Poly(N-isopropylacrylamide) Latices Prepared with Sodium Dodecyl Sulfate

Stable aqueous colloidal dispersions of cross-linked poly(N-isopropylacrylamide), polyNIPAM, were prepared by aqueous dispersion copolymerization of NIPAM with methylenebisacrylamide, BA, in the presence of sodium dodecyl sulfate, SDS. Below 32°C the particles were highly swollen with water and had average diameters in the range 400-800 nm. Above 32°C the average diameters decreased by a factor of 2. Swelling showed the expected trends with cross-linker density, the presence of methanol, and the presence of urea. Also observed was the expected decrease in average particle size with increasing SDS concentration during the polymerization. Flory χ values were estimated from the swelling data. Unexpectedly, the average diameter at 25°C and the electrophoretic mobility of a cleaned latex were greater in 1 × 10-5M KCl than in higher electrolyte concentrations. Also unexpected was the observation that colloidal particles were obtained without BA. The presence of particles rather than isolated linear polyNIPAM chains was explained by chain entanglement.