Molecular orientation and deformation of polymer solutions under shear: A flow light scattering study

The conformational characteristics of polymer coils in dilute solution under shear flow were investigated by means of wide angle laser light scattering in an apparatus similar to one used by Cottrell, Merrill, and Smith (J. Polym. Sci., Polym. Phys. Ed. 1969, 7, 1415).(1) The polymer solution was subjected to steady, homogeneous shear flow in the annulus between concentric cylinders by rotation of the inner cylinder. By examining the angular dependence of the scattered light intensity during shear, the orientation and the lengths of the major and minor axes of the deformed polymer molecules in solution were determined quantitatively. The solutions studied were linear polystyrenes of various molecular weights dissolved in the viscous solvent dioctyl phthalate near the Theta temperature. The sheaf rate dependence of the orientation angle of the deformed molecules was found to agree well with Zimm model predictions. The elastic dumbbell, Rouse, and Zimm models, however, substantially overpredict the degree of deformation of the polymer coils. This is in agreement with results of Link and Springer (Makromol. Chen., Macromol. Symp. 1992, 61, 358; Macromolecules 1993, 26, 464.).(2,3) In addition, the effects of concentration, molecular weight, and polydispersity were systematically studied. The sensitivity of the coil deformation to concentration persisted to concentrations significantly below c*. Polydispersity had a significant effect on both the measured orientation and the deformation of the polymer molecules; the molecular weight dependence of the deformation was weaker than predicted by the Zimm model.