Dynamics of Flexible Polymer Chains in Elongational Flow

Abstract

This paper focuses on the recent developments achieved in our group in the field of flexible polymer chain dynamics in elongational flow. Stress-induced orientation and deformation in flowing polymer solution is being surveyed in a wide variety of manifestations. Two types of flow fields have been investigated: the opposite-jet flow with a stagnation point, and the abrupt contraction flow with open flowlines. Emphasis has been placed on the latter geometry due to its widespread occurrence. Degradation experiments performed with dilute PS solutions in abrupt contraction flow demonstrate a distinct propensity for midchain scission, a feature apparently consistent with highly extended chains. On the other hand, the critical strain-rate for chain fracture (ἐ_f) determined under the same experimental conditions scales with M^-1. The inverse relationship with polymer MW is characteristic of partly uncoiled chains but not of highly extended chains, for which a stronger MW dependence (ἐ_f) ∝ M^-2 is expected. The 'yo-yo' model, which could rationalize these findings, is refuted by fluorescence depolarisation measurements which indicated that central orientation is more progressive than the average segmental orientation. For the first time, polarization-modulated birefringence was used to determine the state of local chain orientation in abrupt contraction flow. Axisymmetric contraction flow is complicated by the large variations in local molecular orientation along different streamlines. To account for this effect, a numerical inverse Abel transform has been devised to compute local birefringence (Δn) from experimental retardation (δ). Results show a steep decrease in birefringence with axial distance. Birefringence profiles determined in a direction perpendicular to the flow show a maximum, not at the centre, but in the vicinity of the orifice walls. This observation is in accord with flow simulation results which indicated the presence of extreme strain rates near re-entrant corners. Molecular extension ratios calculated with the Kuhn-Grün theory gives a qualitative description of polymer uncoiling up to about one third of the chain contour length at the approach of capillary entrance. A notable departure from the affine deformation model is, however, observed with multiple birefringence saturation levels changing with flow rate conditions. This behaviour could be explained by the presence of transient molecular conformations. Efforts are underway to visualize these metastable states by single chain fluorescence video microscopy using UHMW polystyrene randomly grafted with anthracene chromophores.