Abstract
<jats:p>We probe the structural and rheological properties of ionically crosslinked alginate, a model biopolymer hydrogel, using microscopy, rheology and viscosity dependent molecular probes. This combination of techniques enables the quantification and correlation of microstructure, microviscosity, bulk viscoelasticity and yielding dynamics. By adjusting the stoichiometric ratio, R, between the alginate biopolymer and its cation crosslinks, we observe a transition from a homogeneous network-like structure to a coarse bundle-like structure at high (R$>$0.67) stoichiometric ratio. Intriguingly, these bundle-like structures have distinct and counter-intuitive rheological properties. Using molecular probes, we observe a continuous decrease in microviscosity that is correlated with a decrease in bulk elastic modulus and an increase in energy dissipation. This is accompanied by a transition in the hydrogel yielding under strain from a sharp, well‑defined yield point to a continuous ductile-like yielding. We ascribe these surprising transitions to the looser intermolecular interaction between alginate biopolymers in the bundle-like state, as previously predicted by x-ray scattering experiments. These findings reveal new and counter-intuitive structure-property relations that demonstrate high crosslink concentration does not necessarily translate to optimal mechanical performance.</jats:p>