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 fi bers. Both polyglycolic acid (PGA) and poly (glycolide-lactide) copoly mer (PGL) were used for the study. Four different electrolytes were us ed: NaCl, LiCl, MgCl2, and ZnCl2. The electrolyte effect was evaluated in terms of the change in tensile properties, water uptake, and surfa ce morphology of the polymers and fibers. It was found that the NaCl a nd MgCl2 solutions significantly retarded the hydrolytic degradation o f both PGA and PGL as evidenced in the prolonged retention of tensile breaking strength of these fibers when compared to deionized water con trol. Increasing the concentration of the electrolyte retarded the hyd rolytic degradation rate further. These mechanical property data agree d well with the rate and amount of water uptake of PGA and could be co rrelated with the chemical potential difference of water between the e lectrolyte solution and pure water. The effect of electrolyte was furt her analyzed by theoretical calculation. Semiemperical molecular orbit al calculations indicated that hard cations like Mg, Li, and Zn strong ly coordinated to the polar sites of the polymer chain segments (-C = O) and severely disrupted their solvation spheres. Such a disruption w as reflected in the smaller amount and slower rate of water uptake by PGA, and thus a slower rate of hydrolytic degradation as evident in th e retention of tensile breaking strength.