Crystalline Morphology of Electrospun Poly (ε-caprolactone)(PCL

Mar 11, 2013 - Jun , K.; Reid , O.; Yanou , Y.; David , C.; Robert , M.; Geoffrey , W. C.; Craighead , H. G. A scanning tip electrospinning source for...
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Crystalline Morphology of Electrospun Poly(ε-caprolactone) (PCL) Nanofibers Xiaofeng Wang,† Haibin Zhao,‡ Lih-Sheng Turng,*,§ and Qian Li*,† †

Zhengzhou University, Henan 450001, China South China University of Technology, Guangzhou 510006, China § University of Wisconsin−Madison, Madison, Wisconsin 53706, United States ‡

S Supporting Information *

ABSTRACT: The crystalline morphologies of electrospun random and aligned poly(ε-caprolactone) (PCL) nanofibers, obtained by a plate collector and a two-parallel-conductive-plate collector, respectively, were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), two-dimensional wide-angle X-ray diffraction (2D WAXD), and polarized Fourier transform infrared (polarized FTIR) spectroscopy. The fiber orientations and diameters of the aligned nanofibers were found to depend on the gap size of the collector, which was much larger than those previously reported, thus easing and improving sample handling and characterization. The degree of crystallinity of the aligned nanofibers was higher than that of their randomly aligned counterparts. The crystallites in the nanofibers were highly oriented along the nanofiber axis, as were the molecular chains. The estimated crystallite size suggested that a single nanofiber was composed of dozens of nanofibrils and that each nanofibril was further composed of crystallites along the nanofiber axis with an amorphous region of extended PCL molecular chains between neighboring crystallites.

1. INTRODUCTION Poly(ε-caprolactone) (PCL) is a semicrystalline linear aliphatic polyester with repeating O(CH2)5CO units that is both biocompatible and biodegradable. A large amount of research has been conducted on its biocompatibility and efficacy both in vitro and in vivo, resulting in the approval of a number of PCL-based medical and drug-delivery devices by the U.S. Food and Drug Administration (FDA).1 Up to now, PCL has been widely used in bone graft substitutes, drug-delivery systems, and other tissue engineering applications. PCL has a glass transition temperature of −60 °C and exhibits a melting peak around 60 °C. The temperature at which crystallization proceeds most rapidly is in the proximity of 30 °C.2 The crystal structure of PCL is orthorhombic with the following unit cell parameters: a = 7.496 Å, b = 4.974 Å, and c = 17.297 Å.3 The space group is P212121, and the density is 1.146 g/cm3, which suggests that the unit cell contains two chains with opposite orientations (i.e., “up” and “down”), with the planes of the chains rotated about their axis at an angle of 28° with respect to the a axis. Moreover, the c axis coincides with the chain axis in the orthorhombic system of PCL. 3,4 Recently, many fundamental studies have focused on the crystallization and melting of nanoscale polymer domains in physical or chemical confinement states.5−8 However, the crystal morphology and molecular orientation of PCL nanofibers have rarely been investigated, despite their significance in dictating the bulk properties of PCL, including the mechanical properties and biodegradation behavior. In addition, aligned PCL nanofibers can be used in a wide array of applications, including biosensors in the biomedical field and anisotropic biological scaffolds with an orientation that mimics natural tissue in the tissue engineering and regenerative medicine fields. © 2013 American Chemical Society

Electrospinning is a widely used technology for the production of polymer fibers with diameters ranging from several nanometers to a few micrometers. It provides high surface area-to-volume ratios and has been successfully applied in many fields. The electrospinning process employs a strong electrostatic field to initiate a polymer jet that is subsequently stretched, resulting in a sharp decrease in diameter. Therefore, electrospinning can be used to produce ultrathin fibers that cannot be obtained by the more conventional melt spinning processes. In general, electrospun nanofibers are randomly deposited onto a metal collector because of the bending instability associated with a charged jet.9 To study the crystalline morphology of nanofibers, especially their internal structure, the most direct approach is to investigate samples of single nanofibers. However, this is difficult because of complex sample preparation and handling, as well as a lack of related instruments. Nanofibers are too thin to be tested on devices designed for macroscale samples. Although small-angle X-ray scattering (SAXS) is able to obtain morphological information, measuring ultrathin fibers is very difficult at a laboratory level, thus prompting the use of electrospinning as a new instrument to control and study the morphology and chain orientation characteristics of polymer materials.10 Inspired by this prior work, our research aims at studying the properties of fiber bundles instead of single nanofibers. Not only can nanofiber bundles be handled more easily and provide more information, but they can also replicate the real application conditions of nanofibers, as nanofibers are rarely used individually. Randomly Received: Revised: Accepted: Published: 4939

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Figure 1. SEM images of (a) aligned PCL nanofibers and (b) randomly aligned PCL nanofibers. Scale bars are 10 μm (main images) and 2 μm (insets).

form a 14 wt % solution for electrospinning. A voltage of 20 kV and a collection distance of 20 cm from the tip to the collector were applied. To induce uniaxial alignment of the PCL nanofibers, two parallel metal plates placed on top of a copper plate were used as the collector. A schematic of the collecting system is shown in Figure S1 (Supporting Information). Five different gap sizes between the two metal plates, namely, 20, 26, 32, 38, and 44 mm, were used in this study. A flat copper plate was used as a conventional collector to gather randomly aligned PCL nanofibers. After electrospinning, the PCL nanofibers were dried under a fume hood overnight to remove any residual solvents. 2.2. Morphological Characterization. The electrospun nanofibers were first removed from the collector and then cut to prepare the test samples. The morphologies of the electrospun nanofiber samples were observed using a LEO1530 field-emission scanning electron microscope. The accelerating voltage was 5 kV, and all samples were gold-coated before image analysis. 2.3. Analyses of Alignment and Diameter of Nanofibers. To measure the degrees of alignment and diameters of the nanofibers, SEM images of the fiber bundles at a magnification of 10000 were analyzed using Image J software. A fiber angle φ (see Figure S2, Supporting Information) of 0° indicates parallel alignment from the axis of the fiber to the preferred alignment direction, which is perpendicular to the electrodes. A fiber angle of 90° represents perpendicular alignment. The commercial software COMSOL Multiphysics was employed to simulate the electrostatic field distribution around the metal plates (electrodes) to help understand the fiber orientation. 2.4. Crystallization Analyses. Crystallization analyses were carried out using differential scanning calorimetry (DSC) and two-dimensional wide-angle X-ray diffraction (2D WAXD). DSC thermograms were obtained using a Q2000 modulated differential scanning calorimeter under a N2 gas flow (50 mL/min) at a scanning rate of 10 °C from 0 to 100 °C (first heating), from 100 to 0 °C (first cooling), and from 0 to 100 °C (second heating). The first heating scan was used to reveal prior thermal history, whereas the subsequent cooling and heating scans subjected the samples to the same thermal history. The melt enthalpies of the different samples were obtained in the first and second heating scans. The degree of crystallinity (χc) was estimated by assuming that the melting enthalpy per unit mass of a pure crystalline PCL sample was identical to that of a 100% crystalline PCL sample (139.5 J/ g).26

aligned nanofibers can provide some morphological information; however, they cannot be used to determine molecular orientation, crystallite face orientation, and so on, as this information is lost because of their random orientation at a larger scale (i.e., random alignment of nanofibers). Therefore, it is necessary to collect aligned nanofibers bundled together as an anisotropic sample to investigate the morphological properties of the nanofibers. Various methods have been reported for the preparation of macroscopically aligned nanofibers by employing a special fibercollecting system, such as a high-speed rotating drum,11−13 a copper wire drum,14 a scanning tip,15 or conductive plates containing an insulating gap.16 Generally, aligned nanofibers are different from randomly aligned nanofibers based on their morphological features. Most of the aligned nanofibers are oriented in one specific direction rather than in all directions. Such uniaxially oriented nanofibers exhibit anisotropic properties that are important for use in electrical,17 optical,18 mechanical,19 and biomedical20 applications. However, because the modified collecting system introduces some additional forces and/or changes to the existing forces,11−16,21 the differences between randomly aligned and aligned nanofibers go beyond their morphology. So far, studies focusing on the morphology and molecular orientation of polymer chains in macroscopically aligned electrospun nanofibers are very limited,13,21−25 as are the differences between the randomly aligned fibers and their aligned counterparts.10,21 This study aims to investigate the molecular orientation and crystal structure of polymer chains in both randomly aligned and aligned PCL nanofibers. A two-conductive-plate collector with a varying insulating gap between the two plates was employed to prepare aligned PCL nanofibers. This allowed analysis of the crystal structure and molecular orientation, as well as the overall fiber orientation, using an instrument suitable for samples on the macroscopic scale. The effects of the gap size between the metal plates on the macroscopic alignment of the nanofibers were studied. The nanofibers were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarized Fourier transform infrared (polarized FTIR) spectroscopy.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(ε-caprolactone) (CAPA 6500) was purchased from Perstorp (London, U.K.). The molecular weight was 50000 g/mol, and the melt flow index was 7 g/10 min (2.16 kg, 160 °C). PCL pellets were dissolved in a mixture of chloroform and dimethylformamide (DMF) (7:3, v:v) to 4940

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Figure 2. SEM images of aligned nanofibers using different gap sizes of (a) 20, (b) 26, (c) 32, (d) 38, and (e) 44 mm. Images in the right column are at a higher magnification (scale bars are 1 μm in images a′−e′).

and the Pearson VII curve was selected as the fitting function to identify the diffraction peaks. The distribution curves along the azimuthal angle were fitted by PeakFit, and the Gaussian curve was selected as the fitting function. To investigate the molecular orientation in the PCL nanofibers, polarized Fourier transform infrared (FTIR)

X-ray diffraction was conducted on a Bruker/Siemens HiStar two-dimensional diffractometer with a monochromatic Cu Kα point source (0.8 mm) and a 1024 × 1024 area detector. The diffractometer was operated at 40 kV and 20 mA. The diffraction profiles were measured in transmission mode. The diffraction profiles were fitted by using the PeakFit software, 4941

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Figure 3. Simulation results of an electrostatic field of (a) a conventional flat-plate collector and (b−d) a two-parallel-conductive-plate (TPCP) collector at different gap sizes of (b) 20, (c) 32, and (d) 44 mm. The solid curvy lines represent the electric field streamlines, and the arrows represent the bending direction of the electric field streamlines, which is the direction of the electrostatic forces acting on the nanofibers.

Figure 4. Schematic of the electrostatic force applied to a nanofiber when it is deposited around the electrodes: (a) force in three-dimensional space, (b) components of the electrostatic force in the horizontal plane (top view), (c) electrostatic forces viewed from the y axis, and (d) electrostatic forces and gravity force after the nanofiber was deposited onto the electrodes as viewed from the y axis. Fe′ is the attractive force between the positively charged nanofibers and the negatively charged electrodes. Feg is the force between the nanofibers and ground. Fw is the gravity force of the nanofiber.

fibers or in two mutually perpendicular directions for the randomly aligned fibers.

spectroscopy in transmission mode was performed on a Nicolet 6700 FT-IR spectrometer. Randomly aligned and aligned PCL fibrous membranes were suspended in air by a supporter, and the direction of the oscillating electric field (electric vector) of the incident IR beam was applied in a direction parallel or perpendicular to the direction of fiber alignment for the aligned

3. RESULTS AND DISCUSSION 3.1. Electrospinning of Randomly Aligned and Aligned PCL Nanofibers. The aligned PCL nanofibers and the randomly oriented PCL nanofibers (cf. Figure 1) were 4942

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positively charged for a period of time after deposition,27 repel the incoming nanofibers. Therefore, the repulsive force between the incoming nanofibers and the as-deposited nanofibers further facilitates the alignment of the incoming nanofibers parallel to the as-deposited nanofibers based on the principle of lowest energy. As a result, this effect further enhances fiber alignment. In the next section, the main external forcethat is, the electrostatic force (Fe) between the charged fibers and the electrodesis analyzed. 3.1.1. Orientation of Aligned Electrospun PCL Nanofibers. To quantify the degree of orientation of the nanofibers, Herman’s orientation function was introduced. Herman’s orientation function (f) is defined as

collected with different collectors. The orientation of the PCL nanofibers collected by the two-parallel-conductive-plate (TPCP) collector (Figure 1a) was very different from that of the randomly oriented fibers collected by a conventional flatplate collector (Figure 1b). Additional SEM images of aligned PCL nanofibers produced by TPCP collectors of different gap sizes from 20 to 44 mm are shown in Figure 2, and that of a nanofibrous membrane across the electrodes with an even larger gap size of 60 mm is shown in Figure S3 (Supporting Information). The results suggest that aligned PCL nanofibers were successfully obtained using the TPCP collector. This is consistent with the findings of previous studies.16,21,27,28 However, the gap sizes used in this study were much larger than those reported in previous works, in which the gap sizes ranged from several hundred micrometers to several millimeters. With a larger gap, the area of the nanofibrous membrane obtained over a specific period of collecting time increases by tens to hundreds of times, thereby providing an effective way to produce and handle a significant number of aligned nanofibers for more accurate and insightful characterization. The mechanism of fiber alignment can be elucidated by the predicted electrostatic field distributions around the flat-plate and TPCP collectors, as shown in Figure 3. Around the conventional flat-plate collector for randomly aligned nanofibers, the distribution of the electric field is uniform, and the electric field streamlines above the plate are perpendicular to the horizontal collector (cf. Figure 3a). This suggests that the electrostatic forces applied to the charged nanofibers around the collector are in the perpendicular (downward) direction. Therefore, the perpendicular forces near the collector only accelerate the nanofibers without affecting their alignment. However, the electric field distribution of the TPCP collector is nonuniform, as shown in Figure 3b−d, and depends on the gap size. The electric field streamlines directly on top of the gap between the two vertical electrodes are denser than those elsewhere. Furthermore, the electric field streamlines near the electrodes start to bend toward the two vertical plate electrodes. This indicates that the electrostatic force near the electrodes is not perpendicular to the collector, as with the conventional plate collector, but is instead inclined with respect to the vertical direction and that the angle of inclination becomes larger as the gap size increases (cf. Figure 3b−d). Figure 4 shows the various forces acting on the charged nanofibers in the electrospinning process for the aligned nanofibers with the TPCP collector. The electrostatic forces (Fe) of the charged fibers in three-dimensional space are shown in Figure 4a. The two horizontal components of the electrostatic forces, which are perpendicular (f⊥) and parallel (f∥) to the nanofiber axis (Figure 4b), act on the charged nanofibers. The f⊥ component applied to the fibers forms a rotational torque in the horizontal plane that drives the nanofibers into a perpendicular orientation with respect to the edge of the vertical electrode. The fiber rotation mechanism follows the principle of lowest energy,29 which indicates that a falling nanofiber is prone to finding the steadiest orientation in space, namely, perpendicular to the electrodes. Once a fiber has achieved that orientation, it is likely to stay in that orientation and deposit onto the collector. Because the oriented electric field exists in only a relatively small space around the electrodes, not all of the fibers become perfectly aligned in the perpendicular direction. However, the as-deposited fibers (the fibers suspended across the gap), which can remain highly

f = (3⟨cos2 φ⟩− 1)/2

(1)

where ⟨cos φ⟩ represents the average of the squared cosine of angle φ between the fiber axis and the preferred (gap-wise) direction controlled by the TPCP collector (cf. Figure S2, Supporting Information). The value of f is 1 when the nanofibers align perfectly parallel to the gap-wise (alignment) direction, which means the direction perpendicular to the parallel-plate electrodes, and f = −0.5 when the nanofibers align along the long edge of the electrodes. A value of f = 0 signifies random orientation. In this study, the fiber orientation angle, φ, was obtained from imaging analysis. Figure 5a shows the Herman’s 2

Figure 5. (a) Herman’s orientation function of electrospun PCL nanofibers before (■) and after 2 h of spinning (●). (b) Diameters of randomly aligned and aligned nanofibers after electrospinning for 2 h (▲).

orientation functions ( f) calculated from the fiber orientation angles at the very beginning of electrospinning and after 2 h of deposition. It is clear that the Herman’s orientation functions at the very beginning (f f0) and after 2 h (f f2) both decrease as the gap size increases. The reason for this behavior is that, as the gap size increases, the strength of the electric field near the electrodes decreases, as shown in Figure 5b−d. Recall that the density of the electric field streamlines, and thus the strength of the electric force, decreases as the gap size increases from 20 to 44 mm. Therefore, the degree of orientation of the nanofibers decreases as a result of the weakening electric force, because the electric force is needed to rotate and align the nanofibers. As the electrospinning time increases, the fibers become more disordered; that is, f f2 is smaller than f f0, which agrees 4943

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Figure 6. DSC results for the electrospun PCL nanofibers: (a) degree of crystallinity (χc) for both heating scans and (b) DSC curves for the first heating scans of the samples.

with the results of Yang et al.30 In their work, they found that the fibers became disordered at the top layer of the electrospun fiber mesh when the collecting time was longer than 30 min. As the collecting time increased, increasing numbers of electrospun nanofibers were deposited onto the electrodes, which might have shielded and reduced the electric field strength, thus resulting in a decreased electrostatic force. As the collecting time increased, f f2 became smaller than f f0. The collective effect of gap size and time resulted in the smallest value of f f2 being obtained at the largest gap size of 44 mm after 2 h of electrospinning. 3.1.2. Diameter of the Electrospun PCL Nanofibers. Figure 5b shows the diameters of the electrospun nanofibers collected by the conventional flat-plate and TPCP collectors at gap sizes ranging from 20 to 44 mm. Compared to a diameter of greater than 500 nm for the randomly aligned nanofibers, the diameter of the aligned nanofibers was much smaller at around 300 nm. In addition, the diameters of the aligned PCL nanofibers were more uniform. This suggests that the electrostatic field introduced by the two conductive electrodes affected not only the nanofiber alignment, but also the nanofiber diameter. Two dominant effects dictate the diameter of electrospun PCL nanofibers: jet splaying and whipping motion.31,32 First, splaying converts a single jet into many much thinner jets with approximately equal diameters and charges per unit length when the radial forces of the single jet are larger than the cohesive forces within the jet. Second, the charged jets are accelerated to generate a whipping motion, which can further stretch the jet and, as the solvent evaporates, cause the fiber diameter to become smaller. As shown in Figure 4c, the electrostatic force generated by the modified electrostatic field also provides an extra stretching component (f∥) along the nanofiber axis (in addition to the rotating force, f⊥). This force component stretches the fibers as they are deposited onto the collector. Generally, the average diameters of the electrospun PCL nanofibers remain constant as the gap size increases. The electrostatic force decreases when the gap size is increased from 20 to 44 mm, whereas the decrease in force might not be enough to affect the diameters of the PCL nanofibers. This suggests that the modified electrostatic force around the electrodes affects the alignment more than the diameters of the nanofibers.

3.2. Crystalline Morphology of Electrospun PCL Nanofibers. 3.2.1. Degree of Crystallinity and Melting Peak. To investigate the crystallization behavior of the nanofibers, DSC tests were conducted, as shown in Figure 6. It was found that the degree of crystallinity of randomly oriented nanofibers was lower than that of the as-received PCL pellets and of the aligned nanofibers, indicating that the conventional electrospinning process hampered polymer crystallization, as also reported by Zong et al. and Dhanalakshmi et al.34 The reason for this behavior might be that the solvent evaporation rate was relatively high, so that the PCL molecular chains had less opportunity to rearrange, nucleate, and crystallize.35,36 On the other hand, the degree of crystallinity of the oriented nanofibers was higher than that of the randomly oriented nanofibers and close to that of the asreceived PCL pellets. This implies that crystallization was enhanced during the alignment process. As shown in Figure 4b, an extra stretch force component (f∥) provided by the modified electrostatic field played an important role in the crystallization by subjecting the nanofibers to a stronger tensile force and orienting the molecular chains in the nanofiber. Because of the different electrode design and configuration, the aligned nanofibers might still have remained positively charged after being deposited between the parallel electrodes. Thus, the attractive forces between the positively charged nanofibers and the negatively charged electrodes (Fe′), as well as the forces between the nanofibers and ground (Feg) (cf. Figure 4d), might have further enhanced the crystallization, as does the gravity force (Fw).37 As a result, the degree of crystallinity of the aligned nanofibers was higher than that of their randomly oriented counterparts. The degree of crystallinity varied little with the gap size. However, it is interesting to note that the melting point (Tm) shifted to a slightly higher temperature in the first heating as the gap size increased (Figure 6b), which indicates that the crystallites in the PCL nanofibers had a higher degree of crystalline perfection. The glass transition temperature (Tg) of PCL is around −60 °C, which is lower than the temperature at which the nanofibers were collected (i.e., room temperature). The temperature with the maximum crystallization rate is around 30 °C, which is very close to the room temperature. Therefore, the PCL molecular chains in the nanofibers might 4944

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Figure 7. (a) XRD curves of the randomly aligned and aligned PCL nanofibers. (b,c) Two-dimensional WXRD patterns for the 110 and 200 reflections of (b) aligned and (c) random PCL nanofibers. (d) Schematic of the crystal lattice in an electrospun PCL nanofiber.

chains might have had enough time to form an ordered crystalline structure. With a lower rate of solvent evaporation, the crystals might have higher perfection under higher stress.41 Therefore, the melting point (Tm) of the nanofibers increased slightly as the gap size increased. Because several factors affect the degree of perfection of the crystals, more specific studies need to be done to explore the crystallization mechanism in the postdeposition phase. Because most of the solvent evaporates during the electrospinning process, only a small region in the PCL chains would be able to further crystallize. In addition, the solvent evaporation rate likely depends more on the fiber diameter than on the gap size. Thus, the overall degree of crystallinity of the nanofibers at different gap sizes did not show a significant variation. The DSC results suggest that the electrostatic force played an important role in enhancing crystallization by providing additional stretching forces to the nanofibers when they were being deposited onto the collector and also in the postdeposition crystallization phase. 3.2.2. Orientation of Crystal Lattice and Molecular Chains. Two-dimensional WAXD was employed to investigate the molecular orientation and crystal lattice in the electrospun PCL

still have had the mobility needed to further crystallize even after they had been deposited on the collector.38 On the other hand, as the solvent in the surface layer evaporated, a solid layer was formed that prevented further solvent evaporation. Note that, because of the small diameter of the nanofibers, no collapsed cross section of the fiber was observed as the solvent continued to evaporate. Thus, solvent trapped in the center of the nanofibers even after they were on the collector might have allowed the molecular chains to further crystallize.39 Because of these two effects, crystallization of the electrospun PCL nanofibers might have continued after the fibers had been deposited. In this postdeposition crystallization phase, the crystals might have become more perfect under the influence of stretching forces Fw, Fe′, and Feg (cf. Figure 4d). The stretching forces might have become stronger as the gap size increased because, as the gap size increased, a greater mass of nanofibers might have deposited across the gap, leading to an increase in Fw, and more charges might have remained on the fibers, resulting in an increase in Fe′ and Feg. Because of the solidified surface layer trapping the residual solvent in the nanofibers,39,40 the evaporation rate might have become much lower than that in the electrospinning process, which indicates that the PCL 4945

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using two mutually perpendicular polarizations, as shown in Figure 8a. The PCL bands and corresponding peak assignments were based on previously reported analyses. 47,48 The absorbance intensities of the two polarization spectra were almost identical, which suggests that the infrared beam encountered almost the same number of characteristic C OC, CO, CH2, and (CO and CC) bonds and their corresponding changes in dipole moments at any polarization angle. As a result, there were equal absorbance intensities, indicating that the randomly aligned PCL nanofibrous membranes were isotropic and showed similar conformational order. However, because the incident IR beam was focused not on a single nanofiber but rather on a collection of randomly aligned nanofibers, it cannot be concluded whether the molecular orientation in a single nanofiber was isotropic. It is difficult to determine whether the similarity in absorbance intensity of the two mutually perpendicular polarization directions results from random fiber alignment or both random fiber alignment and random molecular orientation. Therefore, the molecular orientation of an individual randomly aligned nanofiber requires further investigation. For the aligned PCL nanofibers, very different absorbance intensities were observed for the FTIR spectra polarized parallel and perpendicular to the fiber axis (cf. Figure 8b). The bands of symmetric COC stretching at 1171 cm−1, asymmetric COC stretching at 1238 cm−1, and (CO and CC) stretching in the crystalline phase at 1293 cm−1 showed higher intensities when the electric vector was parallel to the fiber axis than when it was perpendicular to the fiber axis. This is because, when most of the PCL molecular chains were oriented in a specific direction and the infrared beam was polarized parallel to that direction, the infrared beam encountered a larger number of COC, CO, and CC bonds and the accompanying changes in the dipole moments of their corresponding vibrations. As a result, there were higher absorbance intensities in that specific direction. The difference in the absorbance intensities between parallel and perpendicular polarizations suggests that the molecular chains were oriented along the axes of the polymer nanofibers, which confirms the conclusion from the WAXD results. 3.2.3. Herman’s Orientation Function. The crystallographic analysis provides a qualitative description of the orientation of the molecular chains and crystals in the electrospun PCL nanofibers. However, a more accurate quantitative analysis is needed to reveal more of the crystalline morphological information. To identify the orientation of the crystal lattice face in the nanofibers by XRD, Herman’s orientation function ( f x) was applied. For the XRD results, the calculation of f x was simplified by using the orientation appearing in the azimuthal scan. The value of f x can then be calculated as

nanofibers. Figure 7 shows the 2D WAXD diffraction patterns of the randomly aligned and aligned PCL nanofibers. Both the randomly aligned PCL nanofibers and the aligned PCL nanofibers displayed (110) and (200) diffraction peaks at 21.3° and 23.7°, respectively (cf. Figure 7a). The intensities of the (110) and (200) diffraction peaks of the aligned nanofibers were greater than those of the randomly aligned ones, indicating that the degree of crystallinity in the aligned fibers was higher than that of their random counterparts.42,43 This is consistent with the DSC results. The two smooth diffraction rings of the randomly aligned samples indicated that the azimuthal intensity distribution was homogeneous (cf. Figure 7b), which suggests that the crystal planes were randomly oriented. However, the two discrete diffraction arcs in the aligned samples (see Figure 7c) suggest that the crystal planes in the nanofibers were oriented in a specific direction. In fact, the azimuthal angles between the centers of both diffraction arcs and the fiber axis were approximately 90°, indicating that the angles between the poles of the (110) and (200) crystal planes and the fiber axis were orthogonal.44,45 That is, the normals of both crystal planes (110) and (200) were perpendicular to the fiber axis, which suggests that the crystal lattice was oriented along the fiber axis. In other words, the c axis of the crystal lattice in the electrospun PCL nanofibers was predominantly oriented along the fiber axis (cf. Figure 7d). At the same time, the unit cell of the PCL crystal is orthorhombic, and the c axis coincides with the chain axis in terms of the orthorhombic crystalline structure system. Thus, the polymer chains in the crystal lattices are also oriented parallel to the fiber axis. The electron diffraction pattern of a single randomly aligned PCL nanofiber was reported by Chen et al.,46 who observed two weak arcs of crystal planes (110) and (200) similar to those in Figure 7b, indicating a similar crystalline morphology in the randomly aligned PCL nanofibers. Therefore, the PCL crystal lattices might be oriented parallel to the fiber axis in both randomly aligned and aligned PCL nanofibers, as well as the polymer chains. Polarized FTIR spectra of the randomly aligned and aligned nanofibers are shown in Figure 8. FTIR spectra of randomly aligned (isotropic) PCL nanofibrous membranes were obtained

fx = (180° − Δφ1/2)/180°

(2)

where Δφ1/2 is the half-width of the intensity distribution curve (cf. Figure 9) along the Debye−Scherrer ring.10,49 However, any variation in the alignment of the nanofibers in the sample will affect the orientation distribution of the crystallites observed by XRD.50 In fact, the orientation distribution of the crystallites in the sample ( foverall) is the convolution of the fiber axis orientation distribution, f f, and the orientation distribution of the crystallites within a single fiber with respect to the fiber axis, fc51

Figure 8. Polarized FTIR spectra of (a) randomly aligned nanofibers and (b) aligned nanofibers. R0 and R90 are two mutually perpendicular polarization directions. For aligned nanofibers, they are the directions of polarization parallel and perpendicular, respectively, to the fiber axes.

foverall = fc ff 4946

(3)

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value of fc is about 0.8, which suggests that the crystallites are highly aligned along the nanofiber axis. Nonetheless, not all of the crystallites are perfectly aligned along the nanofiber axis because fc is 0.8 and a value of 1 corresponds to perfect alignment. These result agrees well with the crystallographic analysis from the diffraction patterns of the electrospun PCL nanofibers. 3.2.4. Crystal Size in Electrospun PCL Nanofibers. To estimate the crystallite size, we used the Scherrer equation L

= Kλ /β cos θ

(5)

where ⟨L⟩110 is the crystallite size as estimated from the (hkl) reflection planes, K is a constant (0.9), λ is the wavelength of the incident X-rays (1.542 nm), β is the full width at halfmaximum (FWHM) peak intensity, and θ is the Bragg angle. The parameter β was corrected for peak broadening caused by the slit system of the diffractometer according to the expression β2 = βobs2 − βm2, where βobs is the fwhm and βm is the peak broadening due to the slit system. βm was estimated from the diffraction peak measured from an Al powder sample. Figure 10b shows the crystallite size calculated from the Scherrer equation for the (110) reflection. The crystallite size ⟨L⟩110 of the randomly aligned PCL nanofibers was found to be greater than 140 Å, which was larger than that of the aligned samples (around 130 Å). Compared to the randomly aligned nanofibers, the aligned nanofibers experienced additional stretching forces ( f∥) when they were being deposited. The stretching stress aligned more extended PCL molecular chains along the fiber axis, and these chains could act as row nuclei for subsequent crystallization. The increasing nuclei might result in a decrease in crystallite size. However, ⟨L⟩110 did not show a significant change as a function of gap size. Even though f∥ was applied to the nanofibers when they were being deposited, the crystallite size inside the PCL nanofibers and the degree of crystallinity of the electrospun nanofibers were not affected. 3.2.5. Structure of PCL Nanofibers. In the electrospinning process, the draw ratio of the nanofibers ranges from several hundred to over 10000. This provides an extremely high drawing rate for the nanofibers, which enables the molecular chains to be stretched and oriented along the fiber axis. The elongation process of electrospun nanofibers is similar to the drawing process of melt spinning fibers,52 in which a fibril is a secondary structural unit composed of microfibrils. Each microfibril is further composed of alternating crystalline and noncrystalline segments. For the electrospinning process, as the solvent evaporates, a crystallite oriented along the fiber axis is formed by the local folding of molecular chains. Iwata et al.53 reported that, for a PCL lamellar single crystal, PCL chain folding occurred along the {110} and {010} growth planes, as in other aliphatic polyesters, and the polymer chains mainly aligned perpendicular to the base plane of the crystal. The ⟨L⟩110 crystallite size was about 130 Å, indicating that there were about 32 polymeric chains arranged along the {110} plane in the nanofibers. This might suggest that a bundle of 32 chains arranged along the long axis in a crystallite. Furthermore, some polymer chains extended across the amorphous area between neighboring crystallites along the fiber axis and thus were shared by the neighboring crystallites. Thus, the crystalline and amorphous regions were spaced in an alternating pattern along the fiber axis and shared molecular chains in the amorphous segments that acted as tie chains. Crystallites sharing PCL molecular chains with each other therefore became the constituent elements of a longitudinal nanofibril along the

Figure 9. Azimuthal-scan profiles of the 110 reflection for randomly aligned and aligned PCL nanofibers.

Hence, fc can be obtained as fc = foverall /ff

hkl

(4)

From the fiber angle orientation distribution, it is known that most of the nanofibers orient around the preferred orientation direction (perpendicular to the electrodes). Here, the average of f f0 and f f2 was used as f f in eq 4. Furthermore, foverall substantially equals f x as estimated by the XRD result. Figure 10a shows a plot of f x estimated from XRD and the calculated

Figure 10. (a) Herman’s orientation functions of the electrospun PCL nanofibers f f (●), f value of the overall crystal lattice foverall (■), and f value of the crystal lattice face in a nanofiber fc (▲). (b) Crystallite size estimated from the 110 reflections of randomly aligned and aligned PCL nanofibers (⧫).

value of fc. f x decreases with an increase in gap size, and the same tendency is seen with f f. However, fc increases slightly when the gap size is increased. This suggests that the orientation of the crystallites in the nanofibers is improved as the gap size increases. The reason for this might be that, during the postdeposition crystallization phase, the subsequent tension stress rotates the as-formed crystals, causing them to develop a better orientation.50 Thus, the intrinsic orientation of the crystal planes in the nanofibers can be determined from fc. The 4947

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fiber axis. Figure 11 shows the crystalline structure of a PCL nanofiber in which dozens of nanofibrils consisting of

nanofiber mat. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.-S.T.), [email protected] (Q.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Yiyan Peng from UW−Madison for comments during the preparation of this manuscript. The authors are also grateful for financial support from the Chinese Scholarship Council and the Wisconsin Institutes for Discovery (WID) at the University of Wisconsin−Madison. The authors also acknowledge financial support from the Key Commonwealth Aid Project of Henan Province of China (HNZB[2010] N91) for the polarized FTIR experiments.

Figure 11. Schematic of a nanofibril in a single PCL nanofiber.

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crystalline and amorphous segments form a single nanofiber. The structure in electrospun polyoxymethylene (POM) nanofibers shows similar nanofibrilar bundles,10 and this was also observed by Lim and co-workers.54 In particular, the crystalline structure of electrospun PCL nanofibers can be characterized as follows: (1) The crystallites are highly oriented along the fiber axis, as well are the molecular chains in the crystallites. (2) The crystallites oriented along the fiber axis form the nanofibrils in the nanofibers. (3) Dozens of nanofibrils constitute a single nanofiber. Furthermore, the extended chains from neighboring crystallites form tying chains and amorphous regions between the crystallites.

(1) Chong, E. J.; Phan, T. T.; Lim, I. J.; Zhang, Y. Z.; Bay, B. H.; Ramakrishna, S.; Lim, C. T. Evaluation of electrospun PCL/gelatin nanofibrous scaffold for wound healing and layered dermal reconstitution. Acta Biomater. 2007, 3, 321. (2) Acierno, S.; Di Maio, E.; Iannace, S.; Grizzuti, N. Structure development during crystallization of polycaprolactone. Rheol. Acta 2006, 45, 387. (3) Bittiger, H.; Marchessault, R. H.; Niegisch, W. D. Crystal structure of poly-ε-caprolactone. Acta. Crystallogr. B 1970, 26, 1923. (4) Chatani, Y.; Okita, Y.; Tadokoro, H.; Yamashita, Y. Structural Studies of Polyesters. III. Crystal Structure of Poly-ε-caprolactone. Polym. J. 1970, 1, 555. (5) Ho, R.-M.; Chiang, Y.-W.; Lin, C.-C.; Huang, B.-H. Crystallization and Melting Behavior of Poly(ε-caprolactone) under Physical Confinement. Macromolecules 2005, 38, 4769. (6) Guo, Q.; Thomann, R.; Gronski, W.; Staneva, R.; Ivanova, R.; Stühn, B. Nanostructures, Semicrytalline Morphology, and Nanoscale Confinement Effect on the Crystallization Kinetics in Self-Organized Block Copolymer/Thermoset Blends. Macromolecules 2003, 36, 3635. (7) Jiang, S.; Ji, X.; An, L.; Jiang, B. Crystallization behavior of PCL in hybrid confined environment. Polymer 2001, 42, 3901. (8) Sakurai, T.; Nagakura, H.; Gondo, S.; Nojima, S. Crystallization of poly(ε-caprolactone) blocks confined in crystallized lamellar morphology of poly(ε-caprolactone)-block-polyethylene copolymers: Effects of polyethylene crystallinity and confinement size. Polym. J. 2012, 1. (9) Li, D.; Xia, Y. Electrospinning of Nanofibers: Reinventing the Wheel? Adv. Mater. 2004, 16, 1151. (10) Kongkhlang, T.; Tashiro, K.; Kotaki, M.; Chirachanchai, S. Electrospinning as a New Technique To Control the Crystal Morphology and Molecular Orientation of Polyoxymethylene Nanofibers. J. Am. Chem. Soc. 2008, 130, 15460. (11) Matthews, J. A.; Wnek, G. E.; Simpson, D. G.; Bowlin, G. L. Electrospinning of Collagen Nanofibers. Biomacromolecules 2002, 3, 232. (12) Teo, W. E.; Kotaki, M.; Mo, X. M.; Ramakrishna, S. Porous tubular structures with controlled fibre orientation using a modified electrospinning method. Nanotechnology 2005, 16, 918. (13) Park, J. H.; Kim, B. S.; Yoo, Y. C.; Khil, M. S.; Kim, H. Y. Enhanced mechanical properties of multilayer nano-coated electrospun nylon 6 fibers via a layer-by-layer self-assembly. J. Appl. Polym. Sci. 2008, 107, 2211. (14) Katta, P.; Alessandro, M.; Ramsier, R. D.; Chase, G. G. Continuous Electrospinning of Aligned Polymer Nanofibers onto a Wire Drum Collector. Nano Lett. 2004, 4, 2215.

4. CONCLUSIONS Aligned PCL nanofibers were prepared on a special TPCP collector with a varying insulating gap size. The orientation of the nanofibers was generated by the resulting electrostatic forces. With an increase in gap size or collecting time, the degree of alignment of the nanofibers gradually decreased. In general, the degree of crystallinity of the aligned nanofibers was higher than that of the randomly aligned nanofibers because of additional aligning torque and stretching forces. The tension forces on the nanofibers, as well as the trapped solvent inside the nanofibers, enhanced the degree of perfection of the crystallites formed in the postdeposition stage, resulting in an increased melting point. The crystallites showed a higher degree of orientation in the nanofibers. However, the gap size did not significantly affect the degree of crystallinity or crystallite size when it was increased from 20 to 44 mm. The results demonstrate that, for aligned PCL nanofibers, the polymer chains orient along the nanofiber axis, and at the same time, the crystallites inside the nanofibers also align along the nanofibers. A single PCL nanofiber is composed of dozens of nanofibrils, which, in turn, consist of PCL crystallites aligned along the fiber axis. Extended tying chains from the crystallites constitute amorphous regions in the nanofibers. Highly oriented electrospun PCL nanofibers could find promising applications in the biomedical and tissue engineering fields.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Additional figures, such as a sketch of the two-parallelconductive-plate (TPCP) collector and electrospun PCL 4948

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