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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]

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]

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]