Polymer solutions under elongational flow .1. Birefringence characterization of transient and stagnation point elongational flows

Birefringence measurements are reported in transient and stagnation point elongational flows created by forcing a polymer solution through a narrow contraction. A strongly molecular weight dependent birefringence was observed and attributed to segmental orientation caused by individual chain extension. The birefringence was also found to depend on the position within the flow field, the solvent type, thermodynamic quality and viscosity, the polymer composition, and the type of flow held. Two distinct concentration regimes, corresponding to the dilute and semi-dilute polymer regions, were observed. The transition was marked by a dynamic critical overlap concentration which was lower than that observed under quiescent conditions. In the dilute regime, a laminar how profile was observed and intermolecular interactions were negligible. The modest local orientation and overall chain deformation however indicate that intramolecular hindrances to extension exist. Transient elongational flows were characterized by a 'tube-like' birefringence which had a diameter equal to that of the orifice and extended several orifice diameters into the solution. In contrast, stagnation point elongation flows possessed both the tube-like birefringence and the classical highly localized birefringent 'lines'. The transition between the tubular and linear birefringent zones is a function of the viscous coupling between the polymer coil and the deforming fluid element. This coupling is itself a complex function of temperature, molecular parameters and the flow conditions. Measurements in stagnation point elongational flows have also indicated that the maximum degree of local orientation does not occur at the stagnation point, and data indicate that chains continue to accelerate until a region very close to the orifice. In transient elongational flow the polymer dynamics follow a coil-to-deformed coil transition with birefringence saturation not observed. Copyright (C) 1996 Elsevier Science Ltd.