REMARKABLY SMALL CRITICAL EXPONENT FOR THE VISCOSITY OF A POLYMER-SOLUTION

We have measured the apparent critical exponent y characterizing the d ivergence of the viscosity eta(x)(T - T-c)(-Y) near the liquid-liquid critical point of the mixture polystyrene in diethyl malonate. The dat a span the range in reduced temperature of 10(-4)<(T - T-c)/T-c<10(-1) . The sample was prepared from the same materials used by Gruner et al . in their capillary viscometer [Macromolecules 23, 510 (1990)]; howev er our torsion oscillator viscometer had a shear rate 80 times lower. This increased the range of reduced temperatures where shear effects c ould be neglected. In spite of the large reduction in shear rate and t he different viscometry technique, the parameters fitted to our data a nd those of Gruner et al. are in agreement. For this polymer solution, y is in the range 0.028 +/- 0.003, close to recent results for two ot her polymer solutions measured in capillary viscometers. However, it i s significantly smaller than the exponent for pure fluids (0.041 +/- 0 .001) and simple binary mixtures (0.042 +/- 0.002). It appears that po lymer solutions are in a dynamic universality class different from tha t of simpler fluids.