Birefringence of dilute PS solutions in transient elongational flow

Transient elongational flow, created by forcing a polymer solution across a narrow contraction, is characterized by a high strain rate of limited duration. Due to an inherent short residence time, this type of flow generally is considered as being less efficient in extending isolated flexible molecular coils than "stagnation" point elongational flow. Rheo-optical measurements revealed, nevertheless, a readily detectable birefringence zone above a critical strain rate in the immediate orifice entrance. Birefringence was studied for dilute PS solutions (100-400 ppm) in decalin as a function of fluid strain rate ((epsilon) over dot = 1000-38,000 s(-1)) and polymer molecular weight (M = 1.93-10.2.10(6)). Transient elongational flow is complicated by the presence of local orientation distribution along the different streamlines. To account for this effect, a numerical technique has been devised to compute local birefringence (Delta n) from experimental retardation (delta). Results show a uniform birefringence distribution across the capillary entrance and a steep decrease with the axial distance. Molecular extension ratio calculated with the Kuhn-Grun theory suggests that polymers may uncoil up to one third of the chain contour length at the approach of capillary entrance. Although extension ratio determined at the inlet could be fitted with an affine deformation model, notable departure from this simple representation is observed when molecular strain is calculated along the streamline, (C) 1998 John Wiley & Sons, Inc.