Session:

Metallo-polyelectrolyte complexes (MPEC) represent a unique class of materials where metal ions embedded into soft organic frameworks undergo reversible electrostatic interactions through the formation and dissociation of dynamic bonds. These molecular-level processes give rise to a wide range of material dynamic responses, for example, stimuli-responsiveness, self-healing, dissolution in solvent, and improved toughness through enhanced energy dissipation. We synthesized a 3D-architected MPEC, with dimensions of a millimeter to a centimeter, fabricated via in-situ polymerization of acrylic acid with divalent and trivalent metal ions such as Ni(II) and Al(III) via stereolithography. Water-solvated metal ions dynamically cross-link with the pendant carboxyl group of polyacrylic acid to form a dense network capable of significant mechanical deformation. From the uniaxial tensile test, the resulting materials with trivalent metal-ions are stiff and tough, with a modulus of ~3MPa and elongation strains upward to ~700% in contrast to the divalent metal-containing gels, whose modulus is about 500kPa to 1MPa with elongation upwards of ~1400%. A large hysteresis in stress-strain response and fracture toughness for trivalent metal-containing gels indicates the significant contribution of dynamic bonds in energy dissipation. The valency of the solvated metal cation critically governs the mechanical and rheological properties of the polymer network. Experiments conducted with a Dynamic Mechanical Analyzer (DMA) demonstrate that the association/dissociation of the metal cation cross-linker induces distinct dynamic properties of the gels: the storage modulus, from ~500kPa to ~5MPa, and a loss factor, tan delta, from less than 0.5 (viscous-solid like) to close to 1 (viscous-liquid like) in the ambient temperature, each varying with the valency and concentration of metal ions. Stress relaxation experiments reveal the gels with trivalent metal ions are ~60 times slower, yet the long-term relaxation process was more apparent than the ones with divalent metal ions. The local lability of the metal ions with the polymer chains serves as the origin of global material properties. To probe the mechanisms of local interaction and their manifestation in global parameters, Molecular Dynamics (MD) simulations have been employed. Using MD simulation, we examine the effects of ion valency on ionic binding within polymer chains of an equilibrated gel, both at equilibrium and under stress. Further, fundamental thermal characterization via Differential Scanning Calorimetry (DSC) and Thermo-Gravimetric Analysis (TGA) – including kinetic analysis and thermodynamics of bonding – has demonstrated the role of solvating water in mediating this valency control and has corroborated the greater enthalpic cost of thermally-activated processes in trivalent compared to divalent metal systems. We utilize valency-dependent material properties to control the nonlinear mechanical instability in 3D-printed multi-material architectures. Uniaxial compression and tension experiments conducted at strain rates of 10^-4 to 1 s^-1 demonstrate the strong dependence of instability deformation mode on material compositions and mechanical inputs (applied strain rate or relaxation time). These results demonstrate the synergetic effect of MPEC’s valency-dependent, tunable viscoelastic properties with nonlinear mechanical response of compliant architectures, permitting control of deformation modes in soft mechanical meta-materials.

Co-Author(s): Seneca Velling (California Institute of Technology), Pierre Walker (California Institute of Technology), Cyrus Fiori (California Institute of Technology), Zane Taylor (California Institute of Technology), Zhen-Gang Wang (California Institute of Technology), Julia Greer (California Institute of Technology)

Seola Lee

California Institute of Technology

Appearances