Ruifeng Liang

Rheological characterization of the time and strain dependence for polyisobutylene solutions

Abstract The rheological response of polyisobutylene (PIB) solutions in Decalin and a related standard fluid S1 has been characterized in dynamic oscillatory flow, step strain, step-shear rate and steady shear using a Rheometrics RDSII rheometer. The time dependence represented as a discrete spectrum of relaxation times and the strain dependence characterized as an exponential damping function have been presented as a function of PIB concentration. The relaxation spectrum was calculated from the dynamic storage modulus and loss modulus. The damping function was determined from the non-linear relaxation modulus in a step-strain experiment. The Wagner integral viscoelastic model incorporated with the relaxation and the damping function has been used to predict the stress growth and the steady-shear behaviour, which were compared with the experimental data. A novel extensional rheotester was also used in this study to measure the stretching response of polymer solutions. The data gave a near single relaxation time for each solution, and this single relaxation time obtained from uniaxial extension was correlated to the relaxation spectrum obtained in simple shear.

The experimental observation and numerical prediction of planar entry flow and die swell for molten polyethylenes

We report experimental observations and matching numerical simulations for the planar entry flow and die swell of two high-density polyethylenes (HDPEs) and one low-density polyethylene (LDPE). Experimental data for stress fields, centreline velocities and die swell are reported for each polymer. These results are compared with numerical simulation. The materials are characterized in simple shear using a Wagner integral constitutive equation with a discrete spectrum of relaxation times and a single parameter damping function. The numerical simulation has been carried out using a finite element software package, Polyflow. Self consistency in the stress and die swell data are found for one HDPE, but the other HDPE and the LDPE show an extensional strain hardening response which is not predicted using the simple shear rheology data. In the latter cases, the numerical predictions consistent with entry flow experimental observations can be achieved if extensional flow damping parameters, rather than simple shear damping parameters, are chosen. For the LDPE, an increase in the strain hardening parameter results in the numerical prediction of upstream recirculation vortices in the entry region, which qualitatively agrees with the experimental observations. Apparent inconsistencies in the absolute values of measured and simulated velocity profiles are explained in terms of the 2D nature of the simulation and a 3D component to the experimental flow.

Electrorheological properties of semiconducting polymer‐based suspensions

A new type of semiconducting polymer (oxidized polyacrylonitrile)‐ based electrorheological (ER) suspension is described which shows reasonable ER effects, especially at high temperatures. Rheological properties, such as yield stress, transient stresses, flow curve, dynamic modulus as functions of electric field, particle concentration, water content and temperature, are measured with a modified Rheometrics mechanical spectrometer. The characteristic ER responses in various deformation histories are found to be described approximately by a rate insensitive stress, which can be yield stress, steady shear stress, the recovery stress after cessation of flow, or the product of dynamic modulus and strain amplitude. This stress is dominated only by the electrically induced interparticle forces. These results can be well explained using a spanned strands model.

The gas-assisted extrusion of molten polyethylene

This paper reports for the first time our preliminary findings of a new gas-assisted extrusion process. We have discovered that if gas is injected at a metal die/molten polymer interface at a low flow rate, it is possible to establish a stable gas layer at the interface, which can give rise to an essentially full slip wall boundary condition. We report experimental optical observations, flow birefringence data, pressure difference, and die swell data for both a slit and rod geometry extrusion. We also match some of the experimental results with a viscoelastic numerical simulation. The introduction of wall slip induced by the presence of the gas layer has a profound effect on the magnitude of the die swell observed for polyethylene processed using gas-assisted extrusion. The experiments demonstrate, without ambiguity, that wall boundary conditions can play a crucial role in the overall extrusion flow of high viscosity viscoelastic fluids, such as polyethylene.