Shear-governed microstructural variation and evolution of PPTA in dry-jet-wet spinning process
Graphical Abstract
Introduction
The PPTA fiber is a kind of aramid fiber with excellent mechanical properties, among which the most well-known representative is the Kevlar fiber developed by DuPont in the 1960s. [1], [2], [3], [4], [5], [6], [7] Because of its outstanding modulus, specific strength, and impact toughness, the PPTA fiber has been widely used in ballistic protection products and advanced equipment such as airplanes and rockets. [8], [9], [10] The outstanding mechanical behavior of PPTA fibers is attributed to its high alignment of molecular chains and multi-level microstructure composed of repetitive p-phenylene terephthalamide units. [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]
Electron microscopy and X-ray diffraction are commonly used methods in characterizing the microstructure of finished PPTA fibers. [21], [22], [23], [24], [25], [26] In an X-ray diffraction study, Northolt proposed the model of the crystal and the molecular structure of PPTA. [19] Dobb et al. discovered the pleat supramolecular structure of PPTA fibers using a combination of electron diffraction and electron microscope dark-field image techniques. [18] Near the edge of the fiber, the pleat angle is small, while near the center of the fiber, the fold angle is relatively larger, which is due to the fact that the orientation of molecular chains are not parallel to the fiber axis. Morgan et al. proposed a microstructural model of PPTA fibers based on images from the scanning reflection electron microscopy. [17] A similar microstructural model of PPTA fibers was also proposed by Roenbeck. [14] It has been proposed that the PPTA fiber can be divided into skin part and core part. The skin part is partially made up of parallel chains of molecules, and the core part is composed of molecular bundles with a length of 200 to 250 nm and a width of around 60 nm.
Despite the progress, limited studies and datasets are available on the evolution and variation of PPTA fiber microstructure during fabrication due to the lack of direct observation methods. PPTA fiber is normally manufactured by the dry-jet-wet or wet spinning process because PPTA can be easily dissolved in strong acids such as the sulfuric acid. [27, 28] Fibers produced by such methods often have oriented molecular chains, which largely determine the crystallinity and mechanical properties of PPTA fibers. [14, 27, [29], [30], [31]] When the sulfuric acid solution of PPTA is extruded through the fine orifice of a spinneret, a velocity gradient is generated radially in the cross section of the orifice. As the flow distance increases, Poiseuille flow is developed globally while Couette flow holds locally. [32], [33], [34], [35], [36], [37] Experimentally observing the microstructural evolution and variation of PPTA in such a complex flow environment is challenging.
Molecular simulation has provided an alternative route to elucidate the microstructural changes of PPTA in the solution-based fabrication process. [38], [39], [40], [41], [42], [43] For example, Mogurampelly et al. simulated the microstructural evolution of PPTA molecular chains with both random and parallel orientations in the concentrated sulfuric acid solution. [44] The coarse-grained molecular dynamics (CG-MD) was applied to enable large-scale molecular simulations which were difficult to achieve by all-atom MD. However, this study did not consider the fabrication process or the microstructural formation of fibers. Alternatively, molecular simulation may be coupled with advanced characterization techniques to understand the microstructural evolution of polymers in spinning. For example, small angle X-ray scattering (SAXS) is one of the most popular methods to characterize polymers, which has been applied to study flow-induced crystallization of semicrystalline polymers and tension-induced microstructural defects. [45], [46], [47], [48], [49], [50], [51], [52], [53] By performing SAXS based on MD, Aratsu et al. has studied the microstructure of helical chains and provided molecular insights that are otherwise difficult to achieve by experiments. [54]
This paper aims to investigate the microstructural variation and evolution of PPTA fibers in dry-jet-wet spinning at two distinct length scales. At the microscopic scale, the kinetic motion and alignment of PPTA chains will be simulated by CG-MD in a local environment assuming Couette flow. Microstructural changes due to the solidification and refinement of fibrils will be investigated by MD-based SAXS. At the macroscopic scale, the pointwise Weissenberg number (Wi = shear rate × relaxation time) will be evaluated for the PPTA-H2SO4 solution inside a spinneret hole assuming Poiseuille flow. Finally, the two scales are bridged via the Weissenberg number, enabling the study of how chain alignment varies at different positions of a PPTA fiber, and how chain alignment is influenced by macroscopic parameters (e.g., aspect ratio of the fine orifice). The achieved theoretical insights may guide process design leading to PPTA fibers of improved microstructural quality and desired properties.
Section snippets
Methods
The coarse-grained molecular dynamics (CG-MD) method was used to study the trajectory of PPTA molecular chains in the sulfuric acid solution. The coarse-grained model is an efficient method for the molecular simulation of a massive system including millions of atoms that is impossible for all-atom molecular dynamics (AA-MD) to process and has been widely used in multiscale molecular dynamics simulations in various applications. [55], [56], [57], [58], [59], [60], [61], [62]
Deriving force field
Influence of shear rate on PPTA chain orientation in solution
The CG model of a PPTA-H2SO4 solution is shown in Fig. 3(a), which has a total of 2500 PPTA molecular chains (each containing 8 repeating units) and 200,000 H2SO4 molecules randomly inserted into the box. The total number of particles in the box is 225,000. After relaxation, the box shrank from 90 × 90 × 90 nm to 28.6 × 28.6 × 28.6 nm. The PPTA-H2SO4 models after relaxation and after shear deformation with Wi = 5 are shown in Fig. 3(b) and (c), respectively. Note that the deformed CG system in
Concluding remarks
To sum up, this paper investigates the microstructural variation and evolution in PPTA fibers fabricated via dry-jet-wet spinning by using computational and analytical methods at two length scales. At the macroscopic scale, the sulfuric acid solution of PPTA in the spinneret was analyzed assuming Poiseuille flow, leading to expressions of Wi in terms of the shear rate, flow velocity, spinneret geometry, and radial position. At the microscopic scale, the arrangement of PPTA chains was simulated
CRediT authorship contribution statement
Tong Li: Conceptualization, Methodology, Funding acquisition, Writing – review & editing, Supervision. Zebei Mao: Conceptualization, Methodology, Validation, Writing – original draft. Juan Du: Resources. Ling Liu: Methodology, Writing – review & editing. Bo Wang: Funding acquisition, Writing – review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Authors, Li, and Wang, would like to thank the support from the National Natural Science Foundation of China (12172077, 11825202), Dalian High-Level Talent Innovation Support Program (2019RD04), and the Dalian Science and Technology Innovation Fund (2020JJ25CY011). Author Liu would like to thank the startup fund from Temple University, and CAREER Award No. CBET-1751610 from the National Science Foundation.
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These authors contributed equally to this work and should be considered co-first authors.