The surface tension of aqueous poly(N‐isopropylacrylamide‐co‐acrylamide)

A series of poly(N‐isopropylacrylamide‐co‐acrylamide) copolymers with N‐isopropylacrylamide (NIPAM) to acrylamide (AM) ratios varying from 95/05 to 10/90 was synthesized and surface tensions, cloud point temperatures, and enthalpies of phase separation were measured. At 25°C, 1 wt % poly(N‐isopropylacrylamide) homopolymer has a surface tension of 41.8 mJ/m2. Incorporation of AM moieties in the copolymer increased surface tension approaching the limiting value of 65.3 mJ/m2 which was obtained for polyacrylamide solutions. The surface tension values of copolymer solutions were predicted from the surface tensions of the homopolymers applied to a one‐parameter model analogous to the Margules model for the excess free energy of mixing. Heats of phase separation for the copolymer were less than expected compared with PNIPAM homopolymer. It was proposed that NIPAM moieties directly bonded to acrylamide did not contribute to the enthalpy of phase separation. Finally, surface tension lowering kinetics were slower above the cloud point temperatures because at high temperatures the copolymers were present as colloidally dispersed particles which had to diffuse to the air/water interface, unwrap, and spread to give an adsorbed monolayer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2137–2143, 1999