Polymer International zyxwvutsr 36 (1995) 219-226 zyxwvuts Synthesis of Porous Hydrogel Structures by Polymerizing the Continuous Phase of a Microemulsion Darrell J. Bennett, Robert P. Burford, Thomas P. Davis* zyx & Heath J. Tilley School of Chemical Engineering and Industrial Chemistry, University of New South Wales, PO Box 1, Kensington, NSW 2033, Australia (Received 15 March 1994; revised version received 6 September 1994; accepted 30 September 1994) Abstract: A series of microemulsions have been formulated, with 2-hydroxyethyl methacrylate (HEMA) or HEMA/water/propanol mixtures as the continuous phase and methylcyclohexane as the discontinuous phase. The effect of surfactant type was investigated with the utilization of both anionic and nonionic zyxw sur- factants. The microemulsion continuous phase was polymerized by UV radiation and a thermal post-cure. The resultant polymers were extracted to remove the discontinuous phase and the surfactant. On swelling, the majority of the poly- mers became opaque, although transparent PHEMA hydrogels were synthesized with an improved equilibrium water content (EWC). The cause of opacity was shown by field emission scanning electron microscopy (FESEM). The breakdown in the microemulsion on polymerization is caused by unfavourable interactions between the PHEMA and the stabilizing surfactants causing agglomerization of the discontinuous phase. All the hydrogels were found to have higher water retention than PHEMA, with EWCs of up to 70%. The modified polymers also demonstrated an increased rate of water diffusion into the matrix. A preliminary study of oxygen permeability revealed that a significant improvement had been made over standard PHEMA membranes. The porous structure of the PHEMA gels has been shown to be dependent on the type of surfactant used during syn- thesis. Key zyxwvutsrqp words: microemulsion, hydrogel, poly(2-hydroxyethyl methacrylate), equi- librium water content, oxygen permeability, scanning electron microscopy. INTRODUCTION presence of water in the gel is crucial in contact lens applications as the oxygen permeability of any lens is Poly(2-hydroxyethyl methacrylate) (PHEMA) is widely solely governed by the equilibrium water content used as a hydrogel for contact lenses. The maximum (EWC). The conventional approach to boosting the equilibrium swelling of PHEMA is thermodynamically EWC of PHEMA is to copolymerize HEMA with a limited to 39 wt%. If HEMA is polymerized in the pres- highly hydrophilic monomer such as N-vinyl-2-pyrroli- ence of greater than 39% water then phase separation done. The novel approach taken in this work is to ‘trap’ occurs and an opaque heterogeneous gel forms.’ The a Schulman’s type micr~ernulsion~-~ inside the polymer matrix, in the hope of forming micropores with a very * To whom correspondence should be addressed. small diameter incapable of scattering light. To our Polymer InternntionalO959-8 103/95/$O9.OO zyxwvut 0 1995 SCI. Printed in Great Britain 219