Alkaline and enzymatic degradation of L-lactide copolymers.: II.: Crystallized films of poly(L-lactide-co-D-lactide) and poly(L-lactide) with similar crystallinities

Films of poly(L-lactide-co-D-lactide) [P(LLA-DLA); 95/5] and poly(L-lactide) [i.e., poly(L-lactide acid) (PLLA)] were prepared by crystallization from the melt, and a comparative study of the crystallization effects on the alkaline and proteinase K catalyzed hydrolysis of the films was carried out. The hydrolyzed films were investigated with gravimetry, differential scanning calorimetry, polarimetry, and gel permeation chromatography, and the results were compared with those reported for amorphous-made specimens. The alkaline hydrolyzability of the P(LLA-DLA) (95/5) and PLLA films was determined solely by the initial crystallinity (X-c) and was not affected by the content of the incorporated D-lactide (DLA) unit in the polymer chain; this was in marked contrast to the fact that the enzymatic hydrolyzability depended on not only the initial X-c value but also the DLA unit content. The alkaline hydrolysis rate of the P(LLA-DLA) (95/5) and PLLA films and the enzymatic hydrolysis rate (R-EH) of the P(LLA-DLA) (95/5) films decreased linearly as the initial X-c value increased. This meant that the hydrolyzability of the restricted amorphous regions was very similar to that of the free amorphous regions. In contrast, R-EH of the PLLA films decreased nonlinearly with the initial X-c value, and this nonlinear dependence was caused by the fact that in the PLLA films the restricted amorphous regions were much more hydrolysis-resistant than the free amorphous regions. (c) 2005 Wiley Periodicals, Inc.