The viscoelastic and mechanical behaviors of physically cross-linked copolymer hydrogels synthesized from N,N-dimethylacrylamide (DMA) and 2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate (FOSA) with varying FOSA concentration were studied by rheological and static tensile tests. The strong hydrophobic association of the FOSA moieties in an aqueous environment produced core–shell nanodomains that provided the physical cross-links. These PDMA–FOSA hydrogels exhibited excellent mechanical properties, including a modulus of ∼130–190 kPa, elongation at break of 1000–1600%, and ∼500 kPa tensile strength, depending on the FOSA concentration. The physical gels were more viscous than comparable chemical gels and were much more efficient at dissipating stress. The latter characteristic produced relatively high tensile toughness, ∼4–6 MPa, because of the extra energy dissipation mechanism provided by the reversible, hydrophobic cross-links. The PDMA–FOSA hydrogel exhibited peculiar dynamic behavior which was greatly dependent on temperature. At 25 °C, the hydrogel was highly elastic, but as the temperature increased, its viscous behavior increased and a crossover of the dynamic moduli (i.e., G″ > G′) occurred at 55 °C, as the rheological characteristics of the material went from a viscoelastic solid to a viscoelastic liquid. That behavior is a consequence of the physical nature of the structure of the physical cross-links and the dynamic nature of hydrophobic associations, which are influenced by composition, temperature, and time.

The synthesis of a 4-dibenzocyclooctynol (DIBO) functionalized polyethylene glycol (PEG) and fabrication of hydrogels via strain-promoted, metal-free, azide-alkyne cycloaddition is reported. The resulting hydrogel materials provide a versatile alternative in which to encapsulate cells that are sensitive to photochemical or chemical crosslinking mechanisms.

The shape memory behavior of a series of strong, tough hybrid hydrogels prepared by covalently cross-linking quad-polymers of N,Ndimethylacrylamide (DMA), 2-(N-ethylperfluoro-octanesulfonamido) ethyl methacrylate (FOSM), hydroxyethyl acrylate, and 2-cinnamoyloxyethyl acrylate was investigated. The hybrid hydrogels, which had physical and covalent cross-links, contained ∼60−70% water, were relatively soft and elastic, and exhibited high mechanical strength, extensibility, and fracture toughness. The temporary network was derived from glassy nanodomains due to microphase separation of the FOSM species. The switching temperature for shape memory was the glass transition temperature of the nanodomains. Some creep relaxation occurred in the fixed shape due to viscoelastic effects of the nanodomain cross-links, but shape fixing efficiencies of 84−88% were achieved for the fixed shape after 24 h at 10 °C. Shape recovery to the permanent shape was achieved by reheating the hydrogel to 65 °C and was essentially quantitative.

The origin of poly(N-isopropylacrylamide) (PNIPAM) cononsolvency in tetrahydrofuran-water (THF-water) mixture was studied from the point of view of mixed solvent structure. The dynamic equilibrium of THF-water composition fluctuation in the mixed solvent system was found to be the main variable for this cononsolvency effect. Temperature and THF content dependences of composition fluctuation were studied by a combination of small angle neutron scattering (SANS), dynamic laser light scattering, and viscometry. A lower critical solution temperature (LCST) type phase diagram for THF-water mixture was established by SANS. The composition fluctuation in THF-water system reaches the maximum at about 20 mol % THF content at constant temperature and increases with temperature as getting closer to the phase boundary. This kind of composition fluctuation induces PNIPAM cononsolvency. When the THF content is lower than 4.5 mol %, the composition fluctuation influence of the THF-water structure is quite weak and most of water structure is not disturbed. Then, at low THF content, poly(N-isopropylacrylamide-co-ethylene glycol) (PNIPAM-co-PEG) microgel can still form hydrogen bonds with water and exist in the swollen state. The basic phase transition behavior of the microgel in THF-water is relatively similar to that in pure water, except for the shift of LCST to lower temperature. With THF content increasing to 20 mol %, the influence of composition fluctuation in the THF-water mixture becomes dominant. Solvent-solvent interaction is stronger than mixed solvent-polymer interaction. So PNIPAM does not dissolve in the mixed solvent, and the microgel is in the collapsed state. Further increase in THF content abates the contribution of composition fluctuation, and the structures of mixed solvents tend to be that in pure THF. PNIPAM becomes soluble again via Van der Waals interaction between THF and polymer.