Structural rearrangement and stiffening of hydrophobically modified supramolecular hydrogels during thermal annealing

Dynamic crosslinks formed by thermoreversible associations provide an energy dissipation mechanism to toughen hydrogels. However, the details of the organization of these crosslinks impact the hydrogel properties through constraints on the network chain conformation. The physical crosslinks generated by hydrophobic association of the 2-(N-ethylperfluorooctane-sulfonamido)ethyl methacrylate (FOSM) groups in a random copolymer of N,N-dimethylacrylamide (DMA) and FOSM provide a simple system to investigate how the hydrogel structure (as determined from small angle neutron scattering impacts the mechanical properties of the hydrogel. The initial hydration of the copolymer at 25 °C leads to a kinetically trapped structure with large-scale heterogeneities. Heating the hydrogel at 60 °C, which is above the glass transition temperature for the FOSM domains, allows the hydrogel structure to rearrange to reduce the density of network defects and the structural heterogeneities. That effectively increases the crosslink density of the network, which stiffens the hydrogel and decreases the swelling at equilibrium at 25 °C. The processing history determines how the hydrophobes aggregate to form the physically crosslinked network, whose structure defines the mechanical properties of these hydrogels.