Flow Light Scattering Studies of Polymer Coil Conformation in Solutions in Extensional Flow

The dynamics of isolated polymer chains in dilute solution under steady, extensional flow were investigated by means of flow light scattering. Both the orientation and deformation of the chains during flow were determined by analyzing the angular dependence of the scattered light intensity. The extensional flow field was imparted on the polymeric fluids by a stagnation point flow in the center of a four-roll mill apparatus. The fluids studied were nearly monodisperse high molecular weight polystyrenes (HMPS) of various molecular weights dissolved in either the viscous solvent dioctyl phthalate (DOP) or a mixed solvent of low molecular weight polystyrene (LMPS) and dioctyl phthalate. The flow field in a Newtonian fluid of seeded glycerin was also examined by flow dynamic light scattering techniques and flow visualization to confirm the extensional flow field. Flow dynamic light scattering and flow visualization results verified that the field generated by the four-roll mill was a reasonable approximation of planar extensional flow, under the conditions investigated. Flow (static) light scattering results confirmed that the polymer chains aligned completely with the stretch direction, as expected for this type of flow. The deformation of the chains, however, was significantly less than predicted by elastic dumbbell models. Also studied were the effects of molecular weight and solvent quality on the deformation of the polymer chains. Comparisons are drawn between polystyrene chains in the strong extensional flow field studied here and a steady shearing flow examined in a previous publication as well as with other studies in extensional flows.