Reversible shear gelation of polymer–clay dispersions

Reversible, shear-induced gelation of semi-dilute aqueous colloidal dispersions consisting of monodisperse discoid particles (Laponite) and weakly adsorbing polymer (polyethylene oxide) is studied through a combination of small angle neutron scattering and oscillatory shear. When shaken the samples undergo a dramatic transition from a low viscosity fluid to a self-supporting, turbid gel. This complex non-linear behavior is found to occur over a narrow composition regime near a composition commensurate with saturation of the clay surface with polymer. Through a combination of SANS and rheology, shear gelation is found to occur through the deformation of large stable flocs that expose fresh surface area for the formation of new polymer bridges. At rest, the temporary shear-induced flocs slowly fractionate with time as the polymer desorbs from the clay surface. The shear-induced gelation is time reversible and strongly temperature-dependent suggesting that relaxation is an activated process. Samples showing shear induced gelation are also able to form stiff stable gels which are characteristically similar to pure clay dispersions.     

Orientation and relaxation of polymer–clay solutions studied by rheology and small-angle neutron scattering

The influence of shear on viscoelastic solutions of poly(ethylene oxide) (PEO) and clay [montmorillonite, i.e., Cloisite NA+ (CNA)] was investigated with rheology and small-angle neutron scattering (SANS). The steady-state viscosity and SANS were used to measure the shear-induced orientation and relaxation of the polymer and clay platelets. Anisotropic scattering patterns developed at much lower shear rates than in pure clay solutions. The scattering anisotropy saturated at low shear rates, and the CNA clay platelets aligned with the flow, with the surface normal parallel to the gradient direction. The cessation of shear led to partial and slow randomization of the CNA platelets, whereas extremely fast relaxation was observed for laponite (LRD) platelets. These PEO–CNA networklike solutions were compared with previously reported PEO–LRD networks, and the differences and similarities, with respect to the shear orientation, relaxation, and polymer–clay interactions, were examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3102–3112, 2004     

Shear-induced morphology transition and microphase separation in a lamellar phase doped with clay particles

We report on the influence of shear on a nonionic lamellar phase of tetraethyleneglycol monododecyl ether (C12E4) in D2O containing clay particles (Laponite RD). The system was studied by means of small-angle light scattering (SALS) and small-angle neutron scattering (SANS) under shear. The SANS experiments were conducted using a H2O/D2O mixture of the respective scattering length density to selectively match the clay scattering. The rheological properties show the familiar shear thickening regime associated with the formation of multilamellar vesicles (MLVs) and a shear thinning regime at higher stresses. The variation of viscosity is less pronounced as commonly observed. In the shear thinning regime, depolarized SALS reveals an unexpectedly strong variation of the MLV size. SANS experiments using the samples with lamellar contrast reveal a change in interlamellar spacing of up to 30% at stresses that lead to MLV formation. This change is much more pronounced than the change observed, when shear suppresses thermal bilayer undulations. Microphase separation occurs, and as a consequence, the lamellar spacing decreases drastically. The coincidence of the change in lamellar spacing and the onset of MLV formation is a strong indication for a morphology-driven microphase separation.     

Kinetics of the shear-induced isotropic-to-lamellar transition of an amphiphilic model system: a nonequilibrium molecular dynamics simulation study

The shear-induced isotropic-to-lamellar phase transition in the amphiphilic systems in the vicinity of the quiescent order-to-disorder transition point is investigated by the large-scale parallel nonequilibrium molecular dynamics simulations of simple amphiphilic model systems. There is a shear-induced upward shift of the ordering temperature. The initial isotropic phase orders into a lamellar phase perpendicular to the shear vorticity. The phase diagram as a function of temperature and shear rate is established. The dependency of the ordering transition on interaction strength and shear rate is rationalized by the competition between shear rate and chain relaxation. The time evolution of morphology reveals that the shear-induced ordering proceeds via nucleation and growth, a signature of a first-order phase transition. At low shear rate, a single ordered domain grows after an incubation period. With increasing shear rate ordering speeds up, but eventually develops in a lamellar system with disordered shear bands. The time dependence of the order parameter follows that of the mean-squared end-to-end distance, shear viscosity, and bulk pressure, and follows an Avrami scheme with an Avrami exponent between 2 and 4.     

Wormlike micelles mediated by polyelectrolyte

Aqueous solutions of the anionic surfactant potassium oleate (K-oleate) were studied using small-angle neutron scattering (SANS), steady-state rheology, and cryogenic transmission electron microscopy (cryo-TEM). The micellar structural changes induced by the addition of potassium chloride (KCl) and sodium polystyrenesulfonate (PSS) of different molecular weights were investigated. Upon addition of KCl, a transition from spherical to wormlike micelles was detected from the SANS data and confirmed by the cryo-TEM pictures. The rheological measurements revealed a strong dependence of the low-shear viscosity on the concentration of salt: a broad maximum in the viscosity curve was observed upon addition of KCl, characteristic of the growth of micelles into long worms, followed by branching. The addition of PSS to salt-free solutions of K-oleate had a significant effect on the scattering patterns, revealing partial growth of the spherical micelles into rodlike micelles. In contrast, in the presence of high salt concentrations, addition of PSS to solutions of wormlike micelles did not bring any noticeable modifications in the scattering. However, in the same salt conditions, a clear effect was observed on the low shear viscosity upon addition of PSS, which was found to depend significantly on molecular weight. This suggests a novel way of impacting the viscosity of solutions of wormlike micelles.     

Shear small-angle light scattering studies of shear-induced concentration fluctuations and steady state viscoelastic properties

We aimed at elucidating the influence of shear-induced structures (shear-enhanced concentration fluctuations and/or shear-induced phase separation), as observed by rheo-optical methods with small-angle light scattering under shear flow (shear-SALS) and shear-microscopy, on viscoelastic properties in semidilute polystyrene (PS) solutions of 6.0 wt % concentration using dioctyl phthalate (DOP) as a Theta solvent and tricresyl phosphate (TCP) as a good solvent. In order to quantify the effects of the shear-induced structures, we conducted a numerical analysis of rheological properties in a homogeneous solution based on the constitutive equation developed by Kaye-Bernstein, Kearsley, and Zapas (K-BKZ). In the low-to-intermediate shear rate gamma region between tau(w) (-1) and tau(e) (-1), where tau(w) and tau(e) are, respectively, terminal relaxation time and the relaxation time for chain stretching, the steady state rheological properties, such as shear stress sigma and the first normal stress difference N(1), for the PS/DOP and PS/TCP solutions are found to be almost same and also well predicted by the K-BKZ equation, in spite of the fact that there is a significant difference in the shear-induced structures as observed by shear-SALS and shear-microscopy. This implies that the contribution of the concentration fluctuations built up by shear flow to the rheological properties seems very small in this gamma region. On the other hand, once gamma exceeds tau(e) (-1), sigma and N(1) for both PS/DOP and PS/TCP start to deviate from the predicted values. Moreover, when gamma further increases and becomes higher than gamma(a,DOP) (sufficiently higher than tau(e) (-1)), above which rheological and scattering anomalies are observed for PS/DOP, sigma and N(1) for PS/DOP and PS/TCP are significantly larger than those predicted by K-BKZ. Particularly, a steep increase of sigma and N(1) for PS/DOP above gamma(a,DOP) is attributed to an excess free energy stored in the system via the deformation of interface of well-defined domains, which are aligned into the stringlike structure developed parallel to the flow axis, and stretching of the chains connecting the domains in the stringlike structures. Thus, we advocate that the effect of shear-induced structures should be well considered on the behavior of sigma and N(1) at the high gamma region above tau(e) (-1) in semidilute polymer solutions.     

Temperature-induced intermicellization and contraction in aqueous mixtures of sodium dodecyl sulfate and an amphiphilic diblock copolymer

Aqueous solutions of a thermoresponsive amphiphilic diblock copolymer, containing poly(N-isopropylacrylamide), in the presence of the anionic sodium dodecyl sulfate (SDS) surfactant can undergo a temperature-induced transition from loose intermicellar clusters to collapsed core–shell nanostructures. The polymer–surfactant mixtures have been characterized with the aid of turbidity, small-angle neutron scattering (SANS), intensity light scattering (ILS), dynamic light scattering (DLS), shear viscosity, and rheo-small angle light scattering (rheo-SALS). In the absence of SDS, compressed intermicellar structures are formed at intermediate temperatures, and at higher temperatures further aggregation is detected. The SANS results disclose a structure peak in the scattered intensity profile at the highest measured temperature. This peak is ascribed to the formation of ordered structures (crystallites). In the presence of a low amount of SDS, a strong collapse of the intermicellar clusters is observed at moderate temperatures, and only a slight renewed interpolymer association is found at higher temperatures because of repulsive electrostatic interactions. Finally, at moderate surfactant concentrations, temperature-induced loose intermicellar clusters are detected but no shrinking was registered in the considered temperature range. At a high level of SDS addition, large polymer–surfactant complexes appear at low temperatures, and these species are compressed at elevated temperatures. The rheo-SALS results show that the transition structures are rather fragile under the influence of shear flow.     

Shear-induced parallel-to-perpendicular orientation transition in the amphiphilic lamellar phase: a nonequilibrium molecular-dynamics simulation study

The present work is devoted to a study of the shear-induced parallel-to-perpendicular orientation transition in the lamellar system by the large-scale nonequilibrium molecular-dynamics (NEMD) simulation. An effective generic model-A2B2 tetramer for amphiphilies is used. The NEMD simulation produces unambiguous evidence that undulation instability along the vorticity direction sets in well above a critical shear rate and grows in magnitude as the shear rate is further increased. At a certain high shear rate, the coherent undulation instability grows so large that defects are nucleated and the global lamellar monodomain breaks into several aligned lamellar domains. Subsequently layers in these domains rotate into the perpendicular orientation with the rotation of chains towards the y direction, merge into a global perpendicular-aligned lamellar monodomain, and organize into a perfect well-aligned perpendicular lamellar phase by the migration and annihilation of edge dislocations and disclinations. The macroscopic observable viscosity as a function of time or shear rate is correlated with the structural response such as the mesoscopic domain morphology and the microscopic chain conformation. The onset of undulation instability concurs with the start-up of shear-thinning behavior. During the orientation transformation at the high shear rate, the complex time-dependent thixotropic behavior is observed. The smaller viscosity in the perpendicular lamellar phase gives an energetic reason for the shear-induced orientation transition.     

Phase Transitions in Non-ionic Detergent Micelles

Differential scanning calorimetry (DSC) studies of micellar, 60 mM solutions of the octaethyleneglycol alkylethers C₁₄E₈ and C₁₆E₈ provide evidence for a narrow endothermic transition at 41 and 32°C,respectively, characterized by an enthalpy change of 2 kJ mol⁻¹ for both detergents. The observed thermal transition is indicative of a concerted transition of the surfactant molecules, as illustrated on the basis of a simple molecular model. The effect of co-solvents such as different alcohols on the thermal transition is investigated. Glycerol markedly lowers the transition temperature whereas the transition is absent in the presence of at least 10% ethanol. The calorimetric transition correlates with the temperature dependent increase of viscosity and static light scattering as well as with changes observed by small-angle neutron scattering (SANS). The SANS results provide clear evidence for a distinct structural change occurring at the transition temperature, which is interpreted as a sphere-to-rod transition of the detergent micelles. Moreover, the rod length increases with increasing temperature. We suggest that the process causing the thermal transition acts as the prerequisite of the growth process. This revised version was published online in August 2006 with corrections to the Cover Date.     

Structural Transitions of CTAB Micelles in a Protic Ionic Liquid

Micellar solutions of hexadecyltrimethylammonium bromide (CTAB) in a protic ionic liquid, ethylammonium nitrate (EAN), are studied by shear rheology, polarizing optical microscopy (POM), conductivity measurements, and small angle neutron scattering (SANS). Three concentration regimes are examined: A dilute regime (with concentrations [CTAB] < 5 wt %) consisting of noninteracting spherical micelles, a semidilute regime (5 wt % ≤ [CTAB] ≤ 45 wt %) where micelles interact via electrostatic repulsions, and a concentrated regime (45 wt % < [CTAB] ≤ 62 wt %) where a reversible, temperature-dependent isotropic (L(1)) to hexatic (Hex) phase transition is observed. The L(1)-Hex transition, which has been predicted but not previously observed, is characterized by (1) a sharp increase in the shear viscosity, (2) the formation of focal conical birefringence textures (observed by POM), and (3) enhancement of the crystalline order, evidenced by the appearance of Bragg reflections in the SANS profiles. Ionic conductivity is not sensitive to the L(1)-Hex transition, which corroborates the absence of topological transitions.     

Shear-induced orientation of a rigid surfactant mesophase

An optically clear, crystalline, gel-like mesophase is formed by the addition of water to a micellar solution consisting of a mixture of 0.85 M anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and a 0.42 M zwitterionic surfactant phosphatidylcholine (lecithin) in isooctane. At 25 degrees C and water to AOT molar ratio of 70, the system has a columnar hexagonal microstructure with randomly oriented domains. The shear-induced orientation and subsequent relaxation of this structure were investigated by rheological characterization and small-angle neutron scattering (SANS). The rheological response implies that the domains align under shear, and remain aligned for several hours after cessation of shear. Shear-SANS confirms this picture. The sheared gel mesophase retains its alignment as the temperature is increased to 57 degrees C, indicating the potential to conduct templated polymer and polymer-ceramic composite materials synthesis in aligned systems.     

有机蒙脱土对端羧基聚丁二烯凝胶流变行为的影响

在恒定剪切速率下,利用旋转流变仪研究端羧基聚丁二烯/蒙脱土纳米复合凝胶的流变行为,同时利用X-射线衍射(XRD)和透射电镜(TEM)等表征手段研究其微观结构.研究结果表明,该体系在26～116℃的升-降温过程中,观察到不可逆转变和可逆转变两种流变行为,其中不可逆转变流变行为归因于蒙脱土片层的剥离过程,而可逆转变流变行为...     

Mesophase separation in polyelectrolyte-mixed micelle coacervates

Mesophase separation has been identified in a polycation/anionic-nonionic mixed micelle system formed by the coacervation of poly(diallyldimethylammoniumchloride)/sodium dodecylsulfate-Triton X-100 in 0.40 M NaCl. The resultant dense, optically clear fluid was studied by turbidity, dynamic light scattering (DLS), and rheology. The presence of two diffusion modes in DLS points to microscopic heterogeneity: coexistence of micelle-rich (dense) domains with micelle-poor (dilute) domains. With an increase in temperature above 20 degrees C, the turbidity rises rapidly along with the intensity of the slow mode. The concomitant decrease in the diffusivity of the slow mode signals an increase in the effective viscosity of the dense domain. With further increase in temperature, dramatic shear thinning is observed, and finally, macroscopic phase separation can be identified by centrifugation. At a temperature near that for quiescent phase separation, we observe shear-induced phase separation. We propose a mechanism to explain the connection between temperature- and shear-induced mesophase separation.     

Structural relaxation and rheological response of a driven amorphous system

The interplay between the structural relaxation and the rheological response of a simple amorphous system {a 80:20 binary Lennard-Jones mixture [W. Kob and H. C. Andersen, Phys. Rev. Lett. 73, 1376 (1994)]} is studied via molecular dynamics simulations. In the quiescent state, the model is well known for its sluggish dynamics and a two step relaxation of correlation functions at low temperatures. An ideal glass transition temperature of Tc=0.435 has been identified in the previous studies via the analysis of the system's dynamics in the framework of the mode coupling theory of the glass transition [W. Kob and H. C. Andersen, Phys. Rev. E 51, 4626 (1995)]. In the present work, we focus on the question whether a signature of this ideal glass transition can also be found in the case where the system's dynamics is driven by a shear motion. Indeed, the following distinction in the structural relaxation is found: In the supercooled state, the structural relaxation is dominated by the shear at relatively high shear rates gamma, whereas at sufficiently low gamma the (shear-independent) equilibrium relaxation is recovered. In contrast to this, the structural relaxation of a glass is always driven by shear. This distinct behavior of the correlation functions is also reflected in the rheological response. In the supercooled state, the shear viscosity eta decreases with increasing shear rate (shear thinning) at high shear rates, but then converges toward a constant as the gamma is decreased below a (temperature-dependent) threshold value. Below Tc, on the other hand, the shear viscosity grows as eta proportional, etax 1/gamma, suggesting a divergence at gamma=0. Thus, within the accessible observation time window, a transition toward a nonergodic state seems to occur in the driven glass as the driving force approaches zero. As to the flow curves (stress versus shear rate), a plateau forms at low shear rates in the glassy phase. A consequence of this stress plateau for Poiseuille-type flows is demonstrated.     

Rheo-SANS studies on shear-thickening/thinning in aqueous rodlike micellar solutions

Shear-induced thickening/thinning phenomena of aqueous rodlike micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium p-toluene sulfonate (NapTS) were investigated by means of simultaneous measurements of rheology and small-angle neutron scattering (SANS), the so-called Rheo-SANS. The aqueous CTAB/NapTS solutions were classified into five different categories dependent on their flow behavior and micellar structure. By increasing salt concentration and/or shear rates, the micelles underwent morphological transition from (i) spherical or short rodlike micelles to (ii) long rodlike micelles without entanglements, followed by (iii) those with entanglements. These transitions were recognized as changes in flow behavior from Newtonian to shear-thickening and shear-thinning flow, respectively. In the latter two cases, anisotropic SANS patterns appeared around these critical shear rates. The physical meaning of the anisotropic SANS patterns accompanied by shear-thickening flow behavior is discussed in conjunction with other shear-thickening systems.     

A phenol-induced structural transition in aqueous cetyltrimethylammonium bromide solution

The effect of phenol on the structure of micellar solution of a cationic surfactant, cetyltrimethylammonium bromide (CTAB) was investigated using viscosity, dynamic light scattering (DLS), small angle neutron scattering (SANS) and nuclear magnetic resonance (NMR) techniques. The relative viscosity and apparent hydrodynamic diameters of the micelles in CTAB solution increase initially and then decrease with addition of phenol. SANS studies indicate a prolate ellipsoidal structure of the micelles. The axial ratio of the prolate ellipsoidal micelles increases and then decreases with addition of phenol, consistently with DLS and viscosity measurements. NMR studies confirm the solubilization of phenol to the palisade layer and growth of the micelles at high concentration of phenol as revealed from the broadening of peaks.     

Micelle-vesicle transition of oleyldimethylamine oxide in water

We studied the effects of the degree of ionization() and the surfactant concentration (C_d) on the micelle–vesicle transition in salt-free oleyldimethylamine oxide (OlDMAO) aqueous solutions by the dynamic light scattering (DLS), the hydrogen ion titration, the small angle neutron scattering (SANS), the electrophoretic light scattering (ELS) and viscoelastic measurements. From the study of ionization effects, the micelle–vesicle transition was recognized as a change of aggregate size by the DLS measurement; however, the micelle–vesicle transition was not detected both in the ELS measurement and the hydrogen ion titration, suggesting that the electric properties of the worm-like micelles and the vesicles are very similar despite a large difference of shapes between them. From the results of the SANS, the DLS and the viscosity measurements, it was suggested that a concentration-dependent micelle–vesicle transition took place around C_p = 10 mmol kg⁻¹ for the solutions at = 0.5. In the concentration-range 10 mmol kg⁻¹ < C_d < 150 mmol kg⁻¹, the micelles and the vesicles coexisted. In the concentration region (C_d = 10–50 mmol kg⁻¹), the vesicle size increased with the surfactant concentration.     

Rheological and rheo-SALS investigation of the multi-lamellar vesicle formation in the C12E3/D2O system

Usually in nonionic surfactant aqueous systems of the C(n)E(m) type, a lamellar phase occurs over a wide temperature and concentration range. For some C(n)E(m) surfactants, multi-lamellar vesicle (MLV) formation has been observed when the lamellar phase is subjected to shear flow. This communication reports the shear flow behavior at different shear rate values of a C(n)E(m) (where "n" is 12 and "m" is 3) aqueous system at 34 °C. The typical transient viscosity behavior of the shear-induced MLV formation in C(12)E(3)/D(2)O at 50 wt% of surfactant has been observed. The MLV formation is confirmed by time-resolved rheo-small angle light scattering (SALS) experiments. The experimental data show an intermediate structure that has been attributed to a multi-lamellar cylinders (MLCs).     

Shear-induced conformational ordering, relaxation, and crystallization of isotactic polypropylene

The shear-induced coil-helix transition of isotactic polypropylene (iPP) has been studied with time-resolved Fourier transform infrared spectroscopy at various temperatures. The effects of temperature, shear rate, and strain on the coil-helix transition were studied systematically. The induced conformational order increases with the shear rate and strain. A threshold of shear strain is required to induce conformational ordering. High temperature reduces the effect of shear on the conformational order, though a simple correlation was not found. Following the shear-induced conformational ordering, relaxation of helices occurs, which follows the first-order exponential decay at temperatures well above the normal melting point of iPP. The relaxation time versus temperature is fitted with an Arrhenius law, which generates an activation energy of 135 kJ/mol for the helix-coil transition of iPP. At temperatures around the normal melting point, two exponential decays are needed to fit well on the relaxation kinetic of helices. This suggests that two different states of helices are induced by shear: (i) isolated single helices far away from each other without interactions, which have a fast relaxation kinetic; (ii) aggregations of helices or helical bundles with strong interactions among each other, which have a much slower relaxation process. The helical bundles are assumed to be the precursors of nuclei for crystallization. The different helix concentrations and distributions are the origin of the three different processes of crystallization after shear. The correlation between the shear-induced conformational order and crystallization is discussed.     

Rheology of branched wormlike micelles

The topology of self-assembled surfactant solutions includes varying degrees of micellar branching, ranging from linear wormlike micelles to a micellar network. Micellar branching acts as an effective attraction between micelles such that network condensation can lead to phase separation. Unlike chemical branching in polymers, micellar branches are labile. Movement of branches along a micelle contour has therefore been proposed as a mechanism of stress relaxation that leads to a reduction in the structural relaxation time and thus, the zero-shear viscosity. Branching is also thought to suppress flow alignment, and for lower levels of branching, may also suppress instabilities such as shear banding. The suppression of shear banding can lead to a lesser degree of shear-thinning in the apparent viscosity at higher shear rates, as well as a reduction in extensional thickening. However, for higher levels of branching, shear can induce branching for samples in proximity to such a phase transition, which can result in shear banding due to shear-induced phase separation. Recent modeling and simulations of the energetics of branching, as well as experiments on model systems, show that the reduction in zero-shear viscosity is due to micelle branching. Current research includes efforts to develop a more mechanistic, quantitative understanding of micellar branching and more generally, its effects on micellar solution rheology.