Crosslinked polymers made from soluble monomer precursors are an increasingly important class of soft materials with applications ranging from regenerative medicine to commodity materials. Typically, dynamic moduli of polymers are highly controllable via backbone composition and crosslinking density. Two critical limitation of polymers have been that, once synthesized, the initial modulus of a polymer network is fixed, and polymers typically weaken under repeated strain. This latter feature limits their utility in any applications that require increasing strength during or as a result of processing. Building analogous attributes into polymeric systems could greatly broaden their full potential and utility in applications as stable and mechanically robust adhesives, coatings, fabricated articles, and potentially biomaterials. Imparting strain-induced strengthening to polymer networks represents a major opportunity and unmet need.
Recent innovations have generated dynamically responsive materials with increasing moduli induced via light or heat. However, these methods typically require sophisticated chemistries and would not work for materials in dark environments such as adhesives or heat-sensitive substrates like papers and biomaterial platforms. Attempts to selectively tailor the strength and functionality of synthetic polymer networks by exploiting mechanical deformation have resulted in transient stiffening due to chain associations or elasticity, but have not achieved permanent strain-induced modulus or crosslinking increases across the range of deformations. Thus, strain-induced crosslinking approach provides an alternative, new class of simple, mechanically-driven crosslinking strategies via pressure, elongation or ultrasound that circumvent many limitations of these traditional approaches.
Creation of polymeric materials containing hidden, namely cryptic, binding sites that provide on-demand improvements in mechanical properties is of great interest. While only force is required to achieve such property strengthening, this approach enables material stiffening either during fabrication, such as through extrusion or foaming, or post-adhesion through compression or ultrasound, without any complicated equipment required. Furthermore, this mechanical induction creates fully mechanically sensitive networks and allows for simple, accessible chemistries to afford stiffened, force-responsive polymers. Such strain-induced crosslinking activation would be particularly useful in adhesive applications, coatings, elastomers, structural components, and the like.