Solution-Electrospun Poly(ethylene terephthalate) Fibers: Processing

Oct 1, 2012 - ... zero-shear viscosity (η0) revealed that two scaling laws existed for the present solutions, i.e., ... European Polymer Journal 2016...
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Solution-Electrospun Poly(ethylene terephthalate) Fibers: Processing and Characterization Chi Wang,* Ming-Feng Lee, and Yi-Jiun Wu Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan S Supporting Information *

ABSTRACT: Electrospun poly(ethylene terephthalate) (PET) fibers were prepared from a trifluoroacetic acid (TFA)-based solvent. Rheological studies revealed the concentration (ϕ) dependence of the specific viscosity (ηsp) to be ηsp ∼ ϕ3.7 for PET/TFA solutions in the entangled regime. The determined entanglement concentration (ϕe) was higher using a lowermolecular-weight PET. To obtain bead-free fibers, the minimum concentration for the electrospinning was 0.8−1.0ϕe owing to the high volatility of TFA solvent, which significantly enhanced the chain network strength during jet whipping. The doublelogarithmic plots of the jet (dj) and fiber (df) diameters versus the zero-shear viscosity (η0) revealed that two scaling laws existed for the present solutions, i.e., dj ∼ η00.06 and df ∼ η00.77. The microstructural evolution of the electrospun PET fibers from stepwise annealing to crystal melting was investigated by simultaneous small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) measurements using synchrotron radiation sources. The conformer transformation from gauche to trans was monitored by in-situ Fourier transform infrared spectral measurement. In the absence of any WAXD reflection, the as-spun PET fibers possessed a SAXS scattering peak, indicating the presence of a mesomorphic phase with an interdomain distance of 6.8 nm. At annealing temperatures (Ta) higher than 100 °C, the mesomorphic phase gradually transformed into imperfect triclinic crystals and reached its saturation at 130 °C. Further increased Ta perfected the triclinic structure without altering fiber crystallinity until the initial crystal melting at 218 °C, at which a significantly increased long period was detected. When the electrospun PET fibers were embedded in an isotactic polypropylene (iPP) matrix, surface-induced crystallization occurred to develop a transcrsytalline layer of iPP monoclinic crystals at the interface.

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Fine PET fibers with diameters of 0.2−1.0 μm become available by electrospinning, a promising technology for obtaining submicrometer polymeric fibers. The final fiber diameter mainly depends on three processing variables (solution flow rate, applied voltage, and tip-to-collector distance) and on the solution properties (viscosity, conductivity, and surface tension). Several review articles have addressed the detailed processing mechanism and potential applications of derived nanofibers.11−13 Electrospun PET fibers have been obtained using different solvent systems.14−17 Previous papers have focused on the effects of processing parameters on the final fiber diameter without discussing their impacts on the morphology of the electrified cone and liquid jet. To gain a better understanding of how to manipulate the electrified jets to produce fibers with small diameters, a systematic study on the correlation of the cone−jet−fiber morphology is required. More importantly, a detailed investigation on the crystalline structure of electrospun PET fibers via X-ray scattering is not yet available. To date, as-spun

INTRODUCTION Poly(ethylene terephthalate) (PET) is one of the important polyesters with wide applications ranging from functional fibers to automotive parts because of its excellent mechanical properties and thermal stability. Both academic and industrial interest have led to extensive studies on the orientation-induced crystallization of PET in the past decades1−10 owing to its sophisticated structural evolution and practical implications in optimizing its properties. In addition to its triclinic crystalline form, PET forms a mesomorphic phase, which has an intermediate structure between crystal and amorphous. The mesomorphic phase has a smectic structure, which may act as a precursor for crystallization.5 For melt-quenched amorphous PET, Yeh and Geil1 have been the first to show evidence on its ball-like (nodule) structure with a typical nodular size of 4.5− 11.0 nm based on electron microscopy studies. Kaji et al.10 then attributed this special morphology to an apparent precursor for the spinodal-assisted crystallization. Further studies on the mesomorphic phase obtained by drawing amorphous unoriented samples at temperatures lower than the glass transition temperature (Tg) suggest that it is in the metastable state and may transform into the usual triclinic structure by heating above Tg.5,7 © 2012 American Chemical Society

Received: January 17, 2012 Revised: August 25, 2012 Published: October 1, 2012 7939

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PET fibers have been reported to be in the amorphous state based on wide-angle X-ray diffraction (WAXD).17 High-strength polymer fibers, such as Kevlar and PET fibers,18,19 are frequently incorporated into semicrystalline polymers to reinforce mechanical strength for practical application. Compared with conventional fibers with diameters of 10−100 μm, electrospun fibers provide two important benefits because of their small diameter. First is the enhancement of the fiber modulus with decreasing fiber diameter,20 plausibly due to the improved molecular orientation in the electrospun fibers. Fiber surface-induced crystallization of the matrix is likely to develop the so-called transcrystalline layer (TCL), which improves the interfacial strength. Electrospun nanofibers, which possess a high surface-to-volume ratio, are considered to be an ideal candidate not only as a reinforcing filler but also as a nucleating agent to trigger the crystal morphology of the matrix. In the present study, the effects of PET solution rheologies on the morphologies of the Taylor cone, electrified jet, and asspun fibers were studied. TFA is found to be an excellent solvent for PET electrospinning to produce fine diameters. Several scaling exponents are derived to correlate the fiber diameter with the processing parameters. The chain conformation and crystalline state of PET in the electrospun fiber during stepwise annealing were also investigated using some analytical tools, mainly, simultaneous wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). By stepwise heating, the submicrometer-sized fibers provide a unique geometry to elucidate the crystallization mechanism of oriented PET chains. The as-spun fibers are found to be in the mesomorphic state with an interdomain distance of about 6.8 nm. The phase transformation from mesomorphic to triclinic occurs between 100 and130 °C, followed by gradual crystalline perfection at higher annealing temperatures. In the final section of the paper, the as-spun PET fibers are embedded in the isotactic polypropylene (iPP) matrix to study the fiber surfaceinduced crystallization of iPP. Despite its fine diameter, the electrospun PET fibers act as an effective nucleating agent for the iPP matrix to enhance the crystallization rate.

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were measured using a Brookfield viscometer (LVDV-1+, spindle 18, and cup 13R) and a Consort conductivity meter (C832), respectively. For the electrospinning process, a needle with inner and outer diameters (Do) of 0.508 and 0.813 mm, respectively, was used as the spinneret. The prepared solution was delivered by a syringe pump (Cole-Parmer) at a controlled flow rate (Q; ranging from 0.3 to 3.0 mL/h) to the needle. In the needle, a positive voltage (V; ranging from 6 to 12 kV) was applied by a high-voltage source (Bertan, 205B) to provide a sufficient electric field for electrospinning. To construct a needle-to-plate electrode configuration, a steel net (30 × 30 cm2) was used as the collector of the electrospun fibers at a fixed tip-to-collector distance (H) of 140 mm. The detailed processing conditions have been provided in our previous paper.22 During electrospinning, several CCDs were used to capture the images of the Taylor cone and electrified jet, from which the cone height (Hc) and the site of initial jet whipping (i.e., the distance from the needle end, Lj) were measured. The jet diameter (dj) at Lj was measured by a laser diffraction technique. For the present needle-to-plate electrode configuration, the nominal electric field strength was expressed by V/H, but the real electric strength was highly nonuniform and concentrated at the needle end according to simple finite element analysis.22 The calculated electric strength exponentially decayed with the distance from the needle end. Thus, a larger Lj eventually led to a lower field strength for jet whipping. In other words, the electric field strength at the initial jet whipping (Ej) plays an important role in determining the final diameter of the as-spun fibers. In the present study, the magnitude of Ej was theoretically calculated using the FLUX2D9.10 software together with the experimentally obtained Lj. The calculated Ej was considered as a quantitative “estimate” of electric field for the jet whipping process since the field associated with the jet itself was not taken into account.22 The morphology of the as-spun fibers was observed using scanning electron microscopy (SEM) system (Hitachi S4100). The fiber diameters were measured within electron micrographs from a population of ∼500 fibers, from which the average fiber diameter (df) and the corresponding standard deviation were determined. Microstructural Development of PET Fibers during Annealing. To characterize the structural evolution during stepwise heating, fibers with an average diameter of 613 nm obtained from the 20 wt % PET1 solutions were used. A differential scanning calorimetry (DSC) thermogram was obtained using a DSC (PerkinElmer, DSC7) under a nitrogen atmosphere at a scanning rate of 10 °C/min. Fourier-Transform Infrared (FTIR) Measurements. The FTIR spectra of the samples were obtained using a PerkinElmer Spectrum 100 spectrometer with a resolution of 2 cm−1 and 64 scans. The in-situ FTIR spectra of the samples during stepwise annealing at different temperatures (Ta) were obtained to determine the chain conformational change. A Mettler heating stage (FP900) was used to control the temperature, and the temperature protocol is shown in Figure S2. The IR band near 973 cm−1 is attributed to the antisymmetric C−O stretching mode and is associated with the trans conformer of the −O−C−C− group in the crystalline phase.23 In the present study, the as-spun fibers showed a broad IR band centered at 966 cm−1, which gradually shifted during annealing to 971 cm−1 and sharpened at 210 °C (Figure S3). During annealing, the absorbance of this band (denoted by A968) was obtained to represent the relative crystallinity of the PET fibers. The band located at 1340 cm−1 was attributed to the CH2 wagging of the ethylene glycol segments in the trans conformer, whereas the band at 1370 cm−1 was to that in the gauche conformer. PET in the crystalline phase assumes a full trans conformation. Hence, the absorbance of the 1340 cm−1 band (A1340) was also determined to provide the relative crystallinity. However, the glycol segments of PET in the amorphous phase are mostly in the gauche conformation, whereas 10−12% are in the trans conformation.24 The position of the 1340 cm−1 band remained unchanged during stepwise annealing (Figure S3). Simultaneous SAXS/WAXD Measurements. Using the same temperature protocol (Figure S2), simultaneous SAXS/WAXD measurements were conducted using the wiggler beamline BL23A of the National Synchrotron Radiation Research Center (Taiwan) with

EXPERIMENTAL SECTION

Solutions Properties and Electrospinning Process. PET pellets having two molecular weights (MWs) were obtained from Nan-Ya Plastics Co. (Taiwan) and Shin-Kown Co. (Taiwan). The lowand high-MW batches were denoted as PET1 and PET2. Trifluoroacetic acid (TFA; 99 vol % pure) was purchased from Alfa Aesar Co., Ltd., and used as the solvent to prepare the electrospinning solution. Prior to solution preparation, the as-received pellets of PETs were dried under a vacuum at 60 °C for 24 h. The advantages of TFA include its high dielectric constant (42.1), low boiling point (72.4 °C), and low surface tension (13.4 dyn/cm). These characteristics favor the production of fine fibers by electrospinning. The viscosity of TFA (ηs) is 0.81 cP. At room temperature, homogeneous solutions with high PET concentrations (up to 20 wt %) were readily obtained. Using an Ubbelohde viscometer, the intrinsic viscosity [η] of PET1 solutions was determined to be 0.638 dL/g, whereas that of PET2 solutions was 0.936 dL/g (Figure S1, Supporting Information). On the basis of the Mark−Houwink−Sakurada equation, with the reported values of K = 1.4 × 10−3 and a = 0.64,21 the average MWs of PET1 and PET2 were calculated to be 14 260 and 25 960 g/mol, respectively. Solutions were prepared on a weight basis, and the volume fraction (ϕ) was calculated from the pure component density (ρPET = 1.34 and ρTFA = 1.48 g/ cm3), assuming a negligible volume change during mixing. The zeroshear viscosity (η0) and conductivity (κ) of the prepared solutions 7940

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an 8 keV (wavelength λ = 1.55 Å) beam. Specimens were sealed in Al pans with a Kapton windows ca. 2 mm in diameter for the X-ray beam. Using a linear position-sensitive detector for WAXD data acquisition, the 2θ range for the WAXD profile covered the range of 8°−18°. Gaussian curves were used to describe the crystal reflections.25 After the deconvolution of the (01̅1) and (010) reflections at 2θ = 16.3° and 17.2° (Figure S4), the crystal length along the (010) normal plane, D010, was estimated using the Scherrer equation: D010 = kλ /β010 cos θ010

(1)

where k is a constant about unity and β010 is the integral width of the (010) reflection peak.25 D010 was used to represent the lateral dimension of the PET lamellae. SAXS data were collected by two-dimensional (200 × 200 mm2) proportional counters in a master-slave mode. The two-dimensional SAXS pattern was azimuthally integrated to obtain the intensity profile as a function of the scattering vector q (= 4π sin θ/λ, where 2θ is the scattering angle). The interdomain distance or long period (interlamellar spacing) developed in the fibers was calculated from the equation L = 2π/qm, where qm is the peak value found in the Lorentzcorrected SAXS (q2I(q) versus q) plot. On the basis of the lamellar stack model, the scattering invariant Qs was calculated according to the relation2,3 Qs =

∫0

∞

lin q2I(q) dq ∼ ϕϕ (1 − ϕlin)(ρc* − ρa )2 s

Figure 1. Concentration dependence of the specific viscosity for the PET solutions. The MW of PET1 and PET2 are 14 260 and 25 960 g/ mol, respectively. The entanglement concentration ϕe and final slope for the entangled regime are determined to be 12 wt % (∼13.1 vol %) and 3.56 for the PET1 solution and 10.5 wt % (∼11.5 vol %) and 3.82 for the PET2 solution.

(2)

tration ϕ* was estimated by 1/[η] to be 1.16 vol %, leading to the ϕe/ϕ* ratio of ∼11 for the PET1 solution. On the other hand, the ϕ* value for the PET2 solution is 0.79 vol %, giving rising to the ϕe/ϕ* ratio of ∼14. These derived ϕe/ϕ* ratios are also consistent with those reported for polymers dissolved in good solvents. The pioneering work by McKee et al.30 has pointed out the importance of chain entanglements in forming electrospun fibers. In the absence of chain entanglements, only spherical particles are produced. They showed that the minimum concentration required to obtain bead-free fibers is 2.0−2.5 ϕe for the PET copolymer solutions using a mixed solvent of 70/30 (w/w) CHCl3/DMF.30 Figure 2 shows the collected products from the electrospinning process of the PET solutions with different concentrations. To obtain bead-free fibers, the minimum concentration required for the PET1 and PET2 solutions are 12 and 8 wt %, respectively. Solutions with concentrations higher than these respective values possess a deformable chain network to prevent network rupture during electrospinning, thereby yielding uniform fibers. In contrast with McKee et al.,30 the present solutions indicate that the minimum concentration for producing uniform PET fibers is in the range of 0.8−1.0 ϕe, possibly due to the high volatility of TFA. Compared with the CHCl3/DMF cosolvent, the high volatile TFA solvent provides an additional advantage of a high dielectric constant, which enhances the electric stretching force during processing. To reveal the effect of the concentration on electrospinning, only entangled solutions yielding uniform fibers were studied. Prior to electrospinning, the processing window for the stable cone−jet electrospinning mode22b was constructed to determine the common processing variables of 9.3 kV and 1 mL/h. Under the same processing variables, the measured H c decreases, but the Lj increases with increasing concentrations (Table 1). On the other hand, a negligible effect is observed on dj (ca. 4−5 μm). The diameter of the electrospun fibers increases with increasing solution concentration. PET fibers with a diameter of 210 nm are obtained from the 8 wt % solution, whereas a 17 wt % solution yields fibers with a diameter of 1.65 μm. The final fiber diameter is related to dj

ϕs and ϕ are the stack fraction and linear crystallinity of PET within the lamellar stacks. ρc* and ρa are the effective crystalline density of the lamellae and amorphous density, respectively. All X-ray data were corrected for beam fluctuations, sample absorption, and background scattering. The details of the experimental setup and data analysis can be found elsewhere.26 Electrospun-Fiber-Induced Transcrystallization of iPP. The same batch of iPP powder used in our previous TCL study19 was used in the present work. It had a viscosity average MW of 2.8 × 105 g/mol. The testing fibers were covered with iPP powders and placed in a wellcontrolled hot stage (Linkam, THMS600) at 200 °C for 10 min to homogenize the iPP melts. The samples were then cooled to room temperature at a rate of 4 °C/min. Dry nitrogen was introduced to eliminate possible thermal degradation. The crystallization of iPP on the PET fibers was monitored using a polarized optical microscope (POM, Leica DMLP) equipped with phase contrast lens. The surface of the fiber was investigated by atomic force microscopy (AFM) in the tapping mode of operation. The AFM observations were performed in air at room temperature with a Nanoscope Multimode IIIa (Digital instruments) apparatus. lin

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RESULTS AND DISCUSSION Entanglement Concentration (ϕe) and Concentration Effect on Spinnability. Figure 1 shows the plots of the specific viscosity (ηsp = η0/ηs − 1) versus the volume percentage of the investigated PET solutions. As expected, ηsp increases with increasing PET content. For the PET1/TFA solution, a constant slope of 3.56 is reached at a concentration of 13.1 vol %. Above this critical value (ϕe), a semidilute solution regime with entangled PET chains is obtained. For PET2 possessing a higher MW, the ϕe required to reach the entangled solution regime is reduced to 11.5 vol % with a terminal slope of 3.82. The derived exponents of 3.56 and 3.82 are in fair agreement with the theoretical prediction for entangled solutions in a good solvent (∼3.9).27 ϕe can also be estimated by the simple relation 2Me/MW, where Me is the entanglement MW for undiluted PET melt (1170 g/mol).28 Based on this relation, the derived ϕe is 16.4 vol % for the PET1 solution and 9.0 vol % for the PET2 solution. These values are relatively consistent with those determined from the present rheological measurements. The overlapping concen7941

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Figure 3. Solution viscosity effect on the terminal jet diameter dj (open symbols) and fiber diameter df (filled symbols) for PET/TFA solutions under the same processing variables of Q = 1 mL/h, H = 14 cm, and 9.3 kV. Dashed line exhibits the data obtained from ref 30 expressed by df = 50η00.8. Note that the present PET solutions give a similar exponent (∼0.77) but a much lower prefactor of 5.8.

Our derived exponent for the η0 dependence on df are in good agreement with the polyester solution (∼0.80, shown by the dashed line in Figure 3)30 and the poly(methyl methacrylate) solution (∼0.72).32 Based on the derived ϕ−ηsp relation (Figure 1), the concentration dependence of df was scaled to be df ∼ (ϕ/ϕe)2.83. Similar exponents have been reported as ca. 2.6 for PET copolymer fibers,30 3.1 for poly(methyl methacrylate),32 and 3.0 for polyurethane fibers.33 As shown in Figure 3, the derived intercept is 5.8, which is about 1 order lower than that (dashed line) derived by McKee et al.30 using a different solvent system (CHCl3/DMF) for the PET copolymers and processing variables (Q = 3 mL/h and 18 kV at H of 24 cm). As previously reported,22 lower Q and better solvent properties (e.g., a good solvent with a high dielectric constant) favor the formation of fine fibers. Thus, in addition to the low Q applied in the present study, the results suggest that TFA is a better solvent than the CHCl3/DMF cosolvent for PET to obtain thinner fibers. Effects of Processing Variables. To elucidate the effects of Q and V on the processing, the 15 wt % PET2 solution was electrospun at a fixed H of 14 cm. According to the processing window for a stable cone−jet electrospinning mode, the available V range for a given Q of 1 mL/h is 8.5−10.2 kV, whereas the Q range available for a fixed V of 9.3 kV is 0.3 to 3.0 mL/h. By varying either V or Q, the measured quantities of Hc, Lj, dj, and df are summarized in Table 2. The table also

Figure 2. SEM images of electrospun fibers from PET1 solution (left column) and PET2 solution (right column). The scale bar is 50 μm.

and to the effectiveness of jet whipping (as indicated by Ej).22 Thus, a straight jet with a smaller dj and a shorter Lj is desirable for producing electrospun fibers with a smaller df. Generally, two stages of jet stretching are considered during electrospinning. The first jet stretching occurs at the cone apex up to the straight jet end, which proceeds to jet whipping. At this stage, the level of jet diameter reduction was estimated by D0/dj to be ∼200. Further jet stretching occurs in the whipping region. The amount of diameter reduction estimated by the ratio of dj/df is found to be 3−15 for the studied solutions. Based on this argument, the initial jet stretching associated with the induced electrostatic forces at the cone surface is the dominant stage. Provided that Ej is higher for a more effective jet whipping, smaller fibers are produced. Figure 3 shows the double-logarithmic plots of dj and df versus η0, from which two scaling laws are derived as dj ∼ η00.06 and df ∼ η00.77. These plots indicate that η0 plays an insignificant role in varying the dj, which is consistent with the theoretical derivation that dj is independent from solution viscosity.31 The superposition of the data obtained from the PETs with different MWs suggests that η0 is the key parameter in determining the final fiber diameter.

Table 1. Effects of Solution Concentration on the Morphologies of Cone/Jet/Fiber during Electrospinninga wt (%)

η0 (cP)

κ (μS/cm)

Hc/D0

Lj/D0

Ej (kV/m)

8 10 12 15 17

88 153 287 644 1087

1.60 1.58 1.40 1.30 1.20

0.63 0.63 0.50 0.33 0.25

7.2 6.9 12.8 20.6 21.8

198 209 97 54 50

dj (μm) 3.67 4.83 3.94 4.85 4.50

± ± ± ± ±

0.08 0.16 0.07 0.17 0.17

df (nm) 210 336 406 860 1653

± ± ± ± ±

81 107 187 257 490

a PET2/TFA solution, Q = 1 mL/h, H = 14 cm, 9.3 kV, Do = 0.813 mm. Key: η0, shear viscosity; κ, conductivity; Hc, cone height; Do, outer needle diameter; Lj, distance from the needle end to the straight jet end; Ej, electric field at the straight jet end; dj, diameter of straight jet end; df, diameter of electrospun fiber.

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Table 2. Effects of Applied Voltage and Flow Rate on the Morphologies of Cone/Jet/Fiber during Electrospinninga voltage (kV)

Q (mL/h)

Hc/D0

Lj/D0

Ej (kV/m)

8.5 8.9 9.3 9.7 10.2 9.3 9.3 9.3 9.3 9.3 9.3 9.3

1.0 1.0 1.0 1.0 1.0 0.3 0.5 1.0 1.5 2.0 2.5 3.0

0.63 0.58 0.42 0.38 0.38 0.36 0.41 0.55 0.74 0.97 1.18 1.43

11.8 12.4 14.1 14.9 15.8 11.5 12.4 13.2 13.7 13.4 12.4 12.6

98 97 86 84 82 111 102 94 89 92 102 99

dj (μm) 5.29 5.24 5.14 4.92 4.84 1.86 3.44 5.36 6.19 6.93 8.57 9.86

± ± ± ± ± ± ± ± ± ± ± ±

0.14 0.19 0.08 0.14 0.13 0.07 0.12 0.16 0.18 0.17 0.31 0.63

df (nm) 917 831 861 798 835 727 778 850 877 906 985 1050

± ± ± ± ± ± ± ± ± ± ± ±

326 262 270 289 248 250 164 275 275 381 347 348

15 wt % PET2/TFA solution, H = 14 cm, Do = 0.813 mm. Key: Q, flow rate; Hc, cone height; Do, outer needle diameter; Lj, distance from the needle end to the straight jet end; Ej, electric field at the straight jet end; dj, diameter of straight jet end; df, diameter of electrospun fiber. a

displays the calculated Ej at the location where the jet whipping is initiated (z = Lj). For a fixed Q, the application of a larger V leads to a smaller Taylor cone and a longer straight jet with a slightly smaller dj. Owing to a larger Lj, the Ej for jet whipping is somewhat reduced, leading to an insignificant effect on the fiber diameter. The magnitude of Ej is higher than the nominal electric field (60.7−72.6 kV/m) determined by V/H. With increasing Q, the Taylor cone enlarges and Lj slightly increases, leading to a slight reduction in Ej for the jet whipping process. Both dj and df also apparently increase. Interestingly, a simple scaling law is observed for the dj−Q relation, i.e., dj ∼ Q0.66, suggesting that Q is a more controllable variable than V in manipulating the free jet diameter. Gaňań -Calvo31 has theoretically derived an empirical relation for the Q dependence of the jet diameter to be dj ∼ Q0.5. For a given solution, the final fiber diameter is crucially determined by two measured quantities, namely, dj and Ej, which the whipping jet experiences at Lj. With enhanced Ej, the whipping process becomes more effective in further reducing the jet diameter because of the electric repulsion among the liquid segments of the whipping jets. Thus, a straight jet with a shorter length and smaller dj is believed to yield PET fibers with a smaller df. In essence, there should exist an intimate correlation between dj and df. Owing to the coupled effects of solvent evaporation and jet whipping, the theoretical derivation of the dj−df correlation essentially becomes rather difficult, if not impossible. In the present paper, we tentatively attempt to construct the log−log plot of the measured df versus dj to reveal the possible relation. As shown in Figure 4, a master curve is obtained by the expression df ∼ dj0.31, regardless of the processing variables used (Table 2, despite the difference in Ej). The derived exponent ∼0.31 is obviously lower than unity, which is calculated based on the simple solid conservation, i.e., df = ϕ0.5dj1.0, in the absence of jet stretching. This correlation suggests that jet stretching associated with the whipping process also plays an important factor in determining the final fiber diameter. In Figure 4, the two independently measured quantities (dj and df) follow the relation of df ∼ dj0.31 for a given solution (fixed ϕ) under a similar Ej for jet whipping. In Figure 3, on the other hand, not only the volume fraction of polymer but also the magnitude of Ej is significantly altered. The genuine relation of dj−df is further complicated by both parameters (ϕ and Ej). As pointed previously, the fiber diameter depends upon the

Figure 4. Correlation between fiber diameter df and jet diameter dj obtained from electrospinning of the 15 wt % PET2/TFA solution by changing the flow rate (circle symbols) and applied voltage (triangle symbols), respectively [H = 14 cm].

processing variables and solution properties. A more precise description is that these six main parameters may change the dj and Lj (and the corresponding Ej), which, in turn, determine the final fiber diameter. For data shown in Table 1, we fixed the processing variables but changed the polymer concentration (mainly altered was the solution viscosity). The results demonstrate the viscosity plays a negligible effect on dj, but a pronounced effect on Lj. For the viscoelastic forces in the spinning line, shear viscosity is dominant in the Taylor cone and straight jet region since shear stresses induced by the electrostatics at the fluid surface are transferred into the fluid core. On the other hand, elongational viscosity becomes dominant during jet whipping because of the repulsive force between electrified jet segments. To a first approximation, the elongational viscosity (ηe) is related to the shear viscosity by ηe ∼ 3η0 at low strain rates. In this paper, we tentatively used η0 to account for the flow dynamic effect. In Table 2, we fixed the polymer concentration (thus the viscosity) but changed the processing variables (Q and V). The results demonstrate these two variables play a negligible effect on Lj (and Ej), but a pronounced ef fect on dj. These data provided a feasible analysis of the dj−df relation as shown in Figure 4 since the solution is fixed. It evidenced that for a given solution the fiber diameter can be controlled by the dj through a careful manipulation of Q and V. 7943

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Recently, Helgeson et al.13 derived an empirical equation to predict the fiber diameter using two major dimensionless groups. We have tried to apply this relation to our system, but unfortunately no good agreement was obtained. It may also indicate that modeling the electrospinning process is not an easy task. This is the reason why we tried to provide a simple approach by using two measured quantities (dj and Lj) to phenomenologically describe how to manipulate the fiber diameter. Fiber Characterization and Phase Transition during Stepwise Annealing. The DSC heating scan of the as-spun fibers obtained from the 20 wt % PET1 solution is illustrated in Figure 5. Tg of 78.0 °C, a cold crystallization temperature (Tc)

the melting endotherm 45.4 J/g, which indicates that the asspun fibers may not be in the amorphous state. A careful examination of the curve reveals that the initial temperatures for cold crystallization (Tc,i) and crystal melting (Tm,i) are 104.6 and 218.2 °C, respectively. An evident enthalpy recovery peak is observed at 81.5 °C with an endothermic enthalpy of 5.0 J/g. In cooling from 300 °C, the DSC curve shows a crystallization peak at 195.0 °C with a crystallization enthalpy of 38.4 J/g (not shown for brevity). The second heating curve does not exhibit the cold crystallization process and demonstrates a doublemelting behavior with the melting peaks located at 242.2 and 251.3 °C having a melting enthalpy of 38.7 J/g. Figure 6 shows the simultaneous WAXD/SAXS intensity profiles of the PET fibers annealed at different Ta values. At 40 °C, no detectable WAXD peak is observed. However, the SAXS curve exhibits a scattering shoulder at 0.9 nm−1 (inset), indicating the presence of a density fluctuation of 6.8 nm. Thus, a long-range ordered structure is developed in the as-spun fibers in the absence of triclinic crystallites. The ordered structure is likely to be associated with the mesomorphic phase, which has been widely reported for drawn amorphous PET fibers.5,10 To validate this hypothesis, AFM was adopted to trace the morphological feature. The images and height profiles are shown in Figure 7a. A ball-like structure is clearly recognized, but their average diameter is quite large (ca. 30 ± 5 nm) compared with that derived from SAXS. In some regions, the fusion of these ball-like structures is seen. A careful examination of the height profile reveals that regular spacings with sizes of 4−6 nm, as evidenced by the wavy feature, are uniformly distributed on the fiber surface. This finding suggests that within the large ball-like structure a fine and ordered texture related to the mesomorphic phase is developed. In other words, the as-spun amorphous PET fibers are not featureless but possess the mesomorphic phase (likely to be smectic) having an interdomain distance of ∼5 nm, in fair agreement with that derived from SAXS. The formation of the ball-like structure and the mesomorphic phase is attributed to the phase separation of the PET/TFA solution during electrospinning due to the fast solvent evaporation.

Figure 5. DSC heating trace of electrospun PET fibers. The initial and peak crystallization temperatures are denoted by Tc,i and Tc, respectively. The initial crystal melting and apparent melting temperatures are shown by Tm,i and Tm, respectively.

of 122.3 °C, and a melting temperature (Tm) of 254.3 °C are observed. The enthalpy change associated with the crystallization exotherm is 18.9 J/g, which is much lower than that of

Figure 6. Simultaneous (a) WAXD and (b) SAXS profiles of PET fibers during stepwise annealing at different temperatures, Ta. The inset shows the magnification of SAXS profiles at 40, 60, 80, and 100 °C to show the presence of a scattering hump at 0.8−1.2 nm−1. The fibers are obtained from the 20 wt % PET1 solution. 7944

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Figure 7. AFM images and the corresponding height profiles of (a) asspun PET fibers and (b) thermally treated PET fibers at 200 °C for 10 min. The width of images (a) and (b) is 300 and 500 nm, respectively. The fiber axis is vertical. The height profile along the fiber axis is shown by the green line, and that perpendicular to the fiber axis is shown by the red line. The arrows are for the eye guide on the image. In (a), fusion of ball-like domains is seen at the left-bottom corner, and the average domain diameter is 30 ± 5 nm. The height trace reveals the wavy features with a regular spacing of ∼5 nm. These wavy features are likely to be relevant with the mesomorphic domains detected by SAXS. In (b), the ball-like domains are still recognized, but the regular wavy features disappear after annealing at 200 °C.

Figure 8. Ta dependence of relative crystallinity by WAXD intensity of (010)/(011̅ ) reflections and SAXS invariant (top), absorbance of 968 and 1340 cm−1 band (middle), and long period as well as crystal size along (010) plane normal (bottom) for electrospun PET fibers during stepwise annealing.

Upon annealing, no apparent variation in the WAXD profile is seen until Ta = 110 °C, where a diffraction hump at 2θ = 16°−18° has emerged (Figure 6a). At Ta = 200 °C, two broad peaks centered at 16.3° and 17.3° are initially recognized. They are associated with the (01̅1) and (010) reflection planes of the triclinic lattices. Further annealing up to 230 °C results in both reflection peaks sharpened and reaching their maximum intensities. At 250 °C, the intensity of both reflections decreased due to crystal melting. The SAXS curves reveal the clearly discernible intensity upturn in the low q region, which may be attributed to the ball-like structure and the development of microvoids in the PET fibers during solution electrospinning.34,35 At Ta = 80 °C, which is slightly higher than Tg, a relaxation of the amorphous PET chains in the vicinity of the microvoid occurs, leading to intensity reduction in low q regions (0.1−0.6 nm−1). As Ta is elevated to 110 °C and higher, a marked increase in the SAXS intensity and a shift in the SAXS maximum to a smaller q are observed. The shift corresponds to an increase in the long period with increasing annealing temperature. At 250 °C, the SAXS intensity is decreased due to crystal melting. At a given Ta, the fiber crystallinity and crystal size along the (010) normal were determined from the WAXD profiles. The scattering invariant and L were determined from the SAXS curves. The variation in these morphological data with Ta is plotted in Figure 8. The figure also shows the Ta dependence of the A1340 and A968 IR bands, which are relevant to PET crystallinity. The WAXD results indicate that triclinic crystals start to appear above 100 °C. Fiber crystallinity increases with increasing Ta, levels off at 130 °C, and remains constant until 240 °C, at which crystal melting occurs. The evolution of triclinic crystals is also confirmed by the FTIR results. Interestingly, a sudden increase in L occurs from 6.8 nm at 100 °C to 8.5 nm at 110 °C, which

suggests that the metastability of the rapidly frozen mesomorphic phase develops during electrospinning. In the temperature range of 110 °C < Ta ≤ 130 °C, where the mesomorphic phase transforms into triclinic crystals, L remained unchanged but Qs rapidly increases. Further heating to 170 °C results in slightly increased L possibly due to the thermal expansion of the lamellae along its thickness direction. Nevertheless, the crystal length along the (010) normal is constant at ∼5 nm, which suggests that the lateral size of the PET lamellae is restricted. The restriction of lateral lamellar growth is attributed to the domain confinement set by the spatial region initially occupied by the mesomorphic phase. In the Ta range of 170−210 °C, fiber crystallinity is unchanged. However, a fast increase in L accompanied by a rapid increase in size of the (010) normal observed. Thus, in this temperature range, the thickness of PET lamellae and their lateral sizes increase. In other words, the lateral growth of crystalline domains (lamellae) is likely to occur through the “staggering and chain folding”2 of neighboring lamellae. This growth behavior of crystalline domains is also confirmed by the AFM image of the PET fibers after thermal annealing at 200 °C for 10 min. As shown in Figure 7b, the wavy feature associated with the “initial mesomorphic domains” disappears because of the fusion of neighboring lamellae. However, the large ball-like structure is still preserved after annealing at 200 °C. Crystal perfection is inferred from the sharpening of the WAXD reflections. A similar finding of increased size of “mosaic blocks” building up the crystalline lamellae has been reported by Fischer et al.2 No variation in fiber crystallinity is observed. 7945

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fibers lose its ability to induce the TCL of iPP. The parameter Tmax is a measure of the nucleating ability of a given fiber.19 A higher Tmax corresponds to a better nucleating ability of the fibers. The isothermal results show that Tmax for the electrospun PET fibers is about 136 °C, which is identical to that of conventional PET fibers with diameters of 12 μm.19 The induction time for TCL formation increases with increasing crystallization temperature. Despite their small fiber diameters, electrospun PET fibers effectively act as a nucleating agent for iPP. The enhancement of iPP crystallization is extraordinary, considering the large surface area provided by the submicrometer-sized fibers.

Hence, ϕs and ϕlin may remain unchanged.3 Based on the progressive increase in Qs, a continuously increasing effective crystal density within the Ta range of 130−210 °C can be inferred from eq 2, providing additional evidence of crystal perfection within the triclinic lamellae. A similar annealing effect on the measured Qs has been reported for crystal-free PET films either in the undrawn state2 or in the highly oriented state.3 Above 210 °C, partial crystal melting leads to the reorganization of the triclinic structure to form thicker lamellae with a larger lateral dimension. Therefore, the structure developed in the as-spun PET fibers is in the mesomorphic phase, which serves as a precursor for subsequent crystallization on annealing. By heating from 100 to 130 °C, imperfect triclinic crystals of PET appear and grow within the domains preoccupied by the mesomorphic phase. Further heating leads to the crystal perfection in the absence of crystallinity increment until 200 °C, at which neighboring short PET lamellae merge. Electrospun-Fiber-Induced Transcrystallization of iPP. The addition of fibers in the semicrystalline matrix not only improves the mechanical properties but also changes the crystalline structure of the matrix. The formation of a TCL on the fiber surface is essentially desirable to enhance the interfacial strength.36 It is already known that conventional PET fibers with a diameter of ∼12 μm can induce transcrystallization of iPP.19 The unsolved issue is that: can the PET fibers with a submicrometer diameter also induce the iPP transcrystallization? In spite of the submicrometer diameter, the electrospun PET fiber preferentially induces the nucleation of iPP on its surface (Figure 9) through a cooling

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CONCLUSIONS The spinnability of PET/TFA solutions with various concentrations was investigated, and the results were correlated with their rheological properties. The minimum concentration of PET solution that yields bead-free fibers is ca. 0.8−1.0 ϕe because of the high volatility and large dielectric constant of the TFA solvent. Some scaling laws are derived to describe the variable dependence of the final fiber diameter. Given the phase separation during solution electrospinning, the as-spun PET fibers have a mesomorphic structure with a regular interdomain distance of 6.8 nm in the absence of a crystalline order. Via simultaneous WAXD and SAXS as well as in-situ FTIR, the structural evolution during stepwise annealing in the electrospun fibers is elucidated. Upon heating, the mesomorphic phase serves as a precursor for crystallization to develop the triclinic crystal. Owing to the confinement effect, the lateral sizes of the PET lamellae growing within the phase-separated domains are restricted for temperatures lower than 170 °C. Above 200 °C, the fusion of short PET lamellae occurs, leading to significant increments in the long period and crystal size along the (010) normal. Electrospun PET fibers readily induce the formation of the TCL of the iPP matrix, similar to conventional micrometersized fibers. The presence of PET fibers in the iPP matrix tends to enhance the crystallization rate because the fibers act as a powerful nucleating agent owing to their large surface areas.

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ASSOCIATED CONTENT

S Supporting Information *

Determination of intrinsic viscosity of PET solutions; temperature protocol used for stepwise annealing of the samples; FTIR spectra of electrospun PET fibers at different Ta; deconvolution of WAXD profiles of PET fibers at different Ta. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 9. Surface-induced crystallization of iPP by the electrospun PET fibers. (a), (b), and (c) are POM images at 128, 126.5, and 120 °C, respectively. The sample is cooled at a rate of 4 °C/min from 200 °C after holding for 10 min.

*Tel +886-6-2757575 ext 62645; Fax +886-6-2344496; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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process at a rate of 4 °C/min from 200 °C. The initial nuclei at the fiber surface appear at 131 °C, and the iPP lamellae grow perpendicular to the fiber axis to form a thick TCL layer at 126.5 °C, at which only few iPP spherulites are observed in the bulk. Only α-form iPP transcrystallites are observed on the PET fiber surface. Isothermal transcrystallization was also performed to detect the maximum temperature Tmax, above which the PET

ACKNOWLEDGMENTS This work was financially supported by National Science Council of Taiwan (NSC 98-2221-E-006-005-MY3), National Synchrotron Radiation Research Center (NSRRC, 2009-2-0475), and Industrial Technology Research Institute (ITRI). The assistance of simultaneous SAXS/WAXD experiments from 7946

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(18) Sun, X.; Li, H.; Lieberwirth, I.; Yan, S. Macromolecules 2007, 40, 8244. (19) Wang, C.; Liu, F. H.; Huang, W. H. Polymer 2011, 52, 1326. (20) Pai, C. L.; Boyce, M. C.; Rutledge, G. Polymer 2011, 52, 2295. (21) Moore, W. R.; Sanderson, D. Polymer 1968, 9, 153. (22) (a) Wang, C.; Chien, H. S.; Hsu, C. H.; Wang, Y. C.; Wang, C. T.; Lu, H. A. Macromolecules 2007, 40, 7973. (b) Wang, C.; Cheng, Y. W.; Hsu, C. H.; Chien, H. S.; Tsou, S. Y. J. Polym. Res. 2011, 18, 111 . In this study, finite element analysis was applied by using FLUX2D9.10 software to calculate the electric field of electrospinning space without considering the presence of electrified cone/jet. However, the presence of straight jet is likely to interact and interfere with the external electric field to further alter the Ej for the whipping process. To reveal its effect, a stagnant Taylor cone and jet was attempted to adhere at the needle end to see the effect of “induced electric field by the electrified cone/jet” on the calculated Ej. The calculated Ej is similar with that derived without considering the stagnant fluid. It indicated that the induced electric field by the charged cone/jet is negligible in comparison to the high external field applied (80−100 kV/m). Another plausible account is that the electrified jet is too tiny, in comparison with the needle diameter and tip-to-collector distance, to significantly alter the electric field set up mainly by the external source (V/H). (23) Ito, M.; Pereira, J. C.; Hsu, S. L.; Porter, R. S. J. Polym. Sci., Polym. Phys. 1983, 21, 389. (24) Kirov, K. R.; Assender, H. E. Macromolecules 2005, 38, 9258. (25) Wang, Z. G.; Hsiao, B. S.; Fu, B. X.; Liu, L.; Yeh, F.; Bauer, B. B.; Chang, H.; Schultz, J. M. Polymer 2000, 41, 1791. (26) Jeng, U. S.; Su, C. H.; Su, C. J.; Liao, K. F.; Chuang, W. T.; Lai, Y. H.; Chang, J. W.; Chen, U. J.; Huang, Y. S.; Lee, M. T.; Yu, K. L.; Lin, J. M.; Liu, D. G.; Chang, C. F.; Liu, C. Y.; Chang, C. H.; Liang, K. S. J. Appl. Crystallogr. 2010, 43, 110. (27) Adam, M.; Delsanti, M. J. Phys. (Paris) 1983, 44, 1185. (28) Fetters, L. J.; Lohse, D. J.; Richter, D.; Witten, T. A.; Zirkel, A. Macromolecules 1994, 27, 4639 . TFA may react with hydroxyl end groups in PET, but there is no significant degradation of the polymer chains.29 Thus, the chain entanglements in the electrospinning solution are not influenced by the possible esterfication reaction. (29) Kenwright, A. M.; Peace, S. K.; Richards, R. W.; Bunn, A.; MacDonald, W. A. Polymer 1999, 40, 2035. (30) McKee, M. G.; Wilkes, G. L.; Colby, R. H.; Long, T. E. Macromolecules 2004, 37, 1760. (31) Gaňań -Calvo, A. M. Phys. Rev. Lett. 1997, 79, 217. (32) Gupta, P. G.; Elkins, C.; Long, T. E.; Wilkes, G. L. Polymer 2005, 46, 4799. (33) Demir, M. M.; Yilgor, I.; Yilgor, E.; Erman, B. Polymer 2002, 43, 3303. (34) Statton, W. O. J. Polym. Sci. 1956, 22, 385. (35) Wang, C.; Hsieh, T. C.; Cheng, Y. W. Macromolecules 2010, 43, 9022. (36) Quan, H.; Li, Z. M.; Yang, M. B.; Huang, R. Compos. Sci. Technol. 2005, 65, 999.

Drs. U-Ser Jeng and Chun-Jen Su in NSRRC is highly appreciated. The helpful comments provided by the reviewers are also appreciated.

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NOMENCLATURE dj, diameter of straight jet end (μm); df, diameter of electrospun fiber (nm); Do, outer diameter of needle (mm); D010, crystal dimension along the (010) plane normal (nm); Ej, electric field at the straight jet end, determined by finite element analysis (kV/m); H, tip-to-collector (working) distance (cm); Hc, cone height, measured from the needle end to the apex of Taylor cone (mm); L, long period of lamellar crystal (nm); Lj, distance from the needle end to the straight jet end (mm); q, scattering vector (nm−1); Q, solution volume flow rate (mL/h); Qs, SAXS scattering invariant (−); V, applied voltage (kV); Ta, annealing temperature for electrospun fibers (°C); Tc, peak temperature of cold crystallization (°C); Tc,i, initial temperature of cold crystallization (°C); Tg, glass transition temperature of electrospun fibers (°C); Tm, peak temperature of crystal melting (°C); Tm,i, initial temperature of crystal melting (°C); η0, zero shear viscosity of solution (cP); ηs, shear viscosity of solvent (cP); ηsp, specific viscosity of solution (−); κ, solution conductivity (μS/cm); ρ, density (g/cm3); ϕ, volume fraction of polymer in the solution (−); ϕ*, overlapping concentration (−); ϕe, entanglement concentration (−); ϕs, lamellar stack fraction in electrospun fibers (−); ϕlin, linear crystallinity within lamellar stacks in electrospun fibers (−).

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