Nonstop Monomer-to-Aramid Nanofiber Synthesis with Remarkable

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Nonstop Monomer-to-Aramid Nanofiber Synthesis with Remarkable Reinforcement Ability Jun Mo Koo,†,‡ Hojun Kim,† Minkyung Lee,† Seul-A Park,† Hyeonyeol Jeon,† Sung-Ho Shin,† Seon-Mi Kim,† Hyun Gil Cha,† Jonggeon Jegal,† Byeong-Su Kim,§ Bong Gill Choi,∥ Sung Yeon Hwang,*,†,⊥ Dongyeop X. Oh,*,†,⊥ and Jeyoung Park*,†,⊥

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†

Research Center for Bio-based Chemistry, Korea Research Institute of Chemical Technology (KRICT), Ulsan 44429, Republic of Korea ‡ Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, SE-100 44, Stockholm, Sweden § Department of Chemistry, Yonsei University, Seoul 03722, Republic of Korea ∥ Department of Chemical Engineering, Kangwon National University, Samcheok, Gangwon-do 25913, Republic of Korea ⊥ Advanced Materials and Chemical Engineering, University of Science and Technology (UST), Daejeon 34113, Republic of Korea S Supporting Information *

ABSTRACT: Aramid nanofibers (ANFs), typically produced by exfoliating aramid microfibers (Kevlar) in alkaline media, exhibit excellent mechanical properties and have therefore attracted increased attention as nanoscale building blocks. However, the preparation of aramid microfibers involves laborious and hazardous processes, which limits the industrial-scale use of ANFs. This work describes a facile and direct monomer-to-ANF synthesis via an as-synthesized intermediate low-molecular-weight poly(p-phenylene terephthalamide) (PPTA) without requiring the environmentally destructive acids and high-order shearing processes. Under the employed conditions, PPTA immediately dissociates and self-assembles into ANFs within a time period of 15 h, which is much shorter than the time of 180 h (not including the Kevlar preparation time) required for the Kevlar-to-ANF conversion. Interestingly, the fabricated ANFs exhibit nanoscale dimensions and thermoplastic polyurethane (TPU) reinforcing effects similar to those of Kevlar-derived ANFs; i.e., a 1.5-fold TPU toughness improvement and a maximum ultimate tensile strength of 84 MPa are achieved at an ANF content of only 0.04 wt %. Remarkable reinforcement ability investigated by comprehensive analytical data comes from ANFs, which disturb ordered hydrogen bonding in hard segments and induce strain hardening along the elongation pathway. Thus, the developed approach paves the way to industrial-scale production of ANFs and related nanocomposites.

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INTRODUCTION

high-pressure water jet atomization treatment were required after refluxing aramid microfibers in 10 wt % NaOH solution.11 Kotov’s research group reported a facile top-down strategy of the physicochemical Kevlar disintegration and self-assembly into negatively charged aramid nanofibers (ANFs) of high aspect ratio with length up to 10 μm using a KOH/DMSO solution.12,13 A large number of reports describe the preparation of ANFs as nanoscale building blocks for hybrid polymeric materials via layer-by-layer assembly,14−18 vacuum filtration,18−28 spinning,29 solution casting,30−35 solvent exchange,36−38 and dipping,39 and ANF nanocomposites have already been utilized in the fabrication of super reinforced materials, 17−24,30−38 membranes, 14,38,39 supercapacitors,15,16,25,29 conductors,26,37 and so forth.27,28 As nanofillers,

p-Aramid microfibers, also known as Kevlar, exhibit a remarkable tensile strength of ∼3.6 GPa and a modulus of ∼120 GPa due to intermolecular hydrogen bonding between the repeating units of poly(p-phenylene terephthalamide) (PPTA) and aromatic stacking along the fiber axis.1−4 Importantly, the molecular orientation accounting for these excellent mechanical properties is achieved by an environmentally destructive and energy-consuming process of air-gap wet spinning. Despite their high strength and stiffness, aramid microfibers cannot be easily utilized as reinforcing fillers because of their poor adhesion to and miscibility with polymer matrices.5,6 Currently, surface modification of microfibers is believed to be the only way of increasing their affinity toward polymer matrices.7−10 The alternative approach of downsizing the fiber diameter to the nanoscale level is difficult to achieve because of the inherent inertness of p-aramid; 30 cycles of © XXXX American Chemical Society

Received: November 8, 2018 Revised: January 7, 2019

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DOI: 10.1021/acs.macromol.8b02391 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Syntheses of PPTA and Kevlar. (b) Photographic and FE-SEM images of PPTA precursors (i: crude PPTA-O solution in NMP (2.0 wt %); ii and iii: crude PPTA-2 solution in NMP (7.2 wt %); iv and v: refined PPTA-1; vi and vii: commercial Kevlar microfibers after air-gap wet spinning). (c) AFM height images of ANFs (viii: ANF-O prepared from PPTA-O; ix: ANF-C prepared from PPTA-2; x: ANF-R prepared from PPTA-1; xi: ANF-K prepared from Kevlar).

DMSO mixtures with NMP and N,N′-dimethylacetamide (DMAc) further reduced the ANF production time to less than one-twelfth of that required in the case of pure DMSO, as verified by UV/vis spectroscopy. The reinforcing effects of ANFs were investigated by preparing thermoplastic elastomer nanocomposites, which are independent from molecular weights of PPTAs. Notably, ANFs enhanced the tensile properties of thermoplastic elastomers even at extremely low loadings by disturbing ordered hydrogen bonding in hard segments and inducing strain hardening along the elongation pathway.

ANFs possess innate stiffness and additional beneficial anionic, hydrogen-bonding, and π−π interactions on the interface, which, however, have not been fully exploited yet. In particular, the realization of polymer nanocomposites by maximizing the effect of aramid nanofillers has attracted increased attention, but further experimental improvements are still required for the successful implementation of this approach. Notably, ANFs are prepared from Kevlar and derived readymade products (i.e., staple, flocs, and pulp) after completing the laborious processes of (1) synthesis and purification of PPTA in N-methyl-2-pyrrolidone (NMP), (2) solvation of PPTA in corrosive sulfuric acid, (3) fastidious spinning/ drawing by application of high shearing forces, and (4) washing and drawing (Figure 1a). Once oriented, aramid fibers can hardly be redissolved even in sulfuric acid because of the presence of strong intermolecular hydrogen bonds. For similar reasons, it generally takes more than a week to exfoliate Kevlar to ANFs, although ultrasonic pretreatment of aramid broken paper has been shown to reduce the ANF preparation time from 9 to 5 days.40 On the other hand, the bottom-up ANF synthesis from monomers requires a high degree of shearing to assemble PPTA chains into nanosized aggregates during polymerization in a special reactor and long-chain branched comonomer on the side of PPTA chains to avoid interfibril aggregation.41−43 Thus, this method represents the penalty for the reduced aspect ratio and ANF content. Herein, we developed a facile and one-pot ANF synthesis directly from 1,4-phenylenediamine and terephthaloyl chloride monomers involving the formation of rough (unprocessed) PPTAs as an intermediate state and bypassing polymer purification, sulfuric acid-mediated dissolution, and fiber spinning processes (Figure S1). Interestingly, the use of

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RESULTS AND DISCUSSION Nanoscale PPTA domains along the fiber axis can be exfoliated to nanofibers by deprotonation of amide groups, which weakens hydrogen bonding (Figure 1). Similarly to other reports,12−39 atomic force microscopy (AFM) and fieldemission scanning electron microscopy (FE-SEM) imaging of Kevlar-derived ANFs (ANF-K) revealed the presence of clustered wrinkled nanofibers with diameters of 30−70 nm (Figure S3a). Considering that the dissolution of ANFs occurs in nanoscale PPTA domains regardless of their macro shape, we attempted to directly use rough PPTA in place of microfibers. Various types of bulk PPTA synthesized from 1,4-phenylenediamine and terephthaloyl chloride in NMP (high-molecular-weight PPTA-1 synthesized using pyridine and CaCl2, PPTA-2 synthesized using pyridine and excluding CaCl2, and PPTA-O synthesized by a one-pot technique in diluted NMP) were used to compare the effects of molecular weight and cosolvent on ANF properties. ANFs were prepared by stirring PPTAs in KOH/DMSO solution at room temperature (25 °C); specifically, ANF-R was prepared from B

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Macromolecules Table 1. PPTA and ANF Preparation Conditions and Parameters synthesis of PPTAs code Kevlar PPTA-1 PPTA-2 PPTA-O

additivesa pyridine, CaCl2 pyridine pyridine

preparation of ANFs

concn in NMPb (wt %)

Mw(PPTA)c (kg mol−1)

code

source

7.2

43 10

ANF-K ANF-R

Kevlar refined PPTA-1

0.2/0/0/100 0.2/0/0/100

7.2 2.0

3.2 2.6

ANF-C ANF-O

crude PPTA-2 crude PPTA-O

0.2/0.2/2.8/97.2 2.0/2.0/100/900

PPTA/pyr-HCl/NMP/ DMSOd (weight ratio)

dissolution timee (h)

Mw(ANF)c (kg mol−1)

180 70

15 4.3

20 15

1.8 1.0

a 1,4-Phenylenediamine and terephthaloyl chloride were polymerized in NMP with additives. bCalculated weight percent of theoretical PPTA yield per NMP. cWeight-average molecular weight calculated based on inherent viscosity (Tables S1 and S2). dComposition of feed used for ANF solution preparation (pyr-HCl: pyridine hydrochloride). The KOH/PPTA weight ratio was set to 1.5. eDetermined by UV/vis spectroscopy.

Figure 2. (a) ANF color changes with time at room temperature (25 °C). (b) Plot of maximum absorption intensity of the 100-fold diluted ANF solution vs dissolution time (λ = 337 nm for ANF-K and ANF-R; λ = 331 nm for ANF-C and ANF-O). (c) DLS-determined size distributions of various ANFs (intensity-mean diameters and standard deviation presented).

refined PPTA-1 after washing out salt and solvent residues, ANF-C was prepared from crude PPTA-2 codissolved in NMP (7.2 wt %), and ANF-O was prepared from a dilute solution of crude PPTA-O in NMP (2.0 wt %). The one-pot preparation of ANF-O was performed without any work-up stages (Movie S1), and detailed procedures used for PPTA synthesis and ANF preparation are described in Table 1, Figure S2, and Tables S1 and S2. The surface of refined PPTA obtained by crushing crude PPTA with a 1:1 (v/v) water/methanol mixture featured disordered nanoscale PPTA domains, although no harsh shear forces or spinning processes were applied (Figure 1b). In all

cases, irrespective of the refined/crude nature of rough PPTAs, the color of ANFs changed from orange to wine-red upon dispersion (Figure S1). Notably, ANF-C-Ca prepared from CaCl2-containing crude PPTA-1 could not be effectively exfoliated into nanofibers, presumably because the basicity of the in situ formed Ca(OH)2 was insufficiently high for the formation of the methylsulfinyl carbanion that is generally prepared by the reaction of KOH with DMSO (Figure S4).44,45 The time of PPTA disintegration and self-assembly into ANFs was determined by the naked eye as the point when PPTA sludge disappeared and was also verified by UV/vis C

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Figure 3. (a) Tensile stress−strain curves of TPU and TPU/ANF nanocomposites (inset: magnified view of initial curves). (b) Tensile modulus− strain curves and theoretical changes of nanofiller orientation. Initial nanocomposite modulus was well fitted by the isotropic model (EI: dark gray zone), while the modulus at >1000% elongation was fitted by the longitudinal model (EL: light gray zone) (inset: magnified view of initial curves). (c) Reinforcement performances of ANFs in TPU/ANF films and other polymer/ANF nanocomposites reported in the literature (Table S6). (d) Ashby plot of UTS increase per unit (1 wt %) filler loading versus UTS of TPU elastomer composite for TPU/ANFs and other TPU composites reported in the literature (Table S7). (e) DSC (first scan) and (f) one-dimensional small-angle X-ray scattering profiles of TPU and TPU/ANF-K films.

maximum absorbance was plotted as a function of time. As a result, dramatically reduced dissolution times were observed, e.g., ∼180 h for Kevlar vs ∼70 h for refined PPTA-1, which was

spectroscopy (Figure 2a,b and Figure S5). The clear supernatants of ANF-containing solutions were collected and diluted 100-fold for UV/vis absorbance measurements, and D

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Nevertheless, ANF-O did not show marked degradation signs until a temperature of 350 °C, which was outside the processing temperature range of most commercial polymers. Moreover, the thermal stabilities of ANFs were inferior to those of PPTAs because of the negative charge and broadened distribution of the former. ATR-FTIR spectroscopy revealed that the amide vibrational bands of ANFs were blue-shifted relative to those of their precursors, which was indicative of moderate hydrogen bond decrease, and only marginal differences were observed between the spectra of ANFs from different sources. The crystalline structure of ANFs was confirmed by glazing incidence X-ray diffraction patterns, which showed characteristic peaks of the (110) and (200) planes of Kevlar.30,40 Subsequently, we proposed that despite their lower molecular weight, the prepared ANFs should exhibit reinforcing effects comparable to those of classical Kevlarderived ANFs in view of the similar size distributions of these nanofillers, additionally featuring the advantage of increased accessibility in terms of time and cost. To test this hypothesis, we selected thermoplastic polyurethane (TPU) as a polymer matrix because nanofibers constituting aromatic amides are expected to engage in π−π and hydrogen-bonding interactions with the hard segment of aromatics and urethane bonds and thus exhibit good affinity for these moieties and provide effective reinforcement.49−51 To date, numerous attempts have been made to enhance the tensile properties of TPU by using various surface-modified (to increase the affinity for the polymer matrix) fillers such as clays,52−57 carbon nanomaterials,58−62 and natural fibers,63−66 and products,67,68 albeit with unsatisfactory success. Herein, following a previously reported TPU film characterization method,50,51 we prepared TPU/ ANF nanocomposite films by solution casting and evaluated their tensile properties. Solutions of commercial-grade TPUs (containing ester-type polyol as soft segments and methylene (bisphenyl isocyanate)/1,4-butanediol as hard segments) in DMSO were mixed with ANF solutions,69 and the solid content/solution and ANF/TPU weight ratios were fixed at 20 and 0.04 wt %, respectively (Figure S12). Notably, no expected reinforcement effects (but rather tensile property deterioration) were observed for free-standing films obtained by casting freshly prepared nanocomposite solutions (Figure S13). Additionally, numerous macroscopic aggregates were observed, which indicated the poor miscibility of the composite with TPU, contrary to a previous report.18 On the basis of the above result, we concluded that filler performance can be improved by allowing more time for the diffusion and equilibration of polymer chains and nanofillers in the viscous solution and therefore extended the aging time to 4 days at room temperature (25 °C). As a result, the reinforcing effect at an ANF loading of 0.04 wt % (400 ppm) was dramatically enhanced irrespective of nanofiller molecular weight, as reflected by the increases of modulus at 100% strain (1.2− 1.4-fold), ultimate tensile strength (UTS; 1.5−1.8-fold), and toughness (1.3−1.5-fold) (Figure 3a and Table S5), which was ascribed to the fact that all ANFs exhibited similar sizes. The degree of variations in tensile properties among ANF series was within an acceptable similarity range. Furthermore, the minimum content of ANFs required to increase both UTS and toughness by a factor of ∼1.5 has previously been reported as 0.1 wt % (for ANF-functionalized graphene sheets inside PMMA; Figure 3c and Table S6).30 The reinforcement effectiveness of a given filler can be expressed as the increase

ascribed to the low-molecular-weight and loose packing density of rough PPTA. Surprisingly, the ANF dissolution time could be further decreased to ∼20 h when crude PPTA-2/NMP cosolvent heterogeneous mixtures were used. To distinguish the effects of molecular weight and packing density from that of the cosolvent, the dissolution time of ANF-K was investigated as a function of cosolvent (NMP and DMAc) in DMSO (Figures S6 and S7). An increase of NMP content to 20 wt % resulted in a 2-fold reduction of dissolution time, whereas an increase of dissolution time was observed at higher NMP contents (>50 wt %). Similar results were obtained when DMAc was used instead of NMP within the range of 10−50 wt %, and the observed behavior was explained by considering the Hansen solubility parameter. For PPTA, the value of the above parameter (23.0) is similar to those of NMP (22.9) and DMAc (22.7), which suggests that better swelling of PPTA domains facilitates the approach of sodium methylsulfinyl carbanions (Table S3).46 However, at low DMSO contents, the amount of the above anions generated by the KOH-DMSO reaction decreases, which results in a concomitant reduction of the base-mediated ANF dissolution efficiency. Finally, for ANF-O, in which case the content of NMP was increased to 10 wt % and the molecular weight of PPTA was further reduced to 2.6 kg mol−1, the dissolution time was as low as ∼15 h. Thus, the use of rough PPTA resulted in 9-fold (20 h, ANF-C) and 12fold (15 h, ANF-O) productivity increases compared to the case of Kevlar (180 h). To check whether the molecular weight of PPTA is reduced during its base-mediated dissolution to form ANFs, we compared inherent viscosities in concentrated sulfuric acid by acidifying ANF dispersions to re-form PPTA,41 showing that the ANF/PPTA precursor molecular weight ratios were around 0.35−0.55 (i.e., Mw(Kevlar) = 43 kg mol−1 → Mw(ANF-K) = 15 kg mol−1 and Mw(PPTA-O) = 2.6 kg mol−1 → Mw(ANF-O) = 1.0 kg mol−1). It is attributed to the cleavage of PPTA amide bonds caused by alkali reagents. Although the molecular weights of ANFs varied, the average diameter of ANFs measured by AFM and FE-SEM was within the range 30−70 nm in all cases (Figure 1c and Figure S3). The average diameters of ANF-K, -R, -C, and -O were measured by AFM and found to be 40 ± 3, 39 ± 3, 49 ± 5, and 49 ± 4 nm, respectively. In a suspension, dispersed ANFs are surrounded by solvent molecules and rotate to form a hypothetical sphere, the diameter of which can be determined by dynamic light scattering (DLS) measurements. Importantly, since the prepared ANFs exhibited comparable chemical compositions, widths, and rodlike structures, and could therefore be viewed as a homologous series, the diameter of the above sphere was concluded to be a valid parameter for relative length comparison. In particular, repeated measurements revealed that all ANFs exhibited DLS sizes of 200−300 nm irrespective of the PPTA source (Figure 2c, Figure S8, and Table S4), which indicated that low-molecular-weight PPTA molecules contained a sufficient number of hydrogen bonding and aromatic stacking interactions to form self-assembled nanofibers with a size comparable to that of Kevlar-derived ANFs.4,47,48 ANFs were additionally characterized by thermogravimetric analysis (TGA), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray diffraction (Figures S9−S11). Thermal stability decreased with increasing end-group concentration per polymer chain (∝ 1/Mw), i.e., was in the order of ANF-K > ANF-R > ANF-C > ANF-O and Kevlar > PPTA-1 > PPTA-2 > PPTA-O. E

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Figure 4. (a) Synchronous and asynchronous 2D correlation spectra of TPU and TPU/ANF-K films at 0, 100, and 200% elongation. (b) Peak sequence and description of 2DCOS measurement results. (c) Polarized optical microscopy images of (i) pristine TPU, (ii) 200% elongated TPU, (iii) pristine TPU/ANF-K, and (iv) 200% elongated TPU/ANF-K films.

of UTS per filler loading ([fold-1] wt %−1), and the corresponding data for TPU elastomer composites containing ANFs and other fillers are plotted in Figure 3d and listed in Table S7. The highest UTS of 84 MPa and the highest reinforcement performance of 20 [fold-1] wt %−1 was observed for TPU/nonmodified ANFs, which suggested that among the tested fillers ANFs exhibit the best affinity for the TPU matrix. At this point it is interesting to compare these results to the previous record of 62 MPa/7.2 [fold-1] wt %−1 observed for a cellulose nanocrystal/TPU nanocomposite.63 To visualize the superior tensile properties of TPU/ANFs, a rectangular film with a cross-sectional area of 0.15 mm2 was used for a weightlift experiment and was shown to withstand a weight of 10 kg without breaking while TPU without ANF disconnected (Movie S2). ANF loading of 0.04 wt % exhibited the best

performance among 0.02−0.16 wt % with good reproducibility (Table S5). The origin of this unprecedented reinforcement effect was further probed by instrumental techniques. Differential scanning calorimetry (DSC) analysis of TPU/ANF-K revealed that the melting enthalpy of pristine TPU at 145 °C was drastically reduced, and three ill-defined melting points of 96, 148, and 161 °C were observed (Figure 3e and Table S8). Notably, the characteristic points in the DSC curves of series of TPU/ANF composites were almost identical (Figure S14), which indicated that the addition of ANFs reduced the molecular packing density and affected the molecular packing structure in the hard domain of TPU. However, the glass transition temperatures (Tgs) of soft and hard segments were unaffected by ANF introduction, and the degree of hard and F

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all elongations (Figure S20). 2DCOS is a versatile spectral analysis method that can emphasize the transformation of various chemical constituents not readily observable in onedimensional (1D) spectra.81,82 Figure 4a shows synchronous and asynchronous 2D IR COS spectra obtained utilizing a series of 1D IR spectra. The autopeaks observed along the diagonal line of synchronous spectra indicate the intensity changes of the CO band (1750−1650 cm−1), while intensity changes of two bands in the same and opposite directions are represented by red and blue colors (positive and negative cross-peaks), respectively. When designated cross-peaks from the synchronous match the sign of the asynchronous, the intensity on the x-axis occurs before that on the y-axis, and vice versa. Information on sample cross-peaks is presented in Table S9, and their sequential order and description are provided in Figure 4b. Interestingly, TPU and TPU/ANF-K featured completely different sequences despite the identical number of cross-peaks. The sequence of pristine TPU corresponded to 1691 cm−1 (hydrogen (H)-bonded CO in ordered hard domain) > 1732 cm−1 (free CO) > 1709 cm−1 (H-bonded CO in disordered hard domain), which indicated that the force is first received in the rigid domain.83,84 The observed intensity variation indicated that upon TPU stretching the degree of microphase separation and the relative content of the ordered hard domain increased, while the relative content of the disordered hard domain decreased. For TPU/ANF-K, the sequence corresponded to 1736 cm−1 (free CO) > 1713 cm−1 (H-bonded CO in disordered hard domain) > 1656 cm−1 (H-bonded CO in ordered soft domain). Interestingly, the above result indicates that soft domains, comprising free CO and disordered domains, were affected in preference to hard/ordered domains, since ANF incorporation deteriorated the crystallinity of TPU. In contrast to the case of pristine TPU, the degree of microphase separation increased concomitantly with the relative content of the disordered hard domain for TPU/ANF-K. On the basis of the obtained results, we suggested that disrupted lamellar ordering and disordered H-bonding in nanocomposites increased the mobility of nanofillers and thus facilitated their alignment along the longitudinal direction on the elongation pathway. The matrix orientations of pristine TPU and TPU/ANFs in films elongated to 0 and 200% were observed by a polarized optical microscope equipped with a λ retardation plate (Figure 4c and Figure S21),85 and the images of both nonstretched films appeared dark, which implied that their constituent molecules were disordered. However, at an identical microscopic setting, the image of the elongated TPU/ ANF-K nanocomposite was much brighter and more bluecolored than that of TPU; i.e., the nanocomposite matrix and filler became well oriented upon elongation because of hard domain disruption. The mechanical behavior of the TPU/ANF nanocomposite was described by the Halpin−Tsai ̈ equation using the tensile moduli of the TPU matrix (5−50 MPa) and ANF filler (120 GPa), filler aspect ratio (100−1000), and filler content (0.0004). Theoretically, upon the incorporation of ANFs, the tensile modulus of TPU could be increased by a factor of only 1.03−1.3 if ANFs were isotopic (EI), whereas the corresponding increase could be as high as 1.08−1.8-fold when ANFs were fully longitudinally aligned (EL) (Figure S22).86,87 Figure 3b indicates that the initial modulus of nanocomposite exceeded that of pristine TPU by factors of 1.0−1.3, in agreement with the isotropic model. However, at >1000% elongation, the above increase was as high as 1.5−2.5-fold,

soft segment microphase separation was concluded to remain unchanged.70−72 The above conclusions were supported by the results of ATR-FTIR spectroscopy and dynamic mechanical analysis (DMA). The ATR-FTIR spectra of TPU and TPU/ ANFs showed no differences. In particular, the intensities of both free CO stretching peaks at 1727 cm−1 and hydrogenbonded CO stretching (disordered) peaks red-shifted to 1703 cm−1 remained unchanged, which suggests that the degree of microphase separation was not altered despite the interfering effect of ANFs on the molecular ordering of hard domains (Figure S15).51 The absence of the degree of microphase separation changes was confirmed by DMA; i.e., identical soft segment Tgs of −35 °C were determined from the maximum tan delta curve, indicating the mechanical transformation of the material from the glassy to rubbery state (Figure S16). The X-ray diffraction pattern of the pristine TPU matrix showed a broad amorphous peak with maximum intensity at 2θ = 18.68°, and the corresponding interchain spacing was determined to be 0.475 nm, which is relevant to the short-range order of TPUs.52,73,74 In the case of TPU/ ANF-K, the above peak lost intensity and shifted to lower angles, which was ascribed to the decreased crystallinity of the hard domain and an increased interchain spacing of 0.492 nm (Figure S17). Moreover, wide-angle X-ray scattering patterns revealed a slight broadening of short-range order upon the intercalation of ANFs into the TPU matrix (from 0.468 nm (TPU) to 0.474 nm (TPU/ANF-K); Figure S18). The tiny shoulder peak at 0.421 nm observed for TPU/ANF-K corresponded to reflection from the (110) plane of PPTA units.30,75 The small-angle X-ray scattering pattern of TPU showed an intense peak and thus confirmed the nanoscale ordering of hard domains, and the decreased peak intensity observed for TPU/ANF-K was indicative of reduced density difference between hard and soft segments (Figure 3f). Additionally, upon ANF incorporation, the position of the above peak shifted to a higher distance (from 2.40 to 2.53 nm); i.e., the spacing between hard domains slightly increased due to the increased intermolecular spacing. In addition, other TPU/ANF films showed the identical trend (Figure S19). Thus, X-ray studies revealed that ANF incorporation increased the intermolecular distance and hard domain size of TPU but decreased the density of the hard domain.52,76−79 Based on the results of the above analyses, the infiltration of ANFs resulted in a moderate molecular-to-lamellar-level ordering disruption and decreased the melting enthalpy of hard domains while preserving the degree of microphase separation. Nevertheless, these insights do not fully explain the remarkable mechanical enforcement observed upon ANF incorporation. Moreover, the low content of ANFs did not allow the observation of the corresponding bond stretchings and interchain spacings. Interestingly, the tensile moduli (derivative of stress) of TPU/ANFs in the initial state were 1.0−1.3-fold higher than that of pristine TPU, whereas at >1000% elongation, the corresponding factor was higher (1.5−2.5) because of straininduced hardening (Figure 3b).67 In view of the above, we suggested the occurrence of microphase or hydrogen bonding variation along the elongation pathway. To confirm/disprove this hypothesis, we traced the stretching variations of salient chemical bonds in films elongated to 0, 100, and 200% by ATR-FTIR spectroscopy and two-dimensional correlation spectroscopy (2DCOS).80 As expected, the ATR-FTIR spectra of TPU and TPU/ANF-K showed no apparent differences at G

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yield (inherent viscosity (IV) = 1.84 dL g−1). PPTA-2 (7.2 wt % of target PPTA in CaCl2-free NMP): The amounts of materials and the operation sequence were identical to those used to prepare PPTA-1, except for the absence of CaCl2. Polymerized crude PPTA (∼5 g, 7.2 wt %) was sampled for preparing ANF-C, and the remainder was purified as described above to afford refined PPTA-2 in 96% yield (IV = 0.89 dL g−1). PPTA-O (2.0 wt % of target PPTA in CaCl2-free NMP): A 250 mL dried RBF was charged with PPD (0.90 g, 8.32 mmol), pyridine (1.33 g, 16.8 mmol), and NMP (90 mL), and the resulting mixture was stirred at 2 °C for 60 min under N2 for complete dissolution. A solution of TPC (1.69 g, 8.32 mmol) in NMP (10 mL) was prepared in a separate vial. The PPD solution was transferred into a blender, and the TPC solution was injected into the blender in two equal parts at 10 s intervals upon intermittent grinding. For PPTA-O characterization, the reactant (2.0 wt %) was crushed with DI water (700 mL) and methanol (700 mL), and the precipitated PPTA was filtered off, washed with DI water and acetone, and dried under vacuum at 25 °C to afford refined PPTA-O in 93% yield (IV: 0.77 dL g−1). Preparation of Aramid Nanofibers (ANFs). ANF-K (from Kevlar): A 250 mL dried RBF was charged with Kevlar (0.20 g), KOH (0.30 g), and DMSO (100 g), and the obtained mixture was magnetically stirred mildly at room temperature (25 °C) and a relative humidity of 20%. Dissolution was considered to be complete when the solution turned dark red and did not contain any sludge. Additionally, the extent of dissolution was verified by saturated UV/ vis absorption measurements. Based on the obtained results, complete dissolution took ∼180 h. In the case of ANF-K, a certain amount of DMSO was replaced with NMP or DMAc, i.e., NMP/DMSO weight ratios of 0/100, 5/95, 10/90, 20/80, 30/70, and 50/50 and DMAc/ DMSO weight ratios of 0/100, 5/95, 10/90, 20/80, 30/70, and 50/50 were used (Figures S6 and S7). ANF-R (from refined PPTA-1): Refined PPTA-1 (0.20 g) was used instead of Kevlar. Complete dissolution took ∼70 h. ANF-C-Ca (from crude PPTA-1 containing NMP, CaCl2, and pyridine-HCl): Crude PPTA-1 (3.32 g; = 0.2, 2.8, 0.19, and 0.14 g of PPTA, NMP, pyridine-HCl, and CaCl2, respectively) was used instead of Kevlar. The total solvent weight equaled 100 g (NMP/DMSO weight ratio of 2.8/97.2 was used). Aramid nanofibers were not formed since PPTA could not be dissolved. ANF-C (from crude PPTA-2 containing NMP and pyridine-HCl): Crude PPTA-2 (3.18 g; = 0.2, 2.8, and 0.19 g of PPTA, NMP, and pyridine-HCl, respectively) was used instead of Kevlar. The total solvent weight equaled 100 g (NMP/DMSO weight ratio of 2.8/97.2 was used). Complete dissolution took ∼20 h. ANFO (directly from as-synthesized PPTA-O containing NMP and pyridine-HCl): After the preparation of PPTA-O, as described in the above section, KOH (3.00 g, 53.5 mmol) and DMSO (900 g) added into the blender were crushed upon 2 min grinding. The reaction mixture was transferred into a 2 L dried RBF and mildly stirred at 25 °C. The total solvent weight equaled 1000 g (NMP/DMSO weight ratio of 1/9 was used). Complete dissolution took ∼15 h. Preparation of TPU/ANF Nanocomposites. Commercial-grade TPU with ester-type polyol soft segments and methylene (bisphenyl isocyanate)/1,4-butanediol hard segments (5.0 g) was dissolved in DMSO (20 g) upon 3 h mechanical stirring at 150 °C under nitrogen. The obtained solution was cooled to room temperature, and ANF solution (2 mg/g) was injected using a gastight syringe. The solids content of the solution was fixed at 20 wt %, and the ANF/TPU weight ratio equaled 0.04 wt % (400 ppm). The TPU/ANF solution was mechanically stirred for 4 h at room temperature (25 °C) and then allowed to stand for 4 days, poured into a steel mold, and slowly dried by increasing the temperature (25−90 °C) over 7 days. Characterization. The IV (ηinh) of PPTA in sulfuric acid was measured at a concentration of 0.5 g dL−1 by an Ubbelohde viscometer (IIc, diameter = 1.36 mm). Based on the measured IV, the weight-average molecular weight (Mw) of PPTA was calculated as41

which was well fitted by the longitudinal model; i.e., ANFs were aligned along the drawing axis upon elongation. Thus, the stress hardening behavior of the composite along the elongation pathway notably deviated from that of pristine TPU because of the concomitant tensile modulus increase. The degree of ANF orientation at low filler loading seems to be a key parameter determining the strain hardening behavior of the TPU matrix. Compared to TPU, TPU/ANFs featured a 1.5−2.5-fold higher tensile modulus at extreme elongation and a 1.5-fold higher toughness, additionally exhibiting the highest UTS (84 MPa) among the known TPU composites.52−68 This exemplary nanocomposite system will facilitate the development of various ANFs in different polymer nanocomposites.

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CONCLUSION In summary, we investigated the accelerated preparation of ANFs using different cosolvents/PPTA precursors and investigated the mechanical reinforcement performance of the thus-obtained nanofibers. Notably, the nanofiber production time could be decreased by decreasing the molecular weight of as-synthesized PPTA intermediates and by dilution of DMSO with suitable cosolvents. Under optimal conditions, the time required for the one-pot preparation of ANFs (ANFO; (1) synthesis of PPTA in NMP, (2) addition of KOH/ DMSO, (3) dissolution and production of nanofibers) equaled only 15 h, which was markedly lower than that required for ANF production from Kevlar and thus represented a large increase in cost-effectiveness. Although ANFs prepared from different sources had different molecular weights, they featured similar hydrodynamic sizes and therefore exhibited similar reinforcing effects. In particular, ANF incorporation at a very low loading of 0.04 wt % increased the toughness of TPU by a factor of 1.5 and achieved a record high UTS of 84 MPa, which was ascribed to the disruption of hard domain crystallinity upon ANF introduction and the disordered hydrogen bonding and strain hardening along the elongation pathway. Thus, the developed technique allows the cost-effective large-scale production of ANFs, which is expected to expand the scope of their industrial applications.

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EXPERIMENTAL SECTION

Materials. 1-Methyl-2-pyrrolidinone (NMP, 99%), dimethyl sulfoxide (DMSO, 99.9%), N,N-dimethylacetamide (DMAc, 99%), and sulfuric acid (99.9%) were purchased from Sigma-Aldrich (USA). Pyridine (99%), KOH (85%), and anhydrous CaCl2 (anhydrous, 96%) were sourced from Alfa Aesar (USA). 1,4-Phenylenediamine (PPD, 98%) and terephthaloyl chloride (TPC, 99%) were purchased from TCI (Japan). Kevlar 49 thread was sourced from Dupont Inc. (USA). Thermoplastic polyurethane (TPU, 5575AP grade) was obtained from Dongsung Corp. (Korea). All chemicals were used without further purification unless stated. Synthesis of Poly(p-phenylene terephthalamide)s (PPTAs). PPTA-1 (7.2 wt % of target PPTA in CaCl2-containing NMP): A 500 mL dried round-bottom flask (RBF) was charged with PPD (5.84 g, 54.0 mmol), pyridine (8.54 g, 108 mmol), CaCl2 (8.79 g, 79.2 mmol), and NMP (160 g), and the obtained mixture was stirred at 2 °C for 60 min under N2 for complete dissolution. A solution of TPC (10.96 g, 54.0 mmol) in NMP (20 g) was prepared in a separate vial. The PPD solution was transferred into a blender (Tefal BL311E, 500 W, France), and the TPC solution was injected into the blender in two equal parts at 10 s intervals upon intermittent grinding. The obtained crude PPTA (∼5 g, 7.2 wt %) was sampled for preparing ANF-C-Ca, and the remainder was crushed with deionized (DI) water (700 mL) and methanol (700 mL), filtered, washed with DI water and acetone, and dried under vacuum at 25 °C to afford refined PPTA-1 in 97%

M w = 3902.4ηinh1.556 H

(1) DOI: 10.1021/acs.macromol.8b02391 Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

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Mw measurements were performed as follows. A solution of ANFs (0.1 g) in DMSO (50 mL) was poured into aqueous H2SO4 (0.1 M) and crushed for 30 min at room temperature (25 °C). The precipitate was filtered off, repeatedly washed with water and methanol, dried under vacuum at 25 °C for 12 h, and then dissolved in sulfuric acid for viscosity measurements. Atomic force microscopy (AFM) measurements were conducted using a MultiMode V instrument (Veeco, USA) in tapping mode and in an ambient atmosphere. Field-emission scanning electron microscopy (FE-SEM) imaging was performed using a Tescan MIRA3 (Czech Republic) instrument with an in-beam detector. Samples prepared on silicon wafers were sputtered with Pt (Quorum Technologies Q150T S, UK) and investigated. UV/vis absorption spectra were performed on a UV-2600 spectrophotometer (Shimadzu, Japan). For the preparation of each 100-fold diluted ANF/DMSO solution, the clear supernatant (40 μL) was mixed with DMSO (4 mL), and the mixture was stored in a freezer to prevent any reactions prior to measurements. Particle size distribution was analyzed by dynamic light scattering (DLS; Zetasizer Nano ZS90, Malvern Instruments, UK) at a wavelength of 633 nm and a scattering angle of 90°. Thermogravimetric analysis (TGA; PerkinElmer Pyris 1 TGA, USA) was conducted by heating samples from 30 to 800 °C at a rate of 10 °C min−1 in a flow of nitrogen. Tensile properties of at least three samples were measured using a universal testing machine (UTM; Instron 5943, UK) using a 1 kN load cell and a constant speed of 100 mm min−1 at room temperature (25 °C). The tested specimens had a dog-bone shape with length × width × thickness = 25.5 mm × 3.11 mm × 3.1 mm, respectively. Differential scanning calorimetry (DSC; Q-2000, TA Instruments, USA) measurements were conducted within a temperature range of −80 to 190 °C in a nitrogen atmosphere at a heating/cooling rate of 10 °C min−1. Fourier-transform infrared (FT-IR) spectra of TPU and TPU/ANF films were recorded on a Nicolet iS50 spectrometer (Thermos-Fisher Scientific, USA) in the range 650−4000 cm−1 using the attenuated total reflectance (ATR) technique. For each spectrum, a total of 64 scans with a spectral resolution of 4 cm−1 were collected. A smart iTR diamond ATR accessory (face angle = 45°, Thermo-Fisher Scientific, USA) was used for ATR measurements. Two-dimensional correlation spectroscopy (2DCOS) measurements to afford synchronous/ asynchronous 2D correlation spectra were performed using a custom-built 2DShige program (freely downloadable software developed by Prof. Shigeaki Morita (Osaka Electro-Communication University, Japan)). The perturbation used in 2DCOS experiment was the elongation. Pristine TPU and TPU/ANF-K were stretched to 0, 100, and 200% of its original length in a dog-bone shape, and the IR spectrum of each was measured at the center point of the dog bone at a room temperature of 25 °C. Dynamic mechanical analysis (DMA) was performed using a DMA Q800 instrument (TA Instruments, USA) at a fixed frequency of 1 Hz. The test specimens were barshaped films with approximate dimensions of 13 mm × 5 mm × 0.3 mm. Measurements were performed at a heating/cooling rate of 3 °C min−1 from −80 to 100 °C in an atmosphere of N2. Wide-angle X-ray diffraction patterns were collected using Cu Kα1 radiation (λ = 1.5406 Å) produced by an X-ray generator (single, 220 V, 3 kW), and scanning was performed in a 2θ range of 5°−40° (grazing incidence mode at an angle of 0.8° for ANF samples). One-dimensional wide-/ small-angle X-ray scattering profiles were obtained using synchrotron radiation (λ = 1.28 Å) of the 3C SAXS I beamline, Pohang Accelerator Laboratory (PAL). The optical textures of TPU samples were analyzed by polarized optical microscopy (POM; Zeiss Axio Imager, Germany) with a first-order retardation λ/4 plate placed between cross-polarizers at an angle of 45°. All POM images were collected at identical camera and light source settings such as light intensity and exposure time. The sample film was placed on the microscope, and POM images were obtained by rotating the sample from 0° to 90°. The position affording the lowest light intensity was chosen as 0° (standard).

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02391.

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Preparation process, polymerization information, and results of PPTAs and ANFs, AFM and FE-SEM images, UV/vis and ATR-FTIR spectra, DLS size, and TGA data of ANFs, change of ANF−C-Ca color with time, and ANF-K color with time observed for different NMP, and DMAc contents, Hansen solubility parameters of PPTA and solvents, preparation process, and tensile, thermal, ATR-FTIR, 2DCOS, DMA, X-ray, and POM data, and theoretical moduli calculation of TPU/ANF nanocomposite, including Tables S1−S9 and Figures S1− S22 (PDF) Movie S1 (AVI) Movie S2 (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hyeonyeol Jeon: 0000-0001-9176-2913 Hyun Gil Cha: 0000-0002-1228-4464 Byeong-Su Kim: 0000-0002-6419-3054 Bong Gill Choi: 0000-0002-5452-4091 Sung Yeon Hwang: 0000-0002-4618-2132 Dongyeop X. Oh: 0000-0003-3665-405X Jeyoung Park: 0000-0002-9369-1597 Author Contributions

J.M.K. and H.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Technology Innovation Program (10070150) funded by the Ministry of Trade, Industry & Energy (MI, Korea) and Korea Research Institute of Chemical Technology (KRICT) core project (SI1941-20, KK1941-30). S.Y.H. acknowledges support from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2018R1C1B6002344). We are thankful to Mr. Jehan Kim at the Pohang Accelerator Laboratory for X-ray diffraction measurements with synchrotron radiation (Beamline 3C) and Dongsung Corp. for providing thermoplastic polyurethanes.

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REFERENCES

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DOI: 10.1021/acs.macromol.8b02391 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02391 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02391 Macromolecules XXXX, XXX, XXX−XXX