of surfactant molecules at the interface and the higher moduli (G' and G').     

Nonlinear microrheology: bulk stresses versus direct interactions

In passive microrheology, the linear viscoelastic properties of complex fluids are inferred from the Brownian motion of colloidal tracer particles. Active (but gentle) forcing may also be used to obtain such linear-response information. More significant forcing may drive the material significantly out of equilibrium, thus potentially providing a window into the nonlinear response properties of the material. In leaving the linear-response regime, however, the theoretical underpinning for passive microrheology is lost, and a variety of issues arise. Most generally, what exactly can be measured, and how can such measurements be interpreted? Here we motivate and discuss a variety of theoretical issues facing the interpretation of active microrheology. First, in the continuum limit, the inhomogeneous velocity field around the probe gives rise to rheological inhomogeneities, whereupon an assumed generalized Stokes drag yields a weighted average of the viscosities around the probe rather than the (homogeneous) viscosity measured macroscopically. We then explicitly treat the material microstructure using a model system (a large colloidal probe pulled through a dilute suspension of small bath particles). We examine the different sources of stress upon the probe particle (e.g., direct probe-bath collisions as well as microstructural deformations within the bulk suspension) and discuss their analog (or lack thereof) in the corresponding macrorheological system. We discuss several crucial issues for the interpretation of nonlinear microrheology: (1) how to interpret the inhomogeneous and nonviscometric nature of the deformation field around the probe, (2) the distinction between direct and bulk stresses and their deconvolution, and (3) the (Lagrangian) time-dependent nature of the stress histories experienced by material elements as they advect past the probe. Having identified these issues, we briefly discuss adaptations of the basic technique to recover bulk rheology more faithfully. Whereas we specifically discuss a model colloidal suspension, we ultimately envision a technique capable of measuring the nonlinear rheology of general materials.     

一水甲酸锂晶体三阶非线性光学性质理论研究

用含时藕合微扰Hartrre一Fork（CPHF）方法，有限场MP2理方法以及有向气 体方法，首次分别在分子和晶体水平上计算了一水甲酸锂晶体三次谐波极化率，表 明超分子γ(一3ω；ω,ω,ω)数值与超分子链长有线性关系；晶体x（一3ω;ω, ω,ω）计算值主要受到电子相关影响，其次受到基组和局域场影响，在非共振 条件下，频率色散的影响甚小'最后，估算了一水甲酸锂的三次谐波系数．     

WHAT ARE THE DIFFERENCES OF POLYMER SURFACE RELAXATION FROM THE BULK?

The polymer surface relaxation in thin films has been a long debating issue.We report a new method on studying surface relaxation behaviors of polymer thin films on a solid substrate.This method involved utilizing a rubbed polyimide surface with a pretilting angle in a liquid crystalline cell.Due to the surface alignment,the liquid crystals were aligned along the rubbing direction.During heating the liquid crystalline cell,we continuously monitored the change of orientation of the liquid crystals.It is u...     

Microstructure and dynamics in lyotropic liquid crystals. Principles and applications of nuclear spin relaxation

Nuclear spin relaxation of quadrupolar nuclei provides access to a wide range of properties of lyotropic liquid crystals, ranging from the molecular ordering and dynamics at the interface to the macroscopic viscoelastic behaviour. We emphasize here the unique capability of the spin relaxation method to provide detailed geometric and dynamic information relating to the microstructure of lyotropic liquid crystals, i.e. the metric, curvature, and fluctuations of the dividing interface that separates polar and non-polar regions. This information is conveyed to the spin system via the translational diffusion of surfactants or counterions over the interface. The general principles of the spin relaxation method, as applied to lyotropic liquid crystals, are described, with emphasis on the model-independent information content of the relaxation observables and on the relation to microstructure. Specific results for lamellar, hexagonal, cubic, and nematic phases are also described.     

Apparent microrheology of oil-water interfaces by single-particle tracking

We investigate the dynamics of charged microparticles at polydimethylsiloxane (oil)-water interfaces using Pickering emulsions as an experimental template. The mobility of the charged particles depends largely on the viscoelastic properties of the oil phase and the wettability of the solid particles. In addition, we have explored the potential of developing microrheology at liquid-liquid interfaces from the single-particle tracking technique. The apparent loss modulus, storage modulus, and relaxation time of the oil-water interfaces obtained from singe-particle microrheology depend strongly on the surface nature of the tracer particles, especially when the oil phase is viscoelastic.     

MULTI-SCALES RELAXATION PROCESSES OF SHORT FIBER OF NEMATIC LIQUID CRYSTALLINE COPOLYMER IN POLYCARBONATE MATRIX

Multi-scales relaxation processes of short fiber of a nematic liquid crystalline copolymer(LCP)in polycarbonate matrix were investigated.First,the structure relaxation of LCP was studied by rheology.The relaxation spectrum of the nematic liquid crystalline copolymer at 295℃was calculated from the combined dynamic modulus.There are three kinds of relaxation mechanisms for nematic liquid crystalline copotymer:the relaxation of chain orientation,the relaxation of deformed polydomains and the coalescence of ...     

Microstructure and dynamics in lyotropic liquid crystals. Principles and applications of nuclear spin relaxation

Abstract

Nuclear spin relaxation of quadrupolar nuclei provides access to a wide range of properties of lyotropic liquid crystals, ranging from the molecular ordering and dynamics at the interface to the macroscopic viscoelastic behaviour. We emphasize here the unique capability of the spin relaxation method to provide detailed geometric and dynamic information relating to the microstructure of lyotropic liquid crystals, i.e. the metric, curvature, and fluctuations of the dividing interface that separates polar and non-polar regions. This information is conveyed to the spin system via the translational diffusion of surfactants or counterions over the interface. The general principles of the spin relaxation method, as applied to lyotropic liquid crystals, are described, with emphasis on the model-independent information content of the relaxation observables and on the relation to microstructure. Specific results for lamellar, hexagonal, cubic, and nematic phases are also described.     

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。     

Structure-property relations in linear and crosslinked liquid crystalline polymers

This paper reviews structure-property relations in liquid crystalline side group polymers, as investigated by X-ray scattering of fibres, by small angle X-ray scattering in solution, by dielectric relaxation measurements and by melt rheology, as well as synthetic ways to “combined liquid crystalline polymers”. The synthesis of liquid crystalline elastomers from side group, main chain and combined liquid crystalline polymers is described. First structure-property relations are discussed.     

Shear-induced topology changes in liquid crystals of the soybean lecithin/DDAB/water system

The viscoelastic behavior of the two different liquid crystalline lamellar phases and the liquid crystalline cubic phase of the mixed soybean lecithin/DDAB system in water was studied through rheology, with mechanical parameters studied as a function of composition. The swollen or diluted lamellar region is formed by vesicles, and its characteristic flow curve presents two-power law regions separated by a region where viscosity passes through a maximum. Yield stress and shear-dependent flow behavior were also observed. The microstructure suffers transformation under shear stress, and rheological response shifts from thixotropic to antithixotropic loops. Similar rheological behavior has been observed for samples in the collapsed or concentrated lamellar region, at the water-rich corner of the phase diagram. Vesicle formation may therefore occur by shearing the initial stacked and open bilayers. However, concentrated lamellar samples in the water-poor part of the phase diagram are less sensitive to shear effects and show plastic behavior and thixotropy. All lamellar samples manifest high elasticity. The dynamic responses of both lamellar topologies, i.e., vesicles and open bilayers, are comparable and exhibit an infinite relation time. The bicontinuous cubic, liquid crystalline phase is highly viscous. Its dynamic response cannot be modeled by a Maxwell model.     

Rheology and dynamics of micellar cubic phases and related emulsions

The rheological behavior of micellar cubic phases in C12EO25 systems and related emulsions has been investigated. In the aqueous C12EO25 binary system, the transition from the cubic phase to the micellar solution is associated with a sudden drop in viscosity and with a small enthalpy of transition. The elastic modulus and viscosity of the cubic phases show a maximum with concentration but remain very high within the range of existence of the cubic phase. Several relaxation processes seem to be present in binary cubic phases, and some of them occur in a time scale that can be followed by both rheology and dynamic light scattering measurements. Upon addition of a small amount of oil (decane), the rheological behavior changes remarkably. As the oil fraction increases, the relaxation times also increase and, finally, highly concentrated, gel-like emulsions are obtained. Contrary to conventional concentrated emulsions, the viscosity of cubic-phase-based emulsions is decreased by increasing the fraction of the dispersed phase. The non-Maxwellian rheological behavior at low oil fractions is described according to the model of slipping crystalline planes, modified by using a distribution of bulk relaxation times, and good fitting to the experimental data is obtained.     

Organic reactions in liquid crystalline solvents. 5. : Intramolecular phenolic quenching of aromatic ketone triplets as a probe of solute conformational mobility in liquid crystals.

The effects of liquid crystalline order on the confomational motions involved in end-to-end, intramolecular triplet quenching of aromatic ketones by a suitably situated phenolic moiety have been investigated. Triplet state quenching has been monitored using two independent probes: Norrish II fragmentation quantum yields of valeryl-substituted derivatives and direct lifetime measurements in methyl-substituted ketones, using the corresponding anisyl-substituted ketones as models for triplet state behaviour In the absence of phenolic quenching.Quantum yields for Norrish II fragmentation in the liquid crystalline solvents have been estimated using 4-methoxyvalerophenone (MVP) as the actinometer. The ability of liquid crystalline solvents to inhibit intramolecular phenolic quenching is dependent on both phase type and solute length. In one case, Intramolecular quenching Is completely suppressed In smectic phases, indicating that the conformational motions involved in achieving the quenching geometry are slowed by a factor of at least 10³ relative to their rates in non-viscous, isotropic solvents. The Norrish II product ratios from photolysis of MVP in the liquid crystalline solvents are also affected dramatically by liquid crystalline order.     

Rheological properties of lyotropic liquid crystals encapsulating curcumin

This study constructed new curcumin-loaded lyotropic liquid crystals containing pharmaceutically accepted oil, and ethyl oleate (EtOL). Three liquid crystalline phases including lamellar, hexagonal, and cubic phases were identified by means of the polarized optical microscopy and rheology method. By analyzing the shear viscosity (η_0.1), the viscosity of curcumin-liquid crystals is smaller than those without curcumin. Dynamic rheological results show that: Dissolved curcumin in EtOL can make the elastic modulus of hexagonal and cubic phase increase compared with that without curcumin, while the elastic modulus of lamellar phase decreases. Dissolved curcumin in Brij 97 can lead to the decreasing of the elastic modulus for cubic and lamellar phases, whereas it has little influence on hexagonal phase. When the curcumin is solubilized in both EtOL and Brij 97, the elastic modus of hexagonal phase increase, the elastic modus of lamellar and cubic phases decrease compared with that without curcumin. Furthermore, three temperature turning points were identified by the change in the slope of tanδ (G″/G′) for curcumin-hexagonal liquid crystal. These studies might be a help to study the storage of drug carrier and in vitro release properties of lyotropic liquid crystals containing curcumin.     

Using particle tracking to probe the local dynamics of barley β-glucan solutions upon gelation

The sol-gel transition of aqueous barley β-glucan solutions which undergo gelation with ageing has been studied by conventional bulk rheology, phase contrast microscopy and particle tracking microrheology. Characterisation of the primary structure of the β-glucan isolate was carried out by enzymic methods and HPLC. The Brownian diffusion of fluorescent microspheres (0.75 μm diameter, carboxylate-coated particles) was used to probe the spatial mechanical properties of the gelling systems at the scale of microns; the potential use of passive particle tracking to study biopolymer gelling systems that present spatial heterogeneities is thus explored. For the β-glucan gels cured at 25°C both microrheology and bulk rheology revealed that with increasing the polysaccharide concentration the gelation time decreased, while the gelation rate and gel strength of the barley β-glucan gels increased. The particle tracking method had higher sensitivity and could map molecular ordering and structural heterogeneities in the evolving polysaccharide network at a micro-level. That is, different size pores were generated upon ageing with regions of depleted or less amount of β-glucan molecules. Furthermore, this method could detect changes in the fine structure of the system before such events can be registered by bulk rheological measurements; i.e. microheterogeneity and aggregation of β-glucan chains were revealed by particle tracking at earlier temporal stages of the experiment.     

Synthesis and Characterization of Thermotropic Liquid Crystalline Poly(ester-imide)s

A series of thermotropic liquid crystalline poly(ester-imide)s was synthesized by melt polymerization of diacetoxynaphthalene acid and n-(ω-carboxyalkylene) trimellitic imides. All polymers with 2,6 substituent positions (n-2,6 PEIM) on the napthalene ring exhibit liquid crystalline phases, whereas polymers with 2,7 substituent positions (n-2,7 PEIM) do not. This result suggests that the kink structure of n-2,7 PEIMS would hinder the formation of liquid crystalline polymer. The copoly(ester-imide)s with an irregular sequence of aliphatic units and aromatic mesogens showed the liquid crystallinity with the lower transition temperatures and a lesser tendency to crystallize than homopoly(ester-imide)s. The semicrystalline polymers with more regular monomeric sequence in the main chain showed the hysteresis of viscoelastic property in the temperature cycle. A nematic glassy copolymer gave the higher molecular orientation to the fiber than a semicrystalline polymer. © 1997 John Wiley & Sons, Ltd.