Pyrolysis kinetics of poly(L-lactide) with carboxyl and calcium salt end structures

To clarify the pyrolysis mechanism Of poly(L-lactide), which has been reported as complex, the thermal decomposition of carboxyl type and calcium ion end capped PLLA (PLLA-H and PLLA-Ca, respectively) was investigated by means of thermogravimetric analysis (TG), and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). The TG data revealed that PLLA-Ca has a lower pyrolysis temperature (220-360 degreesC) than that of carboxyl type PLLA-H (280-370 degreesC). The apparent activation energy of the decomposition reaction was estimated from TG curves at different heating rates by plural methods to be 176 and 98 U mol(-1) for PLLA-H and PLLA-Ca, respectively.. Further kinetic studies indicated that PLLA-H degraded mainly through a random reaction with a pre-exponential factor A = 2.0 x 10(12) s(-1), whereas PLLA-Ca degraded by way of a 1st-order reaction with A = 8.4 x 10(1) s(-1). Pyrolysis products of PLLA-H were composed of lactides and other cyclic oligomers, while the degradation products of PLLA-Ca were principally lactides. The main reaction pathway for PLLA-H pyrolysis was regarded as the random transesterification, whereas for PLLA-Ca pyrolysis the unzipping depolymerization process was dominant. (C) 2002 Elsevier Science Ltd. All rights reserved.