Synthesis and Characterization of Hydrogels from Graft Copolymers

Abstract

Graft copolymers of well-defined structure composed of a 2,3-dihydroxypropyl methacrylate backbone and uniform molecular weight polystyrene branches were prepared by the macromonomer method and studied for their application as hydrogels utilizing their microdomain structure. Film specimens prepared by solution casting of the graft copolymer swelled immediately in water to form stable hydrogels which were characterized by measuring the equilibrium degree of swelling and the oxygen permeability in water. Swelling behavior of the hydrogel was also characterized by the small angle X-ray scattering (SAXS) measurement. It is clearly shown that the microphase separated structure of the graft copolymers is considerably regular and the swelling behavior and the oxygen permeability coefficient of hydrogels can be well-controlled by both the architecture of preformed graft copolymers and the microdomain structure in the cast specimens. Hydrogels prepared from the nonionic water soluble polymers like poly(2-hydroxy-ethyl methacrylate), poly(propylene glycol monoacryl.ate), poly(vinylalcohol), and polyacrylamide have been proposed for medical applications such as surgical implants ,. soft contact lenses, slow release devices and some others. 1-12 These polymeric materials can swell in water and contain large amount of water which is considered to be essential to their slight irritation and biocom-patibility for living tissues because the inter-facial free energy between water-swollen gel and the aqueous biological environment is very small and the inner water provides good permeability for oxygen, metal ions, and other metabolites. However, hydrogels at high swelling state often show insufficient mechanical strength which has been a large difficulty for their wide application. Yamashita et al. reported on hydrogels prepared from poly(2-hydroxyethyl meth-acrylate (HEMA)-g-styrene) utilizing the microphase separated structure in the copoly-mer.13 They showed that the hydrogels were transparent and had much better mechanical strength compared with the crosslinked HEMA homopolymer. Furthermore, they observed pronounced thromboresistance with the hydrogel in the blood compatibility experiment. It is known at present that the microphase separated structure of the materials in biomedical applications including hydrogels acts a quite important role in improving both their mechanical strength and biocompatibility. 14-16 We have been studying the synthesis and characterization of tailor-made graft copoly-mers having controlled graft number and length by the macromonomer method to 1033