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Searle, R. J. (1988) Modification of the mechanical properties of synethetic hydrogels by various techniques. Doctoral thesis, Polytechnic of North London.
A study in the field of hydrogel reinforcement has been carried out, the reinforced hydrogel being a copolymer of 2-hydroxyethyl methacrylate and methacrylic acid, crosslinked with ethylene glycol dimethacrylate and having an equilibrium water content at pH 8.5 of 74% by weight. Three methods of incorporating hydrophobic groups into the polymer have been investigated, in order to compare the reinforcing effects of each, viz. reinforcement by the addition of polymeric fillers; by forming interpenetrating polymer networks (IPNs): and by preparing terpolymers of 2-hydroxyethyl methacrylate/methacrylic acid with hydrophobic monomers. The hydrogels were prepared by redox polymerisation in IMS solution. A method has been developed for producing poly(2-hydroxyethyl methacrylate/methacrylic acid)/poly(methyl methacrylate) IPNs. They were prepared by drying the gels, swelling them in a solution containing monomers, initiators, crosslinker and suitable solvents, and then exposed to ultra-violet light. Filled hydrogels were formed by adding quantities of a PMMA dispersion in IMS to the hydrogel polymerisation mixture. The properties which represent best the level of mechanical reinforcement are the tear strength or tearing energy, tensile strength and modulus of the material. The tensile strengths of the various types of reinforced gel were compared as a function of composition, and of water content. It was observed that the addition of filler to the gels gave no improvement in tensile strength compared with the terpolymers, although the tensile strength increased with increasing filler content. However, it was not possible to produce a monodispersed dispersion of PMMA in IMS, of small particle size. Using the IPN method, hydrogels of high water content and high strength have been produced. The reproducibility of the process is so far poor, due to its complex nature, but it merits much further investigation. It was found that the tensile strength did not depend on the level of crosslinker in the hydrophobic component of the IPN. Stress-strain curves of several terpolymer hydrogels showed an increase in slope at high strains, thought to be caused by the presence of hydrophobic phase domains. Tearing energies of hydrogels maybe measured by the "trouser" method, which has been previously applied to dry rubbers. Terpolymers containing EHA or styrene show particularly high tearing energies at a given mol % hydrophobic monomer. The homo-IPN hydrogels were translucent.
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