Volume 49, Issue 3, May 2005
Index of content:
Study of the stick-slip phenomenon of linear low-density polyethylene in a brass die by using electrical measurements49(2005); http://dx.doi.org/10.1122/1.1888645View Description Hide Description
Extrusion through brass dies has been found to be useful to eliminate sharkskin and stick-slip instabilities in linear low-density polyethylene. However, the physicochemical interaction of polyethylene melts with brass dies is not well understood. In the present work, such an interaction is analyzed by using electrical measurements. Experiments were performed in a single screw extruder at a temperature of with a metallocene linear low-density polyethylene and a brass die. The interaction between the polymer melt and the die changed with time. Different flow curves were obtained as the extrusion time was increased to reach an invariant flow curve. An irreversible transition from slip to no-slip at the die wall under stable flow was observed, consistent with a change in the sign of the measuredelectric charge. The critical shear stress for the onset of the stick-slip also increased with the extrusion time. Electrical measurements reproduced the stick-slip dynamics with fidelity. Pressure oscillations were accompanied by in-phase variations of electrical charge during the stick-slip, from which a cohesive failure is suggested for slip in this flow regime. For long extrusion times, a new phenomenon was observed, which is the disappearance of pressure and electric charge oscillations, but in the presence of a nonmonotonic flow curve. It is suggested that the density of adsorbed chains at the wall increases with time because of contamination of the die wall, ending in a “dry brush” regime that suppresses pressure oscillations and produces a true plateau in the flow curve.
Measuring the transient extensional rheology of polyethylene melts using the SER universal testing platform49(2005); http://dx.doi.org/10.1122/1.1896956View Description Hide Description
We use a new extensional rheology test fixture that has been developed for conventional torsional rheometers to measure the transient extensional stress growth in a number of different molten polyethylene samples including a linear low density polyethylene (Dow Affinity PL 1880), a low density polyethylene (Lupolen 1840H), and an ultrahigh molecular weight polyethylene (UHMWPE). The transient uniaxial extensional viscosity functions for the linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) samples have both been reported previously in the literature using well-established instruments and this allows us to benchmark the performance of the new test fixture. Transient stress growth experiments are carried out over a range of Hencky strain rates from 0.003 to and the data show excellent agreement with the published material functions. At deformation rates greater than a true steady state extensional viscosity is not obtained in the LDPE samples due to the onset of necking failure inthe elongating strips of polymer; however, the limiting values of the transient extensional viscosity at the onset of sample failure agree well with previously published values for the steady state extensional viscosity. This apparent steady-state extensional viscosity first increases with deformation rate before ultimately decreasing as approximately . In addition we perform extensional step-strain measurements at small Hencky strains and demonstrate good agreement with the relaxation modulus obtained from shear rheometry. Extensional creep measurements are performed over a range of constant imposed tensile stresses and also agree well with the measured shear creep compliance. Finally, tensile stress relaxation experiments are carried out after a range of imposed Hencky strains. These tests demonstrate that following large extensional deformations the tensile stresses relax nonlinearly and also that, beyond a critical strain, the material is unstable to viscoelastic necking and rupture. Additional transient extensional stress growth measurements using highly entangled linear UHMWPE samples show greatly reduced strains to failure, that are in agreement with the predictions of the Considere theory.
49(2005); http://dx.doi.org/10.1122/1.1896957View Description Hide Description
Polylactides (PLAs) have been known for several decades and have recently gained considerable commercial significance. This development makes it highly desirable to have the rheological properties of these materials well characterized and reduced to useable mathematical models. However, comprehensive rheological characterization of PLAs is not yet available from the literature. In this study, rheological and thermal measurements were made on a comprehensive and well-characterized set of homopolymers and copolymers spanning wide ranges of molecular mass and stereoisomer proportions (L content). For all compositions within the weight average molecular weight range of and a reference temperature of 180 °C, the zero shear viscosity is described by the relationship , the plateau modulus is , and average WLF parameters are and ; the later correspond to a Vogel temperature of 288.25 K. The values of the glass transition temperatures at infinite molecular weight for 100%, 80%, and 50% L content are 60.2, 56.4, and 54.6 °C, respectively. Based upon a chain packing model, molecular parameters determined include a packing length of 2.51 Å, a tube diameter of 47.7 Å, and a characteristic ratio of , independent of stereoisomeric composition. The critical molecular weight for entanglement, , is found to be near while the molecular weight between entanglements, , is near . Reasons for accepting these values over previous literature studies are given; the available data imply linear polylactides are typical glass-forming polymers.
49(2005); http://dx.doi.org/10.1122/1.1879043View Description Hide Description
The apparent slipflows of incompressible and viscoplastic (Herschel–Bulkley) fluids in plane Couette, capillary, and rectangular slit dies under fully developed, isothermal, and creeping flow conditions were analyzed assuming that the apparent slip layer consists solely of the binder and its thickness is independent of the flow rate. Both the drag-induced (plane Couette) and pressure-induced (capillary and slit) flows generate the same dependencies of the wall-slip velocity on the wall shear stress. Navier’s slip coefficient, which relates the wall-slip velocity to the shear stress, is similar for all three flows and is a function of the thickness of the apparent slip layer and the shear viscosity of the binder. The assumed apparent slip mechanism provides methodologies for the determination of the slip velocity values that are consistent with the traditional Mooney method and furthermore allows the determination of the true shear rate of the suspension at the wall and the yield stress. The analysis of the slip data of various concentrated suspensions of rigid particles reveals that, as a first approximation, the apparent slip layer thickness is related to the particle diameter and the ratio of the volume loading level over the maximum packing fraction of the particles.
49(2005); http://dx.doi.org/10.1122/1.1879040View Description Hide Description
We perform systematical viscometric investigations on the dilute solutions of a model electroluminescent conjugated polymer, poly(2-methoxy-5-(-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV). Results on low-shear viscosity measurement reveal markedly close correspondence between viscometric and aggregation properties. Peculiar viscometric features of MEH-PPV solutions include (1) the polymer contribution to the solutionviscosity exhibits a slow yet persistent decrease during constant-temperature aging, and (2) the same quantity exhibits thermal irreversibility as the solution has been subjected to a short-term thermal treatment. These viscometric features and corresponding aggregation properties are bridged using kinetic-theory/thermodynamic arguments and molecular dynamics simulations. Primary implications as to the effects of the choice of solvent,solvent quality, concentration, aging, and short-term thermal treatment on the aggregation properties agree fairly well with recent optical observations. Overall, the present study suggests that rheological characterizations may be very useful for gaining additional insights into the aggregation properties for conjugated polymer solutions.
49(2005); http://dx.doi.org/10.1122/1.1895799View Description Hide Description
The transient structural evolution of polystyrene colloidalgels with fractal structure is quantified during start-up of steady shear flow by time-resolved small-angle light scattering and rheometry. Three distinct regimes are identified in the velocity-gradient plane: structural orientation, network breakup, and cluster densification. Structural anisotropy in the first regime is a universal function of applied strain. Flow cessation in this regime shows a lack of structural relaxation for , where Pe is the Peclet number. In the second regime, the anisotropy attains a maximum value before monotonically decreasing. The volume fraction dependence of the critical strain for maximum anisotropy follows the scaling: . Here and are the backbone and cluster fractal dimensions, respectively. This scaling agrees with the simple model of a gel network that ruptures after the cluster backbone is extended affinely to its full length. Rheological measurements demonstrate that the maximum anisotropy coincides with a maximum in shear stress. Qualitative differences between the transient anisotropy of fractalgels comprised of spheres and rods support the conclusion that the microstructural origin of the anisotropy maximum in sphere aggregates is the free rotation of singly connected regions of the gel backbone.
49(2005); http://dx.doi.org/10.1122/1.1879042View Description Hide Description
Kannan et al. [J. Rheol.46, 979–999 (2002)] studied the problem of fiber spinning for polymers that are largely atactic in nature, within a thermodynamic framework developed by Rao and Rajagopal [Z. Angew. Math. Phys.53, 365–406 (2002)], where significant crystallization does not take place. Here, we develop a model within the same framework that can take into account flow-induced crystallization and the anisotropy of the crystalline part of the semicrystalline polymer. We find that the predictions of the model agree very well with experimental data.
49(2005); http://dx.doi.org/10.1122/1.1879041View Description Hide Description
The slump test, originally used to determine the “workability” of fresh concrete, has since been used in many industrial fields (e.g., mining and food industries). It offers a quick and easy way to measure the yield stress of suspensions or pasty materials. The model used for estimating the yield stress from the measured conical slump was first written by Murata [Mater. Struct.98, 117–129 (1984)], corrected by Schowalter and Christensen [J. Rheol., 42, 865–870 (1988)] and adapted for a cylindrical geometry by Pashias [J. Rheol.40, 1179–1189 (1996)]. However, a discrepancy between experimental and predicted slumps still appears in the case of conical slumps [Clayton et al., Int. J. Miner. Process.70, 3–21 (2003)] and for high-yield stress materials. In the present paper, we extend the theoretical analysis of this simple practical test by including different flow regimes according to the ratio between the radius and the height of the slumped cone. We propose analytical solutions of the flow for two asymptotic regimes, namely and . We finally compare the predictions of these solutions in terms of yield stress and previous expressions to three-dimensional numerical simulations and experimental data on a large range of yield stresses. This makes it possible to clarify the field of validity of the different approaches and provide further practical tools for estimating the yield stress of coarse materials.
The rheology and microstructure of acicular precipitated calcium carbonate colloidal suspensions through the shear thickening transition49(2005); http://dx.doi.org/10.1122/1.1895800View Description Hide Description
The shear rheology and shear-induced microstructure of poly(ethylene glycol) (PEG)-based suspensions of acicular precipitated calcium carbonate (PCC) particles of varying particle aspect ratio (nominal , 4, 7) are reported. These anisotropic particle suspensions demonstrate both continuous and discontinuous reversible shear thickening with increasing applied shear rate or stress similar to that observed for suspensions of spherical colloidal particles. The critical volume fraction for the onset of discontinuous shear thickening decreases as the average particle aspect ratio is increased. However, the critical stress for shear thickening is found to be independent of particle anisotropy and volume fraction. Rather, it can be predicted based on the minor axis diameter of the particles and is found to agree with values for near hard-sphere suspensions. Small angle neutron scattering during shear flow (Rheo-SANS) demonstrates that long-axis particle alignment with the flow direction is maintained throughout the range of shear stresses investigated, including the shear thickening regimes for both continuously and discontinuously shear thickening PCC/PEG suspensions. Rheo-SANS and transient rheological experiments indicate that this reversible shear thickening is a consequence of lubrication hydrodynamic interactions and the formation of transient hydroclusters of flow-aligned particles.
49(2005); http://dx.doi.org/10.1122/1.1895801View Description Hide Description
Although the stress of oscillatory shear flow can be decomposed into elastic and viscous parts in the linear regime, it is not yet known how to decompose the stress of a large amplitude oscillatory shear (LAOS) flow. We developed a method of analyzing LAOS data, which decomposes the stress into elastic and viscous components on the basis of a sound mathematical and physical foundation. This method is based on the symmetry of the stress and is a generalization of linear viscoelasticity from the viewpoint of geometry. The proposed method is more powerful than previous methods such as Fourier transform analysis and the Lissajous plot, in that it is more sensitive to the presence of nonlinearities and it is easier to determine nonlinear parameters.
49(2005); http://dx.doi.org/10.1122/1.1888665View Description Hide Description
The mechanical properties of three-dimensionally organized nanocompositematerials composed of nanometer-sized inorganic particles dispersed in micelle cubic crystals is studied rheologically. The influence of parameters including temperature, relative concentrations, and the relative size of particles and the micelles that make up the cubic crystal is explored using oscillatory shear and creep measurements. These parameters have a significant influence on the mechanical properties of the final nanocompositematerials. Additionally, we find that the properties of the samples depend strongly on the temperature profile that is used to form the micelle crystal. Results indicate the importance of particle-template stoichiometry and relative particle size (to the micelle size) on the mechanical properties of the nanocomposites.
The effect of block copolymer architecture on the coalescence and interfacial elasticity in compatibilized polymer blends49(2005); http://dx.doi.org/10.1122/1.1888625View Description Hide Description
The effect of block copolymer architecture on the suppression of dropletcoalescence and on the interfacial elasticity was studied in immiscible blends of polydimethylsiloxane(PDMS) and polyisoprene (PI) with a droplet-matrix morphology. The PDMS-PI diblock copolymers used in this study to compatibilize the blends differ in molecular weight and degree of asymmetry of the blocks. The general Palierne model with an interfacial shear modulus was used to analyze the dynamic measurements performed after different shear histories. It was shown that the coalescence suppression is more effective when the amount of compatibilizer increases and when the overall molecular weight of the block copolymer increases. When comparing the coalescence behavior of a blend and the inverse blend, it was shown that coalescence is suppressed more when the longest block of the block copolymer is located in the matrix. The interfacial relaxation time increases with molecular weight of the blocks for symmetric block copolymers. Asymmetry of the blocks also causes it to increase. A scaling relation is proposed for the interfacial relaxation time of PI/PDMS blends. This master curve is extended for varying viscosity ratios of the blend and with data of a polydimethylsiloxane (PDMS)/polyisobutylene (PIB) blend and a polymethylmethacrylate (PMMA)/polystyrene (PS) blend.