In Situ Formation of Nanohybrid Shish-Kebabs during Electrospinning

Article pubs.acs.org/Macromolecules

In Situ Formation of Nanohybrid Shish-Kebabs during Electrospinning for the Creation of Hierarchical Shish-Kebab Structures Matthias M. L. Arras,† Richard Jana,† Mike Mühlstad̈ t,† Stefan Maenz,† Joseph Andrews,† Zhiqiang Su,‡ Christian Grasl,§,∥ and Klaus D. Jandt*,†,⊥ †

Chair of Materials Science, Otto Schott Institute of Materials Research, Faculty of Physics and Astronomy, Friedrich Schiller University Jena, Löbdergraben 32, 07743 Jena, Germany ‡ State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China § Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, AKH, Ebene 4L, Währinger Gürtel 18-20, 1090 Vienna, Austria ∥ Ludwig Boltzmann Cluster for Cardiovascular Research, AKH, Ebene 1Q, Währinger Gürtel 18-20, 1090 Vienna, Austria ⊥ Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7, 07743 Jena, Germany ABSTRACT: We successfully tested the hypothesis that the nanohybrid shish-kebab (NHSK) structure, i.e., polymer kebabs on a CNT shish, can be created in situ during electrospinning. In addition, the resulting nanofibers containing the NHSKs were used to create a hierarchical structure by introducing a second level of shish-kebabs: the nanofiber shish-kebab (NFSK), i.e., polymer kebabs on the nanofiber shish. A multiwalled CNT/ poly(ε-caprolactone) (MWCNT)/(PCL) solution was electrospun to form nanofibers of Ø ≈ 100 nm. The MWCNTs aligned in fiber direction within the MWCNT/PCL nanofibers. The first time creation of the NHSK during electrospinning was confirmed by bright field diffraction contrast in the transmission electron microscope. The NFSK was created by incubating the NHSK containing nanofibers in a supersaturated PCL solution. A simple model explaining the in situ NHSK formation is presented. The hierarchical structure based on the shish-kebab morphology is proposed as a high-performing building block in future advanced materials.

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INTRODUCTION Hierarchical design of materials is one key to achieve outstanding performance.1 Nature applies this principle very effectively in the form of hierarchical nanocomposites to yield material properties superior to the single components.2,3 For example, the nested hierarchy of collagen fibrils spans at least six levels from the single chain α helix to the fibril bundle, with each hierarchical level being self-similar to the former, i.e., fibershaped.4 Here, we present a self-similar two-level hierarchical structure based on the shish-kebab morphology, i.e., a shishkebab morphology on different length scales. This is important and promising because the shish-kebab morphology in semicrystalline polymers is the origin of high strength polymer fibers and films. In addition, carbon nanotubes (CNTs) with their outstanding properties form the basis of this two-level hierarchical structure. The presented hierarchical material features the novelty of being a shish-kebab material on different length scales, the creation of a PCL/CNT hybrid shish-kebab, and the in situ hybrid shish-kebab formation during electrospinning. Shish-Kebab Morphology. The shish-kebab morphology consists of a polymer row crystal (shish) which is usually overgrown in situ by plate-like folded lamellae crystals (kebab). © XXXX American Chemical Society

This shish-kebab resembling polymer morphology was found in the late 1950s by Keller.5 In general, semicrystalline polymers crystallized under shear-flow show a pronounced shish-kebab morphology:6−8 The shear flow elongates the polymer chains which undergo coil−stretch transition and eventually crystallize in form of a row crystal (shish, often also referred to as row nuclei/crystal). Subsequently, less stretched chains overgrow the shish as folded lamellae crystals to form the kebabs.6 A recent perspective on the molecular origin of the polymer only shish-kebab morphology has been published by Kimata et al.9 The unique shish-kebab morphology is fundamental to the outstanding properties of polymer fibers and films and, thus, is a very important structure on its own. However, most important for this study is as presented in the following, that the polymer shish-kebab morphology may be a general concept which can be applied to a variety of suitable shish-like structures.10 Nanohybrid Shish-Kebab (NHSK). Recently, the Li group reported the creation of the CNT hybrid shish-kebab, i.e., a Received: January 22, 2016 Revised: March 29, 2016

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DOI: 10.1021/acs.macromol.6b00153 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules shish-kebab morphology where the polymer shish is replaced by a CNT.11−13 They coined the term nanohybrid shish-kebab (NHSK). Considering the outstanding properties of CNTs,14 the NHSK is a very promising structure, especially for the reason that the NHSK is a physical solutiona to the problems accompanied by the fabrication of CNT/polymer nanocomposites, i.e., (re)agglomeration and filler−matrix adhesion. The physical functionalization of CNTs in CNT/polymer nanocomposites by polymer crystal growth on the CNTs13 can provide both a means to avoid reagglomeration of CNTs due to the screening of the strong intertube van der Waals interactions and an ideal, interlocked integration into the matrix. The unique shape of the crystals protruding perpendicularly from the CNT is thought to increase the interfacial shear strength and, thus, the nanocomposite‘s strength.10,15 Primarily, polymers with a zigzag chain conformation may form NHSKs. Accordingly, NHSKs have been reported for the following polymers: poly(ethylene),12 poly(amide),16 poly(vinyl alcohol),17 and poly(vinylidene fluoride).18 In contrast, it has also been reported that isotactic poly(propylene) can form an NHSK as well, but only on CNT bundles.19 However, to the best knowledge of the authors and despite its zigzag main chain, a poly(ε-caprolactone) (PCL) NHSK has not been reported yet. The formation mechanism of the NHSK is still under investigation.19 Especially information on the early stage of nucleation remains unclear: One notion is that polymer chains directly nucleate on the surface of the CNT via “soft epitaxy”,13 i.e., the folded lamellae crystals which forms the kebab grow directly by matching to the carbon atoms on the surface of the CNT. Another explanation assumes that a rowlike polymer nucleus forms first, which is much longer than the height of the subsequently homogeneously nucleating folded lamellae crystals (kebab).20 Nanofiber Shish-Kebab (NFSK). Another engineered shishkebab invented by the Li group is the electrospun nanofiber shish-kebab (NFSK).21 In the NFSK, the polymer shish is replaced by an electrospun nanofiber and the kebab crystallization is induced subsequently, similarly to the NHSK formation.12,13 Electrospinning is a versatile technique for the creation of next-generation textiles which can be applied for e.g. high performance filters22 or biocompatible scaffolds.23 Although it is possible to control the deposition of electrospun nanofibers, e.g. fiber alignment24 and “direct writing”,25 it is not possible to weave the fibers during electrospinning. Thus, the NFSK might be a means to achieve better mechanical interconnection between different layers. A precise control of the mechanical properties of the electrospun fiber mat is very important for one of electrospinning’s main applicationsthe biomedical field.24 In addition, for the filter application the NFSK will even increase the surface area of the mat. Futhermore, the NFSK offers additional tailorability because the polymer of the nanofiber can in principle be different from the shish.26 Creation of a Self-Similar Hierarchy Based on the Shish-Kebab Morphology. Here, as a proof-of-principle we aimed to create a self-similar two-level hierarchical structure based on the shish-kebab morphology: The first level consists of NHSKs. These are embedded within the top level, the NFSK. In Figure 1 a schematic drawing of the devised two-level hierarchical nanocomposite is shown. The corresponding legend is shown in Table 1. Only recently was the superiority in mechanical properties demonstrated, which nanocomposites

Figure 1. Schematic of the shish-kebab nanocomposite featuring a selfsimilar two-level hierarchy. Around the multiwalled carbon nanotube (MWCNT) in the electrospun MWCNT/polymer nanofibers a nanohybrid shish-kebab (NHSK) forms (see inset). The top-level of the shish-kebab is created on the electrospun fiber by postspinning solvent based crystallization. The structure is called nanofiber shishkebab (NFSK). For a detailed legend see Table 1.

Table 1. Legend of Schematic Depictions Used Throughout the Text for the MWCNT/Poly(ε-caprolactone) (PCL) System

containing NHSK have over those containing only CNTs.15 NHSKs forming directly during fiber formation have already been reported,27,28 but the formation of NHSKs during electrospinning has not been reported so far. Since a precompounding creation of NHSKs is very time-consuming and, thus, costly, the NHSK formation during electrospinning can be considered an important progress. However, none of the studies on electrospun CNT/semicrystalline polymer nanofibers report the formation of the NHSK.29−31 PCL is used throughout this paper, i.e., for the NHSK as well as for the NFSK, because the NHSK should be obtained in situ during electrospinning and PCL is a zigzag forming polymer which can easily be electrospun to form small nanofibers. As mentioned above, a PCL NHSK structure has neither been reported yet. In conclusion, we test the hypothesis that it is possible to obtain a PCL NHSK in situ during electrospinning and to subsequently create a two-level hierarchical structure, i.e., NHSKs within NFSKs.

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MATERIALS AND METHODS

Materials. PCL with a mass average molar mass of Mw = 65 kg mol−1 and a molar mass dispersity Đ of 1.5 (both manufacturer specification), chloroform, N,N-dimethylformamide, and pentyl acetate were purchased from Sigma-Aldrich (Sigma-Aldrich Chemie GmbH, Schnelldorf, Germany). The chemical vapor deposition produced multiwalled carbon nanotubes (MWCNTs) named Baytubes were obtained from Bayer Technology Services Ltd. According to the manufacturer, they feature an average outer diameter of 13−16 nm and a length of 1−10 μm. All materials were used as received. B


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Macromolecules Electrospinning MWCNT/PCL Nanofibers. For the electrospinning of PCL fibers, a previously reported method was used.32 In brief, 10 wt % PCL was dissolved under stirring in a solution of 90 wt % chloroform and 10 wt % N,N-dimethylformamide and was subsequently electrospun with a self-made electrospinner. To electrospin the MWCNT/PCL nanocomposite fibers the MWCNTs were first dispersed in a 90/10 (wt %/wt %) mixture of chloroform and N,N-dimethylformamide by ultrasonication (ultrasonic cleaner USC 300TK, 45 kHz, 80 W sonic power, VWR International bvba, Leuven, Belgium) for 30 min. Immediately after ultrasonication, the MWCNT solution was added to a similar stock solution of PCL to finally yield a 10 wt % PCL solution in a 90/10 (wt %/wt %) mixture of choloroform and N,N-dimethylformamide with a resulting MWCNT content per PCL of 3 wt %. This MWCNT/PCL solution was ultrasonicated for another 30 min. Subsequently, the solution was filled into a syringe and electrospun using a blunt-ended 20 G needle. The distance to the target was 10 cm, the applied high voltage was 18 kV, and a flow rate of 100 μL min−1 was used for electrospinning. Depending on the planned characterization method, the solution was electrospun directly either on aluminum foil or on a TEM copper grid mounted on top of an aluminum foil. Thermal Characterization. To determine an integral effect of the MWCNTs on the electrospun PCL nanofibers, the PCL nanofibers and the MWCNT/PCL nanofibers were analyzed by differential scanning calorimetry (DSC) (NETZSCH DSC 204 F1 instrument, NETZSCH GmbH, Selb, Germany). Approximately 5 mg of the respective electrospun mat was heated to 150 °C at a rate of 10 °C min−1 under a constant nitrogen flow, held at the final temperature, and cooled down to room temperature with the same rate again. The cycle was run twice to provide information about the sample immediately after electrospinning and subsequently without the effect of the processing conditions/thermal history. The melting and crystallization point Tm and Tc were determined at the peak temperature of the respective curves. Microscopy. Scanning Electron Microscopy. To observe the nanofiber geometry, the fibers were electrospun (see above) on an aluminum foil, eventually postprocessed, and coated with a layer of gold prior to transferring it to the scanning electron microscope (SEM, Leica LEO 440i, Leica, Bensheim, Germany). The SEM was operated at 20 kV, and the standard Everhart−Thornley-type secondary electron detector was used. Fiber Diameter Distribution. ImageJ (1.48t)33 was used to manually measure the thickness of the electrospun fibers imaged by SEM at suitable magnification. To generate the fiber diameter distribution, a total of n = 50 different fiber diameters were measured. Transmission Electron Microscopy. Transmission electron microscopy (TEM) was used to investigate the inner structure of the electrospun nanofibers. For the TEM, the nanofibers were electrospun (see above) directly on a TEM copper grid and mounted on a singletilt TEM holder. The specimen were imaged at 300 kV with a JEOL 3010 (JEOL Ltd., Tokyo, Japan), and bright field images as well as selected area electron diffraction (SAED) patterns were recorded. The latter reveal the crystalline structure and orientation within the electrospun MWCNT/PCL fibers. To successfully record the SAED pattern, a very low beam current and thus electron dose were used. The SAED patterns were taken before the bright field images to reduce the beam damage encountered before recording the SAED pattern. Creation of the Hierarchical Shish-Kebab Morphology. To create level II of the self-similar hierarchy, the MWCNT/PCL nanofibers were decorated with PCL lamellar crystals; i.e., the NFSK was created. The NFSK was created according to a recently reported method.21 Briefly, the electrospun MWCNT/PCL nanofiber mat was immersed in a supersaturated 1 wt % PCL/pentyl acetate solution for 30 min, subsequently withdrawn, and rinsed with fresh pentyl acetate. Pentyl acetate did not dissolve PCL at room temperature but did so at elevated temperatures,b yet PCL did only precipitate visually from the solution after several hours at room temperature. Therefore, prior to immersion of the electrospun nanofiber mat, the PCL/pentyl acetate solution was heated to 60 °C under thorough stirring to fully

dissolve the PCL. Afterward, the PCL solution was cooled to room temperature through cooling the container in running tap water to yield the supersaturated PCL solution. The overall experimental setup for the creation of the self-similar two-level hierarchical structure based on the shish-kebab morphology is shown in Figure 2.

Figure 2. Schematical depiction of the hierarchical material creation process. MWCNT/PCL nanofibers are electrospun. The NHSK forms directly during electrospinning (level I). The MWCNT/PCL nanofibers are subsequently incubated in PCL/pentyl acetate solution to give the NFSK (level II).

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RESULTS AND DISCUSSION The overall aim of this study was to create a self-similar twolevel hierarchical structure based on the shish-kebab morphology. Therefore, the PCL NHSK should be created inside the PCL NFSK. To achieve this goal, MWCNT/PCL nanofibers were electrospun and characterized by DSC and SEM. Subsequently, the existence of the NHSK morphology inside the e-spun fiber was confirmed by TEM. In a last step the NFSK was created by growing PCL kebabs on the electrospun MWCNT/PCL nanofibers which was confirmed by SEM. The section ends with a simple model of the NHSK and NFSK formation mechanism. Thermal Characteristics. DSC scans of the electrospun PCL and MWCNT/PCL fiber mats were performed to investigate the change in the melting and crystallization temperature upon adding MWCNTs to the PCL fibers. Changes in the crystal characteristics may reflect a distinct interaction between the PCL and the MWCNTsa prerequisite for the formation of the NHSK. The DSC heating and cooling scans are depicted in Figure 3. The first DSC heating run still contained information about the crystals which formed during the electrospinning process, i.e., crystals which crystallized under shear6 and simultaneous solvent evaporation.34 One can observe that the pristine PCL nanofibers showed a broader endotherm peak with a higher peak melting temperature Tm of 61.1 °C than the MWCNT/PCL nanofibers, which showed a Tm of 60.4 °C. The nonequilibrium conditions during electrospinning probably led to the formation of a large number of differently sized nuclei which evolved to crystals of varying thermal stability. We suggest that the higher melting fraction of crystals consists of row crystals which appear frequently during electrospinning.35 Since the MWCNTs may act as heterogeneous nuclei for chain folded crystals,12,13 fewer row crystals should be present in the MWCNT/PCL sample. This rationalizes why the first run of the MWCNT/PCL sample is rather identical to the quiescently crystallized MWCNT/PCL as measured on the second run. C


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Figure 4. Scanning electron microscopy (SEM) image of electrospun MWCNT/PCL nanofibers. The inset shows the fibers’ diameter distribution.

investigations, MWCNTs were not found in these ultrathin nanofibers. Infrequently, fibers of considerably larger diameter than the mean were observed: fibers of approximately 1 μm. A bimodal fiber distribution is not uncommon in electrospinning.32 However, we think that the addition of MWCNTs may have added to the bimodal fiber diameter distribution. MWCNT agglomerates were observed to temporarily clog the orifice until the pressure increased sufficiently leading to lessstable spinning conditions. MWCNT agglomerates manifested themselves as thicker beads in the electrospun fiber mat. These beads pulled thicker fibers to the target, and hence, a pronounced bimodal fiber distribution was observed. Similar behavior and fiber morphology of MWCNT/polymer composite nanofibers have already been reported previously,29 but without a discussion of their origin. Here, the spread in fiber diameter is appreciated, as it allows to select those fibers suitable for TEM investigations directly on the TEM grid. It will depend on the specific application of the presented NHSK reinforced PCL fibers if further optimization of the electrospinning process toward a more monomodal fiber diameter distribution would be beneficial. One approach would be to tailor the degree of MWCNT dispersion in the polymer by covalent or non-covalent functionalization. The authors intentionally refrained from using chemically modified MWCNTs or low molar mass detergents because it was anticipated that MWCNTs dispersed and thereby may show a reduced possibility to participate in the formation of NHSKs. In our experience the used MWCNTs which are up to 10 μm in length form agglomerated nests which are hard to disentangle in a usual ultrasonication bath.37 Therefore, in future studies the use of shorter MWCNTs may be useful to improve the MWCNT dispersion in the spinning solution. Inner Structure of the Nanofibers. For individual nanofiber characterization TEM was used. TEM images of the electrospun MWCNT/PCL nanofibers are shown in Figure 5. In Figure 5a, a TEM image of two MWCNT/PCL nanofibers is shown which locally contain no MWCNTs. The nanofibers had a smooth surface and were straight over dimensions which are usually useful to observe with the TEM (several micrometers). The two nanofibers in the image lay on top of each other with no visible feature of nanofiber coalescence. The inset in Figure 5a shows the SAED pattern of the two nanofibers. The widest aperture was used, and the selected

Figure 3. Differential scanning calorimetry (DSC) heating (top) and cooling (bottom) curves for the MWCNT/PCL electrospun fibers. Tm and Tc are the peak melting and cooling temperature, respectively. As can be deduced from the increase in Tc (bottom), MWCNTs are good nucleation agents for PCL, which means that PCL and MWCNT interact strongly which is a prerequisite for the formation of the NHSK. The second cooling run is omitted because it is identical to the first one. For the first heating run the curves were offset by 5 W g−1.

For the cooling runs (Figure 3, bottom), only the first run is displayed because it was identical to the second run for both PCL and MWCNT/PCL samples. The peak crystallization temperature Tc of 23.6 °C for the pristine electrospun PCL fibers was increased by over 7 to 30.9 °C upon addition of the MWCNTs. As anticipated, the MWCNTs acted as heterogeneous nuclei for crystallization, which reduced the degree of undercooling necessary to observe crystallization at the given cooling rates of 10 °C min−1. The increase of Tc upon addition of MWCNTs has been observed for a number of different semicrystalline polymers.36,37 A comprehensive study on the crystallization of PCL in MWCNT/PCL bulk composites has recently been published by Wurm et al.38 which underlined the strong nucleation ability of MWCNTs on PCL. Morphology of Electrospun Fibers. The surface morphology of the MWCNT/PCL nanofibers was evaluated by SEM. The inner structure of the MWCNT/PCL nanofiber was investigated by TEM to investigate if the composite nanofibers were created successfully. As a guide to the reader, all micrographs in this paper feature a small bar on the top left corner displaying a scheme of the depicted structure (see Table 1 for reference). This info bar is marked by the boxed i symbol. Nanofiber Surface. An SEM image of the electrospun MWCNT/PCL nanofiber mat is depicted in Figure 4. The mean nanofiber diameter and its standard deviation were approximately 110 ± 100 nm. The inset in Figure 4 shows the fiber diameter distribution. The thinnest nanofibers had diameters in the single-digit nanometer range and are thus among the thinnest reported so far.39 According to the TEM D


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As visible in Figure 5b, under standard imaging conditions (mass thickness contrast) in the TEM, individual PCL crystals in the nanofiber or around the MWCNT were not visible. When one emphasized the diffraction contrast, e.g., by lowering the beam current, imaging a low-index zone axis, and selecting a small objective aperture (see Figure 6a), we found that it is

Figure 5. Transmission electron microscopy (TEM) images of electrospun MWCNT/PCL nanofibers.

diffraction area corresponds approximately to the whole displayed image. A typical polymer fiber diffraction pattern was visible which usually shows reflexes on the meridional and equator (parallel and perpendicular to the fiber direction) planes. Here, the equatorial (110) reflex was the main reflex observed. This is in agreement with the calculation of the PCL structure factor40 which predicts that the (110) reflex has the highest intensity. The reflex is relatively sharp which means that the polymer crystallites in the nanofiber were mostly aligned with the c-axis of the unit cell (c-axis is parallel to the polymer chain’s zigzag backbone) in nanofiber direction. This confirms a previously reported SAED pattern of electrospun PCL very well.41 Even the nanofiber as small as 30 nm (labeled 2, Figure 5a) in diameter gave an electron diffraction pattern which indicates that these nanofibers are highly crystalline, too. A high degree of crystallization and crystal orientation is common for electrospun semicrystalline polymer fibers.30,42 As has been recently shown, electrospun semicrystalline fibers often show a very high content of row crystals35 due to the high shear rates and the elongational flow present. This is consistent with the observed SAED pattern. A SAED reflex of the MWCNT was not visible in the SAED pattern. This observation is consistent with the findings of Salalha et al.42 We suggest that the low exposure times for the SAED pattern of approximately 1 s which were used to clearly capture the PCL reflexes before they broadened to an amorphous halo led to the nonvisibility of the MWCNT reflexes: in comparison to the fiber, the diffraction volume of the MWCNT was only roughly 1%, as the MWCNT radius was 10% of the fiber’s. In Figure 5b, a MWCNT/PCL nanofiber with a MWCNT inside is shown. The MWCNT was highly aligned along the MWCNT/PCL nanofiber axis. This is in line with the wellestablished theory of Jeffrey43 for the general case of orientation of ellipsoidal particles and specifically with the results of Dror et al.,30 who reported that CNTs can be well aligned by the electrospinning process. As visible in the inset, the fiber was still crystalline and showed pronounced crystal orientation. Only a minor broadening in the PCL crystal orientation was observed. In conclusion, the fundamental requirement for this current study, to obtain a crystalline nanofiber with aligned MWCNTs within, has been achieved. Level I Shish-Kebab: The NHSK. In this section evidence for the formation of a MWCNT shish-kebab, termed NHSK, which is a MWCNT (shish) overgrown by lamellae polymer crystals (kebabs), in the PCL system is presented.

Figure 6. NHSK visualized in the TEM by diffraction contrast. The inset in (c) and (d) shows a sketch of the respective NHSK: The kebabs are formed by folded lamellae crystals. The reader is referred to the digital version of this figure for magnified observation.

possible to reveal the NHSK in the TEM: This can be seen in the image sequence of Figures 6b−d, which provides evidence for the NHSK inside a MWCNT/PCL nanofiber. To render the small PCL crystals forming the NHSK together with the MWCNT visible, they have to be oriented suitably (low-index zone axis) and similarly. Fortunately, the individual PCL crystals are linked in orientation by the MWCNT, which is usually straight over 100 nm and thereby introduces long-range order between individual PCL crystals. Thus, if one of the PCL crystals of the NHSK was aligned correctly, a sufficient number of neighboring NHSK PCL crystals were, too. In contrast, this long-range order was absent for the oriented but not perfectly adjusted crystals of the polymer nanofiber matrix,c and thus, the nanofiber crystals in the background showed less diffraction contrast and only gave a uniform gray background.44 Therefore, PCL NHSK crystals can be distinguished from the PCL crystals in the fiber, although the PCL crystal orientation in the shish is principally congruent to the one of the PCL in the regular fiber (c-axis in fiber direction). Figure 6b shows an overview of a MWCNT/PCL nanofiber imaged with emphasis on bright field diffraction contrast. A high number of MWCNTs were presentmost easily discernible at the rim of this nanofiber. In addition, diffraction contrast patterns can be observed in Figure 6b. The diffraction contrast patterns are depicted as close-ups in Figures 6c,d. E


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in the strain field during electrospinning. The strain field may be locally changed due to the disentanglement process of multiple MWCNTs during the electrospinning process.37 As long as the fundamental formation mechanism of the NHSK remains unknown, it will limit the applicability of this promising nanostructure. We feel that the myriad of different CNTs with their different surface properties which are mostly used in parallel in virtually all of the reported studies on the NHSK hamper a clear insight into the formation mechanism. Unless, from a technological point of view, it may be beneficial that the NHSK formation is not overselective, without distinguishing between different CNTs, it will remain largely unknown how the NHSK formation mechanism can adapt to the different boundary conditionsmay it be the same, similar, or completely different for different CNTs. Nevertheless, we will present a simple model of the NHSK formation during electrospinning. Although the in situ formation of an NHSK during electrospinning offers easy processability, it can also be of interest to electrospin NHSK blends, i.e., a system where the NHSK forming polymer is different from the host material of the electrospun fiber. One example would be to electrospin PCL which contains preassembled poly(ethylene) kebab covered CNTs. Level II Shish-Kebab: The NFSK. In the previous section the existence of the level I shish-kebab morphology has been confirmed; i.e., the NHSK morphology was observed inside the MWCNT/PCL nanofibers. This section presents the data on the formation of the level II shish-kebab; i.e., the NFSK. Both levels form a hierarchical structure. As shown in Figure 2, the as-spun MWCNT/PCL nanofibers (see Figure 4) which contain the level I shish-kebabs were immersed in supersaturated 1 wt % PCL/pentyl acetate solution. Figure 7 shows SEM images at different magnifica-

In Figure 6c, an NHSK-like morphology is visible, consisting of a MWCNT reaching from the upper right to the lower left (marked by arrows) and two prominent black domains approximately 30 nm apart. The black domains are attributed to highly ordered crystalline PCL regions. The crystalline regions are folded lamellae crystals stacked on the MWCNT. The gray parts in between the dark domains were either less crystalline or less highly ordered and, thus, diffracted fewer electrons. One may note that the lower black domain seems to consist of more than one black domain. The smaller, yet highly periodic diffraction contrast pattern running in parallel to the 30 nm one is depicted in Figure 6d. Again, a MWCNT was centered below the periodic diffraction pattern consisting of small black domains. The dimensions of the MWCNT are smaller than presented before; however, it is also known that CNTs produced by the chemical vapor deposition method are not uniform in structure. Here as well, the black domains are thought to be lamellae crystals. The mean distance between the lamellae crystals in this case is approximately 5 nm. This can be considered a low value, but similar single digit thicknesses have been found for other nanoconfined PCL crystallization conditions.45 Additional small-angle X-ray scattering measurements of the crystal thicknesses were not feasible because the electrospinning setup does not allow us to produce highly oriented fibers in its present form. The origin of the small crystal thickness may lie in the crystal growth kinetics. Electrospinning is a very fast process (jet velocities on the order of 10 m/s in 10 cm target distance are common);37 therefore, the time frame for crystal growth is probably very small. We believe that the 30 nm blocks that can be seen in Figure 6c consisted of the same small units as in Figure 6d but cannot be as clearly resolved because they are more in the center of the fiber, and thus electrons are increasingly randomly diffracted before they can be detected in the TEM. An alternative explanation for the origin of the black domains may be the observation of moiré pattern of graphite/graphene ribbon impurities.46 However, it is evident that the patterns only occur perpendicular to the drawing direction over the whole length of about a micrometer (see Figure 6b). For the moiré pattern this would require a very special arrangement of all of these ribbons. We believe this is very unlikely and are therefore convinced that the black patterns are due to PCL crystals. Normally, a polymer only shish-kebab features gap distances starting in the lower third of the double-digit nanometer range.47 Yet Zhang et al.20 observed NHSK formation with kebabs just 5 nm thick. Therefore, we propose that our periodic lamellae (see Figure 6d) are also part of a NHSK. Besides the notion that MWCNTs can act as a very effective heterogeneous nucleation agent for PCL,38 the NHSK structure in the CNT/ PCL system has not yet been reported until today (see Figure 6). Here, evidence for the existence of the NHSK morphology in the MWCNT/PCL system was presented. This adds to the idea that zigzag forming polymers can readily form the NHSK. Nevertheless, during the electrospinning process a high shear rate is already introduced which can greatly assist in the formation of the shish-kebab morphology.7 The author’s efforts to grow NHSKs in the PCL system in a quiescent solution based process have only had limited success so far. The different sizes of the NHSKs can be attributed to differences in the MWCNT surface properties (diameter, chirality, defect density) and/or to local differences

Figure 7. SEM images of the NFSKs formed after incubating the MWCNT/PCL nanofibers in supersaturated 1 wt % PCL/ pentlyacetate solution for 30 min. (a)−(c) showing different magnifications. The shish-kebab morphology is clearly visible. F


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Macromolecules

may allow the stretched polymer chains to relax as the shear rate decreases further down the spinning path. Then the high nucleation ability of the MWCNTs (see DSC section) leads to the formation of chain-folded crystals on the MWCNT resulting in the NHSK. It is likely that competition between the crystal growth of row nuclei nucleated crystals formed in the sheat of the nanofiber and folded lamellae crystals as nucleated on the MWCNT occurs. Therefore, in nanofibers without MWCNT is it likely to observe mainly row crystals, which is in agreement with the recent literature.35 NFSK Formation during Incubation. The facile formation of the NHSK on the surface of the nanofiber is a direct consequence of the high content of row crystals in the nanofiber surface35 (see Figure 9). When incubated in a

tions after incubation. The overview in Figure 7a shows the typical appearance of a shish-kebab morphology of etched bulk samples, where parallel crystal patterns along one direction appear. The average distance between the kebabs is approximately 2 μm here. The arrows in the figure exemplary indicate the direction of the underlying nanofiber exemplarily. The area surrounded by the white rectangle is magnified and displayed in Figure 7b. Here, it can be seen that the individual kebabs already started to coalesce. This could not only lead to a mechanical reinforcement by interlocking of the kebabs but also by the additional binding of the coalescent crystals. The area surrounded by the white dashed rectangle was magnified a second time. The second close-up is shown in Figure 7c. The NFSK structure is clearly visible on this micrograph. The large PCL kebab, formed perpendicular to the PCL nanofiber, had a diameter of approximately 4 μm. As anticipated, the NFSK growth is not affected by the addition of MWCNTs to the nanofibers and the results are similar to those of Wang et al.,21 who used a polymer only system to form an NFSK. Again as the NFSK is rather generic in nature, it may also offer the opportunity to create a hierarchical structure with different polymer combinations.26 The presented self-similar hierarchical structure with two levels is a proof-of-principle and could be extended to higher orders, e.g., by aligned deposition of the electrospun nanofibers24 prior to NFSK formation, which are subsequently combined to thicker threads which might then be decorated by an 3D printing technique with polymer to give the next self-similar level. Model. NHSK Formation during Electrospinning. Figure 8 shows a simple model which can explain the formation of different sized NHSKs during electrospinning.

Figure 9. Model of the NHSK formation process. The level II kebabs can nucleate directly on the row crystals present on the surface of the electrospun nanofiber.

supersaturated PCL solution which contains relaxed PCL chains, folded lamellae crystals can readily grow epitactically on the surface of the row crystals. This process mimics the formation of the shish-kebab morphology in a stepwise fashion.

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CONCLUSION Here we presented the creation of a self-similar hierarchical structure based on the shish-kebab morphology. To this end, the direct formation of nanohybrid shish-kebabs during electrospinning has been successfully realized for the first time. In addition, the existence of nanohybrid shish-kebabs in a novel polymer system, i.e, with poly(ε-caprolactone), was reported. Nanofiber shish-kebabs were successfully prepared on electrospun poly(ε-caprolactone) nanofibers containing nanohybrid shish-kebabs resulting in a hierarchical structure. This work proposed and advanced a way to bridge the gap between the outstanding nanofiller carbon nanotubes and the macroscopic world in a way which will likely contribute to future high-performance materials.

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Figure 8. Model of the NHSK formation process. For details see text.

In Figure 8a, an overview of the electrospinning MWCNT/ PCL solution is presented. As depicted in Figure 8b, the MWCNTs align very efficiently in the elongational flow as the fiber thins. This behavior is predicted by the theory of Jeffrey and well understood.43 The solvent in the thinning fiber starts to evaporate via the surface of the nanofiber, and the stretched polymer chains will start to crystallize in a row-crystal fashion (not shown). The elongational flow is very effective to promote the coil−stretch transition.48 Evidence for a high fraction of row-crystals in electrospun fibers has recently been reported.35 In contrast to the sheath of the nanofiber, and as shown in Figure 8c, the core of the nanofiber is still rich in solvent which

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +49 (0)3641 947730; Fax +49 (0)3641 947731 (K.D.J.). Present Addresses

R.J.: Computational Materials Science, Institute for Applied Materials, Karlsruhe Institute of Technology, Engelbert-ArnoldStraße 4, 76131 Karlsruhe, Germany. J.A.: Department of Electrical & Computer Engineering, Duke University, Durham, NC 27708, USA. Notes

The authors declare no competing financial interest. G


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ACKNOWLEDGMENTS J.A. gratefully acknowledges financial support by the DAAD RISE program. ADDITIONAL NOTES Covalent functionalization, in contrast to physical functionalization, is accompanied by the change of sp2-hybridized carbon atoms to sp3-hybridized carbon atoms in the outer carbon layers, which greatly (often adversely) affects the CNT properties. b According to our investigation, the cloud point temperature of PCL in pentyl acetate was approximately 35 °C; data not shown. c This can be inferred form the fact that the SAED diffraction pattern in Figure 5 shows smeared reflexes instead of points. a

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In Situ Formation of Nanohybrid Shish-Kebabs during Electrospinning

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