Investigation of structural conversion and size effect from stretched bundle of electrospun polyacrylonitrile copolymer nanofibers during oxidative stabilization

Abstract

The ultimate goal of this research is to develop continuous carbon nanofibers with superior mechanical strength. The hypothesis is that aligned and stretched polyacrylonitrile (PAN) copolymer nanofibers may be an innovative type of precursor, because the nanofibers possess high degrees of morphological and structural perfections; and due to size effect (i.e., the nanofibers are ~20 times thinner than the conventional microfibers), the formation of structural heterogeneities (e.g., sheath/core structure) can be effectively prevented/mitigated. In this study, a bundle consisting of aligned PAN copolymer nanofibers was first prepared by electrospinning (with nanofibers collected by a flowing water system) followed by 3 times stretching; the oxidative stabilization of precursor nanofiber bundle was then investigated. Compared to conventional microfibers, electrospun nanofibers had smoother surface, higher density, and higher degrees of crystallinity and macromolecular orientation. As a result, the activation energy for cyclization of nitrile groups was higher; nevertheless, the diffusion of oxygen molecules was easier and the distribution of oxygen was more uniform particularly when the stabilization temperature was 240°C or higher. Such a situation led to the reduction of activation energy for oxidative dehydrogenation, facilitated the structural conversion into aromatic and supra-molecular structures, and also prevented the formation of sheath/core structure.