Effects of Surfactants on Thermally Collapsed Poly(N-isopropylacrylamide) Macromolecules

Poly(N-isopropylacrylamide) macromolecules (PNIPAM) dissolve in water at temperatures below 33 °C; at higher temperatures, due to hydrophobic interactions, they precipitate out of solution. Addition of a small quantity of charged surfactant resolubilizes the precipitated polymer. We have used small angle neutron scattering to determine the structures of aqueous solutions of PNIPAM and sodium dodecyl sulfate (SDS). At temperatures above the precipitation temperature of the pure polymer solution, two different structures have been found, depending on the ratio of surfactant to polymer. At high ratios, SDS micelles bind to individual macromolecules and pull them into solution; the distance between consecutive micelles in a “necklace” is found to remain constant at around 60 Å. At low ratios, the macromolecules collapse to form small colloidal particles rather than forming necklaces with micelles spaced further apart. The collapse transition is abrupt in the sense that no intermediate states between the stretched necklaces and the collapsed colloidal particles have been found. A critical number of micelles per unit length of polymer is required for complete solubilization.