Shear-governed microstructural variation and evolution of PPTA in dry-jet-wet spinning process

https://doi.org/10.1016/j.ijmecsci.2022.107950Get rights and content

Highlights

  • Flow-induced shearing dominates microstructural evolution of PPTA fibers.

  • Shearing rate mediates chain alignment during the manufacturing process.

  • Larger aspect ratios of spinnerets improve the quality of PPTA fiber products.

Abstract

A kind of aramid fiber with excellent mechanical properties, PPTA often shows process-induced, position-dependent molecular chain alignment that greatly impacts its mechanical performance. Direct observation of the chain reorientation along with other microstructural variation and evolution characteristics during fiber spinning has been a challenge for experiments. This study develops a coarse-grained molecular dynamics (CG-MD) model to investigate the shear-governed microstructural evolution of PPTA during dry-jet-wet spinning considering various shear deformation and rates. By using MD-based X-ray scattering, the shearing effects on microstructures are revealed systematically for PPTA both in the solution and after drying. The MD results are further coupled with a macroscopic analysis assuming Poiseuille flow to demonstrate how PPTA chain alignment varies in the cross section of a fiber, and how geometry of the fine orifice of a spinneret governs the shear deformation and chain alignment. These insights are expected to guide future process design to improve the quality and properties of PPTA fibers made via dry-jet-wet spinning.

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.

References (73)

  • Y. Rao et al.

    Structure-property relation in poly(p-phenylene terephthalamide) (PPTA) fibers

    Polymer

    (2001)
  • M.G. Northolt

    X-ray diffraction study of poly(p-phenylene terephthalamide) fibres

    Eur Polym J

    (1974)
  • B. Wang et al.

    Multiscale insights into the stretching behavior of Kevlar fiber

    Comput Mater Sci

    (2020)
  • R. Ramachandramoorthy et al.

    In situ electron microscopy tensile testing of constrained carbon nanofibers

    Int J Mech Sci

    (2018)
  • J.C. Cheng et al.

    Ballistic impact experiments and modeling on impact cratering, deformation and damage of 2024-T4 aluminum alloy

    Int J Mech Sci

    (2022)
  • Z. Wei et al.

    Analysis of ductile damage and fracture under reverse loading

    Int J Mech Sci

    (2022)
  • Y. Li et al.

    Deformation twinning in single-crystal Mg under high strain rate tensile loading: a time-resolved X-ray diffraction study

    Int J Mech Sci

    (2022)
  • S. Oliver et al.

    In-situ measurements of stress during thermal shock in clad pressure vessel steel using synchrotron X-ray diffraction

    Int J Mech Sci

    (2021)
  • A.D. Smith et al.

    Applying a combination of laboratory X-Ray diffraction and digital image correlation for recording uniaxial stress-strain curves in thin surface layers

    Int J Mech Sci

    (2020)
  • S.J. Picken et al.

    Molecular and macroscopic orientational order in aramid solutions: a model to explain the influence of some spinning parameters on the modulus of aramid yarns

    Polymer

    (1992)
  • X. Zhang et al.

    Poly(p-phenylene terephthalamide) modified PE separators for lithium ion batteries

    J Membr Sci

    (2019)
  • R. Kotek et al.

    7 - Production methods for polyolefin fibers

  • D.T. Grubb et al.

    Molecular chain orientation in supercontracted and re-extended spider silk

    Int J Biol Macromol

    (1999)
  • D. Lin et al.

    Macromolecular structural evolution of polyimide chains during large-ratio uniaxial fiber orientation process revealed by molecular dynamics simulation

    Chem Phys Lett

    (2020)
  • J. Cui et al.

    Three-dimensional simulation of lateral migration of fiber in a laminar channel flow

    Int J Mech Sci

    (2022)
  • W.-M. Zhang et al.

    Gaseous slip flow in micro-bearings with random rough surface

    Int J Mech Sci

    (2013)
  • S. Alexandrov et al.

    Couette flows of rigid/plastic solids: analytical examples of the interaction of constitutive and frictional laws

    Int J Mech Sci

    (2001)
  • Y. Liu et al.

    Mathematical modelling of flow field in 3-dimensional additive printing

    Int J Mech Sci

    (2022)
  • Z. Pan et al.

    Deformation mechanisms of TRIP–TWIP medium-entropy alloys via molecular dynamics simulations

    Int J Mech Sci

    (2022)
  • C. Zhang et al.

    Strong strain hardening in graphene/nanotwinned metal composites revealed by molecular dynamics simulations

    Int J Mech Sci

    (2021)
  • Q. Lv et al.

    Effects of single adatom and Stone-Wales defects on the elastic properties of carbon nanotube/polypropylene composites: a molecular simulation study

    Int J Mech Sci

    (2017)
  • H. Nada et al.

    Molecular chain plasticity model similar to crystal plasticity theory based on change in local free volume and FE simulation of glassy polymer

    Int J Mech Sci

    (2015)
  • K. Yashiro et al.

    Molecular dynamics simulation of polyethylene under cyclic loading: effect of loading condition and chain length

    Int J Mech Sci

    (2010)
  • K. Yashiro et al.

    Molecular dynamics simulation of deformation behavior in amorphous polymer: nucleation of chain entanglements and network structure under uniaxial tension

    Int J Mech Sci

    (2003)
  • V. Wahl et al.

    Specific surface, crystallinity, and dissolution of lyophilized fibrinogen. A study by combined small- and wide-angle X-ray scattering (SWAXS)

    Eur J Pharm Biopharm

    (2015)
  • K. Mohammadi et al.

    Analysis of mechanical and thermal properties of carbon and silicon nanomaterials using a coarse-grained molecular dynamics method

    Int J Mech Sci

    (2020)
  • Cited by (1)

    1

    These authors contributed equally to this work and should be considered co-first authors.

    View full text