Properties of amphoteric polyurethane waterborne dispersions. III. Isoelectric points and precipitation

The isoelectric points of amphoteric polyurethane (APU) with hydrophobic soft segments and pendent -COOH and -CH2N(CH3)(2) groups and the effect of inorganic salts on the isoelectric points were investigated by the conductivity titration method. The pH values at isoelectric points agreed with the theoretical isoelectric point equations of Patrickios and Merle with pK(a) = 6.1 and pK(b) = 8.3, even though, as an ionomer, APU has a low density of ionizable groups on the macromolecular chains and takes a micellar state in aqueous media. The precipitation of APU waterborne dispersions also was observed around the isoelectric points and around lower or higher pH values with the ultraviolet spectrophotometric method. (C) 2002 Wiley Periodicals, Inc.