Studies on the thermal degradation of poly (p-phenylene benzobisoxazole)

Thermal decomposition of poly(p-phenylene benzobisoxazole) (PBO) has been studied between room temperature and 1123 K. Two types of material (regular and high-modulus) were studied, which yielded almost equivalent results. Thermogravimetry and differential thermal analysis allowed establishing the main stages of the pyrolytic degradation of the material. On the basis of the thermal analysis results, samples were decomposed at several controlled temperatures and characterized by elemental analysis, infrared spectroscopy, X-ray diffraction, atomic force microscopy, and scanning tunneling microscopy. At temperatures below 933 K the polymer retains its original conformation and becomes stabilized by enhancement of its crystallinity. The decomposition takes place in a single step and the main changes occur within a very narrow temperature interval (983-993 K). Formation of polyaramides as intermediates in the decomposition process was detected. These amide bonds subsequently degrade by homolytic breaking, yielding nitriles. The final carbonaceous residue is rich in nitrogen and retains a certain degree of anisotropy, a fact that was explained by the conservation of crystallinity at an intermediate decomposition stage.