Investigating the Kinetics and Structure of Network Formation in Ultraviolet-Photopolymerizable Starch Nanogel Network Hydrogels via Very Small-Angle Neutron Scattering and Small-Amplitude Oscillatory Shear Rheology

While photopolymerization has been broadly used to fabricate hydrogels, the kinetics of the structural evolution of such hydrogels during photopolymerization and how the kinetics relate to ultimate hydrogel properties are not well-understood. Herein, small-amplitude oscillatory shear rheology (bulk scale) and time-resolved very small-angle neutron scattering (vSANS, microscale) are used in tandem to investigate the kinetics of the photopolymerization of methacrylated starch building blocks with different concentrations, charges (cationic (+), anionic (−), or neutral (0)), and morphologies (soluble branched starch, starch nanoparticles, or combinations thereof). Starch nanoparticles (SNPs) enabled the fabrication of much denser hydrogels than soluble starch but took longer to gel due to the reduced conformational mobility of the polymerizable methacrylate groups on the SNPs. The addition of charge (cationic or anionic) increases the bulk gelation time while significantly reducing the observed changes in the fluid scale and correlation length, suggesting less covalent crosslinking and inherent SNP deformation during photogelation; indeed, the fluid exponent analysis suggests that charged SNPs deswell upon crosslinking, consistent with the behavior of microgels in colloidal crystals, while uncharged SNPs swell due to competition between inter- and intraparticle crosslinking. The combination of shear rheology and vSANS measurements can thus inform the design of new photopolymerizable hydrogels with targeted comprehensive properties.