Olney Medal Address

Adsorption at the Liquid/Solid Interface:Conductive Textiles Based on Polypyrrole

By Hans H. Kuhn, Milliken Research Corp., Spartanburg, SC

Adsorption at the liquid/solid inter-face is not only of great impor-

tance in many fields of natural sciencessuch as biology and geology, but playsa significant role in the field of textilefinishing. In most cases of textile fin-ishing, the interface is between the fab-ric (solid) and water. Processes such as

ABSTRACTAdsorption at the liquid-solidinterface is widely used by the textiieindustry in processes such asdyeing, finishing, wastewatertreatment, and many others.Precipitates of nano dimensionsformed in situ have been used toform coherent, usually amorphous,fiims of metai oxides, sulfides, etc. onsoiid surfaces including textiles.

This phenomena has beensuccessfuiiy used to poiymerizepyrroie on the surface of textilefibers, fabrics, or yarns. While pyrroleis particularly suitable for thisprocess, other monomers such asaniline or thiophene may be used.Polypyrrole is one of the moreenvironmentally stable conductivematerials and the above process hasbeen successfully used in industrialproduction. The electrical propertiesand the environmental stability of theproduct depend on a number ofvariables such as concentration ofthe chemicals and particularly thedopants used. Furthermore, theenvironmental stability of theconductive textiles can besignificantly improved by the use ofcertain additives in thepolymerization.

Numerous applications have beenfound for conductive textiles,especially in the field where theirmicrowave adsorption characteristicsare highly desirable. This is the casein military applications likecamouflage and stealth technology.

KEY TERMSAdsorptionConductive TextilesOlney Address

scouring, dyeing, and application ofsoil release finishes all depend on theadsorption at the liquid/solid interface.While many of these interactions occurat the molecular level, similar reactionscan be conducted with colloidal par-ticles of nanometer size or even withdispersions of particles in aqueous so-lutions.•'"•' These reactions may bedriven by chemisorption, physisorp-tion, or possibly only by hydrophobicbonding. This paper discusses one ofthese reactions which leads to an en-tirely new and exciting product.

Conducting polymers have beenknown for more than two decades.They consist of conjugated unsaturatedpolymers of which polyacetylene is thebasic structure:

The oxidized polymer contains cat-ionic moieties that are neutralized ordoped with a counter-ion. Thesecharge-defects, called bipolarons, areelectron holes that serve as the chargecarriers and allow the fiow of electronsalong the polymer chain, similar to amolecular wire." Weight-based con-ductivities approaching that of copperhave been reported for doped poly-acetylene. Unfortunately, polyacety-lene is highly unstable and loses itsproperties if not held under anhydrousand oxygen free environment. Thereare, however, a number of polymersthat are much more stable under ambi-ent conditions such as polypyrrole,polythiophene, and polyaniline. These

polymers can be prepared by theoxidative polymerization of theirrespective monomers which may beconducted either chemically or electro-chemically.^"'' Usually these polymer-izations are conducted in dilute solu-tions. If the monomers are watersoluble, which is the case, for instance,for aniline and pyrrole, the polymeriza-tion can be conducted in aqueous so-lutions. The conductivity of tJiese poly-mers are not quite as high as inpolyacetylene and lay in between theconductivity of metals and semi-conductors.

Polymerization of Pyrroie

The oxidative polymerization of pyr-.role has been examined by a large mmi-ber of investigators and is believed tofollow the mechanism shovm in Fig.1.«

Some of the lower molecular weightoligomers have actually been isolatedduring the polymerization.^ Pyrrole issoluble in water to about 6%. Thissolubility drastically decreases for thedimer and oligomers which causes pre-cipitation of the polymerizing species.If the polymerization is conducted inthe presence of a high surface areastructure such as a textile fabric, theentire polymerization occurs on thesurface provided without any visibleprecipitate in the aqueous phase. For.the chemical polymerization of pyr-role, iron (HI) salts have been found" toyield superior products over other oxi-dizing agents such as hydrogen perox-ide or alkali persulfates. It is suspectedthat complex formation between the

Fig. 1. Polymerization mechanism for pyrrole.

December 1997 OCO Textiie Chemist and Colorist 17

Fig. 2. Polypyrrole on knitted nylon fabric. Mag-nification 200X.

tri-valent iron and pyrrole and the rela-tively low oxidation potential of thisoxidant may be responsible for thesefindings. ^

To directly obtain the oxidized anddoped form of the polypyrrole, at least2.33 moles of iron (III) salt are neces-sary. Iron perchlorate has been shownto yield the best products, but for safetyand economic reasons, iron chloride ispreferred and yields products of simi-lar properties. If no other dopants areadded to the reaction mixture, thepolypyrrole obtained is doped withchloride ions. However, it is possible toadd other anions, particularly aryl-sulfonic acids, which compete with thechloride ion for the doping of the poly-mer. If the organic sulfonic acid isbarely soluble in water, chloride dop-ing can be suppressed so that eventu-ally less than 10% of the dopant is pro-vided by chloride ions. ^

It was found that the polymerizationis typical of a second order autocatalyt-ic reaction with a rate constant of 1.3 x10" M' sec" at 25C in the presence oftextile fibers. The rate is considerablyslower if no fibers are present resultingin arate constant of 3.2 x 10' M' sec' ^"

These reactions work very well ifconducted at relatively low concentra-tions, preferably 0.01 to 0,02 moles/li-ter of the monomer. To obtain thickercoatings, resulting in higher conduc-tivities, it is often desirable to usehigher concentrations of pyrrole. Un-der these conditions, polymerizationmay occur in the liquid phase resultingin the deposition of particulate mate-rial on the fabric surface. To slow thereaction, it has been found that the ad-dition of a mild complexing agent foriron [III) compounds, such as 5-sul-fosalicylic acid, works exceedinglywell to provide the necessary control.The addition of one mole of the sali-cylic acid derivative per mole of ironreduces the rate by a factor of approxi-mately five.'^'^

As one can see from Figs. 2-5, theself-assembled layers of polypyrroleconsists of a smooth, coherent film

Fig. 3. Cross section of nylon 6 fiber treatedwith polypyrrole. The dark area shows thepolypyrrole coating. Magnification 960X,

composed of nanometer size nucleiwhich have grown into each other dur-ing the polymerization. It is known thatself-assembled thin layers of polymersdeposited on substrates often result ina higher order than polymers obtainedby precipitation from solution. Wehave indeed been able to show asimilar effect in our process. X-ray pho-toelectron spectroscopy analysis indi-cates that polypyrrole powders pro-duced by the chemical oxidation ofpyrrole in water consists of about 45%of the desirable 1,4-coupling. with theremainder consisting of a mixture ofnon-conjugated polymers resultingfrom substitution in the 2 and 3 posi-tions and other oxidized species. Con-ducting the polymerization on thesurface of quartz fibers results in aproduct where the 1,4 coupling is in-creased to almost 70%, showing a sig-nificantly higher degree of order in theself-assembled thin film. These find-ings are further supported by the in-creased volume conductivity found inthe adsorbed polypyrrole on textilefabrics.'-'

At this point it may be appropriateto discuss some of the units used in thefollowing discussions. The volumeconductivity, also called specific con-ductivity, is expressed in Siemens/cm.It is the reciprocal of the specific resis-tance, which is defined as the resis-tance measured in ohms along 1 cm of

material with a cross-sectional area of1 cm . For thin films such as textiles,however, the term of surface or sheetresistance is used, measured accordingto AATCC Test Method 76-1987. Theresistance is measured with the sepa-ration of the resistance probes equal totheir length and is reported in ohms/square. The resistance is the same for1 cm , 1 in.^, etc. The stability of con-ductive fabrics is expressed as RQ/RXwhere H represents the original sur-face resistance and H^ is the surfaceresistance at time t.

Figs. 2 and 4 show that there is nofiber-to-fiber bonding in the final prod-uct. As a result, the textile propertiessuch as breaking strength, elongation atbreak, etc. are largely unaffected.'^Drape and formability of the textile arepreserved as is the capability of beingsaturated by resins for composite struc-tures. Depending on the amounts ofmonomer used and the weight andconstruction of the fabric, surface resis-tances of 10-10,000 ohms/sq can beproduced. Surprisingly, no significantdifferences could be found by the useof different fibers. Polyester, nylon,rayon, acrylics, Kevlar. Nomex, wool,and cotton all seem to perform verysimilarly. Glass fibers, however, are toohydrophilic and must be treated withaminosilane to respond properly to thetreatment with polypmole.

As stated previously, differentcounterions or doping agents can beused in the pohTnerization of p\Trole.It is one of the most important variablesin this process, affecting not only thedegree of conductance, but surpris-ingly also the degree of stability. Awide range of properties can be ob-tained by simply adding variousorganic sulfonic acids to the polymer-ization mixture (Tabie I). The perfor-mance, in terms of conductance andstability, improves with the concentra-tion of sulfonic acid used. The perfor-mance is further improved if sparinglysoluble aromatic sulfonic acids, suchas naphthalene-2-sulfonic acid andparticularly ant hraqu i none-2-sulfonic

Fig. 4. Polypyrrole on S-2 glass fiber. Magnifi-cation 1000X.

Fig. 5. Polypyrrole on glass fiber.tion 100,000X.

18 Textile Chemist and Coiorist 000

Table I. Polypyrrole on 100% Textured Woven Polyester Fabric*

Dopant

Hydrochloric acid'Sulfuric acid''Methanesuifonic acidTrifluoromethanesulfonic acidBenzenesulfonic acidBenzenesulfonic acidp-Toluenesulfonic acidp-Toulenesulfonic acidp-Ethylbenzenesulfonic acidp-Ethylbenzenesulfonic acid1,3-Benzenedisulfonic acid2-Naphthalenesulfonic acid1,5-Naphlhalenedisulfonic acid^1,5-Naphthalenedisulfonic acid*'2-AnthraqL)inonesu!fonic acid'2-Anthraquinonesulfonic acid'2-Anthraquinonesulfonic acid'

Amount(%owf)

-101010

10010

10010

100101010

100245

Resistance(Ohms/sq)

12508300110050052217624078

14560

276688974504641

Stability

.08

.08

.05

.05

.16

.25

.24

.45

.36

.65

.35

.70

.39

.38

.63

.65

.65

^Polymerized at room temperature using 1.33 g/L pyrrole, 2.4 moles of ferric chloride in a 1:30 fabric to liquorratio for 4 hours. '^0 hrs, 100C. ' Polymerized with ferric chloride only. ''Polymerized with ferric sulfate only.^Disodium salt. 'Sodium salt.

Table II. Elemental Analysis of Polypyrrole Powders^

Atomic Ratio

DopantFerric chloridep-Toluenesulfonic acidp-Toluenesulfonic acid2-Anthraquinonesulfonic acid

Concentration(in g/L)

3.310.03.3

ci/r.22.15.11.07

(Cl+S)/N O/N"S/N

.12

.18

.27

«Polymerized in water at room temperature using 0.2 mol/L of pyrrole and a ferric chloride/pyrrole ratio of 2.4in the absence of textiles. ''In excess of oxygen in sulfate ions.

.22

.27

.29

.34

.83

.83

.57

.36

acid are used.^^ Analytical investiga-tions reveal that the less water solubleacids suppress the doping with chlo-ride ions resulting in doping predomi-nantly with the aryl sulfonates. It isfurther suspected that the relativelyplanar structure of naphthalene- oranthraquinone-2-sulfonic acid allowsstacking of these molecules betweenthe layers of the conjugated polymer.This facilitates hopping of electronsfrom chain to chain where defects inthe chain occur, a process necessary toachieve reasonable conductancevalues.

If pyrrole is oxidized electrochemi-cally, appropriate doping can be ob-tained by the selection of the electro-lyte used. If, however, ferric chloride isused for the chemical oxidation of pjT-role, doping with chloride ions, whichare present in a relatively high concen-tration, is always possible. It has beensuggested that the use of iron (III) salts,derived from the desired organic sul-fonic acid, when used as a dopingagent would eliminate this problem.This is indeed possible, for instance,with the commercially available ferrictosylate.'^ The 3.5 times higher mo-lecular weight together with a manyfold higher price makes this approacheconomically unattractive.

Only very few publications discussthe elemental analysis of conductingpolymers. This may originate from the

fact that the results always indicate thepresence of excess oxygen, which isoften explained by the adsorption ofoxygen and carbon dioxide on the sur-face of the polymer. Recently, Martinand Lei have shown that polypyrrolesynthesized in the presence of a smallamount of water is most likely a co-polymer of pyrrole and 3-hydroxy-pyrrole.'^ While this substitution inposition three does not reduce thelength of the conjugated chain, andwith it the volume conductivity, it cer-tainly makes it more sensitive to oxida-

tion and the formation of defects in thechain. Table II shows the amount ofexcess oxygen, determined by directoxygen analysis, for polypyrrole syn-thesized with various doping agents.When the polymer is doped with chlo-ride only, enough excess oxygen isfound for 80% of the pyrrole moietiesto be hydroxy functionalized. Whereasin the presence of anthraquinone-2-sulfonic acid, this substitution is re-duced to about 30% of the pyrrolemolecules. This may present an addi-tional reason for the observed improve-ment in stability.

Tbere have been reports that theaddition of certain chemicals can im-prove the properties of the polypyrroleobtained. Such improvements have, forinstance, been reported for the additionofp-nitrophenol.^^It was found in thisstudy that the addition of the sparinglywater-soluble 2,4-dihydroxybenzo-phenone (DHBP) leads to an interestingimprovement in the stability of thepolypyrrole deposited onto the surfaceof textiles. DHBP is, for all practicalpurposes, insoluble in water. It can,however, be dissolved in methanol. Ifsuch a solution is added to an aqueoussolution of ferric chloride, the complexformed between the iron and the DHBPshows a limited water solubility. Fig.6 shows the normalized stability of apolypyrrole-coated fabric polymerizedin the presence of DHBP at elevatedtemperature as a function of time.

The initial increase in conductanceis dependent upon the DHBP/p)Troleratio and is more pronounced forhigher values. The initial increase inconductivity is most likely due to mor-phological changes in the depositedpolymer.^^ The mechanism for thisimprovement has not been established.There are reasons to believe that thecompound is not incorporated into the

Fig. 6. Degradation of polypyrrole-coated polyester fabric at 100C with and without DHBP.

December 1997 CCO Textiie Chemist and Colorist 19

Ml

Time (hours)100

Fig. 7. Conductivity decay of polypyrrole-coated PET Fabric at 100C {•). Best fit diffusion con-trolled curve (—). Best fit first-order decay curve ( ).

polymer. It is suspected that thedihydroxybenzophenone may furtherreduce the doping with chlorine ions,but this has not yet been determined.

Numerous papers discuss the degra-dation of polypyrrole and find either adiffusion-controlled or first-order de-cay reaction. Although the exactmechanism for degradation has notbeen established, oxidation of the poly-mer backbone leading to non-conju-gated defects is generally thought to beresponsible for the loss of conductivitywith time. Samples aged at the el-evated temperature under argon anddoped only with toluenesulfonic acidshow no degradation over time. Thedecay resembles diffusion-controlledkinetics at the initial stages of degrada-tion and first order kinetics during thelater stages (Fig. 7).

Applications

This study identified a number of ap-plications where the physical proper-ties of the textile substrate, such asstrength, drape, etc., combined withthe electrical and microwave proper-ties of the polypyrrole coating are ofimportance. These applications are instatic dissipation, EMI-shielding, com-posite structures, and particularly mili-tary applications where the microwaveresponse from polypyrrole-coated fab-rics is in the desired range.

The static dissipative characteristicsof polypyrrole-coated textiles are ex-cellent and independent of the relativehumidity of the environment. In addi-tion, the environmental stability is notof primary concern as the change inresistance even from 10* to 10 ohms/sq will not significantly affect staticdissipation. Applications such as in

sanding belts, transport belts, filtration,and others are promising.

Applications for electromagneticshielding have been found in a numberof areas where the elimination of elec-tromagnetic interference is of impor-tance, such as in electronic packaging,electronic instruments, power genera-tion equipment, and many others. "- ^

Textile fabrics have wide applica-tions in the field of reinforced compos-ite structures. It is essential that goodadhesion to the fiber substrate is ob-tained. Polypyirole-coated quartz fab-rics used in composite structures withepoxy resins for aircraft wings haveshown strength properties comparableto the uncoated control. One of the ben-efits from encapsulating the conduc-tive fabrics into a polymer matrix is theincrease in environmental stability ob-tained. Such combinations have beenfound suitable for practical applica-t ^

The microwave response of poly-pyrrole-coated fabrics in the range from10-10^ ohms/sq renders them particu-larly suited for military applicationssuch as camouflage, decoys, edgecards, etc. Composites of polypyrrole-coated fabrics behave as a continuousconductive media as opposed to agranular media with disperse polymerparticles such as carbon. The dielectricbehavior of these coated fabrics is sig-nificantly different from that of agranular composite in that it may bepossible to introduce substantialdielectric loss without significantly al-tering the real part of the dielectricfunction. ^ The microwave response toconducting materials is highly depen-dent on their surface resistance. Highlyconducting materials, such as metals.

are highly refiective and insulating ma-terials transmit GHz radiation. Theability to tailor the sheet resistance inpolypyrrole-coated fabrics enables theproduction of materials with tunablereflection, transmission, and absorp-tion properties. Mult ispee tral fabricscombining the radar properties ofpolypyrrole-coated fabrics with visible,near infrared, and thermal camouflageproperties are used to produce camou-flage nets for military applications.

A number of military applicationsuse polypyrrole coated fabrics to altertheir radar signature. Materials withconductivity gradients, such as edgecards, and layered structures, such asJaumann absorbers and Salisburyscreens, represent some of the militaryapplications of these fabrics.- - ^

Conclusions

Using the interaction at the liquid/solid interface, we have been able toself-assemble a layer of the conductivepolymer polypyrrole at the surface oftextiles such as fabrics, yams, or loosefibers. The electrical properties andenvironmental stability of these prod-ucts depend upon the processing con-ditions and particularly on thecounterions used. Applications ofthese commercially available products(sold under the trademark Contex) instatic dissipation, EMI shielding, com-posite structures, and military applica-tions have been discussed.

Acknowledgement

The author would like to thank thefollowing co-workers who made thispublication possible: A. D. Child, envi-ronmental stability: R. V. Gregory ofClemson University, kinetics; W. C.Kimbrell. s>Tithesis; and M. L. Sullivan,microscopy. Additional thanks are dueto A. D. Child for the extensive helpwith the preparation of the manuscript.The author is espacially indebted to theMilliken Research Corp. for allowong thepublication of this investigation.

Author's Address

Hans H. Kuhn. Milliken ResearchCorp.. M-405. P.O, Box 1927, Spar-tanburg. SC 29304; telephone 864-503-2320; fax 864-503-2417.

References

1. Kissa, I. and R. Dettre. TexÜle Research four-nal, Vol. 45, No. l l , November 1975. p773.

2. Lakshmi. B, B.. P. K. Dorhout. and C. R. Mar-tin. Cbemistr}' of Materials. Vol. 9 No. 3,March 1997. p857.

3. Marco, F. W.. U.S. Patent No. 4 420 5r 7^ 1993.4. Shirakawa, H. S.. Journal ofPoh^nr ~ience

Part A: Polymer Chemistry, V¿]. " ¡^ ^3'September 1996. p2529 and referer erein

5. Kudo. Y., Synthetic Metals. Vol ',^ 'April 1996, pl7 and references then-, ^ " '

6. Heywang. G.. Symposium MateriajjQ^

20 Textile Chemist and ColoristCOD Vol. 29, No.