Article pubs.acs.org/Langmuir
Intrinsically Conductive Polymer Fibers from Thermoplastic trans1,4-Polyisoprene Peng Han, Xiaohong Zhang, and Jinliang Qiao* SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013, People’s Republic of China S Supporting Information *
ABSTRACT: Herein, we report a new strategy to prepare conductive polymer fibers to overcome the insurmountable weakness of current conductive polymer fibers. First, special thermoplastic polymers are processed into polymer fibers using a conventional melt-spinning process, and then the nonconductive polymer fibers are converted into intrinsically conductive polymer fibers. Using this new strategy, intrinsically conductive polymer fibers have been prepared by melt spinning low-cost thermoplastic trans-1,4-polyisoprene and doping with iodine, which can be as fine as 0.01 mm, and the resistivity can be as low as 10−2 Ω m. Moreover, it has been found that drawing can improve the orientation of trans-1,4-polyisoprene crystals in the fibers and, thus, the conductivity of the conductive polymer fibers. Therefore, conductive fibers with excellent conductivities can be prepared by large drawing ratios before doping. Such conductive polymer fibers with low cost could be used in textile, clothing, packing, and other fields, which would benefit both industry and daily life. The newly developed method also allows one to produce conductive polymers of any shape besides fibers for antistatic or conductive applications.
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with an electron donor or acceptor.19−22 It was reported that the electrical conductivity of iodine-doped polyisoprene was in the range of 10−2−10−1 Ω−1 cm−1, which is more than 10 orders of magnitude higher than that of the pristine state. The mechanism was the production of conjugated fragments during the doping process.22 Nevertheless, such non-conjugated conductive polymers have not yet been used to produce conductive polymer fibers. On the other hand, conductive polymer composite fibers are produced by adding conductive fillers, such as carbon black,23 carbon nanotubes, 24−33 graphene,34 metal powder,35−39 and other conductive particles40,41 into polymer fibers. Without the processing problems, however, these conductive polymer composite fibers suffer from the large usage of conductive fillers, high expense, and instability after drawing. It is well-known that the mechanical properties of fibers with low drawing ratios are usually rather poor. However, during the routine drawing process, the formed conductive network in conductive polymer
INTRODUCTION As the information technology improves, smart textile and clothing with wearable electronic devices are becoming more and more popular.1−12 A large number of polymer materials will be highly required to produce some special parts, such as the shell of electronic devices, circuit boards, and even electrodes. Conductive polymer fibers are one of the most important and useful materials for weavable, flexible, bendable, stretchable, and lightweight.1 However, current conductive polymer fibers cannot meet such demands. Dependent upon the nature of the conducting component, conductive polymer fibers are mainly divided into intrinsically conductive polymer fibers and conductive polymer composite fibers. Intrinsically conductive polymers fibers are made of conjugated polymers having metallic or semiconducting conductivities. Since their discovery, conjugated polymers have been paid much attention for their wide applications in solar cells,13−16 chemical sensors,17 and other fields.18 However, conjugated polymers are all difficult to be processed into fiber because they cannot be melt processed and some of them (e.g., polyaniline) are even insoluble in solvent. Non-conjugated polymers with double bonds in the backbone were found to be conductive by doping © XXXX American Chemical Society
Received: April 7, 2016 Revised: April 25, 2016
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DOI: 10.1021/acs.langmuir.6b01333 Langmuir XXXX, XXX, XXX−XXX
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at a rate of 10 °C/min, then cooling to −20 °C at the same rate, and then heating to 200 °C again. The drawing process of the fibers was manipulated on an Instron 3366 machine with a drawing rate of 5 mm/min at room temperature. Preparation of Fibers. The TPI was melt-extruded on a Haake Mini Lab machine at 120 °C. Dies of different diameters were used on the exit to obtained fibers of different sizes. Because of the swelling, the diameter of the fiber was about 0.2 mm bigger than that of the die used. The primary fibers were then placed in a dryer with saturated iodine vapor for over 48 h. The resistance of the fibers was measured as described in the instrumentation part. The melt extrusion described above can only produce fibers with diameters larger than 0.7 mm. To produce fine fibers, a melt-spinning process was introduced. The polymer TPI was melt-extruded through a spinneret with 24 holes of 0.2 mm at 145 °C and then collected on a rotating roll with a rate of 20 m/min. The diameter of the fibers was about 0.02 mm. The rotating roll can speed up to 50 m/min without breaking the fibers, and the diameter of the fibers can be as fine as 0.01 mm. Fine conductive fibers can be obtained by reacting them with iodine vapor for over 48 h. Time of Reaction in the Iodine Vapor. To understand the reaction of the fibers in the iodine vapor, fibers of 0.7 mm were placed in the iodine vapor and taken out after a certain time interval (i.e., 1, 4, 8, 24, 48, 72, and 144 h) to measure the resistance. Influence of Drawing. Drawing is a normal routine process to deal with fibers. We attempted to draw the fibers below their melting point. Fibers with the diameter of 1.2 mm were drawn to 0.7 mm, of which the drawing ratio is about 3. Then, the two kinds of fibers of the same diameter of 0.7 mm but with or without drawing were placed in the same iodine vapor environment for reaction. After over 48 h, the two different kinds of fibers were taken out to measure the resistivity. DSC Measurements. DSC data of fibers with a diameter of 0.7 mm and the iodine-doped fibers with the same diameter were obtained. The DSC measurements were performed by heating from −20 to 200 °C at a rate of 20 °C/min, then cooling to −20 °C at the same rate, and then heating to 200 °C again. At both 200 and −20 °C, the temperature was kept for 2 min for balance.
composite fibers will be destroyed because the conductive fillers will separate from each other by drawing. Therefore, the mechanical properties of conductive polymer composite fibers are rather poor as a result of their low drawing ratios. Obviously, new types of conductive polymer fibers with good conductivities and mechanical properties are needed. Herein, we report a new strategy to prepare conductive polymer fibers to overcome the insurmountable weakness of current conductive polymer fibers. First, special thermoplastic polymers are processed into polymer fibers using a conventional melt-spinning process, and then the nonconductive polymer fibers are converted into intrinsically conductive polymer fibers. Using this new strategy, intrinsically conductive polymer fibers were prepared by melt spinning thermoplastic trans-1,4-polyisoprene (TPI) and doping with iodine, as shown in Scheme 1. Briefly, isoprene, a low-cost monomer from the Scheme 1. Intrinsically Conductive Polymer Fibers by Melt Processing and Doping with Iodine
petrochemical industry, was polymerized into TPI using a similar polymerization process and catalyst with polypropylene.42 The resultant TPI was then processed into fibers through conventional melt-spinning process, which could be converted into intrinsically conductive polymer fiber afterward by iodine doping.
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RESULTS AND DISCUSSION Relation of Conductivity to Doping Time. To prepare intrinsically conductive TPI fibers with a high conductivity, it is important to know the mechanism of converting nonconductive TPI fiber into conductive polymer fiber. According to the work of Dai and White,22 the doping reaction should be first an addition reaction of iodine to the double bonds and then an elimination reaction of hydrogen iodide. The addition reaction was a slow step, and the elimination was a fast step. Our experimental results showed that the TPI converting process followed the mechanism by Dai and White. A timedependent resistivity measurement was carried out for the TPI converting process. Fibers with a diameter of 0.7 mm were placed in saturated iodine vapor and taken out to measure resistivity at a certain time interval. The results are shown in Figure 1. It can be found that, at the first 8 h, the resistivity of the TPI fibers merely decreased. After about 24 h, the resistivity of the TPI fibers dropped remarkably. After 48 h, the resistivity reached a relatively small value and then leveled off, indicating that the doping reaction completed in 48 h. DSC data in Figure 2 showed that the melting heat flow peak at around 50 °C of the TPI fiber of 0.7 mm had disappeared as well as the recrystallization heat flow peak at 7 °C after doping with iodine for 48 h. The results also confirmed that the doping had been completed from outside to inside for the fiber of 0.7 mm after 48 h. Conductive Fibers with Different Sizes. According to the above mechanism, it is easy for us to control the doping
EXPERIMENTAL SECTION
Materials. TPI was purchased from Qingdao Tpi New Material Co., Ltd. Different kinds of TPI are sorted by Mooney viscosity. TPIs of 24, 38, 45, 54, 58, and 85 Mooney units are used in the experiments. Iodine was a product of Sinopharm Chemical Reagent Co., Ltd. Instrumentation. The resistance of the fibers was measured with a Keithley 6517B electrometer produced by Keithley Instruments, Inc. A single fiber was put between two clips at a distance (L) of 15 mm, and then voltage was applied to obtain resistance. The resistivity ρ was calculated from the measured resistance R using eq 1 as follows:
ρ = RA /L = πRd 2/4L
(1)
where A was the area of the cross-section and d was the diameter of the fiber. A wide-angle X-ray diffraction (WAXD) experiment was carried out on a Bruker D8 DISCOVER two-dimensional (2D) X-ray diffractometer. The X-ray was generated using an IμS microfocus X-ray source incorporating a 50 W sealed-tube X-ray generator with a Cu target. The wavelength is 0.1542 nm. The power of the generator used for measurement was 45 kV and 0.9 mA. The X-ray intensities were recorded on a VÅNTEC-500 2D detector system with a pixel size of 68 × 68 μm2. The distance from the sample to detector was 82.8 mm. The spot size of the beam was 0.5 mm. The exposure time was 2 min. Small-angle X-ray scattering (SAXS) was conducted using a Bruker NANOSTAR SAXS instrument. The IμS-type generator was operated at 40 kV and 650 μA. Cu radiation with a wavelength of 0.1542 nm was used for the experiment. The scattering data were collected by a Hi-Star area detector, which has the resolution of 1024 × 1024 pixels and pixel size of 100 μm. The detector to sample distance was 1053 mm. Differential scanning calorimetry (DSC) data were obtained on a Pyris 1 instrument. The temperature was heating from −20 to 200 °C B
DOI: 10.1021/acs.langmuir.6b01333 Langmuir XXXX, XXX, XXX−XXX
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Figure 2 showed that the melting heat flow peak at around 50 °C of the TPI fiber of 0.7 mm has disappeared as well as the recrystallization heat flow peak at 7 °C after doping with iodine for 48 h, which means that the doping has been completed from outside to inside for the fiber of 0.7 mm. Why did the conductive fibers of 0.02 and 0.01 mm have even a better conductivity? WAXD was introduced to help us understand this phenomenon. As seen in panels a and c of Figure 3, the WAXD results showed that the crystals of TPI fiber of 0.01 mm have better orientation than TPI fiber of 0.7 mm. It is clear that crystal orientation is another important factor to increase the conductivity of the conductive fiber. To confirm this conclusion, two conductive fibers with the same diameter of 0.7 mm but with or without drawing were compared to eliminate the size effect. It is surprising to find that the resistivity of the drawn fiber decreases by about 2 orders of magnitude, as shown in Table 1. It is well-known that the drawing process could increase the degree of polymer orientation; therefore, WAXD was introduced again to see the difference of the orientation before and after drawing. As seen in panels a and b of Figure 3, the WAXD results showed that the TPI crystals in the drawn fiber were more oriented than that in the non-drawn fiber. SAXS results also confirmed a higher order of orientation in the drawn fibers than in the nondrawn fibers, as shown in Figure S1 of the Supporting Information. In the meantime, it is also found that the crystal peaks in both drawn and non-drawn fibers were the same. The lamellar distances were 0.47, 0.39, 0.33, and 0.29 nm after calculation. The results indicated that there were two kinds of crystalline forms in the fibers: monoclinic (α) and orthorhombic (β).43,44 After doping with iodine, WAXD results of the two fibers were shown in Figure S2 of the Supporting Information. The non-drawn fibers showed the TPI crystalline signal with very weak intensity compared to before doping and also very low orientation, while the drawn fibers showed almost no TPI crystalline signal but a visible high orientation. The results implied that the oriented TPI crystal were almost completely converted in the drawn fibers and the high orientation was in favor of high conductivity. It should be mentioned that the amount of TPI crystals may be too little to be detected; therefore, there was no visible TPI crystal melting peak on the DSC results of doped TPI fibers, as shown in Figure 2. It is extremely important to know that drawing can help us to prepare a conductive fiber with a high conductivity. Conductive polymer fibers, especially conductive polymer composite fibers, usually lose conductivity after drawing because the drawing process would break the conductive network of the fibers. However, in our case, the drawing process did not reduce the conductivity because it has nothing to do with the conductive network of the fiber. Orientation had been proven to be an important effect to the conductivity in some conjugated films or fibers, where the parallel direction to the orientation showed increased conductivity.45−50 Thus, we can conclude that the high degree of orientation is another factor of high conductivity of the conductive fibers. Moreover, it could be an effective way to improve the conductivity of the conductive fibers by drawing before doping with iodine. That is to say, conductive fibers with excellent conductivity are possible to be prepared by large drawing ratios before doping.
Figure 1. Resistivity of the conductive fibers dependent upon the doping time in the iodine vapor.
Figure 2. DSC data of TPI fibers with a diameter of 0.7 mm (a) before and (b) after iodine doping.
time to obtain desired resistivity for certain applications. Also, it should be mentioned that the doping takes place at room temperature. Therefore, the preparation process does not seem difficult to scale up. Indeed, we did successfully fabricate TPI fibers continuously by melt spinning through a spinneret with 24 tiny holes and fibers with a diameter of 0.02 mm to be collected on a rotating roll with a rate of 20 m/min. The rate of the collecting roll can speed up to about 50 m/min without breaking the spinning process by which TPI fibers with a diameter as fine as 0.01 mm were prepared. After the reaction in iodine vapor for over 48 h, all of the raw TPI fibers were converted to conductive fibers. The resistivity of these conductive fibers is shown in Table 1, from which we can see Table 1. Resistivity of Iodine-Doped TPI Fibers with Different Diameters diameter of fibers (mm) 1.7 1.2 0.7 0.7a 0.02 0.01 a
resistivity (Ω m) 2.9 6.3 3.1 5.0 5.0 1.0
× × × × × ×
104 103 103 101 10−2 10−2
The diameter of the fiber was drawing from 1.2 to 0.7 mm.
that the resistivity is reduced at least by 5 orders of magnitude, considering that the resistivity of TPI fiber is more than 109 Ω m. It is interesting to find that the finer the fiber, the higher the conductivity. The resistivity of the finest fiber of 0.01 mm can be as low as 10−2 Ω m, 11 orders of magnitude lower than that of the pristine TPI fiber. Influence of Drawing. It is important to know that finer fiber has better conductivity. One could automatically realize that the finer fiber can be doped more completely from outside to inside of the fiber; therefore, the better conductive fiber can be obtained from the finer fiber. However, DSC results in C
DOI: 10.1021/acs.langmuir.6b01333 Langmuir XXXX, XXX, XXX−XXX
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Figure 3. WAXD images of TPI fibers: (a) fibers with a diameter of 0.7 mm without drawn, (b) fibers with a diameter of 0.7 mm drawn from a diameter of 1.2 mm, (c) fibers with a diameter of 0.01 mm by spinning, and (d) diffraction peaks of the above three different kinds of fibers.
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CONCLUSION In conclusion, a new process to prepare conductive polymer fibers has been developed and noble intrinsically conductive polymer fibers have been prepared by melt spinning thermoplastic TPI and doping with iodine. Moreover, it has been found that drawing can improve the orientation of TPI crystals in the fibers and, thus, the conductivity of the conductive polymer fibers. Therefore, conductive fibers with excellent conductivities can be prepared by large drawing ratios before doping. Such conductive polymer fibers with low cost could be used in textile, clothing, packing, and other fields, which would benefit both industry and daily life. The newly developed method also allows one to produce conductive polymers of any shape besides fibers for antistatic or conductive applications.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01333.
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REFERENCES
More details of the SAXS (Figure S1) and WAXD (Figure S2) data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Prof. Liming Dai from Case Western Reserve University for his helpful advice and suggestions. This work was financially supported by the postdoctoral foundation of SINOPEC Beijing Research Institute of Chemical Industry. D
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