This has further inspired the creation of supramolecular networks where metal-coordination complexation is the sole crosslinking mechanism. These fully transient, musselinspired hydrogels typically comprise multiarm polymers, end-functionalized with metal-coordinating ligands, such as His or DOPA, with metal ions serving as the crosslinking sites. [2,[20][21][22] Both His and DOPA are bidentate ligands that can coordinate with metal ions in either tris-, bis-, or mono-modalities, or are left free (unbound) (Figure 1a). [2,4] The tris-and bis-modalities serve as crosslinking sites to create a percolated transient hydrogel network, while the mono-modality and free ligands do not support network formation. To coordinate with metal ions, ligands must be deprotonated, and the concentration of available deprotonated ligands relative to the concentration of available metal ions determines the distribution of ligand coordination modalities at equilibrium within the network. [9,10]
Two-part, commercial silicone elastomers are used in a variety of fundamental soft materials research and industrial applications due to their wide availability, ease of use, low cost, and mechanical tunability. This work seeks to create a library of moduli for three common elastomer systems with varied mixing ratios: Sylgard 184, Solaris, and Ecoflex 00-30, as well as provide a comparison of their adhesive properties. Shear storage moduli are quantified using parallel plate oscillatory shear rheology. The work of debonding is measured with spherical probe adhesion testing, and the static, advancing, and receding contact angles are measured via goniometer. Sylgard 184 can have shear moduli ranging from $0.5 kPa-620 kPa, Solaris from $0.6 kPa-175 kPa, and EF from $1.3 kPa-35 kPa measured at a frequency of 0.01 rad/s. In general, increasing mixing ratios creates softer samples. Additionally, softer samples are universally more adhesive, regardless of the material system. When comparing the different material systems, Sylgard 184 is generally the most adhesive, followed closely by Solaris, and then by Ecoflex 00-30. Our study offers a baseline dataset of modulus values and comparative adhesion to help researchers determine an appropriate commercial silicone for their application.
Fully transient, mussel‐inspired hydrogels are supramolecular networks where polymers are crosslinked by reversible, metal‐ligand coordination bonds. In article number 2100319, Niels Holten‐Andersen, Jonathan T. Pham, and co‐workers demonstrate an inverse correlation between tris‐ coordinate crosslinks and peak adhesive stress for hydrogels comprising either histidine‐Ni2+ ion crosslinks or nitrodopamine‐Fe3+ ion crosslinks. This exhibits tunability of supramolecular hydrogels for soft materials and interfacial applications.
Metal‐coordinated hydrogels can form a percolated network with transient bonds due to metal ions‐functional group coordination. Each metal ion can link with more than one ligand, leading to an intricate speciation of bonding modes. While the mechanics of transient gels made with four‐arm polymers are often studied, less is known about how increasing the number of arms affects the modulus. Using shear rheology, we measure the modulus of hydrogels prepared from four‐, six‐, and eight‐armed poly(ethylene glycols), functionalized with histidine ligands that complex with nickel (II) ions. These gels have matched polymer wt% and varied pH to compare their moduli. We consider if the modulus can be described by established polymer network models by calculating the speciation of metal‐coordinated crosslinks, and then incorporating it into a phantom network prediction. We find that (1) increasing the number of polymer arms increases the modulus, (2) the phantom network allows reasonable modulus approximation for four‐arm and six‐arm gels, and (3) the modulus of eight‐arm gels exceed our phantom network prediction. Since polymer cores act as chemical crosslinks and metal‐coordinated crosslinks form network strands, it is possible that increasing the number of metal‐coordinated linkages per molecule reinforces the chemical crosslink at the polymer core.This article is protected by copyright. All rights reserved
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