A team from the Rubinstein and Craig labs present a modified Lake–Thomas theory that accounts for the molecular details of network connectivity upon crack propagation in polymer networks. In comparison with the “strong” mechanophores that can only be activated in the bridging strand, “weak” mechanophores that can be activated both in the bridging strand and in other generations could provide more energy dissipation due to their larger contribution to higher generations of the tree. Article Link

A team from the Craig and Olvera de la Cruz labs report a polyelectrolyte handle for single-molecule force spectroscopy that offers a combination of high attachment forces, good success in obtaining a high-force attachment, a non-fouling detachment process that allows for repetition, and specific attachment locations along the polymer analyte. The polyelectrolyte handle approach will prove useful for selected applications in covalent polymer mechanochemistry. Article Link

A team from the Craig and Johnson labs use mechanophore force probes with discrete molecular weights to determine how the deformation and tension experienced by a strand is influenced by strand length. The combination of discrete strand synthesis and mechanochromism provides a foundation to further test and develop molecular-based theories of elasticity and fracture in polymer networks. Article Link

MONET JHU trainee Herb Wakefield gave an invited talk on "Precise Synthesis and Reactivity of Organometallic Building BLocks of Novel Polymers" at Kenyon College. Following the talk, Herb discussed the process to choose and apply to grad schools with undergraduates in the departments of Chemistry and Biochemistry & Molecular Biology.

A team from the Klausen, Johnson, Craig, and Kulik labs report the geometrically selective synthesis of trans- and cis-silacycloheptenes via a novel synthetic strategy and probe the effect of Si for C substitution on ring-opening metathesis polymerization (ROMP). The results point to the possibility of employing Si-rich polymers to prove the contribution of strand elasticity to the mechanical properties of bulk polymer networks. Article Link

A team from the Steinmetz and Craig labs investigate the effects of linear and multivalent polyethylene glycol (PEG) coatings on the tobacco mosaic virus (TMV) at varying chain lengths on serum protein adsorption, antibody recognition, and macrophage uptake. The work provides insight into the PEGylation of virus-based nanoparticles (VNPs), which may improve the possibility of their implementation in clinical applications. Article Link

A team from the Kalow and Kulik labs investigate the relationship between conjugate acceptor structure, kex, and viscoelasticity for conjugate addition-elimination using a series of dithioalkylidenes and their corresponding hydrogels. By incorporating the corresponding cross-links into photopolymerized hydrogels, they further demonstrated that the hydrogel’s characteristic stress relaxation time (τ) is directly proportional to molecular kex. Overall, they demonstrate that mechanistic insight into cross-link exchange and parametrization of cross-link reactivity enable the design of materials with targeted viscoelasticity. Article Link

When a polyacrylate is crosslinked by weak, strong, or intermediate strength cross-linkers, the Craig, Johnson, and Rubinstein labs find that the weakest cross-linker leads to the strongest network, and vice versa. The effect is large enough (factor of 9-10 in toughness) to be important, and some systematic studies uncover a physical picture at the heart of the phenomenon, in a way that others can actually transfer the insights in the design of other material classes. Article Link

Extending polymer chains results in a positive chain tension, 𝑓 ch , due primarily to conformational restrictions. At the level of individual bonds, however, tension 𝑓 𝑏 is either negative or positive and depends both on chain tension and bulk pressure. Typically, the chain and bond tension are assumed to be directly related. The Rubinstein lab shows by molecular dynamics simulations and polymer theory that in specific systems, however, this dependence may not be intuitive, whereby 𝑓 ch increases while 𝑓 𝑏 decreases, i.e., the entire chain is extended, while bonds are compressed.   Article Link

A team from the Rubinstein lab present an equilibrium statistical mechanical theory for the formation of reversible networks in two-component solutions of associative polymers to account for the phase behavior due to hydrogen-bonding, metal–ligand, electrostatic, or other pairwise heterotypic associative interactions. These results demonstrate that reversibly associating polymers have a large parameter space in terms of molecular design, binding energy, and mixture compositions. The predictions are expected to be useful in the rational design of interacting polymer mixtures and the formation of reversible networks.   Article Link

Scott Danielsen of the Rubinstein lab extends mean-field equilibrium theory for reversible network formation due to heterotypic pairwise interactions in mixtures of associative polymers via a weak inhomogeneity expansion to account for spatial fluctuations due to chemical incompatibility. He shows that the chemical incompatibility between A and B polymers drives a competition between associative and segregative phase separation. The reactive blending of such multifunctional polymers presents the opportunity to envision novel properties, processing conditions, and applications accessible by the tunable production of supramolecular complexes, mesophases, and multicomponent polymer networks.  Article Link

A team from the Johnson lab use ring-opening metathesis polymerization to synthesize terpolymers of (1) a “functional” monomer (e.g., a polyethylene glycol macromonomer or dicyclopentadiene); (2) a monomer containing an electrophilic pentafluorophenyl (PFP) substituent; and (3) a cleavable monomer based on a bifunctional silyl ether. This method is shown to be effective for deconstruction of polyethylene glycol (PEG) based graft terpolymers in organic or aqueous conditions as well as polydicyclopentadiene (pDCPD) thermosets, significantly expanding upon the versatility of bifunctional silyl ether based functional polymers. Article Link