Viscosities of sodium polyacrylate in aqueous sodium chloride

Abstract

Dilute solution viscosity data were obtained for a 98.1% neutralized sodium salt of polyacrylic acid having a viscosity-average degree of polymerization of about 1.9 × 10 4, over a wide range of added salt (NaCl) concentrations. It was found that: 1. 1. The shape of the η sp C curve in pure water is of an entirely different type from that expected from the folding-chain theory of Fuoss and others, but is essentially similar to that which we observed with sodium carboxy methyl celluloses of high degrees of polymerization. The curve increases steadily with increasing C, exhibiting a rather wide plateau region. 2. 2. A marked decrease of the reduced viscosity is brought about by the addition of NaCl, but no indication for the appearance of a maximum in η sp C is observed for the added salt concentration ranges studied. 3. 3. With increasing ionic strength of the medium, the Huggins constant, starting with a value of about 0.6, initially decreases, reaches a minimum with a value of about 0.33, increases again, and tends to 0.85 at the limit of zero ionic strength. This behavior is discussed in connection with the change in degree of coiling of the polyelectrolyte molecule as caused by the change in the ionic strength. In particular, the behavior at sufficiently high ionic strengths is interpreted as a result of hypercoiling of the molecule. 4. 4. The application of the method of Pals and Hermans to our data leads to too high a value for the length of a statistical segment in Kuhn's sense. In contrast to this, the Flory-Fox equation, when applied at the limit of very high ionic strength, gives a very reasonable value for this length, being about 38 A.