TGA/FTIR study on thermal degradation of polymethacrylates containing carboxylic groups

In this work, the thermal degradation of polymethacrylates containing carboxylic groups namely poly(methacryloyloxy butanoic acid), PMBA; poly(methacryloyloxy hexanoic acid), PMHA; and poly(p-methacryloyloxy benzoic acid), PMBeA was investigated by TGA/FTIR. Moreover, in order to shed more light on the reaction pathways during the thermal decomposition of these polymers, an FTIR spectroscopic study of structural changes in the degrading material was performed. By TGA it was observed that PMBA exhibited two well-defined degradation stages at 327 and 450 °C; PMHA presents only one main weight loss at ca. 402 °C although from DTG curve it was noted that the single step degradation was composed by two overlapped peaks located at 414 and 449 °C and a small shoulder at 317 °C; finally PMBeA showed three weight loss regions at 265, 353 and 468 °C. From FTIR analysis of the partially degraded samples it was found that the thermal degradation of these polymers resembled that of polymethacrylic acid, i.e. anhydrides were initially formed and then the modified structure is broken to yield an aromatic structure with phenolic groups. In contrast, the analysis by FTIR of the volatile products from the studied polymers differs notably than those obtained for polymethacrylic acid: β-lactones and γ-lactones were released from PMBA and PMHA, respectively, during its thermal degradation, whereas an ester derivative from benzoic acid evolves from PMBeA probably through depolymerization.