The development of polymers that can spontaneously repair themselves after mechanical damage would significantly improve the safety, lifetime, energy efficiency, and environmental impact of manmade materials. Most approaches to self-healing materials either require the input of external energy, or need healing agents, solvent or plasticizer. Despite intense research in this area, the synthesis of a stiff material with intrinsic self-healing ability remains a key challenge. Our laboratory has recently succeeded in a design of multiphase supramolecular thermoplastic elastomers that combine high modulus and toughness with spontaneous healing capability. In one design, H-bonding brush polymers (HBPs) self-assemble into hard-soft microphaseseparated system, combining the enhanced stiffness and toughness of hybrid polymers with the self-healing capacity of dynamic supramolecular assemblies [1]. In another design, supramolecular ABA triblock copolymers formed by dimerization of 2-ureido-4-pyromidone (UPy) end-functionalized polystyrene-b-poly(n-butylacrylate) (PS-b-PBA) AB diblock copolymers are synthesized, resulting in a self-healing material that combines the advantageous mechanical properties of thermoplastic elastomers and the dynamic self-healing features of supramolecular materials [2]. In contrast to previous self-healing polymers, our systems spontaneously self-heals as a single-component solid material at ambient conditions without the need of any external stimulus, healing agent, plasticizer, or solvent.

Natural materials employ many elegant strategies to achieve mechanical properties required for survival under varying environmental conditions. Thus these remarkable biopolymers and nanocomposites often not only have a combination of mechanical properties such as high modulus, toughness, and elasticity, but also exhibit adaptive and stimuli-responsive properties. Inspired by skeletal muscle protein titin, we have synthesized a biomimetic modular polymer that not only closely mimics the modular multi-domain structure of titin, but also manifests an exciting combination of mechanical properties, as well as adaptive properties such as self-healing and temperature-responsive shape-memory properties.Whereas man-made polymers can be prepared to meet particular parameters one at a time, it remains a challenge to design synthetic polymers with a combination of mechanical properties such as high modulus, toughness, and resilience. A further challenge is to introduce adaptive properties into polymers. In contrast, many smart strategies have evolved in nature to achieve biopolymers possessing excellent combinations of mechanical properties. 1 To survive in often variable environments, natural materials have also evolved to be adaptive, maintaining functions across a range of stress or strain, or changing properties in response to stimuli such as temperature or moisture level. 2 In recent years, the elucidation of molecular mechanisms for natural materials has prompted many biomimetic materials designs. 3 Herein, we report a Supporting Information Available: Synthesis and characterization of monomers and polymers, stress-strain, DMA, X-ray, AFM, and molecular modeling experiments. This material is available free of charge at http//:pubs.acs.org. biomimetic design of a modular polymer that has a combination of high modulus, toughness, and resilience, while possessing adaptive mechanical properties. NIH Public AccessOur biomimetic concept is based on the modular domain design observed in the skeletal muscle protein titin, which possesses a remarkable combination of strength, toughness, and elasticity. 4 The ability of titin to absorb energy by the reversible rupture of intramolecular secondary interactions, followed by re-folding induced recovery, makes it an intriguing model for the design of adaptive materials. Following titin's modular design, our group first synthesized polymers incorporating the quadruple hydrogen bonding 2-ureido-4[1H]-pyrimidone (UPy) motif 5 as the modular domain-forming mimic of the Ig domains in titin. 6 To overcome issues such as structural heterogeneity and inter-chain cross-linking, we further developed a cyclic modular polymer using a peptidomimetic β-sheet dimer. 7 Despite the well-defined single molecule unfolding properties, the synthesis of the second generation polymers is tedious, and the rupture forces of the H-bonded modules are much lower than the UPy dimer. To simplify synthesis and improve mechanical strength, the cyclic modular concept was applied to the UPy core. In our pre...

Polymer, heal thyself! Supramolecular ABA triblock copolymers formed by dimerization of 2-ureido-4-pyrimidinone (UPy) end-functionalized polystyrene-b-poly(n-butyl acrylate) (PS-b-PBA) AB diblock copolymers have been synthesized, resulting in a self-healing material that combines the advantageous mechanical properties of thermoplastic elastomers and the dynamic self-healing features of supramolecular materials.

Under eons of evolutionary and environmental pressure, biological systems have developed strong and lightweight peptide-based polymeric materials by using the 20 naturally occurring amino acids as principal monomeric units. These materials outperform their man-made counterparts in the following ways: 1) multifunctionality/tunability, 2) adaptability/stimuli-responsiveness, 3) synthesis and processing under ambient and aqueous conditions, and 4) recyclability and biodegradability. The universal design strategy that affords these advanced properties involves "bottom-up" synthesis and modular, hierarchical organization both within and across multiple length-scales. The field of "biomimicry"-elucidating and co-opting nature's basic material design principles and molecular building blocks-is rapidly evolving. This Review describes what has been discovered about the structure and molecular mechanisms of natural polymeric materials, as well as the progress towards synthetic "mimics" of these remarkable systems.

For rational design of advanced polymeric materials, it is critical to establish a clear mechanistic link between the molecular structure of a polymer and the emergent bulk mechanical properties. Despite progress towards this goal, it remains a major challenge to directly correlate the bulk mechanical performance to the nanomechanical properties of individual constituent macromolecules. Here, we show a direct correlation between the single-molecule nanomechanical properties of a biomimetic modular polymer and the mechanical characteristics of the resulting bulk material. The multi-cyclic single-molecule force spectroscopy (SMFS) data enabled quantitative derivation of the asymmetric potential energy profile of individual module rupture and re-folding, in which a steep dissociative pathway accounted for the high plateau modulus, while a shallow associative well explained the energy-dissipative hysteresis and dynamic, adaptive recovery. These results demonstrate the potential for SMFS to serve as a guide for future rational design of advanced multifunctional materials.