Cross-Linking Density Controls the Interstitial Pore Size and Elasticity in Polyacrylamide Microgel Packings

Concentrated packings of microgel particles behave like soft elastic solids, in which the macroscopic elastic properties arise from the interactions of the underlying particles. In order to utilize these microgel packings as scaffolds for three-dimensional biological assays, the macroscopic elastic behavior and the size of the interstitial pores that form between particles must be controlled. Here, we explore the effect of cross-linking density on the rheological behavior of packed polyacrylamide microgel particles. At low polymer concentrations, just above the onset of solid-like behavior, the elastic properties of the microgel packings emanate from the interactions of brush-like polymer chains on the corona of the microgels. This behavior depends on the size of the polymer brush relative to the size of the microgel, where increasing the cross-linking density of the microgel particle decreases the brush length, resulting in high shear moduli of the packed microgels. At high polymer concentrations, well above the onset of elastic behavior, we observe a transition in the rheological behavior, in which the loss factor becomes independent of the polymer concentration. In this regime, the elastic shear moduli of microgels with low cross-linking (x-linking) densities follow distinct polymer physics scaling laws for bulk hydrogel networks. Additionally, we explore how these changes in the x-linking density alter the structure of interstitial pores between microgel particles through a combination of cryo-SEM and single nanoparticle tracking. We find that stiffer microgel particles with shorter brush-like coronas not only have larger pore sizes but also nanoparticles exploring the pores are able to escape and enter neighboring pores. Further, we find that the viscosity within the interstitial pores of the microgels with high x-linking density is less than that of the lower x-linked systems having comparable pore sizes. These findings show that the stiffness and pore size of microgel packings can be tuned independently and will guide the development of packed microgels as sacrificial scaffolds for three-dimensional biological applications.