Anomalous Inverse Hysteresis of Phase Transition in Thermosensitive Dextran-graft-PNIPAM Copolymer/Au Nanoparticles Hybrid Nanosystem

An anomalous inverse hysteresis behavior of the thermally induced LCST structural phase transition in dextran-graft-PNIPAM copolymer macromolecules with Au nanoparticles has been observed by means of surface plasmon extinction and dynamic light scattering spectroscopy. Namely, the phase transition at heating occurs at temperature lower than one occurs at cooling. The inverse character of observed hysteresis has been rationalized to be caused by Au nanoparticles which play the key role in the change of the "sign" of the hysteresis. Also, Au NPs counteract the formation of PNIPAM aggregates in the solution. Moreover, it has been shown that the PNIPAM macromolecule shrinking at heating leads to an unexpected decrease of the surface plasmon peak area, its blue-shift, and narrowing. Such effects have been rationalized as the result of the sharp jump-like decrease of the refractive index of PNIPAM polymer occurring at the phase transition. It has been found that the observed LCST phase transition is reversible.