The Manifold Varieties of Poly(2-Hydroxyethyl Methacrylate) Hydrogels-IPNs

The interpenetrating network structure was used to control mechanical properties of hydrogels based on poly(2-hydroxyethyl methacrylate) (PHEMA) (first network (A) or second network (B)) and poly(glycerol methacrylate) (PGMA) (network B). In order to understand the structure, mechanical and swelling properties of sequentially made IPN hydrogels, the swollen PHEMA network microstructure and its formation was investigated by means of swelling and SWAXS experiments. Visually clear and microscopically homogenous hydrogel networks based on poly(2-hydroxyethyl methacrylate) revealed presence of domains of size 1-10nm formed during polymerization in the presence of water. The study was carried out to understand conditions under which the hydrophobic interactions are operative and their effect on the microstructure as well as how they change when the double network structure is introduced. The morphologies of network A ranged from homogenous, non-porous, and optically clear gels, to macroporous gels resulting from phase separation and offering fused-sphere morphology. A cryogel characterized by large elongated and partially interconnected pores of tens to hundreds micrometers were another object for comparison. In most cases, an increase of tensile moduli and improvement of ultimate tensile properties was achieved. A surprisingly high increase in true strength (by a factor of 10 - 30) was achieved when the macroporous PHEMA network A was reinforced by weak PHEMA or PGMA networks B. All these weakly crosslinked IPN gels were optically clear.