Swelling-induced Reversible Wrinkle Morphologies In Crosslinked Layer-by-layer Self-assembly Multilayers

In recent years, surface wrinkling has drawn great research interest. Currently,one of the research focuses is to study the evolution of surface wrinkles to reveal theinternal relationship between the stress field and surface wrinkling.In this thesis, the self-assembled multilayers are incorporated into thefilm/foundation bilayer system to investigate the swelling/deswelling-induced wrinkleevolution for the first time. As one typical example, hydrogen-bonded (PAA/PEG)nmultilayers (PEG: poly(ethylene glycol); PAA: poly(acrylic acid)) are deposited onpoly(dimethyl siloxane)(PDMS) substrate via the simple layer-by-layer (LbL)self-assembly technique, followed by heating treatment for the crosslinked reaction inthe multilayers with greatly enhanced stability. Meanwhile, the labyrinth wrinkles areinduced on the heated (PAA/PEG)n/PDMS bilayer system. The subsequent waterswelling leads to a series of surface morphological evolution on thethermally-crosslinked bilayer system, ranging from the initial labyrinth wrinkles (a) tothe smooth wrinkle-free state (b), the intermediate hexagonally-arranged dimples (c),and the latter segmented labyrinth patterns (d). It is found that the wavelength of theheating-induced wrinkles and the swelling-induced wrinkles are linear with thethickness of the multilayers (i.e., the bilayer number). Upon deswelling by re-heating,the reverse wrinkle evolution happens via the process of d→b, or d→b→a, orc→b→a, or c→b, which is dependent on the re-heating temperature and theswelling-induced patterns before re-heating. The influence of the experimental factorson this reversible wrinkle evolution with the swelling/deswelling processing has beeninvestigated systematically, which include the thickness of the bilayer film and thesubstrate, the pH value of the swelling water, and the additional inert outer-layerrelative to the swelling. The involved mechanism has also been discussed. Obviously,the combined strategy of the simple universal LbL self-assembly technique with theintroduction of the additonal layer endows us with considerable freedoms tocontrollably fabricate functional film/foundation systems for the well-definedwrinkling microstructures, properties and applications.