CORRELATION OF MOLECULAR-ORIENTATION AND SHEAR VISCOSITY OF HIGH MOLAR-MASS POLYMERS IN DILUTE-SOLUTION

The shear induced orientation of high molecular weight polystyrene (M- w congruent to 10(7) g/mol) in dilute solution has been determined by means of wide angle laser light scattering in the flow-plane of a Sear le-type shear cell. The shear dependent molecular orientation is relat ed to the orientation resistance parameter, m, that characterizes the influence of solution properties on the orientational dynamics of the polymer coil. For several solvents from the good to the poor, the shea r dependent variation of m is compared to the viscometric behaviour of the solutions. The experiments show, that a certain critical shear ra te, Bent, defined by a change in the shear dependence of m, correlates with the shear rate characterizing the transition from the non-Newton ian decrease of viscosity to a viscosity plateau. This correlation is quite obvious in the poor solvent trans-decalin and becomes less prono unced with increasing solvent quality. For the given polymer, beta(cri t), is a function of both the solvent viscosity and the intrinsic visc osity. The discussion of different theoretical approaches to the non-N ewtonian how of dilute polymer solutions demonstrates an agreement bet ween theoretical understanding and the observed viscometric behaviour. The observed orientational behaviour below beta(crit), however, is no t explained by present theory. This deviation is attributed to intermo lecular entanglement of the very high molecular weight polymers even a t rather low concentrations. (C) 1997 Elsevier Science Ltd. All rights reserved.