Isolation of Aramid Nanofibers for High Strength and Toughness

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Isolation of Aramid Nanofibers for High Strength and Toughness Polymer Nanocomposites Jiajun Lin, Sun Hwi Bang, Mohammad H. Malakooti, and Henry A. Sodano ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01488 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 15, 2017

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ACS Applied Materials & Interfaces

Isolation of Aramid Nanofibers for High Strength and Toughness Polymer Nanocomposites Jiajun Lin,† Sun Hwi Bang,‡ Mohammad H. Malakooti,‡ and Henry A. Sodano* ,†,‡,§ †Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA ‡Department of Aerospace Engineering, University of Michigan, Ann Arbor, MI 48109, USA §Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA *

Address correspondence to

[email protected]

ABSTRACT: The development of nanoscale reinforcements that can be used to improve the mechanical properties of a polymer remains a challenge due to the long standing difficulties with exfoliation and dispersion of existing materials. The dissimilar chemical nature of common nanofillers (e.g. carbon nanotubes, graphene) and polymeric matrix materials is the main reason for imperfect filler dispersion and consequently, low mechanical performance of their composites relative to theoretical predictions. Here, aramid nanofibers that are intrinsically dispersible in many polymers are prepared from commercial aramid fibers (Kevlar) and isolated through a simple, scalable, and low-cost controlled dissolution method. Integration of the aramid nanofibers in an epoxy resin results in nanocomposites with simultaneously improved elastic modulus, strength and fracture toughness. The improvement of these two mutually exclusive

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properties of nanocomposites is comparable to the enhancement of widely reported carbon nanotube reinforced nanocomposites but with a cost-effective and more feasible method to achieve uniform and stable dispersion. The results indicate the potential for aramid nanofibers as a new class of reinforcements for polymers. KEYWORDS: Aramid nanofibers, nanocomposites, elastic modulus, fracture toughness, fractography 1. INTRODUCTION Densely cross-linked thermosetting polymers, such as epoxy, are widely used for their outstanding stiffness, thermal stability and chemical resistance. However, the presence of a high density of crosslinks also results in polymers that exhibit lower toughness and strain to failure as well as brittle fracture.

While the high rigidity of the polymer is required for structural

applications, the brittle nature of these polymers significantly limits their use in many other applications, particularly at low temperature or high strain rate loading conditions.1-4 The most successful technique over the past three decades for toughening of thermosetting polymers has been the addition of an elastic phase to the epoxy such as carboxyl-terminated (butadiene-coacrylonitrile) (CTBN) elastomers or polyethersulfone thermoplastic polymers.5-7 The elastic phase is chosen to be soluble in the resin, but phase separation during curing leads to the formation of a low modulus elastic phase in the composite. These local regions of reduced modulus adsorb strain energy and act to toughen the polymer.5-7 This technique can drastically enhance the toughness of the composite but can also significantly reduce the modulus and performance at elevated temperature.6,7

High elastic modulus and fracture toughness are

generally mutually exclusive properties in thermoset polymers even after the incorporation of a secondary polymer phase.


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Inspired by stiff and yet tough natural materials that consist of both soft and hard segments, nanoscale reinforcements have been developed to overcome the limitations of pure polymers. Although extensive experimentation and processing is usually required to realize materials with improved properties, the nanoscale size of the fillers in nanocomposites can yield greatly improved properties. Nanoscale reinforcements can provide much larger interfacial area than their macro-scale and micrometer-sized counterparts at the same volume fraction leading to improved stress transfer between matrix and filler.8-10 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 micro-cracking, crack bridging, and interfacial sliding.11-13 Among the wide range of nanoscale reinforcements that have been studied over the past two decades, nanoclay reinforcement is one of the most successful approaches to simultaneously improve strength and toughness of a polymer.4, 14-17 The exfoliated structure of high modulus clay contributes to the stiffness of the nanocomposites while the intercalated clay platelets can improve the fracture toughness. Due to its excellent performance, nanoclay reinforced polymer composites have been widely used in applications such as automotive bumpers and ballistic armors. Another widely studied nanoscale reinforcement that has attracted significant interest is carbon nanotubes (CNTs). CNTs possess exceptionally high elastic modulus and strength, theoretically more than 10 times higher than carbon fibers, which can contribute significantly to the reinforcement of polymers.18 However, CNTs are challenging to work with due to their tendency to aggregate and forming bundles. Aggregation results due to the high surface area of the tubes and their strong Van Der Waals force leading to two decades of research on CNT dispersion.19-21 As a result, the uniform dispersion of CNTs in a bulk polymer, such as epoxy resins, can be difficult to achieve and is often unstable. The high aspect ratio of


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CNTs also leads to waviness and entanglement, which limits load transfer to the tube and can result in a non-uniform stress distribution. Additionally, CNT entanglement also leads to a significant increase in the viscosity of the solution when incorporated beyond 1% volume fraction.22, 23 Another challenge in employing CNTs as a nanoscale reinforcement lies in their smooth and chemically inert surface which leads to poor wetting and weak interfacial bonding thus requiring surface functionalization when interfacial strength is necessary.10,24-26 Although CNTs and nanoclays require consideration of a number of processing challenges, they have been shown to offer significant gains in polymer strength, stiffness and toughness. Unlike inorganic nanomaterials that present weak interaction with matrices, recent developments in organic nanomaterials have shown promise for yielding reinforcements that can be readily dispersed and to produce stable colloids. For instance, graphene oxides with surfaces having abundant functional groups can be uniformly dispersed in polymers to enhance the strength and toughness.27-29 Cellulose nanofiber (CNF) is another organic nanostructure that has been used as reinforcement in polymer nanocomposites. Previous studies have shown that integration of the CNFs into polymers can produce to optically transparent nanocomposites with high modulus.30-32 In a recent study, CNF reinforced polylactic acid (PLA) nanocomposites, with 5% weight concentration of the fillers, resulted in 24% and 22% increase in the tensile modulus and strength of the pure PLA, respectively.33 However, there are challenges associated with using CNFs in composites including their low aspect ratio and hydrophilic nature.32, 34 The low aspect ratio of the CNFs lowers their influence on load transfer and the lack of compatibility with hydrophobic polymer matrices limits their applications. Furthermore, CNFs are sensitive to humidity and their moisture uptake significantly affects their mechanical properties which limits their application in structural materials.35, 36


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Recently, Yang et al. aramid.

37-39

developed an alternative organic nanoscale reinforcement based on

Aramid fibers consist of highly aligned molecular chains of poly (paraphenylene

terephthalamide) (PPTA) that form strong intermolecular bonding through dense hydrogen bonds and π-π stacking, resulting in an elastic modulus and strength comparable to carbon fibers.37-39 Yang et al. used a controlled deprotonation and dissolution process that decreases the effects of intermolecular hydrogen bonds and increases electrostatic repulsion yielding a disassembly of the aramid fiber (Kevlar® fiber) into aramid nanofibers. The ANFs obtained from the reported dissolution of aramid fibers have diameters in range of 5-30 nm and lengths in range of 5-10 µm.37 The ion concentration in solution leads to an equilibration of the electrostatic repulsion, hydrophobic attraction and π-π stacking of the backbones which provides control over the nanofiber dimensions. Following the formation of ANFs, the surface is rich in carboxylic acid, carbonyl and hydroxyl groups while the core maintains the structure of aramid fibers presumably maintaining the high strength and stiffness. Abundant surface functional groups can not only enhance the interfacial strength and load bearing capacity of the nanocomposites, but also increase the compatibility and solubility of nanofibers in the host polymer, making it feasible to achieve a high level of dispersion and to incorporate a high volume fraction of fillers. However, prior efforts have not isolated the ANFs from the solution and therefore this novel material has never been studied as a nanofiller in bulk polymers to provide additional reinforcement. It should be noted that bucky paper like and self-assembled films have been studied since these materials can be obtained directly from the solution without isolation or purification of the ANFs.37-39 In this work, solid ANFs are isolated for the first time from the aramid nanofiber dispersion in order to reveal the potential of these nanoscale building blocks and significantly broaden the plausible applications. The effect of ANFs on the elastic modulus, tensile strength


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and fracture toughness of an epoxy resin is demonstrated by fabrication and characterization of ANFs nanocomposites with different filler weight fractions.

The results are compared to

measured mechanical properties of neat epoxy and modified epoxy with fibrillated and short aramid fibers, as control samples. It is shown that the integration of ANFs in an epoxy resin results in nanocomposites with simultaneously improved elastic modulus, strength and fracture toughness. At last, the fracture surface of compact tension specimens are examined using scanning electron microscopy in order to identify possible toughening mechanisms in modified epoxy resins. 2. EXPERIMENTAL SECTION 2.1. Preparation and isolation of aramid nanofibers. After preparing the dispersed ANFs in solvent by employing Yang’s method,37-39 the nanofibers are precipitated from the mixture by modifying the solubility of nanofibers in the solvent. For preparation of the ANF dispersion, 1 g of aramid fibers (Kevlar® 49, DuPont), 1.5 g of KOH (ACS certified, Fisher Scientific) and 500 mL of dimethyl sulfoxide (DMSO) (ACS certified, Fisher Scientific) were mixed together in a flask. The mixture was stirred magnetically for 7 days at room temperature to obtain a dark red solution of ANFs (as shown in Figure 1A). To isolate the ANFs from the solution, an equal volume of deionized water was added to the solution to decrease the solubility and precipitate the ANFs. The precipitate was then collected by vacuum filtering and washed with deionized water several times to neutralize the pH. Finally, the collected ANFs were washed with acetone (ACS certified, Fisher Scientific) and dried at room temperature under vacuum (Figure 1B). 2.2. Morphological and chemical characterization of aramid nanofibers. The dimension and dispersion of isolated ANFs were characterized by Atomic Force Microscopy (Park AFM XE70) and compared to the ANFs obtained directly from the DMSO solution. The chemical


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composition and crystallinity of ANFs were characterized by Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy.

The FTIR characterization of ANFs is

performed by a Nicolet iS60 spectrometer (Thermo Scientific) with a SMART iTR accessary. Raman scattering spectra of ANFs are obtained on a Renishaw inVia confocal Raman microscope. The FTIR and Raman spectra of macroscale aramid fibers are also obtained to investigate the chemical and morphological changes induced by the ANFs preparation process. 2.3. Fabrication of ANFs reinforced epoxy nanocomposites. To fabricate aramid nanofiber reinforced epoxy, a commercial diglycidyl ether of bisphenol-F (DGEBF) epoxy trademarked Epon 862 (Hexion, Inc.) was used in combination with a commercial polyoxypropylenediamine trademarked EpikureTM 3230 (Hexion, Inc.) as a curing agent.

To prepare nanocomposite

specimens, ANFs were added to the epoxy resin with a weight fraction ranging from 0.2% to 3% of the composites. The mixture was then horn sonicated (Fisher Scientific Model 500) for 10 minutes followed by centrifugal shear mixing (FlackTek speedmixerTM DAC 150.1 FVZ) for 10 minutes at 3000 rpm. A uniform dispersion of aramid nanofibers in epoxy resin was achieved through this mixing process and was proven to be stable over a long period of time without any precipitation, as shown in Figure 1C (images show stability after 1 month). Curing agent 3230 was then added at a weight ratio to Epon 862 of 35:100 and subjected to centrifugal mixing for another 5 minutes followed by degassed thoroughly in a vacuum oven at room temperature before casting into silicone molds. The specimens were cured at 80 ºC for 2 hours followed by 125 ºC for 3 hours. Neat epoxy specimens were prepared without reinforcement by using the same degassing and curing procedure. The prepared specimens with different weight fractions of ANFs are shown in Figure 1D.


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Figure 1. (A) ANFs/DMSO solution, (B) isolated ANFs, (C) aramid nanofiber dispersion in Epon 862 resin with a weight fraction of 0.5% and 2%, one month after mixing, comparing with neat Epon 862 resin. (D) ANFs reinforced epoxy specimens with different weight fraction of ANFs. 2.3. Mechanical characterization of ANFs reinforced epoxy nanocomposites. Tensile tests were performed on the prepared nanocomposites to investigate the role of ANFs on the elastic modulus and tensile strength of the nanocomposites. Tensile testing was conducted according to ASTM standard D638 using type V specimens followed by testing on an Instron universal load frame (Model 5982) with a 100 kN load cell at a cross-head speed of 1 mm/min. For each weight fraction of nanocomposite samples, 24 specimens were tested. Compact tension (CT) tests on the same load frame according to ASTM standard D5045 were performed to evaluate the fracture toughness of aramid nanofiber epoxy composites. A minimum of 8 specimens for each weight fraction was tested at a rate of 1 mm/min. In both tests, epoxy specimens reinforced by aramid


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pulp were prepared as a comparison. Aramid pulp (Kevlar® pulp from DuPont) consists of highly fibrillated, short fibers with a length of 0.5-1.0 mm made by milling aramid fibers (Figure S3). Aramid pulp (AP) reinforced epoxy specimens of the same weight fraction as the aramid nanofiber reinforced epoxy specimens were prepared and tested under the same conditions in both tensile and CT tests.

The fracture surface morphology of the nanocomposites was

investigated by optical microscope and Scanning Electron Microscopy (SEM, JOEL 2010F analytical electron microscope) to understand the possible toughening mechanism of ANFs. 3. RESULTS AND DISCUSSION 3.1. Characteristics of aramid nanofibers. The dimension and morphology of isolated ANFs are shown by AFM images in Figure 2. Figure 2A demonstrates the dried ANFs after spin coating their DMSO solution while Figure 2B is the captured AFM image from the isolated ANFs, ready to be used as a reinforcing nanofiller. Little agglomeration is observed in Figure 2B indicating that the isolated ANFs can be dispersed well in a bulk polymer. Furthermore, the isolated ANFs have diameters in the range of 2-4 nm, smaller than the diameters of ANFs in DMSO solution, which is in the range of 5-10 nm, according to the AFM measurement. This decrease in diameter can be attributed to de-swelling of the nanofibers as the solvent is removed from the ANFs during the separation process. Figure 2C shows the IR spectra of the isolated ANFs to identify the molecular structure of the ANF and compare them to the original aramid fibers.

Clearly, the nanoscale and macroscale aramid fibers have very similar IR spectra,

indicating that their molecular structures are almost the same. The similar patterns in the Raman spectra of ANFs and aramid fibers, shown in Figure 2D indicate that the chemical structure of nanoscale fibers wasn’t changed after the dissolution process. The intensity and sharpness of IR peaks and Raman peaks in the spectra of ANFs slightly decreased comparing to the spectra of


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aramid fibers, which is due to the decreased size of crystallites and broad distributed surface states in ANFs. However, the spectra still show that a substantial crystallinity is retained in ANFs despite of the nanoscale dimension, which is in agreement with the results reported before and indicates that the mechanical properties of macroscale aramid fibers can be retained. 37

Figure 2. Atomic force microscopy images of (A) ANFs in DMSO solution and (B) separated ANFs. (C) FTIR spectra and (D) Raman spectra of ANFs and macroscale aramid fibers. 3.2. Tensile behavior. Tensile tests were performed on the prepared nanocomposites in order to investigate the role of ANFs on the elastic modulus and tensile strength of the nanocomposites. The average Young’s modulus and tensile strength of the samples containing


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different fraction of ANFs are shown in Figure 3. The average Young’s modulus of the aramid nanofiber composites is observed to increase at a low weight fraction of ANFs and continued to increase with higher filler concentration.

At 1.5 wt. % of ANFs, there was an observed

maximum increase of Young’s modulus from 2.54±0.06 GPa for the neat resin to 3.25±0.06 GPa, which correlates to an approximately 28% higher stiffness. The measured average tensile strength also shows a similar trend with a maximum increase of 18.5%, from 70.3±1.1 MPa for neat epoxy samples to 83.3±2.4 MPa for modified epoxy with 1 wt. % of ANFs. It should be noted that the improvement of the elastic modulus and the tensile strength of modified epoxy resins saturates when the content of ANFs is 1.5 wt. %. Although saturation is reached, the elastic modulus and ultimate strength of resins with 2 wt. % and 3 wt. % of ANFs are still higher than neat epoxy. A possible reason for this observation is the presence of voids in the high concentration resin slurry and consequently, the formation of defects in cured nanocomposites. Voids may occur due to the increased viscosity of the epoxy resin when higher weight fractions of ANFs are added, making the degassing process less effective. The result indicates that the ANFs with minimal processing provide a level of reinforcement to epoxy, comparable to CNTs of same concentration yet with a stable and surfactant free dispersion.19-21, 40, 41


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Figure 3. Mechanical properties of aramid nanofiber reinforced epoxy nanocomposites with different weight fractions; (A) Young’s modulus and (B) tensile strength. To further confirm the improvement in Young’s modulus and tensile strength resulting from the addition of ANFs, epoxy specimens reinforced by AP were prepared as a comparison. The measured Young’s modulus, the tensile strength and representative stress-strain curves of the nanocomposites are shown in Figure 4. Compared to neat epoxy, the AP reinforced epoxy also demonstrates an improvement in Young’s modulus of up to 3.03±0.06 GPa at 0.7 wt. %, an improvement of 19%. However, this improvement is lower than the improvement obtained from ANF reinforced samples. Furthermore, the Young’s modulus and the tensile strength drop significantly for samples containing 1 wt. % AP. This sudden decrease demonstrates that the large size of the aramid pulps leads to poor dispersion, most likely due to entanglement of the longer fibrils. The increase in viscosity happens at a much higher concentration of fillers for the aramid nanofiber samples than AP samples because of the more reactive functional groups on the surface and consequently, better physical dispersion and chemical interaction in epoxy matrix. The better dispersion leads to a higher Young’s modulus and tensile strength of ANFs reinforced nanocomposites than AP reinforced epoxy at same weight fraction. Specifically, at the same weight fraction of 1%, ANF reinforced epoxy specimens have a 7.5% higher Young’s modulus and a 15.3% higher tensile strength compared to AP reinforced epoxy. Additionally, as shown in Figure 4C, the maximum elongation of aramid pulp reinforced epoxy is significantly lower than that of ANF reinforced composites and neat epoxy specimens. The epoxies reinforced by AP failed immediately after strain softening, indicating the fracture of the specimens without experiencing large plastic deformations. This observation implies that the aramid pulp is not able to efficiently transfer the load after the yield point and causes stress concentrations and consequently premature failure of the epoxy.

In contrast, the stress-strain behavior of the


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nanocomposites indicates that ANFs can be drawn and stretched with the epoxy matrix, possibly due to their smaller size and better chemical bonding of the large surface area with a high content of polar functional groups. These results identify the advantages of a larger interfacial area and improved dispersion of the ANFs in producing enhanced mechanical properties of the polymer.

Figure 4. Comparison of (A) Young’s modulus and (B) Tensile strength between aramid pulp (AP) reinforced epoxy and ANFs reinforced epoxy with different weight fraction, and (C) their stress-strain curves. 3.3. Fracture properties.

As mentioned earlier, the ultimate goal of fabricating

nanocomposites is to use fillers to simultaneously improve stiffness and toughness of the polymer. It was already shown that ANF reinforced epoxies exhibit higher elastic modulus and tensile strength and withstand large deformations before failure. In order to evaluate the fracture


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toughness of these modified epoxy resins, compact tension (CT) tests were performed on ANFs reinforced epoxy nanocomposites. The critical stress intensity factor (KIc) and fracture toughness (GIc) of the specimens were calculated using Equation 1 and 2, respectively. K Ic =

Pmax f (a / W ) BW 1/ 2

(1)

GIc =

KIc2 (1−ν 2 ) E

(2)

where Pmax is the maximum load measured at failure, B is the thickness, W is the overall length of the CT specimen, a is the initial crack length, v is the Poison’s ratio (assumed to be 0.353 for all cases), E is the Young’s modulus obtained from tensile tests above, and f (a/W) is the calibration fraction related to the ratio of a to W, defined in detail in the ASTM Standard D5045. As shown in Figure 5, the average critical stress intensity factor and fracture toughness of nanocomposites are higher than neat epoxy. The increased toughness of the nanocomposites can be related to the role of ANFs as toughening mechanisms. For instance, the high aspect ratio of the ANFs can cause crack bridging and consequently keep the epoxy intact. It is also possible that ANFs create local regions in the epoxy with different elastic modulus and absorb strain energy and toughen the epoxy similar to the well-known toughening mechanism in elastomeric particle-toughened epoxies. Energy dissipation at the interphase regions due to sliding of the nanofibers can also be considered as another plausible reason for the observed increase in the resistance of the nanocomposites against crack propagation.


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Figure 5. Fracture behavior of aramid nanofiber reinforced epoxy nanocomposites with different weight fractions; (A) Critical stress intensity factor, KIc, and (B) fracture toughness, GIc. The results of the CT test demonstrate that nanocomposites with 1 wt. % ANFs are able to withstand the highest level of fracture load as witnessed by a 20 % higher critical stress intensity factor compared to neat epoxy (Figure 5A). The 1.5 wt. % case provides the highest fracture toughness enhancement; a 23% increase from 737.22±183.26 J/m2 for neat epoxy to 908.65±128.69 J/m2 (Figure 5B). Meanwhile, the critical stress intensity factor is also improved by 18.4%, from 1.52±0.19 N/m3/2 to 1.81±0.13 N/m3/2. Recalling tensile testing results, the nanocomposites containing 1.5 wt. % of ANFs are 28% stiffer in addition to being 22% tougher compared to neat epoxy. The nanocomposites exhibit simultaneous increase in elastic modulus and toughness while the inversely proportional relationship between elastic modulus (E) and fracture toughness (GIc) in Equation (2) implies that an increase in elastic modulus should lower the fracture toughness. These results indicate that the ANFs can be considered as fillers that resolve the conflict between strength and toughness by creating heterogeneous nanocomposites with simultaneously enhanced strength and fracture resistance. CT specimens of AP reinforced epoxy were also prepared as a comparison to further confirm the improvement in fracture toughness resulting from the addition of ANFs. As shown in Figure


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6, the fracture toughness of AP reinforced epoxy is increased up to 927.08±186.48 J/m2, with a 25% improvement from neat epoxy specimens. However, the highest reinforcement due to the integration of APs happens at a weight fraction of 0.5%, when the composites showed a low Young’s modulus. This mechanical behavior of the 0.5% AP nanocomposites indicates that the enhanced fracture toughness is mainly because of the relatively low elastic modulus of the nanocomposite. Furthermore, the critical stress intensity factor and fracture toughness drop significantly for specimens containing 1 wt. % AP, which is possibly caused by poor dispersion and entanglement of the longer fibrils as mentioned above.

Figure 6. Comparison of (A) critical stress intensity factor, KIc, and (B) fracture toughness, GIc between aramid pulp (AP) and aramid nanofiber (ANF) reinforced epoxy specimens with different weight fraction. 3.4. Fractography. In order to study the toughening mechanism of ANFs reinforced epoxy nanocomposites the fracture surfaces of CT specimens are examined using an optical microscope and SEM. As shown in Figure 7A, the neat epoxy specimen exhibits a typical brittle fracture surface showing oriented crack pattern parallel to the crack propagation direction. Presence of a very few ripples in addition to the smooth fracture surface of neat epoxy indicate rapid crack propagating and lack of energy dissipation mechanism. Unlike the neat epoxy specimen, the


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modified epoxy specimens exhibit substantially different fractographic features. Representative fracture surfaces of a 0.5 wt. % ANFs reinforced epoxy are shown in Figure 7B. In general, the addition of ANFs to the epoxy leads to a rougher surface and initiates the formation of crazes which usually do not exist in thermosets such as epoxy because of their high crosslinking density. This formation of crazes could be the result of heterogeneous curing of epoxy due to the introduction of nano-scale reinforcements.42 It is also reported that nanofillers can be rearranged in the precraze regions of glassy polymeric nanocomposites to affect the process of craze formation and propagation.43-46 The addition of nanoscale reinforcements will help initiate crazes and contribute to the toughness of the nanocomposites.

The crazes in epoxy

nanocomposite with 0.5 wt. % ANFs are distorted and tilted while many sub-cracks are generated. This phenomenon is common in nanofiller-toughened epoxy and indicates a craze deflection process where the craze is disturbed and split when it encounters an inclusion.47-48 As expected, increasing the weight fraction of ANFs to 1.5 wt. %, which showed highest level of toughening, significantly affects the ripples and crazes in the fracture surface (shown in Figure 7C). During the fracture of 1.5 wt. % ANF nanocomposites, major crazes with very rough surface are generated along the crack propagation direction, implying that the crack propagation may not be initiated from the pre-cracked edge. In this case, it can be assumed that the failure mode of the toughened epoxy was changed and the fracture was initiated in the middle of specimen. It is possible that the agglomeration of some ANFs caused the stress concentration and consequently failure initiation from these regions.49 The presence of small crazes between the extremely rough surfaces further confirms this hypothesis. Overall, the micrographs of the fracture surfaces indicate that incorporation of ANFs into epoxy acts as an energy absorption mechanics and increases the total fracture toughness compare to neat epoxy. The fracture


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surfaces of a 1.0 wt. % AP reinforced epoxy specimen are shown in Figure 7D as a comparison to ANF reinforced epoxy. During the crack propagation of AP modified epoxy, crazes are initiated at the interface between fillers and epoxy matrix and the short aramid fibers are pulled out from matrix, acting as an energy absorption mechanism

Figure 7. Optical microscope and SEM images of the fracture surfaces: A) neat epoxy, B) 0.5 wt.% ANFs reinforced epoxy, C) 1.5 wt.% aramid nanofiber reinforced epoxy, and D) 1.0 wt. % aramid pulp reinforced epoxy.

4. CONCLUSION


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In summary, this work demonstrates the isolation of uniformly sized ANFs and their application for high strength and toughness polymer nanocomposites for the first time. Isotropic and stable dispersions of ANFs in the epoxy are simply obtained through mixing without further modification of nanofibers. The presence of abundant polar functional groups on the surface of ANFs and their uniform distribution within epoxy matrix provided strong interactions at the interface region and enhanced the overall mechanical properties.

The mechanical

characterization of nanocomposites revealed that epoxy resins achieve higher improvements in elastic modulus and tensile strength by using ANFs rather than aramid pulps. Based on the fractography of the modified epoxy resins, it was suggested that incorporation of aramid fillers can effectively change the failure mode of neat epoxy and improve its fracture resistance. The simultaneous enhancement of strength, stiffness and toughness, distinguishes ANFs from many other nanomaterials for polymer reinforcement. These unique reinforcing effects, in addition to the simple, low cost, and scalable procedure for preparation, make them a unique nanoscale building block.

SUPPORTING INFORMATION Detailed FTIR and Raman spectra of isolated ANFs, XRD patterns of isolated ANFs, aramid pulps and macro-scale aramid fibers and morphology characterizations of aramid pulps (SEM images) are provided in the Supporting Information. A comparison of mechanical properties between the reported ANFs reinforced epoxy nanocomposite and other recent reported nanocomposites is also included.


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Table of Content:


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Table of Content 254x190mm (300 x 300 DPI)


Isolation of Aramid Nanofibers for High Strength and Toughness

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