We present a simple photochemical technique for fabricating responsive polymer layers, enabling the construction of responsive surfaces with unique properties. The approach is based on the photo-cross-linking of copolymers synthesized from N-isopropylacrylamide (NIPAAm) and methacroyloxybenzophenone (MaBP). Ultraviolet (UV) radiation triggers the n,g* transition in the benzophenone groups leading to the formation of a biradical triplet that abstracts a hydrogen from a neighboring aliphatic C-H group leading to a stable C-C bond.

Poly(NIPAAm) undergoes a reversible phase transition at approximately 32oC between a swollen hydrophilic to a collapsed hydrophobic state, thought to be caused by increased hydrophobic attractions between the isopropyl groups at elevated temperatures. We studied the swelling behavior of cross-linked poly(NIPAAm-co-MaBP(3%)) layers in water with neutron reflection. NR measurements permit access to segment density profiles and yields more information than turbidity measurements of bulk samples. Between 15 and 30°C, a weak temperature dependence was observed and the thickness of the swollen poly(NIPAAm) layer decreased from 3.7 to 3 times its dry layer thickness. Between 30 and 35 °C, the layer experienced a sharp reduction in thickness, leveling off at 1.2 times its dry layer thickness. Above 35°C, the thickness remained constant with increasing temperature never collapsing fully to its dry layer thickness. In this state, 35% of the layer volume is due to water, which corresponds to 2 water molecules per polymer segment.