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Different methods for preparing ANFs from PPTA fibers include polymerization induced self-assembly, mechanical exfoliation, electrospinning, deprotonation, and mechanical assisted deprotonation.
1. Aggregation induced self-assembly (PISA)
PISA is a bottom-up method for preparing nanofiber dispersions from monomers through copolymerization reactions in solvents. One of the advantages of this method is that it is easy to modify monomers. By adjusting the monomers or adding copolymerizable monomers during the preparation process, the quality of the resulting ANFs can be adjusted to meet specific requirements. During the interaction between p-phenylenediamine (PPD) and terephthaloyl chloride (TPC), PPTA is formed through condensation reaction, leading to the growth of PPTA chains. In order to control the condensation reaction and obtain ANFs instead of PPTA fibers, methoxypolyethylene glycol (mPEG) is usually introduced in the condensation setup.
2. Mechanical degradation method
PPTA fibers have a typical skin core structure, endowing them with excellent mechanical properties, including high strength and modulus, as well as good plasticity and toughness. Their molecular chains exhibit strong atomic bonding when arranged axially, and weaker intermolecular bonding when arranged radially, resulting in significant anisotropy strength. Under mechanical stress, the outer layer of PPTA fibers can be eroded along the longitudinal direction, resulting in the formation of ANFs.
3. Electrospinning
Electrospinning technology is often used to produce polymer nanofibers. However, there is a major issue with the preparation of ANFs through electrospinning, namely the regular structure of PPTA polymer chains and the strong intermolecular hydrogen bonding between these chains, making it difficult to dissolve in common solvents. Strong acids such as concentrated sulfuric acid can dissolve PPTA fibers, but the corrosiveness of strong acids hinders the widespread adoption of this technology.
4. Deprotonation
The preparation of ANF through deprotonation in DMSO/KOH system is a top-down strategy. During deprotonation, dimethyl sulfoxide (DMSO) removes hydrogen atoms from the amino groups of aromatic fibers (AF). Negative charges gradually accumulate on the aromatic molecular chains, causing electrostatic repulsion and splitting AF into microfibers. As the degree of deprotonation increases, the electrostatic repulsion between aromatic chains is enhanced, thereby breaking the hydrogen bonds between AF molecules and achieving nanoscale AF. In addition, adding proton donors (c (OH -)) can shorten the preparation process and improve preparation efficiency.
Compared with other strategies, this deprotonation process eliminates additional energy expenditure while producing smaller and more uniform sized ANFs, ultimately leading to ANFs thin films with improved performance.
5. Mechanical assisted deprotonation
Mechanical assisted deprotonation can improve the efficiency and performance of ANFs. In recent years, researchers have introduced a new method, namely wet ball milling assisted deprotonation, for the rapid production of ultrafine diameter ANFs. During the ball milling process, zirconium balls utilize their strong shear and impact forces to break down large fibers into smaller microfibers. Ball milling accelerated the deprotonation reaction and increased the contact area between reactants by allowing DMSO/KOH solution to penetrate deep into aromatic fibers, thereby reducing the diameter of ANFs.
Morphology of ANFs and their composite materials
In the field of composite materials, nanofibers (ANFs) can be combined with polymer materials, inorganic materials, and yarns through wet spinning to form one-dimensional composite fibers with excellent strength and toughness. ANFs can also be mixed with conductive materials such as graphene oxide (GO), carbon nanotubes (CNT), and MXene to prepare high toughness nanocomposite conductive fibers through hydrogen bonding between ANFs. Specifically, ANFs/MXene core-shell fibers prepared by concentric wet spinning have a conductive MXene core and a tough ANFs shell, making these fibers highly resistant to chemical corrosion, extreme temperatures, and bending fatigue. ANFs have abundant functional groups on their surface, making them an ideal carrier for introducing other functional groups or molecules through chemical modification. Usually, ANFs are mixed with other functional materials to prepare films, papers, coatings, and fabrics for specific applications.
In addition, ANFs can also become a key component of the development of three-dimensional ANF composite aerogels. ANFs can be used alone or combined with other components to prepare aerogel materials through freeze-drying, 3D printing, solvent exchange and other methods.
At the same time, the hydrogel based on ANFs is soft and elastic, which is very suitable for medical and sensing applications. These hydrogels also have potential in drug delivery and soft robot applications. In various industries such as aerospace, building materials, and automotive, aramid honeycomb materials are highly favored due to their high strength, stiffness, and lightweight properties.
The diverse forms and multifunctional structures of ANFs and their composite materials result in a wide range of functions and properties, making them suitable for various applications in different fields. The composition and structure often greatly affect the performance of ANFs composite materials, while the preparation and molding methods determine the structure of the material. In the future development of ANFs, optimizing and upgrading material preparation methods and designing multifunctional structures will be the focus of research. The correlation between its structure/performance, performance, and application is crucial for realizing the high-value potential of ANF based materials.
Image: Morphology and application of aramid fiber composite materials
Application fields of aramid fiber composite materials
1. Energy equipment
Flexible energy equipment has significant flexibility, can adapt to different working environments, and can meet the deformation requirements of the equipment. In the past 408 years, ANFs have been widely used in fields such as flexible energy devices, flexible batteries, supercapacitors, and frictional nanogenerators. A significant feature of ANFs is the abundance of amide functional groups on their surface, which has been shown to improve the conductivity of the material. By utilizing its excellent mechanical properties, chemical stability, and nanoscale characteristics with highly active amide functional groups, ANFs stand out as the preferred choice for high-performance flexible electronic materials.
2. Protective materials
ANFs nanofibers have multiple significant advantages when integrated into protective composite materials, mainly because they can significantly enhance the mechanical properties of composite materials. These nanofibers have a high specific surface area, allowing them to effectively reinforce composite materials at the microscale. In addition, its excellent strength to weight ratio enables it to enhance the strength of materials while minimizing additional weight. In addition, ANFs exhibit excellent heat and corrosion resistance, electromagnetic shielding, and UV radiation resistance. Therefore, these materials are widely used in various fields such as fire protection, electronic packaging, construction, and military equipment. The unique combination of mechanical and functional properties of ANFs makes them a valuable and multifunctional material for enhancing the performance and durability of protective composite materials in numerous applications.
3. EMI shielding
Compared with traditional metal materials, polymer based EMI shielding materials have the advantages of being lightweight, low-density, flexible, chemically stable, and environmentally friendly. Currently, most polymers do not exhibit electromagnetic shielding capabilities, except for inherently conductive polymers such as PPy, PANI, and polythiophene. In order to endow polymers with EMI shielding properties, conductive and/or magnetic fillers need to be added to the polymer matrix. Therefore, flexible thermal conductive films with good EMI shielding performance composed of polymers or nanofibers (such as CNF, ANFs), carbon based materials (GO, carbon nanotubes), and inorganic compounds have been widely studied.
4. Insulation material
Aramid fiber is currently one of the most suitable raw materials for insulation materials due to its insulation properties, as well as its excellent properties such as ultra-high strength, high modulus, high temperature resistance, acid and alkali resistance, and light weight. The fabric (or paper) woven from aramid fiber is widely used in electrical and electronic insulation, such as the core wire, interlayer and interphase insulation of transformers, slot lining insulation of motors, circuit board substrates, radar antennas. Generally, aramid fiber materials are used as raw materials for insulation cylinders or insulation rods for finished insulation.
5. Adsorbent material
The film and aerogel of ANF have dense structure and abundant pores, which are conducive to the separation, filtration and adsorption of various pollutants. These materials are used for gas and liquid filtration, air and water purification, oil spill cleaning, and chemical adsorption due to the high specific surface area and sufficient adsorption of fibers. Their powerful intermolecular forces, including hydrogen bonds and π - π stacking, can effectively adsorb a variety of molecules, including organic compounds and pollutants. ANFs also exhibit good chemical stability and can maintain adsorption capacity even in harsh environments.
6. Medical applications
Aramid nanofibers have good biocompatibility and biodegradability, and can be used to prepare biomedical materials and drug carriers. For example, drug carriers prepared using aramid nanofibers can achieve precise delivery and controlled release of drugs, which is of great significance for improving the therapeutic effect of drugs and reducing side effects.
In summary, ANFs have become a highly outstanding nano polymer material with broad potential applications. In order to further release its potential and make progress in this field, it is crucial to focus on developing more simple and efficient preparation methods, exploring multifunctional ANF composite coatings and expanding the application range of ANFs composite aerogels. Through continuous research and development work, it is expected that ANFs will gradually play an increasingly important role in the fields of nanotechnology and composite materials, and this trend may open up new exciting possibilities for innovation and sustainable solutions in various industries
