We report on the formation of stimuli-responsive structured hydrogel thin films whose pattern geometry can be adjusted on demand and tuned reversibly by varying solvent quality or by changing temperature. The hydrogel films, ∼100 nm in thickness, were prepared by depositing layers of random copolymers comprising N-isopropylacrylamide and ultraviolet (UV)-active methacryloyloxybenzophenone units onto solid substrates. A two-beam interference pattern technique was used to cross-link the selected areas of the film; any unreacted material was extracted using ethanol after UV light-assisted cross-linking. In this way, we produced nanoholes, perfectly ordered structures with a narrow size distribution, negligible tortuosity, adjustable periodicity, and a high density. The diameter of the circular holes ranged from a few micrometers down to several tens of nanometers; the hole periodicity could be adjusted readily by changing the optical period of the UV interference pattern. The holes were reversibly closed and opened by swelling/deswelling the polymer networks in the presence of ethanol and water, respectively, at various temperatures. The reversible regulation of the hole diameter can be repeated many times within a few seconds. The hydrogel sheet with circular holes periodically arranged may also be transferred onto different substrates and be employed as tunable templates for the deposition of desired substances.
Keywords: PNIPAm; UV cross-linking; azobenzene-containing molecules; hydrogels; photosensitive polymers; stimuli-responsive structured polymer films.