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.

We report on the surface characterization of commercial poly(p-phenylene benzobisoxazole) (PBO) fibers. Several solvents (hexane, acetone, and ethanol) were employed to remove the sizing present on the fiber surfaces. The surface properties of the different samples were studied by means of inverse gas chromatography (IGC) at infinite dilution using nonpolar n-alkanes and molecules with different acid−base characteristics (benzene, pyridine, acetonitrile, nitromethane, tert-butyl alcohol). Complementary information was obtained by using thermogravimetric/differential thermal analysis (TG/DTA) and atomic force microscopy (AFM). Results showed that, whereas the sizing of high-modulus (HM) fibers can be gradually washed off as the polarity of the solvent increases, only ethanol washing allowed to detect changes in the surface characteristics of PBO as-spun (AS) fibers, with hexane and acetone having little or no effect. In fact, it is believed that the effect of any coating applied eventually to the PBO AS fibers is masked by the presence of poly(phosphoric acid) (PPA) or partially coagulated PPA/PBO residuals on their surfaces, which in turn determine their surface properties. In any case, both standard finishes and contaminants strongly decrease the surface energy of the PBO fibers. According to the experimental results, it was assumed that PBO pristine fiber surfaces could be obtained by ethanol washing. Further differences were found between the cleaned AS and HM fibers. AFM measurements showed microfibrils present in the HM fibers that are somewhat wider than their AS counterparts. The presence of voids at the surface level of the cleaned PBO AS fibers brought about strong changes in adsorption energetics. No such changes were detected in the case of cleaned HM fibers. The contaminants of the AS fibers confer them a more acidic character than that of the rest of samples studied. Sizing/contaminants removal on both fiber types leads to an increase in the number or strength of basic sites. However, the cleaned fibers kept an amphoteric character based on the strong specific interactions existing between pyridine and the fiber surfaces. Ab initio calculations carried out on an appropriate model confirmed the presence of positively charged carbon atoms in the PBO monomers which should act as electron acceptors. Moreover, PBO fibers are prone to exert important π−π interactions as it was found both experimentally (relatively high −Δ values of benzene) and predicted from the charge distribution obtained from theoretical calculations.

The swelling of a polymer surface has been monitored in real time on the nanometer scale by atomic force microscopy (AFM). After modification by oxygen plasma treatment, poly(p-phenylene terephthalamide) (PPTA) displays a characteristic nanostructured surface morphology consisting of high-lying features alternating with topographically depressed areas. Selective swelling of the least cross-linked, depressed areas after the adsorption of ambient water or water from saturated humid atmospheres was observed by tapping mode AFM operated in the attractive interaction regime. The swollen areas could be distinguished from the nonswollen ones by local variations in the sample indentation made by the AFM tip when imaging in the tapping mode repulsive interaction regime. Monitoring the swelling of the plasma-treated polymer surface provided a means to reveal the nanometer-scale heterogeneity that this type of treatment creates on the polymer surface, which is something that would not be possible otherwise. Measurement of AFM tip-sample adhesion forces evidenced rapid water adsorption onto the oxygen plasma-treated surface, supporting the idea of water-induced swelling. This high hydrophilicity was interpreted as arising from the incorporation of polar oxygen functionalities, as demonstrated by X-ray photoelectron spectroscopy (XPS).

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