Nonuniversal nature of dynamic critical anomaly in polymer solutions

Critical anomaly of viscosity has been studied for ideal polymer solutions, focusing on its dependence on the molecular weight of polymer $M_w.$ According to the conventional understanding that polymer solutions should belong to the same dynamic universal class as classical fluids, the critical exponent of viscosity $y_c$ should be a universal constant (∼0.04). Contrary to this, we find that $y_c$ significantly decreases with increasing $M_w.$ This unusual behavior can be explained by the dynamic coupling of critical concentration fluctuations with an additional slow viscoelastic mode intrinsic to polymer solutions. Our dynamic light scattering measurements support this picture.