Materials often exhibit a trade-off between stiffness and extensibility; for example, strengthening elastomers by increasing their cross-link density leads to embrittlement and decreased toughness. Inspired by cuticles of marine mussel byssi, we circumvent this inherent trade-off by incorporating sacrificial, reversible iron-catechol cross-links into a dry, loosely cross-linked epoxy network. The iron-containing network exhibits two to three orders of magnitude increases in stiffness, tensile strength, and tensile toughness compared to its iron-free precursor while gaining recoverable hysteretic energy dissipation and maintaining its original extensibility. Compared to previous realizations of this chemistry in hydrogels, the dry nature of the network enables larger property enhancement owing to the cooperative effects of both the increased cross-link density given by the reversible iron-catecholate complexes and the chain-restricting ionomeric nanodomains that they form.

In this Article we described a ruthenium-catalysed carbonyl addition method for alcohol production via simple unsubstituted hydra-zone intermediates, but we inadvertently omitted the citation of two papers that had previously reported a similar carbanion reactivity 1,2. In these papers, the authors illustrated a series of substituted hindered hydrazones (for example, tert-butyl-, trityl-and diphenyl-4-pyri-dylmethyl) for additions to carbonyl compounds; however, to yield the target alcohols under these circumstances, the lithium salts of these hydrazones had to be pre-formed, with subsequent CC bond formation and removal of bulky substituents on azo-intermediates via radical decomposition. References 1. Baldwin, J. E. et al. Azo anions in synthesis: use of trityl-and diphenyl-4-pyridylmethylhydrazones for reductive C−C bond formation. Tetrahedron 42, 4235−4246 (1986). 2. Baldwin, J. E., Bottaro, J. C., Kolhe, J. N. & Adlington, R. M. Azo anions in synthesis. Use of trityl-and diphenyl-4-pyridylmethyl-hydrazones for reductive CC bond formation from aldehydes and ketones. J. Chem. Soc. Chem. Commun. 22−23 (1984). Addendum: Aldehydes as alkyl carbanion equivalents for additions to carbonyl compounds © 2 0 1 7 M a c m i l l a n P u b l i s h e r s L i m i t e d , p a r t o f S p r i n g e r N a t u r e. A l l r i g h t s r e s e r v e d .

Understanding the fundamental wetting behavior of liquids on surfaces with pores or cavities provides insights into the wetting phenomena associated with rough or patterned surfaces, such as skin and fabrics, as well as the development of everyday products such as ointments and paints, and industrial applications such as enhanced oil recovery and pitting during chemical mechanical polishing. We have studied, both experimentally and theoretically, the dynamics of the transitions from the unfilled/partially filled (Cassie-Baxter) wetting state to the fully filled (Wenzel) wetting state on intrinsically hydrophilic surfaces (intrinsic water contact angle <90°, where the Wenzel state is always the thermodynamically favorable state, while a temporary metastable Cassie-Baxter state can also exist) to determine the variables that control the rates of such transitions. We prepared silicon wafers with cylindrical cavities of different geometries and immersed them in bulk water. With bright-field and confocal fluorescence microscopy, we observed the details of, and the rates associated with, water penetration into the cavities from the bulk. We find that unconnected, reentrant cavities (i.e., cavities that open up below the surface) have the slowest cavity-filling rates, while connected or non-reentrant cavities undergo very rapid transitions. Using these unconnected, reentrant cavities, we identified the variables that affect cavity-filling rates: () the intrinsic contact angle, () the concentration of dissolved air in the bulk water phase (i.e., aeration), () the liquid volatility that determines the rate of capillary condensation inside the cavities, and () the presence of surfactants.

We investigated the morphology, topology, and mechanical characteristics of a loosely cross-linked epoxy network as a function of the varying content of catechol moieties capable of forming reversible, ionic iron− catecholate cross-links. The primary epoxy network structure was kept fixed by a constant mole ratio of difunctional poly(ethylene glycol), monofunctional diluent, and diamine cross-linking agent in all samples. We then systematically replaced the catechol monoepoxide diluent by methyl glycidyl ether, which is incapable of participating in ionic complex formation. This allows the effects of the catechol content on network properties to be isolated and analyzed. Our results support a model in which increasing the concentration of catechol moieties promotes the formation of closely spaced iron− catecholate complex sites. This enables cooperative interactions between netpoints and produces a dramatic improvement in tensile properties. Such ionic interactions are thus a promising approach to creating stiff, strong, and tough load-bearing polymer networks.

Ensembles of amino acid side chains often dominate the interfacial interactions of intrinsically disordered proteins; however, backbone contributions are far from negligible. Using a combination of nanoscale force measurements and molecular dynamics simulations, we demonstrated with analogous mussel-mimetic adhesive peptides and peptoids both 34 residues long that highly divergent adhesive/cohesive outcomes can be achieved on mica surfaces by altering backbone chemistry only. The Phe, Tyr, and Dopa containing peptoid variants used in this study deposited as dehydrated and incompressible films that facilitated analysis of peptoid side chain contributions to adhesion and cohesion. For example, whereas Phe and Dopa peptoids exhibited similar cohesion, Dopa peptoids were ∼3 times more adhesive than Phe peptoids on mica. Compared with the peptides, Phe peptoid achieved only ∼20% of Phe containing peptide adhesion, but the Dopa peptoids were >2-fold more adhesive than the Dopa peptides. Cation−π interactions accounted for some but not all of the cohesive interactions. Our results were corroborated by molecular dynamics simulations and highlight the importance of backbone chemistry and the potential of peptoids or peptoid/ peptide hybrids as wet adhesives and primers.

Using a surface forces apparatus (SFA), we quantitatively study the influence of surface damage on the contact mechanics of self-mated glassy polystyrene (PS) films. We use the SFA to measure the contact radius, surface profile, and normal force between the films, including the adhesion force. The molecular weight (MW) of the polymer influences the repeatability of the adhesion measurements and the effective surface energy calculated using the Johnson−Kendall−Roberts (JKR) theory. For low-MW PS (MW = 2.33 kDa), the effective surface energy increases over repeated adhesion cycles as the films become progressively damaged. For high-MW PS (MW = 280 kDa), the effective surface energy is constant over repeated adhesion cycles, but hysteresis is still present, manifested in a smaller contact radius during compression of the surfaces than during separation. Our results demonstrate that while the JKR theory is appropriate for describing the contact mechanics of glassy polymer thin films on layered elastic substrates, the contact mechanics of low-MW polymer films can be complicated by surface damage to the films.

Impurity incorporation into nanoparticles is modeled using thermodynamics. For small particles, entropically driven impurity incorporation is reduced, rendering doping difficult. We show that the free energy of surface impurities in small nanoparticles is lower than core impurities, surface doping therefore occurs preferentially. A critical size for core doping is identified, below which it is energetically unfavorable. In all cases, core impurity concentration is reduced as particle size decreases. We show larger than bulk impurity concentrations are possible, corresponding to increased alloying.

The adhesion force between individual human hair fibers in a crosshair geometry was measured by observing their natural bending and adhesive jumps out of contact, using optical video microscopy. The hair fibers' natural elastic responses, calibrated by measuring their natural resonant frequencies, were used to measure the forces. Using a custom-designed, automated apparatus to measure thousands of individual hair−hair contacts along millimeter length scales of hair, it was found that a broad, yet characteristic, spatially variant distribution in adhesion force is measured on the 1 to 1000 nN scale for both clean and conditioner-treated hair fibers. Comparison between the measured adhesion forces and adhesion forces modeled from the hairs' surface topography (measured using confocal laser profilometry) shows they have a good order-of-magnitude agreement and have similar breadth and shape. The agreement between the measurements and the model suggests, perhaps unsurprisingly, that hair−hair adhesion is governed, to a first approximation, by the unique surface structure of the hairs' cuticles and, therefore, the large distribution in local mean curvature at the various individual contact points along the hairs' lengths. We posit that haircare products could best control the surface properties (or at least the adhesive properties) between hairs by directly modifying the hair surface microstructure.