Abstract
As the latest generation of high-performance building materials at the nanoscale, para-aramid nanofibers (ANFs) have attracted considerable attention in the scientific community in recent years due to their unique ability to form high-porosity solids and a rare combination of physical properties. However, the presence of strong hydrogen bonds and other intermolecular interactions in aramid macromolecules poses significant challenges to their large-scale and sustainable production. These interactions result in lengthy processes and the need for highly corrosive solvents. In this study, by innovatively leveraging the efficient exfoliation properties of polymer nanosheets, we successfully shortened the preparation time of para-aramid nanofibers by 25 to 20 times (from 1 week to 4 minutes) while increasing the concentration by 10 times. Through molecular intercalation-induced method, we prepared novel ribbon-like aramid nanofibers. Multi-scale simulation studies revealed that the exfoliation process of nanofibers originates from the intercalation of alcohols at the nanoscale interface. In pilot-scale tests, 1000 kilograms of nanofiber dispersion were successfully prepared in just half an hour. These breakthrough results verify the effective recyclability of aramid materials, enabling their successful application in the preparation of various multifunctional nanofiber composites.
Calculation-Related Figures and Text
Figure 1. Depolymerization process of poly(p-phenylene terephthalamide) (PPTA) microfibers.
(a) Alcohol molecule intercalation induces the splitting of PPTA microfibers into dispersed ANFs. (b) Optical microscope images of PPTA microfiber depolymerization in DMSO + potassium hydroxide + isopropanol solution; scale bar = 20 μm. (c) Atomic force microscope (AFM) image of dispersed ANFs. The inset shows the Tyndall effect of 1 wt% ANF dispersion. (d) Molecular structure of PPTA microfibers. (e) Molecular structure of PPTA microfibers after deprotonation by potassium hydroxide in solution. (f) Deprotonated PPTA structure after charge redistribution. (g) Process of isopropanol molecule intercalation between ANFs. (h) The left Y-axis and bar chart represent the time required for complete depolymerization of 1 wt% ANF dispersion in the presence of different alkyl alcohol molecules, and the X-axis shows different alkyl alcohols. The formulas below the X-axis and the ball-and-stick models on the bar chart illustrate the steric hindrance effects and conformational differences of these alcohols. The right Y-axis and orange line represent the hydrogen bond (HB) binding energy between different alcohols and deprotonated PPTA. The steric hindrance, conformation, and polarity of intercalated molecules have significant impacts on the depolymerization process. (i) Comparison of depolymerization time between mechanical fibrillation and molecular intercalation methods.
Figure 2. Morphology and assembled structure of ribbon-like nanofiber arrays (ANFs).
(a) Thickness distribution of ribbon-like ANFs measured by AFM. (b) AFM image of ANFs. (c) AFM image of ribbon-like ANFs showing narrow thickness distribution and ribbon-like morphology. (d) Magnified AFM image of a single ribbon-like ANF. (e) Height profile of a single ribbon-like ANF showing ribbon width. (f, g) SEM images of assembled ribbon-like ANFs showing layered films and spliced ribbon-like structures. (h) XRD curves of films formed by ANFs prepared via three different methods, showing anisotropic distribution of crystal planes in films formed by molecularly intercalated ANFs. (i) Two-dimensional wide-angle X-ray diffraction (2D-WAXD) image of traditional ANF film. (j) 2D-WAXD image of ribbon-like ANF film showing obvious preferential orientation of the 200 crystal plane.
Figure 3. In-situ characterization of the disassembly process via molecular intercalation.
(a−c) SEM images showing different stages of PPTA microfiber disassembly within 1, 2, and 3 minutes after contact with the solution. (d−f) TEM images demonstrating the disassembly of micron-scale PPTA fibers into nanofibers at different magnifications. (g) TEM image showing ANFs with uniform diameter distribution and high aspect ratio. (h) AFM image showing the network structure of fine ANFs. (i) XRD patterns of PPTA microfibers and ANFs prepared by three different methods as listed in the legend, showing high crystallinity of ANFs after disassembly. (j) Cumulative spider diagram of graph theory parameters for three aerogels. (k) SEM image of ANF aerogel prepared by traditional mechanical nanofibrillation (DMSO + potassium hydroxide); (l) Nanofibrillation process in DMSO + potassium hydroxide + water; (m) Molecular intercalation process in DMSO + potassium hydroxide + IPA. (n−p) Corresponding graph theory intercalation of ANF networks.
Figure 4. Characterization and mechanical analysis of molecular intercalation-assisted disassembly process.
(a) UV-visible spectra of ANF dispersions at different mixing times. (b) Atomic Dipole Corrected Hirshfeld (ADCH) charge distribution before and after deprotonation in DMSO solvent. (c, d) Dynamic light scattering (XPS) spectra of PPTA microfibers and their surface-treated counterparts. (e) Raman spectra of PPTA microfibers, ANF dispersions after different mixing times, and PPTA nanofibers (ANF). (f, g) Comparison of intermediate states of isopropanol and DMSO molecules during permeation through nanochannels between two typical deprotonated interfaces dominated by different interactions: the first is dominated by π-π stacking and van der Waals forces, and the second by hydrogen bonding. (h) Number of molecules entering the nanochannels during permeation. (i, j) Simulation snapshots showing the separation process of deprotonated PPTA-PPTA crystal interfaces after intercalation of isopropanol and DMSO molecules. (k) Comparative analysis of interface peak force and fracture energy during the separation of PPTA-PPTA crystal interfaces under different conditions. (l) In-situ X-ray diffraction (XRD) analysis of PPTA microfiber dissociation process, revealing differences in dissociation rates of different crystal planes.
Figure 5. Large-scale production and applications of aramid nanofibers (ANFs).
(a) Different types of waste aramid materials. (b) Recycled ANFs and separated non-aramid components in composites. (c) ANF-based aerogels, hydrogels, and transparent films. (d) Intrinsic viscosity and average molecular weight of pristine aramid materials and recycled ANFs. (e-g) Performance of 1000 kg of 1 wt% ANF dispersion in pilot-scale tests; the container of ANFs in (g) has a capacity of 50 liters. (h) Semi-solid ANF dispersion with a solid content of 10 wt%; scale bar = 1 cm.
Core Content of Calculations
The computational work focused on the molecular intercalation mechanism, interfacial interactions, and separation processes, covering three levels:
1. Electronic structure calculations (DFT):
· Analyze charge redistribution of para-aramid (PPTA) chains before and after deprotonation.
· Calculate binding energies between different alcohol molecules and deprotonated PPTA chains.
· Simulate the static process and energy changes of isopropanol (IPA) molecule insertion between PPTA chains.
1. Molecular dynamics simulations (MD):
· Simulate the permeation kinetics of solvent molecules (IPA/DMSO) in PPTA crystal nanochannels.
· Compare separation behaviors of different interfaces (π-π stacking-dominated vs. hydrogen bond-dominated).
· Quantify interface peak force and fracture energy to evaluate the weakening effect of intercalated molecules on interface bonding.
1. Graph Theory (GT) analysis:
· Quantify the network structure of ANF aerogels from SEM images.
· Compare ANF network parameters (e.g., average degree, average clustering coefficient) prepared by different methods.
Key Computational Parameters and Methods
1. DFT calculation parameters:
· Software and functionals: Gaussian 09 (B3LYP functional), VASP (PBE-GGA).
· Basis set and solvent model: 6-311++G** basis set, SMD implicit solvent model (DMSO environment).
· Correction method: Grimme DFT-D3 correction for van der Waals forces.
· Charge analysis: Atomic Dipole Corrected Hirshfeld (ADCH) charges (Figure 4b).
· Binding energy formula: EBinding = EPPTA...Alcohol − (EPPTA + EAlcohol)
1. MD simulation parameters:
· Force field: ReaxFF reactive force field (describes non-bonding interactions such as hydrogen bonds and van der Waals forces).
· Simulation conditions: NVT ensemble, 300 K, time step 0.25 fs, simulation duration 1 ns (permeation process) and 200 ps (interface separation).
· Interface separation settings: Constant stretching speed of 1 m/s, spring constant of 1000 kcal/mol·Å².
1. GT analysis parameters:
· Network parameters: Average Degree, Average Clustering Coefficient, path length, etc. (Table S1).
Computational Conclusions
1. Essence of intercalation mechanism: IPA molecules form strong hydrogen bonds with oxygen atoms of deprotonated PPTA chains (optimal binding energy), preferentially intercalate into π-π stacking-dominated interfaces, and significantly weaken interlayer forces.
2. Kinetic advantage of interface separation: The mixed solvent of IPA and DMSO reduces interface peak force and fracture energy more effectively than pure DMSO (Figure 4k), increasing separation efficiency by 25 to 20 times.
3. Morphological selectivity: XRD and MD simulations (Figure 4l) show that IPA preferentially intercalates along the (200) crystal plane (π-π stacking plane), resulting in ANFs with a ribbon-like morphology (thickness ~2 nm, width 20-30 nm).
4. Difference in network structure: GT analysis indicates that ANF networks prepared by molecular intercalation have higher local connection density (Figure 3j), explaining the excellent mechanical and thermal insulation properties of their aerogels.
Summary
Through multi-scale correlation of electronic structure, molecular dynamics, and macroscale networks, the computational part demonstrates that IPA intercalation can achieve efficient and controllable preparation of ANFs by precisely regulating the cleavage path of non-covalent bonds. This mechanism provides a theoretical basis and design strategy for the green recycling of high-performance nanomaterials.
Time：2025-11-06
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