Forced Phase Separation by Laser-Heated Gold Nanoparticles in Thermoresponsive Aqueous PNIPAM Polymer Solutions

We have investigated the formation, growth, and dissolution dynamics of aggregates of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM) that form around laser heated gold nanoparticles (GNPs). The aggregates show an initial rapid growth followed by a slow long-term tail that is caused by the temperature dependent induction time until phase separation sets in. The maximum aggregate radius is determined by the distance from the GNP where the temperature crosses the binodal. Melting and evaporation of the GNP can be identified as characteristic steps in the aggregate size as a function of the heating laser power. After prolonged exposure, the polymer concentration inside the aggregate increases considerably. GNPs get immobilized at the perimeter, and a stepwise increase of the laser power results in onionskin-like growth shells. After switching the laser off, the system returns to the homogeneous phase and the growth shells are radially repelled by the high osmotic pressure within the volume previously occupied by the aggregate.