Thermal Degradation of Carboxymethyl Starch–g-Poly(lactic acid) Copolymer by TG–FTIR–MS Analysis

Abstract

In this study, carboxymethyl starch (CMS) was grafted with l(+)-lactic acid (LA) in a CMS/LA molar ratio of 1/36, resulting in the CMS–g-PLA copolymer. The grafting reaction was carried out via a solution polycondensation procedure, in the presence of stannous 2-ethyl hexanoate [Sn(Oct)2] as a catalyst. Poly(lactic acid) (PLA) was synthesized under the same conditions with the copolymer, for comparative analyses of the thermal properties. The CMS–g-PLA copolymer, CMS, and PLA were structurally characterized by Fourier transform infrared (FTIR) spectroscopy, while thermal degradation products were characterized by thermogravimetry analysis coupled with Fourier transform infrared spectroscopy and mass spectrometry (TG–FTIR–MS). The thermal degradation was achieved in a nitrogen atmosphere, in a temperature interval of 30–600 °C. The thermal degradation of CMS and PLA was studied under the same conditions to highlight the products resulted by the thermal decomposition of CMS–g-PLA copolymer. By processing the achieved data via thermal degradation at three different heating speeds (5, 7.5, and 10 °C min–1) and using the multivariate nonlinear regression method (MNLR), the kinetic parameters were determined. The dependence of the activation energy of the degradation process versus the extent of conversion was evaluated using the Friedman and Ozawa–Flynn–Wall model-free analyses. This variation suggests that the degradation process is a complex one and can be divided in one or two steps, depending on the chemical structure of the compounds. The kinetic parameters and the most probable thermal degradation mechanisms were given.