SEMI-METALLIC, STRONG CONDUCTIVE POLYMER MICROFIBER, METHOD AND FAST RESPONSE RATE ACTUATORS AND HEATING TEXTILES
A method comprising: providing at least one first composition comprising at least one conjugated polymer and at least one solvent, wet spinning the at least one first composition to form at least one first fiber material, hot-drawing the at least one fiber to form at least one second fiber material. In lead embodiments, high-performance poly(3,4-ethylenedioxy-thiophene)/poly(styrenesulfonate) (PEDOT/PSS) conjugated polymer microfibers were fabricated via wet-spinning followed by hot-drawing. In these lead embodiments, due to the combined effects of the vertical hot-drawing process and doping/de-doping the microfibers with ethylene glycol (EG), a record electrical conductivity of 2804 S·cm−1 was achieved. This is believed to be a six-fold improvement over the best previously reported value for PEDOT/PSS fibers (467 S·cm−1) and a twofold improvement over the best values for conductive polymer films treated by EG de-doping (1418 S·cm−1). Moreover, these lead, highly conductive fibers experience a semiconductor-metal transition at 313 K. They also have superior mechanical properties with a Young's modulus up to 8.3 GPa, a tensile strength reaching 409.8 MPa and a large elongation before failure (21%). The most conductive fiber also demonstrates an extraordinary electrical performance during stretching/unstretching: the conductivity increased by 25% before the fiber rupture point with a maximum strain up to 21%. Simple fabrication of the semi-metallic, strong and stretchable wet-spun PEDOT/PSS microfibers can make them available for conductive smart electronics. A dramatic improvement in electrical conductivity is needed to make conductive polymer fibers viable candidates in applications such as flexible electrodes, conductive textiles, and fast-response sensors and actuators.
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This application claims priority to U.S. provisional application Ser. No. 62/086,885 filed Dec. 3, 2014, which is hereby incorporated by reference in its entirety.
BACKGROUNDPart I:
Fiber-shaped conductive materials are attractive for use in applications ranging from simple textiles to complex multimaterial piezoelectric fibres and supercapacitors (a listing of cited references 1-48 is provided hereinbelow for Part I).1-6 Conjugated polymer fibers, featuring tunable electrical conductivity, have been extensively investigated from both fundamental and application perspectives to understand their electrical and mechanical properties and their practical use in conducting textiles, organic electronics, sensors and actuators.7-12 The main techniques for processing polymer fibers are dry-spinning, wet-spinning, melt-spinning and electrospinning.13 In particular, wet-spinning is an important process to produce continuous polymer microfibers by submerging the spinneret in a coagulation bath that causes the fiber to solidify.13,14
Among the popular conjugated polymers, polypyrrole (PPy), polyaniline (PANI) and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT/PSS) have been successfully spun into microfibers by the wet-spinning process.8,9,11,15 PEDOT/PSS has been found to have the good spinnability and its preparation technique can be scaled up to an industrial process.16 In the past decade, doping and/or de-doping with various polar solvents has been carried out to enhance the conductivity of PEDOT/PSS films and fibers by two or three orders of magnitude.9,7-21 Doping PEDOT/PSS involves mixing it with a small amount of secondary dopant (with a high boiling point and <6 wt %) to change its molecular structure to a more conductive state; de-doping requires partially removing amorphous PSS by washing it with polar solvents. Depending on grade of the employed pristine PEDOT/PSS, as-spun microfibers (without any doping) generally display low electrical conductivity (from 1 to 74 S·cm−1).8,9 Their electrical conductivity could be improved up to 467 S·cm−1 by using an ethylene glycol (EG) de-doping process, which partially removes the amorphous and insulating PSS phase.8 Jalili et al.9 showed that by combining doping and de-doping, the electrical conductivity of PEDOT/PSS microfibers could reach 351 S·cm−1. Spinning PEDOT/PSS with functionalized carbon nanotubes into fibers led to limited improvement in conductivity (400 S·cm−1).22,23 To meet the requirements for use as flexible electrodes, conductive textiles and fast-response sensors and actuators, such conductive polymer fibers must have dramatically improved electrical conductivity.
A need exists for improving commercial prospects to find better materials and processing methods which provide improved electronic and mechanical properties and combinations of these and other properties. See, for example, U.S. Pat. Nos. 7,132,630; 7,886,617; 8,501,317; WO 2007/099889; and US Pat. Pub. 2008/0099960.
SUMMARYEmbodiments described and/or claimed herein include polymers and compositions, and methods of making and using such polymers and compositions, including devices. The polymers and compositions can be in fiber form or also called a fiber material.
For example, one aspect is for a method comprising: providing at least one first composition comprising at least one conjugated polymer and at least one solvent, wet spinning the at least one first composition to form at least one first fiber material, and hot-drawing the at least one first fiber material to form at least one second fiber material.
In a preferred embodiment, a method is provided comprising: providing at least one first composition comprising PEDOT/PSS and water, wet spinning the at least one first composition to form at least one first fiber material, hot-drawing the at least one first fiber material to form at least one second fiber material, wherein the method further comprises doping of the first composition and de-doping of the first fiber material.
Other embodiments are the materials provided by these and other methods described herein.
In preferred embodiments and working examples described further hereinbelow, semi-metallic, strong, and stretchable wet-spun PEDOT/PSS conjugated polymer microfibers were fabricated. For some embodiments, the novelty of the fabrication technique lies in a two-step method (wet-spinning followed by immediate hot-drawing) that greatly promotes molecular alignment. When combined with, for example, doping/de-doping by ethylene glycol (EG), it achieves what is believed to be a record electrical conductivity as high as 2804 S·cm−1. This is believed to be a six-fold improvement over the best value previously reported for fibers (467 S·cm−1) and double the best value reported for PEDOT/PSS films treated with EG (1418 S·cm−1).24 It was also found that these highly conductive fibers display a semiconductor-metal transition at 313 K. They also display superior mechanical properties: for example, the Young's modulus is 8.3±0.4 GPa, the tensile strength is 409.8±13.6 MPa and the elongation at break for these fibers is 21.2±1.4%. These results show that the combined properties of the preferred fibers have been remarkably improved in comparison with previously reported PEDOT/PSS fibers, as shown in
References cited herein can be used by one skilled in the art in the practice of the claimed inventions.
Priority U.S. provisional application Ser. No. 62/086,885 filed Dec. 3, 2014 is hereby incorporated by reference in its entirety including all figures and working examples and associated text linked to the figures and working examples.
One lead aspect provides for a method comprising: providing at least one first composition comprising at least one conjugated polymer and at least one solvent, wet spinning the at least one first composition to form at least one first fiber material, hot-drawing the at least one fiber to form at least one second fiber material. Each of these elements is now described in more detail.
For example, the providing step is not particularly limited as one can, for example, make a first composition or one can purchase a first composition. If one purchases a first composition, one can alter it after purchase. For example, one can dilute a purchased solution.
A fiber material can include, for example, filaments, yarns, and ribbons.
First CompositionThe first composition can comprise at least one conjugated polymer and at least one solvent, each of which is described further hereinbelow.
The first composition can be adapted for wet-spinning as known in the art. See, for example, Handbook of Conducting Polymers, 3rd Ed.; Conjugated Polymers, Processing and Applications, Eds. Skotheim and Reynolds, CRC Press, 2007, wherein Chapter 2 (2-1 to 2-72) describes fiber spinning of conjugated polymers.
The conjugated polymer and solvent can be in the form of a dispersion. The conjugated polymer may be in the form of small nano- or micro-sized particles. The solvent can act as a carrier even if the conjugated polymer is not fully soluble in the solvent. In other embodiments, the conjugated polymer may be in the form of a solution, not a dispersion.
In one embodiment, the first composition has a concentration of at least 11 mg/mL, or at least 15 mg/mL of conjugated polymer including in combination, if present, with polymeric dopant. In another embodiment, the first composition has a concentration of at least 22 mg/mL of conjugated polymer including in combination, if present, with polymeric dopant.
Conjugated PolymerConjugated polymers are known in the art. See, for example, Handbook of Conducting Polymers, 3rd Ed; Conjugated Polymers, Processing and Applications, Eds. Skotheim and Reynolds, CRC Press, 2007. They can also be called conducting polymers. The conjugated polymer can have extended conjugation along the backbone as known in the art.
Typical examples of conjugated polymers including polythiophene, polyaniline, polypyrrole, polyphenylene vinylene (PPV), polyacetylene, and the like. The conjugated polymer can be soluble or insoluble.
The conjugated polymer can be doped as known in the art. In one embodiment, the first composition comprises a polymeric dopant for the conjugated polymer. In one embodiment, the first composition comprises a polymeric dopant for the conjugated polymer which is a polyelectrolyte polymer. In one embodiment, the first composition comprises a polymeric dopant for the conjugated polymer which is polystyrene sulfonate (PSS). The polyelectrolyte, such as PSS, can be used in different forms with different forms of counterion including the acid and salt forms.
In one embodiment, the conjugated polymer comprises a polythiophene. In one embodiment, the conjugated polymer comprises a 3,4-di-substituted polythiophene. The substituents at the 3- and 4-positions can be, for example, alkoxy or polyether substituents, and they can be joined together to form a bridge if desired. The bivalent bridging moiety can be, for example, a bivalent alkyleneoxy group such as, for example, ethyleneoxy or propyleneoxy.
In one embodiment, the conjugated polymer comprises PEDOT which is known in the art. See, for example, PEDOT: Principles and Applications of an Intrinsically Conductive Polymer, Elschner et al., 2011. The method of making PEDOT is known and use of terms like the monomer, EDOT, and the doped form of the polymer, PEDOT:PSS are known.
SolventSolvent means a liquid to carry the conjugated polymer in the first composition whether or not a true solution is formed. The solvent can include one or more compounds and can be called also a “solvent system.” One compound can be water. The water can be mixed with one or more organic compounds and organic solvents.
In one embodiment, the first composition is an aqueous dispersion. In one embodiment, the first composition comprises water and at least one polar solvent, including for example at least one protic polar solvent. Aprotic solvents can be used.
Examples of solvents include DMSO, NMP, or an alcohol such as ethanol.
Wet-SpinningWet-spinning is known in the art. See, for example, Handbook of Conducting Polymers, 3rd Ed.; Conjugated Polymers, Processing and Applications, Eds. Skotheim and Reynolds, CRC Press, 2007, including Chapter 2 (2-1 to 2-72).
For sake of clarity, one embodiment is that the wet-spinning is not an electro-spinning process.
In one embodiment, the wet spinning is carried out with a coagulation bath comprising a mixture of solvents such as, for example, a mixture of two organic solvents such as, for example, a mixture of a C2-C5 ketone and a C2-C5 alcohol, such as for example, a mixture of acetone and isopropyl alcohol. The volume ratio of the two solvents can be, for example, 3:1 to 1:3, or 2:1 to 1:2, or about 1:1.
The spin-dope preferably comprises 1.1 wt. % to 6 wt. %, or 1.1 wt. % to 3.1 wt. %, or 2.2 wt. % to 3.1 wt. %, conjugated polymer based on the total weight of the spin-dope. These relatively low concentrations of conductive polymer in the spin-dope enable the resulting conductive polymer fiber to have for at least some embodiments good spinability, a high conductivity, and a higher modulus.
The extruded conjugated polymer fibers can be spun directly into the coagulation medium without an air gap. The coagulation medium can be contained in a coagulation bath.
The extruded conductive polymer fibers may be spun horizontally, vertically or even under an angle to the vertical direction. In an embodiment, the extruded conductive polymer fibers can be spun directly into the coagulation bath in vertical direction. Extruding conductive polymer fibers in a vertical direction is especially preferred as the density of the spun conductive polymer fibers is higher than the density of the coagulation medium. At start-up of the process, the extruded conductive polymer fibers will go to the bottom of the coagulation bath where the conductive polymer fibers can be picked up and collected through the coagulation bath vertically. The spun conductive polymer fibers enter the coagulation medium directly to coagulate the conductive polymer fibers to increase the strength of the conductive polymer fibers and to ensure that the “wet” conductive polymer fibers are strong enough to support their own weight. The extruded conductive polymer fibers are stretched by gravity forces and are supported by the liquid coagulation medium and are not break up into smaller pieces under their own weight. Vertical spinning can be partially helpful for the electrical conductivity and modulus enhancement and essential for the following step of hot-drawing and temperature control.
First Fiber Material and Hot-DrawingThe first fiber material is in essence an intermediate as it will usually be subjected to hot drawing processes and other processing steps as described herein.
After the wet fiber is taken from the coagulation medium, it can be immediately heated and subjected to a vertical hot-drawing process, as shown in
Hot drawing temperature around the fiber can be controlled by one or more (e.g. two) hot plates with a distance around 4 cm and monitored by, for example, a thermal couple.
In one embodiment, the hot drawing is done at a temperature of 50° C. to 140° C., and in another embodiment, the temperature is 50° C. to 120° C., and in another embodiment, the temperature is 60° C. to 90° C.
Second Fiber Material and Overall Fiber Spinning ProcessThe second fiber material can be collected and dried (or dried and collected) as known in the art.
The overall process can be carried out with a draw ratio which is at least 3, or at least 4, or at least 5, or at least 10.
In one embodiment, the second fiber material has a diameter of at least one micron, or at least 2 microns, or at least 3 microns, or at least 5 microns. In another embodiment, the second fiber material has a diameter of at least 10 microns.
The fiber length ranging, for example, from a micrometer to virtually endless. In one embodiment, the fiber length is at least 10 cm, or at least one meter, or at least 10 m, or at least 100 m. In another embodiment, the fiber length is at least 500 m. In another embodiment, the fiber length is at least 1,000 m.
Secondary Doping StepIn one embodiment, the method further comprises a secondary doping of the first composition. In one embodiment, the doping is carried out with at least one organic solvent such as, for example, an oxygenated solvent such as an alkylene glycol. In one embodiment, the doping is carried out with ethylene glycol. Poly(ethylene glycol) can be used.
The working examples hereinafter further describe embodiments for the doping step.
De-Doping StepIn one embodiment, the method further comprises de-doping of the first fiber material before hot drawing. In one embodiment, the method further comprises doping of the first composition, and the method further comprises de-doping of the first fiber material before hot drawing.
In one embodiment, the de-doping is carried out with at least one organic solvent such as, for example, an oxygenated solvent such as an alkylene glycol. In one embodiment, the de-doping is carried out with ethylene glycol.
The working examples hereinafter further describe embodiments for the de-doping step including coupling of doping and de-doping steps.
Fibers and Properties of FibersOne embodiment is a fiber material prepared by the methods described herein. These include intermediate fibers before all processing steps are finished as well as final fiber materials including fiber materials which are disposed into articles and devices.
In one embodiment, the fiber material shows a conductivity of at least 300 S·cm−1, or at least 368 S·cm−1 or at least 500 S·cm−1. In another embodiment, the fiber material shows a conductivity of at least 2,000 S·cm−1 or at least 2,800 S·cm−1.
In one embodiment, the fiber material shows a cross-over temperature of semiconductor-to-metal transition of at least 25° C. In another embodiment, the fiber material shows a cross-over temperature of semiconductor-to-metal transition of at least 40° C.
In one embodiment, the fiber material shows a Young's modulus of at least 5 GPa, a tensile strength of at least 300 MPa, and an elongation at break of at least 10%. In another embodiment, the bundle of fibers shows a Young's modulus of at least 8 GPa, a tensile strength of at least 409 MPa, and an elongation at break of at least 21%.
Methods of Use and ApplicationsThe fiber materials can be used in many applications including, for example, actuators, including electromechanical and electrochemical actuators, various sensors including vapor and humidity sensors and strain sensors, resistive heaters and resistive heating textiles, electromagnetic interference shielding, flexible electrodes, and wearable electronics.
Fiber extensional actuators can be prepared which convert electricity or other input energy to mechanical energy at a low voltage and a very high frequency (e.g., up to 10 Hz) in air.
Additional embodiments are provided in the following non-limiting working examples (including Parts I and II).
WORKING EXAMPLES Results and Discussion (Part I)The Effect of Hot-Drawing on the Conductivity of as-Spun PEDOT/PSS Fibers.
The Effect of EG Doping and/or De-Doping.
To obtain the highest possible electrical conductivity of the conjugated polymer fibers, doping and/or de-doping processes were applied to these fibers, as shown in Table A (Supplemental Working Example Information, hereinafter). First, doping was accomplished by adding 3 wt % EG with respect to the concentrated PH1000 dispersion (22 mg/mL). The average diameter of the fabricated fibers (EG/(PEDOT/PSS) fiber) was 9.7±1.4 μm (
The effect of de-doping was studied separately. Specifically, the as-spun fibers produced from the concentrated PH1000 dispersion (22 mg/mL) were de-doped by directly immersing the fibers in an EG bath for 1 hour and a conductivity of 1304±56 S·cm−1 was achieved. While not limited by theory, perhaps the main reason for the conductivity enhancement by de-doping is the partial removal of amorphous PSS.8
Earlier research has shown that an effective way of achieving the highest conductivity (1418 S·cm−1) of PEDOT/PSS films is to use both doping and de-doping processes together.24 Here, we achieved a striking increase in the electrical conductivity of the fibers: from 607±60 S·cm−1 to 2804±311 S·cm−1. This value is six times better than best value for previous reported fibers (467 S·cm−1) and twice the best value (1418 S·cm−1) of PEDOT/PSS films with EG treatment.24 Although a conductivity for PEDOT/PSS film ranging from 2400 to 4380 S·cm−1 has been achieved by treating the film with sulfuric acid, the use of strong and corrosive acid will cause safety concerns and is undesirable in commercial device fabrication.18,25 In this work, by assisting the wet-spinning process with hot-drawing and doping/de-doping the fibers with EG, the inventors can achieve a high conductivity that is comparable with the highest values for PEDOT/PSS films. The inventors also demonstrate the electrical properties of our fibers by wiring a light-emitting diode (LED) with two PEDOT/PSS conductive fibers. The fiber resistance was low enough to light the LED at 6 V, as shown in
Apart from the electrical conductivity at room temperature (RT), a second important factor characterizing electrical transport in conductive fibers is the dependence of conductivity on temperature.
To understand the electrical properties of the conductive fibers, the inventors studied the surface and inner structural changes of the fibers. The inventors used X-ray photo-electron spectroscopy (XPS) to investigate the as-spun PEDOT/PSS fibers and EG/(PEDOT/PSS) fibers before and after EG de-doping. High-resolution XPS of the S 2p core-level spectra are shown in
AFM images were taken to probe the changes in the surface microstructures of all fibers. The root mean square (rms) roughnesses measured from height images of as-spun PEDOT/PSS fiber and EG/(PEDOT/PSS) fiber were 11.2 and 14.1 nm, respectively. After de-doping with EG, the roughnesses increased to 21.8 and 18.0 nm, respectively (
To investigate the inner microstructural changes between the as-spun PEDOT/PSS fiber and the EG/(PEDOT/PSS)/EG fiber, the samples were cut into nanofilms by Focused Ion Beam (FIB) along the fiber axis direction and investigated by High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM). The average thickness of the films was determined to be 56.7 nm from the effective mean free path.36
To understand the electrical transport in the fibers, the fiber microstructure was investigated by transmission Wide Angle X-ray Scattering (WAXS). The fibers are bundled and aligned perpendicularly to scatter the x-ray beam, as shown in
The EG doped fibers have a similar structure as the as-spun fibers but they have better crystallinity.
An important feature of these fibers is that they can be easily shaped into highly flexible structures, which is crucial to successful applications requiring stretchable electronics and functional fabrics. To illustrate this, a bundle of fibers was wound around a thin glass capillary (diameter: 5 mm) to form a helical shape after drying (inset in
Next, the inventors investigated the single fiber mechanical behavior.
Our results indicate that doping and/or de-doping the fibers with EG improved the mechanical properties. These fibers had a tensile strength of 409.8±13.6 MPa and a Young's modulus of 8.3±0.4 GPa, which are three and 1.5 times higher than previously reported, respectively.9 Generally, while not limited by theory, the inventors attribute these improvements to the partial removal of amorphous PSS (
where σm is the final tensile stress and D is the plastic strain, as shown in
The superior mechanical and electrical properties motivated the inventors to further investigate the degradation of the mechanical and electrical properties by stretching/unstretching. First, the inventors performed cyclic loading/unloading tests (
The relative resistance change (ΔR/R0) was also monitored under cyclic loading/unloading with progressive extension of the as-spun PEDOT/PSS fiber and EG/(PEDOT/PSS)/EG fiber (
R=1/πr2σ (2)
where C is the electrical conductivity, and l and r are the length and the radius of the fiber, respectively. To determine the intrinsic conductivity change, the pure geometric factor was calculated and subtracted from the measured resistance. The relationship between the relative resistance change and the conductivity change can be expressed by:
ΔR/R0=−Δσ/σ+Δl/l−2Δr/r (3)
For small changes Δl/l=ε1 and Δr/r=νεs1, where ε1 is the strain in the fiber direction, and ν is the Poisson's ratio. substituting these values in equation (3) gives
ΔR/R0=−Δδ/σ+ε1(1+2ν) (4)
where F=ε1(1+2ν) comes only from the change in the cross-section of the fibers, whereas the first part on the right-side term, Δσ/σ, reflects changes in the conductivity of the material that can also be dependent on the strain.
For the as-spun PEDOT/PSS fibers, the relative resistance change is 0.23 at 13% strain level. Assuming ν=0.34,44 one can calculate F=0.22 and Δσ/σ≈0. Therefore, the variation in resistance can be mostly attributed to the change in the geometry of the sample. However, it is important to highlight that the EG/(PEDOT/PSS)/EG fiber exhibits a much lower variation in resistance. At the same strain level of 13%, the resistance change is only 0.05, which is almost 3.5 times lower than that of the as-spun fiber. With the previous values, one finds that ΔΓ/σ=0.17, suggesting an increase in the conductivity of the fiber.
Correlation Between the Microstructure and the Electrical and Mechanical Properties.The morphology, microstructure and electromechanical analyses of the fibers provide some insight into the correlation between the PEDOT/PSS microstructure and the electrical and mechanical behaviors. While not limited by theory, the stages of the molecular level deformation of PEDOT/PSS fiber are proposed in
To address the mechanism that leads to electrical conductivity changes in the fibers, a schematic deformation of PEDOT/PSS grains in fibers before and after EG de-doping is displayed in
In conclusion, in the working examples, the inventors systematically studied the effects of hot-drawing and EG doping/de-doping on the conductivity of wet-spun PEDOT/PSS microfibers. Specifically, hot-drawing can improve the conductivity from 187 to 368 S·cm−1, and EG doping these fibers (3 wt %) further improves the conductivity from 368 to 607 S·cm−1. Finally, with EG de-doping the EG doped fibers, the conductivity value reaches as high as 2804 S·cm−1, the highest value in conjugated polymer fibers. All the fibers in this study display a semiconductor-metal transition around 313 K. The results also show a clear correlation between the microstructure and the electrical and mechanical properties. In particular, the inventors found a maximum Young's modulus of 8.3 GPa for the most conductive fiber, which corresponds an increase of 41% over as-spun PEDOT/PSS fibers. The enhanced properties resulted from microstructural refinement, which was achieved by (1) preferred alignment of PEDOT molecule chains through hot-drawing, (2) reduction in the electrostatic interaction of PEDOT and PSS by EG doping and (3) partial removal of amorphous PSS from the fibers by EG de-doping. The fibers with enhanced properties also show superior stretchability and are able to retain high stiffness with an obvious increase in electrical conductivity (25%) at strain levels as high as 21%. These results can provide a foundation for performance maximization of conjugated polymer microfibers and pave the way for stretchable electronics in the form of fiber structures.
Experimental Methods (Part I) Materials.The PEDOT/PSS aqueous dispersion (Clevios™P and PH1000) was purchased from HC Starck, Inc. Ethylene glycol (EG), Isopropyl alcohol (IPA), and acetone were purchased from Sigma-Aldrich.
Preparation of Highly Spinable Inks.PEDOT/PSS inks: 10 mL of water was evaporated from 20 mL of the PH1000 dispersion (11 mg mL−1) at 50° C. to increase the viscosity of the ink. Then 0.3 g of EG was mixed into the concentrated PH1000 dispersion (22 mg mL−1) by a magnetic stirrer for two hours to enhance the electrical conductivity.17,47 Then it was homogenized at 20,000 rpm for 5 mins using a T18 homogenizer (IKA) and followed by 20 mins bath sonication using a Brason 8510 sonicator (250 W, Thomas Scientific) at room temperature. Finally, the dispersion was degassed in a vacuum oven at room temperature (21° ° C.) before wet-spinning.
Wet-Spinning of PEDOT/PSS Fibers.The spinning formulation was loaded into a 5 mL glass syringe and spun into a coagulation bath though a metal needle with an inner diameter from 100 to 220 μm. The flow rate of the ink was controlled between 2 to 50 μL min−1 by using a syringe pump. The fibers were collected vertically onto a 50 mm winding spool, which gives a line speed of 2 to 4 m min−1. The air temperature along the path of the fiber was controlled by two vertically located hot-plates (see
Electrical resistance of the fibers was measured by using an Agilent 1252B multimeter. The electrodes on the fiber were made by connecting a copper wire to the fiber surface with silver epoxy. The distance between two contacts was about 10 mm. The temperature-dependent DC electrical conductivity was measured by the two probe method in a temperature controllable chamber, in which highly pure N2 was purged at a flow rate of 200 mL min−1 to protect the sample and measurement electronics from the humid air. The electrical conductivity measurements of the specimens were carried out in the temperature range from −150 to 220° C. with a heating rate of 5° C. min−1. At least three measurements were conducted for each type of fiber.
Scanning electron microscopy (SEM) was performed using a Quanta 3D (FEI Company). TEM samples were prepared by focused ion beam (FIB) cutting with help from the same SEM machine. The conductive fibers were first fixed on the SEM holder with silver epoxy. The samples were tilted 52° and cuts were made along the fiber axis direction. The ion beam source was a field-emission FIB with a Pt ion emitter, an ion beam voltage of 30 kV and a beam current of 0.47 nA.
X-ray photoelectron spectroscopy (XPS) analyses were carried out with a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα X-ray source (hv=1486.6 eV) operating at 150 W, a multichannel plate and delay line detector under a vacuum of 1×10−9 mbar. All spectra were recorded using an aperture slot of 300×700 μm. The survey and high-resolution spectra were collected at fixed analyzer pass energies of 160 eV and 20 eV, respectively. The XPS peaks were analyzed using a Shirley-type background and a nonlinear least-squares fitting of the experimental data based on a mixed Gauss/Lorentz peak shape. XPS quantification was performed by applying the appropriate relative sensitivity factors (RSFs) to the integrated peak areas.
Atomic Force Microscopy (AFM) images of conductive polymer fibers were taken using an Agilent 5400 (Agilent Technologies) microscope in the tapping mode over a window of 1 μm×1 μm. The fibers were fixed on glass slides with a thin layer of epoxy adhesive.
TEM images of conductive polymer fibers were taken by using a Titan G2 80-300 CT (FEI Company) at an accelerating voltage of 300 kV equipped with a field-emission electron source. The PEDOT/PSS thin films cut from fibers by FIB were analyzed with High-Angle Annular Dark-field Scanning Transmission Electron Microscopy (HAADF-STEM) observation modes at a dose of 187 e Å-2. The HAADF-STEM micrographs were recorded with an analog detector (E. A. Fischione, Inc). The entire image acquisition as well as processing of the data was accomplished by using the GMS v1.8.3 microscopy suite software (Gatan, Inc).
Transmission Wide-angle X-ray Scattering (WAXS) measurements were performed on the D-line, Cornell High Energy Synchrotron Source (CHESS) at Cornell University. The fibers were aligned vertically into a bundle and placed perpendicularly into a monochromatic x-ray beam with the wavelength of 0.115 nm. The scattering patterns were collected by a CCD detector (Medoptics) with a pixel size of 46.9 μm at a distance 100 mm away from sample. The exposure time was 10 s. To present the anisotropic scattering, the plots were integrated along the horizontal and vertical directions in the ±5° region by the Fit2d program.48
Normal WAXD tests were performed from 2 to 350 in a continuous mode using a Bruker D8 Advance powder X-ray diffractometer, with Cu-Kα radiation (λ=1.54 Å) at 40 kV and 40 mA. To generate peaks with relatively high intensity, a slow increment at 0.020 and a slow scan speed at 12 sec/step was applied, and the percentage of crystallinity was determined by the Diffract.EVA software (Bruker). Raman spectra were collected using a LabRAM Aramis Raman spectrometer (Horiba, Ltd.) on casted films and fibers using a 632 nm laser.
The mechanical behavior of the fibers was measured by an 5966 Instron universal testing machine at a strain rate of 5% min−1. The tests were performed inside an enclosure to protect the fibers from environmental disturbances. 2 cm long fibers were prepared and fixed on a paper card. The tensile strength, Young's modulus and elongation were calculated, and the values were collected from at least 10 tests for each formulation. The electrical resistance change of the fibers was monitored using an U1252B digital multimeter. A cyclic loading/unloading program was applied to the fiber with an incremental extension of 0.2 mm at each cycle and then releasing to a load of 1 mN. The resistance data were captured every 1 s during the test. Two ends of the samples were connected with copper wires and painted with silver epoxy, followed by sealing the silver epoxy area by epoxy glue.
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Research has showed that fiber spinnability can be greatly affected by various physical parameters, such as composition of the coagulation bath, the ink concentration and viscosity, the spinning speed and nozzle diameter. (See Reference S-1 and Reference S-2). Among these parameters, viscosity and spinning speed can play important roles in obtaining continuous fibers.
Spinning Dope Formulation:
Pristine PHI 000 dispersion (11 mg/mL) can form fibers over a wide range of spin speeds from 0.3 to 150 μL/min. However, fibers can easily break in the coagulation bath or in the air due to the low viscosity of the dispersion. Higher concentrations of PHI 000 dispersion (22 mg/mL) greatly improve the stability of the wet-spinning process due to the easier entanglement of the polymer chains. To improve the conductivity of the fibers, EG was mixed with the PH1000 dispersion (22 mg/mL) (this process was called EG doping). We found that 3 wt % of EG was the optimized concentration for doping concentrated PH1000 dispersion (22 mg/ml), and the final conductivity was as high as 607.0±60.2 S·cm−1.
Fiber Hot-Drawing:
After the “wet” fiber was taken from the coagulation bath, it was immediately heated and subjected to a vertical drawing process, as shown in
By fixing the dispersion concentration at 22 mg/ml, and keeping the same collection speed, two kinds of coagulation baths were tried. Conductivity of the fibers was 231±12 S·cm−1 from the acetone bath, while the conductivity increased to 368±34 S·cm−1 by using an acetone/IPA (volume ratio 1:1) bath. Research has shown that the conductivity of PEDOT/PSS films can be enhanced from 0.30 to 468 S·cm−1 by IPA dip-treatment for 10 min. (See Reference S-4). As the continuous wet-spinning process ends up with a dip time of 10 s for the fibers in the bath, the conductivity enhancement should be minimal. However, introducing IPA could greatly reduce the number of pores inside the fibers, which would result in better mechanical properties. (See Reference S-5, Reference S-6). Thus, in this study, a mixture of acetone/IPA (volume ratio 1:1) was used as the coagulation bath to maximized the fiber quality.
Spinnability:Spinnability of the inks was monitored in an acetone/IPA (volume ratio 1:1) tank by a high speed camera. Commercial Clevios PH1000 ink after homogenization and sonication was first spun through a 50 μm glass nozzle. The concentration of prepared PH1000 ink (11 mg/mL) was relatively smaller compared with earlier studies, e.g., Clevios P (13 mg/mL), PH500 (11 mg/mL) and even Orgacon dry inks (15-30 mg/mL). See Reference S-5, Reference S-7, Reference S-8. It is noted that PH1000 ink in the current study had a wide range of spin speeds from 0.3 to 150 μL/min.
B. Morphology of Conductive Polymer Fibers (SEM, AFM)
To understand the electrical transport in the fibers, microstructure characterization of the fibers was carried out by normal XRD and Raman spectroscopy.
See
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Additional embodiments are provided hereinbelow for Part II including further working examples which supplement Part I. Conductive fibers with enhanced physical properties and functionalities are needed for a diversity of electronic devices. Here, we report very high performance in the thermal and mechanical response of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) microfibers when subjected to an electrical current. These fibers were made by combining hot-drawing assisted wetspinning process with ethylene glycol doping/de-doping can work at a high current density as high as 1.8×104 A cm−2, which is comparable to that of carbon nanotube fibers. Their electrothermal response was investigated using optical sensors and verified to be as fast as 63° C. s−1 and is comparable with that of metallic heating elements (20-50° C. s−1). We investigated the electromechanical actuation resulted from the reversible sorption/desorption of moisture controlled by electro-induced heating. Results revealed an improvement of several orders of magnitudes compared to other linear conductive polymer-based actuators in air. Specifically, the fibers we designed here have a rapid stress generation rate (>40 MPa s−1) and a wide operating frequency range (up to 40 Hz). These fibers have several characteristics including fast response, low-driven voltage, good repeatability, long cycle life and high energy efficiency, favoring their use as heating elements on wearable textiles and as artificial muscles for robotics.
IntroductionFor Part II, additional introduction is provided. Electroactive materials that convert electrical energy to thermal or mechanical energy have great potentials for many applications, including heating components for wearable textiles and artificial muscles for robots (for Part II, a new listing of references is provided, A1-A64). (A1-4) Among these electroactive materials, conductive polymers are very promising for these types of applications because they can be easily shaped into low-voltage driven actuators and sensors. (A5-7) One of the most widely used conductive polymers, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), is ideal for such applications because it is lightweight, and has good processability, tunable electrical conductivity, suitable mechanical properties and good thermal stability. (A8-12)
Previous works have attempted to design electroactive materials based on PEDOT/PSS. The conversion of electrical energy into thermal energy has been realized using PEDOT/PSS microfilms or silver nanowire/PEDOT/PSS transparent nanofilms as heaters, although the heaters had slow response times (15-80 s). (A13-14) Meanwhile, the conversion of electrical energy into mechanical energy has been widely investigated in PEDOT/PSS film or papers. The well-known actuation mechanism is strongly related to the highly hygroscopic nature of PEDOT/PSS. When PEDOT/PSS is subjected to an electrical current, Joule heating induces the desorption of water, which subsequently results in a volume contraction. Actuation in PEDOT/PSS-based devices thus results from an electro-thermal-mechanical coupling, a multiphysics nature that facilitates various actuation stimuli. (A13, A15-19) For example, a (PEDOT/PSS)/elastomer bilayer actuator was developed to achieve a bending motion under multiple control stimuli, such as electrical current, heat and humidity changes. (A20) To date, most research on PEDOT/PSS-based electroactive materials has focused on two-dimensional films or coated papers. (A13, A16, A17, A19) Therefore, opportunities to design PEDOT/PSS fibers or wires for specific applications, such as functional textiles, or to increase their sensitivity in general remain outstanding.
In a previous study, we investigated the electromechanical actuation properties of low conductivity (PEDOT/PSS)/polyvinyl alcohol fibers, which can generate a maximum actuation stress of 11 MPa at 8 V. (A21) However, they feature a low response time (80 s), which leads to a slow stress generation rate (0.14 MPa s−1) and a narrow operating frequency window (<1 Hz). Other conductive polymer fiber-based actuators have also been developed, where their actuations are controlled by oxidation-reduction reactions in liquid. These materials can produce large actuation strain but have limited cyclability and the technique requires complicated counter electrode, electrolyte and device packaging. (A6, A22)
A direct consequence of low efficiency in converting electrical energy to thermal or mechanical energy is an increase in power consumption. Although it has been shown that short-time application of high voltages does not harm the polymer, long term operation results in electrical failure of the materials. (A23) Small size microactuators should also feature enhanced response rate and be powered with coin batteries. (A5) In addition, increasing the conductivity of the material can improve response rate and permit the use at low voltage power sources. In our previous study, we fabricated highly conductive polymer microfibers by combining hot-drawing assisted wet-spinning process with chemical treatment (ethylene glycol doping/de-doping). (A24) Results showed that the performance of these fibers was superior to that of other conductive polymer fibers, with conductivity ranging from 368 to 2804 S·cm−1 and Young's modulus values as high as 8.3 GPa. (A25, A26) These improvements favored these fibers for use as actuators. For instance, their high conductivity allows them to withstand high current density for long periods of time before electrical failure, and their high Young's modulus imparts good structural stability to the systems based on these fibers.
Here, we demonstrate fast electrothermal and electromechanical responses of a new generation of PEDOT/PSS fibers made by combining hot-drawing-assisted wet-spinning process and chemical treatment. By increasing the conductivity of the fibers, current density can be considerably increased a maximum current density of 1.8×104 A cm−2, which is two orders of magnitude of copper wires and comparable to carbon nanotube fibers, indicating their high potential for used as interconnects in a electrical circuit. Within the operating range before electrical failure, the electrothermal response was investigated systematically using fiber Bragg gratings (FBGs). The single conductive polymer fiber shows rapid electrothermal responsiveness and high reversibility at low voltages. Based on these features, the fiber bundle was demonstrated as wearable heaters on a glove. The fibers also showed remarkable electromechanical behavior induced by Joule heating and featured in fast response, long cycle life and excellent repeatability. Additionally, we show that bundles of novel fibers could be used as actuators, which can lift a load much heavier than itself. The availability of these fibers with high conductivity, high ampacity, and rapid electrothermal and electromechanical responsiveness could facilitate the development of, for example, electronic interconnects, wearable heating textiles and artificial muscles.
Experimental (Part II) MaterialsThe PEDOT/PSS aqueous dispersion (Clevios™ PH1000) was purchased from HC Starck, Inc. Ethylene glycol (EG), isotropyl alcohol (IPA) and acetone were purchased from Sigma-Aldrich.
Preparation of PEDOT/PSS FibersFiber preparation strategies are systematically illustrated in
Thermogravimetric analysis (TGA) was performed to measure the water absorbed in the fibers using a TG 209 F1 instrument (NETZSCH Company) under a nitrogen purge. Samples were heated from 25 to 800° C. at a heating rate of 10° C. min−1.
Characterization of Properties and Demonstration for Applications as Heating ElementsTo assess the potential of these fibers for applications as heating elements, their electrothermal response needed to be accurately calculated. We followed a previous report based on fiber Bragg gratings (FBG) to make temperature measurements for single PEDOT/PSS fibers subjected to electrical stimulus; FBGs have been proven to be suitably efficient both experimentally and numerically. (A27) Measurements were performed inside a chamber to avoid environmental disturbances, and the experimental set-up is sketched in
ΔλB/λB=KTΔT (1)
where, KT, is the thermo-optic coefficient related to the temperature sensitivity of the FBG. This sensitivity was calibrated by immersing it with a thermocouple in a beaker filled with water, and the sensitivity of the FBG was determined to be 9.9 μm ° C.−1. (A27) We used 5-mm long FBGs written on SMF28e standard fibers, and the coating was removed. The Bragg wavelength of the sensor was 1530 nm at room temperature. The radiation/convection heat transfer was also estimated by measuring the temperature at about 700 mm away from the PEDOT/PSS fibers by using another FBG (FBG2.a), which was connected to the second channel (CH2) (
To test the suitability of the fibers as actuators, the following tests were conducted. Demonstration of a PEDOT/PSS fiber bundle actuator was performed under an isotonic measurement, where a load was attached to the movable end of the bundle to eliminate bulking. Displacement at the movable end was measured with a video camera, and thermal images of the fiber bundle during the actuation test were taken with an SC7000 thermographic camera. Single-fiber actuation test was performed under isometric conditions (the length of the fiber remained to be constant during the duration of the test). All tests were performed by applying a preload of 0.5 mN, which kept the fibers straight. Two types of tests are performed by applying either a constant voltage or a harmonic voltage variation with prescribed frequency. In both cases, the force generated by the fiber was monitored, where sampling rate depend on frequency. The data acquisition rate was every 100 ms at 0.02, 0.1, 0.25 and 0.5 Hz; every 20 ms for 1, 2 and 5 Hz; every 2 ms for 10, 20 and 40 Hz. All tests were performed at room temperature (21° C.) with a relative humidity of 60%. The actuation stress values were determined by normalizing the measured load to the fiber cross-sectional area measured before the test. Dynamic mechanical analysis (DMA) was performed on arrays made from 15 fibers of approximately 6 mm in length on a Q800 instrument (TA Instruments) in tension mode. DMA measurements were performed between 25 and 120° C., at 1 Hz and at a heating rate of 3° C. min−1 in air.
Results and Discussion High Current Density of PEDOT/PSS FibersBefore any deeper investigation into the electrothermal and electromechanical behavior, we need to determine the maximum current density (denoted as jb) that each formulation could sustain before reaching electrically induced failure. By gradually increasing the applied voltage on the fibers, heat induced by Joule heating finally result in the fiber breakage. The current carrying capacity is defined here as a maximum current density at which two ends of the fiber fixed on a paper card and show a constant resistance during the experiment. The method for measuring resistance is described earlier by a two-probe method. If the temperature increase in the fiber, caused by an increase in electrical resistance, does not stabilize then at a certain temperature, the fiber breaks.
The SEM images in
In fact, the weakest part of the fibers is the amorphous nonconductive PSS interface that connects PEDOT/PSS grains. The decomposition temperature of PSS phases is low enough to cause an electrical current that results in over-heating followed by failure of the material (
Chemical treatment through EG doping/de-doping not only improves the maximum current density, but also increase the conductivity of PEDOT/PSS fibers (
Table 2 compares the performance of various electrically driven heaters prepared by different methods. The first-order response time of PEDOT/PSS fibers is comparable with tungsten wires or copper interconnects, which need several hundred mini-seconds to several seconds. (A36, A37) Moreover, these fibers have the fastest response rate (63° C. s−1) compared with other types of heaters. Nanomaterial-based heaters, such as silver nanowires (AgNWs), carbon nanotubes (CNTs) and graphene, generally need a long response time for heating because substrates hinder the measured heating rate as compared to a free-standing arrangement. Some other PEDOT based heaters, such as PEDOT/PSS film and PEDOT nanofiber mat, have a response time several times longer than our PEDOT/PSS fibers due to their low conductivities. (A13, A54) To summarize, the merit of PEDOT/PSS fibers as heaters lies in (1) very fast response which is comparable with metal wires; (2) low density, high conductivity, high flexibility, stretchability and better tolerance to frequent bending and contact as compared to carbon fibers and metallic wires (A57); and (3) good spinnability for directly co-spinning with nonconductive polyester fibers from different nozzles, which can be easily twisted and woven into textiles for the application of wearable heaters.
As we know that the temperature range required for wearable applications is much lower than the maximum temperature generated by the fiber (90° C.) and the initial thermal degradation temperature of the fiber (265° C.), the thermal stability of the PEDOT/PSS fiber is estimated to be appropriate for wearable heating textiles.
We applied voltages to a similar fiber bundle as that used in the heatable gloves and found that it displayed linear electromechanical motion. The preliminary actuation performance of the fibers presented as a bundle actuator containing 118 fibers with 14 cm in length under isotonic conditions, as show in
By measuring the tensile actuation of single fibers, we learned more about what affects their actuation performance (
Square-wave voltages at different frequencies were applied to further investigate the high frequency performance of PEDOT/PSS fiber actuators. Inset curves in
Finally,
In summary, in Part II, we systematically studied the current density, electrothermal and electromechanical responses of highly conductive PEDOT/PSS fibers. We construct these fibers by a combination of thermomechanical (hot-drawing-assisted wetspinning) and chemical treatment (EG doping/de-doping). We found that high-performance PEDOT/PSS fibers can carry high current density that is comparable with carbon nanotube fibers, making them promising candidates for use as interconnects in circuits. These fibers could also be used as heating elements in wearable textiles because they enable rapid heating at low operation voltages and show excellent heating repeatability. Moreover, the unique electromechanical response of PEDOT/PSS fibers surpasses other conductive polymer based actuators working in air. They feature with low-driven voltage, fast response time (<0.5 s), a wide frequency window (up to 40 Hz), excellent repeatability (10,000 cycles) and controllability in air. We demonstrated that a PEDOT/PSS fiber bundle can be used as a glove heater powered by a 9-V battery. The fiber bundle can also be used as a linear actuator to lift a load that is 150 times of its own weight. Our results provide the basis bear the potential of these fibers to be implemented in wearable heating textiles and in microelectromechanical systems that need actuators.
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Claims
1. A method comprising: providing at least one first composition comprising at least one conjugated polymer and at least one solvent, wet spinning the at least one first composition to form at least one first fiber material, hot-drawing the at least one first fiber material to form at least one second fiber material.
2. The method of claim 1, wherein the method further comprises doping of the first composition.
3. The method of claim 1, wherein the method further comprises de-doping of the first fiber material before hot drawing.
4. The method of claim 1, wherein the method further comprises doping of the first composition and de-doping of the first fiber material before hot drawing.
5. The method of claim 2, wherein the doping is carried out with at least one organic solvent.
6. The method of claim 2, wherein the doping is carried out with ethylene glycol.
7. The method of claim 3, wherein the de-doping is carried out with at least one organic solvent.
8. The method of claim 3, wherein the de-doping is carried out with ethylene glycol.
9. The method of claim 1, wherein the conjugated polymer comprises a polythiophene.
10. (canceled)
11. The method of claim 1, wherein the conjugated polymer comprises PEDOT.
12. The method of claim 1, wherein the first composition comprises a polymeric dopant for the conjugated polymer.
13-14. (canceled)
15. The method of claim 1, wherein the first composition is an aqueous dispersion.
16-19. (canceled)
20. The method of claim 1, wherein the hot drawing is done at a temperature of 50° C. to 140° C.
21-26. (canceled)
27. A method comprising: providing at least one first composition comprising PEDOT:PSS and water, wet spinning the at least one first composition to form at least one first fiber material, hot-drawing the at least one first fiber material to form at least one second fiber material, wherein the method further comprises doping of the first composition and de-doping of the first fiber material before hot-drawing.
28. A fiber material prepared by the method of claim 1.
29. (canceled)
30. The fiber material of claim 28, wherein the fiber material shows a conductivity of at least 2,000 S·cm−1.
31. The fiber material of claim 28, wherein the fiber material shows a cross-over temperature of semiconductor-to-metal transition of at least 25° C.
32. The fiber material of claim 28, wherein the fiber material shows a cross-over temperature of semiconductor-to-metal transition of at least 40° C.
33. The fiber material of claim 28, wherein the fiber material shows a Young's modulus of at least 5 GPa, a tensile strength of at least 300 MPa, and an elongation at break of at least 10%.
34. The fiber material of claim 28, wherein the fiber material shows a Young's modulus of at least 8 GPa, a tensile strength of at least 409 MPa, and an elongation at break of at least 21%.
Type: Application
Filed: Dec 3, 2015
Publication Date: Dec 28, 2017
Applicant: KING ABDULLAH UNIVERSITY OF SCIENCE AND TECHNOLOGY (Thuwal)
Inventors: Jian ZHOU (Thuwal), Er Qiang LI (Thuwal), Gilles LUBINEAU (Thuwal), Sigurdur THORODDSEN (Thuwal), Matthieu MULLE (Thuwal)
Application Number: 15/525,005