Effect of surfactant concentration on the complex structure of poly(N -isopropylacrylamide)/sodium n-dodecyl sulfate in aqueous solutions

The effect of surfactant concentration on the complex structures of the thermosensitive poly(N-isopropylacrylamide) (PNIPAM) with sodium n-dodecyl sulfate (SDS) in dilute aqueous solutions and below the lower critical solution temperature were investigated using viscometry, two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY), and pulsed-field gradient diffusion nuclear magnetic resonance (NMR). The interactions of PNIPAM and SDS were found to exhibit strong dependences on the SDS concentration. The amount of polymer-bound and free SDS at various SDS concentrations was calculated from the diffusion coefficients of SDS. At SDS concentrations below the critical aggregation concentration (CAC) of around 0.86 mM, the interaction between PNIPAM and SDS was very minimal, resulting in almost no change in the intrinsic viscosity and hydrodynamic size of the polymer. At SDS concentrations above 0.86 mM but below 7.0 mM, the intrinsic viscosity of the polymer as well as the amount of polymer-bound SDS and the intensities of the cross-peaks in the 2D NOESY NMR spectra all increased linearly with increased SDS concentration, resulting from enhanced interaction between PNIPAM and SDS. However, the amount of polymer-free SDS remained almost constant at around 0.86 mM near the CAC. At SDS concentrations above 7.0 mM, almost no more SDS was adsorbed onto PNIPAM and the amount of polymer-bound SDS remained constant, indicating the saturation of the PNIPAM chain with SDS.