Selective enzymatic degradations of poly(L-lactide) and poly(epsilon-caprolactone) blend films

Solution cast films were prepared from poly(L-lactide) (PLLA) and poly(epsilon-caprolactone) (PCL) as well as from three blends, namely B75, B50, and B25 with PLLA/PCL proportions of 75/25, 50/50, and 25/75, respectively. The enzymatic degradation of square samples (10 x 10 x 0.2 mm) cut from the films was investigated at 37 degrees C in a pH = 8.6 Tris buffer containing proteinase K or in a pH = 7.0 phosphate buffer containing Pseudomonas lipase. It was confirmed that proteinase K can degrade amorphous domains of PLLA, but cannot degrade crystalline PLLA or PCL. In contrast, Pseudomonas lipase can degrade both amorphous and crystalline PCL but cannot degrade PLLA. The two faces of solution cast films showed different morphologies due to the solvent evaporation process. The lower face appeared more crystalline than the upper face because of the plasticizing effect of solvent entrapped inside which allowed crystallization to proceed. Therefore, the lower face was more resistant to enzymatic attack by proteinase K in the cases of PLLA and the blends. The two polymers in the blends exhibited well separated crystalline domains. PCL seemed to constitute the continuous phase of the blends with formation of large size spherulites when the PCL content was over 50%. The selective degradation of PCL or PLLA components revealed the inner morphology of the blends where microspherelike or islandlike patterns were observed.