Morton M. Denn

Disappearance of extrusion instabilities in brass capillary dies

We have extruded a linear low-density polyethylene through capillary dies fabricated from stainless steel and brass. We confirm a result first reported by Ramamurthy [Ramamurthy, A. V., “Wall Slip in Viscous Fluids and Influence of Materials of Construction,” J. Rheol. 30, 337–357 (1986)]: sharkskin can be eliminated by the use of a brass die. We also find a substantially enhanced throughput from the brass die relative to the stainless-steel die at stresses in the range where sharkskin is observed with the latter. Finally, the large pressure oscillations and periodic extrudate distortions observed in the “slip–stick” region with stainless steel are absent with brass, where the transition to the “upper branch” of the flow curve is more gradual.

The effect of die materials and pressure-dependent slip on the extrusion of linear low-density polyethylene

The flow of linear low-density polyethylene through stainless-steel slit dies occurred at shear rates approximately 12% higher than in identical α-brass dies at the same wall shear stresses, indicating near-wall slip. The flow curves were independent of gap spacing. We show through the slip theory of Hill and co-workers [J. Rheol. 34, 891–918 (1990)] that a measurable gap dependence of the flow curve is not a necessary consequence of wall slip; the flow curves for both stainless steel and α-brass dies can be fit with the same rheological parameters, with a difference in the work of adhesion accounting for the differences in the flow curves. X-ray photoelectron spectroscopy revealed differences in the chemistry of brass surfaces with different pretreating, corresponding to small differences in flow curves. Fluorocarbon-coated die surfaces showed no more slip than stainless steel, while the flow curve with gold-coated surfaces followed stainless steel at intermediate stress and brass at high stress.

Surface mobility and slip of polybutadiene melts in shear flow

Surface mobility and wall slip of entangled polybutadiene melts were studied with attenuated-total-reflectance infrared spectroscopy at stresses characteristic of the sharkskin, spurt, and melt-fracture regimes. Small-scale slip, accompanied by an apparent decrease in transverse mobility, occurs in the sharkskin regime, but at a stress above the visual onset of sharkskin in capillary viscometry. Simulations cannot distinguish between a cohesive mechanism and a lubrication mechanism that might follow from a stress-induced phase transition, but an adhesive failure seems to be excluded. The near-surface length scale is of the order of four to six times the equilibrium root-mean-square end-to-end distance, and the estimated slip velocity is insensitive to molecular weight. Strong slip occurs in the spurt regime, either at the wall or within one radius of gyration. Substantial apparent slip occurs with a fluorocarbon surface, but the mechanism does not appear to be an adhesive failure; there seems to be a substantial decrease in the friction coefficient of chains over a distance of order 300 nm or more from the fluorocarbon surface, and the transverse chain mobility in this region appears to be enhanced rather than retarded. Overall, the results of this study indicate that the influence of the wall extends farther into the sheared melt than would be expected from the chain dimensions, except in the case of strong slip.

Spatial development of director orientation of tumbling nematic liquid crystals in pressure-driven channel flow

The Leslie-Ericksen equations in the tumbling regime were solved for developing flow between parallel plates to study the two-dimensional spatial development of director orientation. Calculations were carried out using the physical properties of 8CB. Texture development is governed by the number of director rotations required to reach the equilibrium distribution for fully-developed flow, with complex textures developing at large values of both the tumbling parameter (e) and the Ericksen number (Er). The entry length for structure for a given value of e is a non-monotonic function of Er, and entry lengths of >40 can occur for the parameter range studied here when Er is near a value corresponding to a discontinuous jump in the number of rotations required to reach equilibrium.

Rheology of a viscoelastic emulsion with a liquid crystalline polymer dispersed phase

The steady-shear viscosity and first normal stress difference and the dynamic storage and loss moduli have been measured for a blend consisting of a thermotropic liquid crystalline polymer dispersed in a thermoplastic fluoropolymer matrix. The components are immiscible and nonreacting. Consistency with the Palierne emulsion theory for viscoelastic blends is possible if and only if the interfacial tension contribution is negligible for droplets that are comparable in size to a liquid crystalline domain or smaller, while retaining the effect for larger droplets. Steady shear results are approximately described by the scaling of the Doi–Ohta theory, but there is a significant reduction in the excess shear stress over a finite shear-rate range for the lowest concentration, which contains the smallest droplets.

A mesoscopic theory of liquid crystalline polymers

A kinetic equation for polydomains is used to develop a formal structure for a mesoscopic constitutive theory of textured liquid crystalline polymers. The lowest-order approximation for the texture terms leads to the Reference 1991 theory, but with a different meaning for the model parameters. Predicted macroscopic order parameters are in the range observed experimentally.

Squeeze flow between finite plates

Abstract The general solution for squeeze flow between closely-spaced plates of arbitrary shape includes an in-plane potential flow whose components satisfy the Cauchy–Riemann conditions, and the velocity field and the pressure are both determined by the boundary conditions at the edge of the plates. In contrast, the velocity field for the infinite-plate limit only requires boundary conditions at the surfaces of the plates. The infinite-plate problem is singular, and makes sense only as a limit of a sequence of flows in finite geometries, each of which has a well-defined coordinate origin.

Visualization of the flow of a thermotropic liquid crystalline polymer in a tube with a conical contraction

Abstract Flow visualization experiments have been used to characterize the fluid dynamics of a thermotropic liquid crystalline polymer, Vectra A, in the barrel and conical entry of a capillary rheometer. The displacement profiles indicate shear-thinning in the straight section, with a power law index much lower than that measured rheometrically. The contraction flow is complex and three-dimensional. There are asymmetrical vortices and signs of macroscopic structural changes. For flow in a corner, the profiles display patterns that cannot be predicted by the shear-thinning effects alone. These characteristics are in striking contrast to the results on lyotropic solutions in thin slit geometries (see J.N. Baleo and P. Navard, J. Rheol., 38 (1994) 1641 – 1655, and B.D. Bedford and W.R. Burghardt, J. Rheol., 38 (1994) 1657 – 1679).

Technical note: Strain‐measurement error in a constant‐stress rheometer

We describe an error in the strain measurement capability of our Rheometrics Stress Rheometer (RSR). This problem may be common to other constant stress rheometers, and calibrations should be carried out to enable compensation in data analysis. With the RSR a shear stress is applied by a rotating fixture and the resulting shearing motion is measured and recorded