From PPTA Broken Paper: High-Performance ANFs and Their

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From PPTA Broken Paper: High-Performance ANFs and Their Application in Electrical Insulating Nanomaterials with Enhanced Properties Bin Yang, Meiyun Zhang, Zhaoqing Lu, Jingjing Luo, Shunxi Song, and Qiuyu Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01311 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Sustainable Chemistry & Engineering

From PPTA Broken Paper: High-Performance ANFs and Their Application in Electrical Insulating Nanomaterials with Enhanced Properties Bin Yang,*, † ‡, § Meiyun Zhang, ‡, § Zhaoqing Lu, ‡ JingJing Luo, ‡ Shunxi Song, ‡ Qiuyu Zhang, *, † † School of Science, Key Laboratory of Space Applied Physics and Chemistry of Ministry of Education, Northwestern Polytechnical University, Xi’an, 710072, China; ‡ College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science & Technology, Xi’an, 710021, China; § State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, 200051 ABSTRACT: PPTA paper serves as a promising matrix candidate that exhibits reduced overall weight and possesses exceptional inherent dielectric strength, thermal durability as well as excellent mechanical property and processing flexibility. However, PPTA broken paper in mill is regarded as the solid waste and is landfilled eventually, resulting in the severe environmental pollution and waste of resources. In this work, for the first time, an efficient and high value-added approach is proposed for the recovery of aramid nanofibers (ANFs) from PPTA broken papers by controlled deprotonation in KOH/DMSO system. A stable and uniform ANFs/water dispersion and the insulating ANFs-films were successfully obtained. In an effort to explore those possibilities, effect of ultrosonicon on the compact structure of PPTA broken paper was firstly investigated. Then the formation process, micromorphology, thermostability, crystalline structure and formation mechanism of ANFs, and the key performances of the ANFs-films were subsequently investigated and evaluated. The resultant insulating ANFs-films display higher mechanical performances and dielectric strength than that of the well-known Nomex® insulating paper T410. This paper not only opens the possibility of how to recycle PPTA broken paper in an effective and high value-added strategy, but also offers an alternative access to advanced nanocomposites with excellent performances. KEYWORDS: PPTA, PPTA paper, ANFs, Nanofibers, Insulation, FRPs recycle

vented by Stephanie Kwolek in 1965. PPTA fiber has been extensively applied as protective clothing, asbestos substitutes, industrial filters, sport fabrics, electrical insulation, and fiberreinforced Polymers (FRPs).5 Among those applications, FRPs have been highlighted as important to materials field inspired by the rapid developments and burgeoning demands on updated technologies and materials. When it comes to FRPs, the recycling of carbon fiber-reinforced polymers (CFRPs) composites has become the focused attention in academia. CFRPs can be further processed into recycled carbon fibers, fibrous fragments and fillers through the different treatments such as mechanical treatments (shredding, crushing and milling), thermal treatment (pyrolysis and oxidation) and chemical treatments (sonochemical and selective cleavage) et al.6-8 Compared to the other matrixes (such as resin or ceramic matrix) of FRPs, PPTA paper serves as a more promising matrix candidate that has reduced overall weight and exhibits exceptional mechanical property, dielectric performance, thermal durability, as well as remarkable flexibility of processing.9 Owing to these favorable performances of PPTA paper, it has diverse application prospects, such as high-temperature resistance insulating materials and lightened-weight honeycomb structure

INTRODUCTION Sustainable development is of great significance to social and economic development, which has become the driving force for the design and production of industrial processes. Owing to the characteristics of non-degradable or poor degradation selectivity of polymers, it has caused a large number of polymer wastes in polymer materials field. In general, the degradation of polymers must be performed under extreme conditions, such as high temperature and pressure. It is therefore crucial to recycle the waste polymers composites effectively, which remains a huge challenge up to now. Poly (p-phenylene terephthalamide) (PPTA) fiber, commonly known as Kevlar® or Twaron®, is prepared by a dry-jet wet spinning process.1 Due to its fully extended macromolecular chains that interact with each other via strong and highly directional hydrogen bonds, PPTA fiber possesses high strength and modulus, exceptional thermal resistance and chemical inertness.2-3 Kevlar microfiber is anisotropic and its stiff and highly-aligned backbone results in a modulus of 85 GPa and a tensile strength of 3.9 GPa along the fiber axis.4 Therefore, PPTA fiber has drawn increasingly greater attention of researchers owning to its excellent performances since it was in-

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paper-based materials, which have been extensively applied in military and civil fields.10-12 PPTA paper, a combination of PPTA flocs and PPTA pulp at a certain percentage, is made through a wet forming papermaking process firstly and followed by a thermal calendaring process as shown in Figure 1. PPTA flocs show poor interfacial adhesion with other matrix due to the rigid molecular chain oriented along the fiber axis and few active functional groups exposed on its surface13. Therefore, weak bonding force could be formed in the PPTA paper made up of PPTA flocs and pulp, which definitely leads to the inferior quality of PPTA-based paper14. Thus, a thermal calendaring process with high temperature and linear pressure is indispensable for significantly improving mechanical properties of PPTA base paper.15 Inevitably, a great deal of broken paper accounting for 10~15% could be generated during the processes of forming, thermal calendaring and post-processing of PPTA paper. Usually, the broken paper generated by papermaking process needs to be further treated through repulping to achieve its recycling and waste recovery. However, the PPTA broken paper, especially for those originated from the thermal calendaring process exhibits extraordinary high strength, which makes it much more difficult

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for repulping into fibers or fibrils in a hydrapulper. Moreover, it is known that PPTA can only be dissolved under the rigorous conditions, such as the concentrated sulfuric acid, limiting other reuse method of dissolution by chemical solvent.16 PPTA broken paper in mill is accordingly regarded as the solid waste and is landfilled eventually, which not only leads to the severe environmental pollution due to its non-biodegradability, but also causes a waste of resources. Consequently, a cost-effective and high value-added approach to recycle PPTA broken paper has been remained an enormous challenge. Among all the research works reported, however, there are very few reports about the recycling of PPTA paper. Only Muhammad has ever reported that recycled PPTA fiber derived from the textile products was employed to prepare the cost-effective and cut-resistant gloves.17 Naruse Shinji18 also reported the method of grouting and smashing to recycle aramid paper. However, there are three main disadvantages in aforementioned method: (1) only uneven fibers with quantities of undissociated PPTA paper scraps could be obtained; (2) the recycle process caused high-energy consumption; (3) mechanical recycling treatments would detrimentally affect the properties of PPTA fibers.19

Figure 1. Schematic of the papermaking forming process and applications of PPTA paper nanoparticles (AgNPs)/ANFs,25 polyurethanes (PUs) /ANFs,26 polyethersulfone (PES)/ ANFs27, and carbon nanotubes (CNTs)/ ANFs.28-29 Combining with the significant interfacial bonding improvement effects of ANFs, all of those advanced nanocomposites exhibit excellent performances and achieve a combination of desirable properties not available in any single material. Herein, for the first time, we propose a novel and high valueadded approach to recycle the PPTA broken paper, and a stable and uniform ANFs/water dispersion is successfully obtained through the deprotonation and partial protonation recovery procedures. Ultrasonic treatment was carried out to accelerate the deprotonating process. The micromorphology, thermostability, crystalline structure, as well as formation process and mechanism of ANFs are well characterized and evaluated by various

Nanoscale polymeric fibers are expected to bring about exceptional mechanical and optical properties in composite materials related to the better integration of different properties between fibers and polymers.20 The design of high-performance nanocomposites will also benefit from the availability of novel polymeric matrices. Recently, Kotov group first proposed the deprotonation of macroscopic Kevlar® fibers to prepare PPTA nanofibers (aramid nanofibers for short, ANFs) in dimethylsulphoxide (DMSO) in the presence of KOH.21 Since then, ANFs have been widely used as polymeric nanoscale building blocks for advanced nanocomposite matrices.22 ANFs can offer an effectively alternative access to high-performance nanocomposites with extremely high strength and heat resistance, such as poly(ethylene oxide) (PEO)/ANFs,23 graphene(GO)/ANFs,24 Ag


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ACS Sustainable Chemistry & Engineering ment (AI-7000-NGD, Goodtechwill, China) at a cross head speed of 5 mm/min， and the size of sample is 40 mm × 15 mm (length × width) with a gage length of 20 mm. The thickness and tear strength were measured in accordance with TAPPI Standard Method T411 (T411 om-89) and T414 (T414 om-88), respectively. The dielectric strength was tested on a breakdown strength tester (ZJC-50kV, Air Times, China) according to ASTM D-149. In order to improve the accuracy of experimental data, five specimens for each sample were used for testing.

testing methods. The key performances of the insulating ANFsfilms are subsequently explored. As a new attempt to understand how to recycle the PPTA broken paper, this work offers feasibility for recycling PPTA broken paper in a cost-effective and high value-added way, and provides an effectively alternative access to high-performance nanocomposites at the same time.

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

Materials. PPTA flocs and PPTA pulp were provided by the Dupont Group. The average length and diameter of the PPTA fibers were 4.0 mm and 12.0 µm, respectively. The average length and the fibrillation degree (characterized by the parameter of Canadian freeness standard, CSF ) of the PPTA pulp were 0.9 mm and 210 ml, respectively. Dimethyl sulfoxide (DMSO, 99.5%) and potassium hydroxide (KOH) were purchased from Tianjin Da Mao Chemical Reagent Co., Ltd. Sample preparation (1) Preparation and Pretreatment of PPTA Broken Paper. PPTA broken paper with a basis weight of 45 g/m2, comprising 30% PPTA flocs and 70% PPTA pulp, was made using a BBS-3 handsheets former (ERNSTHAAGE, Germany). It was further hot-calendared on a thermal calendar (Shanghai New Apollo Control Equipment Co., Ltd, China) under the linear pressing pressure of 150 kN/m at 280℃. Then, PPTA broken paper was cut into PPTA paper pieces with the specification of 0.5 cm×0.5 cm. PPTA broken paper without ultrasonic treatments was the control sample. Another PPTA broken paper was ultrasonically treated using an ultrasonic generator under the power of 800 W for 40 min. (2) Fabrication and Isolation of ANFs. By employing Yang’s method,21-22, 28 1 g of PPTA broken paper treated with / without ultrasound and 1.5 g KOH were added into 500 mL of DMSO which was magnetically stirred for 5 and 9 days at room temperature to form a homogeneous and transparent dark red solution of ANFs/DMSO, respectively. The Colloidal ANFs were isolated from the ANFs/DMSO solution by employing water as a proton donor to perform the protonation recovery.30 Here, 200 ml of deionized (DI) water was injected into 100 ml ANFs/DMSO dispersion firstly, and then continuously magnetic stirred for 1 h to generate a uniform ANFs/DMSO/H2O solution. Then ANFs were filtered using a G4 sand core funnel under the vacuum pressure. The residual KOH and DMSO remained in ANFs were repeatedly washed and removed by DI water and alcohol. The colloidal ANFs were finally obtained, and could easily disperse in water. (3) Fabrication of ANFs-films. ANFs-films were achieved from the colloidal ANFs by the method of vacuum-assisted filtration (VAF). The obtained ANFs-films were further dried in oven at 105 ℃ for 10 min. The ANFs-film obtained from PPTA paper without/with ultrasonic treatment was called as ANFs-film 1 and ANFs-film 2, respectively. Characterization The morphologies of ANFs were observed by a Transmission electron microscopy (TEM) (Tecnai G2 F20 S-TWIN, FEI, US). The microstructures ANFs-films were observed by using a S-4800 scanning electron microscope (SEM, Hitachi, Japan) operating under the secondary electron mode at an accelerating voltage of 5 kV. Fourier transform infrared (FT-IR) spectra were recorded on a Vertex-70 (Bruker, Germany) spectrometer with an attenuated total reflectance (ATR) in a scanning range of 400 to 4000 cm-1. X-ray diffraction (XRD) characterization was performed using a D8 Advance (Bruker, Germany) with CuKα (λ=0.1542 nm) generating at 40 kV and 40 mA, and a scanning speed of 4 °·min-1. X-ray photoelectron spectroscopy (XPS) was performed using an Axis Supra (Kratos Analytical, UK) with a monochromatic Al Kα source (1486.6 eV). Atomic-force microscopy (AFM) experiments were performed using a Agilent 5100 (Agilent, USA) in a tapping mode. Thermogravimetric analysis (TGA) was carried on the Synchronous TG-DSC thermal analyzer (STA449F3, NETZSCH, Germany) with a heating rate of 10 ℃/min in nitrogen gas atmosphere with temperature ranging from 30 to 800 ℃. The mechanical stress, strain and stiffness of ANFs-films were measured on a tensile testing instru-

In previous reports, ANFs are prepared from the commercial Kevlar® or Twaron® fibers by controlled deprotonation and dissolution process16, 21. The individual fiber was disposed directly into a KOH/DMSO system, which significantly provides a much larger interfacial area than the macroscale and micrometer-sized PPTA broken paper. Additionally, the thermal calendaring process is of considerable significance in the PPTA paper manufacturing process, as it brings a remarkable improvement on microstructure as well as overall mechanical properties to the PPTA paper-based materials.31 Figure 2(a, b) show the internal and cross structures of the PPTA paper after the hotcalendaring process. In our previous work, the distinctive features of PPTA pulp (rough, curled, ductile and highly fibrillated ) and its significant reinforcement, bonding and filling potential in PPTA paper sheets were reported.32 The PPTA floc fibers deformed and distributed evenly as the framework material, determining the well-integrated structure and mechanical properties of the paper. Whilst PPTA pulp with highly dispersed slender thread-like micro-fibrils plays the role as the matrix material, which remarkably improves the interfacial bonding and fills the small pores among PPTA fibers simultaneously. Consequently, the hot-calendared PPTA broken paper with extraordinary compact structure reduces the contacting area and decreases reaction activity of PPTA fibers with KOH/DMSO system.

Figure 2. SEM images of surface and cross section of PPTA broken paper: (a, b) Before ultrasonic treatment; (c, d) After ultrasonic treatment

Thus, in order to accelerate the process of deprotonation, the ultrasonic treatment was carried out to dissociate the PPTA broken paper into fibers. Huang studied the influence of ultrasonic treatment on the interfacial property of the fiber reinforced epoxy resin composites. The results suggested that the



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ultrasonic treatments significantly increased the wettability and the oxygen content of the fiber surface due to the ultrasonic cavitation effect.33 Figure 2(c, d) show that the surface and cross section of the ultrasonically treated PPTA paper exhibits obvious roughness and relatively loose structure, and some PPTA fibers and pulp were stripped under the ultrasonic cavitation. Also, the etching effects on the surface of PPTA fibers could be observed in the inset image of Figure 2(c) due to its skin-core structure34. The ultrasonic treatment on the PPTA broken paper not only enhances the content of the active groups on fiber surface, but also endows it with a loose structure and an etched surface, which further provides higher contacting area and reaction accessibility of PPTA broken paper with KOH/DMSO system than that without the ultrasonic treatment. As a result, the issue of time-consuming of fabrication of ANFs could be figured out by the ultrasonic pretreatment. The PPTA broken paper without the ultrasonic treatment took 9 days to form a dark red solution of ANFs. While, the ultrasonically treated PPTA broken paper took 5 days. As seen in Figure 3a, the ultrasonically treated PPTA broken paper sheets were entirely dissociated into separated fibers after 24h. A bright yellow solution with long PPTA fibers precipitated at the bottom could be observed. As the reaction proceeded, the color changed from bright yellow to dark red, and the amount of the precipitated individual fibers in the system dramatically reduced to form a transparent and homogeneous ANFs/DMSO dispersion at the fifth day (Figure 3b). As shown in Figure 3c, the colloidal ANFs were finally achieved by injecting deionized water into ANFs/DMSO solution, and it could be easily dispersed in water. Inspired by Jiang et al.35 and Duan et al.36, the colloid ANFs could also be utilized to fabricate the ultralight, thermally insulating and compressible sponges by freezedrying, which are the promising candidates for potential applications in thermal insulation, light weight construction, high temperature filtration, sensors, and catalyst carrier for high temperature reactions. As shown in the Figure 3(b), ANFs/water solution with a concentration of 0.2 wt.% also show a transparent state, which can be used to prepare the thin films with high transparency, superior performance and thermostability by the method of VAF.

Figure 3. Digital photos of ANFs formation process: (a) Formation of ANFs from the beginning to the fifth day; (b) Partial protonation recovery of ANFs/DMSO solution; (c) Digital photo of colloidal ANFs

As mentioned earlier, it is the ultrasonic cavitation effect that remarkably looses the compact structure of PPTA broken paper and noticeably etches the individual fiber, which brings about higher contacting area and reaction accessibility of PPTA fibers with KOH/DMSO system. As shown in Figure 4a 1, PPTA broken paper presents dense structure and low transparency. The PPTA chain consisting of an alternation of rigid aromatic rings and amide groups is fully extended as shown in Figure 4b 1. The outstanding performances of PPTA fibers can be attributed to the higher degree of conjugation and more rigid geometry of the para linkages, combined with the greater chain orientation and stronger hydrogen bonding. The skin-core structure model of PPTA illustrated by Li et al. is shown in Figure 4c.37 Each chain has the characteristic of high strength anisotropy, with strong atomic bonds along the axial orientation and weak interchain bonds along radial orientation, such as the hydrogen bond and van der Waals’ interaction. As shown in Figure 4d 1, the PPTA broken paper has been transformed into individual fibers after 24 h. The “skin” layer of fibers are etched dramatically under the effects of ultrasonic cavitation and the attack of KOH/DMSO. As the reaction proceeded, the “skin” layer of fibers began to dissolve and fall off from the surface and the “core” layer was exposed to the KOH/DMSO system. When the reaction time reached 48 h, the fiber split into long slender sub-micron fibrils along its longitudinal direction (Figure 4d 2). It suggests that the diameter of ANFs can be reduced to nanoscale dimension, while the length of ANFs can be retained of the fiber length along its longitudinal direction to some extent, which is helpful to obtain high aspect-ratio of ANFs. The presence of KOH/DMSO decreases the effects of intermolecular hydrogen bonds and increases electrostatic repulsion yielding a disassembly of the PPTA broken paper into ANFs. The deprotonation of PPTA broken paper in the DMSO/KOH system involves two steps, namely the abstraction of mobile hydrogen from N-H groups (as shown the scissors in Figure4b


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1), and substantial destruction of intramolecular hydrogen bonding between the polymer chains, resulting in a negative-charged polymer chains(Figure 4b 2). Therefore, the final transparent dark red ANFs/DMSO solution can be dispersed stably under the balance of the π-π stacking in the polymer backbone, elec-

trostatic repulsion and Van der Waals force between the polymer chains.38 And most importantly, the deprotonation and the destruction of chains are limited and do not occur within individual polymer chains, which ensures that the ANFs possess high strength, stiffness and toughness.21

Figure 4. Formation process and mechanism of ANFs: (a) Digital images of PPTA broken paper and ANFs-film; (b) Molecular chains structure of PPTA fibers, ANFs in DMSO, and ANFs in water; (c) Skin-core structure model of PPTA37; (d) SEM images of ANFs formation process, d1-d2 are ultrasonic-treated PPTA broken paper in KOH/DMSO system for 24 h, 48 h respectively; (e) Schematic diagram of ANFs networks in ANFs-film. The previous studies shown that the ANFs/DMSO solution is very sensitive to humidity and cannot exists for long time.39 ANFs are quite prone to precipitate, resulting in thin fragmental films even in an ambient environment. It can be problematic that if the final nanofibers can only exist stably in the KOH/DMSO dispersion, which extremely restricts the application of ANFs in the water-soluble polymer and hydrophilic materials. Figure 4b 3 illustrates the molecular chains structure of ANFs after the protonation recovery process. DI water was employed as a proton donor to execute protonation recovery of

ANFs in the KOH/DMSO system. Finally, the negatively charged ANFs in water solution were achieved, which are uniformly dispersed as displayed in Figure 3b. It has been reported that the protonation-recovered ANFs have smaller diameter of the sample than that in DMSO, which provides additional opportunities to ANFs for the better combination with other polymers.30 A freestanding ANFs-film with the thickness of 22 ± 3 µm fabricated from ANFs/water solution was obtained by VAF is presented in Figure4a 2, and it shows a high transparency with a transmittance of ~80 % at the wavelength of 550 nm.



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the range of diameters of ANFs prepared by the ultrasonically treated PPTA broken paper reach 15~20 nm, showing a highly uniformity in diameters. The thinner diameter of ANFs in ANFs-film 2 attributes to the higher activity and reaction accessibility of PPTA broken paper with KOH/DMSO. From the nanomaterials perspective, thinner diameter of ANFs can remarkably bring about the better performances of materials. The 3D images obtained in AFM characterization are shown in Figure 5(e, f), the roughness of ANFs-film1 and ANFs-film2 are 37.3 nm, 26.5 nm, respectively. This probably due to the different diameters of ANFs. Larger and wider distribution of diameters of ANFs lead to the rougher surface of ANFs-films.

Also, the transparency of ANFs-film could be tailored by controlling the thickness of the film. Figure 4e also illustrates the schematic diagram of ANFs networks formed by the strong hydrogen bonding due to abundant polar functional groups and high aspect-ratio of ANFs, which contributes to the high performance of ANFs-films.

Molecular structures of ANFs in the ANFs-films and PPTA broken paper was confirmed by FTIR spectra, as displayed in Figure 6a. The stretching vibration peaks of N-H bond and C=O bond are at 3305 cm-1and 1643 cm-1, respectively. The absorption peaks at 1545 cm-1 is assigns to the bending vibration of N-H bond, and the peak at 1510 cm-1 corresponds to the stretching vibration of C=C in benzene rings. Obviously, ANFs and the macroscale PPTA broken paper have very similar IR spectra, indicating that their molecular structures are almost the same. However, compared with PPTA broken paper, the intensity and sharpness of N-H stretching vibrations peaks of ANFs in the ANFs-films have dramatically decreased due to the broader distribution of bond lengths and surface states of the fibers.21 Also, the decreased crystallinity of ANFs is another main reason for the differences of FTIR spectra. More importantly, Yang et al.21 reported that the deprotonation degree of PPTA fibers in DMSO/KOH system is limited, which was beneficial to greatly guarantee the integrity of molecular chain of PPTA.

Figure 5. Micromorphology characterization of ANFs and ANFs-film: (a, b) TEM images of ANFs fabricated from PPTA broken paper treated without / with ultrasonic; (c, d) SEM images of ANFs-film 1 and ANFs-film 2; (e, f) AFM 3D images of the surface of ANFs-film1 and ANFs-film 2 The slender ANFs fabricated from PPTA broken paper that was treated without / with ultrasonic can be observed in TEM images (Figure 5a, 5b), which indicates that the nanoscale aramid fibers have been successfully fabricated from PPTA broken paper. The length of ANFs appear to be several microns. It is also indicates that the length of ANFs can be retained of the fiber length along its longitudinal direction to some extent. The surface morphologies of the ANFs-films prepared by filtration of ANFs are shown in Figure 5c, 5d. Both of the ANFs-films show the compact structures with entangled ANFs, which accordingly contribute to the comprehensive properties of nanomaterials. Additionally, the ANFs are uniformly dispersed with entangled morphologies. Nevertheless, the ANFs in ANFs-film1 and ANFs-film 2 show a bit difference in the distribution of diameter and uniformity. The range of diameters of ANFs prepared by the PPTA broken paper are 18~25 nm, presenting a relatively poor uniformity. There are some ANFs whose diameter is thicker than the others could be seen in Figure 5c, approximately reaching 30~35 nm. In contrast, as shown in Figure 5d,

Figure 6. FTIR spectra, XRD patterns and XPS C 1s spectra of PPTA broken paper and ANFs-films: (a) FTIR spectra; (b) XRD patterns; (c, d) XPS C 1s spectra As displayed in Figure 6b, XRD measurements of PPTA broken paper show peaks at diffraction angle 2θ (Bragg angle) of 20.5° for the (110), 22.6° for the (200) and 28.5° for the (004) reflection-planes, respectively. The ANFs-films have the same diffraction peaks but the peak at 22.6° for the (200) reflection is very weak compared with PPTA broken paper, which is


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different from the XRD pattern of ANFs film reported before by Yang et al.21 The different results can be explained by the distinguishing formation process and mechanism of the ANFsfilms. Yang’s ANFs-film was made directly from ANFs/DMSO solution by the method of layer-by-layer (LBL) assembly, in which a piece of glass slide coated with poly(diallyldimethylammoniumchloride) (PDDA) was dipped into ANFs/DMSO solution and intermediately rinsed with water to fabricate the ANFs-film by the electrostatic attraction between the negatively charged ANFs and positively charged PDDA. The ANFs-film prepared by LBL was fully protonated after the film was achieved, which remarkably guaranteed the successful reconstruction of the molecular chains of negatively charged ANFs. However, for our ANFs-films, the protonation was recovered before the film was formed. Note that it was partial protonation recovery in order to achieve negatively charged ANFs in water solution. According to the Scherrer equation D = 0.89λ/βcosθ, where D is the size of crystallites, λ is the wavelength of X-ray beam, β is the line broadening at half the maximum intensity (FWHM), θ is the Bragg angle. The diameters of ANFs in ANFs-film1 and ANFs-film 2 can be calculated to be ca. 16.75 nm and 14.5 nm, respectively, based on the broadening of (110) reflection. The sizes of crystalline domains match very well with the diameter of ANFs from SEM observations (Figure 5a, 5b). The crystallinity of PPTA broken paper, ANFs-film1 and ANFs-film2 are 67.4 %, 54.1 %, and 50.5 %, respectively, which was obtained by JADE5.0 software. It indicates that the mechanical performances of ANFs might not be retained very well of the excellent mechanical properties of PPTA fibers. The surface chemistry of ANFs was characterized by X-ray photoelectron spectroscopy (XPS). The C 1s spectra of PPTA broken paper and ANFs-film 2 are shown in Figure 6c and 6d, respectively. The peaks from C 1s spectra for PPTA broken paper are assigned to C-C (284.6 eV), C-N (286.1 eV), C=O (287.8 eV). Compared with the PPTA broken paper, the change of ANFs-film 2 in C 1s binding energy at 290.1 eV can be assigned to the newly created carboxylic group (-COOH) resulted from the cleavage of the amide bond to produce nanofibers from PPTA fiber. The surface chemistry is of great significance to interfacial interactions. With the increase in functional groups on the surface of ANFs, it can be more reactive and induce interfacial interactions with other matrixes.40 Taken together the XRD and XPS characterizations, it can be predicted that the mechanical performances of ANFs might not as good as that of PPTA fibers due to decrease in crystallinity. Thus, the mechanical properties of the resulting ANFs-films are further discussed in the following part.

per shows an ultimate strength (σ) of 82 ± 3 MPa and stiffness (E) of 2.94 ± 0.5 GPa. It exceeds the tensile strength of commercial Nomex® paper T410 with the strength 78 MPa, which offers high inherent dielectric strength, mechanical toughness, flexibility and resilience and is used in almost every known electrical sheet insulation application.41 The ANFs-film1 prepared by PPTA broken paper without ultrasonic treatment is found to have an ultimate strength (σ) of 122 ± 5 MPa and stiffness (E) of 6.40 ± 0.4 GPa, while ANFs-film2 fabricated by ultrasonically treated PPTA broken paper has an σ of 136 ± 6 MPa and E of 7.78 ± 0.3 GPa, increased by 11.5 % and 21.5 %, respectively. The action activity and accessibility of PPTA broken paper with KOH/DMSO system are highly increased attributes to the cavitation effect of ultrasonic treatment, leading to a thinner diameter with more homogeneous distribution of diameter. ANFs-films with the dense and uniform structure were achieved due to strong hydrogen bonding and interspersed morphologies. Interestingly, both the σ and E of ANFs-film 2 remarkably increased by ~65% and ~2.6 times compared with the macroscale PPTA broken paper. Compared with the PPTA broken paper, nanoscale ANFs possess large aspect ratio and abundant polar functional groups, which are beneficial for forming ANFs-film networks via strong hydrogen bonding, improving the distribution of stress over polymer chains, and accordingly ensured a higher ultimate strength and modulus of ANF films. It is worth noting that the higher ultimate strength and stiffness of ANFs-film indicate that the ANFs derived from the PPTA broken paper have retained the substantial mechanical properties, despite a reduction in the crystallinity. It also obviously surpassed the well-known Nomex® paper T410. From a fracture mechanics prospective, the nanoscale reinforcement can also increase the resistance of the polymers to initiation and propagation of a crack through mechanisms such as microcracking, crack bridging, and interfacial sliding.30,42-44 The nanoscale ANFs provide much larger interfacial area than their macroscale and micrometer-sized counterparts at the same volume fraction, leading to improved stress transfer between matrix and reinforcement.45 Similar to ANFs, cellulose nanofibers (CNF) is known as a new nanomaterial due to its appealing mechanical properties, easy accessibility, high potential for functionalization, and impressive environmental sustainability. Films made from CNF have high E (13 GPa) and σ (223 MPa). Despite those excellent performances of CNF, their hygroscopic nature and poor thermostability makes them very sensitive to high humidity and high temperature, reducing their mechanical performance under extreme conditions, such as in hygrothermal environment.46-49

The tensile strength and stiffness of the PPTA broken paper and ANFs-films are illustrated in Figure 7a. PPTA broken pa-



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Figure 7. Performances of PPTA broken paper and ANFs-films: (a) stress–strain curves. Inset: summary of stiffness and ultimate strength; (b) TGA curves and derivative thermogravimetry (DTG) curves. Inset: summary of TGA and DTG detailed information Additionally, the ultimate strength of the ANFs-film 2 was higher than the 79 MPa reported for a pure ANFs-film of Kevlar by the method of VAF,24 and also higher than the 66 MPa reported for ANFs-film by the bottom-up polymerization induced self-assembly and VAF process.50 However, ANFs-film fabricated from Kevlar® fibers via LBL method has increased ultimate strength of 179 MPa comparted to our ANF-Film.28 There are two main reasons for the difference, namely the negative effects of hot-calendaring process on PPTA fibers and the film forming method. The fibers in PPTA broken paper probably be damaged to some extent during the thermal calendaring process compared with the original PPTA fibers Moreover, the ANFsfilm prepared by LBL completely accomplish the protonation recovery process and remarkably ensure the reconstruction of PPTA molecular chain, which accordingly retain the performance of PPTA fibers to the maximum extent. LBL is a kind of assembly technology that is mainly applicable for the preparation of ultrathin films (nanometers in the thickness). Although its high accuracy enabled by the step-by-step deposition is easy to form the highly ordered multilayer films21, 28,LBL has been extremely restricted in the practical application due to the challenging of complicated procedures and highly time-consuming. As for VAF, the fast film production rate makes it an attractive alternative method for the design of composite films with programmable properties51. Moreover, the preparation process of ANFs films by the method of VAF is very similar to the traditional papermaking process in general terms, which mainly includes the furnish preparation, draining, forming, press and drying procedures. Therefore, VAF is highly regarded as a simple, efficient and feasible approach for the ANFs film formation, which also makes the industrial development of ANFs films in the future more feasible and prospective.

paper, respectively. In addition, DTG results also show that both the decomposing rate of PPTA broken paper and ANFs-films reached the maximum around 570 ℃. It was the partial protonation recovery of PPTA molecular chains and decrease of crystallinity that might resulted in lower thermal stability of ANFsfilms. Generally, ANFs fabricated by the PPTA broken paper show an excellent thermal stability, which makes it great potential applications in middle and high-end fields such as aerospace, high temperature insulation and filtration. This suggests that the preparation of ANFs derived from PPTA broken paper not only solves the problem of resources waste, but also provides a high-valued approach to recycle the PPTA broken paper generating in the papermaking process. More importantly, ANFs, acting as high-performance products, are indeed advantageous for preparation of high-performance nanomaterials, which will enable the design of many advanced functional materials ranging from flexible display materials to membranes, filters, insulators and electrodes applications become more feasible and promising. Insulation is one of the major factors that should be taken into consideration in many electric power apparatuses, which provides support, cooling and high dielectric strength as well for the windings.52 Electrical transformers typically have windings of conducting wire which must be separated by a dielectric (i.e. non-conducting) material. Most insulation materials used in the construction of transformer are made from organic compounds, which will decompose and deteriorate at high temperature. It will lead to the failure of the insulation materials and dramatic reducing of service life of transformers. Insulating paper, acting as the most widely used insulation material, suffering from the beat and damage by the insulating oil oxidation products during the its service time. The reaction of paper degradation is not reversible, therefore life of the insulating paper determines that of transformer.53 Consequently, the performances of insulating paper, such as the mechanical properties and dielectric strength, are of great significance to the service life of electrical apparatuses. Nomex® T410, the prior insulation choice for a majority of today’s electrical equipment applications, has earned global reputation for its ideal balance of properties and proven performance in a wide range of applications. As shown in Figure 8a,

PPTA is well known for its impressive thermal stability and the initial decomposition temperature could reach 500 ℃. In this work, ANFs-films also show excellent thermal stability as shown in Figure 7b, which is comparable to the PPTA broken paper. The TGA results show that the temperature at 10% weight loss (TG10%) and the residual mass fraction at 800 ℃ of ANFs-film 2 in nitrogen gas were up to 525.8 ℃ and 44.7 %, slightly decreased from 547.9 ℃ and 51.5% of PPTA broken


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the dielectric strength of Nomex® T410 with the thickness of 0.05 mm is 17 kV/mm, which exceeds the PPTA broken paper of 15.9 kV/mm. Interestingly, the dielectric strength of ANFsfilm fabricated by the PPTA broken paper reaches up to 100.1 kV/mm, which is ~5 times more than that of Nomex® T410 paper. Except for the nanoscale size of fibers, strong hydrogen bonding and abundant polar functional groups on ANFs, the prominent dielectric strength of ANFs-films can be attributed to the dense structure as illustrated in Figure 8c. Compared with PPTA paper in which many small holes and voids exist in the loose structure (Figure 8b), dense structure of ANFs-film will avoid the electric breakdown via the electrochemical breakdown. Moreover, nanoscale fibers are beneficial for the distribution of stress over polymer chains, which has effectively avoided the fast breakage of the weakest bond(s) taking place in polymer chains. In addition, the maximum dielectric strength of Nomex® T410 with the thickness of 0.76 mm is only 27

kV/mm, which is also much lower than that of ANFs-films in this work. It is however noted that the tear index of ANFs-film is lower than that of Nomex® T410. It is well known that the length of fibers is the crucial factor determining the tear strength of paper54. Compared with the millimeter-sized fibers in PPTA paper (several millimeters), the fiber length of ANFs are only several micrometers, which result in the decrease of the tear strength of ANFs-film. It suggests that the difference in length of fibers is mainly responsible for the different tear index, which should be further studied for the improvement of tear strength of ANFs assembled insulating ANFs-films. In a word, it is encouraging that the ANFs-films derived from PPTA broken papers possess extraordinary dielectric strength as well as tensile strength could significantly reduce the thickness of the insulation layer, driving the compactly designed and low weight transformers become more efficient, durable and reliable.

Figure 8. Performances and microstructures of PPTA paper and ANFs-films: (a) Dielectric strength and tear strength; (b) Schematic diagram of microstructure of PPTA paper; (c) Schematic diagram of microstructure of ANFs-film which is promising in the fields of flexible display materials, membranes, filters, insulators and electrodes and so on.

CONCLUSION In summary, a novel, effective and high value-added approach for recovering ANFs from the ultrasonically treated PPTA broken paper is demonstrated. Negatively charged ANFs with the diameters of 15~20 nm uniformly dispersed in water were finally achieved. It is worth noting that the ANFs derived from the PPTA broken paper retain the substantial mechanical properties despite reduction in the degree of crystallinity. The obtained ANFs-films via VAF method exhibit remarkably mechanical performances and highly temperature resilient, especially their dielectric strength is ~5 times higher than that of the well-known Nomex® T410 insulating paper. It is encouraging that such ANFs-film will bring about compactly designed and low weight transformers with improved reliability, durability and efficiency. We believe that the method demonstrated here not only provides an effective and high value-added way to recycle PPTA broken paper, but also can be used to design many ANFs-films based advanced functional nanomaterials,

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to express their gratitude to National Key Research and Development Program of China (2017YFB0308300) for primary financial support of this research. Strong support of this work by Shaanxi Overall Planning Innovative Engineering Project of Science and Technology (2016KTCQ01-87) and Open Fund of



State Key Laboratory for Modification of Chemical Fibers and Polymer Materials (LK1601) are also acknowledged.

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Brief Synopsis: High-performance aramid nanofibers (ANFs) that recovered from PPTA broken paper, providing a possible strategy for recycling the polymer composites wastes.

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Figure 1. Schematic of the papermaking forming process and applications of PPTA paper 87x48mm (300 x 300 DPI)



Figure 2. SEM images of surface and cross section of PPTA bro-ken paper: (a, b) Before ultrasonic treatment; (c, d) After ultrasonic treatment 112x78mm (300 x 300 DPI)


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Figure 3. Digital photos of ANFs formation process: (a) For-mation of ANFs from the beginning to the fifth day; (b) Partial pro-tonation recovery of ANFs/DMSO solution; (c) Digital photo of colloidal ANFs 149x138mm (300 x 300 DPI)



Figure 4. Formation process and mechanism of ANFs: (a) Digital images of PPTA broken paper and ANFsfilm; (b) Mo-lecular chains structure of PPTA fibers, ANFs in DMSO, and ANFs in water; (c) Skin-core structure model of PPTA37; (d) SEM images of ANFs formation process, d1-d2 are ultrasonic-treated PPTA broken paper in KOH/DMSO system for 24 h, 48 h respectively; (e) Schematic diagram of ANFs networks in ANFs-film. 149x139mm (300 x 300 DPI)


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Figure 5. Micromorphology characterization of ANFs and ANFs-film: (a, b) TEM images of ANFs fabricated from PPTA broken paper treated without / with ultrasonic; (c, d) SEM images of ANFs-film 1 and ANFs-film 2; (e, f) AFM 3D images of the surface of ANFs-film1 and ANFs-film 2 189x225mm (300 x 300 DPI)



Figure 6. FTIR spectra, XRD patterns and XPS C 1s spec-tra of PPTA broken paper and ANFs-films: (a) FTIR spec-tra; (b) XRD patterns; (c, d) XPS C 1s spectra 133x108mm (300 x 300 DPI)


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Figure 7. Performances of PPTA broken paper and ANFs-films: (a) stress–strain curves. Inset: summary of stiffness and ultimate strength; (b) TGA curves and derivative thermogravimetry (DTG) curves. Inset: summary of TGA and DTG de-tailed information 62x24mm (600 x 600 DPI)



Figure 8. Performances and microstructures of PPTA paper and ANFs-films: (a) Dielectric strength and tear strength; (b) Schematic diagram of microstructure of PPTA paper; (c) Schematic diagram of microstructure of ANFs-film 73x34mm (600 x 600 DPI)


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From PPTA Broken Paper: High-Performance ANFs and Their

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