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Functionalization of xanthan hydrogels is of interest for biomaterial applications. The authors report characterization of electrostatic complexation of xanthan with a recombinant collagen-inspired triblock protein polymer. This polymer has one charged polylysine end-block that can bind to xanthan by electrostatic interactions, and another end-block that can self-assemble into thermosensitive collagen-like triple helices; the end-blocks are connected by a neutral, hydrophilic, mostly inert random coil. The protein modifies the xanthan/protein composite hydrogels in three ways: (a) a significant increase in storage modulus, (b) thermosensitivity, and (c) a two-step strain softening in nonlinear rheology. [ABSTRACT FROM AUTHOR]

Thestructure and linear viscoelastic behavior of four differentmodel star polymer melts were investigated experimentally. The starpolymers were prepared via different synthetic routes based on atomtransfer radical polymerization (ATRP). Stars with small elongated(linear backbone) cores exhibited slight differences in the asymmetryof the core, which however did not affect the rheological propertiessignificantly. The relaxation behavior of these stars with an asymmetriccore was well-described by available tube models. On the other hand,stars with large cross-linked cores exhibited a core–shellmorphology and their stress relaxation was dominated by a power-lawdecay over about 8 decades, akin to gel-like soft systems. This behaviorreflected their liquid-like ordering and small intercore distances,and bares analogueies to that of interpenetrating soft colloids andmicrogels. [ABSTRACT FROM AUTHOR]

We report on a detailed rheologicalinvestigation of well-defined symmetric entangled polymer stars oflow functionality with varying number of arms, molar mass of the arms,and solvent content. Emphasis is placed on the response of the starsin simple shear, during start-up, and for relaxation upon flow cessation.To reduce experimental artifacts associated with edge fracture (primarily)and wall slip, we employ a homemade cone-partitioned plate fixturewhich was successfully implemented in recent studies. Reliable datafor these highly entangled stars could be obtained for Weissenbergnumbers below 300. The appearance of a stress overshoot during start-upwith a corresponding strain approaching a value of 2 suggests thatin the investigated shear regime the stars orient but do not stretch.This is corroborated by the fact that the empirical Cox–Merxrule appears to be validated, within experimental error. On the otherhand, the (shear) rate dependent steady shear viscosity data exhibita slope smaller than the convective constraint release slope of −1(for linear polymers) for the investigated range of rates. The broadnessof the stress overshoot reflects the broad linear relaxation spectrumof the stars. The initial stress relaxation rate, reflecting the initialloss of entanglements due to the action of convective constraint releasein steady shear flow, increases with Weissenberg number. More importantly,when compared against the relevant rates for comb polymers with relativelyshort arms, the latter are slower at larger Weissenberg numbers. Atlong times, the relaxation data are consistent with the linear viscoelasticdata on these systems. [ABSTRACT FROM AUTHOR]

We report on the shear flow start-up and the relaxation upon flow cessation of anionically synthesized comb polymers of different chemistries. The experimental data, obtained with a cone partitioned-plate geometry in order to avoid artifacts, showed that the start-up shear flow of combs exhibits systematic dependencies on the branching structure. They were interpreted by invoking dynamic dilution and hierarchical relaxation, which are known to control the linear viscoelastic response. For all combs studied here, the backbones remained entangled after dynamic dilution due to branch relaxation. We combined the important molecular parameters (i.e., the number and molar mass of the branches) into a single parameter, the number of entanglements of the dynamically diluted backbone, ZBBDIL., which we found to be the main scaling parameter for the observed nonlinear flow behavior. The steady viscosities as function of Weissenberg number were less shear-thinning compared to linear analogues, and the higher the amount of branching, the less thinning they became, reflecting a broader relaxation spectrum, and being consistent with the behavior of commercial branched polymers. The strain at maximum viscosity was higher for combs in comparison to linear polymers, a finding attributed to nonlinear hierarchical relaxation. The maximum in viscosity (scaled with steady viscosity) became lower with increased degree of branching due to the action of dynamic dilution. The viscosity peaks became broader for combs with an increased degree of branching, which is again a reflection of a broader relaxation spectrum. The initial relaxation rate upon cessation of steady shear increased with shear rate and seemed to reflect the loss of entanglements of combs in steady shear due to the action of convective constraint release. The relaxation was found to be independent of branching structure, suggesting that for the time ranges considered here, the loss of orientation of the backbones scales with the longest relaxation time, and is hence an effect of linear relaxation mechanisms (i.e., mainly reptation of the backbone). [ABSTRACT FROM AUTHOR]

Theeffect of using a viscoelastic suspending medium on the in-planehydrodynamic interaction between two equally sized spheres in shearflow is studied experimentally to understand flow-induced assemblybehavior (i.e., string formation). A counterrotating device equippedwith a Couette geometry is used together with quantitative videomicroscopy.To evaluate the effects of differences in rheological properties ofthe suspending media, fluids have been selected that highlight specificconstitutive features. These include a reference Newtonian fluid (N),a constant-viscosity, high-elasticity Boger fluid (BF), a wormlikemicellar surfactant solution with a single dominant relaxation time(WMS), and a broad spectrum shear-thinning elastic polymer solution(ST). As expected, the trajectories are symmetric in the Newtonianfluid. In the BF, the midpoints of the spheres are observed to remainin the same plane before and after the interaction, as in the Newtonianfluid, although the path lines are in this case no longer symmetric.Interactions in the ST and WMS are highly asymmetric. Two fundamentallydifferent kinds of path lines are observed in the WMS and ST: reversingand open trajectories. The type of trajectory depends on the initialconfiguration of the spheres with respect to each other and on theshear rate. On the basis of the obtained results, shear-thinning ofthe viscosity seems to be the key rheological parameter that determinesthe overall nature of the interactions, rather than the relative magnitudeof the normal stress differences. [ABSTRACT FROM AUTHOR]

A cone-partitioned-plate fixture for the ARES rheometer (TA instruments, DE) has been designed, implemented, and validated. This geometry allows measuring the nonlinear shear flow properties of samples, which display edge fracture in regular cone-and-plate geometries, such as polymer melts and concentrated solutions. Reliable bulk shear flow data can be obtained with these systems at high rates and strains, using very small sample quantities. Measurements can be performed at temperatures ranging from at least - 50 °C up to over 200 °C in a controlled nitrogen environment. An extensive set of start-up shear measurements on moderately entangled linear monodisperse polyisoprene (60 kg/mol) and polystyrene (182 kg/mol) melts to validate the design are presented and discussed with focus on the Cox-Merz rule and the characteristics of the stress overshoot. With this new geometry, the range of artifact-free data is extended by a decade in Weissenberg number (WiD). It is shown that the obtained results compared well with the limited experimental data available from the literature on monodisperse polymer melts. Furthermore, we show that, for the investigated range of WiD in start-up shear, the two monodisperse linear polymers of different chemistry with nearly the same number of entanglements, scale identically. [ABSTRACT FROM AUTHOR]

The effect of the viscoelastic nature of the suspending medium on the rotation of spherical particles in a simple shear flow is studied experimentally using a counter-rotating device. To evaluate the effect of variations in rheological properties of the suspending media, fluids have been selected which highlight specific constitutive features. These include a reference Newtonian fluid, a constant viscosity, high elasticity Boger fluid, a single relaxation time wormlike micellar surfactant solution, and a broad spectrum shear-thinning elastic polymer solution. It is shown that particle rotation slows down, when compared to the Newtonian case, as elasticity increases, in qualitative agreement with computer simulation studies. Despite the variation in constitutive properties and the wide range of time scales of the fluids, it is found that the Weissenberg number suffices to scale the data: the dimensionless rotation speed of the spheres in the different fluids scales onto a single master curve as a function of the Weissenberg number. This indicates that the slowing down in rotation finds its main origin in (indirect) normal stress effects. [ABSTRACT FROM AUTHOR]

In inertialess suspensions of rigid particles, the rotational motion of each particle is governed by the so-called freely rotating condition, whereby the total torque acting on the particle must be zero. In this work, we study the effect of viscoelasticity of the suspending liquid on the rotation period of a sphere by means of three-dimensional finite element simulations, for conditions corresponding to a macroscopic shear flow. The simulation results capture the slowing down of the rotation, relative to the Newtonian case, which was recently observed in experiments. It is shown that such a phenomenon depends on the specific constitutive equation adopted for the viscoelastic liquid. Analysis of transients shows a clear correlation between rotation rate and the development of first normal stress difference. [ABSTRACT FROM AUTHOR]

We present an experimental investigation of the curing kinetics and viscoelasticity for a number of "vitrimers" recently developed by Leibler and coworkers. 1-3 Vitrimers are covalently crosslinked networks that can relax stress at elevated temperatures due to thermoreversible bond-exchange reactions. The chosen formulations are composed of diglycidyl ether of bisphenol A, commercial fatty acid mixtures and an appropriate catalyst. The effects of the catalyst and functionality of the curing agents on the kinetics of the curing reactions were systematically investigated using rheometry. The curing kinetics followed the Arrhenius law and the catalyst drastically accelerated the reactions. Time-temperature superposition was used to construct master curves of the small-strain amplitude oscillatory shear moduli over wide ranges of frequencies for the cured networks. Terminal relaxation was not reached in oscillatory experiments for temperatures up to 130 °C and creep and stress relaxation experiments were used to probe the long-time relaxation. The shift factors displayed a Williams-Landel-Ferry dependence on temperature which could be divided into two regions, one above 70 °C, where the dynamics appeared to be controlled by the catalyst, and one below, controlled by the monomeric friction and the free volume of the network. The moduli of the vitrimers obeyed the classical rubber theory well, indicating that the curing reactions proceeded to completion. Furthermore, we systematically and reproducibly observed a double relaxation behavior for the vitrimers, i.e. next to the rubbery plateau at high frequencies, the storage modulus displayed a secondary plateau at lower frequencies before reaching terminal relaxation at even lower frequencies. Interestingly, 70 °C was found to be the transition point in agreement with the shift factors. To the best of our knowledge, the double relaxation behavior has not been previously reported in experimental works and recent theories do not incorporate an explanation for this behavior. Consequently, future investigations concerning the viscoelasticity of other "vitrimer-chemistries" are important to assess if the double relaxation is a universal fingerprint for vitrimers or if it is specific to the here-investigated formulations based on commercial fatty acid mixtures.

Flow-fields are typically used to intimately mix large μm-sized particles with polymer melts. Here we show, using rheology, X-ray scattering, and electron microscopy, that shear flows do not improve the spatial dispersion or ordering of spherical nanoparticles (NP) grafted with polymer chains over the ranges of flow fields realizable in our experiments in the melt state. In the absence of flow, grafted NPs robustly self-assemble into a variety of superstructures when they are added to a homopolymer matrix with the same chemistry as the NP grafts. We find that isolated particles and spherical NP clusters remain dispersed but do not flow align. On the other hand, anisotropic NP assemblies initially break and their constituent building blocks (strings or sheets) flow-align locally. At very large strains, they coarsen into large aggregates, reflecting the dominance of interparticle attractions over flow fields and thermal energy.

Ordering induced by shear flow can be used to direct the assembly of particles in suspensions. Flow-induced ordering is determined by the balance between a range of forces, such as direct interparticle, Brownian, and hydrodynamic forces. The latter are modified when dealing with viscoelastic rather than Newtonian matrices. In particular, 1D stringlike structures of spherical particles have been observed to form along the flow direction in shear thinning viscoelastic fluids, a phenomenon not observed in Newtonian fluids at similar particle volume fractions. Here we report on the formation of freestanding crystalline patches in planes parallel to the shearing surfaces. The novel microstructure is formed when particles are suspended in viscoelastic, wormlike micellar solutions and only when the applied shear rate exceeds a critical value. In spite of the very low volume fraction (less than 0.01), particles arrange themselves in 2D crystalline patches along the flow direction. This is a bulk phenomenon because 2D crystals form throughout the whole gap between plates, with the gap thickness being much larger than the particle size. Shear flow may hence be an easy method to drive particles into crystalline order in suspensions with viscoelastic properties. The crystalline structure reported here could be used to design new materials with special mechanical, optical, thermal, or electric properties.