Effects of sodium dodecyl sulfate on structures of poly(N-isopropylacrylamide) at the particle surface.

Abstract

The effect of sodium dodecyl sulfate (SDS) on the poly(N-isopropylacrylamide) (PNIPAM) tethered to nanoparticles was experimentally investigated using dynamic light scattering below the lower critical solution temperature. A mean-field analytical model was used to calculate the parameters of interfacial PNIPAM-SDS complexes. Particularly, the magnitude of SDS adsorption energy obtained decreases with decreasing excluded volume parameter, implying that the partially collapsed PNIPAM brush virtually favors the adsorption of SDS onto the PNIPAM chains. A self-consistent field theory (SCFT) model was used to get a detailed quantitative description of monomer density distribution. By lowering the solvent quality, a number of phenomena related to the noncontinuity of monomer density are revealed. These phenomena are either referred to as the vertical phase separation or as its precursor, which can be delayed and eventually eliminated as the SDS coverage is increased. The distribution of free chain ends was calculated using the SCFT model. Increasing the SDS coverage gives rise to a broader and more asymmetric distribution of free chain ends, accompanied by a considerable expansion of the dead zone (below which the free chain ends do not exist), but lowering the solvent quality has opposite influences. The relative thickness of the dead zone can be scaled to a master curve, regardless of SDS coverage.