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Title: Cross-slot rheology of polymers
Author: Coventry, K. D.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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This thesis describes a novel experimental system for the study of polymer flow in a cross-slot using a Cambridge Multi-Pass Rheometer (MPR). Cross-slot flow is capable of generating pure rotation-free extensional flow and the rheological information from the MPR is in a format which can easily be compared with the results of numerical simulations. Using only a small quantity of polymer, this technique provides a quantitative test of the performance of constitutive equations and numerical solvers in extensional flow. The cross-slot apparatus was developed and experiments were studied principally using optical birefringence. It was found that the different shapes of the birefringence patterns observed reveal useful information about the rheology and molecular structure of the polymers. For unbranched low molecular weight polymers at low extension rates, an almost Newtonian response was observed, generating a symmetric fringe pattern. as the level of branching, the molecular weight or the extension rate increased, the fringe pattern became more elongated, suggesting stronger viscoelastic effects. For highly branched polymers a very localised stress concentration was observed along the exit symmetry plane. Pioneering experiments with monodisperse polymer have been particularly revealing – a flow transition is observed at extension rates approximately equal to the inverse of the relaxation time. At low flowrates the response was almost Newtonian, but for extension rates above the inverse of the relaxation time a non-homogeneous stress pattern was observed suggesting possible melt fracture of the fluid in the continuum. The experiments have been compared with numerical simulations using generalised Newtonian, integral-Wagner, Rolie-Poly and pom-pom constructive equations. It has been found that the viscoelastic numerical simulations revealed many of the important features observed from the experiments. However, the quantitative value for stress predicted in some case was higher than the measured value.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available