Abstract: The viscosity method serves as a convenient and widely adopted technique for determining the relative molecular weight of polymeric materials, with relevant experiments being incorporated into undergraduate curricula including physical chemistry, polymer physics and chemistry, and biochemistry. However, students frequently encounter issues with poor linearity in viscosity curves when applying this method to measure the relative molecular weights of polymers such as polyvinyl alcohol. This study demonstrates that the linearity of viscosity curves can be significantly improved by introducing single or mixed soluble salt systems into polymer solutions, which modulate the curve's characteristics through “upward” and “downward” bending effects. Through systematic investigation of influencing factors including effective capillary inner diameter and kinetic energy correction, we identified an optimal mixed salt system (0.125 mol·L^-1 LiCl + 0.25 mol·L^-1 KCl) that achieves a linear correlation coefficient (R²) exceeding 0.95. The method's applicability was further validated by determining the relative molecular weights of dextran 20, dextran 100, and polyethylene glycol using this optimized salt system. This improved viscosity method not only enhances experimental outcomes but also encourages students to integrate multidisciplinary theoretical knowledge into practical analysis, stimulates deeper scientific inquiry, fosters research interest, and significantly develops experimental skills and innovative research capabilities.

Key words: Viscosity method, Superpolymer, Relative molecular weight, Mixed salt system