Comparative study on hydrolytic degradation and monomer recovery of poly( l-lactic acid) in the solid and in the melt

Abstract

The hydrolytic degradation of poly( l-lactide) (PLLA) and the formation of its monomer in the solid and in the melt were investigated at 120–150 °C (in the solid), at 160 °C (in the solid up to 40 min and in the melt exceeding 40 min), and at 170–190 °C (in the melt). Such state difference caused the difference in the degradation behavior of PLLA and the behavior of lactic acid formation, although the degradation of PLLA proceeds via a bulk erosion mechanism, regardless of its state. The crystalline residues were formed at the degradation temperatures below 140 °C, but not at the degradation temperatures above 160 °C. The lactic acid yield exceeding 95% can be successfully attained for all the temperatures of 120–190 °C. The activation energy for hydrolytic degradation values of PLLA were 69.6 and 49.6 kJ mol −1 for the temperature ranges of 120–160 °C (in the solid) and 170–250 °C (in the melt), respectively, and are compared with the reported values.