Vapor Phase Deposition of Electroactive Poly(3,4-ethylenedioxythiophene) onto Electrospun Commodity Polymer Nanofibers

Summary

This paper describes a simple technique for coating electrospun PAN nanofibers with electroactive polymers using vapor deposition. The effects of the oxidant FeCl₃ on the deposition of poly(3,4-ethylenedioxythiophene) coatings on PAN were investigated to understand and optimize polymer growth, fiber diameter, and overall mechanical strength of the nanofiber mats.

Abstract

This study investigates the preparation of polyacrylonitrile (PAN) nanofibers through electrospinning to create highly porous and strong materials for applications in water purification, electrocatalysis, and biomedicine. The uniformly white PAN nanofiber mats were cut into 2 cm x 2 cm coupons to ensure consistency. After electrospinning, these nanofibers were coated with an electroactive polymer (EAP) using chemical vapor deposition, with iron (III) chloride (FeCl₃) serving as an oxidant for polymerizing 3,4-ethylenedioxythiophene (EDOT) into poly(3,4-ethylenedioxythiophene) (PEDOT). The study examined the impact of different FeCl₃ concentrations on PEDOT deposition on the PAN coupons. PEDOT deposition led to an increase in coupon weight. Scanning electron microscopy (SEM) revealed increases in the diameter of the nanofibers treated with increasing FeCl₃ oxidant concentration, although higher FeCl₃ concentrations caused inter-fiber bridging, implying a concomitant decrease in inter-fiber spacing. Energy dispersive X-ray spectroscopy (EDS) was used to confirm the presence of Fe, Cl, and S in the nanofibers, with sulfur content rising with FeCl₃ concentration used, suggesting increased PEDOT deposition efficiency with increasing oxidant concentration. Mechanical testing showed that PEDOT-coated PAN fibers had improved tensile strength and toughness in the hydrated state compared to pure PAN nanofibers. These results highlight the crucial role of FeCl₃ concentration in influencing the morphology and properties of PAN-PEDOT composites, enhancing their suitability for applications such as water purification, tissue engineering, biosensing, catalysis, and energy storage.

Introduction

Electrospinning is a technique utilized for the fabrication of nanofiber-based materials for applications including tissue engineering, drug delivery, biosensing, food encapsulation, insulating materials, energy storage and dissipation, catalysis, and filtration. The versatility of this technique allows for different fiber arrangements and morphological structures, supporting innovations in multiple industries^1,2,3,4,5,6. Electrospinning is a voltage-driven fabrication technique that produces small fibers from a polymer solution. The basic setup includes a syringe fitted with a spinneret (a stainless-steel needle or capillary extrusion tube) that is filled with a polymer solution, a syringe pump, a high-voltage power source, and a collector, as shown in Figure 1A. The syringe pump ensures a constant flow of the polymer solution. Using a rotating drum as a collector allows for the preparation of larger, more uniform fiber mats; rotation at low speeds yields randomly oriented fibers, while rotation at high speeds yields fibers oriented in the direction of drum rotation³. The speed necessary for fiber alignment is polymer solution-specific and can vary depending on factors such as solution viscosity, surface tension, and concentration. The process begins when an electric field is established between the needle tip and the collector, resulting in charges accumulating on the liquid surface. When electrostatic repulsion overcomes the surface tension, the liquid polymer solution forms a Taylor cone, leading to a jet of charged liquid moving toward the collector. As the solvent evaporates, solid polymer fibers are deposited onto the collector^4,5. Electrospun nanofibers have been made from several polymers including polyacrylonitrile (PAN), polystyrene (PS), poly(vinyl alcohol) (PVA), polycaprolactone (PCL), poly(ethylene oxide) (PEO), poly(lactic-co-glycolic acid) (PLGA), nylon, poly(vinylidene fluoride)/polyurethane and polyvinylpyrrolidone^{1,7,8,9,10,11,12}.

The fabrication of electrospun nanofiber-based materials using electroactive polymers (EAPs) is of particular interest. EAPs are materials with conjugated backbones that can be reversibly switched between multiple oxidation states using either chemical or electrochemical processes. This modification leads to changes in properties such as color, conductivity, reactivity, and volume¹³. This versatility makes EAPs well-suited for applications such as energy conversion and storage, electrochromics, actuators, sensors, and bioelectronics devices. EAP nanofibers, more specifically, have potential applications in energy storage, sensors, actuators, separations, nerve/tissue engineering, and biosensing¹⁴. Electroactive polymers (EAPs) pose several challenges during electrospinning due to their low viscosity and high conductivity which can hinder stable jet formation and lead to jet instability and bead formation instead of continuous fibers. Their sensitivity to electric fields can cause premature deformation or activation, resulting in inconsistent fiber formation. Additionally, EAPs typically suffer from poor solubility, requiring the use of specifically suitable solvents that affect solution viscosity, surface tension, and dielectric properties, complicating the process. The mechanical properties of EAPs, such as low elasticity and flexibility, can lead to jet breakage and fiber inconsistency. At the same time, the electrospinning process parameters, including voltage, needle-collector distance, and flow rate, need careful optimization, making the process more complex and less reproducible^15,16,17,18. While there are several notable exceptions, in most cases, it is not possible to electrospin solutions of EAPs without the incorporation of additional polymers, called carrier polymers, that are capable of electrospinning¹⁹.

Alternative methods have been devised for the preparation of EAP nanofiber-based materials. Blending EAPs with other polymers or nanoparticles yields properties that are a hybrid of the two polymers¹. While blending EAPs with carrier polymers enables electrospinning, the desirable properties of the EAP — notably its electroactivity (ability to change its properties in the presence of an electric field) and its conductivity — may be lost when blended. Instead, coating nonelectroactive electrospun nanofibers with EAPs can be a way to provide the electroactivity and conductivity of the EAP at the surface while having a mechanically robust polymer at the core of the nanofibers. EAP coatings can be accomplished using electrodeposition, spray coating, dip coating, or vapor-phase deposition. However, electrodeposition has several drawbacks. It requires conductive substrates, limiting its use with non-conductive materials unless they undergo pre-treatment to enhance conductivity. Achieving uniform coatings on complex or porous surfaces can be challenging due to uneven electric fields, resulting in variable deposition thicknesses. Controlling the morphology, thickness, and crystallinity of the coating also requires precise tuning of the electrolyte composition and applied voltage. Additionally, large-scale or three-dimensional objects may not receive uniform coatings, particularly in recessed areas. The risk of contamination from impurities in the electrolyte or on the electrode surface can introduce defects, while energy-intensive processes further complicate the technique¹⁹. Dip coating is a process where substrates are immersed in an EAP polymer solution and then withdrawn at a controlled rate to create a thin film. Alternatively, dip coating of a substrate with a monomer solution could be followed by a secondary polymerization step. The choice of solvent is critical; it must dissolve the polymer or monomer without dissolving the underlying fibers. However, the formation of non-conformal coatings or aggregation can be a drawback, particularly on complex surfaces. Spray coating efficiently covers large or complex surfaces and can be combined with other methods for multi-layer applications²⁰, but spray coating also requires careful solvent selection to avoid dissolving the substrate polymers. Vapor phase deposition, specifically chemical vapor deposition (CVD), typically avoids solubility issues, produces uniform, thin coatings with excellent adhesion, allows coverage of complex geometries, and is a scalable process. CVD is a widely used method for uniformly coating substrates with various materials, such as carbon, metal oxides, and EAPs, by vaporizing precursors, transporting them to the substrate, and enabling adsorption, nucleation, and growth under controlled conditions. In the case of CVD of EAPs, the process typically involves vaporizing monomers and/or oxidants to enable in situ oxidative polymerization to take place onto the fibers. CVD offers advantages like uniform and conformal coatings, precise thin film formation, scalability for industrial applications, and versatility in material deposition, enhancing the functionality of nanofibers for diverse applications²¹. CVD was used by Zhu et al. for the coating of calcinated NiCo₂O₄ nanofibers originally made with polyvinylpyrrolidone with the EAP polypyrrole²².

In our work, nanofiber mats have been prepared from PAN as substrates for the CVD of EAPs. PAN is an inexpensive commodity polymer that is insoluble in aqueous media, is environmentally stable, and has high strength and modulus. Recent study by Sapountzi et al.²³ has demonstrated the successful deposition of various materials on PAN nanofibers using CVD techniques. Specifically, they electrospun solutions of PAN containing the oxidant ferric chloride and then exposed the fibers to pyrrole and/or pyrrole-3-carboxylic acid vapors to produce core-shell nanofibers of PAN and EAPs for glucose sensing²³.

In the CVD of EAPs, the concentration of oxidant is crucial to enable the oxidative polymerization of electroactive monomers like pyrrole and aniline onto electrospun nanofibers, significantly influencing the deposition rate and properties of the resulting conducting polymers. Ferric chloride (FeCl₃) is commonly used as an oxidizing agent, forming monomer radical cations that react to create polymers^24,25. Higher FeCl₃ concentrations increase the polymerization and deposition rates due to more available oxidizing species, while lower concentrations slow these processes. Sapountzi et al. showed that higher FeCl₃ concentrations resulted in faster deposition rates and more uniform coatings of polypyrrole (PPy) onto PAN nanofibers and vice versa²³. With low FeCl₃ concentrations, the limited availability of oxidizing species slows down the formation of radical cations and the subsequent polymerization process^25,26. On the other hand, Xue et al. reported that an optimal FeCl₃ concentration of 0.25 M resulted in a uniform and highly conductive PPy coating onto PAN nanofibers, while higher concentrations led to over-oxidation and degradation of the polymer²⁷. Wissmann et al. studied the deposition of polyaniline (PANI) onto PAN nanofibers and observed that increasing the FeCl₃ concentration from 0.1 M to 0.5 M significantly increased the deposition rate and conductivity of the PANI coating, but further increasing the concentration resulted in non-uniform deposition. Zhang et al. reported the deposition of a copolymer of aniline and o-anisidine onto PAN nanofibers using different FeCl₃ concentrations. They found that an optimum FeCl₃ concentration of 0.25 M resulted in a uniform and highly conductive coating, while higher concentrations led to over-oxidation and degradation of the polymer, as identified by the lower conductivity of the material²⁸. Thus, careful selection of oxidant concentrations is critical to ensure optical EAP coatings.

This research demonstrates the use of CVD for the preparation of poly(3,4-ethylenedioxythiophene) (PEDOT)-coated PAN nanofibers. This research thus focuses on the process which involves electrospinning PAN nanofiber initially, followed first by infusing these PAN nanofibers with FeCl₃ at various concentrations and then by the deposition of EAPs onto the FeCl₃-ladden fibers, as shown in Figure 1. The study explores the influence of different concentrations of FeCl₃ on the deposition rate of PEDOT onto the nanofibers. Mechanical testing assesses the effect of PEDOT coatings on the mechanical properties of the materials, particularly in high-humidity environments indicative of the material's pertinency to biomedical and water purification applications. The optimization of FeCl₃ concentration is pivotal to ensuring efficient and precise deposition of EAPs, leading to uniform coatings and improved material properties. The absence of comprehensive studies examining the effects of FeCl₃ concentrations and VPD setup characteristics on the deposition rate and functionality of EAP-coated PAN nanofibers underscores the significance of this research. The results of this work are especially significant given the differences observed in the EAP deposition at lower FeCl₃ concentrations compared to previous studies, highlighting the critical role of the vapor phase deposition (VPD) setup.

Protocol

1. Experimental setup

1. Electrospinning of PAN nanofibers
   1. Dissolve 1 g of PAN in 10 mL of DMF to create a 10% solution.
   2. Assemble the electrospinning setup, ensuring that the high-voltage power supply, syringe pump, and a 20-gauge needle with a flat tip are connected to a grounded collector shown in Figure 1 and Figure 2. Ensure that the electrospinner is used in a fume hood or connected to ventilation ducting to minimize exposure to solvent vapors.
   3. Cover the metal collection drum, which will receive the polymer nanofibers, with wax paper for easy fiber retrieval and minimal cross-contamination between samples.
   4. Fill the syringe with the PAN solution and connect it to the spinneret needle using a 0.8 mm ID polytetrafluorethylene (PTFE) tube with Luer adapters.
   5. Set the flow rate of the syringe pump to 0.04 mL/min. The flow rate can be adjusted as needed depending on the polymer solution properties.
   6. Apply a voltage of 18.9 kV between the spinneret and the grounded collector, which is set to rotate at 1,200 rpm^29,30. Follow proper guidelines during electrospinning as the process involves high voltage.
   7. Allow the charged jet to initiate from the spinneret, where it solidifies into continuous nanofibers as the solvent evaporates. Ensure that the applied voltage, solution flow rate, and drum rotation speed are consistent across all batches to achieve uniform fiber morphology and diameter.
   8. Allow the PAN nanofibers to deposit on the grounded collector coated with wax paper for approximately 4.5 h. After this time period, unwrap the wax paper from the drum by removing the tape.
2. Preparation of PAN coupons
   1. Dry the PAN sheet mat in a vacuum desiccator at room temperature for 72 h to remove any residual solvent or water adsorbed during processing.
   2. Cut the dried fiber mats into 2 cm x 2 cm coupons with scissors or a razor blade. Note the positions of the coupons with respect to the direction of drum rotation, as shown in Figure 3. Organize coupons into groups of triplicates, producing replicates from the same column of the fiber mat to ensure consistency.
3. Vacuum system assembly
   1. Assemble the chemical vapor phase deposition system using an enclosed steel container with a glass lid fitted with a silicone gasket to precisely control vacuum conditions, as shown in Figure 4.These containers are commercially available as vacuum degassing chamber kits from a variety of vendors and are inexpensive.
   2. Equip the system with two valves to manage the vacuum: one valve connects to the vacuum pump and is closed once the desired vacuum is reached; the other valve connects to the atmosphere and is opened to allow controlled release of the vacuum as needed.
   3. Connect a vacuum pump to the system to reduce the pressure in the chamber. Position the container on a hot plate capable of reaching temperatures up to 250 °C to enable thermal management during the deposition process.
4. Vapor phase deposition rack assembly
   1. Develop deposition racks for the project as per the designs for two deposition rack designs described below.
   2. Construct the Generation 1 (G1) deposition rack shown in Figure 5A by wrapping four vertical legs made of stainless steel in aluminum foil to prevent contamination.
   3. Connect the vertical legs with horizontal rods at the top and bottom, forming a hollow cube-like structure for stability.
   4. Wind three horizontal copper wires around the legs to create three tiers or levels for hanging the coupons.
   5. Ensure that the G1 rack is compact, fits within a processing chamber, and allows for easy assembly and disassembly.
   6. Construct the Generation 2 (G2) setup, shown in Figure 5B, using stainless steel for durability and corrosion resistance. Engineering schematics for this system are provided in Supplementary File 1. Design the G2 rack with a circular base and top plate connected by four vertical rods for stability and support. Equip the vertical rods with adjustable clamps to allow customization of the height between the base and top plate. Install a circular wire mesh tray made of stainless steel for hanging coupons.
   7. Ensure precise control over the positioning of electrospun coupons during the deposition process to promote uniform exposure to the vapor phase.

2. Vapor phase deposition

1. Deposition of FeCl₃ on PAN coupons
   1. Weigh the PAN coupons. Deposit FeCl₃ (oxidant) on the PAN coupons (at least three replicates per condition) by soaking them in aqueous FeCl₃ solutions of varying concentrations, ranging from 1 M to 5 M, for 30 min.
      ​CAUTION: Exercise caution when handling ferric chloride (anhydrous), as it is a corrosive irritant that can cause severe eye damage.
   2. Transfer the coupons onto low-lint paper wipes to facilitate osmotic drying, replacing the wipes 2x.
   3. Wrap the coupons in wipes and place them inside a fume hood for 24 h to complete the first stage of the drying process.
   4. Weigh the PAN coupons after 24 h and record the weight loss. Hang the PAN coupons by hooking them onto the metal wire tray prepared in step 1.4.9.
   5. Place a 500 mL beaker containing dry calcium chloride desiccant at the bottom of the setup. Position the entire setup inside a vacuum chamber.
   6. Initiate the vacuum by turning on the pump and leaving the vacuum valve open for continuous air removal, effectively using the vacuum system as a desiccator. Here, the pumping rate was 85 L/min. The final vacuum achieved by this pump and setup was approximately 8000 Pa.
   7. Monitor the weight loss of the coupons at intervals of 1 h, 2 h, 3 h, 24 h, 48 h, and 72 h.
2. PEDOT deposition onto PAN coupons
   1. Hook and hang the PAN coupons on the coupon rack assembly.
   2. Suspend the coupons inside the vacuum system along with an open glass container holding approximately 4 g of the monomer (EDOT). Adjust the hot plate temperature to 55 °C.
   3. Open the valve between the chamber and the vacuum pump and evacuate the chamber until the desired vacuum (8000 Pa) is achieved. Then, close the valve, leaving the chamber under vacuum.
   4. Allow the vapor deposition process to occur for 2 h. After 2 h, open the vacuum release valve to retrieve the PEDOT-coated coupons.
3. Coupon washing
   1. Remove the polymer-coated coupons from the VPD chamber and transfer them to the washing chamber.
   2. Place the coupons over the opening of a porcelain funnel lined with circular filter paper of 5.5 cm in diameter and a pore size of 180 µm, as shown in Figure 6.
   3. Wash each batch of three coupons of a specific concentration together to remove FeCl₃ residue by spraying methanol over the coupons using a squirt bottle.
   4. Ensure thorough removal of FeCl₃ residue by increasing methanol volume with higher concentrations to effectively dissolve and wash away residual oxidant and monomer. Use approximately 200 mL of methanol for coupons dipped in 1 M FeCl₃. Increase the methanol amount by 50 mL for each higher concentration level (e.g., 250 mL for 2 M, 300 mL for 3 M).
   5. After washing, leave the coupons to dry under the hood for 30 min followed by drying in a desiccator with desiccant at room temperature for 1 h and finally preserve them in glass vials.
4. Cleaning of the VPD chamber and pump
   1. Rinse the CVD chamber with acetone to dissolve the unreacted monomer. Dispose of the rinse in the organic waste container.
   2. Place the chamber in a fume hood while empty and heat it at 30 °C for 1 h. Immerse the chamber in a detergent solution for 24 h and then dispose of the detergent.
   3. Heat the chamber with the lid slightly open at 200 °C for 24 h. Perform another round of acetone spraying and dry the chamber in the fume hood at 30 °C for 1 h.
   4. Change the pump oil after each batch of monomer deposition to maintain the integrity of the experiment.

3. Characterization of samples

1. SEM analysis of nanofiber mat
   1. Attach PAN, PAN+FeCl₃, and PAN+FeCl₃+PEDOT samples to aluminum stubs using carbon tape.
   2. Image the samples at a magnification of 5000x, using a spot size of 4.5 nm and an accelerating voltage of 20 kV. Adjust imaging parameters as necessary to investigate sample morphology.
   3. Use the measurement function to determine fiber diameters. Take 12-15 measurements for a sample.
   4. Perform energy dispersive X-ray spectroscopy (EDS) analysis according to the standard operating procedures of the SEM instrument. Obtain the atomic percentage and weight percentage of the elements found in the samples.
2. Mechanical analysis
   1. Laser cut 40 mm x 40 mm cardboard frames with 20 mm x 20 mm centered window openings.
   2. Using a razor blade, precisely cut out 10 mm x 40 mm nanofiber sample strips with the 40 mm edge being parallel to the direction of electrospun nanofiber formation.
   3. Measure the thickness (mm) of each strip using digital calipers at five locations chosen around the center of the specimen where the sample would not be in contact with the frame and about equal distances apart. Take the average as the thickness. Measure the weight (g) of each strip.
   4. Condition the sample either by desiccation (vacuum desiccator filled with calcium sulfate) or soaking in phosphate-buffered saline (10x PBS) for 72 h.
   5. Dab PBS-soaked samples on lint-free tissue wipes. Overlay a 10 mm x 40 mm polymer sample onto a 40 mm x 40 mm frame and tape it down for insertion into the tensile tester.
   6. Load the frame and sample into the tensile tester and test at room temperature and the desired strain rate (0.8 s^-1). The output will be in force and displacement.
   7. Convert the force and displacement into stress-strain. Calculate the density of the sample from the sample dimensions and mass. To calculate stress, divide the force data collected by the tensile tester by the sample's cross-sectional area, then multiply by both the density of the fiber mat and the gauge length used while testing.

Results

In this work, electroactive polymer-coated PAN nanofibers are fabricated by electrospinning to develop highly porous yet strong materials that could be used as filters, absorbents, and photocatalysts for water purification, substrates for electrocatalysis, and scaffolds/matrices for tissue engineering, nerve regeneration, drug delivery, and biosensing. To provide these materials with electroactive properties, PAN nanofibers are coated with the EAP PEDOT by chemical vapor deposition.

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Discussion

In this work, protocols for electrospinning nanofiber mats from commodity polymers, coating these nanofibers with electroactive polymers by vapor phase oxidative polymerization, and characterization of the chemical and mechanical properties of these materials are described.

Electrospinning has become a useful tool for the generation of high surface area materials for a range of applications from biomedicine to sustainability and electronics^1,

Materials

Name	Company	Catalog Number	Comments
10X PBS	Thermo Fisher Scientific	BP399-1	Used to mimic in vivo salt and humidity conditons
Acetone	Thermo Scientific	A412P4	Used to clean the VPD chamber
DMF	EMD Milipore Corporation	DX1727-6	Used as solvent to prepare PAN solutions
EDOT	TCI	E0741	Electroactive monomer used to deposit polymer on to PAN coupons
Electrospinning apparatus	SprayBase	ES30N-5W	Used to produce PAN nanofibers
FeCl3	Thermo Scientific	12357.22	Used to effect polymerization via chemical oxidation of electroactive monomers
Filter paper (180 μm)	VWR	28310-015	Used for filtration
High voltage power supply	Gamma	ES30N-5W	15KV-20KV. For this experiment 18.9KV was used.
Hot plate	Corning	PC420D	Used to heat vacuum chamber
Methanol	Fischer Chemical	A412-4	Used to remove FeCl2 and FeCl3 from EAP-coated PAN
Needle	BDYale	511098	20G
PAN powder	Ambeed	A971305	Used for the preparation of PAN nanofibers. Average MW 150,000 g/mol.
PTFE tubing	Microsolv Tech Corp.	49210-20-C	0.8 mm ID, 1.6 mm OD
Pump	Kozyvacu	ZK220419	Used for creating vacuum inside the chamber
Syringe	Air-Tite	231220	10 mL, disposable syringe
Syringe pump	New Era	NE-300	Flow rate 0.04 mL/min
Vacuum chamber with gauges	BAC Eng	X002APOXUR
Wax paper	Reynolds Kitchen	CM-3CY-06KR	Used to cover metal drum and collecting PAN nanofiber