Influence of Local Environment on Water Properties Within Physically Crosslinked Amphiphilic Copolymer Hydrogels*

The association of hydrophobic moieties provide effective crosslinks in amphiphilic statistical copolymers swollen in water, but also modulate the local properties of water through the nanostructure developed consisting of hydrated and hydrophobic regions. The characteristic nanostructures tend to confine water to sub-5 nm dimensions locally, which significantly influence the crystallization of water within the hydrogels. The influence of the dynamics of the hydrophobic component on freezing behavior is experimentally demonstrated with a family of copolymers of n-octadecyl acrylate (ODA) or ethylhexyl acrylate (EHA) as the hydrophobic comonomer and hydroxyethyl acrylate (HEA) as the common hydrophilic component. The crystallization of ODA leads to significant contrast in nanostructures at some compositions/processing between the family of hydrogels. The low T_g of EHA leads to a viscoelastic liquid for the swollen water copolymer, while hydrogels based on HEA-ODA copolymers are viscoelastic solids. Despite these structural and viscoelastic differences, both hydrogel families inhibit >90 % of the water from crystallization on supercooling at appropriate copolymer compositions.