(A) rheo-optical study on rheological behaviors of concentrated polymer solution with molecular weight distribution

Abstract

The effect of molecular weight distribution on rheological behaviors in inception of shear flow and in steady shear flow of concentrated polystyrene solutions is studied by using the phase-modulated flow birefringence technique. The flow birefringences and orientation angles of the concentrated polystyrene solutions which have large birefringence values in both of transient and steady state are successfully measured by using a modified retardation calculating procedure. The stress-optical coefficient which is necessary to obtain the rheological properties from the optical measurement are obtained for polystyrene solutions of bimodal molecular weight distribution. The stress-optical coefficient of the experimental PS-DEP system is independent of the degrees of mixing of two monodisperse fractions as expected. By using this coefficient, the transient and steady rheological behaviors of the polystyrene solutions with bimodal MW distribution are obtained from the rheo-optical experiments. The effects of the molecular weight distribution are investigated in transient and steady state of shear flow including the linear and nonlinear region. Entanglements of the polymer chains affected the flow behaviors in both of steady state and transient states. The entanglement concepts generally accepted for stress relaxation mechanism could be applied to the flow behaviors of the polystyrene solutions with bimodal molecular weight distribution. The growth of the normal stress of the PS solutions with bimodal MW distribution in shear flow is turned out to be dependent on the entanglements between high molecular weight components themselves. Shear viscosity and coefficient of first normal stress difference in steady shear flow of such solutions can be correlated into master curves with a single parameter, $\gamma\tau_d$(product of shear rate and the longest relaxation time), irrespective of the molecular weight distributions and shear deformations, respectively. This parameter ca...