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

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.