Functional group changes of polyacrylonitrile fibres during their oxidative, carbonization and electrochemical treatment

A unique consideration is intended, concerning the creation of functional groups during the production of carbon fibres from polyacrylonitrile, i.e. oxidation/stabilization, carbonization/pyrolysis and electrochemical modification processes and their role for applications. Acrylic fibres of homopolymer PAN are subjected to thermo-oxidative treatment following a four-stage or a two-stage isothermal programme, and the resulting oxidized fibres are pyrolyzed up to 1000 °C for 30 min or they are flash-pyrolyzed at 1000 °C for 5 min, and the shrinkage and weight loss are determined. According to FTIR spectra, two indices were expressed to follow the dehydrogenations and cyclizations (nitrile groups) and the band intensity of carbonyl groups to follow the oxidation. The carbon fibres produced from the four-stage oxidation were examined using SEM/EDS. Based on SEM/EDS analysis, an oxygen content of approx. 8.7 at.% in the fibres is needed for the stabilization of the fibres during the oxidation by a four-stage treatment up to 300 °C. These carbon fibres were electrochemically modified by Cyclic Voltammetry in narrow and wide potential regions, using aqueous H2SO4. The electrochemical treatment of carbon fibres in the wide region leads to the introduction of an additional amount of oxygen of about 5 at.%, exhibiting also acidic and basic groups in adequate amount. The formation of oxygenous groups on carbon fibres during the three processes is indirectly followed by their ability to adsorb methylene blue or alizarin yellow dyes, based on the concept of “acid–base interaction”. Considering a model of a graphitic monolayer of a basic square that includes 9 rings, the O atoms introduced in this segment were estimated. Carbon fibre applications depend on the amount and type of oxygenous groups that can be targeted by selecting proper treatment conditions.