Reversibly growing crosslinked polymers with programmable sizes and properties

Abstract Planaria can integrate external nutrients to grow or consume their bodies to degrow for adapting to environmental changes. In contrast, synthetic organic materials, especially those with crosslinked structures, are normally static and cannot post-vary their sizes and properties without compromising material mechanical performances for sustainable use. Inspired by Planaria, we here report a growing-degrowing strategy of enabling thermosetting materials to either absorb or release components for continuously changing their sizes, shapes, compositions, and a set of properties simultaneously. The strategy is based on the monomer-polymer equilibrium of networks in which supplying or removing small polymerizable components would drive the networks toward expansion or contraction. Using acid-catalyzed equilibration of siloxane as an example, we demonstrate that the size and mechanical properties of the resulting silicone materials can be significantly or finely tuned in both directions of growth and decomposition. The equilibration can be turned off to yield stable products or reactivated again. During the degrowing-growing circle, material structures are selectively varied either uniformly or heterogeneously, by the availability of fillers. Our strategy endows the materials with many appealing capabilities including environment adaptivity, self-healing, and switchability of surface morphologies, shapes, and optical properties. Since monomer-polymer equilibration exists in many polymers, we envision the expansion of the presented strategy to various systems for many applications.