RHEO-MECHANICAL INVESTIGATION ON THE NETW ORK STRUCTURE OF ELECTRODE CONTACT GELS AND HPLC METHOD FOR DETERMINATION OF THE MONOMER RESIDUE

Hydrogels were synthesized by photoinitiated copolymerization of acryl ic acid and triethylene glycol dimethacrylate (TEGMA). In less than on e minute an aqueous reaction mixture underwent UV-curing to yield tran sparent gels that retain their shape. The correlation between structur e and properties makes them suitable for the use in medical electrodes . The viscoelastic properties and plateau moduli (G(P)') were determin ed as functions of the cross-linker concentration by means of rheo-mec hanical oscillation measurements. The molecular weights between two en tanglement points were calculated from the G(P)' values and vary from 4.6.10(5) to 1.7.10(6) g mol(-1), depending on the concentration of TE GMA. The low cross-linking efficiency of 1% estimated from G(P)' is pr obably caused by network defects and was not influenced by the degree of crosslinking. Thermal initiators were employed and time-dependent m easurements of the storage modulus (G') were taken in order to study t he kinetics of gel formation. It was thus possible to compare the effe ctiveness of different initiators and their influence on the elasticit y of the hydrogels. In addition, the content of acrylic acid in the pr epared gels was determined by selective extraction and HPLC separation of the monomer from the complex multicomponent mixture. It was found that the residual monomer content of about 0.40% w/w could be reduced to almost zero by increasing the duration of exposure. Finally, the ef fect of the cross-linker concentration on the monomer content in the h ydrogels was also investigated.