Investigation of critical concentrations of poly(vinyl pyrrolidone) in N, N-dimethylformamide by a viscosity technique

The viscosity of solutions of poly(vinyl pyrrolidone) (PVP) in N,N-dimethylformamide over a wide range of concentrations and at different temperatures was measured. The polymer solutions exhibited a critical concentration c**, separating dilute solutions into extremely dilute solutions and dilute solutions. The viscosity–temperature dependence is discussed on the basis of a formula of the correlation between the true and the experimental relative viscosity. The results revealed that an abnormal viscosity of neutral PVP solutions in the dilute solution region (c › c**) and extremely dilute solutions (c ‹ c**) can be ascribed to interface effects. The polymeric solutes were readily adsorbed on the wall surface of the cone-plate rheometer, which greatly influenced the viscosity measurements of the PVP solutions and resulted in the apparent abnormal viscosity behaviour. The intrinsic viscosity [η] and the Huggins constant kH, were calculated. The solute adsorption behaviours and structural information of the polymer were discussed in depth.