Linking Molecular Behavior to Macroscopic Properties in Ideal Dynamic Covalent Networks

Abstract

Dynamic covalent networks (DCvNs) are increasingly used in advanced materials design with applications ranging from recyclable thermosets to self-healing hydrogels. However, the relationship between the underlying chemistry at the junctions of DCvNs and their macroscopic properties is still not fully understood. In this work, we constructed a robust framework to predict how complex network behavior in DCvNs emerges from the chemical landscape of the dynamic chemistry at the junction. Ideal dynamic covalent boronic ester-based hydrogels were used as model DCvNs. We developed physical models that describe how viscoelastic properties, as measured by shear rheometry, are linked to the molecular behavior of the dynamic junction, quantified via fluorescence and NMR spectroscopy and DFT calculations. Additionally, shear rheometry was combined with Transition State Theory to quantify the kinetics and thermodynamics of network rearrangements, enabling a mechanistic understanding including preferred reaction pathways for dynamic covalent chemistries. We applied this approach to corroborate the "loose-bolt" postulate for the reaction mechanism in Wulff-type boronic acids. These findings, grounded in molecular principles, advance our understanding and rational design of dynamic polymer networks, improving our ability to predict, design, and leverage their unique properties for future applications.

Physical networks from entropy-driven non-covalent interactions

“…b Schematic of one binding interaction and the associated equations that relate the thermodynamics to the equilibrium constant (K eq ), the dissociation constant (k d ), and the fraction of bound crosslinks (θ). c Calculated curves of θ versus temperature using the thermodynamic parameters for PSNP/HPMC-C 12 interactions and other notable physical interactions [28][29][30]33,36 using Equation 7 (Supplementary Discussion). d Calculated k d values for these same systems as a function of temperature from Equation 10 (Supplementary Discussion).…”

“…While these materials are often required to exhibit precise mechanical properties over a range of operating temperatures such that thermal responsiveness must be considered in their design, they typically weaken (i.e., become more liquid-like, soften, and relax stress faster) with increasing temperature. Indeed, standard mathematical frameworks for describing temperature-dependent viscoelasticity of dynamically crosslinked materials such as time-temperature superposition assume network weakening at elevated temperatures [28][29][30][31][32][33][34][35][36] .…”

Rheological and mechanical properties of dynamic covalent polymers based on imine bond

“…In comparison to the association energy of physical interactions (typically 1–5 kcal mol −1 ), dynamic covalent bonds are much stronger, of which the bond energy is as high as 50–150 kcal mol −1 9 . It is known that, to enable the dynamic macroscopic properties, the lifetime of dynamic linkages would approach the observable time scales (10 −3 s ~ 1 min) at certain conditions 10 . Thus, tailoring the kinetics of the corresponding reversible or exchange reaction would adjust the lifetime of dynamic covalent bonds and therefore shape the dynamic macroscopic properties 11 …”

Designing Hydrogels for 3D Cell Culture Using Dynamic Covalent Crosslinking

“…[ 71 ] Bond kinetics (i.e., kinetics of bond formation or bond breaking) influence the formation of the network whereas dissolution of the network by the reverse reaction contributes to relaxation of the hydrogel network, thereby limiting the impact of K eq on material properties. [ 59,71,72 ] The modulus, which is dictated by the number of crosslinks, and the relaxation dynamics, which are controlled by the rate constant (k ‐1 ) are both critical parameters to dynamically crosslinked hydrogels. [ 71 ] Table 1 summarizes experimentally determined K eq values, which are helpful to predict the stability of hydrogel networks but cannot be used exclusively without considering bond kinetics.…”

“…[ 21,100 ] The network dynamics and kinetics of boronic‐ester hydrogels have been extensively characterized in order to evaluate the impact of temperature, polymer content, network architecture, and sugar concentration on stability. [ 59,101 ] Although boronate esters have been used for other biomedical applications including drug delivery, they have not yet become widely adopted for 3D culture models. [ 99 ]…”