Enzymatic hydrolysis of poly(lactide)s: Effects of molecular weight, L-lactide content, and enantiomeric and diastereoisomeric polymer blending

Nonblended poly(L-lactide) (PLLA) films having different molecular weights and nonblended poly(lactide) (PLA) films, enantiomeric blend films from PLLA and poly(D-lactide) (PDLA), and diastereoisomeric blend films Of poly(DL-lactide) (PDLLA) with either PLLA or PDLA, having different L-lactide (LLA) contents (X(LLA)s) in the range of 0-0.99, were amorphous made by melt-quenching. The effects of molecular weight, X-LLA, and average L- and D-lactyl unit sequence length (l(L) and l(D), respectively) on the enzymatic hydrolysis of the films were investigated in the presence of proteinase K. The enzymatic hydrolysis rate (R-EH) Of PLLA estimated by gravimetry increased monotonically with the inverse of number-average molecular weight (M-n). The extrapolation of R-EH of PLLA to M-n(-1) = 0, where no exo-chain-scission takes place, gave a positive R-EH value (1.75 mug/(mm(2).h)), meaning that the enzymatic hydrolysis of PLLA proceeds via both endo- and exo-chain-scission. The REH of the nonblended films as well as the enantiomeric and diastereoisomeric blend films decreased monotonically with the decease in XLLA and finally became zero for X-LLA below 0.3. The R-EH values of the blend films of PLLA and PDLLA with PDLA (l(D) = infinity) were lower than expected, while the REH values of the blend films of PLLA with PDLLA (l(D) = 4) agreed completely with the expected values. These results reveal that the nonblended PLA films are enzymatically hydrolyzable when X-LLA and l(L) are higher than 0.3 and 3, respectively, and l(D) is lower than 10 and that the presence of long D-lactyl unit sequences (l(D) > 4) as in PDLA hinders the enzymatic hydrolysis of long L-lactyl unit sequences even when long D- and L-lactyl unit sequences are present in the different molecules.