THE EFFECT OF IONIC ELECTROLYTES ON HYDROLYTIC DEGRADATION OF BIODEGRADABLE POLYMERS - MECHANICAL AND THERMODYNAMIC PROPERTIES AND MOLECULAR MODELING

The purpose of this study was to examine the effect of electrolytes on the hydrolytic degradation of synthetic biodegradable polymers and fibers. Both polyglycolic acid (PGA) and poly (glycolide-lactide) copolymer (PGL) were used for the study. Four different electrolytes were used: NaCl, LiCl, MgCl2, and ZnCl2. The electrolyte effect was evaluated in terms of the change in tensile properties, water uptake, and surface morphology of the polymers and fibers. It was found that the NaCl and MgCl2 solutions significantly retarded the hydrolytic degradation of both PGA and PGL as evidenced in the prolonged retention of tensile breaking strength of these fibers when compared to deionized water control. Increasing the concentration of the electrolyte retarded the hydrolytic degradation rate further. These mechanical property data agreed well with the rate and amount of water uptake of PGA and could be correlated with the chemical potential difference of water between the electrolyte solution and pure water. The effect of electrolyte was further analyzed by theoretical calculation. Semiemperical molecular orbital calculations indicated that hard cations like Mg, Li, and Zn strongly coordinated to the polar sites of the polymer chain segments (-C = O) and severely disrupted their solvation spheres. Such a disruption was reflected in the smaller amount and slower rate of water uptake by PGA, and thus a slower rate of hydrolytic degradation as evident in the retention of tensile breaking strength.