How does the anionic surfactant SDS affect the association of hydrophobically end-modified PNIPAM chains in aqueous solution?

Abstract

The studies on the interaction between thermo-responsive polymer PNIPAM and various ionic surfactants have been extensively investigated in the past few years. Unfortunately, the role of the hydrophobic end-group which is inevitably introduced by the initiator or chain transfer agent was generally overlooked in related studies. In this work, to show the interplay between the hydrophobic end-group and the surfactant molecule, five n-dodecyl-terminated poly(N-isopropylacrylamide)s (PNIPAMs-C12H25) with different chain lengths were prepared by RAFT polymerization. The unambiguous molecular parameters were obtained by GPC, LLS, 1H-NMR and UV–Vis measurements. Further study on the association behaviors in SDS aqueous solutions showed that, these PNIPAM-C12H25 chains can form uniform spherical micelles accompanied with a trace amount of large irregular aggregates in pure water at 20 °C, and the size-molar mass relation of aggregates presents a scaling law. The addition of SDS could continuously weaken the association of these alkyl-terminated PNIPAMs, reflected in the decrease of average hydrodynamic radius (<Rh>) and average aggregation number (<Nagg>) of aggregates. Also, it could endow the PNIPAM chains the polyelectrolyte characteristics, reflected in the emergency of a slow relaxation mode in dynamic LLS measurement. More interestingly, the reversal of the dependence of cloud point on the chain length was observed at high SDS concentration, which may be attributed to the preferentially van der Waals interaction between the SDS dodecyl tail and the PNIPAM dodecyl chain end.