MELT STABILITY STUDY OF VARIOUS TYPES OF POLY(L-LACTIDE)

Poly(L-lactide) (PLLA) prepared by ring-opening melt polymerization us ing tin-octoate as catalyst undergoes a rapid melt degradation. The me lt stability of PLLA was evaluated by using a Haake melt rheometer at various temperatures. The degradation rate constants were calculated f rom the changes in the melt viscosity for as-polymerized, precipitated , and acid treated PLLA. Furthermore, stability measurements were perf ormed for as-polymerized PLLA modified with various types of peroxides (hydroperoxide, dialkyl peroxide, peroxy ester, and diaroyl peroxide) . The energy of activation for the melt degradation of as-polymerized PLLA was calculated to 119.4 kJ/mol at temperatures between 180 and 21 0 degrees C. Precipitation of PLLA gave a reduction of the tin catalys t content in the polymer, and this also reduced the extent of the melt degradation. A further reduction of the tin content, and accordingly a reduction in the melt degradation, was achieved after acid treatment of the polymer. Addition of peroxide in open air reduced the degradat ion rate constant to at least half the value of as-polymerized PLLA. H owever, in an inert atmosphere the dialkyl peroxide did not retard the melt degradation.