9 - Dilute Solution Viscometry

Abstract

One of the most characteristic features of a dilute polymer solution is that its viscosity is considerably higher than that of the pure solvent. This arises because of the large differences in size between polymer and solvent molecules, and can be significant even at very low polymer concentrations, especially for polyelectrolytes and polymers with high molecular weights. Dilute solution viscometry is concerned with accurate quantitative measurement of the increase in viscosity and allows determination of the intrinsic ability of a polymer to increase the viscosity of a particular solvent at a given temperature. This quantity provides a wealth information relating to the size of the polymer molecular shape, degree of polymerization and polymer–solvent interactions. Most commonly, however, it is used to estimate the molecular weight of a polymer. This involves the use of semi-empirical equations which have to be established for each polymer/solvent/temperature system by analysis of polymer samples whose molecular weights are known. Thus the estimates of molecular weight are not absolute. Nevertheless, in comparison to other methods for characterization of polymers in solution (e.g. membrane osmometry and light scattering), dilute solution viscometry is simple, fast and inexpensive. It also has the advantage that it is applicable over the complete range of attainable molecular weights. For these reasons, dilute solution viscometry has been the most widely used method of polymer characterization since the birth of polymer science and continues to be used today.