BIREFRINGENCE OF DILUTE PS SOLUTIONS IN TRANSIENT ELONGATIONAL FLOW

Transient elongational flow, created by forcing a polymer solution acr oss a narrow contraction, is characterized by a high strain rate of li mited duration. Due to an inherent short residence time, this type of flow generally is considered as being less efficient in extending isol ated flexible molecular coils than ''stagnation'' point elongational f low. Rheo-optical measurements revealed, nevertheless, a readily detec table 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 = 1.93-1 0.2.10(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 co mpute local birefringence (Delta n) from experimental retardation (del ta). Results show a uniform birefringence distribution across the capi llary entrance and a steep decrease with the axial distance. Molecular extension ratio calculated with the Kuhn-Grun theory suggests that po lymers may uncoil up to one third of the chain contour length at the a pproach of capillary entrance. Although extension ratio determined at the inlet could be fitted with an affine deformation model, notable de parture from this simple representation is observed when molecular str ain is calculated along the streamline, (C) 1998 John Wiley & Sons, In c.