Fibril microstructural changes of polyacrylonitrile fibers during the post-spinning process

Abstract

Ultrathin sectioning and solvent etching were used to assess microstructural changes in polyacrylonitrile (PAN) fibrils following a dry-jet wet spinning process. Morphologic assessment revealed the coexistence of fibrils and lamellae within the PAN fibers, with a frill-like structure found on the fibrils. The size of the lamella-like layers was consistent with a crystal size of 13.2 nm as characterized by X-ray diffraction and microfibrils were composed of alternate dark lamella-like crystal layers and surrounded by bright amorphous chains. Further, fibril arrangement followed a weave-like pattern. During the post-spinning process, break-rearrangement of chain-folded lamellae had an important role in fibril structure evolution. The high stream stretching and heat treatment induced the breaking of lamellae and the formation of new lamellae through the molecular chains rotating and slipping, resulting in a decrease in voids and the coalescence of microfibrils. Consequently, the fibrils were arranged in a more orderly manner and interlinked more tightly, a process conducive for the preparation of high quality PAN fibers.