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The work contained in this thesis is concerned with the flow behaviour of polymer solutions and melts. It is a theoretical study, though it refers extensively to experimental work by others. Calculations are analytic where possible, though some numerical work is required. The principal source is the theoretical model of linear polymer melts developed by Doi and Edwards. The presentation is as follows: Chapter 1 reviews the experimental properties of polymeric liquids and some of the phenonenological schemes used to describe them. Review material extends into chapter 2 which covers molecular theory, the Doi-Edwards model, and developments in the last few years. Chapter 3 applies the model to the 'spurt effect', a well known unstable flow of linear polymer melts. We show that this may be explained quantitatively by the multivalued constitutive behaviour of the model. Chapter 4 examines the stability of the flow field associated with the spurt effect and shows how certain pulsing periodicities in the flow may be related to the 'normal stress' behaviour of the fluid. Chapter 5 is a case study of a more complex branched polymer: the 'H-polymer'. It is shown that both the linear and non-linear viscoelastic response may be calculated in a straightforward extension of the Doi-Edwards model, comparing favourably with experiments to date.

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No / Spectroscopic techniques have the potential to provide powerful, molecular-specific, non-invasive measurements on polymers during melt processing operations. An exploration is reported of the application and assessment of sensitivity of in-process vibrational spectroscopy¿on-line mid-infrared (MIR), on-line near-infrared (NIR), in-line NIR and in-line Raman¿for monitoring of single screw extrusion of high-density polyethylene and polypropylene blends. These vibrational spectroscopic techniques are compared with novel in-line ultrasound velocity measurements, which were acquired simultaneously, to assess the sensitivity of each method to changes in blend composition and to explore the suitability for their use in real time process monitoring and control.

No / The finite element simulation of a selection of two- and three-dimensional flow problems is presented, based upon the use of four different constitutive models for polymer melts (Oldroyd-B, Rolie-Poly, Pom-Pom and XPP). The mathematical and computational models are first introduced, before their application to a range of visco-elastic flows is described. Results demonstrate that the finite element models used here are able to re-produce predictions made by other published numerical simulations and, significantly, by carefully conducted physical experiments using a commercial-grade polystyrene melt in a three-dimensional contraction geometry. The paper also presents a systematic comparison and evaluation of the differences between two- and three-dimensional simulations of two different flow regimes: flow of an Oldroyd-B fluid around a cylinder and flow of a Rolie-Poly fluid into the contraction geometry. This comparison allows new observations to be made concerning the relatively poor quality of two-dimensional simulations for flows in even quite deep channels.

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No / A small-scale flow cell has been developed and used for small-angle neutron scattering (SANS) investigations of polymer melts in Poiseuille flow through a 4:1 contraction. The cell enables the investigation of polymer melt flow subject to a volumetric flow rate of up to 6 cm3 s-1, at pressures up to 10 MPa, temperatures up to 230°C, and a melt viscosity up to 65000 Pas. The cell has recirculating flow path and a relatively small capacity (circa 200 g of polymer) so that polymers with novel and well-defined molecular architectures may be investigated. The details of its construction and operation are described. When two walls of the cell are composed of zero order birefringent sapphire, both small-angle neutron scattering and birefringence studies can be undertaken in the same cell providing a link between macroscopic and molecular level descriptions of the influence of melt flow. Both birefringence and the first melt flow SANS data for a monodisperse, linear polystyrene are presented. These demonstrate the capability and potential of the apparatus to provide data which provide a crucial test for molecular theories of the rheology of entangled polymer melts. However, the use of sapphire windows limits the maximum flow rate that can be used and higher flows necessitated an all aluminum flow cell to cope with the higher pressures developed in flow. Clear evidence of a stretching of the molecule in the direction of the melt flow and a contraction perpendicular to the flow direction has been provided

No / Using SANS to map the deformation of individual polymer chains in the vicinity of re-entrant corners in a contraction−expansion flow, we show that stress singularities at such corners, predicted by formulations of fluid dynamics that lack a molecular basis, do not cause extreme deformation of the chains. Multiscale modeling based on a nonlinear tube theory incorporating appropriate relaxation processes quantitatively reproduces the observed scattering, thus providing further evidence for the universality of the tube model for polymer flow.

La silice de précipitation, poreuse à l'échelle de 10 nm, a de nombreuses applications industrielles dans lesquelles elle est mélangée avec des fondus de polymère, composés de molécules de dimension nanométrique. La surface de la silice est de haute énergie, si bien qu'elle tend à être recouverte par la plupart des liquides. Par conséquent, lorsqu'une goutte de liquide est déposée sur la surface de la silice, un film " précurseur " s'étale au-devant de celle-ci, avec une épaisseur de l'ordre du nm. A l'aide d'observations macroscopiques et par imagerie ellipsométrique, nous avons mis en évidence que le polybutadiène et le polystyrène sont en conditions de mouillage pseudo-partiel avec la silice : une goutte macroscopique coexiste à l'équilibre avec le film précurseur en raison de la présence de forces à longue portée attractive à l'échelle du film. Le film précurseur est en fait formé de molécules quasi-isolées qui diffusent avec un coefficient de diffusion égal au coefficient d'auto diffusion mesuré en volume. Ceci nous donne des indications précieuses sur les interactions polymère/silice. Par ailleurs nous observons dans certains cas la croissance d'une instabilité, prenant la forme d'un film " secondaire " d'épaisseur supérieure à celle du film précurseur. Le polydiméthylsiloxane est quant à lui en conditions de mouillage total sur la silice, et seul un film existe à l'équilibre, les forces à longues portées étant répulsives. L'imbibition de la silice poreuse peut également se relier à ces observations. Finalement, nous déduisons que le film précurseur pour des faibles masses de polymères a peu d'impact sur la dynamique d'imbibition de pastilles de silice poreuse. / Precipitated silica, which is porous at the 10 nm scale, has various industrial uses where it is mixed with polymer melts, with characteristic molecular sizes in the nanometer range. Having a high surface energy, silica tends to be covered by most liquids. As a consequence, when a liquid droplet is deposited on silica surfaces, a thin “precursor” film spreads in front of the droplet, with a thickness of a few nanometers. By combining macroscopic observations and ellipsometry imaging, we found that polybutadiene and polystyrene melts on silica are in pseudo-partial wetting conditions, for which a droplet coexists with a precursor film at equilibrium, due to attractive long range forces at the film length scale. The precursor film is composed of quasi-isolated molecules diffusing in two dimensions with a diffusion coefficient equal to the bulk self-diffusion coefficient. This provides valuable information on the polymer/silica interactions. Furthermore, we occasionally observe the growth of an instability, as a “secondary” film which is thicker than the precursor film. In contrast, polydimethylsiloxane melts are in total wetting conditions on silica: at equilibrium, a polymer film covers the silica surface and no droplets coexist with the film, due to repulsive long range forces. Our observations of the imbibition of porous silica by polymer melts were related to these results. Eventually, for low molar mass polymers, we find that the precursor film has no significant impacts on porous silica pellets imbibition.