A RHEOOPTICAL STUDY OF SHEAR-THICKENING AND STRUCTURE FORMATION IN POLYMER-SOLUTIONS .2. LIGHT-SCATTERING ANALYSIS

Light scattering calculations based on Anomalous Diffraction Theory (A D), Rayleigh spheroids, and flexible macromolecules are used to propos e a phenomenological explanation for the relationship between shear-th ickening and structure formation in polymer solutions. Quantitative co mparisons are made to experimental data for the rheo-optical behavior of fractionated polystyrene solutions presented in part I of this pape r. Results from the ADA calculations suggest that the viscosity and di chroism behavior can be attributed to the production and growth of mic ron-size, optically isotropic structures during flow. The saturation d ichroism behavior exhibited by the solutions which shear thin can be a ttributed to the formation of entanglement regions which achieve a fix ed size and act as Rayleigh spheroids in their scattering behavior. Th e magnitude and shear rate dependence of the observed birefringence ca n be accounted for on the basis of the non-linear, flexible macromolec ule model, implying that birefringence is governed by the polymer chai ns remaining in solution which do not take part in the structure forma tion. The latter result is consistent with the experimental observatio n that the birefringence dependence on shear rate is the same whether the solution exhibits shear thickening or shear thinning in its viscos ity behavior.