Molecular Control via Dynamic Bonding Enables Material Responsiveness in Additively Manufactured Metallo-Polyelectrolytes

Metallo-polyelectrolyte Complexes (MPEC) are a class of soft materials that exhibit unique mechanical and physical properties through reversible electrostatic interactions between dynamic crosslinkers (multivalent metal ions) and charged polymer chains. These molecular-level processes give rise to a wide range of material dynamic responses, for example, stimuli-responsiveness, self-healing, and high toughness through enhanced energy dissipation. The current state-of-the-art fabrication method based on the multistep nature of solution-based metallo-polyelectrolyte synthesis produces materials that suffer from poor long-term stability and inhomogeneity or relies on covalent crosslinking as a backstop for indirect synthesis. The range of scales involved in determining the behavior of MPECs (local bond liability, polymer configurations, and macroscale behavior) further presents computational challenges, which limits the broader application of models to guide experimental methods.<br/><br/>In this work, we demonstrate a facile, single-step fabrication method for MPECs via stereolithography, which produces homogeneous, stable, and high-longevity materials using a straightforward synthesis route. Then, aided by the robust fabrication platform, we present a theory-guided, physically-informed multi-scale study of MPECs. We have developed a roadmap that outlines the effect of different chemical species on the additively manufactured MPEC properties which can be used to tailor its functionality and responsiveness at material level. Our fabrication method enables easy compositional tuning by a simple swap of metal salt precursors during the resin formulation, which allows the wide selection of metal ions and produces a variety of metal-coordinated polymers.<br/><br/>We demonstrate the tunability of mechanical response by adjusting metal ion valency and polymer charge sparsity. We find that mono-, di-, and trivalent metal ions afford control of the coordination environment and bond strength, which propagate to the macroscale properties where higher valency ions result in stiffer and tougher materials. Polyanion charge sparsity, regulated by the pH of the precursor photoresin, also impacts the phase behavior of the gels, leading to changes in the mechanical response. Molecular Dynamic (MD) simulations and diverse polymer characterization methods (morphology, thermal and mechanical) demonstrate that the combination of these parameters controls the extent of dynamic crosslinking present in the system and the emerging polymer configurational distribution governing material properties of MPEC gels. We believe the development of a simple synthesis pathway via additive manufacturing, informed and guided by experimental investigation and molecular modeling, provides a comprehensive understanding of the parameter space and enables the selective design of advanced compliant and functional metallo-polyelectrolytes.