Poly( l-lactide): VI Effects of crystallinity on enzymatic hydrolysis of poly( l-lactide) without free amorphous region

Abstract

Poly( l-lactide) films without free amorphous region were prepared to have different crystallinities ( x cs) and therefore different crystalline thicknesses ( L cs) by allowing their crystallization to come to completion at different annealing temperatures ( T a s) from the melt. Their enzymatic hydrolysis catalyzed by proteinase K was investigated by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The change in molecular weight distribution and surface morphology of the PLLA films during hydrolysis revealed that enzymatic hydrolysis of the PLLA films proceeded mainly via the surface erosion mechanism. Etching the inside of PLLA spherulites resulted from preferred enzymatic hydrolysis of the PLLA chains in the restricted amorphous region between the crystalline regions. Enzymatic hydrolysis rate ( R EH) of the PLLA films decreased monotonously with increasing the initial x c and L c according to the equation, R EH[μ g/(mm 2·h)]=0.36[1−0.01· x c (%)]. The PLLA chains in the restricted amorphous region between the crystalline regions was found to be rather hydrolysis-resistant compared with those in the free amorphous region as in a completely amorphous film. The lowest molecular weight specific peak in GPC spectra was ascribed to the PLLA chains of one fold in the crystalline region. The appearance and remaining of the low molecular weight specific GPC peaks ascribed to the chains of two- and three-fold in the crystalline region during enzymatic hydrolysis strongly suggest that the enzymatic hydrolysis of the PLLA films took place predominantly at the free and tie chains in the restricted amorphous region rather than at those in the folding surface of the PLLA lamellae. The relationship between the melting temperature ( T m) and L c was found to be T m K =472 [1−1.46/ L c (nm)] for PLLA films enzymatically hydrolyzed for 40 h.