Photo-CuAAC Induced Wrinkle Formation in a Thiol-Acrylate Elastomer via Sequential Click Reactions

Two sequential click reactions are implemented in a facile methodology used to generate well-defined spatiotemporally controlled and persistent wrinkles on the surface of an elastomer. The click thiol-Michael addition reaction was utilized to form a cross-linked polymer with residual, reactive alkyne sites that remained tethered throughout the network The latent, unreacted alkyne sites are subsequently reacted with diazide monomers via a photoinduced Cu(I)-catalyzed alkyne azide cycloaddition (CuAAC) reaction to increase the cross-link density. Increased cross-linking raised the modulus and glass transition temperature from 1.6 MPa and 2 degrees C after the thiol acrylate reaction to 4.4 MPa and 22 degrees C after the CuAAC reaction. However, the second-stage photopolymerization of the CuAAC reaction is significantly spatially restricted via limited Cu(II) ion diffusion into the thiol acrylate elastomer, thereby creating the desired cross-linking gradient throughout the depth of the initial network and leading to the formation of a highly cross-linked skin layer. This approach leads to the formation of well-defined, persistent, reproducible wrinkles on the surface of the material with wavelength and amplitude of 8.50 +/- 1.60 and 1.41 mu m, respectively, for a polymer with a 1280 mu m total film thickness. Control over the wavelength and amplitude of these wrinkles using the resin film thickness is further demonstrated by studying the surface profiles using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Additionally, this novel technique allows the spatial selectivity of wrinkle formation with a wrinkled area that is only 8 mu m wider than the photomasked area. This strategy represents a unique approach to generate photodirected wrinkling on the entire surface of the elastomer in one single step.