Tunable Mechanical Anisotropy, Crack Guiding, and Toughness Enhancement in Two‐Stage Reactive Polymer Networks

Abstract

Using simple and inexpensive processing methodologies afforded by two‐stage reactive polymer networks (TSRPs) tunable mechanical anisotropy is displayed, defect‐independent guiding of cohesive fracture paths through soft material is demonstrated for the first time, and bio‐inspired microstructures are shown to enable performance enhancement beyond what is anticipated by the rule‐of‐mixtures in composites. The ability to pattern rubbery (stage I) and glassy (stage II) domains within a TSRP using photomasks and UV light is investigated through atomic force microscope (AFM) nanomechanical mapping techniques. AFM modulus mapping shows that the resulting stiffness anisotropy between stage I and stage II regions is length scale dependent. A gradient interface in elastic modulus between stage I and stage II materials is observed and, when patterned with an angled stage I pathway, the gradient interface exhibits remarkable resilience during failure, repeatedly deflecting cracks away from stage II regions, even while turning cracks at angles up to 135° When stage I and stage II domains are patterned in a nacre‐inspired microstructure, toughening beyond rule‐of‐mixtures’ prediction is observed.