The Relevancy Research On Electrospun Polyacrylonitrile Copolymer Nanofibers

That graphite-like structures of carbon fibers have high degrees of orientation along the fiber axes endue it with remarkable mechanical properties. In the production or processing of fiber, the stretching is an indispensable mean to improve the physical-mechanical properties of fiber. During the stretching the macromolecular chains or aggregation structure units of the fibers extend and have high degrees of orientation along the fiber axes. Meanwhile, it is usually accompanied with other structure characteristics variation, such as density, crystallinity, and thermal behavior variation.Using a representative polyacrylonitrile (PAN) copolymer(SAF 3K provided by the Courtaulds, Ltd in the UK) as the initial sample, continuous and uniaxially aligned polyacrylonitrile nanofiber bundles were prepared by a nanomaterials processing technique of electrospinning at the condition of the optimization electrospun processing parameters (16 wt.% PAN copolymer solution, high DC voltage was 25 kV, the gap distance was 13 cm, the spinneret consisted of a high-density poly- propylene tube with the inner diameter of 1.0 in., and a stainless steel hemispherical head which had an orifice with the diameter of 1 mm at the center). Instead of using roller, new flow water bath combining with existing wet spinning process and static water bath collection method was adopted as the collector. The influence of the water bath temperature on the nanofiber using flow water bath collection was studied in this paper, and we got the optimization water bath temperature to use for the following experiment.The nanofiber bundles were subsequently stretched in boiling water to different times of their original lengths. Morphologies, aggregation structures, and properties of the nanofiber bundles before and after stretching were investigated by instrumentations including, but not limited to, scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC), and the results were compared to those of as-electrospun PAN nanofiber bundles using roller collector which were prepared at the same electrospun processing parameters. In addition, effects of stretching on the oxidative stabilization process of electrospun nanofiber were also investigated.The study revealed that:(1) Using low water bath temperature during the nanomaterials processing technique of electrospinning was beneficial to prepare nanofibers with smooth surfaces, few defects, uniform diameter distributions and integrate crystal grains. In addition, it could improve thermal performances. (2) As compared to the as-electrospun nanofiber bundles using roller collector, the as-electrospun nanofiber bundles using water bath collector possessed higher degrees of nanofiber alignment and macromolecular orientation. Stretching stress would be more effective role for electrospun nanofiber bundles using water bath collector. (3) Post-stretching could promote diameter thinning and improve aggregation structure of as-electrospun nanofiber bundles. (4) With the same dealing with the stretching process, the as-electrospun nanofiber bundles using water bath collector could be stretched into four times of their original lengths, and the deformation rate increased slowly, and the stretching process effectively reduced the diameters of nanofibers by～44.1%; while the as-electrospun nanofiber bundles using roller collector only could be stretched into three times of their original lengths, and the deformation rate increased sharply, and the stretching process reduced the diameters of nanofibers by～14.4%. (5) The stretching process substantially improved the degrees of crystallinity and macromolecular orientation of the polyacrylonitile copolymer in the stretched nanofiber bundles, which were stretched in boiling water into four times of their original lengths, by 72% and 100%, respectively. The acquired values of crystallinity and macromolecular orientation for the polyacrylonitrile copolymer were 73.5% and 81.5%, which were similar to those in the SAF 3K microfiber bundles. (6) The studies on thermo-chemical behaviors of as-electrospun and stretched bundles indicated that, with the increase of stretching ratio, the activation energy, the exothermic onset temperature, the peak temperature, thermal rate of the as-electrospun nanofiber bundles using water bath collector were lower while the heat evolved was higher; additionally, Aromatization index, ring closure index, content of Oxygen elements, density of PAN pre-oxidized nanofibers were all higher. This suggested that the stretching process would romote cyclization of -CN of electrospun PAN nanofibers, improve thermal performances, and facilitate the oxidative stabilization of PAN copolymer for the development of continuous nano-scaled carbon fiber bundles with superior mechanical strength.