Volume 50, Issue 6, November 2006
Index of content:
New capillary rheometer allowing for small-angle x-ray scattering experiments inside the die. Application to the extrusion of block copolymers, their macroscopic defects, and their structure50(2006); http://dx.doi.org/10.1122/1.2279527View Description Hide Description
Extrusion rates are often limited by the surface cracks originating at the die exit. These cracks can be so severe in the case of certain polymers that the phenomenon known as “flow split” occurs. Previous work by the same authors explains the macroscopic mechanisms leading to flow split observed during the extrusion of polystyrene-block-poly(ethylene-co-buylene)-block-polystyrene block copolymers in their microphase separated state. The present paper looks at the effects of extrusion on the structure of these block copolymers at regimes showing the different characteristic defects of the fluids. The structure of extruded samples is examined by small-angle x-ray scattering (SAXS). A comparison between the relaxation times characteristic of the copolymers and the time necessary to prepare the samples shows that quenching of samples the flow of which is stable at the die exit is practically impossible with a standard capillary rheometer. Hence, in situ experiments inside the die are necessary to understand the structure changes occurring during the extrusion process. For this reason, a new capillary rheometer equipped with a beryllium die, transparent to x rays, is developed and preliminary tests are performed. The results validate the design and show the feasibility of on-line experiments. Moreover, an interpretation that relates the macroscopic defects and structure observed with SAXS experiments is proposed.
Hydrophobic interactions in associative polymer/nonionic surfactant systems: Effects of surfactant architecture and system parameters50(2006); http://dx.doi.org/10.1122/1.2355530View Description Hide Description
The rheological behavior of a comb-like associative polymer with pendant hydrophobes in the presence of nonylphenol polyethoxylate (NPe) nonionic surfactants is examined. In particular, the effects of surfactant concentration, polymer concentration, and surfactant’s hydrophilic–lipophilic balance (HLB) are investigated with the latter being varied by using surfactants with different number of ethoxylate units (, 8, 12, and 15). Steady shear and dynamic measurements are performed and the zero/low shear viscosity, the number and average life time of the hydrophobic junctions, and the numbers of polymerhydrophobes and NP8 surfactant molecules per hydrophobic junction are determined. The trend of zero/low shear viscosity with increasing NP8 surfactant concentration exhibits four distinct regions, each of them related to molecular-level changes: (1) surfactant addition to the hydrophobic junctions (no change in from that of pure hydrophobically modified alkali–soluble emulsion polymer solution); (2) formation of mixed micelles containing surfactants and several polymerhydrophobes ( increases); (3) saturation of each polymerhydrophobe by individual surfactantmicelles ( decreases); and (4) formation of free surfactantmicelles ( is constant). The viscosity trend of the polymer/NP8 surfactantsystems with increasing surfactant concentration does not change upon increasing polymer concentration from to . Moreover, the number of surfactant molecules bound to polymerhydrophobes at the viscosity maximum is the same for both polymer concentrations. In terms of the effects of surfactant HLB on polymer/NPe surfactantssystems, we find essentially similar trends of zero/low shear viscosity with increasing surfactant concentration for systems containing surfactants of higher HLB (NP12 and NP15). However, the viscosity maximum is lower, and viscosity in the fourth region gradually decreases for systems containing higher HLB surfactant. In contrast, systems containing a surfactant with lower HLB (NP6) exhibit a different behavior with a continuous increase in zero/low shear viscosity with increasing surfactant concentration.
50(2006); http://dx.doi.org/10.1122/1.2357595View Description Hide Description
We investigate the concentration dependence of the characteristic relaxation time of dilute polymer solutions in transient uniaxial elongational flow. A series of monodisperse polystyrene solutions of five different molecular weights with concentrations spanning five orders of magnitude were dissolved in two solvents of differing solvent quality (diethylphthalate and oligomeric styrene). Optical measurements with a capillary breakup extensional rheometer of the rate of filament thinning and the time to breakup in each fluid are used to determine the characteristic relaxation time. A criterion for a lower sensitivity limit is introduced, in the form of a minimum concentration necessary for experimental resolution of the effects of polymeric viscoelasticity. This criterion is validated by experiment and comparison to numerical calculations with a multimode bead-spring dumbbell model. These calculations also rationalize previous paradoxical observations of extensional thinning in fluid threads of ultradilute polymer solutions in which stress relaxation apparently occurred faster than predicted by the Zimm theory. Above this minimum sensitivity limit we show that the effective relaxation time of moderately dilute solutions in transient extensional flow rises substantially above the fitted value of the relaxation time extracted from small amplitude oscillatory shear flow and above the Zimm relaxation time computed from kinetic theory and intrinsic viscosity measurements. This effective relaxation time exhibits a power-law scaling with the reduced concentration and the magnitude of the exponent varies with the thermodynamic quality of the solvent. The scaling of this “self-concentration” effect appears to be roughly consistent to that predicted when the dynamics of the partially elongated and overlapping polymer chains are described within the framework of blob theories for semi-dilute solutions.
50(2006); http://dx.doi.org/10.1122/1.2357190View Description Hide Description
We describe the design and construction of a new microrheometer designed to facilitate the viscometric study of complex fluids with very small sample volumes and gaps of micrometer dimensions. The flexure-based microgap rheometer (FMR) is a shear-rate-controlled device capable of measuring the shear stress in a plane Couette configuration with directly controlled gaps between 1 and . White light interferometry and a three-point nanopositioning stage using piezo-stepping motors are used to control the parallelism of the upper and lower shearing surfaces, which are constructed from glass optical flats. A compound flexure system is used to hold the fluid sample testing unit between a drive spring connected to an “inchworm” motor and an independent sensor spring. Displacements in the sensing flexure are detected using an inductive proximity sensor. Ready optical access to the transparent shearing surfaces enables monitoring of the structural evolution in the gap with a long working-distance video microscope. This configuration then allows us to determine the microgap-dependent flow behavior of complex fluids over 5 decades of shear rate. We demonstrate the capability of the FMR by characterizing the complex stress and gap-dependent flow behavior of a typical microstructured food product (mayonnaise) over the range of gaps from and stresses from . We correlate the gap-dependent rheological response to the microstructure of the emulsion and changes induced in the material by prolonged shearing.
50(2006); http://dx.doi.org/10.1122/1.2351880View Description Hide Description
This article investigates how rheological properties of polypropylenes with different molecular structures influence their foaming behavior. The molecular structure of the different polypropylenes is analyzed by size exclusion chromatography coupled with a light scattering detector, and by rheological means, such as the molar mass dependence of the zero shear-rateviscosity. The main focus of the rheological experiments is laid on the strain hardening and failure behavior of the melts in uniaxial elongational flow. For all linear polypropylenes investigated, a rupture of samples occurred before the maximum strain—accessible with the instrument used—was achieved. For the linear polypropylenes, a growth of the stress to rupture and the elongation at break were found with increasing molar mass, which go along with an increase of the expansion ratio in the foaming experiments. Besides the linear polypropylenes, which do not show strain hardening, several so-called high melt strength polypropylenes were investigated. It was found that the strain hardening of those polypropylenes causes not only a high melt strength, but also a more uniform sample deformation in the elongational experiments, and thus high elongations at break or even no rupture of the samples. Due to the superior homogeneity of deformation, the foams of polypropylenes, which show pronounced strain hardening at the strain rates relevant for foaming, possess higher expansion ratios than linear samples of the same melt strength. Furthermore, it was found that the foaming behavior of the polypropylenes is governed by the elongational behavior of the melts independently of the molecular constitution, which is responsible for it. This means that different molecular structures, which cause a similar elongational behavior of the melt, lead to a comparable foaming behavior.
Effect of configuration-dependent intramolecular hydrodynamic interaction on elastocapillary thinning and breakup of filaments of dilute polymer solutions50(2006); http://dx.doi.org/10.1122/1.2357592View Description Hide Description
We use a new constitutive model for the polymer stress in a dilute polymer solution to predict elastocapillary thinning and breakup of a thin filament of the solution. The constitutive model accounts for the effects of finite chain extensibility and configuration-dependent intramolecular hydrodynamic interaction, and is used in the simple stress balance equation proposed by Entov and Hinch [Entov, V. M., and E. J. Hinch, J. Non-Newtonian Fluid Mech.72, 31–53 (1997)] for situations where inertial effects are negligible. In their seminal study, Entov and Hinch showed that during the period where the elasticpolymer stresses are dominant, the filament radius decreases exponentially with time. We find that configuration-dependent hydrodynamic interactions cause the time constant in this exponential decay to depend on concentration, as observed in recent experiments. Moreover, the phenomenon of coil-stretch hysteresis permits a large polymer stress even though the transient Weissenberg number during elastocapillary thinning decreases below the critical value of for the coil-to-stretch transition of a dilute polymer solution in an extensional flow. As a consequence, the Weissenberg number does not have a lower bound of as predicted originally by Entov and Hinch using a simpler constitutive model that does not account for configuration-dependent hydrodynamic interactions.
Characterization of sparsely long chain branched polycarbonate by a combination of solution, rheology and simulation methods50(2006); http://dx.doi.org/10.1122/1.2357188View Description Hide Description
We characterize sparsely and randomly branched polycarbonate and fractions thereof by a combination of solution and rheological techniques, and we compare the observations with the results of Monte Carlo simulations as well as tube-based modeling. On-line coupling of size exclusion chromatography with intrinsic viscosity (SEC-IV) yields values for the viscosity branching index , which deviate significantly from linear behavior above approximately 0.3 branches per chain. The experimental also follows closely the predictions for the geometric branching factor calculated by Monte Carlo simulations of the polymer structure across the entire molar mass distribution . The Monte Carlo simulations provide an explicit description of the molecular architectures present and can therefore in principle be used to test models suitable for sparsely branched polymers. Experimental dynamic moduli of unfractionated and fractionated samples are compared with predictions of a tube-based model recently published in van Ruymbeke et al. [J. Non-Newt. Fluid Mech.128, 7–22 (2005b)]. When the model is calibrated for polydisperse linear polycarbonate, discrepancies between the predicted and measured dynamic moduli are clearly observed at levels well below the detection limit of branching by the SEC-IV coupling method. The comparison between observed and predicted moduli is therefore a very sensitive method for the detection of sparse long chain branching in poorly and moderately entangled polymers, of which polycarbonate is a good example.
50(2006); http://dx.doi.org/10.1122/1.2337259View Description Hide Description
We carried out systematic creep tests after different times of rest and over sufficiently long times with pasty materials of various internal structures in a Couette geometry. From an analysis of the data taking into account the inertia of the system and the heterogeneous distribution of stress, we show that: (i) for a stress below the yield stress these materials remain solid but undergo residual, irreversible deformations over long time which exhibit some trends typical of aging in glassy systems; (ii) as a result of thixotropy (or aging) in the solid regime the elastic modulus increases logarithmically with the time of rest; (iii) in the liquid regime the effective behavior of the material can be well represented by a truncated power-law model; (iv) a fundamental parameter of the solid-liquidtransition is a critical effective shear rate (associated with the yield stress) below which the material cannot flow steadily.
Effect of charged polyelectrolytes on the electrophoretic behavior, stability, and viscoelastic properties of montmorillonite suspensions50(2006); http://dx.doi.org/10.1122/1.2355653View Description Hide Description
This work is devoted to the study of the rheological properties of sodium montmorillonite suspensions in aqueous media containing polyelectrolytes in solution. Two different polyelectrolytes are employed: polyacrylic acid (PAA) and polyethyleneimine (PEI). PAA can bear negative charge, thus acting as a polyanion, while PEI can be considered as a polycation, although the charge of both polymers is strongly dependent on of the solution. The rheological behavior of clay suspensions is essentially determined by the electric potential of the faces and edges of the laminar clay particles. In order to analyze the changes in the interfacial electric potential of clay surfaces, the zeta potential of clay particles was estimated from electrophoresis measurements for different solution compositions. The yield stress and the storage modulus of the suspensions were determined demonstrating that only in some cases the storage modulus can be correlated with the changes in electrostatic interactions between particles. In particular, in clay/PEI suspensions at neutral-basic the changes in the viscoelastic properties do not match with those in surface-to-surface electrostatic interactions. Different mechanisms are proposed to explain the wide variety of rheological phenomena observed.
50(2006); http://dx.doi.org/10.1122/1.2358724View Description Hide Description
Associating polymers are hydrophilic long-chain molecules containing a small amount of hydrophobic groups (hydrophobes), and behave as flocculants in aqueous suspensions. The effects of associating and nonassociating polymers with molecular weights of about on the rheological behavior are studied for silica, latex, and mixed suspensions. Because the hydrophobes adsorb onto hydrophobic surfaces and water-soluble chains onto hydrophilic surfaces, two single suspensions are highly flocculated by a bridging mechanism. The complex suspensions which are prepared by mixing the silica and latex suspensions with the associating polymer at the same concentration show a drastic reduction in viscosity. Since the long chains of associating polymer attach to the surface at many points, the adsorption onto silica surfaces is dominant in the complex suspensions. The hydrophobes extending from the chains adsorbed onto silica particles can adsorb onto latex particles. The associating polymer acts as a binder and the hetero-flocculation between silica and latex is induced. The formation of composite particles in which the silica particles are covered with latex particles is primarily responsible for the drastic decrease in viscosity.