Dual Macroscale Wrinkling Mechanisms for Complex Surface Patterning in Photopolymer Resins

Hierarchical and topographical surface structures are abundant in nature, from the beetle carapace to plant leaves. The interactions between hierarchically structured, natural surfaces and water continue to inspire researchers; these natural designs have been translated to engineering strategies for antifogging coatings and membrane filtration processes. In this work, photoinduced phase separation (PIPS) is utilized to generate naturally inspired hierarchical topographical features on polymer surfaces in a single-step procedure. These complex designs are accessible when a polymer coating undergoes PIPS since both the classic surface wrinkling phenomenon, facilitated by oxygen quenching of radicals, as well as additional patterning from the phase-separated morphology yield topographies on multiple length scales. We demonstrate this patterning in a photopolymerizable resin system composed of acrylonitrile and 1,6-hexanediol diacrylate (co)monomers with poly(methyl methacrylate) as an inert polymer additive. A wide range of surface features arise from this resin system, which were characterized via imaging and optical profilometry. Furthermore, the topographies that formed were mapped to resin formulation parameters (e.g., cross-linking fraction, inert polymer loading) and experimental conditions (e.g., UV intensity). Our analysis highlights how diffusional constraints vary based on cross-link density and thus impact the scale and quality of topography that forms during polymerization. Increased cross-linker loading results in a kinetically trapped system, where the rapid network formation inhibits surface relaxations. Conversely, with decreased cross-linking, mechanical gradients due to radical quenching can evolve, and when PIPS arises simultaneously multiple scales of patterning form on a single surface, spanning from the micro to macroscale. By linking the observed kinetics with the final surface morphologies, we detail how PIPS can be utilized in tandem with traditional wrinkling to tailor hierarchical surface morphologies in rapid, ambient photopolymerizations for the facile production of tunable coating topographies.