ELSEVIER PI1 SO142-9612(96)00058-O Biomoterids 17 (1996) 2259-2264 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 014%9612/96/$15.00 Effect of cross-linking agents on the dynamic mechanical properties of hydrogel blends of poly(acrylic acid)- poly(viny1 alcohol-vinyl acetate) zyxwvutsrqponmlkjihgf J.V. Cauich-Rodriguez, S. Deb and R. Smith IRC in Biomedical Materials, Queen Mary and Westfield College, Mile End Road, London E7 4NS, UK A range of hydrogels were prepared by blending aqueous solutions of poly(vinyl alcohol-vinyl acetate) with poly(acrylic acid) in various proportions. The effects of two cross-linking agents (glyoxal and glutaraldehyde) and subsequent thermal treatment on the properties of the blends are discussed. Dynamic mechanical analysis (DMA) of the xerogels indicated complete miscibility of the various blends which was evident from the appearance of a single glass transition temperature (T,) in the presence of either glyoxal or glutaraldehyde at all thermal treatments studied. A 50150% wtlwt blend was found to have the highest storage modulus and was thus selected for further study. Hydrogels prepared with glutaraldehyde without subsequent thermal treatment exhibited higher storage modulus values than those prepared using glyoxal when tested isothermally at 20°C in a water bath. A further increase in the storage modulus was observed when these hydrogels were thermally treated at 120 or 150°C. In a non-isothermal study on the cross-linked hydrogels, no variation in storage modulus was observed. Broad peaks were observed in tan 6 plots, these peaks shifting towards higher frequencies as the degree of cross-linking increased in the hydrogel. 0 1966 Elsevier Science Limited. Keywords: Hydrogels, polyacrylic acid, poly(vinyl alcohol-vinyl acetate), dynamic mechanical analysis Received 7 September 1995; accepted 19 March 1996 Hydrogels are an interesting and unusual class of materials that are rapidly gaining importance as biomaterials. The unique properties that hydrogels display originate from a network structure which allows the retention o’f a considerable amount of water without dissolution of the polymer itself. For biomedical uses, this high water content gives rise to minimal interfacial tension with surrounding biological fluids, gas permeation, diffusion of low-molecular- weight compounds and reduced mechanical and frictional irritation to tissue. Hydrogels are ge:nerally characterized by their hydrophilicity, the most important aspect being the amount of water that they are able to absorb, often defined as the equilibrium water content (EWC). In biomedical applications, high w ater content is an attractive attribute related to higher oxygen permeability and low cell adhesion and protein absorption. However, there are some disadvantages associated with high water content, the most important being a decrease in mechanical strength. A number of approaches have been adopted to improve the mechanical properties of hydrogels including Correspondence to Dr R. Smith. increasing the cross-linking density, copolymerization with bulky hydrophobic monomers and grafting to rigid substrates. Improved mechanical properties were achieved by Peppas and Merrill1 by electron beam irradiation of poly(viny1 alcohol) (PVA) solutions and reinforcement by a dehydration-annealing process in the already cross-linked network. Watase and Nishinari’ and Urushizaki et ~1.~ have demonstrated that the storage modulus in PVA hydrogels is increased with the number of freeze-thaw cycles and the concentration of PVA in the gel. Bo4 claimed to improve the mechanical properties of PVA hydrogels by cross-linking with epichlorohydrin and subsequent annealing. We have recently prepared hydrogels with poly(viny1 alcohol-vinyl acetate) cross-linked with glutaraldehyde and reinforced with poly(viny1 pyrrolidone)5. Most mechanical property data reported in the literature for hydrogels are obtained in tension/ compression. Dynamic mechanical measurements on hydrogels are rare2,3S6-8, although this technique has been widely used to analyse structural and intrinsic property changes in a variety of materials. Considering that many applications of hydrogels involve the material being subjected to cyclic loads of varying magnitudes, the importance of dynamic mechanical 2259 Biomaterials 1996, Vol. 17 No. 23