Toughening hydrogels through force-triggered chemical reactions that lengthen polymer strands
Longer and stronger; stiff but not brittle: Hydrogels are highly water-swollen, cross-linked polymers. Although they can be highly deformed, they tend to be weak, and methods to strengthen or toughen them tend to reduce stretchability. A new paper out of MONET now reports a strategy to create more durable hydrogels. Wang et al. introduce a toughening mechanism by storing releasable extra chain length in the stiff part of a double-network hydrogel. A high applied force triggers the opening of cyclic strands that are only activated at high chain extension.
Zi Wang, Xujun Zheng, Tetsu Ouchi, Tatiana Kouznetsova, Haley Beech, Sarah Av-Ron, Takahiro Matsuda, Brandon Bowser, Shu Wang, Jeremiah Johnson, Julia Kalow, Bradley Olsen, Jian Ping Gong, Michael Rubinstein, and Stephen Craig
Single-event Spectroscopy and Unravelling Kinetics of Covalent Domains Based on Cyclobutane Mechanophores
Mechanochemical reactions that lead to an increase in polymer contour length have the potential to serve as covalent synthetic mimics of the mechanical unfolding of noncovalent “stored length” domains in structural proteins. Here we report the force-dependent kinetics of stored length release in a family of covalent domain polymers based on cis-1,2-substituted cyclobutane mechanophores. The stored length is determined by the size (n) of a fused ring in an [n.2.0] bicyclic architecture, and it can be made sufficiently large (>3 nm per event) that individual unravelling events are resolved in both constant-velocity and constant-force single-molecule force spectroscopy (SMFS) experiments. Replacing a methylene in the pulling attachment with a phenyl group drops the force necessary to achieve rate constants of 1 s–1 from ca. 1970 pN (dialkyl handles) to 630 pN (diaryl handles), and the substituent effect is attributed to a combination of electronic stabilization and mechanical leverage effects. In contrast, the kinetics are negligibly perturbed by changes in the amount of stored length. The independent control of unravelling force and extension holds promise as a probe of molecular behavior in polymer networks and for optimizing the behaviors of materials made from covalent domain polymers.
B. H. Bowser, S. Wang, T. B. Kouznetsova, H. K. Beech, B. D. Olsen, M. Rubinstein, and S. L. Craig, J. Am. Chem. Soc., online access.
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.
S. P. O. Danielsen, H. K. Beech, B. M. El-Zaatari, X. Wang, D. J. Lundberg, G. Stoychev, L. Sapir, S. Wang, Z. Wang, T. Ouchi, P. N. Johnson, Y. Hu, S. L. Craig, J. A. Kalow, J. A. Johnson, B. D. Olsen, and M. Rubinstein, Chem. Rev., 2021.
A collaborative effort from ten(!) MONET researchers has been selected to be featured in ACS Editors’ Choice, through which it is sponsored for immediate, open access by ACS due to its potential for broad public interest, an honor given to only one article from the entire ACS portfolio each day of the year.
PolyDAT: A Generic Data Schema for Polymer Characterization
Tzyy-Shyang Lin, Nathan J. Rebello, Haley K. Beech, Zi Wang, Bassil El-Zaatari, David J. Lundberg, Jeremiah A. Johnson, Julia A. Kalow, Stephen L. Craig, and Bradley D. Olsen
We know now what a molecular substituent effect on reactivity might feel like to the touch, thanks to this work by Shu, Haley, and colleagues! Read all about it in work just published in J. Am. Chem. Soc.
Mechanism Dictates Mechanics: A Molecular Substituent Effect in the Macroscopic Fracture of a Covalent Polymer Network
Shu Wang, Haley K. Beech, Brandon H. Bowser, Tatiana B. Kouznetsova, Bradley D. Olsen, Michael Rubinstein, and Stephen L. Craig
In 1907, the 1st synthetic polymer, Bakelite, was invented by Leo Baekeland. Since that time, synthetic polymers have become ubiquitous in the daily lives of billions of people around the globe due to their wide-ranging properties and utility. In recent years, the environmental and biological dangers of polymers have been increasingly made known to both the scientific community and the general public. Despite the awareness of these dangers, the common slime demonstration has remained a staple for those seeking to introduce polymers in educational settings. In this talk, Dr. Boyd will discuss the current state of environmental polymer contamination and propose alternate ways in which polymers can be introduced into STEM education.
Congratulations to Julia Kalow, recently named one of only 4 recipients of the 2021 Marion Milligan Mason award by the AAAS! The biennial award, which grants $55,000 each to four or five early-career female scientists conducting basic research in the chemical sciences, are funded from a $2.2 million bequest to AAAS in Mason’s will to both support women in chemistry and honor her own family’s commitment to women’s education. The award winners are selected through a two-stage review process. Research proposals are subjected to criteria that include the potential of their research to advance knowledge and understanding in their field and beyond and to benefit society.