Mechanics of a Dual Cross-Link Gel with Dynamic Bonds: Steady State Kinetics and Large Deformation Effects

We develop a three-dimensional continuum theory for a dual cross-link gel with permanent and transient bonds. This theory connects the breaking and re-forming kinetics of the transient bonds to the large deformation behavior of the gel. Although based on similar principles as our previous theory for the same gel, the new theory addresses a limitation of the previous theory and agrees equally well with experimental data. Specifically, in the new theory we assume that breaking and re-forming kinetics of transient bonds reaches a steady state. This assumption leads to fewer material parameters and enables the new theory to capture the healing process of the dual cross-link gel. In addition, the steady state assumption also allows the new theory to reduce to classical linear viscoelastic theory in the limit of infinitesimal strains, where the kinetics of bond breaking and re-formation can be linked directly to the relaxation function. We also extend the theory to account for strain stiffening under very large deformation. The extended theory with strain stiffening agrees well with experimental data for nominal strain up to 400%. Finally, we extend our theory to gels with multiple types of dynamic bonds which has been recently exploited as a strategy to engineer viscoelastic behavior of gels

Characterizing Adhesion between a Micropatterned Surface and a Soft Synthetic Tissue

The work of adhesion and work of separation are characteristic properties of a contact interface that describe the amount of energy per unit area required to adhere or separate two contacting substrates, respectively. In this work, the authors present experimental and data analysis procedures that allow the contact interface between a soft synthetic tissue and a smooth or micropatterned poly­(dimethylsiloxane) (PDMS) substrate to be characterized in terms of these characteristic parameters. Because of physical geometry limitations, the experimental contact geometry chosen for this study differs from conventional test geometries. Therefore, the authors used finite element modeling to develop correction factors specific to the experimental contact geometry used in this work. A work of adhesion was directly extracted from experimental data while the work of separation was estimated on the basis of experimental results. These values are compared to other theoretical calculations for validation. The results of this work indicate that the micropatterned PDMS substrate significantly decreases both the work of adhesion and work of separation as compared to a smooth PDMS substrate when in contact with a soft synthetic tissue substrate

Time Dependent Behavior of a Dual Cross-Link Self-Healing Gel: Theory and Experiments

Recent experiments have shown that hydrogels with enhanced toughness can be synthesized by incorporating self-healing physical cross-links in a chemically cross-linked gel network. These gels exhibit rate dependent mechanical behavior, suggesting that improved mechanical properties are closely tied to the breaking and reattaching of temporary cross-links in the gel network. In this work, the connection between rate dependent mechanical behavior and kinetics of breaking and reattachment of temporary cross-links is quantified using a three-dimensional finite strain constitutive model. The parameters of the model are fitted using relaxation and constant strain rate tests in uniaxial tension of a model dual-cross-link gel. The stress versus time curves of more complex strain histories, involving loading followed by unloading at different rates, is successfully and quantitatively predicted by our model. Such modeling strategy combining physically based kinetics and three-dimensional large strain mechanics shows great promise for quantitative modeling of soft biological tissues and synthetic counterparts containing dynamic bonds

Graphene Blisters with Switchable Shapes Controlled by Pressure and Adhesion

We created graphene blisters that cover and seal an annular cylinder-shaped microcavity in a SiO₂ substrate filled with a gas. By controlling the pressure difference between the gas inside and outside of the microcavity, we switch the graphene membrane between multiple stable equilibrium configurations. We carried out experiments starting from the situation where the pressure of the gas inside and outside of the microcavity is set equal to a prescribed charging pressure, <i>p</i>₀ and the graphene membrane covers the cavity like an annular drum, adhered to the central post and the surrounding substrate due to van der Waals forces. We decrease the outside pressure to a value, <i>p</i>_e which causes it to bulge into an annular blister. We systematically increase the charging pressure by repeating this procedure causing the annular blister to continue to bulge until a critical charging pressure <i>p</i>_cⁱ is reached. At this point the graphene membrane delaminates from the post in an unstable manner, resulting in a switch of graphene membrane shape from an annular to a spherical blister. Continued increase of the charging pressure results in the spherical blister growing with its height increasing, but maintaining a constant radius until a second critical charging pressure <i>p</i>_c^o is reached at which point the blister begins to delaminate from the periphery of the cavity in a stable manner. Here, we report a series of experiments as well as a mechanics and thermodynamic model that demonstrate how the interplay among system parameters (geometry, graphene stiffness (number of layers), pressure, and adhesion energy) results in the ability to controllably switch graphene blisters among different shapes. Arrays of these blisters can be envisioned to create pressure-switchable surface properties where the difference between patterns of annular versus spherical blisters will impact functionalities such as wettability, friction, adhesion, and surface wave characteristics

Gold-Catalyzed Enantio- and Diastereoselective Syntheses of Left Fragments of Azadirachtin/Meliacarpin-Type Limonoids

Meliacarpin-type limonoids are an important class of organic insecticides. Their syntheses are challenging due to their chemical complexity. Here, we report the highly enantio- and diastereoselective synthesis of the left fragments of azadirachtin I and 1-cinnamoylmelianolone, being two important family members of meliacarpin-type limonoids, via pairwise palladium- and gold-catalyzed cascade reactions. Gold-catalyzed reactions of 1,7-diynes were performed as model studies, and the efficient construction of tetracyclic late-stage intermediates was achieved on the basis of this key transformation. Our unique route gave both of the left fragments in 23 steps from the commercially available chiral starting material (−)-carvone. This study significantly advances research on the synthesis of the meliacarpin-type limonoids

Synthetic Progress toward Azadirachtins. 1. Enantio- and Diastereoselective Synthesis of the Left-Wing Fragment of 11-<i>epi</i>-Azadirachtin I

A highly enantio- and diastereoselective synthesis of the left-wing fragment of 11-<i>epi</i>-azadirachtin I characterized with the pairwise use of palladium- and gold-catalyzed cascade reactions is presented. By enlisting a sequence of stereocontrolled transformations, our 21-step route established the stereocenters of the left-wing fragment from one chiral starting material, (−)-carvone, which would significantly facilitate the synthetic studies of the azadirachtin-type limonoids

Effective Chirality Transfer in [3+2] Reaction between Allenyl-Rhodium and Enal: Mechanistic Study Based on DFT Calculations

Theoretical calculation was performed to study the chirality transfer in a newly reported intramolecular [3+2] cycloaddition of enal and alleno rhodium species, generated <i>in situ</i> from an enynol precursor. [3.3.0] bicyclic system which contains two bridgehead quaternary carbons that can be achieved, the chirality of which are controlled by those of the starting material, and the product stereoselectivity is only determined by the α-position of the acetylene moiety. Density functional theory calculations predicted that only the <i>cis</i> [3.3.0] bicyclic product could be generated, regardless of either <i>erythro</i> or <i>threo</i> substrate, which was also confirmed by experimental observations

Gold-Catalyzed Enantio- and Diastereoselective Syntheses of Left Fragments of Azadirachtin/Meliacarpin-Type Limonoids

Meliacarpin-type limonoids are an important class of organic insecticides. Their syntheses are challenging due to their chemical complexity. Here, we report the highly enantio- and diastereoselective synthesis of the left fragments of azadirachtin I and 1-cinnamoylmelianolone, being two important family members of meliacarpin-type limonoids, via pairwise palladium- and gold-catalyzed cascade reactions. Gold-catalyzed reactions of 1,7-diynes were performed as model studies, and the efficient construction of tetracyclic late-stage intermediates was achieved on the basis of this key transformation. Our unique route gave both of the left fragments in 23 steps from the commercially available chiral starting material (−)-carvone. This study significantly advances research on the synthesis of the meliacarpin-type limonoids

Gold-Catalyzed Enantio- and Diastereoselective Syntheses of Left Fragments of Azadirachtin/Meliacarpin-Type Limonoids

Meliacarpin-type limonoids are an important class of organic insecticides. Their syntheses are challenging due to their chemical complexity. Here, we report the highly enantio- and diastereoselective synthesis of the left fragments of azadirachtin I and 1-cinnamoylmelianolone, being two important family members of meliacarpin-type limonoids, via pairwise palladium- and gold-catalyzed cascade reactions. Gold-catalyzed reactions of 1,7-diynes were performed as model studies, and the efficient construction of tetracyclic late-stage intermediates was achieved on the basis of this key transformation. Our unique route gave both of the left fragments in 23 steps from the commercially available chiral starting material (−)-carvone. This study significantly advances research on the synthesis of the meliacarpin-type limonoids

Adhesion, Stiffness, and Instability in Atomically Thin MoS₂ Bubbles

We measured the work of separation of single and few-layer MoS₂ membranes from a SiO_<i>x</i> substrate using a mechanical blister test and found a value of 220 ± 35 mJ/m². Our measurements were also used to determine the 2D Young’s modulus (<i>E</i>_2D) of a single MoS₂ layer to be 160 ± 40 N/m. We then studied the delamination mechanics of pressurized MoS₂ bubbles, demonstrating both stable and unstable transitions between the bubbles’ laminated and delaminated states as the bubbles were inflated. When they were deflated, we observed edge pinning and a snap-in transition that are not accounted for by the previously reported models. We attribute this result to adhesion hysteresis and use our results to estimate the work of adhesion of our membranes to be 42 ± 20 mJ/m²