Remarkably small critical exponent for the viscosity of a polymer solution

Abstract

We have measured the apparent critical exponent y characterizing the divergence of the viscosity η∝(T−Tc)−y near the liquid–liquid critical point of the mixture polystyrene in diethyl malonate. The data span the range in reduced temperature of 10−4<(T−Tc)/Tc<10−1. The sample was prepared from the same materials used by Gruner et al. in their capillary viscometer [Macromolecules 23, 510 (1990)]; however our torsion oscillator viscometer had a shear rate 80 times lower. This increased the range of reduced temperatures where shear effects could be neglected. In spite of the large reduction in shear rate and the different viscometry technique, the parameters fitted to our data and 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 other polymer solutions measured in capillary viscometers. However, it is significantly smaller than the exponent for pure fluids (0.041± 0.001) and simple binary mixtures (0.042±0.002). It appears that polymer solutions are in a dynamic universality class different from that of simpler fluids.