Abstract
A standard protocol is proposed which has been used to study comparatively the degradation mechanism of bioresorbable poly(α-hydroxy acids) with respect to macromolecular structural characteristics and solid-state morphologies. As a first approach, the hydrolytic degradation of poly(dl-lactic acid) (PLA50) parallelepipedic specimens (15 mm×10 mm×2 mm), processed by compression moulding and machining, was investigated in two aqueous media: iso-osmolar saline and pH 7.4 phosphate buffer. Various techniques (namely weighing, size-exclusion chromatography (SEC), potentiometry, cryometry and enzymatic assay) have been applied to these specimens in order to monitor the degradation. Data show conclusively that the degradation of massive PLA50 specimens proceeds more rapidly in the centre than at the surface. This feature has been related to the formation of an outer layer of slowly degrading polymer, which is caused by surface phenomena and entraps degrading macromolecules. Only oligomers can diffuse and dissolve in the surrounding media. Accordingly, the number of carboxylic groups present in the inner part of the degrading specimens becomes larger than at the surface and accelerates ester bond cleavage. The resultant autocatalytic mechanism explains well the fact that partially degraded PLA50 exhibits bimodal SEC chromatograms although this polymer is amorphous.
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Li, S.M., Garreau, H. & Vert, M. Structure-property relationships in the case of the degradation of massive aliphatic poly-(α-hydroxy acids) in aqueous media. J Mater Sci: Mater Med 1, 123–130 (1990). https://doi.org/10.1007/BF00700871
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DOI: https://doi.org/10.1007/BF00700871