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Strong and well-engineered interfaces between dissimilar materials are a hallmark of natural systems but have proven difficult to emulate in synthetic materials, where interfaces often act as points of failure. In this work, curing reactions that are triggered by exposure to different wavelengths of visible light are used to produce multimaterial objects with tough, well-defined interfaces between chemically distinct domains. Longer-wavelength (green) light selectively initiates acrylate-based radical polymerization, while shorter-wavelength (blue) light results in the simultaneous formation of epoxy and acrylate networks through orthogonal cationic and radical processes. The improved mechanical strength of these interfaces is hypothesized to arise from a continuous acrylate network that bridges domains. Using printed test structures, interfaces were characterized through spatial resolution of their chemical composition, localized mechanical properties, and bulk fracture strength. This wavelength-selective photocuring of interpenetrating polymer networks is a promising strategy for increasing the mechanical performance of 3D-printed objects and expanding light-based additive manufacturing technologies.

Mechanical response of ceramic matrix composites is critically dependent on properties of fibers and fiber coatings. In the present study, we examine effects of these and other constituent properties on the tensile response of unidirectionally-reinforced composites through a combination of existing models of fiber fracture, Monte Carlo simulations of concurrent matrix and fiber fragmentation, and complementary experimental measurements. The results show that, contrary to prevailing understanding, the process of fiber fragmentation cannot go to completion in a uniaxial tensile test. At low stresses (before the maximum), fiber breaks are essentially random throughout the bundle; in contrast, at the stress maximum, all additional breaks are localized to regions close to the fracture plane (within a distance dictated by the characteristic transfer length). Therefore, the resulting distributions in fiber pullout length and strength of pulled-out fibers do not match predictions from existing models. Although the composite stress-strain response associated with fragmentation can be accurately predicted by existing fragmentation models, accurate prediction of the point of instability requires consideration of the local response of fibers in the most heavily strained regions (within matrix crack planes). The response in the latter regions leads to instability at stresses and strains that are lower than those obtained from the average stress-strain response. Additionally, the composite strength and failure strain are found to be sensitive to the fiber volume fraction and the matrix strength distribution. One implication is that the full potential of the fibers may not be realizable in composites of practical interest.

A novel methodology for printing 3D objects with spatially resolved mechanical and chemical properties is reported. Photochromic molecules are used to control polymerization through coherent bleaching fronts, providing large depths of cure and rapid build rates without the need for moving parts. The coupling of these photoswitches with resin mixtures containing orthogonal photo-crosslinking systems allows simultaneous and selective curing of multiple networks, providing access to 3D objects with chemically and mechanically distinct domains. The power of this approach is showcased through the one-step fabrication of bioinspired soft joints and mechanically reinforced “brick-and-mortar” structures.