Stress overshoot of polymer solutions at high rates of shear; polystyrene with bimodal molecular weight distribution

Overshoot of shear stress, sigma, and the first normal stress difference, N -1, in shear flow was investigated for dilute solutions of polystyrene with very high molecular weight in concentrated solution of low M PS. In the ca se that the matrix was a nonentangled system, behavior of overshoot was sim ilar to that of dilute solution of high M PS in pure solvent. The magnitude s of shear, gamma(sigma m) and gamma(Nm), corresponding to the peaks of sig ma and N-1 lay on the universal functions of (gamma) over dot tau(R), respe ctively, proposed for dilute solutions in pure solvent. Here tau(R) is the Rouse relaxation time for high M PS in the blend evaluated from dynamic mod ulus at high frequencies. In the case that the matrix was an entangled syst em, an additional a peak was observed at high rates of shear at times corre sponding to gamma(sigma m) = 2-3. This peak can be assigned to the motion o f low M chains in entanglement network. When the matrix was entangled, stre ss overshoot was observed even at relatively low rates of shear, say (gamma ) over dot tau(R) < 10(-2). This is probably due to the motion of high M ch ains in entanglement of all the chains. In this case the gamma(sigma m) and gamma(Nm) values were higher than those expected for entangled chains of m onodisperse polymer in pure solvent. (C) 2000 John Wiley & Sons, Inc.