Morphological investigation on melt crystallized polylactide homo- and stereocopolymers by enzymatic degradation with proteinase K

Polylactide (PLA) homo- and stereocopolymers containing 100, 98, 96, 94, and 92% L-lactyl units, respectively, were synthesized by ring opening polymerization Of L-lactide and DL-Lactide, using zinc lactate as catalyst. Differential scanning calorimetric analysis measurements show that incorporation Of D-lactyl units leads to decrease of the crystallization rate of the copolymers. However, the crystallization mechanism and the amount of crystallizable fraction are not affected. The enzymatic degradation was performed at 37 degrees C in a pH 8.6 Tris buffer containing proteinase K. Two distinct morphologies were obtained by melt crystallization for PLA films with ca. 80 mu m of thickness. It is confirmed that proteinase K can degrade both the free and confined amorphous regions. Lamella stacks in spherulites retain their orientation during enzymatic degradation. PLA crystal morphologies are affected by the content Of D-lactyl units. Factors such as the nucleus location and the D-lactyl units' exclusion as amorphous fraction were considered to elucidate the observed PLA spherulite morphologies. Infrared spectroscopy and mass loss measurements were also combined to better understand the degradation behaviors. (C) 2008 Wiley Periodicals, Inc.