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The effects of thermally reduced graphene sheets (TRG) on the phase separation in poly[(α-methyl styrene)-co-(acrylonitrile)]/poly(methyl-methacrylate) blends were monitored using melt rheology, conductivity spectroscopy, and electron microscopic techniques. The TRG were incorporated in the single-phase material by solution mixing. The composite samples were then allowed to phase separate in situ. The thermodynamics of phase separation have been investigated by monitoring the evolution of the storage modulus (G′) as a function of temperature as the system passes through the binodal and the spinodal lines of the phase diagram. The phase separation kinetics were probed by monitoring the evolution of G′ as a function of time at a quench depth well in the spinodal region. It was observed that TRG significantly influenced the phase separation temperature, the shape of the phase diagram and the rate of phase separation. The state of dispersion of TRG in the blends was assessed using electron microscopy and conductivity spectroscopy measurements. Interestingly, the composite samples (monophasic) were virtually insulators at room temperature, whereas highly conducting materials were obtained as a result of phase separation in the biphasic materials.

The properties of filled polymers and nanocomposites are strongly linked to the adequate dispersion of the solid phase into a polymeric matrix. However, obtaining the degree of dispersion within a polymer composite system is far from trivial. Typical methods for microstructural analysis such as electron and optical microscopy or scattering methods only investigate the local microstructure. In addition they are either labor intensive or may yield data that are difficult to analyze. Methods that focus more strongly on the end‐use properties, such as conductivity or bulk moduli, offer a global view of the material performance, but are of course post factum. The rheological properties of (nano)particle filled matrices in the molten state offer a cost‐effective alternative to evaluate the dispersion quality and can even be used during a production process. Moreover, it does not necessitate optical transparency, adequate scattering contrast, or conductivity. Typically, rheological data have been mainly used in qualitative terms and it is as yet unclear if the quality of dispersion can be quantified. The present work contains a systematic evaluation of methods to characterize the dispersion quality from rheology. Using a well‐defined model sample it is shown how the dispersion quality can be assessed quantitatively by studying the viscoelastic properties as a function of volume fraction. Moreover, a novel method is proposed, where the time evolution of the linear and nonlinear rheological properties at a fixed volume fraction is studied and analyzed.

In this work, the break-up dynamics of a single, Newtonian droplet dispersed in an immiscible Newtonian matrix undergoing a controlled mixture of shear and extensional flow is investigated using a homemade eccentric cylinder device. Hereto, different representative supercritical flows with varying shear/elongation balance have been applied. The effect of viscosity ratio is explored using droplets with a viscosity ratio of 0.1 and 1.3, respectively. In all cases, break-up is observed to proceed through an “end-pinching” mechanism. The experimentally obtained break-up times have been compared with scaling relations known from literature for simple shear and purely extensional flow. It is found that the global break-up dynamics is still shear dominated, even for conditions where the elongational contribution comprises a substantial amount (up to 30% on average) of the mixed flow.

Conducting polymeric materials with stable phase microstructures have a range of potential applications. In this work, it is investigated whether phase separation in polymer blends can be used as a tool to create well dispersed conducting filler rich domains in 3D with controlled morphology, potentially resulting in more effective percolation. The effect of amine functionalized multiwall carbon nanotubes (NH2-MWCNTs) on the thermally induced phase separation processes in poly[(α-methyl styrene)-co-acrylonitrile]/poly(methyl methacrylate) (PαMSAN/PMMA) blends was monitored by melt rheology, conductivity spectroscopy, and microscopic techniques. Electron microscopic images revealed that the phase separation resulted in a heterogeneous distribution of NH2-MWCNTs in the blends. The migration of NH2-MWCNTs is controlled by the thermodynamic forces that drive phase separation and led to an increase in their local concentration in a specific phase resulting in percolative “network-like” structure. Conductivity spectroscopy measurements demonstrated that the blends with 2 wt % NH2-MWCNTs that showed insulating properties for a one phasic system revealed highly conducting material in the melt state (two phasic) as a result of phase separation. By quenching this morphology, a highly conducting material with controlled dispersion of MWCNTs can be achieved. Furthermore, the role of NH2-MWCNTs in stabilizing the PMMA droplets against flow induced coalescence in 85/15 PαMSAN/PMMA blends was also established for the first time. It was observed that at a typical loading of 1.25 wt % NH2-MWCNTs the coalescence was completely suppressed on a practical time scale.

The controlled generation of 2D aggregate networks is studied experimentally using micrometer-sized polystyrene latex particles attached to the oil−water interface. Starting from an initially crystalline monolayer, appropriate combinations of carefully added electrolyte and surfactant enable control over both the fractal dimension and the kinetics of aggregation. Remarkably, the colloidal crystals formed upon first spreading remain stable, even for days, when substantial amounts of electrolyte are added to the aqueous phase. Pressure−area isotherms reveal a slow time evolution of the electrostatic dipole−dipole interaction. When the electrostatic interaction has been sufficiently weakened, aggregation proceeds in well-defined, reproducible manner. The aggregation process is analyzed using quantitative video microscopy. The evolution of the cluster size distribution and its moments is characterized, and static and dynamic scaling exponents are derived to identify the nature of the aggregation process. In the range of concentrations studied here, the kinetics all agree with a “fast”, diffusion-limited cluster type of aggregation. However, the fractal dimension strongly depends on the composition of the aqueous subphase. Rather dense structures are found when only electrolyte is used, whereas more open structures are obtained when even small amounts of surfactant are added. It is suggested that this structural dependency is related to the effect of surfactant on the contact angle and its consequences for the anisotropic nature of the capillary interactions.

In this work, the structure development in immiscible polymer blends in confined geometries is systematically investigated. Poly(dimethylsiloxane)/poly(isobutylene) blends with a droplet-matrix structure are subjected to simple shear flows. The confined environment is created by using a Linkam shearing cell in which the gap is systematically decreased to investigate the transition from “bulk” behavior toward “confined” behavior. Small-angle light scattering experiments in a confinement, which have not yet been reported in the literature, and also microscopy are used to observe the morphology development during steady-state shearing and relaxation. These experiments indicate that the size and relaxation of single droplets in a confined environment are still governed by the relations that describe the structure development in bulk situations. Yet, depending on the applied shear rates and blend concentrations, the droplets organize in superstructures such as pearl necklaces or extended superstrings in a single layer between the plates. These structures are stable under flow. To observe a single layer, a critical ratio of droplet size to gap spacing is required, but this ratio is clearly below the one already reported in the literature.

The rheology and morphology of highly compatibilized droplets has been studied, using both rheology and small angle light scattering. It is shown that linear viscoelastic measurements are a powerful tool to investigate the effectiveness of a compatibilizer to suppress coalescence. Moreover, such highly compatibilized droplets are insensitive to deformation and breakup. The consequences for the rheological behaviour of the blend are discussed.

Melt rheology and morphology of nylon-6/ethylene propylene rubber (EPR) blends were studied as a function of composition, temperature, and compatibilizer loading. Uncompatibilized blends with higher nylon-6 content (N90 and N95) and rubber content (N5 and N10) had viscosities approximately intermediate between those of the component polymers. A very clear negative deviation was observed in the viscosity–composition curve over the entire shear rate range studied for blends having composition N30, N50, and N70. This was associated with the interlayer slip resulting from the high-level incompatibility between the component polymers. The lack of compatibility was confirmed by fracture surface morphology, given that the dispersed domains showed no sign of adhesion to the matrix. The phase morphology studies indicated that EPR was dispersed as spherical inclusions in the nylon matrix up to 30 wt % of its concentration. A cocontinuous morphology was observed between 30 and 50 wt % nylon and a phase inversion beyond 70 wt % nylon. Various models based on viscosity ratios were used to predict the region of phase inversion. Experiments were also carried out on in situ compatibilization using maleic anhydride–modified EPR (EPR-g-MA). In this reactive compatibilization strategy, the maleic anhydride groups of modified EPR reacted with the amino end groups of nylon. This reaction produced a graft copolymer at the blend interface, which in fact acted as the compatibilizer. The viscosity of the blend was found to increase when a few percent of modified EPR was added; at higher concentrations the viscosity leveled off, indicating a high level of interaction at the interface. Morphological investigations indicated that the size of the dispersed phase initially decreased when a few percent of the graft copolymer was added followed by a clear leveling off at higher concentration. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 252–264, 2004

Suspensions of monodisperse spheres dispersed in a liquid polymer have been studied in shear flow. The particle diameters range from 0.18 to 2.7 μm. The measured properties include linear dynamic moduli, steady-state shear viscosities, and nonlinear stress relaxation moduli. At small volume fractions and/or large particle sizes, the linear and nonlinear behavior can be explained on the basis of hydrodynamic effects caused by the presence of the particles. In that case, the nonlinear relaxation moduli display time−strain separability. The concentration dependence of the damping function at large strains is similar to that of the linear properties. At small interparticle distances, a weak particulate structure develops that mainly alters the low shear viscosities and the low-frequency moduli. The effect of this structure on the stresses can be gradually eliminated by applying larger strains, as demonstrated by the strain dependence of the nonlinear relaxation moduli in step−strain experiments.

A systematic overview of the behaviour of immiscible blends under various transient flow conditions is presented. On the one hand, it has been observed that the rheological response reflects very clearly the actual microstructure of the blend. It is shown that specific rheological measurements can serve as techniques to probe in‐situ and time‐resolved the flow‐induced blend morphology. On the other hand, models that have been developed to describe the flow behaviour and the corresponding morphology evolution during transient shear flow are discussed. The approach that was used to arrive at these models was to introduce the physics of the morphological processes into the existing blackbox model of Doi‐Ohta.

In emulsions and immiscible polymer blends linear conservative dichroism is known to be very sensitive to flow-induced microstructural changes during flow. The same holds for the excess or interfacial contribution to the first normal stress. Similarities have already been observed between these two properties in blends. Here it is investigated whether this similarity reflects a more quantitative relationship. The interfacial normal stress can be described by means of an interface anisotropy tensor. Therefore, the measured dichroism will be compared with the calculated components of this anisotropy tensor for different flow histories. These include steady-state shear flow, sudden increase in shear rate, relaxation with droplet breakup, and oscillatory flow. In all cases a proportionality factor is obtained. For strongly deformed droplets direct theoretical evidence for a quantitative relationship between rheology and rheo-optics in blends is provided. To extend this relationship to a larger window of shear rates, the size dependence of the scattering has to be taken into account.

Abstract:  Mechanical spectroscopy is used to probe the structure of lyotropic liquid crystalline polymers during flow and after the cessation of flow. The oscillatory flow is either parallel or perpendicular to the steady-state flow. The resulting moduli provide information about the time- and shear-dependent microstructure, including anisotropy. Two different concentrations of poly(benzylglutamate) (PBG) in m-cresol and a concentrated hydroxypropylcellulose (HPC) solution, also in m-cresol, are investigated. In all cases, the orthogonal superposition moduli evolve differently from the parallel ones. The former are less sensitive to the flow-induced changes in structure than the latter ones. Together with the lack of sensitivity of the superposition moduli to texture refinement during flow, this suggests a strong relation between director orientation and superposition moduli. After the cessation of flow the parallel moduli decrease for the PBG solutions, whereas the opposite is observed in the HPC solutions. A comparison with the orthogonal moduli provides a direct measure of anisotropy. At rest, the PBG solutions tend toward a higher degree of anisotropy while the HPC solutions become more isotropic. In the latter systems, all moduli are much larger, reflecting a larger contribution from the texture.

Abstract: Immiscible polymer blends are known to display an unusual elastic recovery after stress release. Recoil after steady-state shearing is well understood and obeys specific scaling relations. Releasing the stress before the steady-state morphology has been reached results in a more complex elastic recovery, including very large final values. This behaviour is investigated systematically. Model blends are used, consisting of nearly inelastic components; hence the measured recoil can be attributed totally to contributions from the interface. The instantaneous structure at the onset of the recoil can vary greatly in transient experiments, ranging from slightly deformed droplets to highly elongated filaments. The effects of this initial structure on the ultimate recoil and time scale of the recovery are studied. The morphological changes during recovery are considered as well. It is demonstrated that they can be computed from the normal stresses during stress relaxation with comparable initial morphologies. This indicates that the same morphological changes occur during stress relaxation and constrained recoil. A scaling relation for the recoil curves has been derived from the Doi-Ohta theory, which is confirmed by the experiments.

Abstract: Due to the interfacial tension, immiscible blends can show an elastic recovery that is substantially larger than that of their pure components. Here it is attempted to relate the elastic recovery after steady shear flow to the underlying morphology. On the one hand, the predictions of the Palierne and the Doi-Ohta models are calculated for the flow conditions during recoil. On the other hand, systematic recoil experiments after steady state shearing have been performed on a model blend. As the component polymers hardly show any recoil under the stresses applied in these tests, the measured recovery can be attributed completely to the action of the interface. Comparison of the model predictions with the experimental results shows that the recoverable strain can be derived quantitatively from the linear Palierne theory. Although the droplet deformation remained limited during the preshear, the retardation time predicted by this model has to be multiplied by the aspect ratio of the droplet phase to the power 2/3 to describe the experiments. For conditions in which the material does not show an intrinsic length scale, particular scaling relations as derived from the Doi-Ohta theory are found to apply also to recoil.

Relaxation after cessation of flow is studied in mixtures of immiscible polymer fluids by means of linear conservative dichroism. Depending on the previous shear history, relaxation is known to proceed by either droplet retraction or break-up. Here, scaling relations for the evolution of the dichroism for both mechanisms of relaxation are presented. In addition a direct analysis of the rheo-optical relaxation curves is given. Starting from a suitable model for droplet retraction and applying Rayleigh–Gans–Debye scattering theory, this particular type of dichroism relaxation can be described quite accurately. A similar approach has been applied to relaxation by droplet break-up as induced by interfacial instabilities (Rayleigh instabilities). Tomotika theory is used to model the evolution of the droplet shape. The theoretical result deviates systematically from the experiments which is attributed to end-pinching. Lacking a suitable model for the latter, it is taken into account by an empirical correction similar to that used to describe the normal stress relaxation. With the rheo-optical technique the evolution of the relaxation time as a function of the initial aspect ratio of the droplets can be studied. In addition the effect of concentration on the relaxation time is investigated.

Abstract: The mesoscopic models for the rheological properties of liquid crystalline polymers proposed by Larson and Doi in 1991 and Kawaguchi and Denn in 1999 are based on phenomenological expressions that describe the evolution of the defect density and the contribution of the “texture” to the stress. In the present work, we attempt to assess some of these assumptions by monitoring how the energy stored in the texture of liquid crystalline materials evolves during shear flows. For that purpose, strain recovery is measured as a function of the applied strain for flow reversal and intermittent flow. Solutions of poly-benzylglutamate in m-cresol, hydroxypropylcellulose in water and a nematic surfactant solution are used as model systems. Although the behaviour is described qualitatively by the model, discrepancies between the predictions and the experiments are observed, especially when the shear history includes rest periods.

An improved procedure is proposed to solve the problem of measuring strong birefringence on flowing samples which have a retardation d larger than p/2 The procedure can be applied to devices which are based on the modulation of the polarization vector with a high-speed rotating half-wave plate. It uses two optical configurations which have different responses to the retardation. By combining the information obtained from the two configurations, one can easily and accurately determine the actual retardation of the sample, irrespective of the quadrant in which the measured retardation is located, provided the sample is isotropic. This technique can also overcome the problem of large errors occurring in the vicinity of d = (2m+1)p/2 due to the limited sensitivity around these values of d. The use of the technique is illustrated with measurements on an isotropic solution of poly(benzyl glutamate) in m-cresol, which is strongly birefringent during shear flow.

Experiments combining mechanical rheometry with polarimetry (birefringence and scattering dichroism) have been conducted on a 6% solution of polystyrene (1.86x106 molecular weight) in dioctyl phthalate. Birefringence is used to measure the extent of segmental orientation, whereas the dichroism is sensitive to orientation and deformation of concentration fluctuations associated with the process of flow-induced phase separation. The results indicate that these fluctuations grow predominately along the neutral (or vorticity axis) of a simple shear flow. At higher rates of shear, orientation in the flow direction is favored. The transition in orientation direction is accompanied by time-dependent behavior in the optical properties of the solution during shear and the onset of shear thickening of the viscosity and the first normal stress difference coefficient.

In lyotropic liquid crystalline polymers, flow is known to induce either director tumbling or flow alignment, depending on the shear rate. Recently the existence of an intermediate transition region, a “wagging” regime, has been predicted on the basis of molecular theories. Here, rheological and rheo-optical techniques have been applied to assess the physical reality of the wagging regime in a solution of poly(benzyl glutamate). Stationary rheological and rheo-optical characteristics indicate a gradual change from tumbling to flow aligning when the shear rate is increased, without any divergent behavior in between. When transient measurement techniques are used, however, distinct differences can be observed over a very narrow shear rate range, immediately before the onset of flow alignment. This intermediate region is characterized by strongly fluctuating transients of the stresses and the linear conservative dichroism, especially when starting from a flow-aligned initial condition. In this manner evidence is provided for the existence of an intermediate regime, which can be associated with the predicted wagging regime.

Abstract: The effect of fillers on the flow curves of polymeric liquid crystals is investigated. Suspensions of polystyrene particles in liquid crystalline solutions of hydroxypropylcellulose (HPC) in water are used. By reducing the HPC concentration an isotropic solution can be prepared. It serves as a reference to isolate the effect of the isotropic/anisotropic structure of the suspending medium on the rheological behaviour. Suspensions in the isotropic solution behave as expected for filled viscoelastic matrices in general. In the anisotropic medium the shear rate rather than the shear stress seems to govern the changes in the relative viscosity. This behaviour is clearly different from isotropic viscoelastic media. The most dramatic effect however is that even small amounts of particles eliminate or drastically shift the region of negative normal stress differences. As far as the structure is concerned, microscopic observations show that particles align in anisotropic as well as in isotropic media. At rest or at relatively low shear stresses the liquid crystalline structure is, in the present case, hardly affected by the presence of the particles. If anything, it becomes more homogeneous.

Recent developments in the theoretical understanding of the rheology of polymeric liquid crystals include the contribution of viscous and textural forces, the mathematical structure of the molecular theory and possible constitutive equations for these materials. At the same time the role of the various forces has been determined experimentally in different ways. The variable microstructure during flow and after stopping the flow has been studied by means of scattering techniques in which light, X-rays and neutrons have been used to supplement rheological techniques. A more complete and complex picture is now emerging for the flow-induced texture over a wide range of shear rates and the concentration range from the isotropic→nematic transition up to the melt. Whereas previous results referred to main chain liquid crystals, new results on side chain polymeric liquid crystals have elucidated the contributions of the mesogenic side groups and the flexible backbone.

Rheo-optical techniques are used here to probe the development of the morphology in transparent polymeric emulsions. The evolution of the droplet geometry is monitored in situ by means of linear conservative dichroism and flow small-angle light scattering (SALS). An interrupted step-up in shear rate is used to generate distorted droplets. Once the flow rate is stopped the droplets can relax either by retraction or by breakup. These relaxation mechanisms can be distinguished by the techniques used here, and structural relaxation times can be derived from either dichroism or light scattering experiments. During filament breakup the SALS pattern exhibits two secondary streaks that can be associated with capillary instabilities. From the kinetics of the corresponding relaxation, the breakup time of the fibrils can be estimated. The combination of size information derived from light scattering experiments with breakup times deduced from dichroism measurements or SALS experiments also provides a new rheo-optical method for estimating the interfacial tension in transparent immiscible blends.

The effect of processing conditions (essentially the flow conditions) on the mophology of incompatible, immiscible polymer blends was investigated. In simple shear flow the steady‐state mophology, that is, droplet size, of such blends is a single‐valued finction of the shear rate whenever a critical shear rate is exceeded. Below this critical value it becomes uely dificult to reach a dynamic equilibrium between droplet breakup and coalescence, which gives rise to a hysteresis region for the size of the inclusions. Such regions were identified and studied for model‐blend systems. The critical shear rate decreases with increasing concentration of the droplet phase. A large ratio of droplet over matrix viscosity seems to have the opposite effect. The hysteresis region is bounded by the critical curues for droplet breakup and coalescence. The accessibility of this re gion can therefore be used to evaluate models for the said phenomena in nondilute systems. Droplet breakup is nearly independent of droplet concentration, while elasticity of the dispersed phase seems to facilitate breakup. As expected, coalescence is enhanced by higher concentrations. Droplet elasticity induces a certain degree of “rigidity” to the interface, and therefore the mobile interface model for coalescence is not suitable for such systems.

In this paper we study how the steady state morphology during shear of an incompatible blend can be affected by the initial conditions. By means of rheological experiments it is shown that below a critical shear rate multiple steady states can be obtained, the final morphology being then determined by the initial conditions of the blend. Above the critical shear rate the morphology is univocally determined by an equilibrium between break-up and coalescence. The critical shear rate is identified as the value at which the break-up limiting curve crosses the coalescence one. The applicability of different coalescence theories has been investigated by changing the viscosity ratio. The different coalescence theories can all describe the experimental results with a reasonable precision. The accuracy of the fully mobile interface theory seems to increase with decreasing viscosity ratio, whereas the opposite holds for the immobile interface theory. The partially mobile interface theory describes the various results equally well.

Mechanical and optical rheometric measurements are reported on solutions of polystyrene dissolved in dioctyl phthalate, a solution that can undergo an apparent phase separation upon the application of shear. Solutions prepared using three molecular weights ranging from one to four million were studied. Time-temperature superposition was observed to apply for these solutions up to and including the onset of an apparent shear thickening of the steady shear and first normal stresses. Optical measurements employing turbidity and scattering dichroism determined that concentration fluctuations were enhanced by flow and grew parallel to the vorticity axis below the critical velocity gradient for the onset of the apparent shear thickening effect. Prior to the onset of thickening, the fluctuations were observed to rearrange and orient parallel to the flow direction. Second normal stress difference measurements indicate these solutions have a negative ratio of the second to the first normal stress differences. It is interesting to point out that the ratio tends to zero in the vicinity of the shear rate range at which shear thickening occurs.

Nearly all the available information on the transient flow behaviour of liquid crystalline polymers has been obtained on model systems, especially on solutions of polybenzylglutamate (PBG) and hydroxypropylcellulose (HPC). The assessment of rheological models has been based almost entirely on these model systems. It is not clear how much of the available theoretical and experimental knowledge can be applied to systems of industrial relevance, which have quite different molecular structures. Here, an industrial lyotropic system, poly(p-phenylenetherephthalamide) (PpPTA) in sulphuric acid (TWARON from AKZO), is investigated. Various techniques to study transient behaviour are used, these include measurements of transient shear and normal stresses after sudden changes in shear rate, dynamic moduli and stress relaxation after cessation of flow and elastic recoil. At all shear rates studied the PpPTA solution is shear thinning, and the first normal stress difference remains positive. For the stress transients a strain scaling applies reasonably well as it did in model systems. The moduli increase with time upon cessation of flow, indicating that the molecules become less oriented in the previous flow direction. This particular behaviour is similar to that of HPC. Transients also resemble more closely those of HPC rather than those of PBG. This latter difference might be attributed to the higher flexibility of HPC and PpPTA chains as compared with PBG molecules.

Some unusual rheological features of a liquid crystalline solution of hydroxypropylcellulose (HPC) in water have been investigated. Measurements have been performed by using a variety of different apparatuses with cone and plate geometries. Particular attention has been devoted to the experimental procedures, including the use of different sealing techniques, which are necessary to avoid solvent evaporation during the very long transients. Shear fracture effects, and their dependence on the type of sealing agents have also been studied. In steady shear, the HPC solution shows some rheological features which are common to other lyotropic systems, such as a three-region viscosity curve, and a double sign change in the first normal stress difference vs shear rate curve. The structural changes which take place after cessation of shear flow have been investigated by following the evolution of the dynamic moduli as a function of the time elapsed after the shear flow is stopped. It was found that the rate of the previously applied shear strongly affects both the kinetics and the asymptotic, long time values of the dynamic properties. Possible explanations for such behavior in terms of microstructure evolution are presented and discussed.

It has been observed previously that xanthan gum (XG) and hydroxypropylmethyl cellulose (HPMC) show different drug release behavior. In order to clarify these findings, the rheological properties of both polymers have been determined by oscillatory as well as by steady shear measurements. Aqueous solutions of 4 and 7% (w/w) polymer have been used to simulate the outer surface of a hydrated tablet. The dynamic moduli, i.e., storage modulus (G′) and loss modulus (G″) of the two polymers have been determined in pure water and USP phosphate buffer pH 7.4 at different dilutions. In this concentration range XG solution exhibits “gel-like” behavior, while HPMC behaves as a typical polymer solution. These findings are quite consistent with the reported higher ability of XG matrices to retard drug release than HPMC matrices for controlled-release formulation. The effects of differences in drug solubility and acidity, as well as the addition of lactose, and of the ionic strength of the medium on the rheological properties of XG and HPMC solutions have been studied in detail. Among these parameters, only the salt concentration exerts an enhancing effect on both moduli of XG, while no detectable influence on HPMC solution could be observed.

A new technique has been developed to position the cup rigidly into the acetabulum during total hip joint replacement. After positioning the flanged cup on the acetabulum by means of screws, a low-viscosity cement is injected through a hole in the rim of the cup. A comparison with the standard cementing technique was made on the basis of in vitro experiments. For this purpose a mold was constructed to produce castings simulating the human acetabulum. The distribution of the cement, the pressure developed in the acetabulum, and the penetration of the cement are compared for the two cementing techniques.

Opposed nozzle flow is used to measure the elongational properties of wormlike micelles. Well-characterized semidilute micellar solutions of CPCl/Sal in concentrated brine are used. By varying temperature and concentration, the viscoelastic properties (τR, G0) of the equilibrium solutions can be controlled and the influence of viscoelasticity on elongational viscosity probed. Results indicate three regimes of steady elongational behavior:  a constant viscosity at low elongational rates, an increase to a maximum viscosity, and then a subsequent drop in viscosity. Scaling of the macroscopic elongational properties with linear viscoelastic parameters and similarities between these solutions and entangled polymer solutions indicate that semidilute CPCl/Sal solutions do not exhibit significant changes in micellar structure and may provide model systems for studying the influence of complex flow fields on entangled polymer solutions.

Hydrogels are widely used in the development of bioinks for extrusion-based 3D bioprinting (EBB). The rheological properties of these materials have already been studied extensively in the literature, and characteristics such as shear-thinning, yield stress and quick structural recovery upon flow cessation are deemed key properties to obtain bioinks of superior performance in EBB. However, there are still inconsistencies in the usage of shear rheology for the characterization of hydrogels for EBB. Therefore, this review discusses the rheometrical techniques applied to the development and characterization of inks for extrusion-based 3D bioprinting. More specifically, the focus will be on the applicability of shear rheometry in EBB as well as important underlying rheological concepts. Rheological tests were categorized according to the nature of the applied stimuli during rheometrical testing, e.g., continuous or oscillatory shear rheometry. In the first class of tests, the flow curves and the shear-thinning behaviour of Power-law fluids are tackled, and the relevance of this effect is discussed in the context of EBB and cell viability. The importance of rapid structural recovery of the material after shear-thinning is illustrated with examples. Then, the Herschel-Bulkley model for yield stress fluids and their connection with improved shape fidelity is highlighted. Oscillatory tests, such as amplitude-, frequency-, temperature- and time-sweeps, and their most relevant applications in EBB, such as gel strength and flowability, gel/liquid-like states, transition temperatures and processing temperatures, and equilibrium modulus after crosslinking are closely reviewed. In addition, relationships between rheological testing and the determination of process parameters are given in a section dedicated to the flow characteristics in EBB. Finally, we offer some future directions for rheology of EBB inks, aiming at the conception of next-generation bioinks.

Droplet breakup in immiscible polymer blends after a sudden substantial increase in shear rate is studied using small-angle light scattering experiments. During the breakup of the fibrils, secondary streaks are visible in the scattering pattern, which indicate the occurrence of Rayleigh instabilities. A high-resolution camera allows quantitative determination of the evolution of the wavelength of the disturbances during the disintegration process. It is shown that this process is time-controlled rather than strain-controlled as suggested by theory.