Volume 59, Issue 5, September 2015
Index of content:
59(2015); http://dx.doi.org/10.1122/1.4923405View Description Hide Description
A common approach to prevent or alleviate wall slip in rotational rheometry is to utilize roughened geometries. While this is a helpful strategy, the presence of the surface roughness introduces a fundamentally different boundary condition, as the flow trough the porous structure needs to be accounted for. In the present work, we investigate the use of rough surfaces in rheometry, starting with simple Newtonian fluids. Both structured and randomly structured surfaces are used, i.e., a cross-hatched plate and a sandpaper disk, respectively. We show that the fluid flows within the roughened layer and that the flow in the gap of the rheometer can be modeled as a flow over and through a porous medium. Stokes' equation and Brinkman's equation can be coupled at the interface through a suitable stress boundary condition recently developed in the literature that takes the momentum transfer to both the fluid and the solid substrate properly into account [Minale, Phys Fluids 26, 123102 (2014b)]. The predictions of this new model are compared with the experimental results on Newtonian reference fluids and show excellent agreement for the case of the cross-hatched geometry, and satisfactory agreement for the geometry with sandpaper. The analysis provides a way to correct the apparent viscosity by dealing with the extra dissipation in the rough layer using an extrapolation length.
59(2015); http://dx.doi.org/10.1122/1.4927919View Description Hide Description
There have been a number of studies of a series of branched metallocene polyethylenes (BMPs) made in a solution, continuous stirred tank reactor (CSTR) polymerization. The materials studied vary in branching level in a systematic way, and the most highly branched members of the series exhibit mild strain hardening. An outstanding question is which types of branched molecules are responsible for strain hardening in extension. This question is explored here by use of polymerization and rheological models along with new data on the extensional flow behavior of the most highly branched members of the set. After reviewing all that is known about the effects of various branching structures in homogeneous polymers and comparing this with the structures predicted to be present in BMPs, it is concluded that in spite of their very low concentration, treelike molecules with branch-on-branch structure provide a large number of deeply buried inner segments that are essential for strain hardening in these polymers.
Determination of molecular weight distribution and composition dependence of monomeric friction factors from the stress relaxation of ultrahigh molecular weight polyethylene gels59(2015); http://dx.doi.org/10.1122/1.4928072View Description Hide Description
The stress relaxation of ultrahigh molecular weight polyethylene (UHMWPE) gels in a broad composition range ( ) was studied within the framework of the reptation model. The molecular weight distribution (MWD) was determined at an optimum gel concentration ( ) by regularization analysis of the stress relaxation data. The calculated relaxation moduli [ ] by using MWD evaluated at a certain composition (UHMWPE gels, ) were consistent with the experimental of other compositions in a broad range ( ). That is, the MWD (regularization method) was self-consistent within the framework of the reptation model. The composition dependence of the monomeric friction factor ( ) of the UHMWPE gels was examined by use of the double reptation (DR) model with the determined MWD. The Ferry's free volume model of the molecular weight dependence of can be extended to the composition dependence of . In the low-concentration gels ( ), the experimental relaxed faster than expected by the DR model.
Using viscoelastic properties to quantitatively estimate the amount of modified poly(lactic acid) chains through reactive extrusion59(2015); http://dx.doi.org/10.1122/1.4928071View Description Hide Description
In this study, the dynamic viscoelastic properties of three structurally modified poly(lactic acid) (PLA) samples processed through reactive extrusion (REX) were analyzed. While classical chromatographic and spectroscopic techniques exhibited limited sensitivity to the presence of topological changes, rheological measurements confirmed the presence of nonuniform branched macromolecules, holding sparsely long chain branches. According to the processing conditions used, the flow activation energy and the thermorheologically simple behavior remained roughly unaffected for PLA-REX containing an amount of modified chains up to 24%. Distinctly separated relaxation processes in a broader transition zone were observed in the complex viscosity function (|η*(ω)|) of all PLA-REX samples. The “extended Carreau–Yassuda” model and an extension of the Havriliak–Negami model, proposed in this work, were used to capture the main characteristics of |η*(ω)| experimental data. Both fitted models were inverted to molecular weight distribution (MWD) spectrum using the numerical inversion technique of Shaw and Tuminello, and these were compared with size exclusion chromatography MWDs. It was shown that the resolution of the predicted bimodal MWDs was enhanced when the model used to fit |η*(ω)| data was exempted from the Cox-Merz rule and included a complex time dependence. Based on the MWDs deduced from solely melt measurements, a procedure was described to quantitatively estimate the amount of modified chains.
Rheological characterizations of wormlike micellar solutions containing cationic surfactant and anionic hydrotropic salt59(2015); http://dx.doi.org/10.1122/1.4928454View Description Hide Description
Aqueous micellar solutions of cationic surfactant cetyltrimethylammonium bromide (CTAB) and organic hydrotropic salt 3-hydroxy naphthalene-2-carboxylate (SHNC) in the semidilute regime have been characterized by linear and nonlinear rheology, and dynamic light scattering. The strong hydrophobicity and naphthalene structure present in the SHNC induces significant growth of CTAB wormlike micelles and promotes stable micellar network formation. Focusing primarily on 75 mM CTAB/SHNC solution, we correlate the rich rheological behavior with structural transitions of the micellar network under different deformation histories with temperatures in the range of 20 °C 40 °C. Viscous dissipation dominates at low temperature, while short range interactions among micellar head groups, reorganization of micellar networks play important roles at higher temperatures, leading to complex stress responses under large deformations. The influence of double benzene rings on the response of transient and large amplitude oscillatory shear flows in the system was further elucidated by comparing the rheological behavior of CTAB/SHNC with CTAB/NaSal at the same salt and surfactant concentrations. Our studies distinguished SHNC as a stable hydrotrope in a semidilute cationic surfactant system under thermal variations, with potential applications such as drag reduction and fracturing fluids in oil recovery.
59(2015); http://dx.doi.org/10.1122/1.4929398View Description Hide Description
Various algorithms have been proposed to solve the interconversion equation of linear viscoelasticity when Prony series are used for the relaxation and creep moduli, G(t) and J(t). With respect to a Prony series for G(t), the key step in recovering the corresponding Prony series for J(t) is the determination of the coefficients of terms in J(t). Here, the need to solve a poorly conditioned matrix equation for the is circumvented by deriving elementary and easily evaluated analytic formulae for the in terms of the derivative of the Laplace transform of G(t).
59(2015); http://dx.doi.org/10.1122/1.4928951View Description Hide Description
The rheology of a semidilute dispersion of colloids having short-ranged attractions is studied. A depletion potential is chosen as a model for the attractive interaction and arises from nonadsorbing polymer dispersed with the colloids. The complex viscosity of these materials can be calculated by investigating their response to weak oscillatory shear. A first order expansion in small rates of deformation is used to solve for the microstructure and stress in the dispersion. Additionally, the effect of hydrodynamic interactions is studied via the excluded annulus model, which views the radius at which hard-sphere interactions occur as a barrier that resides beyond the hydrodynamic radius of the particles. This treatment allows a continuous variation of the hydrodynamic interaction strength. The viscoelastic response exhibits a sharp transition when going from weak attraction to strong attraction. Below a critical strength, increasing the interparticle attraction reduces the low frequency viscosity. Strong attractions increase the viscous response and delay the onset of an elastic plateau at high frequencies. An asymptotic analysis shows that the stress response is a result of the interplay of two length scales: The range of attraction and the diffusive boundary layer around particles. Independent of the strength of hydrodynamic interactions, the complex viscosity can be described with high accuracy by two well characterized viscoelastic models: At low frequencies, a Maxwell mode; at high frequencies, the purely repulsive hard-sphere response. Our results demonstrate that beyond hydrodynamic interactions the strength and range of the interaction potential between particles plays a central role in setting the viscoelasticity.
59(2015); http://dx.doi.org/10.1122/1.4929486View Description Hide Description
The rheology and shear-induced structures of a series of self-assembled surfactant wormlike micelles (WLMs) with varying levels of branching are measured using rheo- and flow-small angle neutron scattering (SANS). The degree of branching in the mixed cationic/anionic surfactant (cetyltrimethylammonium tosylate/sodium dodecyl benzene sulfonate) WLMs is controlled via the addition of the hydrotropic salt sodium tosylate and verified by cryo-transmission electron microscopy. The linear viscoelasticity of the low salt (linear) micellar solutions is well described as an extended Maxwell (Oldroyd-B) fluid, and samples exhibit shear banding under steady-shear flow. The linear viscoelasticity of more highly branched solutions deviates from Maxwellian behavior, where the plateau in gradually increases in slope with increasing salt content. The higher salt solutions exhibit a shear thinning regime, followed by a shear thickening regime at high shear rates. Micelle segmental alignment in the flow-gradient plane is a nonmonotonic function of salt level and radial position. Spatially resolved measurements of the segmental alignment corroborate shear banding in the linear WLMs, and the absence of shear banding with branching. Rheo-SANS measurements show that the onset of shear thickening at high rates corresponds to a structural transition. The results of this study link micellar microstructure and topology to the measured shear rheology of WLM solutions.