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Rheology of polymer melts, polymer processing

Main research domains

  • determination of volume changes of polymers
  • digital image processing of polymer extrudates
  • qualitative analysis of flow instabilities of polymer extrudates using fractal analysis
  • Poiseuille and/or Couette flow of polymer liquids in annular geometries
  • rheology of polymers in extensional flows
  • contraction and expansion flow of semiconcentrated and concentrated fibre suspensions

Most important results

  • The critical values of shear stresses and shear rates determine the onset of oscillatory flow of polymer melt extrudates. There were proposed the mixture rules relating the critical value of shear stress (rate) of blend to the critical values of shear stresses (shear rates) of the individual pure components, weight fractions, and interaction parameters. A good agreement between these rules and experiments was shown.
  • Molecular theory was used for describing effects of macromolecular structure of molten polymers on volume changes and consequently rheological properties. The predicted effects show that at constant temperature and pressure the mass density of a polymeric melt decreases with increasing shear rate. Mass density variations are considered to result from variation of molecular packing since motion prevents molecules from packing as efficiently as they might in equilibrium at rest.
  • The modified version of fractal analysis was derived and applied to the qualitative analysis of polymer extrudates with the aim to differ the onsets of the individual flow instabilities. The theoretical results were applied to the blends of polyamide and polyethylene.
  • It was shown that axial flow of power-law fluids in concentric annuli has a quasisimilarity solution in a broad range of flow behaviour and consistency indices, and an aspect ratio k of the inner diameter to the outer one. This solution makes it possible to transform the dependence of the flow rate vs. pressure drop for individual k to a common graph for all kł.4. The procedure fully eliminates the necessity of determining the location of maximum axial velocity.
  • The approximate relation flow rate vs. pressure drop for the problem above was derived for almost whole region of aspect ratios and flow behaviour indices. The corresponding relation was also obtained for the tangential flow. The deviations of these simple relations from the exact ones are fully negligible.
  • A novel rheometer generating uniaxial extensional flow with superposed oscillations was used for study of viscoelastic behaviour of polymeric melts (polyethylene, ethylene based ionomers). In comparison with classical technique of extensional rheology, the material functions give information about elastic and viscous melt deformation separately. For quantifying melt behaviour, a modified Doi-Edwards molecular approach was used.
  • It was shown that dilute fibre suspensions behave similarly to polymeric solutions in contractions but not in expansions. Study also indicated that the fibres fail to flow affinely with the bulk flow. This is particularly apparent in expansion flow where the fibres tend to an orientation which is transverse to the flow direction and also close to boundary walls where substantial fibre-free regions develop.