Kinetically Controlled Morphology in Copolymer-based Hydrogels Crosslinked by Crystalline Nanodomains Determines Efficacy of Ice Inhibition

Confinement of water within physically crosslinked hydrogels can alter the physics associated with the water within the hydrogel. When water is confined between hydrophobic nanodomains that act as physical crosslinks, crystallization of water can be dramatically inhibited. In prior work, the inter-nanodomain spacing was controlled by changing the fraction of hydrophobic segments in the polymer, which leads to an inverse correlation between water content and confinement extent. However, water content and confinement effect have not been decoupled. Here, we decouple confinement and water content through kinetically controlled nanostructure in a hydrogel crosslinked by crystalline nanodomains based on 2-hydroxyethyl acrylate (HEA) and n-octadecyl acrylate (ODA). Zone annealing of the dry copolymer alters the structure of the crystalline ODA domains that effectively crosslink the hydrogel. Despite these changes in the ODA structure, the average hydrogel water content was not significantly altered. However, the unfrozen water inside the hydrogel varies from 60 to 99 wt%, determined from DSC and in-situ WAXS. The antifreeze efficacy is found to correlate directly with the structure where smaller spacing leads to higher unfrozen water fraction.