Network and swelling parameters of chemically crosslinked thermoreversible hydrogels

Two series of hydrogels of poly(N-isopropylacrylamide-co-acrylic acid) have been prepared by copolymerisation in solution using tetrafunctional N,N′-methylene bis-acrylamide and a novel octafunctional crosslinker glyoxal bis(dially acetal) (GLY) as crosslinker. These thermoreversible hydrogels were swollen to equilibrium in water at 301 K and examined by gravimetric, dimensional and compression–strain measurements. The influence of nature and content of crosslinker on swelling ratio, polymer–water interaction parameter, elastic moduli and effective crosslinking density (νe) is reported and discussed. Both series exhibit low crosslinking efficiencies expressed as νe/(theoretical crosslinking density calculated from feed composition). The extremely low efficiencies for the GLY-crosslinked gels is responsible, at least in part, for the ultra-high swellability of these hydrogels.