3D mapping of nanoscale crosslink heterogeneities in microgelsElectronic supplementary information (ESI) available: Detailed synthesis and molecular characterization of compounds; hydrogels and microgels preparation details; additional DLS and SEM results; video of dye diffusion from hydrogel matrix. See DOI: 10.1039/c8mh00644j

The majority of swollen polymer networks exhibit spatial variations in crosslink density. These spatial heterogeneities are particularly important in colloidal gel particles, or microgels, where they manifest themselves on the nanoscale and impact mechanical and transport properties. Despite their importance, the real space nanostructure of these heterogeneities at the individual particle level has remained elusive. Using state of the art super-resolution microscopy known as Whole cell 4Pi Single Molecule Switching Nanoscopy (W-4PiSMSN) we demonstrate 3D nanoscale mapping of spatial crosslink heterogeneities in a model system of poly( N -isopropylacrylamide) colloidal gel particles containing a novel fluorophore tagged crosslinker. We reveal the presence of higher crosslink density clusters embedded in a lower crosslink density matrix within the core of individual microgel particles, a phenomenon that has been predicted, but never been observed before in real space. The morphology of the clusters provides insight into the kinetics of microgel formation. This study also provides proof-of-concept 3D super-resolution imaging of spatial heterogeneities in bulk hydrogels. The majority of gels exhibit nanoscale spatial variations in crosslink density. We present the first 3D super-resolution microscopy images of dye tagged cross-link distributions in microgels and hydrogels. The morphology of nanoscale features never imaged previously in microgels, are revealed.