Microstructure of fibrils separated from polyacrylonitrile fibers by ultrasonic etching

Abstract

Polyacrylonitrile (PAN) fiber is an important precursor fiber for high performance carbon fiber. The properties of the final carbon fiber depend strongly on the nature of the PAN fibers. The PAN fibrils were separated successfully from fibers by ultrasonic etching and were systematically investigated by field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). It is found that in certain ultrasonic etching conditions (at 75±2ºC for 6 h with a frequency of 40 kHz) the PAN fibers are dissolved in the 95 wt.% aqueous dimethylsulphoxide (DMSO) solution; the fibrils consisting of numerous periodic lamellae with thickness of 3045 nm and perpendicular to the fiber axis are separated in the 90 wt.% aqueous DMSO solution; and the fibrils with smooth surface exfoliated from the PAN fibers are obtained in the 7090 wt.% aqueous DMSO solutions. Inner periodical structure of fibrils was observed in HRTEM, which indicates that there are different densities and two phases in fibrils. The PAN fibers are dissolved layer by layer with increasing ultrasonic etching time. The fiber surface experiences ultrasonic cleaning, selective etching, excessive etching and dissolution, and then the sublayer experiences the same process. There are numerous periodic lamellae in fibrils of nascent fibers. This means that the fibrils with lamellae are formed by orientation and crystallization in shearing field of spinning pipe and drawing stress field of coagulation bath. carbon fibers, PAN fibers, fibrils, microstructure Citation: Wang Q F, Wang C G, Yu M J, et al. Microstructure of fibrils separated from polyacrylonitrile fibers by ultrasonic etching.

Correlation between fibril structures and mechanical properties of polyacrylonitrile fibers during the dry‐jet wet spinning process

“…Figure shows the SEM morphology of the PAN fiber after ultrasonic etching. Comparing to Figure , the cracks appeared on the fibers and the surface became rougher, which were resulted by the dissolution of the amorphous phase and the separation of fibrils by ultrasonic etching . The surfaces of PAN Fibers S1 and S2 [Figure (a,b)] were consisted of a mass of oriented microfibrils, which arranged along the fiber axis.…”

“…Comparing to Figure 4, the cracks appeared on the fibers and the surface became rougher, which were resulted by the dissolution of the amorphous phase and the separation of fibrils by ultrasonic etching. 23,24,33 The surfaces of PAN Fibers S1 and S2 [ Figure 5(a,b)] were consisted of a mass of oriented microfibrils, which arranged along the fiber axis. In addition, the microfibrils were formed by the phase separation and the molecular chains orientation under the extensional stress field of the coagulation bath.…”

“…Craig et al 22 found the existence of fibril-void structures in PAN fibers, originated during coagulation process and further developed through stretching and collapsing. Wang et al 23,24 combined the ultrasonic breakup and solvent etching to separate the fibril elements from PAN fibers and proposed the formation process of the periodic lamellae structures in microfibrils. Li et al 25 suggested that the fibrils served as the self-reinforcing elements to enhance the material mechanical properties.…”

Visualization of microfibrillar elements in cross‐section of polyacrylonitrile fiber along the fiber spinning line

“…Figure shows the SEM morphologies of cross‐section of the PAN fibers after ultrasonic etching. Compared with Figure S3, the flat and smooth cross‐section became rough after ultrasonic etching, because the amorphous phase between the microfibrils appeared to be dissolved by DMSO solvent (Kunzmann et al, ; Wang, Wang, Bai, et al, ; Wang, Wang, Yu, et al, ). And then the microfibrillar nodes were carved out more distinctly (Figure a–f).…”

“…Even though the microfibrillar element acts as one of the minute fibrous unit making up a fiber, it is still a very complicated system itself. Such complexity stems from the broad dimension distribution (Tucker & George, ), their spatial arrangement (orientation, bundle formation) (Wang et al, ; Wang et al, ), interlinking lamellae between fibrils (Chen et al, ), and different morphologies and chain stack (Kunzmann, Schmidt‐Bilkenroth, Moosburger‐Will, & Horn, ). In addition, due to the big length to diameter ratio of the microfibrils, the previous research mainly focuses on the longitudinal morphologies of the microfibrils (Chen et al, ; Kunzmann et al, ; Wang, Wang, Bai, et al, ; Wang, Wang, Yu, et al, ; Warner, Uhlmann, & Peebles Jr, ).…”