Poly(L-lactide). X. Enhanced surface hydrophilicity and chain-scission mechanisms of poly(L-lactide) film in enzymatic, alkaline, and phosphate-buffered solutions

Attempts were carried out to enhance the surface hydrophilicity Of poly(L-lactide), that is, poly(L-lactic acid) (PLLA) film, utilizing enzymatic, alkaline, and autocatalytic hydrolyses in a proteinase K/Tris-HCL buffered solution system (37degreesC), in a 0.01N NaOH solution (37degreesC), and in a phosphate-buffered solution (100degreesC), respectively. Moreover, its chain-scission mechanisms in these different media were studied. The advancing contact-angle (theta(a)) value of the amorphous-made PLLA film decreased monotonically with the hydrolysis time from 100degrees to 75degrees and 80degrees without a significant molecular weight decrease, when enzymatic and alkaline hydrolyses were continued for 60 min and 8 h, respectively. In contrast, a negligible change in the theta(a) value was observed for the PLLA films even after the autocatalytic hydrolysis was continured for 16 h, when their bulk M-n decreased from 1.2 X 10(5) to 2.2 X 10(4) g mol(-1) or the number of hydrophilic terminal groups per unit weight increased from 1.7 X 10(-5) to 9.1 X 10(-5) mol g(-1). These findings, together with the result of gravimetry, revealed that the enzymatic and alkaline hydrolyses are powerful enough to enhance the practical surface hydrophilicity of the PLLA films because of their surface-erosion mechanisms and that its practical surface hydrophilicity is controllable by varying the hydrolysis time. Moreover, autocatalytic hydrolysis is inappropriate to enhance the surface hydrophilicity, because of its bulk-erosion mechanism. Alkaline hydrolysis is the best to enhance the hydrophilicity of the PLLA films without hydrolysis of the film cores, while the enzymatic hydrolysis is appropriate and inappropriate to enhance the surface hydrophilicity of bulky and thin PLLA materials, respectively, because a significant weight loss occurs before saturation of theta(a) value. The changes in the weight loss and theta(a) values during hydrolysis showed that exo chain scission as well as endo chain scission occurs in the presence of proteinase K, while in the alkaline and phosphate-buffered solutions, hydrolysis proceeds via endo chain scission. (C) 2002 Wiley Periodicals, Inc.