Thermal stability of poly (L-lactide): influence of end protection by acetyl group

Thermal stability of end-protected Poly (L-lactide) (PLLA) was studied by dynamic thermal degradation and pyrolyzate analyses. The treatment of PLLA by acetic anhydride resulted in the acetylation of end hydroxyl groups, and at the same time a decrease in the residual Sri content in the polymer. The thermal degradation of the acetylated PLLA-Ac showed a shift to a 40-50 degreesC higher degradation temperature range than that of untreated, high Sri content PLLA, but exhibited nearly the same degradation behavior as the untreated PLLA with a comparable Sri content. Purified metal-free PLLA-H showed good thermal stability, having the highest degradation temperature range. Interestingly, despite the end-protection, the acetylated metal-free PLLA-H/Ac decomposed at almost the same temperature as that of PLLA-H. From pyrolyzate and kinetic analyses, it was found that the contribution of the hydroxyl-end acetylation to the stability of PLLA was negligible, except for the stabilization effect due to the elimination of residual Sri during the acetylation process. (C) 2004 Elsevier Ltd. All rights reserved.