Hydrolytic degradation of poly(rac-lactide) and poly[(rac-lactide)-co-glycolide] at the air-water interface

Abstract

The understanding of the simultaneous transport and chain-scission phenomena involved in the hydrolysis of bulk-degrading polymers requires the experimental separation of chain cleavage and water diffusion. The hydrolytic chain cleavage of poly(rac-lactide) rac-(PLA) and poly[(rac-lactide)-co-glycolide] (PLGA) is analysed on the basis of monolayer degradation experiments combined with an improved data reduction procedure. Different, partly contradictory models of the hydrolytic degradation and erosion mechanism of PLA and PLGA, namely random chain scission and chain-end scission, are discussed in the literature. The instantaneous linear area reduction observed for the polymer Langmuir films indicates a chain-end scission mechanism. As monolayers of end-capped and non-end-capped polymers degrade with exactly the same rate, the observed differences in the degradation kinetics of bulk samples do clearly result from differences in the water penetration into these polymers. A pronounced auto-inhibition effect is observed for the polymers degraded at initially high pH of the aqueous subphase in the absence of buffers.