Index of content:
Volume 50, Issue 5, September 2006

Optical microstructure and viscosity enhancement for an epoxy resin matrix containing multiwall carbon nanotubes
View Description Hide DescriptionThis paper describes rheological measurements and associated optical microstructural observations of multiwall carbon nanotubes(MWCNTs) suspended in an epoxy resin matrix. The base epoxy resin was found to be essentially Newtonian, and the progressive incorporation of nanotubes enhanced the low shear rate viscosity of the suspension by nearly two decades. At higher shear rates, the suspensionviscosity asymptotically thinned to the viscosity of the matrix alone. The low shear rate viscosity enhancement was correlated with the optical observations of interconnected aggregates of carbon nanotubes, which themselves were induced by the low shear conditions. Intermediate shear rates resulted in a reduction in the size of the aggregates. High shear rates appeared to cause nearcomplete dispersal of the aggregates. From these results it is conjectured that for this suspension,shear thinning is connected with the breaking of the interconnected networks between nanotubes and or aggregates of nanotubes, and not by nanotube alignment.

Spurt and melt flow distorsions of linear styreneisoprenestyrene triblock copolymers
View Description Hide DescriptionThe steady state rheological behavior of three narrow molecular weight distribution styreneisoprenestyrene (SIS) triblock copolymers with different molecular weights and polystyrene (PS) contents was investigated by means of a capillary rheometer using the roundhole and slit die geometry. A monodisperse polyisoprene was also investigated to allow a comparison with the flow behavior of the pure elastomer. Like polyisoprene, all the copolymers exhibit spurt marked by a plateau of the shear stress versus flow rate curve and by a discontinuity of the entrance pressure when a critical shear rate is reached. Near the glass transition of the PS domains, the flow curve is mainly influenced by the PS content and exhibits, with increasing flow rate, first yield stress, and then spurt. On the contrary, at higher temperatures, yield stress and spurt disappear and the rheological behavior is qualitatively driven by the overall molecular weight as for linear polymer melts. The values of the critical stress for spurt are significantly larger than that of the polyisoprene matrix and the influence of the PS content on and the sequence of melt flow distorsions were discussed.

A comparison of simple rheological models and simulation data of nhexadecane under shear and elongational flows
View Description Hide DescriptionThe microscopic origins of five rheological models are investigated by comparing their predictions for the conformationtensor and stress tensor with the same tensors obtained via nonequilibrium molecular dynamics simulations for nhexadecane. Steadystate simulations were performed under both planar Couette and planar elongational flows, and the results of each are compared with rheological model predictions in the same flows, without any fitting parameters where possible. The use of the conformationtensor for comparisons between theory and experiment/simulation, rather than just the stress tensor, allows additional information to be obtained regarding the physical basis of each model examined herein. The character of the relationship between stress and conformation is examined using model predictions and simulation data.

Interfacial stickslip transition in simple shear of entangled melts
View Description Hide DescriptionThis article describes a systematic investigation of a discontinuous interfacial stickslip transition (SST) in simple shear of monodisperse entangled 1,4polybutadiene (PBD) and polyisoprene (PIP) melts with different molecular weights and architecture, using a specially designed controlledforce shear rheometer. The magnitude of the transition is found to be determined by the level of chain entanglement. Specifically, the dependence of extrapolation length on molecular weight as and of the melt viscosity as is consistent with the observations based on capillary rheometric studies [X. Yang et al., Rheol. Acta37, 415–423 (1998)]. The interfacial nature of the flow behavior is explicitly demonstrated by a surface treatment of the shearing plates and dependence of the abrupt increase of the apparent shear rate on the gap distance as well as by particle tracking velocimetry. The critical stress for different molecular weights of PBD and PIP is about 0.2 and , respectively, independent of molecular weight and architecture. These results are consistent with the previous conclusion of an interfacial SST as the origin of the discontinuous spurt flow behavior observed with pressuredriven capillary rheometry. The critical stress for the SST is found to be lower in simple shear flow. Finally, chain architecture is observed to also influence the magnitude of the SST apart from the level of chain entanglement.

Mechanisms of steadyshear rheology in polymernanoparticle composites
View Description Hide DescriptionWe use coarsegrained computer simulations to delineate the mechanisms governing the steadyshear rheology of polymernanoparticle composites. Our studies specifically focus on the regimes where the particle sizes and the interparticle distances become comparable to the polymer sizes and where the interactions between polymer and particles become relevant in influencing the dynamical characteristics. Our results suggest the shear rheology of the composite is very similar to that of colloidalsuspensions in a simple fluid when polymer rheology, the particleinduced changes in the polymer rheology and the polymer slip effects are accounted. At dilute and semidilute nanoparticle concentrations, the composite shear rheology is shown to be dominated by the shear thinning of the polymer chains which in turn is modified by the presence of the particles. For higher particle loads, the polymeric contribution to the rheology becomes much less important and the shear rheology is dominated by the particles stresses. Using our results we suggest how a simple empirical model can be constructed for the shear rheology of the composite over the entire range of volume fractions. Our results and mechanistic explanations are in very good agreement with associated experimental observations.

RheoSANS investigation of acicularprecipitated calcium carbonate colloidal suspensions through the shear thickening transition
View Description Hide DescriptionThe rheology and particle alignment of suspensions of anisotropicprecipitated calcium carbonate particles of various aspect ratios is investigated using smallangle neutron scattering simultaneous with rheological measurement (RheoSANS). RheoSANS experiments were performed at concentrations from dilute to those exhibiting continuous and discontinuous shear thickening behavior. Long axis flow alignment is evident in all systems over the range of shear rates investigated. The flow alignment is shown to increase with particle aspect ratio, the applied shear stress (up to the point of shear thickening), and particle loading. For samples exhibiting shear thickening behavior, the highest degree of flow alignment occurs at the critical stress associated with the onset of shear thickening. Irrespective of particle aspect ratio, a gradual reduction in flow alignment with increasing applied stress is observed beyond the critical stress in continuously shear thickening samples, whereas discontinuously shear thickening samples maintain nearly the same degree of flow alignment within the shear thickening regime. The results are shown to be consistent with hydrocluster formation as the mechanism of shear thickening. The critical volume fraction for discontinuous shear thickening is shown to coincide with the equilibrium isotropicnematic phase transition at higher aspect ratios.

Oscillatory shear of suspensions of noncolloidal particles
View Description Hide DescriptionThe rheological behavior of noncolloidal suspensions in oscillatory shear flow was studied by measuring the complex viscosity as a function of total strain. The rheology was evaluated in the Couette and parallelplate geometries for five different suspension systems at a volume fraction of 0.40. As a reference, the steady shear rheology was also evaluated in both geometries. For steady shear in the Couette geometry, suspensions showed a decrease in viscosity as a result of shearinduced particle migration, whereas in the parallelplate geometry, no change in viscosity was observed over a similar total strain. The oscillatory shear rheology was observed to have a strong dependence on the applied strain amplitude. At each amplitude, the steady value of the complex viscosity was preceded by a drift. The complex viscosity decreased with total strain for high strain amplitudes and increased for low amplitudes. The transition point at which the qualitative behavior changed occurred at an amplitudetogap ratio between 0.1 and 0.5 and was independent of the particle size distribution and suspension system. The large strain required to reach a steady value for the complex viscosity at the lower amplitudes suggests that previous measurements may not have been reported with respect to their steady values. The oscillatory shear results were independent of the shear cell geometry, indicating that shearinduced migration was of no consequence and that the observed behavior was instead due to changes in the suspension microstructure.

The effect of free surfactant and grafted surfactant surface coverage on the rheology of organoclay dispersions
View Description Hide DescriptionThis work uses rheology to probe the solidlike network formed in organically modified montmorillonite clay dispersions. All dispersions are based upon two commercial organoclays, 15A and 20A, which differ only in the quantity of surfactant used in the cation exchange reaction performed to render the natural Namontmorillonite organophilic. In both cases, the amount of surfactant used in the cation exchange reactions is in excess of the cation exchange capacity of montmorillonite. Removal of unexchanged, free surfactant and subsequent thermogravimetric analysis demonstrates that the asreceived, unexchanged, and surfactant extracted organoclays possess different levels of free surfactant and grafted surfactant surface coverage. The four resulting organoclays were dispersed in xylene at two concentrations and subjected to oscillatory and steady shear experiments in a controlledstress rheometer. Experimental data show that all dispersions exhibit a solidlike response at low applied stress followed by yielding above an apparent yield stress. The presence of free surfactant is shown to significantly weaken the organoclay network, while a modest increase in surfactant surface coverage leads to a considerably more solidlike dispersion. The influence of free surfactant and surfactant surface coverage on organoclay exfoliation, tactoid size, and tactoid interactions are discussed.

Analysis of the rheotens experiment with viscoelastic constitutive equations for probing extensional rheology of polymer melts
View Description Hide DescriptionA model for the rheotens experiment is proposed to probe extensional rheology of polymer melts relevant to fabrication. The philosophy of the model is the coupling of the filament transport phenomena equations with kinetic theory constitutive equations of the differential type. A modified Giesekus (MG)constitutive equation seems to outperform other constitutive equations of the kinetic theory type, such as original Giesekus (OG) and finitely extensible nonlinear elastic (FENE)like, in terms of fitting and predictive capability of force rheotens curves at different process conditions for (i) substantially linear homogeneously branched polymers (AFFINITY™ polyolefin plastomers), (ii) linear low density polyethylene (LLDPE) (ASPUN™ fiber grade resins) and (iii) dow LDPE melts, with a variety of melt indexes and polydispersities . The model is a useful tool for estimation of an apparent elongational viscosity versus strain rate curve from raw rheotens data. It predicts a dominating extension thinning behavior for both melts of AFFINITY and ASPUN. For LDPE, the model predicts a pronounced strain hardening effect with a maximum in the elongational viscosity at a certain strain rate followed by extension thinning, behavior consistent with long chain branched melts. The MG rheotens model naturally predicts a smooth elongational viscosity curve with no singularity in its slope, which is a potential advantage relative to previous modeling approaches.

Oscillation superimposed on steady shearing: Measurements and predictions for wormlike micellar solutions
View Description Hide DescriptionMeasurements are made of the viscoelastic properties of a wormlike micellar solution undergoing steady shearing. The wormlike micellar solution has a clearly defined Maxwell response in oscillation. The features (for example, the plateau of the elastic modulus,, and the peak of the viscousmodulus,) are extremely well defined, allowing the differences in elastic properties brought about by steadyshearing to be easily discerned. An important feature of the solution studied is that it displays a plateaulike region with almost constant shear stress over a range of steady shear rates. We find that as the steady shear rate is increased: (i) becomes negative at low frequencies; (ii) the plateau value of decreases; (iii) the frequency at which the plateau value of is reached shifts to higher frequencies; (iv) the width of the peak decreases; (v) the position of the peak shifts to higher frequencies; and (vi) the height of the peak increases then decreases. The constitutive equations of Bautista et al. [J. NonNewtonian Fluid Mech.80, 93–113 (1999)] and Boek et al. [J. NonNewtonian Fluid Mech.126, 39–46 (2005)] were used to fit and predict bulk rheology measurements. Neither model predicts the observed rise in normal stress, despite fitting the plateau region in the shearstress versus shearrate curve. It proved difficult to predict transient shear stress data when both increases and decreases in the shear rate were considered. The features found in combined steady and oscillatory flow are predicted qualitatively but not quantitatively. In particular, the reduction of the plateau of is predicted to occur only at much higher shear rates than found experimentally. We also note that the commercial software controlling the rheometer does not interpret the raw data correctly in the regime where is negative.