Volume 59, Issue 4, July 2015
Index of content:
Determination of characteristic lengths and times for wormlike micelle solutions from rheology using a mesoscopic simulation method59(2015); http://dx.doi.org/10.1122/1.4919403View Description Hide Description
We apply our recently developed mesoscopic simulation method for entangled wormlike micelle (WLM) solutions to extract multiple micellar characteristic lengths and time constants: i.e., average micelle length, breakage rate, and entanglement and persistence lengths, from linear rheological measurements on commercial surfactant solutions, one containing sodium lauryl one ether sulfate (SLE1S), and the other containing both SLE1S and cocamidopropyl betaine, as well as a perfume mixture, in both cases with a sample salt (NaCl) added. Measurements include both mechanical rheometry and diffusing wave spectroscopy, the latter providing the high-frequency data needed to determine micelle persistence length accurately. By fitting the experimental data ( and ) across the entire frequency range through our iteration procedure, the method is of practical use in predicting micellar parameters, which are difficult to obtain from other theoretical or experimental methods. The dependence of micellar parameters on added salt concentration, and the effect of micelle breakage mechanisms on viscoelasticity of WLM solutions, are also discussed.
59(2015); http://dx.doi.org/10.1122/1.4919405View Description Hide Description
We report in situ small-angle x-ray scattering studies of a prealigned cylindrically ordered styrene-ethylene butylene-styrene block copolymer melt subjected to extensional flow. Samples are prepared via lubricated planar extensional flow and tested using two initial conditions: “parallel” with cylindrical microdomains oriented along the extensional flow direction and “perpendicular” with domains aligned transverse to the flow direction. The experiments employ a counter-rotating drum extensional flow fixture housed in an oven designed for in situ synchrotron access. The impacts of initial condition, extension rate, and final Hencky strain on the melt structure are analyzed both during and following flow. Stress and flow kinematics are strongly influenced by the initial sample orientation. While parallel samples exhibit uniaxial deformation, perpendicular samples exhibit planar extensional kinematics, attributed to susceptibility to compression along the cylindrical microdomain axis due to a microscopic buckling instability. Scattering data reveal both anisotropic deformation of microdomain spacing and reorientation induced by the flow. Persistence of higher order reflections confirms hexagonal packing throughout the flow process, and strong alignment along the stretching direction is attained for both initial conditions. Flow-induced deformation of microdomain spacing and mechanical stress relax on similar time scales upon flow cessation, while negligible relaxation of orientation is observed.
59(2015); http://dx.doi.org/10.1122/1.4919970View Description Hide Description
The flow of dry and wet granular media is investigated in a Couette geometry using magnetic resonance imaging in order to test the applicability of the “fluidity model” for nonlocality in these materials. Local volume fraction measurements show that the systems become heterogeneous during flow. We find that the nonlocal rheology of suspensions can be correlated using the fluidity model, but the length scale that emerges is not a material property and the model cannot be used for predictive purposes. Rather, the suspension behavior is fully explained as a consequence of stress-driven particle migration and the resulting concentration gradient. The conclusion is less strong for the dry granular system, but it appears likely that the apparent nonlocal behavior is simply due to the formation of a shear band caused by granular dilatancy.
Adjustable rheology of fumed silica dispersion in urethane prepolymers: Composition-dependent sol and gel behaviors and energy-mediated shear responses59(2015); http://dx.doi.org/10.1122/1.4922010View Description Hide Description
Variation of colloidal and interfacial interactions leads to a microstructural diversity in fumed silica dispersions exhibiting absolutely different sol- or gel-like rheological responses. In this study, fumed silicas with different surface areas (200–400 m2/g) and surface characteristics (hydrophilic or hydrophobic) are dispersed into moisture-cured polyurethane. The microstructures investigated using transmission electron microscope are associated perfectly with three different rheological behaviors: (i) Sols with well-dispersed silica aggregates, (ii) weak gels with agglomerate-linked networks, and (iii) strong gels with concentrated networks of large agglomerates. Though sols and gels are well distinguished by shear thickening or sustained thinning response through steady shear flow test, it is interesting that the sols and weak gels exhibit a uniform modulus plateau-softening-hardening-softening response with increasing dynamic strain at frequency 10 rad s−1 while the strong gels show a sustained softening beyond the linear regime. Furthermore, the onset of softening and hardening can be normalized: The two softening are isoenergetic at mechanical energies of 0.3 J m−3 and 10 kJ m−3. On the other hand, the hardening is initiated by a critical strain of 60%. The mechanisms involved in the generation of the sol- and the gel-like dispersions and their structural evolutions during shear are thoroughly clarified in relation to the polyols, the characteristic and content of silica and the curing catalysts.
59(2015); http://dx.doi.org/10.1122/1.4922060View Description Hide Description
We have developed a variant of the pom-pom model that qualitatively describes two surprising features recently observed in filament stretching rheometer experiments of uniaxial extensional flow of industrial branched polymer resins: (i) Overshoots of the transient stress during steady flow and (ii) strongly accelerated stress relaxation upon cessation of the flow beyond the overshoot. Within the context of our model, these overshoots originate from entanglement stripping (ES) during the processes of normal chain retraction and branch point withdrawal. We demonstrate that, for a single mode, the predictions of our overshoot model are qualitatively consistent with experimental data. To provide a quantitative fit, we represent an industrial melt by a superposition of several individual modes. We show that a minimal version of our model, in which ES due to normal chain retraction is omitted, can provide a reasonable, but not perfect, fit to the data. With regard the stress relaxation after (kinematically) steady flow, we demonstrate that the differential version of tube orientation dynamics in the original pom-pom model performs anomalously. We discuss the reasons for this and suggest a suitable alternative.
Identification of broad-spectrum viscoelastic parameters: Influence of experimental bias on their accuracy and application to semihard-type cheese59(2015); http://dx.doi.org/10.1122/1.4922221View Description Hide Description
This paper sets out a method to extract Maxwell model parameters from experimental compression-relaxation tests and investigates common experimental sources of bias when dealing with viscoelastic materials. Particular attention was given to viscoelastic materials that relax stress quickly. The proposed method differs from the methods usually used in that it takes into account the stress that can relax when a material is submitted to compression before proper relaxation. Among the experimental biases that can affect the tests, this study investigated the impact of the geometry defects of the samples, of the sensitivity of the rheometer used and of the compression speed on the characterization of the material. The uncertainties caused by these biases were then propagated in the proposed method. The proposed method was used to study the evolution of the viscoelastic properties of semihard cheese during ripening. Variability between cheeses proved to be greater than the uncertainty of the proposed method, and no tendency could be established, meaning that the viscoelastic parameters were considered constant during ripening.
59(2015); http://dx.doi.org/10.1122/1.4922486View Description Hide Description
This study was aimed at improving the process rheology of polylactide (PLA) melts by means of two strategies. First, PLAs of different branched structures, i.e., star shaped, comblike, and hyper branched, were synthesized and blended with a linear grade analog. Shear and extensional flow rheometry tests were performed on pure materials and their blends to evaluate their rheological properties. It was shown that the presence of branched poly(L-lactide) (PLLA) increased the shear thinning, shear and extensional viscosity, and elastic modulus of linear PLLA at the same time; the star shaped PLLA providing the most significant change. Second, poly(D-lactides) (PDLA) with similar molecular architectures were synthesized to have a double branching effect. In addition to the presence of branched architecture, physical cross-links due to the stereocomplex formation exist between PLLA and PDLA chains. Based on the rheological characterizations in shear and extensional mode, a greater improvement in PLA melt rheological properties was observed for blends containing stereocomplex structure as compared to linear/branched enantiopure blends.
59(2015); http://dx.doi.org/10.1122/1.4922653View Description Hide Description
Granular materials do not always flow homogeneously like fluids when submitted to external stress, but often form rigid regions that are separated by narrow shear bands where the material yields and flows. This shear localization impacts their apparent rheology, which makes it difficult to infer a constitutive behavior from conventional rheometric measurements. Moreover, they present a dilatant behavior, which makes their study in classical fixed-volume geometries difficult. These features led numerous groups to perform extensive studies with inclined plane flows, which were of crucial importance for the development and the validation of the -rheology. Our aim is to develop a method to characterize granular materials with rheometrical tools. Using rheometry measurements in an annular shear cell, dense granular flows of 0.5 mm spherical and monodisperse beads are studied. A focus is placed on the comparison between the present results and the -rheology. From steady state measurements of the torque and the gap under imposed shear rate and normal force FN , we define an inertial number I. We show that, at low I (small and/or large FN ), the flow goes to a quasistatic limit, and the response in terms of dimensionless stress or internal friction coefficient—μ—and solid concentration—ϕ—profiles is independent of the inertial number. Upon increasing I (large and/or small FN ), dilation occurs and ϕ decreases while μ increases. The observed variations are in good agreement with previous observations of the literature [Jop et al., Nature 441, 727–730 (2006) and Hatano, Phys. Rev. E 75, 060301 (R) (2007)]. These results show that the constitutive equations and of granular materials can be measured with a rheometer.
59(2015); http://dx.doi.org/10.1122/1.4922851View Description Hide Description
Various algorithms for determining relaxation time spectrum have been to the fore, because relaxation spectrum plays an important role in interconversion between various viscoelastic functions. In this study, a new algorithm is developed by means of double-logarithmic power series approximation and the Levenberg–Marquardt method. Compared with most previous algorithms, new algorithm is applicable not only to dynamic data but also to the Laplace transform of relaxation modulus, which is available for optical data from the generalized Stokes–Einstein equation or creep data with ringing. It is worth noticing that the new algorithm determines continuous relaxation spectrum without any additional parameters and precedent procedures. This paper suggests such framework of newly developed algorithm and its applications to various kinds of viscoelastic data.
59(2015); http://dx.doi.org/10.1122/1.4922795View Description Hide Description
The scaling relations established for the relaxation modulus of concentrated solutions of polystyrene (PS) in oligomeric styrene [Wagner, Rheol. Acta 53, 765–777 (2014); Wagner, J Non-Newtonian Fluid Mech. (2015)] are applied to the solutions of PS in diethyl phthalate (DEP) investigated by Bhattacharjee et al. [Macromolecules 35, 10131–10148 (2002)] and Acharya et al. [AIP Conf. Proc. 1027, 391–393 (2008)]. The scaling relies on the difference ΔTg between the glass-transition temperatures of the melt and the glass-transition temperatures of the solutions. ΔTg can be inferred from the reported zero-shear viscosities, and the Baumgaertel, Schausberger, and Winter (BSW) spectra of the solutions are obtained from the BSW spectrum of the reference melt with good accuracy. Predictions of the extended interchain pressure (EIP) model, which is based on the assumption that the relative interchain pressure in the melt and in the solutions is identical, are compared to the steady-state elongational viscosity data of PS/DEP solutions. The Rouse stretch relaxation times as calculated from the respective scaling relation are a factor of 2–3 higher than the Rouse time defined by the experimentally observed upturn of the elongational viscosity at . This may be caused by DEP being a good solvent at room temperature and/or polymer degradation. Using the experimentally determined Rouse times, quantitative agreement is obtained between experimental data and model. Except for a possible influence of solvent quality, linear and nonlinear viscoelasticity of entangled PS solutions can thus be obtained from the linear-viscoelastic characteristics of a reference polymer melt and the shift of the glass-transition temperature between melt and solution.