Time-of-Flight Secondary Ion Mass Spectrometry Studies of in Vitro Hydrolytic Degradation of Biodegradable Polymers

Abstract

The in vitro hydrolytic degradation at the surface of six biodegradable polymers, namely polyglycolic acid (PGA), poly(lactic acid) (PLA), random copolymer poly(lactic-co-glycolic) acid (PLGA), poly(sebacic acid) (PSA), and two random copolymer poly(fumaric-co-sebacic) acid (PFS) of different compositions, has been studied using time-of-flight secondary ion mass spectrometry (ToF SIMS). A distribution of hydrolysis products, namely oligomer molecules, were observed as a series of intact molecular ions in the ToF SIMS spectra for all polymers studied. In most cases, the molecular ion peak in each repeating pattern is the most intense peak. Analysis of the intensities of the molecular ions in the distribution (e.g., approaching Mn) allows chemical kinetic information to be obtained from the ToF SIMS spectra of hydrolyzed samples. In the case of polyglycolic acid, a maximum in the distribution of the molecular ion peaks is observed which changes with respect to the hydrolysis time. For PGA, the average molecular weight of the hydrolysis products can be calculated from the ToF SIMS spectra, and a good linear relationship between the molecular weight and hydrolysis time was obtained. In the case of polyanhydrides, molecular ion peak intensities of the hydrolysis products decrease exponentially with respect to the molecular weight of the hydrolysis products. The slope of log peak intensity versus the molecular weight of hydrolysis products was found to vary with respect to the hydrolytic degradation rates of the polymer. The use of such in vitro kinetic information may be possible to estimate the degradation rate of the surface of the polymer. These data could be used for rapid screening of formulations and preparation of new materials.