Mortise-and-tenon joints have been widely used for thousands of years in wooden architectures in virtue of their artistic and functional performance. However, imitation of similar structural and mechanical design philosophy to construct mechanically adaptive materials at the molecular level is a challenge. Herein, we report a mortise-and-tenon joint inspired mechanically interlocked network (MIN), in which the [2]rotaxane crosslink not only mimics the joint in structure, but also reproduces its function in modifying mechanical properties of the MIN. Benefiting from the hierarchical energy dissipative ability along with the controllable intramolecular movement of the mechanically interlocked crosslink, the resultant MIN simultaneously exhibits notable mechanical adaptivity and structural stability in a single system, as manifested by decent stiffness, strength, toughness, and deformation recovery capacity.

A Mortise-and-Tenon Joint Inspired Mechanically Interlocked Network

Molecular photoactuators can control shape and chemical or physical properties of the responsive system they are embedded in. These effects are usually mediated by supramolecular interactions and can be amplified to perform work at the micro- and macroscopic scale, for instance, in materials and biomimetic systems. While many studies focus on the observable outcome of these events, photoresponsive structures can also translate their conformational change to molecular components and perform work against random Brownian motion. Stereochemical cascades can amplify light-generated motion to a distant moiety of the same molecule or molecular assembly, via conformationally restricted stereogenic elements. Being able to control the conformation or motion of molecular systems remotely provides prospects for the design of the smallest machines imaginable. This Focus Review emphasizes the emergence of directed, coupled motion of remote functionalities triggered by light-powered switches and motors as a tool to control molecular topology and function.

Directing Coupled Motion with Light: A Key Step Toward Machine-Like Function

A critical review on performance and phase separation of thermosetting epoxy asphalt binders and bond coats

Polymer adhesion is ubiquitous in both the natural world and human technology. It is also a complex multiscale phenomenon, such that the solution of adhesion problems requires a convergence of chem...

Polymer Adhesion: Seeking New Solutions for an Old Problem#

We report a straightforward chemical strategy to tackle current challenges of irreversible deformation in low T g vitrimers at operating temperature. In particular, vinylogous urethane (VU) vitrimers were prepared where reactive free amines, necessary for material flow, were temporarily shielded inside the network backbone, by adding a small amount of dibasic ester to the curing mixture. The amines could be released as reactive chain ends from the resulting dicarboxamide bonds via thermally reversible cyclisation to an imide moiety. Indeed, (re)generation of the required nucleophilic amines as network defects ensured reprocessing and rapid material flow at higher temperature, where exchange dynamics are (re)activated. As a result, VU vitrimers were obtained with limited creep at service temperature, yet with good reprocessability at elevated temperatures. Thus, by exerting strong control on molecular level over the availability of exchangeable functional groups, a remarkable improvement of VU properties was obtained.

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Suppressing Creep and Promoting Fast Reprocessing of Vitrimers with Reversibly Trapped Amines.

Polymer networks are complex systems consisting of molecular components. Whereas the properties of the individual components are typically well understood by most chemists, translating that chemical insight into polymer networks themselves is limited by the statistical and poorly defined nature of network structures. As a result, it is challenging, if not currently impossible, to extrapolate from the molecular behavior of components to the full range of performance and properties of the entire polymer network. Polymer networks therefore present an unrealized, important, and interdisciplinary opportunity to exert molecular-level, chemical control on material macroscopic properties. A barrier to sophisticated molecular approaches to polymer networks is that the techniques for characterizing the molecular structure of networks are often unfamiliar to many scientists. Here, we present a critical overview of the current characterization techniques available to understand the relation between the molecular properties and the resulting performance and behavior of polymer networks, in the absence of added fillers. We highlight the methods available to characterize the chemistry and molecular-level properties of individual polymer strands and junctions, the gelation process by which strands form networks, the structure of the resulting network, and the dynamics and mechanics of the final material. The purpose is not to serve as a detailed manual for conducting these measurements but rather to unify the underlying principles, point out remaining challenges, and provide a concise overview by which chemists can plan characterization strategies that suit their research objectives. Because polymer networks cannot often be sufficiently characterized with a single method, strategic combinations of multiple techniques are typically required for their molecular characterization.

Molecular Characterization of Polymer Networks.

Here, we report the effect of force applied to the biaryl backbone of a bisphosphine ligand on the rate of oxidative addition of bromobenzene to a ligand-coordinated palladium center. Local compressive and tensile forces on the order of 100 pN were generated using a stiff stilbene force probe. A compressive force increases the rate of oxidative addition, whereas a tensile force decreases the rate, relative to that of the parent complex of strain-free ligand. Rates vary by a factor of ∼6 across ∼340 pN of force applied to the complexes. The crystal structures and DFT calculations support that force-induced perturbation of the geometry of the reactant is negligible. The force-rate relationship observed is mainly attributed to the coupling of force to nuclear motion comprising the reaction coordinate. These observations inform the development of catalysts whose activity can be tuned by an external force that is adjusted within a catalytic cycle.

Patricia N. Johnson

Papers

Molecular Characterization of Polymer Networks.

Mechanochemical Regulation of Oxidative Addition to a Palladium(0) Bisphosphine Complex.