Index of content:
Volume 55, Issue 5, September 2011
A study on the flow, failure, and rupture mechanisms of low-density polyethylene in controlled-stress uniaxial extensional flow55(2011); http://dx.doi.org/10.1122/1.3596210View Description Hide Description
The main aim of this work is to study the flow, failure, and rupture dynamics of a benchmark low-density polyethylene (BASF Lupolen 1840H) in true tensile creep conditions in both the viscoelastic and elastic deformation regimes. For this, we used a novel extensional rheometer that for the first time allows real controlled-stress conditions to be applied to the materials (as opposed to nominally controlled-stress) until physical rupture occurs (as opposed to being limited by maximum Hencky strain). We observed that constant strain rate was achieved for all flow conditions, and depending on the level of applied tensile stress, one or two states were obtained. In the former case, the observed mode of rupture was ductile, or liquid-like, and in the latter was cohesive, or elastic-like. The coupling at the molecular level between these flow and rupture mechanisms is not yet fully understood, although some recent studies in the literature may be able to offer at least partial explanations.
55(2011); http://dx.doi.org/10.1122/1.3596599View Description Hide Description
The nonlinear rheological behavior of entangled poly(ethylene oxide) (PEO) melts was studied by particle-tracking velocimetry (PTV) technique in a cone-plate geometry. In the absence of wall slip, the PEO melts exhibited internal macroscopic motions after shear cessation, when the imposed strain was larger than a critical value (between 1 and 1.5 strain units). This nonquiescent relaxation behavior of entangled melts has only recently been reported once in the literature involving a room temperature melt, and our work is the first PTV observation of nonquiescent relaxation at high temperature.
55(2011); http://dx.doi.org/10.1122/1.3599676View Description Hide Description
The rheological behavior of ballistic gelatin is studied experimentally by a newly developed optical technique for visualization of gelatin flow between two parallel plates at high shear rates in the range of 2000/s–8000/s. The unsteady motion of this fluid is captured by the displacement profiles of grid lines marked on the gelatin specimen. By utilizing a power-law model for fluid behavior and then solving the unsteady momentum equation of the non-Newtonian fluid, the power-law model parameters have been determined for early part of the shear flow where linearity assumption in velocity profile is valid. The exponent in the power-law model was found to be 2.25 ± 0.43. The diffusion profiles from optical images also compare well with numerical predictions based on extracted rheological properties of gelatin. Finally, it was noted that the model results compare well with those obtained from other experimental methods on the same gelatin material and also with other fluids using similar experimental methods.
Influence of annealing on linear viscoelasticity of carbon black filled polystyrene and low-density polyethylene55(2011); http://dx.doi.org/10.1122/1.3604816View Description Hide Description
Influence of annealing on linear dynamic rheology of carbon black filled polystyrene and low-density polyethylene before and after annealing was investigated in relation to a recently proposed two phase model. Annealing-induced improvements in dynamic complex modulus of filled polymers in the linearity region are related to increases in amplification factor and characteristicmoduli and to retardation in dynamics of the filler phase. It is suggested the characteristicmodulus and dynamics of the filler phase are interrelated to each other, and their relationship is independent of thermal history. Moreover, annealing-induced variations in global viscoelasticity of filled polymers are ascribed to the geometric changes of the filler phase consisting of filler clusters with a constant fractal dimension.
55(2011); http://dx.doi.org/10.1122/1.3606633View Description Hide Description
The effect of nonparallelism in a plate–plate torsional geometry is investigated for confined Newtonian fluids by directly solving Reynolds equation. A nondimensionalization is proposed, and theoretical results are compared to triborheological experiments by Kavehpour and McKinley [Tribol. Lett.17, 327–335 (2004)] in the form of a frictional Stribeck curve.
55(2011); http://dx.doi.org/10.1122/1.3609853View Description Hide Description
A series of controlled microstructure poly(lactide) (PLA) samples were synthesized using a novel chiral dinuclear indium catalyst capable of living polymerization of lactide. PLAs with different ratios of L- to D- monomer ratios of 100:0, 90:10, 75:25, 50:50, and 0:100 were investigated. The relationship between intrinsic viscosity and the absolute molar mass distribution of the samples obtained by light scattering gel permeation chromatography in tetrahydrofuran gives [η] = 0.014 + 0.75 Mw, a scaling law of typical coil dimensions of linear macromolecules in good solvent. The melt rheological study includes determination of zero-shear viscosity and its relationship with the molecular weight, the relaxation spectrum, and its relation with molecular weight characteristics, as well as plateau modulus and other important rheological parameters that are helpful in predicting the linear viscoelasticity of PLA. Emphasis is placed on the uniaxial melt behavior of these polymers. At low temperatures, significant strain hardening is observed, which gradually disappears with an increase in temperature and decrease of Hencky strain rate. The K-BKZ constitutive equation is used to model the experimental data. It is concluded that in spite of their linear structure, PLAs exhibit strain hardening which is not due to strain-induced crystallization, and it is solely due to the dynamics of molecular relaxation.
55(2011); http://dx.doi.org/10.1122/1.3610169View Description Hide Description
Spatially inhomogeneous shear flow occurs in entangled polymer solutions, both as steady state shear banding and transiently after a large step strain or during startup to a steady uniform shear rate.Theoretically, steady state shear banding is a hallmark of models with a nonmonotonic constitutive relation between total shear stress and applied shear rate, but transient banding is sometimes seen in fluids that do not shear band at steady state. We model this behavior using the diffusive Rolie-Poly model in a Newtonian solvent, whose steady state constitutive behavior can be monotonic or nonmonotonic depending on the degree of convective constraint release. We study monotonic steady state constitutive behavior. Linear stability analysis of the startup to a sufficiently high shear rate shows that spatial fluctuations are unstable at early times. There is a strong correlation between this instability and the negative slope of the (time dependent) constitutive curve. If the time integral of the most unstable eigenvalue is sufficiently large, then the system exhibits transient shear bands that later vanish in steady state. We show how perturbations, due to fluctuations or the inhomogeneous stresses, can trigger this instability. This transient behavior is similar to recent observations in entangled polymer solutions.
Shear-induced anisotropy of concentrated multiwalled carbon nanotube suspensions using x-ray scattering55(2011); http://dx.doi.org/10.1122/1.3609854View Description Hide Description
X-ray scattering is used to measure particle orientation in concentrated multiwalled carbon nanotube(MWNT)suspensions under shear flow.MWNTs were dispersed in a Newtonian suspending fluid (uncured epoxy). The dispersions exhibit shear thinning, approaching the matrix viscosity at high shear rates. This is accompanied by progressive development of MWNT orientation along the flow direction with increasing shear rate. The impact of MWNT aspect ratio and concentration on steady-state orientation is explored. In one sample (2 wt. % dispersion of short MWNTs), orientation was measured in both the flow-gradient (1-2) and flow-vorticity (1-3) planes of shear flow to provide a more complete picture of the three-dimensional orientation state. Also in this sample, 1-3 plane measurements were conducted using both small- and wide-angle x-ray scattering (SAXS and WAXS). While the two methods produce qualitatively similar results, WAXS-derived measures of flow-induced anisotropy are consistently larger than SAXS data. In transient measurements following step-down in shear rate,MWNT orientation is found to decrease on similar time scales as viscosity increases. Prolonged growth of storage modulus is observed following flow cessation, accompanied by an unexpectedly rapid partial loss of MWNT orientation. The rheological and orientation data are discussed in terms of distortion, breakdown, and reformation of percolatedMWNT networks in these samples.
55(2011); http://dx.doi.org/10.1122/1.3610439View Description Hide Description
We have applied a series of start-up of uniaxial extensions to very high strain followed by stress relaxation. A potential temperature change was applied during the stress relaxation. We used two thermorheological simple polymers; a linear polystyrene and a branched low density polyethylene. Experiments performed with temperature changes during the stress relaxation were shown to be identical with isothermal ones in the “pseudotime,” within the accuracy of the experiments. This verifies that the pseudotime approach seems to be the general basis for nonisothermal microstructural modeling for flow of polymers. The pseudotime is given as , where a T are the well established time-temperature superposition shift factors, calculated from the past temperatures (at time t′) in a particle path
55(2011); http://dx.doi.org/10.1122/1.3613978View Description Hide Description
Glasses of hard spheres show a single yield stress or strain associated with deforming particle cages to an extent that stress is rapidly relaxed by exchange of nearest neighbors. In this first of two articles on the rheological behavior of suspensions of anisotropically shaped particles (particle diameter ∼300 nm), we demonstrate that particles experiencing volume exclusion interactions at high volume fractions exhibit two maxima in G″, which are interpreted as multiple yielding events. We interpret these results through naïve mode coupling theory (NMCT) where one yield stress is associated with exceeding the entropic barrier constraining rotational motion and the other exceeding the barrier for exchanging nearest neighbors. These results are qualitatively compared to theoretical state diagrams predicting double glass states for hard anisotropic particles. In the second of the two articles, we explore the effects of shape anisotropy and weak attractions on yielding and the onset of shear thickening. This second data set is undertaken with particles of similar shape but larger size (∼1.2 μm) and under conditions where the particles feel sufficient attractions to reduce the gel point below the glass transition observed for purely volume exclusion interactions explored here.
Exploration of the volume fraction above which suspensions of spherical and weakly anisotropic colloid particles cannot flow55(2011); http://dx.doi.org/10.1122/1.3613983View Description Hide Description
In article II of this series, we describe experiments with dense suspensions of spherical and dumbbell shaped colloid particles (particle diameter ∼1.2 μm). The suspensions are sheared with dynamic and continuous stress. Dynamic stress sweeps are used to characterize a dynamic glass transition volume fraction (φ g ) and a dynamic yield stress (τ y *). Both phenomena are understood in terms of activated naïve mode coupling theory. The dynamic yield stress increases with volume fraction and scales with a reduced volume fraction φ* as τ y * ∼ φ*4. Under continuous stress, suspensions discontinuously shear thicken at a critical stress (τ t *), which is independent of particle shape and particle interaction. The volume fraction at the onset of thickening (φ t ) is higher for dumbbell particles compared to spheres. Our results suggest that there is a volume fraction where τ y = τ t , and this volume fraction occurs close to that of random close packing. For larger volume fractions, steady flow is only established through dilation.
55(2011); http://dx.doi.org/10.1122/1.3613948View Description Hide Description
Some of the fluids routinely used in microfluidic devices have micro and nanostructures that can generate viscoelastic behavior. In this paper, an experimental technique is developed to estimate the extensional flow resistance of dilute polymer solutions using flow in a lubricated, converging, and microfluidic channel. The channel has a two-dimensional hyperbolic profile, and the viscous or viscoelastic core fluid is lubricated by a less viscous Newtonian fluid that is introduced through side channels upstream. Differences in flow patterns are observed between miscible and immiscible lubricants. Stable flows are achieved with an immiscible lubricant and with a suitable interfacial tension between the core fluid and the lubricant. Steady extensional flows are established at low Reynolds numbers (Re ≪ 1 for the core fluid), at moderate Hencky strains (3 < ɛ H < 3.9), and over a wide range of Deborah numbers (0 < De < 20). Pressure drop and flow rate measurements are made, and the velocity field is characterized by particle image velocimetry and streak photography. The apparent extensional viscosity, as a function of strain rate, is estimated from the difference between the experimental pressure drop and the calculated pressure drop due to shearing in the core flow. Values of the Trouton ratio, for dilute solutions of high-molecular-weight polyethylene oxide, are O(103), consistent with literature values.
55(2011); http://dx.doi.org/10.1122/1.3606593View Description Hide Description
In this study, we use microparticle image velocimetry (μ-PIV) and adapt a commercial birefringence microscopy system for making full-field, quantitative measurements of flow-induced birefringence(FIB) for the purpose of microfluidic, optical rheometry of two wormlike micellar solutions. In combination with conventional rheometric techniques, we use our microfluidic rheometer to study the properties of a shear-banding solution of cetylpyridinium chloride (CPyCl) with sodium salicylate (NaSal) and a nominally shear-thinningsystem of cetyltrimethylammonium bromide (CTAB) with NaSal across many orders of magnitude of deformation rates (). We use μ-PIV to quantify the local kinematics and use the birefringence microscopy system in order to obtain high-resolution measurements of the changes in molecular orientation in the wormlike fluids under strong deformations in a microchannel. The FIBmeasurements reveal that the CPyCl system exhibits regions of localized, high optical anisotropy indicative of shear bands near the channel walls, whereas the birefringence in the shear-thinning CTAB system varies more smoothly across the width of the channel as the volumetric flow rate is increased. We compare the experimental results to the predictions of a simple constitutive model, and we document the breakdown in the stress-optical rule as the characteristic rate of deformation is increased.