Effect of ionic strength on the rheological behavior of aqueous cetyltrimethylammonium p-toluene sulfonate solutions

The influence of ionic environment on the rheological properties of aqueous cetyltrimethylammonium p-toluene sulfonate (CTAT) solutions has been studied under three different flow fields: simple shear, opposed-jets flow and porous media flow. Emphasis was placed in the experiments on a range of CTAT concentration in which wormlike micelles were formed. It is known that these solutions exhibit shear thickening in the semi-dilute regime, which has been explained in terms of the formation of shear-induced, cooperative structures involving wormlike micelles. In simple shear flow, the zero shear viscosity exhibits first an increase with salt addition followed by a decrease, while the critical shear rate for shear thickening increases sharply at low salt contents and tends to saturate at relatively high ionic strengths. The results are explained in terms of a competition between micellar growth induced by salt addition and changes in micellar flexibility caused by ionic screening effects. Dynamic light scattering results indicate that micelles grow rapidly upon salt addition but eventually achieve a constant size under static conditions. These observations suggest that the wormlike micelles continuously grow with salt addition, but, as they become more flexible due to electrostatic screening, the wormlike coils tend to adopt a more compact conformation. The trends observed in the apparent viscosities measured in porous media flows seem to confirm these hypotheses-but viscosity increases in the shear thickening region-and are magnified by micelle deformation induced by the elongational nature of the local flow in the pores. In opposed-jets flow, the solutions have a behavior that is close to Newtonian, which suggests that the range of strain rates employed makes the flow strong enough to destroy or prevent the formation of cooperative micellar structures.     

Rheological and Rheo-Optical Investigation of Cellulose Ethers in Aqueous Solution

The opportunities provided by rheo-optical techniques for the investigation of cellulose derivatives in solution are discussed. Flow birefringence experiments yield information on the optical properties of the material and allow to distinguish between the contribution of different structural elements like, for example, the dangling chains and the anisotropic stem of fringed micelles on shear thinning and flow orientation, respectively. Rheo-turbidity and rheo-small angle light scattering measurements were used to study the influence of shear on the phase separation that occurs in aqueous methyl hydroxypropyl cellulose (MHPC) solutions upon heating. Both shear induced mixing and demixing were found and also differences between temperature ramp and shear rate ramp experiments were observed. The differences between chemical and physical crosslinking of MHPC were characterized by low amplitude oscillatory shear measurements. Finally, the possibility to prepare microgels by shearing the samples during the crosslinking reaction is reported.     

Genetically Engineered Supercharged Polypeptide Fluids: Fast and Persistent Self‐Ordering Induced by Touch

Mechanically induced disorder–order transitions have been studied in fluid surfactant solutions or polymer thermotropic liquid crystals. However, isothermally induced ordered phases do not persist after cessation of shear, which limits their technological applicability. Moreover, no such stimuli‐responsive materials involving biomacromolecules have been reported although biopolymer liquids are gaining a lot of attention. A biological fluid system is introduced in which anionic polypeptides are complexed with cationic surfactants. The resulting fluids exhibited very sensitive isotropic–nematic transition triggered by shear. The formed liquid crystal was preserved after cessation of mechanical stimulus. Self‐ordering behavior of the material was achieved through water flow and finger pressing. The latter mechanical induction resulted in the formation of complex pattern that can be read out by birefringence, allowing the recording of fingerprint information.     

Structural Changes in Block Copolymer Solutions under Shear Flow as Determined by Non‐Equilibrium Molecular Dynamics

Summary: A non‐equilibrium molecular dynamics computer simulation on microsegregated solutions of symmetrical diblock copolymers is reported. As the polymer concentration increases, the system undergoes phase transitions in the following order: body centered cubic (BCC) micelles, hexagonal (HEX) cylinders, gyroid (GYR) bicontinuous networks and lamellae (L), which are the same morphology reported for block copolymer melts. Structural classification is based on the patterns of the anisotropic static structure factor and characteristic 3‐dimensional images. The systems in the BCC micellar (ρσ³ = 0.3) and HEX cylindrical (ρσ³ = 0.4) phases were then subjected to a steady planar shear flow. In weak shear flow, the segregated domains in both systems tend to rearrange into sliding parallel close‐packed layers with their normal in the direction of the shear gradient. At higher shear rates, both systems adopt a perpendicular lamellar structure with the normal along the neutral direction. A further increase in the shear rate results in a decrease in lamellar spacing without any further structural transitions. Two critical shear rate values that correspond to the demarcation of different structural behaviors were found.

Shear‐induced BCC‐LAM phase transition.     

The effect of shear on ordered block copolymer solutions

Block copolymers are extensively used in solution, especially aqueous solution, because of their amphiphilic character. This leads to the formation of lyotropic mesophases under given conditions of concentration and temperature. In many applications, block copolymer solutions are subjected to shear during processing (for example in drug delivery or when washing in detergent solutions) and thus it is of considerable interest to understand how shear affects the mesophase structure. Recent research has focussed on probing shear-induced structural transformations in lamellar, hexagonal and cubic-packed micellar phases using small-angle X-ray and neutron scattering.     

Alignment of lamellar diblock copolymer phases under shear: insight from dissipative particle dynamics simulations

Sheared self-assembled lamellar phases formed by symmetrical diblock copolymers are investigated through dissipative particle dynamics simulations. Our intent is to provide insight into the experimental observations that the lamellar phases adopt parallel alignment at low shear rates and perpendicular alignment at high shear rates and that it is possible to use shear to induce a transition from the parallel to perpendicular alignment. Simulations are initiated either from lamellar structures prepared under zero shear where lamellae are aligned into parallel, perpendicular, or transverse orientations with respect to the shear direction or from a disordered melt obtained by energy minimization of a random structure. We first consider the relative stability of the parallel and perpendicular phases by applying shear to lamellar structures initially aligned parallel and perpendicular to the shear direction, respectively. The perpendicular lamellar phase persists for all shear rates investigated, whereas the parallel lamellar phase is only stable at low shear rates, and it becomes unstable at high shear rates. At the high shear rates, the parallel lamellar phase first transforms into an unstable diagonal lamellar phase; and upon further increase of the shear rate, the parallel lamellar phase reorients into a perpendicular alignment. We further determine the preferential alignment of the lamellar phases at low shear rate by performing the simulations starting from either the initial transverse lamellar structure or the disordered melt. Since the low shear-rate simulations are plagued by the unstable diagonal lamellar phases, we vary the system size to achieve the natural spacing of the lamellae in the simulation box. In such cases, the unstable diagonal lamellar phases disappear and lamellar phases adopt the preferential alignment, either parallel or perpendicular. In agreement with the experimental observations, the simulations show that the lamellar phase preferentially adopts the parallel orientation at low shear rates and the perpendicular orientation at high shear rates. The simulations further reveal that the perpendicular lamellar phase has lower internal energy than the parallel lamellar phase, whereas the entropy production of the perpendicular lamellar phase is higher with respect to the parallel lamellar phase. Values of the internal energy and entropy production for the unstable diagonal lamellar phases lie between the corresponding values for the parallel and perpendicular lamellar phases. These simulation results suggest that the relative stability of the parallel and perpendicular lamellar phases at low shear rates is a result of the interplay between competing driving forces in the system: (a) the system's drive to adopt a structure with the lowest internal energy and (b) the system's drive to stay in a stationary nonequilibrium state with the lowest entropy production.     

Photorheological Effects in Aqueous Micellar Tetramethylammoniumhydrogen-2-dodecyl Malonate via Photodimerization of Acridizinium Bromide

Aqueous solutions of tetramethylammoniumhydrogen-2-dodecyl malonate (TMHM) were investigated rheologically in the absence and presence of monomeric and photodimerized acridizinium bromide as solubilizates. Upon constant shearing a viscosity increase (accompanied by viscoelasticity) was observed in a certain range of TMHM concentration after an induction period. This indicates the formation of shear induced structures. Viscoelasticity was characterized via stress relaxation experiments revealing a single relaxation process with time constants of 0.2-0.3 s mostly. At certain shear rates a temporal oscillation of viscosity was observed. In the presence of acridizinium bromide the elasticity was reduced and the TMHM concentration range of viscoelastic flow was narrowed while relaxation time constants remained constant. Photodimerization of acridizinium bromide (performed in situ) reverted the effects induced by the monomers. Copyright 1999 Academic Press.     

Kinetics of strain-induced structural changes under high pressure

A mechanism-based microscale kinetic theory for strain-induced structural changes (SCs) (that includes phase transformations (PTs) and chemical reactions (CRs)) is developed. Time is not an independent parameter in this theory; instead, plastic strain is a time-like parameter. Kinetics depends essentially on the ratio of the yield strengths of phases. Stationary and nonstationary solutions of the kinetic equations are analyzed for various cases, including SCs between two phases in an inert matrix and between three phases in silicon and germanium. A number of experimental phenomena are explained, and material parameters controlling the kinetics of strain-induced SCs are determined. This includes the possibility of intensification (or suppression) of SCs at the initial stage of straining by adding a stronger (or weaker) inert phase, zero pressure hysteresis that however has nothing to do with phase equilibrium pressure, the possibility of obtaining some phases (that cannot be obtained under hydrostatic loading) under strains, and the possibility to obtain some phases under relatively small shear, which disappear under larger shear.     

Quasi-anomalous diffusion processes in entangled solutions of wormlike surfactant micelles

In this article, we present a detailed analysis of the dynamic properties of entangled solutions of semi-flexible, threadlike surfactant micelles. These aggregates were formed by self-association processes in aqueous solutions of cationic surfactants such as cetylpyridinium chloride (CPyCl) or cetyltrimethylammonium bromide (CTAB) after the addition of different amounts of sodium salicylate (NaSal). We performed dynamic light scattering (DLS) experiments in combination with rheological measurements in order to investigate the dynamic properties of these viscoelastic surfactant solutions. In all samples, we observed three distinct relaxation regimes: initial monoexponential decay, followed by a power-law behavior at intermediate observation times. A second monoexponential region was detected at very long times, and this terminal regime described the viscoelastic features of the samples. The fast decay mode was induced by local cooperative motions in the gellike network. The intermediate and slowest decay modes point to the existence of quasi-anomalous diffusion processes. These phenomena are characterized by linear-diffusion properties at long times, and they obeyed anomalous logarithmic slow-dynamics behavior at intermediate time zones. The anomalous diffusion properties at intermediate time scales can be induced by the bending motions of the rod-shaped micelles between two entanglement points. This regime, which was more extended at lower temperatures, was described by the power-law form of the correlation function. The power-law exponent depended on the chemical structure of the surfactants and the temperature. The power-law regime shifted toward earlier times as the gellike network evolved. The slowest mode of the correlation function coincided very well with the shear stress relaxation times of the three-dimensional, transient networks. We observed that the temperature dependence of the slowest mode followed Arrhenius laws. This result provides experimental evidence for thermally activated topological relaxation processes of random fluid phases. We obtained activation energies of approximately 30 kcal/mol, and these data coincided well with previously reported literature values, which were determined in similar surfactant solutions. Characteristic "screening lengths", over which viscous effects became important, could also be determined from the activation energy. The elastic modulus G0, calculated from the slowest mode of the correlation function, was in pretty good agreement with rheological data. The light-scattering spectra were consistent with the theoretical model of dynamical coupling of the concentration fluctuations to viscoelasticity. Since only minute sample volumes are required for advanced DLS experiments, this method to extract viscoelasticity is well suited for advanced studies of gellike biomaterials.     

Memory effects across surfactant mesophases

We report a detailed analysis of deuteron NMR spectra of micellar, lamellar, cubic, and hexagonal mesophases in the aqueous non-ionic surfactant system C(12)E(6)/water. Samples are prepared with and without shear. Particular attention is paid to an interesting temperature-driven phase sequence that includes all of the above phases that are studied before and after shear parallel or perpendicular to the magnetic field direction. Surprising memory effects are found across mesophase transitions. These memory effects provide clues to the structure of the various phases.     

Transitional flow behavior of entangled polyisoprene solutions

A contradiction has recently appeared between the current understanding of entangled polymers in nonlinear shear flow and new experimental observations based on highly entangled monodisperse 1,4‐polybutadiene solutions. Using entangled polydisperse solutions, we have examined the universality of the new revelation that any sufficiently entangled and relatively monodisperse polymeric fluids undergo a flow transition in simple shear when the applied stress is comparable to the plateau modulus. Indeed, a similar flow transition, marking the onset of bulk chain disentanglement, as previously observed for polybutadiene solutions in controlled‐stress experiments, can also be observed in less entangled and more polydisperse polyisoprene solutions, whereas controlled‐rate experiments do not reveal any transitional behavior. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4132–4138, 2004     

Structure of electrorheological fluids under an electric field and a shear flow: experiment and computer simulation

It is known that macroscopic properties of colloidal suspensions are often determined by the microstructure of the particles in the suspensions, depending on the interparticle, Brownian, and hydrodynamic (if any) forces. We take electrorheological (ER) fluids as an example. By using a computer simulation and an experimental approach, we investigate the structure of ER fluids subjected to both an electric field and a shear flow. The microstructure evolution from random structure, to chains, and then to stable lamellar patterns, observed in the experiments, agrees very well with that obtained in the simulations. It is shown that the formation of such lamellar patterns originates from the difference between the dipole moment induced in the particles suspended in the ER fluids without shear and the one with shear. The results on the relaxation process of structural formation and the internal structure of layers are also presented. Thus, it seems possible to achieve various structures and hence desired macroscopic properties of colloidal suspensions by adjusting external fields and, simultaneously, a shear flow.     

Rheological measurements and light-scattering experiments of dilute solutions of high molecular polystyrene. Light-scattering of flowing polymer solutions

We describe the behavior of dilute polymer solutions by means of light-scattering under shear flow. Solution properties of polystyrene in benzene over a wide range of molecular weight has been studied to determine the coefficientsa andK of the Mark-Houwink relationship and to estimate the rheological conditions with regard to light-scattering experiments of flowing polymer solutions. The investigations were carried out to measure the shear-rate dependence of macromolecules in solution, e.g., to observe an orientation and changing of the mean-square radius of gyration.     

Orientation behavior of attapulgite nanoparticles in poly(acrylonitrile)/attapulgite solutions by rheological analysis

Nanocomposites based on poly(acrylonitrile) (PAN) and attapulgite (AT) had been prepared by solution blending in dimethyl sulfoxide (DMSO). The rheological properties of the nanocomposites solutions were investigated by HAAKE rheometer with plate‐cone geometry. The steady and oscillation shear experiments were carried out to investigate the effects of clay exfoliation and orientation as well as polymer‐clay interaction. The presence of small amount of AT nanoparticles with large aspect ratio improved the fluidity of these PAN solutions under low shear rate, whereas large amount of AT will reduce its aspect ratio by aggregation and constrain the polymer segment motion in the solutions. The relaxation and disentanglement behaviors of the PAN chains in nanocomposite solutions were also studied by oscillation and thixotropic experiments, from which, it can be concluded that well dispersed and oriented AT nanorods will reduce interaction among macromolecules. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 945–954, 2009     

Shear induced mixing/demixing: blends of homopolymers,of homopolymers plus copolymers,and blends in solution

Shear may shift the phase boundary towards the homogeneous state (shear induced mixing, SIM), or in the opposite direction (shear induced demixing, SID). SIM is the typical behavior of mixtures of components of low molar mass and polymer solutions, SID can be observed with solutions of high molar mass polymers and polymer blends at higher shear rates. The typical sequence with increasing shear rate is SIM, then occurrence of an isolated additional immiscible area (SID), melting of this island into the main miscibility gap, and finally SIM again. A three phase line originates and ends in two critical end points. Raising pressure increases the shear effects. For copolymer containing systems SID is sometimes observed at very low shear rates, preceding the just mentioned sequence of shear influences.     

Drop deformation dynamics and gel kinetics in a co-flowing water-in-oil system

Drop deformation and superimposed gel kinetics were studied in a fast continuous-flow process for a water-in-oil system. Highly monodisperse drops were generated in a double capillary and then deformed passing through a narrowing rectangular channel geometry. Nongelling deformation experiments were used to establish the process and compare it with existing theories. Thereafter, temperature induced drop gelation was included to study its effect on deformation and gel kinetics on short timescales and at high temperature gradients. The disperse phase was a kappa-carrageenan solution with additional sodium and potassium ions for gelation experiments. Sunflower oil was used for the continuous phases. Nongelling experiments showed that shear forces are able to deform drops into ellipsoids. A comparison with the small deformation theory by Taylor was surprisingly good even when drop deformation and flow conditions were not in steady state. Superimposed gelation on the deformation process showed clearly the impact of the altered rheological properties of the dispersed and continuous phase. Deformation first increased on cooling the continuous phase until the onset of gel formation, where a pronounced decrease in deformation due to increasing droplet viscosity/viscoelasticity was observed. Drop deformation analyses were then used to detect differences in gelation kinetics at high cooling rate within process times as short as 1.8 s.     

Local and network structure of thermoreversible polyrotaxane hydrogels based on poly(ethylene glycol) and methylated alpha-cyclodextrins

Thermoreversible gelation and microphase formation of aqueous solutions of a methylated polyrotaxane (MePR) were investigated by means of differential scanning microcalorimetry, rheometry, and X-ray diffractometry (XRD). The aqueous solutions of MePR show a lower critical solution temperature (LCST) and form an elastic gel with increasing temperature. The sol-gel transition of the MePR solutions was induced by formation and deformation of aggregates of methylated alpha-cyclodextrins (alpha-CDs) of polyrotaxane due to hydrophobic dehydration and hydration, respectively. The XRD investigation revealed localization and highly ordered arrangement of methylated alpha-CDs along the PEG chain in the gel. The arrangement of CDs was also reflected by the changes in elasticity and long relaxation behavior of the solution around the sol-gel transition. The quasiequilibrium shear modulus of MePR solutions showed the critical phenomena against temperature. The scaling exponents measured at two different concentrations were almost equal to the values predicted by a gel percolation theory. Therefore, the heat-induced gelation of aqueous MePR solutions is well explained by a model in which clusters assembled with methylated alpha-CDs are gradually connected to the network as the temperature increases.     

Oscillatory shear flow of polymeric systems

On the basis of the Leonov viscoelastic constitutive equation, oscillatory shear flow of elastic fluids in the linear and nonlinear regimes has been considered. The Fourier components and associated phase angles of the shear and normal components of the elastic strain tensor have been found as functions of frequency and deformation amplitude in the range usually employed in experiment, and are presented in a form convenient for further rheological applications. In the linear case, the results correspond to many known theories. In the nonlinear case, the theoretical results have been compared with experiments, on different polymeric systems, with very good agreement being obtained for the shear stress in polymeric solutions but only qualitative agreement for the shear stress and first normal-stress difference in polymer melts.     

Multicompartment Micellar Solutions in Shear: A Dissipative Particle Dynamics Study

Summary: Dissipative particle dynamics simulations were performed to study the effect of shear on the rheological behavior of multicompartment micellar solutions, demonstrating that both shear thickening and thinning can occur, and the macroscopic behavior was elucidated at a molecular level. In addition, a novel shear‐induced morphology of “sphere‐on‐rod” was observed. This work provides useful information towards a complete understanding of the properties and morphologies of multicompartment micelles that is useful for future rational synthesis of novel micelles.

Shear‐induced morphological transitions and rheological behavior in multicompartment micellar solutions formed from star triblock copolymers.     

Shear-induced crystallization in phase-separated blends of isotactic polypropylene with ethylene-propylene-diene terpolymer

The compatibility between isotactic polypropylene(i PP) and ethylene-propylene-diene terpolymer(EPDM) in the blends was studied. SAXS analysis indicates that i PP and EPDM phases in the binary blend are incompatible. Isothermal crystallization behaviors of i PP in phase-separated i PP/EPDM were studied by in situ POM equipped with a Linkam shear hot stage. It was found that typical spherulites of i PP were formed both in neat i PP and in i PP/EPDM blends. The radial growth rate(d R/dt) of spherulites of i PP in the blend was not influenced by EPDM phases. Further investigations on isothermal crystallization of i PP in i PP/EPDM after shear with a fixed shear time showed that the crystallization rate of i PP in the blends increased with increasing shear rates, whereas, the crystallization rate was much lower than that of neat i PP. WAXD results showed that ?-crystal i PP was formed in neat i PP as well as in i PP/EPDM blends after shearing and the percentage of ?-crystal bore a relationship to the applied shear rate. The presence of EPDM resulted in lower percentage of ?-crystal in the blends than that in neat i PP under the same constant shear conditions. SAXS experiments revealed that shear flow could induce formation of oriented lamellae in i PP and i PP in the blends, and the presence of EPDM led to a reduced fraction of oriented lamellae.