NMR study on the effects of sodium n-dodecyl sulfate on the coil-to-globule transition of Poly(N-isopropylacrylamide) in aqueous solutions

A study was conducted to investigate the complex structures and interactions of a polymer and its surfactant in aqueous solutions during the coil-to-globule transition. The investigations focused on the role of sodium n-dodecyl sulfate (SDS) below and above the low critical solution temperature (LCST) and were conducted using a combination of high-resolution ¹H, Overhauser effect spectroscopy (2D NOESY), and pulsed-field gradient (PFG) NMR. Poly(N-isopropylacrylamide) (PNIPAM) was prepared in the laboratory for the investigations through free radical polymerization in benzene, initiated by recrystallized azobis(isobutyronitrile) (AIBN). The resulting PNIPAM sample was carefully fractionated by successive dissolution precipitation cycles in a mixture of extremely dried acetone and n-hexane at ambient temperature. The investigations revealed that the normalized number of repeated PNIPAM units decreased during heating and recovered in the cooling process, indicating the reversibility of the coil-to-globule transition.