Self-Association of Poly(N-isopropylacrylamide) and Its Complexation with Gelatin in Aqueous Solution

The association of poly( N -isopropylacrylamide) (PNIPAM) above its low critical solution temperature (LCST, ∼32 °C) leads to stable aggregates instead of the expected precipitation, which could be attributed to the microphase inversion of the PNIPAM chains. A laser light scattering study showed that the lower association temperature and higher PNIPAM concentration resulted in larger low density interchain aggregates with a cluster structure, while the higher association temperature and lower PNIPAM concentration led to spherical aggregates formed mainly through the packing of individual collapsed chains. Above ∼32 °C, the intrachain coil-to-globule transition competes with the interchain association. Gelatin was encaptured/complexed into the PNIPAM aggregates and the complexation was completely reversible as the temperature varied. The PNIPAM/gelatin complexes with the highest molar mass and density were formed when the PNIPAM/gelatin weight ratio was in the range 1−2 and the temperature was close to ∼32 °C. This would be the optimum condition for the concentration of a dilute protein solution followes by the ultracentrifugation of the PNIPAM-protein complexes.