Orientation of macromolecules and elastic deformations in polymer melts. Influence of molecular structure on the reptation of molecules

Abstract

Melt elasticity has a strong impact on both the processing behaviour of polymers and end use properties of fabricated parts. This paper compiles in the first part relations describing time-dependent and steady-state orientations (flow birefringence) as well as elastic strains for different deformation histories. Analytical expressions based on the relaxation time spectrum and rubber-like liquid theory are obtained for small shear or elongational strains. An at least approximate description is possible for high deformation rates and strains. Some fundamental theoretical predictions are compared with experimental results obtained on polystyrene and polyolefine melts of different molecular structure.

In addition, the second part presents fundamental experimental results on the influence of average molar mass and molar mass distribution on dynamic moduli, viscosity functions, normal stress coefficients, recoverable shear strains, extrudate swell, entrance pressure losses, and flow instabilities. The kind of side groups of the C-C-backbone as well as the type and number of chain branches in polyolefines affects the viscosity level and the temperature dependence (flow activation energy) of the rheological quantities. Long chain branching causes deviations from a thermorheologically simple behaviour. The experimental results are discussed in simple model images, taking into account the reptation motion of the molecules.