In conventional polymer materials, mechanical performance is traditionally engineered via material structure, using motifs such as polymer molecular weight, polymer branching, or copolymer-block design1. Here, by means of a model system of 4-arm poly(ethylene glycol) hydrogels crosslinked with multiple, kinetically distinct dynamic metal-ligand coordinate complexes, we show that polymer materials with decoupled spatial structure and mechanical performance can be designed. By tuning the relative concentration of two types of metal-ligand crosslinks, we demonstrate control over the material’s mechanical hierarchy of energy-dissipating modes under dynamic mechanical loading, and therefore the ability to engineer a priori the viscoelastic properties of these materials by controlling the types of crosslinks rather than by modifying the polymer itself. This strategy to decouple material mechanics from structure may inform the design of soft materials for use in complex mechanical environments.
We have developed model light-emitting metallogels functionalized with lanthanide metal-ligand coordination complexes via a terpyridyl-end-capped four-arm poly(ethylene glycol) polymer. The optical properties of these highly luminescent polymer networks are readily modulated over a wide spectrum, including white-light emission, simply by tuning of the lanthanide metal ion stoichiometry. Furthermore, the dynamic nature of the Ln-N coordination bonding leads to a broad variety of reversible stimuli-responsive properties (mechano-, vapo-, thermo-, and chemochromism) of both sol-gel systems and solid thin films. The versatile functional performance combined with the ease of assembly suggests that this lanthanide coordination polymer design approach offers a robust pathway for future engineering of multi-stimuli-responsive polymer materials.
Reversible cross-links between polymer chains are a promising avenue to engineer the mechanical properties of soft materials and in particular hydrogels. Such developments are however challenged by the complexity of these materials, which unlike conventional, permanently cross-linked gels involve multiple relaxation time scales. To address this challenge, we study a model system of tetra-arm poly(ethylene glycol) hydrogels transiently cross-linked by reversible histidine:Ni2+ coordinate complexes and explore the separate influences of polymer structure and cross-link density on the time-dependent hydrogel rheology. We show that the characteristics of the polymer matrix primarily control the hydrogels’ static elasticity, implying that its dynamics are largely governed by coordinate-bond rearrangement kinetics rather than polymer relaxation time scales. By contrast, the ion concentration has a strong influence on both the hydrogel’s statics and dynamics, and we quantitatively account for the former using a simple model based on the known equilibrium bonding properties of histidine:Ni2+ complexes. Our findings establish specific engineering principles for the viscoelastic mechanics of metal-coordinate hydrogel materials, thus opening new perspectives for the optimization of their use in (bio)functional applications.
Tunable mechanical response under dynamic and static loading is desirable for many technological applications. Traditionally, mechanical performance of polymeric materials is controlled by modulating structural (i.e., molecular weight, chain packing, or cross-link density) or temporal parameters (such as kinetics of the exchange of dynamic cross-linkers). Metal–ligand interactions are uniquely suited to control both structural and temporal parameters as the thermodynamics and kinetics of mechanically active cross-linkers can be varied by careful selection of metal without significant synthetic modification of the polymer backbone. Here, we have demonstrated that it is possible to engineer desired mechanical properties in a metallopolymer with a high degree of tunability by simply changing the type and amount of added metal. Specifically, we cross-linked an imidazole-containing brush copolymer system with the divalent cations of zinc, copper, and cobalt. Using rheology and tensile experiments, we have correlated the emergent mechanical properties to the stoichiometric ratio of ligand to metal as well as the coordination number and ligand exchange mechanism of the imidazole–metal cross-links. In contrary to the general view that unbound free ligands are normally regarded as mechanically inactive dangling chains in metallopolymer networks, this study clearly shows that they can play a critical role in stress distribution and chain relaxation. Importantly, this work shows for the first time that it is possible to simultaneously control both the structure of networks and the temporal response of bulk materials using dynamic association of weak and monodentate ligands with transition metals.
Density functional theory is widely used to predict materials properties, but the local density approximation and generalized gradient approximation exchange-correlation functionals are known to poorly predict the energetics of reactions involving molecular species. In this paper, we obtain corrections for the O2, H2, N2, F2, and Cl2 molecules within the Perdew-Burke-Enzerhof GGA, Perdew-Wang GGA, and Perdew-Zunger LDA exchange-correlation functionals by comparing DFTcalculated formation energies of oxides, hydrides, nitrides, fluorides, and chlorides to experimental values. We also show that the choice of compounds used to obtain the correction is significant, and we use a leave one out cross-validation approach to rigorously determine the proper fit set. We report confidence intervals with our correction values, which quantifies the variation caused by the choice of fit set after outlier removal. The remaining variation in the correction values is on the order of 1 kcal/mol, which indicates that chemical accuracy is a realistic goal for these systems.
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