Plastic deformation analysis of polyacrylonitrile fibers subject to tension: visualization of fibrillation behavior

Abstract

To develop next-generation polyacrylonitrile (PAN) based carbon fibers with enhanced mechanical performance, it is crucial to understand the plastic deformation mechanisms of PAN fibers during stretching process. In this work, the morphological changes of intermediate microfibril structures within PAN fibers at various stages along the stress–strain process were visualized using the ultrathin sectioning technology and electron microscopy. Upon approaching the yield point, the crystalline structure's constraints were compromised, leading to the initiation of microfibril slippage. During the necking process, the varying mechanical responses of the interpenetrated network resulted in a radial gradient in the orientation degree and packing density of microfibrils along the stretching direction. The stretching-induced fibrillation resulted in the alignment of microfibril elements and subsequent recrystallization, thereby facilitating significant macroscopic deformation. The fracture failure of PAN fibers was attributed to the cracking and breakage of the microfibril network, which involved the pull-out of microfibril elements and disentanglement of the interpenetrated network. Furthermore, a novel structural model was developed to elucidate the plastic deformation mechanisms of microfibril elements during macro-drawing of fibers. This model is anticipated to enhance the design and optimization of the microstructure and processing techniques for high-performance PAN fibers.