Swelling and shrinking of poly(N-isopropylacrylamide) chains adsorbed on the surface of polystyrene nanoparticles

As a thermally sensitive polymer, poly(N-isopropylacrylamide) (PNIPAM) is hydrophilic at temperatures below its lower critical solution temperature (LCST, similar to 32 degrees C), but undergoes a coil-to-globule transition in water at higher temperatures. The adsorption of PNIPAM on polystyrene (PS) nanoparticles resulted in a core-shell nanostructure. In the first heating-and-cooling cycle there existed a hysteresis in terms of the thickness of the adsorbed PNIPAM layer; this could be attributed to additional binding of the adsorbed PNIPAM chains to the surface. Moreover, the surface shifted the coil-to-globule transition to a lower temperature and made the transition less sharp. The temperature dependence of the ratio of the radius of gyration to the hydrodynamic radius (<R-g>/<R-h>) revealed that the PNIPAM shell is highly swollen, and the density of the shell in the collapsed state approached that of the PS core.