Effects of Long-term Chemical Treatment on the Electrical Resistance of

Poly(3,4-ethylenedioxythiophene) on Textiles.

University of Massachusetts Dartmouth

Bioengineering DepartmentMTX-463 Senior ProjectChristopher DiFranco

Dr. Qinguo Fan

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Introduction:

• Poly(3,4- ethylenedioxythiophene) or more commonly known as PEDOT is a conjugated polymer that carries positive charges upon doping and is typically used as a transparent, conductive polymer with high ductility in various applications.

• Among conducting polymers, PEDOT is significantly important due to its small band gap, high conductivity and high stability.[1]

• PEDOT’s small band gap structure allows it to be utilized in several applications such as organic light emitting diodes, photovoltaic’s, electroluminescent devices, antistatic coatings and capacitors [2].

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Introduction:

• PEDOT-PSS has been proven to react with high boiling solvents like methylpyrrolidone, dimethyl sulfoxide (DMSO) and sorbitol to increase conductivity by many orders of magnitude bringing its conductivity to 1000 S/cm.

• By themselves conductive polymers lack good mechanical properties due to their structure. They are mostly incorporated on stiff rigid substrates which disguise their flexibility. Typically plastic substrates.

• Using textile substrates allows the polymer to utilize some of its flexibility while increasing its air permeability and to see its potential electronic applications for apparels.

• Conductive textile fabrics can be made with different coating techniques in which the conducting ink or oxidant ink is either applied on the surface (ink jet printing) or diffused into the materials structure [3].

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Literature Review:• Conductive polymers have been a widely studied topic since the first

recorded experiment in1862, when researchers at the college of London Hospital, Letheby attained a partly conductive material by oxidizing aniline in a sulfuric acid solution.

• Since the first experiment conductive polymers researchers have now evolved into testing the conductivity of PEDOT

• Materials can be generally categorized into three groups of electrical conductivity: insulators, semiconductors and conductors. With conductive polymers lying in the semiconductor group.

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Literature Review:• PEDOT is a good conductor because of its conducting mechanism, and

its relation to the doping process. [4]

• While PEDOT is in its neutral state the electrons are delocalized along the conjugated backbones, forming an extended π-system with a completely filled valence band.

• By doping the π-system you either remove (P-doping) or add electrons (N-doping) into the system. These two processes create cations and anions on the backbone that are called polarons while a pair of polarons is known as a bipolaron.

• The vertical up and down movement of the bipolarons as a pair on the polymer chain is responsible for the good conductivity of PEDOT.

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Literature Review:Vapor Phase Polymerization (VPP):• In 1994, De Leeuw reported a direct oxidative polymerization method for

surface film formation using ferric para-toluen sulfonate (Fe (III) tosylate) as the oxidant and imidazole as an inhibitor than can suppress the polymerization of the EDOT monomer until the solvent is evaporated.[5]

• This method increased the available levels of conductivities in PEDOT

Vapor Phase Polymerization Process:

• In VPP the polymerization reaction is carried out in the gas phase and is based on direct oxidation by a catalyst solution.

• The initial experiments for VPP of PEDOT were done by Mohammad et al titled Chemical Vapor Deposition (CVD) process for polypyrrole polymerization in which FeCl3 and H2O2 were used as oxidants.

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Literature Review:• The first VPP of EDOT using (Fe (III) tosylate) as oxidizing agent was

reported by scientists from Hanyang University and Korea Basic Science Institute. In their experiment the surface resistance of 500 ohm/sq was reported for PEDOT films with a thickness of 300 nm.[6]

Simple Schematic of VPP chamber

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Experimental Procedure:• The experiments took place in the Biomanufacturing lab at the Advanced

Technology and Manufacturing Center (ATMC) in Fall River, MA.

• Under the supervision of Dr. Qinguo Fan, and Dr. Chen-Lu Yang

• Testing took twenty four weeks, finishing by May 15th 2012

• Results were submitted to the AATCC Materials Research Poster competition Outcome: Awaiting results

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Experimental Procedure:

o Main Objective:• To analyze the changing properties of VPP-PEDOT coated on textile

substrates after 72 hrs’ chemical treatments in both organic and inorganic solvents.

o Specific Objectives:• To examine the chemical resistance of VPP-PEDOT in some organic and

inorganic chemicals• To track the long term treatment effects on weight• To track the long term treatment effects on electrical resistance• To identify morphological changes of PEDOT layers coated on textiles from

SEM

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Experimental Procedure:

Materials Needed:o Vapor Phase Polymerization of PEDOT

• 3,4-Ethylenedioxythiophene (EDOT) monomer (Sigma Aldrich)• (Fe(III) tosylate),40% in Butanol (Heraeus)• Pyridine 99.8% (Fluka)• Poly(ethylene glycol)PEG Average M.W.400 (Sigma Aldrich)

o Solvents:• NaOH 97% pellets (Acros Organics)• Cyclohexanone 99.8% (Acros Organics)• Tetrahydrofuran (Fisher Scientific)• 91% Isopropyl alchohol (Vi-jon)• Dimethyl sulfoxide DMSO 99.7% (Acros Organics)

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Experimental Procedure:o Fabrics: All from Testfabrics, Inc.

Equipment:• Resistance measured in according to AATCC Test Method 76-2005.

• Extech Instruments Multipro 530 True RMS Multimeter• Pony 3202 Clamps

• Weight measured on Mettler Toledo AX-205 Delta Range Scale• Morphological changes inspected by Joel JSM 5610 Scanning Electron

Microscope (SEM)• Quincy Lab Inc. AF Model 40 Lab Oven set at 100°C

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Experimental Procedure:

• To test resistance before and after treatment electrodes were formed on a rigid surface, and the firm contact between the electrodes and the fabric will be provided by high pressure clamps.

• Surface resistance is given by the following equation.

Where :R is the resistivity in ohms per square O= measured resistance in ohms W =is the width of the specimen D = is the distance between the electrodes

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Experimental Procedure:oThe chemical resistance of Poly-3,4-ethylenedioxythiophene (PEDOT) was tested in both organic and inorganic solvents after being VPP coated on three different textile substrates: Cotton, Poly/cotton, and Polyester, these solvents include:• 10% Sodium hydroxide• Cyclohexanone• Tetrahydrofuran (THF)• 91% Isopropyl alcohol • Dimethyl sulfoxide (DMSO)

o First the samples must be Vapor Phase Polymerized• The samples are submerged into an oxidizing agent of, Fe(III)tosylate in butanol)

mixed with pyridine in molar ratio of 2:1, this serves as an oxidant dopant and base inhibitor while PEG (5wt%) acts as a plasticizer to decrease the electrical resistance.

• Then the samples are hung in the VPP chamber setup where they will be exposed to the EDOT monomer vapor at 50°-55°C for 1 hour while pumping in inert argon gas during polymerization.

• Samples are then removed and flushed with deionized water and ethanol and dried for an hour at 100°C

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Experimental Procedure:•After the Vapor Phase Polymerization step the textile substrates had their weights and resistance measured to record an initial reading.

•The substrates were then submerged into the five solvents for a period of 24 hours, removed and flushed with ethanol and deionized water, dried for 1 hour at 100°C and tested again.

•This process was repeated for 0hr, 24hr, 48hr, and 72hr treatments in each solvent and their data was recorded and graphed.

•After 72hr treatments the samples were observed under SEM for morphological changes.

•From the SEM pictures I was be able to determine if a visual VPP PEDOT layer is still present.

•Also from the SEM I was able to link any correlations between missing PEDOT layer and the substrates weight and resistance change.

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Results: Cotton• 10% NaOH showed the most significant influence on changing the electrical

resistance of VPP PEDOT. After 72hr treatment the resistance increased over 8 times.

• In THF, PEDOT almost lost its electrical conductivity after 24hr treatment, the conductive layer was removed from the cotton substrate after 48hr and after 72hr there was no remaining PEDOT available on the substrate to test resistance.

• For the remaining solvents Cyclohexanone, DMSO and 91% Isopropyl the electrical resistance increased around 2-4 times after 72hrs and most of the conductive layer was left on the substrate.

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Results:• Weight loss of each cotton samples corresponds to its decrease of electrical

conductivity• From analyzing the SEM pictures, textile structures contained less PEDOT

compared with untreated substrates.• While testing THF it was also noted that after 48 hour solvent treatment the

substrates showed color change from dark blue to a green.

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Results:• When studying the cotton substrates under the scanning electron microscope it was

visually clear the PEDOT layer had significantly been removed from both the NaOH 10% and THF solvents causing their resistances to rise until there was no more PEDOT layer left to be conductive in the case of THF.

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Results: Poly/Cotton• 10% NaOH showed the most significant influence on changing the electrical

resistance. After 24hr treatment the resistance increased by almost 11 times however after 72hr treatment the final increase in resistance was 8 times from the initial resistance.

• We observed a drop in resistance at 48hrs to about 4 times the initial resistance in NaOH

• In THF, PEDOT resistance only increased roughly 8 times and after observing SEM images, was clear there was a remaining layer of PEDOT on the substrate.

• The remaining solvents had increases between 3-10 times their initial resistances.

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Results:• The weight loss of the samples corresponded with their decreases in electrical

conductivity with 10% NaOH having also the greatest weight loss after 72 hr solvent treatment.

• The SEM pictures show most of the PEDOT has been removed in 10% NaOH while still some remains in THF

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Results: Polyester• 10% NaOH showed the most significant influence on changing the electrical

resistance. After 72hr treatment the resistance increased almost 7,000 times.• In THF, PEDOT did actually lose its electrical conductivity after 24hr and there

was no more PEDOT remaining on the substrate.• The remaining solvents had changes of electrical resistance between 2-10 times

with the exception of 91% Isopropyl alcohol which experienced a change of about 256 times its initial resistance.

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Results:• The weight loss of the samples treated in 10% NaOH had significant weight

decrease after 72 hours.• The remaining samples did not show significant weight loss.• The SEM pictures showed the PEDOT conductive layer was mostly removed

after 72 hours treatment in NaOH and was still remaining on the remaining solvents

Figure shows: PEDOT coated polyester substrate

Webbing

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Discussion and Conclusion:• The PEDOT coated fabrics demonstrated excellent chemical resistance because it

maintained on the order of magnitude of 1000ohm after treatment in almost all of the solvents.

• Solvent treatment showed to best effective on the cotton substrates for submerging in THF and polyester substrates for 10% NaOH

• 10% NaOH and Tetrahydrofuran (THF) had an increase of electrical resistance which occurred due to the disappearing PEDOT coating from the substrate which indicates its potential to be a good solvent for PEDOT.

• This research allowed me to get firsthand user experience using the scanning electron microscope.

• One difficulty I experienced while using the SEM was how to properly sputter the specimens before using them, it proved to be important to avoid localized charging problems.

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Future Work:

• Find reasons for electrical resistance loss and PEDOT removal from textile substrates.

• Find what could be the reason for the decomposition mechanism on chemical treatment of VPP-PEDOT.

• Analyze the effects on the electrical resistance of VPP PEDOT coated textiles subjected to the vapor phase of the solvents for a given time period instead of the solvents in their liquid phase.

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[4,5] B. Winther-Jensen, K. West. Vapor-Phase Polymerization of 3,4 Ethylenedioxythiophene: A Route to

Highly Conducting Polymer Surface Layers. Macromolecules. 37(2004): 4538-4543.

[6] J. Kim, E. Kim, Y. Won, H. Lee, K. Suh, The Preparation and Characteristics of Conductive Poly(3,4-

ethylenedioxythiophene) Thin Film by Vapor-phase Polymerization, Synthetic Metals, vol:139, pg:485-

489, 2003.

Hung, L.S. 2002, ‘Recent progress of molecular organic electroluminescent materials and devices’,

Materials Science & Engineering, vol. 39, pp. 143–222.

S.O. Kasap, Principles of Electronic Materials and Devices, McGraw Hill Higher Education, s:

0071244581, pg:488-489, 2005.

Catarina Carlberg, Xiwen Chen, Olle Inganas. Ionic Transport and Electronic Structure in Poly(3,4-

ethylenedioxythiophene). Solid State Ionics. 85(1996):73-78

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Polymer Science, vol: 32, pg:876-921, 2007.