Swelling and switching kinetics of gold coated end-capped poly(N-isopropylacrylamide) thin films

Thin thermoresponsive hydrogel films of poly(N-isopropylacrylamide) end-capped with n-butyltrithiocarbonate (nbc-PNIPAM) on silicon supports with a gold layer on top, causing an asymmetric confinement, are investigated. For two different gold layer thicknesses (nominally 0.4 and 5 nm), the swelling and switching kinetics are probed with in situ neutron reflectivity. With a temperature jump from 23 to 40 °C the film is switched from a swollen into a collapsed state. For the thin gold layer this switching is faster as compared to the thick gold layer. The switching is a two-step process of water release and a subsequent structural relaxation. In swelling and deswelling cycles, aging of the films is probed. After five cycles, the film exhibits enhanced water storage capacity. Grazing-incidence small-angle X-ray scattering (GISAXS) shows that these gold coated nbc-PNIPAM films do not age with respect to the inner structure but slightly roughen at the gold surface. As revealed by atomic force microscopy, the morphology of the gold layer is changed by the water uptake and release.