Swelling/deswelling-induced reversible surface wrinkling on layer-by-layer multilayers.

Layer-by-layer (LbL) multilayer film is incorporated in the fabrication of a film/substrate system for the investigation of swelling/deswelling-induced wrinkle evolution for the first time. As one typical example, hydrogen-bonded (PAA/PEG)n (PAA, poly(acrylic acid); PEG, poly(ethylene glycol)) is deposited on a poly(dimethylsiloxane) (PDMS) substrate via the LbL technique. Heating treatment causes the covalent cross-linking reaction to occur in the H-bonded multilayers with simultaneously spontaneous formation of labyrinth wrinkles. Subsequent water immersion leads to the evolution of a series of the swelling-sensitive wrinkles in the thermally cross-linked (PAA/PEG)n/PDMS bilayer, ranging from initial labyrinth wrinkles (a) to an intermediate smooth wrinkle-free state (b), hexagonally arranged dimples (c), and the later-segmented labyrinth patterns (d). Upon deswelling by reheating of the swollen bilayer, the reverse wrinkle evolution happens via the process of d → b, or d → b → a, or c → b, or c → b → a, which is dependent on the reheating temperature and the swelling-induced pattern. We investigate the influences of experimental conditions on the swelling kinetics and the resulting wrinkle evolution, which include the thickness of (PAA/PEG)n, the additionally deposited outermost layer (e.g., Pt and polystyrene), and the swelling solution pH. The involved mechanism has been discussed from the viewpoint of the relation between the wrinkling behavior and the swelling/deswelling-induced stress state. The results indicate that the combined strategy of LbL assembly with the introduction of additional layers endows us with considerable freedom to fabricate multifunctional film/substrate systems and to tune the instability-driven patterns for advanced properties and extended applications.