Volume 46, Issue 2, March 2002
Index of content:
Effect of surfactant on viscoelasticity and shear thickening in aqueous solutions of hydrocarbon end-capped poly(ethylene oxide)46(2002); http://dx.doi.org/10.1122/1.1450561View Description Hide Description
The effect of the anionic surfactantsodium dodecyl sulfate (SDS) on the linear viscoelastic and shear thickening behavior of aqueous solutions of hydrophobically end-capped poly(ethylene oxide) was examined. At low SDS concentration, the linear viscoelastic response is similar to that in pure water. As SDS concentration increases, especially above the critical micellization concentration, deviations from the behavior of the polymer in pure water occur at high frequencies. However, if the data are corrected for an effective viscosity of the surrounding medium the observed behavior of a Maxwellian fluid with a single relaxation time is preserved and it is also possible to superimpose and data at different SDS concentrations. was found to be 0, 0.02, 0.2, and 0.28 Pa s for solutions with and SDS, respectively. Shear thickening was only observed at the highest SDS concentration. It is not conclusive whether shear thickening is present in lower SDS concentrations or not because of the onset of flow instability. Comparing the shear thickening behavior in two systems (model polymers in pure water and with SDS), a common location of shear thickening (on a reduced shear rate scale) was observed. This is consistent with the non-Gaussian chain stretching mechanism where the onset of shear thickening depends only on the polymer molecular weight, and not on the viscosity of the surrounding medium.
Obtaining the shear rate profile of steady laminar tube flow of Newtonian and non-Newtonian fluids from nuclear magnetic resonance imaging and laser Doppler velocimetry data46(2002); http://dx.doi.org/10.1122/1.1446881View Description Hide Description
The problem of extracting shear rate profiles from experimentally measuredvelocity profiles in steady laminar tube flow is formulated as an integral equation of the first kind to reveal its ill-posed nature. A procedure, based on Tikhonov regularization, is then developed to solve this problem. The performance of the procedure is assessed by applying it to the measuredvelocity profile of a number of fluids of increasing rheological complexity. These velocity profiles, taken from recent literature, are representative of what is currently achievable with nuclear magnetic resonance imaging and laser Doppler techniques. The shear rate profiles thus obtained are compared against those obtained by existing methods of treating this problem. For velocity profiles where the associated pressure gradient is available, the outcome of Tikhonov regularization is converted into a viscosity versus shear rate relationship and compared against that obtained by direct viscometric measurement. The results show that Tikhonov regularization is a reliable method and has the added advantage of not requiring the assumption of a rheological constitutive equation. It can therefore be applied with equal ease to non-Newtonian, including those with yield stress, as to Newtonian fluids.
46(2002); http://dx.doi.org/10.1122/1.1450563View Description Hide Description
The role of molecular weight in shear-induced structure transitions, such as striations during shear startup and banding during relaxation, was probed through small-angle light scattering and shear microscopy. The characteristic texture relaxation time for banding, is molecular weight dependent. The minimum molecular weight for these shear-induced complex texture transitions to occur is estimated from as 27 000 for a 37 wt % poly(n-hexyl isocyanate) in 1,1,2,2-tetrachloroethane solution at room temperature. is independent of applied shear rate, implying that it is a material property. Thus, both of these structure transitions appear to occur only after shearing above a threshold Deborah number, below which viscous forces would suppress these structure transitions. Both the strain dependence of the structure transitions during shear startup and the preshear rate dependence of the banding phenomenon exhibit little or no molecular weight dependence. Increasing molecular weight does appear to decrease the characteristic band correlation length and slow the decay rate of the bands.
46(2002); http://dx.doi.org/10.1122/1.1445186View Description Hide Description
We measure the flow kinetics of a polyethylene extruded through the exit of a sapphire capillary tube in order to understand the nature of sharkskin, a surface roughness in the extruded material. Optical velocimetry shows that sharkskin can occur under a variety of polymer/wall boundary conditions; stick, slip, or oscillating stick/slip, demonstrating that the flowboundary condition is not the direct cause of sharkskin. Downstream of the exit, high-speed video microscopy reveals two distinct material failures during each sharkskin cycle, the first is cohesive and splits the material into two regions, the second one occurs at the polymer–wall interface. Upon modification of the surface with a polymer processing additive (PPA), we confirm strong slip at the wall and a suppression of sharkskin, but find that sharkskin does return at sufficiently high flow rates. The extensional strain rate at the onset of sharkskin is significantly higher in the case with PPA than that without. We then empirically define a “reconfiguration rate” and find it is comparable at the onset of sharkskin for the two surface conditions. We use data in the literature to show that the reconfiguration rate also predicts the relationship observed between the onset of sharkskin and the capillary radius.
46(2002); http://dx.doi.org/10.1122/1.1451083View Description Hide Description
We introduce a method using the rheology of concentrated solutions to differentiate sensitively between nonbranched and branched architecture polymer melts. By modeling the concentration dependent shift in zero-shear viscosity using molecular tube theory for melt rheology, it is feasible to quantify the degree of long chain branching as well as the distribution of sizes of such branches, providing the class of branched material is known. The technique detects branched chains with the order of one entanglement length and greater. We apply the technique, already validated against monodisperse controlled-architecture polymers [D. R. Daniels et al. Rheol. Acta 40, 403 (2001)], to a series of industrially produced metallocene-catalyzed polyethylenes with an octene comonomer and two low density polyethylenes, diluted by the short chain alkane squalane. The dilution method compares favorably with direct solution methods, and holds promise of much greater sensitivity.
46(2002); http://dx.doi.org/10.1122/1.1446880View Description Hide Description
A molecular model for wall slip [Y. M. Joshi et al., Macromolecules34, 3412 (2001)] based on recent tube theories is extended to account for the effects of entanglements between tethered chains that occur at higher surface coverage. Three regimes of surface coverage are identified. Regime I is a low surface coverage regime (the mushroom regime), which has been discussed earlier by us. In regime II the tethered chains undergo a cooperative constraint release process due to which the slip velocity increases with surface coverage while the slip length becomes independent of the surface coverage. In regime III the tethered chains start to become entangled with each other thereby causing the interfacial modulus, the critical wall shear stress and the critical shear rate to decrease with surface coverage. Our model is different from previous scaling models in that it provides a constitutive equation for tethered chains. As a result, it offers scope for quantitative prediction of microscopic and macroscopic experimental slip data based solely on molecular information. Our model also predicts scaling laws for the various slip parameters in the three regimes of surface coverage. These laws are in general agreement with previously reported scaling models and experimental data.
46(2002); http://dx.doi.org/10.1122/1.1450562View Description Hide Description
The flow properties of three aqueous polyurethane dispersions are characterized as a function of particle concentration. The particles are highly charged. Using a series of simplified methods pioneered on model suspensions, particle interaction energies capable of describing osmotic pressure, continuous shear, and elastic behavior are developed. Electrostatic forces dominate particle interactions at low volume fractions. At elevated concentrations, short range repulsive interactions dominate flow properties.
46(2002); http://dx.doi.org/10.1122/1.1446882View Description Hide Description
In this paper a method based on attenuated total reflection–Fourier-transform infrared spectroscopy is described to measure the concentration of colloidal particles dispersed in a liquid near a solid boundary in pressure driven flows. The method has been used to obtain measurements of wall depletion in an aqueous suspension of ethylcellulose particles. The method is capable of measuring the particle concentration in a thin layer near the boundary, whose thickness is between 0.3 and 1.4 μm. A considerable decrease in the concentration of particles near the wall is found for a suspension with a particle volume fraction of 0.24 over a wide range of apparent shear rates. The results from the spectroscopic experiments are compared to rheological experiments on parallel disk torsional flow at different gap heights. With this method, the order of magnitude of the thickness of an equivalent fully depleted layer (a layer at the wall assumed to contain only pure solvent and no particles) can be obtained. The results found with the two techniques are in reasonable agreement. The described method is a convenient, rapid, and relatively inexpensive way to measure the particle depletion near the wall in flowing suspensions, compared to existing magnetic resonance imaging and laser-Doppler techniques.
46(2002); http://dx.doi.org/10.1122/1.1445184View Description Hide Description
The viscoelastic behavior of a rosin derivative, dehydroabietic acid, DHAA, has been determined with shear creep and recovery measurements at temperatures between 43 and 88 °C. The viscosity measurements extended up to 162 °C. The glasstemperature (0.2 °C/min cooling) was estimated to be 43.3 °C. Earlier measurements on a commercially produced dehydrogenated rosin called Galex, which was thought to be relatively pure DHAA, had a of 7 °C. A steady-state recoverable compliance, of was extracted from a reduced shear compliance curve. Our measurements on highly purified DHAA showed of However an unmodified rosin does show large recoverable compliances that are comparable to those found in Galex. We have concluded that rosin and Galex are dilute polymer solutions. All of the common viscoelastic functions have been calculated from the retardation spectrum determined, the glassy compliance and the viscosity of DHAA. It is believed that such a complete description of the viscoelastic behavior of an organic nonpolymeric glass former has not previously been presented. At the retardation spectrum was found to be the same as that of all other amorphous glass formers at times between 0.1 and 1000 s within experimental uncertainty.
Investigation of the elongational behavior of polymer solutions by means of an elongational rheometer46(2002); http://dx.doi.org/10.1122/1.1445185View Description Hide Description
An elongational rheometer is used to measure the relaxation times λ and the steady terminal elongational viscosities of aqueous solutions of ionic and nonionic polymers at various concentrations in Θ and in good solvents. In the corresponding nomogram the results yield two typical limiting curves—one for solutions of nonionic polymers and another for solutions of ionic polymers. The curves characterize the elongational behavior of the polymer solutions. The elongational flow results in data that are intermediate to the two limiting curves for polymers in aqueous salt solutions and for aqueous solutions of mixtures of ionic and nonionic polymers. By means of an empirical correlation, the relaxation time and the steady terminal elongational viscosity are normalized such that the nomogram can be applied to polymers in various solvents. Thus a simple and reliable method for determining the behavior of polymer solutions in elongational flows is achieved.
Effects of shear flow on a polymeric bicontinuous microemulsion: Equilibrium and steady state behavior46(2002); http://dx.doi.org/10.1122/1.1446883View Description Hide Description
We have investigated the effects of shear flow on a polymeric bicontinuous microemulsion using neutron scattering,light scattering,optical microscopy, and rheology. The microemulsion consists of a ternary blend of poly(ethyl ethylene) (PEE), poly(dimethyl siloxane) (PDMS), and a PEE–PDMS diblock copolymer. At equilibrium, the microemulsion contains two percolating microphases, one PEE rich and the other PDMS rich, separated by a copolymer-laden interface; the characteristic length scale of this structure is 80 nm. Low strain amplitude oscillatory shear measurements reveal behavior similar to that of block copolymer lamellar phases just above the order–disorder transition. Steady shear experiments expose four distinct regimes of response as a function of the shear rate. At low shear rates (regime I) Newtonian behavior is observed, whereas at intermediate shear rates (regime II) development of anisotropy in the morphology leads to shear thinning. When the shear rate is further increased, there is an abrupt breakdown of the bicontinuous structure, resulting in flow-induced phase separation (regime III). Rheological measurements indicate that the shear stress is almost independent of the shear rate in this regime. Light scattering reveals a streak-like pattern, and correspondingly a string-like morphology with micron dimensions is observed with video microscopy. Upon a further increase of the shear rate (regime IV), the sample resembles an immiscible binary polymer blend with the block copolymer playing no significant role; the stress increases strongly with the shear rate. In some respects these results resemble those from other weakly structured complex fluids (sponge phases, liquid crystals, worm-like micelles, block copolymers,polymer blends, and polymer solutions), yet in important ways this system is unique.