Reverse Kebab Structure Formed inside Carbon Nanofibers via

Aug 27, 2015 - The morphology of polymers inside a confined space has raised great interest in recent years. However, polymer crystallization within a...
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Reverse Kebab Structure Formed inside Carbon Nanofibers via Nanochannel Flow Min Nie,† Dilhan M. Kalyon,‡ and Frank T. Fisher*,† †

Department of Mechanical Engineering and ‡Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States S Supporting Information *

ABSTRACT: The morphology of polymers inside a confined space has raised great interest in recent years. However, polymer crystallization within a one-dimensional carbon nanostructure is challenging due to the difficulty of polar solvents carrying polymers to enter a nonpolar graphitic nanotube in bulk solution at normal temperature and pressure. Here we describe a method whereby nylon-11 was crystallized and periodically distributed on the individual graphitic nanocone structure within hollow carbon nanofibers (CNF). Differential scanning calorimetry and X-ray diffraction indicate that the nylon polymer is in the crystalline phase. A mechanism is suggested for the initiation of nanochannel flow in a bulk solvent as a prerequisite condition to achieve interior polymer crystallization. Selective etching of polymer crystals on the outer wall of CNF indicates that both surface tension and viscosity affect the flow within the CNF. This approach provides an opportunity for the interior functionalization of carbon nanotubes and nanofibers for applications in the biomedical, energy, and related fields.

1. INTRODUCTION In recent years, several methods have been demonstrated to deliver different nanomaterials into graphitic hollow nanostructures (carbon nanotubes and nanofibers) for various applications. For example, Pd nanoparticles carried in water were driven by capillary force to enter carbon nanotubes (CNTs) which were pretreated with acid for increased hydrophilicity.1 Cobalt nanoparticles were fabricated inside single-walled CNTs from the reduction of a metal precursor carried in solution which entered the CNTs via sonication.2 Supercritical fluids were used to deliver gold nanoparticles into carbon nanofibers (CNFs) for catalysis applications.3 On the other hand, the functionalization of the outer surface of carbon nanotubes is a well-studied area,4−10 where nanoparticles, small molecules, or polymer chains have been attached to the outer surface by physical adsorption or covalent bonding methods. The functionalization of CNTs has shown distinct advantages in applications such as catalysis, sensing, nanocomposites, and nanomedicine. Recently, Li et al. found that certain polymers were able to be crystallized on CNTs to form periodic disc-shape polymer nanocrystals, which is referred to as a nanohybrid shish kebab (NHSK) structure.11−15 On the basis of this work, the NHSK structure has been previously studied in our group as a means to enhance interfacial load transfer in polymer nanocomposites.16,17 The mechanism of formation of the NHSK structure is attributed to the soft epitaxy12 of polymers where the epitaxial growth is sizedependent as polymer chains were confined to align along the long axis on the small-diameter CNTs regardless of strict lattice matching. While such a structure shows that the graphitic © 2015 American Chemical Society

surface is a favorable substrate for polymer crystallization, it inspires a question as to whether a “reverse kebab” structure, where polymer crystals are formed on the inner concave wall of a hollow carbon nanostructure, can be achieved. For confined crystallization within nanochannels such as anodic aluminum oxide (AAO), the literature18−23 shows that a polymer melt was able to infiltrate the nanochannel to crystallize into the shape of nanorods or nanotubes. Polymer crystallization within a confined space is of interest as the variation of size, composition, and crystallization conditions can influence the morphology and the properties of materials on the nanoscale. Forming nanocrystals on the inner wall of graphitic nanotubes in bulk solution requires that solvents carrying polymer solutes are able to enter the graphitic nanotube at normal temperature and pressure. It is known that the graphitic carbon structure is nonpolar and that a concave graphite surface with high curvature has even further reduced polarizability which can cause small-diameter CNTs to be nonwetting by other materials.24 On the other hand, most commonly used solvents for polymers generally exhibit high polarity and trap air from the two ends of the nanotube, which hinders the initiation of nanofluidic flow within the nanotubes. In recent years, gases or water flowing through the CNTs has been successfully demonstrated, as driven by pressure or electrophorectic forces when the CNTs are aligned to form a unidirectional nanofluidic Received: June 3, 2015 Revised: July 31, 2015 Published: August 27, 2015 10047

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Langmuir system to transport DNA, dye molecules, or nanoparticles.25−27 Observations with optical microscopy, fluorescence microscopy, fluorescence correlation spectroscopy, and UV−vis spectroscopy were used to illustrate the transport process. However, the observation of a solvent with dissolved polymer entering graphitic nanotubes arbitrarily distributed in a bulk liquid is still lacking. As a result, a direct approach to crystallizing polymers on the inner wall of graphitic nanotubes in the bulk solution remains unexplored. In the current work, we report that under specific experimental conditions a polymer (nylon-11) with a polar solvent is able to enter graphitic carbon nanofibers (CNFs) in a bulk solution and that the graphitic stacked nanocone structure28,29 of the inner wall can act as a periodic substrate to facilitate the formation of polymer nanocrystals (Figure 1).

GC) was used as the solvent for dissolving nylon-11 and dispersing the carbon nanofibers. Phosphoric acid (85%) was purchased from Fisher Scientific and heated to remove water before use. 2.2. Crystallization and Etching. The major steps in fabricating the polymer crystal inside the CNF are as follows. Nylon-11 was dissolved in BDO at 165 °C to form a 0.02 wt % solution. Carbon nanofibers were dispersed in BDO to form a 0.02 wt % suspension by first heating the mixture for 10 min at 100 °C to decrease the viscosity of the solvent and then performing sonication for 20 min using a tip sonicator (Misonix XL2020 sonicator with a microtip probe). The dispersed CNFs and the dissolved polymer were mixed in a 4:1 weight ratio between nylon and the CNF and annealed at 147 °C under quiescent conditions for 1 h to form polymer nanocrystals on and within the CNF. The polymer-decorated CNFs were filtered isothermally and rinsed with acetone. By changing the concentration of the polymer solution, additional experiments were also carried out with ratios of 1:1, 2:1, and 8:1 between the polymer and CNF. Each experiment was performed at least three times. The discussion below is based on the 4:1 sample unless otherwise specified. For comparison, a similar process was used to form polyethylene crystals within the CNF as described in the Supporting Information. In addition, an etching process was developed using phosphoric acid to selectively remove the polymer crystals and observe how the viscosity affects the fluid flow in CNF. In this procedure, to increase the viscosity of the etchant, the phosphoric acid was preheated at 150 °C for 60 min to remove water and then cooled to the etching temperature (35 or 60 °C). A very thin layer of nylon/CNF hybrid was placed on glass fiber filter paper by filtering the sample suspension in acetone, and then 2 mL of acid was added dropwise to the sample surface and removed by vacuum filtration. The vacuum pressure was about 200−300 Torr, under which the acid dried very quickly. The etched sample was rinsed with distilled water and dried by vacuum. 2.3. Sample Characterization. The obtained nanostructures were redispersed with acetone and dried on a silicon substrate or a lacey film copper grid for SEM or TEM characterization. The samples were imaged using an Auriga field emission SEM (Carl Zeiss NTS GmbH, Germany) with an acceleration voltage of 1 kV and an FEI CM20 TEM with an acceleration voltage of 200 kV. It is noted that although the electron beam can affect the polymer crystal, which causes the detailed crystal structure to be unresolvable, during a certain exposure time the electron beam does not alter the shape of the polymer. Therefore, TEM can be used to prove the existence of the polymer. Melting and crystallization behaviors of the aggregates of nyloncoated CNF were tested with a DSC Q100 (TA Instruments). The samples were heated from 25 to 230 °C at 10 °C/min, held isothermally for 1 min, and then cooled at 10 °C/min to 25 °C under a nitrogen environment with a gas flow rate of 50 mL/min. In addition, to selectively melt the lower-melting-point crystal, the sample was also heated at 10 °C/min to 199 °C, held isothermally for 1 min, and then cooled at 10 °C/min to 25 °C. Samples were also characterized using an X-ray diffractometer (XRD, Bruker D5000) with Cu Kα radiation (λ = 1.5418 Å). To prepare these samples, after the aforementioned annealing process, the nylon/CNF hybrid material was collected on glass fiber filter paper by vacuum filtration. The background from glass fiber filter paper was deducted. Also characterized by XRD for comparison was a bulk nylon-11 polymer sample prepared by compression molding at 550 psi and 200 °C, after which the sample was slowly cooled to room temperature.

Figure 1. (a) Schematic of the nylon/CNT NHSK structure.17 (b) Schematic of the periodic distribution of individual polymer crystals on the inner wall of the CNF.

The cavitation effect from sonication and convection during heating were found to be the major driving forces for the nanochannel flow of the solvent and polymer, which is necessary for the growth of polymer crystals inside a tubelike carbon nanostructure. Because this approach does not require alignment techniques or the complex design of nanofluidic systems, it allows a facile fabrication procedure for the interior functionalization of graphitic nanotubes with polymer nanocrystals. The interior functionalization of the graphitic nanotube has significant potential applications in nanoscale chemical reactions,30 filtering,27 and chromatography. With regards to the latter, Singhal et al. demonstrated individual CNTs with inner diameters from 60 to 190 nm as a liquid chromatography column, with smaller multiwalled CNTs inside functionalized with iron oxide acting as the stationary phase to improve separation.31 On the other hand, nylon has also been used as a stationary phase to enhance protein separation in macroscale chromatography.32 Such results suggest that nylon nanocrystals coated on the inner wall of a CNF may act as the stationary phase in a nanoscale chromatography column for protein separation. Additional applications for the CNF with a reverse kebab structure such as superhydrophobic nanochannels which could be used for single water droplet transport33 or electrohydrodynamic spraying devices34 are also envisioned.

3. RESULTS AND DISCUSSION 3.1. Morphology. On the basis of earlier work in generating the NHSK structure,17 nylon-11 and BDO are used here as the polymer and solvent in the crystallization experiments, respectively. The BDO solvent is a glycol with a polar molecular structure and a high dielectric constant. Sonication in BDO and the annealing of the mixture of CNF and nylon solution were found to be two important conditions

2. EXPERIMENTAL SECTION 2.1. Materials. Carbon nanofibers (Pyrograf-III, PR-19-XT-PS) were purchased from Applied Sciences, Inc. Nylon-11 (Polyamide 11, molecular weight 18 000) pellets and poly(methyl methacrylate) (PMMA, molecular weight 540k) were purchased from Scientific Polymer Products, Inc. 1,4-Butandiol (BDO) (Fluka, purity >98.8% by 10048

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Figure 2. (a) SEM image of the CNF outer wall decorated with a quasi-random orientation of polymer crystals. (b) Schematic of the quasi-random orientation of polymer crystals on the outer wall of CNF.

Figure 3. TEM images of (a) as-received CNF and (b) CNF with inner polymer crystals. (c) As-received CNF with a smooth, cylindrical inner wall structure. (d) The inside kebabs formed in this type of CNF nanostructure. The lines in the CNF background perpendicular to the long axis are the rims of the cropped nanocones projected from the upper and bottom halves of the CNF onto the central plane.29

necessary to enable solvent flow through the nanochannel and crystal growth within the CNF. Scanning electron microscopy of the samples shows that the outer wall of the CNF is covered with polymer nanocrystals oriented in different directions (Figure 2). This is similar to what was observed in Li’s work of polyethylene crystals on the outer wall of CNF,12 where it was explained that the orientation of crystal on the outer wall of the CNF is affected by lattice matching whereas on a carbon nanotube this effect is limited by the small-diameter substrate. This suggests that polymer

crystallization is affected by the curvature of the substrate on the nanoscale. Of particular interest here, a comparison of transmission electron microscopy (TEM) images of an as-received CNF (Figure 3a) with a polymer-decorated CNF (Figure 3b) shows that for the latter there appears to be additional material with a triangular cross section periodically distributed on the inner walls of the CNFs, forming a hierarchical structure. The lighter contrast of this inner structure compared to the CNF wall indicates that this material has a lower average atom density than pure carbon, considering the different overall thicknesses 10049

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Langmuir between the two regions when the electron beam passes through the sample. Since the solvent residue was removed after the annealing process, this suggests that nylon is the only possible material which could remain within the CNF and be deposited on the inner wall, as is confirmed later in DSC experiments. Under the same annealing conditions in the dilute solution, the phase structure of the nylon polymer on the inner graphitic wall of the CNF is expected to be the same as that on the outer graphitic wall. As discussed later, DSC and XRD indicate that the nylon polymer on the CNF, including the inner polymer, is in the crystalline phase. Each inner polymer crystal is distributed on an individual cone surface of the CNF, indicating a template effect of the naturally grown inner structure of the CNF. For a quasiisotropic substrate (the outer wall of the CNF), there is less preference for the polymer chain to align, which results in crystal growth in different orientations. However, for the inner wall with higher curvature, the polymer chains are able to align only in the longitudinal direction of the CNF. In addition, on the stacked nanocone structure of the CNFs, large curvature at the step edges of the conical platelets prevents the polymer chains from extending beyond the platelets. Thus, the size of the polymer crystal is in general limited to the exposed surface of each cone, resulting in the periodic distribution of individual polymer crystals (Figure 3b). In some cases CNFs were observed to have a smoother, cylindrical CNT inner wall (shown in Figure 3c) in which the interior polymer crystals were also found (Figure 3d). This indicates that the reverse kebab structure can be formed within carbon nanotubes using the same crystallization procedure. 3.2. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) was used to characterize the phase-transition behavior of polymer crystal on the outer and inner walls of the CNF. As shown in Figure 4a, the heating curves of the DSC results for bulk polymer, polymer/CNT NHSK from our previous work,17 and polymer on the outer and inner walls of the CNF all exhibit melting behaviors of the nylon polymer crystals. Similar to the crystal in the NHSK structure which shows melting points at 197 and 205 °C, the polymer crystal on/in the CNF shows melting points at 196 and 203 °C, while the melting temperature of the bulk polymer is about 191 °C. This indicates that the nylon polymer on the outer and inner walls of the CNF has formed crystal structures similar to those of NHSK, which is attributed to the fact that in both cases the presence of graphitic surfaces affects the nucleation and crystal growth process. However, the melting temperatures of the polymer crystal on the CNF are slightly lower compared to that of the polymer crystal in the NHSK structure on a CNT found in our earlier work,17 which could be due to two reasons. First, the randomly distributed polymer crystal on the outer wall of the CNF may have a smaller crystal size compared to that of the ordered kebab structure on the CNT because for the former the lateral front growth of the edge-on lamella is limited by the surrounding crystals on the substrate. Second, the size of the ring-shaped crystal in the confined space may be smaller compared to the outer kebabs on the CNT due to the fact that in the case of the lamella growing from the outer boundary toward the center of the CNF there is limited space compared to that of crystal growing outward from a center fiber. In the cooling curves shown in Figure 4b, the crystallization temperatures of polymer on the CNT and CNF are very close, which indicates that the graphitic surfaces of both the CNT and

Figure 4. Heating (a) and cooling curves (b) in DSC for nylon-11, nylon/CNT NHSK (from our previous work17), and nylon/CNF with polymer crystals on the outer and inner walls.

CNF have a similar nucleation effect on nylon crystallization independent of the curvature for the polymer melt. The fact that melting and crystallization behaviors of the nylon crystal on the concave wall of the CNF are very similar to that observed for the NHSK structure in previous work17 indicates that both crystallization processes are surface-induced crystallization. The phenomenon of the double endothermic maxima observed in Figure 4a can be caused by the fractionation of the polydispersed polymer as described in the literature.35 It is known that when polymer begins to crystallize, the highermolecular-weight components preferentially crystallize first. In the pure polymer shish kebab structure, two melting peaks were present due to the difference between the crystals of the central fiber formed by the extended chains and thin lamellae formed by the folded chains.36 Therefore, the two peaks on the nylon11/CNT NHSK sample may belong to an inner layer on the CNT and the kebabs.17 A similar phenomenon of a polymer layer on a CNT beneath the kebab structure has also been observed for the polyethylene/CNT hybrid formed under shear.37 To further examine the crystal within the CNF, we used DSC to precisely heat the sample to 199 °C to melt the lowermelting-point crystals. This step was then followed by cooling the sample. After the sample is heated to 199 °C, the inner polymer shows signs of melting, during which the polymer 10050

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Figure 5. (a) DSC heating curves of nylon/CNF samples previously melted at different temperatures. (b) TEM image taken after heating the sample to 199 °C and cooling, showing that the inner polymer has been melted and has formed a meniscus.

forms a meniscus to reduce the surface energy as shown in Figure 5b. This solid−liquid transition during the melting process indicates that the inner polymer is crystal, as the glass transition of an amorphous polymer would not generate such a sharp morphology transformation. The reheating process of the samples which were preheated to 199 °C shows a change in endothermic behavior of the lower-melting-temperature crystal (196 °C), but the higher-melting-temperature crystal at 203 °C seems not to be affected. This confirms that the inner crystal has a melting temperature of 196 °C. In the two melting peaks in Figure 4a, the 196 °C peak has a larger area than the 203 °C peak. However, the volume of the inner crystal is less than that of the outer crystal, which indicates that part of the crystal on the outer surface should contribute to the 196 °C peak. Therefore, polymer crystals on the outer wall have two melting temperatures at 196 and 203 °C. Samples were also prepared using polymer and CNF at different ratios by changing the concentration of polymer solution. As shown in Figure 6, it is observed that by increasing the polymer/CNF ratio from 1:1 to 4:1 there is an increase in the ratio between the area of the 196 and 203 °C peaks, which indicates more of the lower-melting-point crystal forms. When the ratio is increased to 8:1, the two peaks have comparable size similar to the 1:1 sample. Correspondingly, very little inside

polymer crystal was observed in the 1:1 sample via TEM, and a large amount of crystal was formed on the outer surface in the 8:1 sample while the amount of inside polymer is relatively small. This phenomenon may be due to the fact that certain concentrations of polymer allow more material to enter the CNF; however, above a certain concentration this process is limited due to the higher viscosity of the solution. After the samples are heated to 199 °C, the CNFs are slightly “glued together” and difficult to disperse into the individual CNF as the polymer on the outer wall of neighboring CNFs coalesces during the melting process. This was observed for all polymer/CNF ratios. This is consistent with the conclusion that the 196 °C peak also contains part of the crystal from the outer wall of the CNF. According to the literature,35 because the higher-molecular-weight polymer preferentially crystallizes first from solution, the thicker crystal on the outer wall can be formed by the high-molecular-weight component, which is subsequently followed by the lower-molecular-weight component overgrown on the outer wall and entering the space inside the CNF. 3.3. X-ray Diffraction. The XRD results of the nylondecorated CNF and molded bulk nylon-11 polymer are shown in Figure 7. Diffraction peaks at 2θ values of 20.2 and 23.4° (d spacings of 0.44 and 0.38 nm, respectively) belong to the (100) and (110)/(010) planes of the α phase of nylon,38 while the

Figure 6. DSC curves for melting behaviors of the nylon/CNF hybrid prepared with different ratios of nylon and CNF.

Figure 7. XRD results for CNF, nylon-decorated CNF, and bulk nylon-11 samples. 10051

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Figure 8. Effect of surface tension near the ends of the CNF in the bulk liquids. (Left) Meniscus with a large contact angle formed when polar solvent BDO entered the nonpolar graphitic CNF; without a sufficient pressure difference, air trapped within the CNF cannot be removed. (Right) Menscius with a very small contact angle formed for phosphoric acid in the nylon-coated CNF due to the chemical reaction between the acid and the nylon on the inner wall. The capillary flow through the CNF in this case depends on the viscosity of the fluid.

Figure 9. Three stages describing the bubble growth and implosion by ultrasound which allows the liquid to flow into the CNF: (1) Air bubble trapped in CNF acts as a nucleus during sonication. (2) Ultrasound irradiation vaporizes the liquid to enter and grow the bubble. (3) Bubble collapses by implosion when the outer pressure exceeds the inner pressure.

Figure 10. (a) CNF with amorphous polymer PMMA and trapped air bubbles after directly sonicating CNF in PMMA/acetone solution and evaporation of the solvent. The presence of the bubbles indicates that cavitation and bubble implosion cause liquid to flow into the CNF. (b) Sample made by mixing the polymer solution and CNF suspension after sonication separately, which results in a small amount of polymer entering the CNF.

peak at a 2θ value of 26.6° (d spacing of 0.34 nm) belongs to the (002) plane of CNF. As expected, the diffraction peaks of nylon are much smaller than the CNF peak since CNF comprises the majority of the sample. Compared to the bulk nylon sample, which has wide diffraction peaks that are overlapped due to the large amount of amorphous phase, the separated peaks of nylon polymer on CNF indicate that most of the polymer is in the crystalline phase. Due to the fact that the CNF is randomly distributed, the diffraction of the nylon crystal in the nylon/CNF hybrid sample is similar to that of polycrystals. On the basis of the Scherrer equation, τ = ((0.9 λ)/(β cos θ)), where τ is the average crystal size, λ is the wavelength of the X-rays, β is the peak width at half the maximum intensity, θ is the Bragg angle, and the average crystal sizes calculated from the (100) reflection and the (110)/(010) reflection of the nylon/CNF hybrid are 11 and 6 nm, respectively. The average crystal sizes calculated from the (100) reflection and the (110)/(010) reflection in the bulk

nylon sample are 6.1 and 5.3 nm, respectively. The larger crystal size of the polymer on the nylon/CNF hybrid in comparison to the bulk polymer indicates that the nucleation effect of CNF has facilitated the crystal growth. The crystallinity of the bulk nylon sample calculated from the crystalline peaks is about 33% after peak fitting. 3.4. Nanofluidic Mechanism. A key factor in forming the polymer crystal within the CNF is the realization of liquid flow through the CNFs in the bulk solution. BDO is a polar solvent. Although it is unable to measure the contact angle of BDO in CNF directly, we found on the carbon-coated cross section of the AAO membrane which has nanogrooves that the BDO is rolling away from and nonwetting with respect to the surface. We believe the contact angle of BDO in CNF is larger than 90°. When the BDO tries to enter the CNF from the two ends with the nonpolar inner surface, it traps an air bubble inside the tube (Figure 8a). In this case, the prerequisite condition for BDO to flow through the CNF is that the pressure difference between 10052

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Langmuir the two ends ΔP is larger than the surface tension difference 2(cos θA − cos θR)/r, where θA is the advancing angle and θR is the receding angle. This indicates that without a sufficient pressure difference the fluid will not enter the CNF. For the controlled unidirectional flow within aligned CNTs described in the literature,25−27 dc electric fields and pressure are demonstrated to be able to drive the liquid into the nanotube and transport materials through the CNTs. However, in the current experiments where the randomly distributed CNFs are immersed in a bulk liquid, there is no direct external electrical field or pressure on the liquid medium. Under our experimental conditions, only sonication and heating steps are used to achieve liquid flow through the nanochannel to form the reverse kebabs. During this process, we believe that the cavitation effect of sonication is the mechanism responsible for the initiation of the solvent flowing through the nanochannel within the bulk medium. Sonication, a process of acoustic cavitation in which air bubbles are generated, grown, and imploded, can provide an extreme microenvironment for nanomaterials in a liquid.39,40 The air initially trapped in the CNF can act as a nucleus in the bulk solvent and grow into a larger bubble as the liquid is vaporized by the irradiation of the ultrasound. When the bubble implodes (Figure 9), the surrounding solvent is driven into the cavity and subsequently enters the CNF in microseconds. This process is confirmed in a separate experiment when the CNFs are sonicated in acetone which contains a trace amount (1 ppm) of amorphous polymer poly(methyl methacrylate) (PMMA, molecular weight 540k). After one droplet of these dispersed CNFs is dried on a TEM grid by gradually evaporating the solvent at room temperature, it is observed that within certain CNFs there is an amorphous polymer phase with many tiny bubbles “frozen” inside (Figure 10a). Since evaporation occurs at room temperature, these bubbles can be produced only during the sonication process, which suggests that bubble implosion is the driving force in removing the original trapped air and carries the solvent into the CNF. Closely packed small bubbles indicate that they were formed in a very short time period trapped by the highly viscous polymer chains rather than being generated by the vaporization of the solvent from polymer solution, under which conditions a large discrete void should form instead. A higher concentration (for example, 0.001 wt %) of PMMA results in the polymer being deposited only on the outer surface of the tubes, which may be due to an exclusion effect caused by entangled chains at higher concentrations. For comparison, another experiment was performed by sonicating CNF in acetone separately, followed by mixing the CNF suspension and the PMMA solution. The mixture was under the quiescent condition for 2 h. With only convection to bring the PMMA into CNF, much less polymer was observed inside the CNF as shown in Figure 10b. After the processes necessary to remove the initially entrained air and allow the BDO solvent to enter the CNF, convection at high temperature during the crystal annealing process generates the pressure difference necessary to drive the polymer solution to flow into the CNF. It was observed that most CNFs with the inner polymer crystal are less than 5 μm in length, which indicates that the variation of the pressure difference between the two ends limited the probability for the polymer solution to pass through the longer CNFs. The radius of gyration of the nylon polymer may also place a lower limit on the interior radius of the CNFs within which the polymer can enter and hence the conditions under which the

nanocrystals can be formed. It was observed that most inside kebabs are present in the nanofibers with inner diameters larger than 15 nm, whereas for inner diameters of less than 15 nm inside kebabs were not observed (Figure 11). This suggests

Figure 11. Small-diameter (