Force field in coagulation bath at low temperature induced microfibril evolution within PAN nascent fiber and precursor fiber

Abstract

The coagulation process was vital for the microfibril evolution and mechanical properties of polyacrylonitrile (PAN) fibers. The PAN nascent fibers and precursor fibers were prepared by controlling drawing ratio of coagulation bath at low temperature during the dry‐jet wet spinning process. The microfibril morphological changes induced by force fields in coagulation bath were investigated by scanning electron microscopy and high‐resolution transmission electron microscopy. During the coagulation process, the spinning solution evolved into an interconnected network composed of random microfibrils and tie joints as a building block of the network. At low drawing ratio, the random interconnected network existed in nascent fibers. Increasing drawing ratio, the fiber filaments underwent shrinkage and the network was transformed into the transverse lamellae. Furthermore, the lamellar thickness also decreased. After the treatment of post‐spinning, similar transverse lamellae were formed in all of precursor fibers, and random microfibrillar network was stretched and oriented to develop into the aligned microfibrils in precursor fibers. The transverse fold‐chain crystal layers were densely stacked in the microfibrils. Increasing drawing ratio, the lamellae and microfibrils in precursor fibers were packed more densely and orderly. Consequently, the order and homogeneity of microfibrils in nascent fibers and precursor fibers were improved by increasing drawing ratio, which were benefit to increase fiber tensile strength and modulus.

From Microfibrillar Network to Lamellae during the Coagulation Process of Polyacrylonitrile Fiber: Visualization of Intermediate Structure Evolution

“…However, PAN nascent fibers are constituted of a random microfibrillar network. , Meanwhile, the lamellae structures perpendicular to the fiber axis are also discovered in PAN nascent fibers in electron microscope studies . In our previous work, the random microfibrillar network was transformed into transverse lamellae in nascent fibers when the drawing ratio of coagulation bath increased, but the nascent fibers with lamellae structures still showed a microfibrillar network after solution etching of ultrathin section . Consequently, it is inferred that the network and lamellae coexist in some PAN nascent fibers.…”

“…Polyacrylonitrile (PAN)-based carbon fibers are utilized in many applications in automobile, sports, aviation, wind blades, and pressure vessels, and their demand increases very quickly. − They are manufactured by PAN precursor fibers through the thermal stabilization and carbonization. − And their mechanical strength is determined by the quality and microstructures of precursor fibers, which are originated and developed during the stretching process of fiber production. − So far, PAN precursors are prepared by wet spinning, dry-jet wet spinning, and gel spinning. , The fiber-processing conditions have a vital influence on the fiber microstructures and the quality of precursor fibers. , Many studies have been carried out to reveal the evolution mechanisms of PAN fiber microstructures, thereby regulating and controlling microstructures of PAN fibers by optimizing process parameters to achieve excellent mechanical properties. ,, However, with either technique, the coagulation process is an initial and vital stage for structural formation of the PAN fibers, which determines the quality of PAN fibers and then affects the properties of the carbon fibers. − Consequently, we have devoted to explore the formation mechanisms of the PAN fiber microstructures during the coagulation process in this work as well as in previous works. , …”

“…Freeze-drying of wet sample is one of the best methods of water and solvent removal and obtains better quality of final products, compared to any other drying technique. , The fresh fiber samples (directly taken out of coagulation bath) at different stages of coagulation process are frozen in liquid nitrogen and subsequently freeze-dried, which could efficiently retain the intermediate structures of PAN fibers in the coagulation bath. On the other hand, the widely used techniques for determining fiber microstructures involve X-ray diffraction (XRD) and scanning or transmission electron microscopy (SEM or TEM). ,− The XRD method suffers from two limitations for charactering network and lamellae structures. First, it shows averaging results over an entire interrogation volume (rather than being spatially resolved), and the minor morphological changes of the network and lamellae structures are difficult to determine .…”

Designing Materials and Processes for Strong Polyacrylonitrile Precursor Fibers

“…The drawing effects on the microstructural changes and mechanical property improvements have been demonstrated with the dry-jet wet spinning process [ 100 , 101 ]. To determine the most effective drawing process, the microstructures and mechanical properties of the wet- and dry-drawn precursor fibers were evaluated, and the wet-drawn precursor fiber exhibited better modulus and rigidity, while the dry-drawn precursor fiber exhibited slightly higher strength and breaking strain [ 102 ]. Therefore, the combination of wet and dry drawing processes can help optimize the microstructure and mechanical performance.…”

Improving Transverse Compressive Modulus of Carbon Fibers during Wet Spinning of Polyacrylonitrile

“…The total increase in length between the extruded fiber and the final WF is known as the total draw ratio (TDR). Increasing the TDR during the spinning process increases the orientation along the fiber axis [15], increases the crystallinity [16] and the homogeneity [17], and promotes the formation of a rounded cross-section [18]. Surface roughness also increases with drawing [19,20], which must be accounted for when considering its properties in the transverse direction.…”