Dynamics of flexible polymer chains in elongational flow

This paper focuses on the recent developments achieved in our group in the field of flexible polymer chain dynamics in elongational flow. Stress-induc ed orientation and deformation in flowing polymer solution is being surveye d in a wide variety of manifestations. Two types of flow fields have been i nvestigated: the opposite-jet flow with a stagnation point, and the abrupt contraction flow with open flowlines. Emphasis has been placed on the latte r geometry due to its widespread occurrence. Degradation experiments perfor med with dilute PS solutions in abrupt contraction flow demonstrate a disti nct propensity for midchain scission, a feature apparently consistent with highly extended chains. On the other hand, the critical strain-rate for cha in fracture (epsilon (f)) determined under the same experimental conditions scales with M-1. The inverse relationship with polymer MW is characteristi c of partly uncoiled chains but not of highly extended chains, for which a stronger MW dependence (epsilon (f)) proportional to M-2 is expected. The ' yo-yo' model, which could rationalize these findings, is refuted by fluores cence depolarisation measurements which indicated that central orientation is more progressive than the average segmental orientation. For the first t ime, polarization-modulated birefringence was used to determine the state o f local chain orientation in abrupt contraction flow. Axisymmetric contract ion flow is complicated by the large variations in local molecular orientat ion along different streamlines. To account for this effect, a numerical in verse Abel transform has been devised to compute local birefringence (Delta n) from experimental retardation (delta). Results show a steep decrease in birefringence with axial distance. Birefringence profiles determined in a d irection perpendicular to the flow show a maximum, not at the centre, but i n the vicinity of the orifice walls. This observation is in accord with flo w simulation results which indicated the presence of extreme strain rates n ear re-entrant corners. Molecular extension ratios calculated with the Kuhn -Grun theory gives a qualitative description of polymer uncoiling up to abo ut one third of the chain contour length at the approach of capillary entra nce. A notable departure from the affine deformation model is, however, obs erved 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 metastabl e states by single chain fluorescence video microscopy using UHMW polystyre ne randomly grafted with anthracene chromophores.