Elongational flow-induced birefringence of polyacrylamide and poly(acrylamide- co-sodium acrylate)

Aqueous solutions of poly(acrylamide- co-sodium acrylate), in up to 1M NaCl, exhibited a flow-induced negative birefringence above a critical elongation flow rate in a crossed-slot device. Aqueous polyacrylamide, on the other hand, exhibited a positive birefringent pattern just above a critical elongation rate. At slightly higher elongation rates a negative birefringent thin-line region centred in the positive birefringent region appeared. This phenomenon was observed for all polyacrylamide concentrations tested (0.001 to 0.5 wt%; 0.5 to 20 times the quiescent overlap concentration). This change in birefringence sign can be qualitatively mimicked using a simple coil deformation model which includes contributions from intrinsic, microform and macroform optical anisotropy terms. Evidence is also given for this alternating birefringence sign pattern possibly being due to an elongational flow-induced change in the microstructure of the polyacrylamide.