Volume 55, Issue 4, July 2011
Index of content:
Effect of compatibilizer concentration and weight fraction on model immiscible blends with interfacial crosslinking55(2011); http://dx.doi.org/10.1122/1.3571549View Description Hide Description
Reactive compatibilization, in which a compatibilizer is formed by an interfacial coupling between two reactive polymers, is commonly used when blending immiscible homopolymers. We consider reactive compatibilization using two multifunctional reactive polymers, which leads to a crosslinked copolymer at the interface. Experiments were conducted on model blends of polydimethylsiloxane(PDMS) and polyisoprene (PI). Compatibilizer was formed by a chemical reaction between amine-functional PDMS and maleic anhydride-functional PI. Droplet-matrix blends with a PI:PDMS ratio of 30:70 or 70:30 and reactive compatibilizer loadings from 0.1% to 3% were examined by optical microscopy and rheometry. Experiments reveal that the effects of interfacial crosslinking are highly asymmetric, with PI-continuous blends showing altogether different behaviors from PDMS-continuous blends. The PI-continuous blends show unusual features including drop clusters and nonspherical drops. In contrast, PDMS-continuous blends displayed a typical droplet-matrix morphology with round drops that do not appear to stick together. The rheological properties are also asymmetric: The PI-continuous blend showed gel-like behavior in oscillatory experiments, high viscosity, and viscosity overshoots during startup of shear flow, whereas PDMS-continuous blends showed liquidlike behavior that is qualitatively similar to that of compatibilizer-free blends. We speculate that the observed structural and rheological asymmetry is attributable to the asymmetry of the compatibilizer architecture on the two sides of the interface.
55(2011); http://dx.doi.org/10.1122/1.3579161View Description Hide Description
We investigate the response of a well-characterized colloidal star glass to large-amplitude oscillatory stress and strain fields. By combining these measurements with dynamic time sweeps we demonstrate the importance of probing both strain- and stress-induced nonlinear rheology of such complex fluids in order to elucidate the yielding and fluidization behavior. We also show that, due to the strong time dependence, it is essential to perform dynamic time sweeps at different strain and stress amplitudes, which result in different departures of the glass cage from its quiescent quasiequilibrium structure. This allows for steady-state responses to be reached and for nonlinear oscillatory responses to be treated properly while also suggesting that yielding is a gradual process. Further, we use a recently published framework for analyzing nonlinear responses to large-amplitude oscillatory shear [Rogers et al., J. Rheol.55, 435 (2011)], based on the analysis of the whole stress waveforms as a sequence of physical processes, in order to measure the points of static and dynamic yielding. By doing so, we show that the stress-amplitude dependence of the dynamic yield stress can be linked to the strain-rate-amplitude dependence via the form of the steady-state flow curve.
55(2011); http://dx.doi.org/10.1122/1.3574932View Description Hide Description
This paper is concerned with an investigation of the rheological performance of magnetorheological fluids under squeeze flow. Preliminary results on Newtonian fluids are first compared to Stefan’s equation. Then, unidirectional monotonic compression tests are carried out in the presence of uniaxial external magnetic fields at slow compression rates under constant volume operation. Results are compared to Bingham plastic, biviscous, and single chain micromechanical squeeze flow models. Measurements using combined deformation modes (-strain oscillatory shear) suggest a compression-induced shear strengthen effect up to strains of . Particle-level dynamic simulations are in qualitatively good agreement with experimental observations.
A constitutive model for characterization of shear and extensional rheology and flow induced orientation of carbon nanofiber/polystyrene melt composites55(2011); http://dx.doi.org/10.1122/1.3579160View Description Hide Description
We characterize the transient shear and uniaxial extensional rheology of melt phase polystyrene/carbon nanofiber composites and measure orientation development during each flow type. Experimental measurements show an increase in the viscosity of the composite with increasing carbon nanofiber concentration under both types of flow. We present a microstructurally based constitutive model to predict the experimentally observed rheological behavior and evolution of nanofiber orientation. Experimental measurements were performed to determine the average orientation of the carbon nanofibers at various points in time during shear and extensional flows. We demonstrate that the constitutive model predicts the transient shear and extensional viscosities of polystyrene/carbon nanofiber composites and the orientation evolution of nanofibers during flow.
55(2011); http://dx.doi.org/10.1122/1.3579189View Description Hide Description
Under application of an electric field, suspensions of the synthetic clay Na-fluorohectorite in a silicone oil aggregate into chain/columnlike structures parallel to . This microstructuring results in a transition in the suspensions’ rheology, from Newtonian to a shear-thinning with a significant yield stress. We study this electrorheology (ER) as a function of and of the particle volume fraction on samples with a large clay particle polydispersity. The flow curves under fixed shear rate are well fitted by the Cho–Choi–Jhon model [M. Cho et al., Polymer46, 11484 (2005); H. J. Choi and M. Jhon, Soft Matter5, 1562 (2009)]; proper scaling of and of the measured shear stress provides a collapse of all flow curves onto a master curve. The corresponding dynamic yield stress scales as , while the static yield stress inferred from disruption tests behaves as . The bifurcation in the rheology when letting the flow evolve under constant shear stress is also characterized; the corresponding bifurcationyield stress scales as with . All measuredyield stresses increase with ; for the static yield stress, a scaling law is found. The three mutually consistent types of measurements are compared with previous measurements on laponite suspensions, and the rheologies of these two types of samples are discussed in light of existing theories of the ER effect.
55(2011); http://dx.doi.org/10.1122/1.3582848View Description Hide Description
This paper reports experiments on the shear transient response of concentrated non-Brownian suspensions. The shear viscosity of the suspensions is measured using a wide-gap Couette rheometer equipped with a particle image velocimetry device that allows measuring the velocity field. The suspensions made of PMMA particles ( in diameter) suspended in a Newtonian index- and density-matched liquid are transparent enough to allow an accurate measurement of the local velocity for particle concentrations as high as 50%. In the wide-gap Couette cell, the shear induced particle migration is evidenced by the measurement of the time evolution of the flow profile. A peculiar radial zone in the gap is identified where the viscosity remains constant. At this special location, the local particle volume fraction is taken to be the mean particle concentration. The local shear transient response of the suspensions when the shear flow is reversed is measured at this point where the particle volume fraction is well defined. The local rheological measurements presented here confirm the macroscopic measurements of Gadala-Maria and Acrivos [J. Rheol.24, 799–814 (1980)]. After shear reversal, the viscosity undergoes a steplike reduction, decreases slower, and passes through a minimum before increasing again to reach a plateau. Upon varying the particle concentration, we have been able to show that the minimum and the plateau viscosities do not obey the same scaling law with respect to the particle volume fraction. These experimental results are consistent with the scaling predicted by Mills and Snabre [Eur. Phys. J. E30(3), 309–316 (2009)] and with the results of numerical simulation performed on random suspensions[Sierou and Brady, J. Fluid Mech.448, 115–146 (2001)]. The minimum seems to be associated with the viscosity of an isotropic suspension or at least of a suspension whose particles do not interact through non-hydrodynamic forces, while the plateau value would correspond to the viscosity of a suspension structured by the shear where the non-hydrodynamic forces play a crucial role.
55(2011); http://dx.doi.org/10.1122/1.3589798View Description Hide Description
This study examines the linear viscoelastic properties and processing of blends of a metallocene polyethylene with different ultrahigh molecular weight polyethylenes (UHMWPEs). Blend compositions were prepared such that they were below and above the UHMWPE coil overlap concentration, . Linear rheology and processing were found to be very sensitive to molecular weight and concentration, suggesting an effective macromolecular network threshold. The linear viscoelastic behavior of the blends could be described using a simple model based on reptation, implying a segmental interaction of the macromolecules. Extrusionflow curves were identical for the pure material and blends. However, for a given blend composition, the amplitude of the sharkskin distortion diminished as the UHMWPE molecular weight in the blend increased, and completely vanished at a value corresponding to the coil overlap condition. For a given molecular weight of the UHMWPE, it was also possible to obtain distortion free extrudates simply by maintaining the UHMWPE concentration above the corresponding .
Cross-slot extensional rheometry and the steady-state extensional response of long chain branched polymer melts55(2011); http://dx.doi.org/10.1122/1.3589972View Description Hide Description
Stress-optical measurements at a flow stagnation point in confined geometries such as the cross-slot provide an elegant way to perform extensional testing for polymer melts. This technique is especially useful for samples which have a steady-state that cannot be reached (easily) in standard elongational rheometry, for example, highly branched polymers which show a non-homogeneous deformation that occurs in stretching experiments for Hencky strains above 4. In contrast to filament stretching, the cross-slot provides one point at which steady-state extensional flow may be sustained indefinitely. In this study, a Cambridge multi-pass rheometer [Coventry, K. D., and M. R. Mackley, J. Rheol.52, 401–415 (2008)] is used to generate planar elongational flow in a cross-slot geometry for different polyethylene melts. The experimental results are compared to finite element flow simulations using the multi-mode Pompom constitutive equations. The steady-state elongational viscosity at the stagnation point is computed from the flow-induced stress birefringence and the strain-rate determined from numerical calculations of the flow field. We apply this technique to a range of different branched high- and low-density polyethylene melts. This demonstrates both the effectiveness of this technique and shows how the stress distribution in a complex flow depends on molecular structure. Cross slot extensional rheometry therefore provides a very promising technique for parameterizing molecular constitutive equations for LCB melts.
Extensional flow-induced crystallization of isotactic poly-1-butene using a filament stretching rheometer55(2011); http://dx.doi.org/10.1122/1.3593471View Description Hide Description
A filament stretching rheometer is used to investigate the extensional flow-induced crystallization of two commercial grade isotactic poly-1-butene samples. The degree of crystallinity of the stretched fibers is quantified using differential scanning calorimetry measurements as a function of extension rate and accumulated Hencky strains. All the measurements are performed using the Janeschitz-Kriegel protocol. The samples are first melted to erase their thermal and mechanical history. They are then quickly quenched to after which the stretch is imposed. The deformed filament is then allowed to crystallize fully at . The extensional rheology of both the samples shows only minimal strain hardening. For the case of the lower molecular weight sample, the percent crystallinity increases from 46% under quiescent conditions to a maximum of 63% at an extension rate of . This corresponds to an increase of nearly 50% above the quiescent case. The high molecular weight sample shows similar trends achieving an increase in crystallinity of 25%. The experiments show an optimal extension rate for which the extensional flow has the maximum impact on the polymer crystallinity. The percent crystallinity of both the samples is observed to increase with increasing strain for a fixed extension rate. Small angle X-ray scattering shows that the observed increase in crystallinity is likely due to the increasing orientation and alignment of the polymer chains in extensional flows which enhances the thread-like precursors responsible for the formation of the crystals in the shish-kebab morphology.
55(2011); http://dx.doi.org/10.1122/1.3586815View Description Hide Description
In Table II of both printings [Dealy, J. M., J. Rheol.28, 181–195 (1984); 39, 253–265 (1995)] of the official nomenclature of The Society of Rheology, the entry for the “out-of-phase component of ” should be , not . In the table titled “Nomenclature for Small Amplitude Oscillatory Motion” of the preceding proposed official nomenclature [Sieglaff, C. L., Trans. Soc. Rheol.20, 311–317 (1976)], should also be .