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

A team from the Klausen, Kulik, and Craig labs report the preparation of a new class of oligosilane dienes and their acyclic diene metathesis (ADMET) polymers. Their data suggest that substitution of carbon with silicon in linear polymers will have a substantial effect on the mechanical properties of materials, and motivates the design of polymer networks with heavier carbon analogues that will be future synthetic targets.  Article Link

A team from the Olvera de la Cruz and Kalow labs combined coarse-grained molecular dynamics simulations with a Monte Carlo method to investigate the topological structural changes, microscopic dynamics, and linear rheology of unentangled side-chain-linked vitrimers in conjunction with the sticky Rouse model (SRM). The results indicated that the linear rheology of unentangled vitrimers with a fast bond-exchange rate can be analyzed via a single-chain approach based on the SRM. Article Link

A team from the Johnson, Craig, and Kulik labs introduce a new class of polymer metal–organic cage (polyMOC) gels featuring polyethylene glycol (PEG) strands of varied length cross-linked through bis-pyridyl-carbazole-based M6L12 cubes, where M is Pd(II), Pt(II), or mixtures thereof. The work introduces a novel MOC architecture for polyMOC design, shows that polyMOCs can be prepared from mixtures of Pd(II)/Pt(II), and demonstrates that polyMOCs display unique relaxation behavior due to their multivalent junctions, offering a strategy for controlling polyMOC properties independently of their polymer components. Article Link

A team from the Craig and Kulik groups crosslinked polybutadiene backbone polymers by complexation with two different metal salts. Though dynamic mechanical analysis (DMA) and small-angle X-ray scattering (SAXS) indicate that the crosslinking density and topology of the two materials are the same, the material crosslinked with copper ions exhibits a higher extensibility and fracture energy than the polymer crosslinked with iron. Article Link

The Johnson lab reports bifunctional silyl ether (BSE)-containing high-density polyethylene (HDPE)-like materials synthesized through a one-pot catalytic ring-opening metathesis polymerization (ROMP) and hydrogenation sequence. The crystallinity of these materials can be adjusted by varying the BSE concentration or the steric bulk of the Si-substituents, providing handles to control thermomechanical properties. Article Link

Danyang Chen has successfully defended his thesis, "Elasticity and Fracture of Polymer Networks with Entanglements and Weak Crosslinkers". Congratulations Dr. Chen!