Article pubs.acs.org/IECR
Characterization of Fluoropolymer Nanofiber Sheets Fabricated by CO2 Laser Drawing without Solvents Kei Fukuhara,*,† Takemune Yamada,‡ and Akihiro Suzuki‡ †
Department of Molecular Design & Engineering, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan ‡ Interdisciplinary Graduate School of Medicine and Engineering, Yamanashi University, Takeda-4, Kofu 400-8511, Japan ABSTRACT: Nanofibers of tetrafluoroethylene-perfluoroalkyl vinylether (PFA) copolymer were prepared by irradiating PFA fibers with a CO2 laser under supersonic velocities. This is the only method to make PFA nanofibers without the use of solvents, because for the majority of methods for preparing nanofibers, they must be dissolved in a solvent and PFA fibers cannot be dissolved in most solvents, except special fluorinating reagents. The average fiber diameter of PFA nanofibers was approximately 273 nm. As a result, we developed a CO2 laser supersonic multidrawing (CLSMD) in a new way to produce PFA nanofibers in large quantities. CLSMD can make nanofibers continually in a wide area. The length, width, and thickness of pileup nanofiber sheets are 50 cm, 17 cm, and 70 μm, respectively. It was observed that nanofiber sheets have a higher melting point and degree of crystallinity than microfibers by differential scanning calorimetry (DSC) measurements and have larger crystallite size realized by wide-angle X-ray diffraction (WAXD) measurements. Moreover, it was considered that the interior of nanofiber sheets have oriented structurally and the strength of nanofiber sheets was higher than that of microfibers. Nanoparticles were evolved to the surface of nanofiber sheets by annealing.
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fiber of micrometer scale goes through the hole and it is irradiated with the CO2 laser, nanofibers are readily prepared. In comparison with ES,15−19 CLSD has two obvious benefits. First of all, we can make a sample with no solvent and it is applicable to various fibers. Especially, fluorine-based fibers are efficient, because these fibers cannot be dissolved in most solvents except a very dangerous toxic combination of the following: fluorine of high temperature, molten alkali-metal, and special fluorinating reagents like chlorine trifluoride. The second one is we can prevent dispersion into the atmosphere to make nanofibers in a sealed state. Thus, this device is considered to have an environmental benefit. We have already reported fabricated nanofibers in poly(ethylene terephthalate) (PET), poly(ethylene-2,6-naphthalate) (PEN), poly (L-lactic acid) (PLLA), and poly(glycolic acid) (PGA) by the use of CLSD and CO2 laser supersonic multidrawing (CLSMD).29−34 Typical fluoropolymers, like PVDF, ETFE, poly(tetrafluoroethylene) (PTFE), PFA, etc., exhibited superior properties including hydrophobicity, resistance to chemicals, electrical characteristic, low frictional properties, and heat resistance.35 Thus, PTFE and PFA have approximately equal properties. However, PFA is able to melt process whereas PTFE is not. For this reason, PFA is used as an industrial material including a molding process and is useful as filter materials in the electrical component because of particular superiority in chemical characteristics and heat resistance. If PFA becomes superfine-drawn of fibers constituting a filter, it will increase uniformity, increase the ratio of surface area, and
INTRODUCTION Recently, nanotechnology research has received an upsurge of interest for its potential functionalities. Toward this research field, nanofibers have attracted the most attention among all the fibers,1−7 including the fields of electronics, healthcare, and environment engineering. Thus, a study is proceeding with enabled actual products in each field. At first, a dye-sensitized solar cell and a polymer electrolyte (membrane) fuel cell were used in the field of electronics.8,9 Then, a regenerative medicine research project that cultured and multiplied cells on materials called a scaffold in the field of health care was performed.7 Finally, in the field of environmental engineering, there was an advancement of a filter and a purification unit.10−14 Nanofibers have been produced by electrospinning (ES),15−19 melt electrospinning,20 sea−island-type conjugated melt spinning,21 single-orifice melt blowing,22 and jet blowing.23,24 ES is widely used as a nanofiber manufacturing method. However, melt blowing still used by the nanofiber manufacture technology, although there are some problems with productivity and higher cost. Moreover, it has been reported that fabricated nanofibers are created using polymers containing parts of fluorine compounds such as ethylenetetrafluoroethyleneethylenetetrafluoroethylene (ETFE) and polyvinylidene difluoride (PVDF) and combining inorganic polymer with fluorine compounds.25−28 However, manufacturing of nanofibers using perfluoroalkyl vinylether (PFA) is yet to be reported. Therefore, we have reported a carbon dioxide (CO2) laser supersonic drawing (CLSD) as a nanofiber manufacturing method.29−33 At first, we prepared a chamber (box) which made a thin hole through the upper part. Then, vacuuming the chamber, airflow (supersonic velocities) occurs near the hole by a difference of atmospheric pressure. Finally, when an original © 2012 American Chemical Society
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May 3, 2012 June 22, 2012 July 2, 2012 July 2, 2012 dx.doi.org/10.1021/ie301138t | Ind. Eng. Chem. Res. 2012, 51, 10117−10123
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reduce pore diameters, i.e., improve filter performance a lot. In this Article, we reported the wide area and continual manufacturing of PFA nanofiber sheets by the CO2 laser supersonic multidrawing (CLSMD) which exhibited a subtle type of CLSD and subsequent annealing of piled up nanofiber sheets.
There is a vacuum chamber in the center of the apparatus. In the vacuum, there is a conveyer to continually make nanofibers in the wide area. The vacuum chamber is attached to the chamber for fiber supply orifices. Fiber supply orifices are the place that makes thin holes through the upper part. The diameter of orifices is 0.5 mmφ, and the number of orifices is 17. The greater the amount of original fiber passing through the fiber supply orifices with air results in a substantially wide range of nanofiber sheets being produced. In this study, we used 15 holes of orifices. Two Zn−Se windows are to pass through the CO2 laser along the sequence of orifices located on the side of fiber supply orifices. In addition, we prepared a valve on the side of the vacuum chamber and regulated chamber pressures using the pump to evacuate air. The original fiber passing through fiber supply orifices is rolled into a fiber supply spool. The fiber supply spool is controlled by a motor. The vacuum chamber was attached to an X-axis, Y-axis, and Z-axis stage and would be able to move right to left, back and forth, or up and down, and the vacuum chamber was able to move every 0.001 mm. In addition, we attached the turntable spin around. The efficiency of the turntable was to move every 1′ (1′ = 1°/60). We turned the chamber around a little using the turntable after having regulated the laser beam to come to the center of fiber supply orifices (Figure 3). The laser beam
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EXPERIMENTAL SECTION Materials. A diameter of 100 μm PFA original fiber (Gunze Ltd.) was used in this experiment. The structural formula of PFA is shown in Scheme 1, and a scanning electron microscopy
Scheme 1. Structural Formula of PFA
(SEM) micrograph and wide-angle X-ray diffraction pattern of PFA original fibers are illustrated in Figure 1a,b respectively.
Figure 3. Overhead view of the apparatus turning the chamber around various angles of rotation (θ) using the turntable.
Figure 1. (a) SEM micrograph of PFA original fiber and (b) wideangle X-ray diffraction pattern of PFA original fiber of 100 μm in diameter.
power is increasing gradually with the center. Thus, when the Z-axis was regulated, the center where the laser power was the highest came to right immediately below the orifice based on 0.0 mm of the distance from orifices. For the nanofiber manufacturing condition, we changed the laser power, the distance from the orifice, the original fiber supply speed, the chamber pressure, and the angle of rotation. Morphology. The surface observations of produced fibers were investigated by scanning electron microscopy (SEM; JEOL Ltd.) and atomic force microscope (AFM; JEOL Ltd.). The fiber diameter is produced at an average rate that belongs to a total 100th type of lengths observed by SEM photograph. The particle sizes attached to nanofibers were observed by an AFM photograph. Differential Scanning Calorimetry Analysis. Differential scanning calorimetry (DSC; Rigaku Co.) has been used as a measuring tool of calorimetry. DSC scans were performed within the temperature range of 25−350 °C at a heating rate of 10 °C min−1. About 2.00 mg of mass was used for DSC measurements for all samples. The melting point (Tm) was determined by DSC scans. The degree of crystallinity (Xc) of the PFA component was calculated from the heat of fusion (ΔHm) and the 100% crystalline PFA polymer, which was taken as 82 J g−1 according to the following relationship:35
From the wide-angle X-ray diffraction pattern, we can realize that the diffracted beam originated from the reflection (200) on the equator and the presence of crystallite fiber with a high level of orientation.35 Apparatus. We used the device shown in Figure 2 for CLSMD of PFA fibers. The CO2 laser (COHERENT Co.) which we used for this study has an oscillation wavelength of 10.6 μm, a maximum power of oscillation of 40 W, and an official beam diameter of 3.6 ± 0.5 mmφ.
Xc (%) = Figure 2. Apparatus used for CO2 laser supersonic multidrawing. 10118
ΔHm × 100 −82(J/g)
(1)
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Analysis of Wide-Angle X-ray Diffraction. Wide angle X-ray diffraction (WAXD) (Rigaku Co.) measurements were performed with a voltage of 40 kV, current of 300 mA, and irradiation time of 1 h to create copper Cu Kα radiation (λ = 1.542 Å). We read and measured an X-ray diffraction pattern exposed by an imaging plate. From the diffraction pattern of the sample from a figure of diffraction obtained from the sample, we obtained the information about presence or absence of the crystal, orientation state of the crystal, and information about the lattice spacing (d) and the crystallite size (XS).
force) and shear force (downward force) was acting on an original fiber at a distance orifice from 0 to 10 mm. Figures 4
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RESULTS AND DISCUSSION In the CLSMD device, vacuuming the chamber, airflow (supersonic velocities) occurred near the orifices by a difference of an atmospheric pressure. The chamber pressure was converted to the velocity by Graham theory (2) about supersonic flow that occurred just below orifices. The velocity of flow from orifice (ν) was calculated from the atmospheric pressure (P0), the chamber pressure, and the density of air (ρ = 1.2 g L−1), according to the following relationship:
Figure 4. Compressive force at the node of fiber surfaces obtained at various chamber pressures.
1
⎧ 2(P − P) ⎫ 2 ⎬ ν=⎨ 0 ρ ⎭ ⎩
(2)
When this velocity of flow from the orifice abided by the law of conservation of energy, there was a relationship between the temperature of before and after an adiabatic expansion and a Mach number. The following relation was applicable between the ratio of the temperature before the adiabatic expansion (T0) to the temperature after the adiabatic expansion (T) and the Mach number (M):36 −1 ⎧ (γ − 1)M2 ⎫ T ⎬ = ⎨1 + T0 2 ⎩ ⎭
Figure 5. Shear force at the node of fiber surfaces obtained at various chamber pressures.
(3)
Because air consists principally of diatomic gases, γ(Cp/Cv = 1.4) was the ratio of the heat capacity at the constant pressure (Cp) to that at the constant volume (Cv). The flow velocity was converted into the Mach number of the sonic speed at the temperature before the adiabatic expansion: v v M= = C 331.5 + 0.6(T − 273.5) (4)
and 5 showed compressive force and shear force at various chamber pressures (10, 50, and 90 kPa), respectively. It was understandable that compressive force was substantially stronger than shear force. Furthermore, the lower the chamber pressure, the stronger the compressive force was getting. However, with the distance of orifice ranging from 0 to 4.0 mm at 10 kPa of chamber pressure, it seemed that both shear force and compressive force were uneven. It would be worth mentioning that the shear force was uneven at all conditions. Figure 6 shows flow velocity distribution of the air jet (vector representation) at chamber pressure of 10 kPa. Vacuuming at chamber pressure of 10 kPa, because it was confirmed that airflow eddied around just below the orifice, forces dispersed up and down. Especially, compressive force fell into negative territory at a distance from the orifice of about 1.0 mm. PFA nanofibers prepared by CLSD29−33 were nearly the size of a palm, and the average fiber diameter was 273 nm with attached fine particles of around 40 nm in diameter (Figure 7). PFA fibers produced nanoparticles only, because surface energy was low compared to PFA fibers with reported fibers like PET and PLLA. In this experiment, laser power, chamber pressure, the fiber supply speed, and collecting time were kept constant at 30 W, 54 kPa, 8.0 m min−1, and 5 min, respectively. However, the method of fiber preparation was shown in Figure 7b. If the fiber supply speed was too fast, then a little amount of microfibers was being prepared; on the other hand, a large number of nanofibers was fabricated only when the fiber supply speed became significantly slow. Then, PFA fibers were
Table 1 shows the velocity of flow from orifice and the Mach number at various chamber pressures. Table 1. Velocity of Flow from Orifice (v) and the Mach Number (M) at Various Different Chamber Pressures (P) P/kPa
V/m s−1
M at 25 °C
94 76 44 6
100 200 300 396
0.289 0.577 0.866 1.15
Thus, it was analyzed by 3D FEM (Computer simulation) that velocities of flow from the orifice and forces acting on the original fiber occurred from just below the orifices. The analysis condition was defined as the temperature of 25 °C under the compaction property of existing air and as the diameter of orifices of 0.5 mmφ. Moreover, the change from kinetic energy to thermal energy at all areas was taken into account. An analysis result was calculated that compressive force (pushing 10119
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Figure 6. Flow velocity distribution of the air jet (vector representation) at chamber pressure of 10 kPa.
continuously collected on the rotating conveyer at a width of 25 cm and length of 50 cm, to produce big nanofiber sheets. The optimum preparation conditions for homogeneous PFA nanofiber sheets are as follows: orifice numbers of 15, orifice diameter of 500 μm, laser power of 40 W, original fiber supply speed of 0.1 m min−1, chamber pressure of 94 kPa, conveyer speed of 0.03 m min−1, and collecting time of 9 ks. Figure 8 shows a photograph of the pileup nanofiber sheets (17 cm in width, 50 cm in length, and 70 μm in thickness) which was prepared by the renewed CLSMD. The sheets were constructed by the nanometer sized fibers (average fiber diameter of around 370 nm) with attached fine particles (Figure 9). In other conditions, nanofibers were not connected on the conveyer. For example, when the laser power was less than 40 W, PFA original fibers did not produce nanofibers beyond the certain distant regime. No nanofiber has been produced when the original fiber supply speed was faster than 0.1 m min−1. To draw, nanofibers need to pull the original fibers down under supersonic velocities and at the same time fuse heat of the laser. Thus, we realized that the laser power would have enough output to draw all original fibers passing through the orifices. The nanofiber sheet manufacturing depends on the angle of rotation (θ). With θ ranging from 0° 00′ to below 0° 35′, the produced fibers differed in diameter, but at 0° 35′, there was no difference of average fiber diameter in the distance from the laser to the fiber supply orifice (Figure 10). Nanoparticles were attached to the surface of PFA nanofiber sheets. Thus, for changing the porous film, it had been annealed at 300 °C below the melting point. SEM photographs of the annealed sheets were shown in Figure 11a−c. The (a) coalescence of fine particles, (b) connection and entangling
Figure 8. Photograph of PFA nanofiber sheets obtained by CLSMD.
Figure 9. Histogram of fiber diameter distributions with an inset of a SEM micrograph at magnifications of 10 000 fibers for fibers with nanofiber sheets prepared by CLSMD.
of fibers, and (c) breakage of the lattice for 1, 2, and 3 min, respectively, was observed. Finally, the obtained porous structure was composed of fibers with an average fiber diameter of 686 nm at 2 min. The sample had crumbled in several seconds when it was annealed at more than 300 °C. The reason behind this was the thick diameter of fiber ascribed by the thermal contraction.
Figure 7. (a) Photograph of PFA nanofibers obtained by CLSD, (b) histogram of fiber diameter distributions with an inset of a SEM micrograph of nanofibers (average fiber diameter of 273 nm), and (c) AFM image of nanoparticles that attached with nanofibers (around 40 nm in diameter). 10120
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Figure 10. Variation in the average diameter with distance (Xp) from the laser beam side of the conveyer for the fiber sheets obtained at various angles of rotation (θ).
Figure 12 and Table 2 show DSC curves, the melting point (Tm), and the degree of crystallinity (Xc) for the original fiber, nanofiber sheets, and annealed sheets obtained at various annealed times (Tan). Annealed conditions of PFA nanofiber sheets were at 300 °C for 1, 2, and 3 min. At first, the melting point increased from 308.6 °C in original fiber to 314.1 °C in nanofiber sheets. Then, the degree of crystallinity increased from 26.5% in original fiber to 51.6% in nanofiber sheets. When microfibers like original fibers changed to nanofibers like drawn sheets, the melting point was raised to about 7 °C. This result was obtained when the degree of crystallinity was also increased nearly two times. With increasing crystallinity, it is thought that the packing of crystal became stronger by making nanofiber from microfiber in the lamella structure of the nanofiber interior. Therefore, the melting point was increased. Thus, the degree of crystallinity was enhanced as result of annealing of nanofiber sheets. Xc of annealed samples for 1, 2, and 3 min were 54.1, 54.8, and 60.6%, respectively. However, annealed sheets were about 1 °C lower than nanofiber sheets in the melting point. From these results, when nanofiber sheets were annealed, crystal architecture of PFA nanofiber has been increased by means of lamella structure, but packing of crystal became weak. Figure 13 shows WAXD profiles inserting X-ray diffraction patterns. Table 3 shows the diffraction angle (2θ), the lattice spacing (d), and the crystallite size (XS) of original fiber, nanofibers sheets, and annealed nanofiber sheets obtained at various annealed times (Tan). In all conditions, these Bragg diffractions indicated strong diffraction peaks at around 2θ = 17° were the (200) crystallographic planes35,37 and Debye− Scherrer ring was seen. These diffraction peaks at 2θ = 16.86, 16.88, 17.09, 17.14, and 17.06° corresponded to the lattice spacing of d = 5.25, 5.24, 5.18, 5.16, and 5.19 Å, respectively. Whereas, in crystallite size, the original fiber was Xs = 73 Å, nanofiber sheets were Xs = 90 Å. In addition, annealed nanofibers for 1, 2, and 3 min were Xs = 120, 128, and 131 Å. Each sample did not change the diffraction angle and the lattice spacing, but the crystallite size changed significantly. The
Figure 12. DSC curves of original fiber, nanofiber sheets, and annealed sheets obtained at annealed times (Tan).
Table 2. Melting Point (Tm) and the Degree of Crystallinity (Xc) of the Original Fiber, Nanofiber Sheets, and Annealed Sheets sample
Tm/°C
Xc/%
original fiber nanofiber sheets Tan= 1 min Tan = 2 min Tan = 3 min
308.6 314.1 313.8 313.1 313.5
26.5 51.6 54.1 54.8 60.6
Figure 13. WAXD profiles of the original fiber, nanofiber sheets, and annealed sheets obtained at various annealed times (Tan), inset of Xray diffraction patterns.
Figure 11. SEM photographs of annealed nanofiber sheets at 300 °C for (a) 1 min, (b) 2 min, and (c) 3 min. 10121
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Notes
Table 3. Diffraction Angle (2θ), the Lattice Spacing (d), and the Crystallite Size (Xs) of the Original Fiber, Nanofiber Sheets, and Annealed Sheets sample
2θ
d/Å
Xs/Å
original fiber drawn nanofiber Tan = 1 min Tan = 2 min Tan = 3 min
16.86 16.88 17.09 17.14 17.06
5.25 5.24 5.18 5.16 5.19
73 90 120 128 131
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Mr. K. Kuroda in Gunze Ltd. for helpful discussions. This work was supported by JSPS Grant-In-Aid for Scientific Research B (21350121) from the Ministry of Education, Science, Sports, Culture and Technology of Japan.
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crystallite size of nanofiber sheets was larger than the original fiber; it would be worth mentioning that annealed nanofibers were larger than the original fiber. The lamella structure of the nanofiber interior was readily realized by this WAXD and DSC analysis. The Debye−Scherrer ring usually means an amorphous structure, but these samples were a nonwoven fabric. It was considered that they were in the crystal structure, if a drawn piece of nanofiber interior could be observed by X-ray diffraction and scattering. However, because the melting point increased about 7 °C in comparison with the original fiber and the degree of crystallinity also was enhanced nearly twice in DSC measurement, it seemed that crystal structure became enriched. Then, we thought that interior nanofiber sheets have orientation structures; moreover, nanofiber strength was higher than the original fiber.
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CONCLUSION PFA nanofibers were prepared as nearly the size of a palm by CLSD, and the average fiber diameter was 273 nm with attached fine particles of around 40 nm in diameter. As a result, the nanometer sized PFA nanofiber sheets have been successfully prepared by carbon dioxide gas laser drawing of original fibers (of 100 μm in diameter) passing through the orifice at ultrasonic speed and subsequently piled up by rotation of movable plates. Optimum conditions for PFA nanofibers are established through this investigation, and the size of the nanofibers was determined. The melting point increased from 308.6 °C in the original fiber to 314.1 °C in nanofiber sheets. Then, the degree of crystallinity increased from 26.5% in the original fiber to 51.6% in nanofiber sheets. For crystallite size, nanofiber sheets (Xs = 90 Å) were larger than the original fiber (Xs = 73 Å). Moreover, it seems that the interior of nanofiber sheets have oriented structures, and the strength of nanofiber was higher than that of the original fiber. After being annealed at 300 °C for 2 min, finally, we could obtain porous nanofiber sheets composed of fibers with an average fiber diameter of 686 nm, and the degree of crystallinity was 54.8%. CLSMD can be used for all thermoplastic polymers without the use of solvents. It can be used to produce nanofibers in a large extent because the fiber is supplied at a constant speed and is continuously irradiated by the laser beam. CLSMD can be used to easily prepare various nanofiber sheets using the CO2 laser irradiation. CLSMD is a novel method for producing nanofiber sheets using only the CO2 laser. It is especially useful that PFA fibers cannot be dissolved in most solvents, except special fluorinating reagents.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 10122
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Industrial & Engineering Chemistry Research
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dx.doi.org/10.1021/ie301138t | Ind. Eng. Chem. Res. 2012, 51, 10117−10123
