Structure Dependent Ice Inhibition in Physically Crosslinked Hydrogels by Crystallization of Hydrophobic Crosslinks

Confinement of water at the nanoscale can enable significant supercooling that is not possible in bulk water, but common strategies to confine water lead to low water content. We have recently demonstrated near complete ice inhibition in physically crosslinked hydrogels that use the hydrophobic association of perfluorinated moieties to crosslink and confine the water. However, the confining structure and water content in these hydrogels are coupled, which leads to some questions about the underlying physics. Here we demonstrate a similar route to modulate the nanostructure of a physically crosslinked hydrogel based on 2-hydroxyethyl acrylate (HEA) and n-octadecyl acrylate (ODA) without changing the copolymer composition and water content by crystalline crosslinks. Crystallization in the dry state leads to sheets of crystalline ODA, while melting and re-crystallizing the ODA in a hydrated copolymer leads to spherical nanodomains. The temperature-dependent structure was probed by SAXS, WAXS and SANS. DSC revealed that crystallization in the state leads to significantly more unfrozen water inside the hydrogel (10 to 15 wt%). The hydrogel morphology appears to control the unfrozen water content in these hydrogels under cryogenic temperatures.