Polyaniline as a Potential Radar Absorbing...

Defence R&D Canada

DEFENCE DÉFENSE&

Polyaniline as a Potential Radar

Absorbing MaterialPreliminary Experiments

Dr. Trisha A. HuberDRDC Atlantic

Mr. David R. EdwardsUniversity of Victoria Co-op Student

Technical Memorandum

DRDC Atlantic TM 2003-153

September 2003

Copy No.________

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

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Copy No: _________

Polyaniline as a Potential Radar Absorbing Material Preliminary Experiments

Dr. Trisha A. Huber DRDC Atlantic

Mr. David R. Edwards University of Victoria Co-op Student

Defence R&D Canada – Atlantic Technical Memorandum DRDC Atlantic TM 2003-153 September 2003

Abstract Conducting polymers offer many advantages over traditional radar absorbing materials (RAM), such as ferrites and carbon black. Conducting polymers are lightweight, corrosion-resistant, and relatively inexpensive. In addition, conducting polymers exhibit a high degree of tailorability and good matrix adhesion. The morphology of a polymer, and thus the conductivity, is highly dependent on reaction conditions, such as temperature, dopant, and oxidant/monomer ratio. Polyaniline has been synthesized under a variety of reaction conditions, including varying dopant, temperature, and oxidant:monomer ratio. The results of the synthetic experiments will be presented. In addition, the polymer has been incorporated into an acrylic matrix under a variety of conditions. The effect of these processing conditions on the conductivity and permittivity has been investigated, and the results will be discussed.

Résumé

Les polymères conducteurs offrent de nombreux avantages par rapport aux traditionnels matériaux absorbant les ondes radar, comme les ferrites et le noir de carbone. Ils sont légers, résistent à la corrosion et coûtent relativement peu. En outre, ils sont très adaptables et adhèrent bien à une matrice. La morphologie et, par conséquent, la conductivité d’un polymère dépendent énormément des conditions de réaction, comme la température, le dopant et le rapport oxydant/monomère. La polyaniline a été synthétisée dans diverses conditions de réaction, dont divers dopants, diverses températures et divers rapports oxydant/monomère. On présente les résultats d’expériences de synthèse. De plus, le polymère a été incorporé dans une matrice acrylique dans diverses conditions. On a étudié l’effet des conditions de traitement sur la conductivité et la permittivité, et on examinera les résultats obtenus.


i



ii

Executive summary

Introduction

Conducting polymers are organic in nature, thus interact favourably with matrix materials; this interaction results in excellent adhesion, thus a more robust material. Traditional radar absorbing materials (RAM), such as ferrites or carbon black, are fraught with problems relating to their cost, environmental integrity, and ease of application. In most cases, these substances must be incorporated into a matrix, however, the resulting composite exhibits poor mechanical properties due to the poor matrix adhesion (interfacial interaction). Breakdown of the composite necessitates renewal or reapplication of the coating or tile. In addition, in the case of the ferrites, corrosion is an issue, particularly in the harsh environmental conditions out at sea. Furthermore, the ferrite-based RAM are relatively dense, and therefore add significant weight to the structure. Conducting polymers appear to offer a viable alternative, as they offer improvements over traditional RAM particularly with respect to weight, corrosion, cost, and matrix adhesion. Furthermore, the properties of conducting polymers may be tailored, rendering these materials more flexible than traditional RAM. Indeed there are already a number of studies, which indicate that conducting polymers have electromagnetic shielding and absorption properties.

Principal Results

Polyaniline was synthesized under a variety of reaction conditions – variable dopant, temperature, and oxidant/monomer ratio. The degree to which these synthetic parameters affect the conductivity of the product was assessed. In addition to the synthetic parameter investigation, the effect of several processing parameters in the formation of acrylic composites was also studied. The effect of the processing parameters on the conductivity and permittivity of the composites was also determined.

Significance

These results are significant as they indicate which synthetic parameters are the most influential in tailoring the properties of the polymer. In addition, the results of the processing parameter investigation indicate how sensitive the electrical properties are to the history of the composite.

Future Plans

Future investigations include the preparation and characterization of polyaniline derivatives designed to improve dielectric loss and solution-processibility, as well as focussing on selectively producing polyaniline in the form of fibres or microtubules.

Huber, T.A., Edwards, D.R.. 2003. Polyaniline as a Potential Radar Absorbing Material: Preliminary Experiments. TM 2003-153 DRDC Atlantic.


iii

Sommaire

Introduction

Étant donné leur nature organique, les polymères conducteurs interagissent favorablement avec des matériaux utilisés comme matrice; cette interaction entraîne une excellente adhérence et permet donc d’obtenir un matériau plus robuste. Les matériaux traditionnels absorbant les ondes radar, comme les ferrites ou le noir de carbone, comportent de nombreux problèmes liés à leur coût, à leur intégrité environnementale et à leur facilité d’application. Dans la plupart des cas, ces substances doivent être incorporées dans une matrice. Néanmoins, le composite obtenu présente des propriétés mécaniques médiocres en raison d’une faible adhérence à la matrice (interaction inter-faciale). Comme le composite se dégrade, il faut reconstituer le revêtement ou la tuile ou encore procéder à une nouvelle application. En outre, dans le cas des ferrites, la corrosion est un problème, tout particulièrement dans les rigoureuses conditions en mer. Par ailleurs, comme les matériaux à base de ferrite sont relativement denses, ils augmentent considérablement le poids de la structure. Les polymères conducteurs semblent constituer une solution de rechange viable, puisqu’ils offrent des améliorations par rapport aux matériaux traditionnels absorbant les ondes radar, tout particulièrement en ce qui concerne le poids, la résistance à la corrosion, le coût et l’adérence à la matrice. De plus, comme leurs propriétés peuvent être adaptées, les polymères conducteurs peuvent être utilisés avec une plus gande souplesse que les matériaux traditionnels absorbant les ondes radar. En effet, on a déjà publié un certain nombre d’études indiquant que les polymères conducteurs possèdent des propriétés de protection et d’absorption électromagnétiques.

Principaux résultats

La polyaniline a été synthétisée dans diverses conditions de réaction, soit divers dopants, diverses températures et divers rapports oxydant/monomère. On a évalué dans quelle mesure ces paramètres de synthèse influent sur la conductivité du produit, ainsi que l’effet de plusieurs paramètres de traitement sur la formation de composites acryliques. On a également déterminé l’effet des paramètres de traitement sur la conductivité et la permittivité des composites.

Importance des résultats

Ces résultats sont importants, car ils indiquent quels paramètres influent le plus sur l’adaptation des propriétés du polymère. En outre, les résultats des études portant sur ces paramètres révèlent combien sont sensibles les propriétés électriques aux traitements subis par le composite.


iv

Plans futurs

Parmi les autres recherches qu’il y aurait lieu d’effectuer, on compte la préparation et la caractérisation de dérivés de la polyaniline conçus pour réduire la perte diélectrique et améliorer la facilité de traitement de la solution, ainsi que l’étude de la production sélective de polyaniline sous forme de fibres ou de microtubules.

Huber, T.A., Edwards, D.R.. 2003. Polyaniline as a Potential Radar Absorbing Material: Preliminary Experiments. TM 2003-153 DRDC Atlantic.


v

Table of contents

Abstract........................................................................................................................................ i

Résumé ........................................................................................................................................ i

Executive summary ................................................................................................................... iii

Sommaire................................................................................................................................... iv

Table of contents ....................................................................................................................... vi

List of figures .......................................................................................................................... viii

List of tables ............................................................................................................................ viii

Introduction ................................................................................................................................ 1

Experimental............................................................................................................................... 7 Materials ........................................................................................................................ 7 Synthetic Conditions ..................................................................................................... 7 Plaque Preparation......................................................................................................... 7 Conductivity Measurements .......................................................................................... 8 Permittivity Measurements............................................................................................ 8

Results and Discussion ............................................................................................................... 9 Effect of Oxidant/Monomer Ratio................................................................................. 9 Effect of Dopant ............................................................................................................ 9 Effect of Temperature.................................................................................................. 11 Stability of Conductivity ............................................................................................. 11 Permittivity .................................................................................................................. 13 Processing Conditions ................................................................................................. 14


vi

Conclusion................................................................................................................................ 18

References ................................................................................................................................ 19

List of symbols/abbreviations/acronyms/initialisms ................................................................ 21

Distribution list ......................................................................................................................... 23


vii

List of figures

Figure 1. Polypyrrole.................................................................................................................. 2

Figure 2. Polyaniline .................................................................................................................. 3

Figure 3. Variable Oxidation State Octamers of Aniline ........................................................... 3

Figure 4. Polyaniline Charge Carriers ........................................................................................ 4

Figure 5. Dopant Structures........................................................................................................ 7

Figure 6. Conductivity Measurements........................................................................................ 8

Figure 7. Plot of Conductivity versus Oxidant/Monomer Ratio............................................... 10

Figure 8. Effect of Dopant on Conductivity ............................................................................. 10

Figure 9. Plot of Conductivity versus Synthesis Temperature ................................................. 12

Figure 10. Conductivity as a Function of Sample Age............................................................. 12

Figure 11. Sample Permittivity Data ........................................................................................ 13

Figure 12. Permittivity, Loss Tangent, and Conductivity as a Function of PAni/PTSA Loading15

Figure 13. Permittivity, Loss Tangent, and Conductivity as a Function of Plasticizer Content16

Figure 14. Permittivity, Loss Tangent, and Conductivity as a Function of Heating Time....... 16

List of tables

Table 1. Conductivity of Various Materials ............................................................................... 5


viii

Introduction

The goal of stealth technology is to make one’s military assets invisible to the enemy. In terms of radar, one needs to minimize the radar cross section (RCS), which is a measure of detectability of a target. There are several ways to reduce RCS, involving either altering the shape or the surface characteristics of the target by active or passive means.[1] Altering the shape of a target must take place at the design phase, thus for existing military assets, altering the surface characteristics is the only viable alternative. The use of radar absorbing materials (RAM) is one way of altering the surface characteristics, and they function by absorbing and redirecting high frequency electromagnetic (EM) radiation. The use of RAM is extremely practical as these materials can be applied in the form of a coating or a tile.

In general, RAM operate by interacting with EM radiation in the microwave region, thus must be electrically conductive or magnetic. The requirement for an electrically conductive or magnetic material, is clear, although there are other properties which are highly desirable. A material with the ability to absorb over a wide frequency range (broadband absorber) would be more effective at reducing RCS than a narrowband absorber. Other desirable properties include lightweight (especially for aircraft), durable and corrosion resistant (therefore low maintenance), and low cost (military assets possess a large surface area).[1]

Materials used traditionally as RAM consist of ferrites, which are magnetic, or carbon based RAM, often carbon black, which is conductive. Ferrites are problematic due to their high density, and their tendency to corrode. Carbon black exhibits poor matrix adhesion, resulting in loss of integrity of the coating or composite. There is obviously a need for RAM with superior properties.

A relatively new type of material, known as conducting polymers, exhibits many of the desirable RAM properties, which is why their viability is currently being investigated. Conducting polymers are polymers that possess a conjugated π system, which means the backbone is made up of alternating single and double bonds. These polymers may be chemically or electrochemically oxidized or reduced, meaning that π electrons are removed or added; this process is known as doping. In 1976, Shirakawa, Heeger, and MacDiarmid discovered that treatment of semiconducting polyacetylene with iodine, produced a polymer exhibiting conductivity comparable to metals.[2] The conductivity had increased 11 orders of magnitude! This phenomenon was also found to occur with other conjugated polymers, such as polypyrrole, polythiophene, and polyaniline. The importance of this discovery and the vast potential of these materials are exemplified by the awarding of the Nobel Prize in Chemistry to these scientists in 2000. Applications for conducting polymers may be found in a large number of fields including anti-static devices, electromagnetic shielding, sensors, actuators, lightweight energy storage, displays, and electronic components, just to name a few.[3]

Their versatility becomes obvious when one considers their favourable properties, including their environmental stability, low density (therefore lightweight), the fact that their conductivity may be tuned, and their corrosion resistance. In addition, they exhibit better matrix adhesion than other conducting materials, and this allows for their ease of incorporation into various coatings and commodity polymers. Moreover, conducting

DRDC Atlantic TM 2003-153 1

polymers may be deposited onto fabric,[4-8] and in some cases may be made solution-processible,[9-13] such that ink-jet printing of a conducting polymer solution is possible. Ink-jet printing may be used to print a pattern or pixels onto a substrate.[14] In short, conducting polymers are extremely versatile, and many of their properties address some of the shortcomings of traditional RAM.

The oxidation of neutral (or undoped) polypyrrole to produce a conductive, doped polymer, illustrated in Figure 1, also applies to polythiophene, in which a sulfur atom replaces the NH moiety. The polymer is oxidized by removing electrons from the π system, resulting in a positively charged backbone chain; this process is known as p-doping (p for positive). The neutral polymer may also be reduced, yielding a negatively charged backbone (known as n-doping). The positive charge in the backbone is delocalized over many monomer units, and is compensated by the presence of a counter anion, represented by A-. The most common method of preparing conducting polymers is by oxidative polymerization. In this method, the polymerization and the oxidation occur in the same process. As the polymerization of pyrrole proceeds, the chain is oxidized. Oxidative polymerization may be carried out either chemically or electrochemically. The dramatic increase in conductivity that occurs upon doping is due to an increase in the number of charge carriers, as well as the introduction of energetically accessible states in the band gap, which is the energy gap between the valence band and the conduction band.[2]

NH

NH

NH

oxidant NH

NH

NH

+A-

n n

Figure 1. Polypyrrole

Polyaniline is structurally different from polypyrrole and polythiophene in that the heteroatom, nitrogen, is not part of the ring (see Figure 2, below). Oxidation of the polyaniline chain can occur with concurrent loss of amine protons, resulting in the formation of neutral quinoid units, as opposed to a positively charged backbone. Quinoid units contain imine nitrogens, and the greater the number of quinoid units, the higher the oxidation state of the polyaniline.[15]


2

NH NH NN

y 1-y

NH NH

y 1-y

HHN

+N

+

A-

amine imine

HA

Figure 2. Polyaniline

There is a wide range of oxidation states of polyaniline with the most reduced form, called leucoemeraldine, consisting of all benzenoid units (y = 1), and the most oxidized form, called pernigraniline, consisting of quinoid units (y = 0). The wide range of oxidation states is illustrated with the octameric oligomer, in Figure 3. The middle oxidation state, y ~ 0.5, is known as emeraldine, due to its dark green colour when doped (undoped it is blue), and it is the most conductive. Although polyaniline can be oxidatively doped, the most common method of doping polyaniline is known as acid (or proton) doping. In acid doping, the number of π electrons is unchanged, but many of the nitrogen atoms are protonated. Protonation of the imine nitrogens, which are more basic than the amine nitrogens, and are thus protonated preferentially, results in the formation of charge carriers. As with the polypyrrole or polythiophene, acid doping results in an increase in the number of charge carriers as well as the formation of new energy states within the band gap.[15]

NH2NH

NH

NH

NH NH

NH

NH

NH2NH

NH

N

N NH

NH

NH

NH2N

N

N

N NH

NH

NH

NH2N

N

N

N NH

N

N

NHN

N

N

N N

N

N

leucoemeraldine

protoemeraldine

emeraldine

nigraniline

pernigraniline

Figure 3. Variable Oxidation State Octamers of Aniline


3

The initial product of imine protonation undergoes a geometrical relaxation, in which the electrons rearrange, yielding an unpaired electron and a positive charge on the imine nitrogen atoms. This charge carrier is known as a bipolaron, but is believed to be relatively unstable, and undergoes a redistribution of charge and spin to yield the more stable polaron.[2]

NH NH

y 1-y

HHN

+N

+

xgeometrical relaxation

NH NH

y 1-y

HHN

+N

+

xbipolaronredistribution of charge and spin

HHN

+N

HN

+N

H

xpolaron

Figure 4. Polyaniline Charge Carriers

To put their electrical conductivity in perspective, Table 1 lists a number of materials, in order of decreasing conductivity from top to bottom. Undoped conducting polymers are insulating or semi-conducting at best, due to the large band gap between the valence and conduction bands, but when doped, can achieve conductivity approaching that of metals. Undoped polyaniline is insulating at ~ 10-10 S/cm, which is on par with glass, but its conductivity can reach ~ 10 S/cm when doped, an increase of 11 orders of magnitude.[14]


4

Table 1. Conductivity of Various Materials

MATERIAL CONDUCTIVITY (S/cm)

gold, silver, copper ~ 106

doped trans-polyacetylene ~ 105

doped polyaniline ~ 101

germanium ~ 10-2

silicon ~ 10-6

undoped trans-polyacetylene ~ 10-6

undoped polyaniline ~ 10-10

glass ~ 10-10

quartz ~ 10-12

from reference [14]

Conductivity depends on three parameters: the number of charge carriers, the charge and mobility of the carriers,

σ = nqη (1)

where n = number of charge carriers, q = charge, and η = mobility

For a conducting polymer there are two contributions to the conductivity: the intrachain conductivity, where charge transport occurs along the backbone of the chain, thus is dependent on the conjugation length, and interchain conductivity, where the charge hops from one chain to the next, and depends on the morphology, or structure, of the polymer.

σ = σintrachain + σinterchain (2)

It has been determined that the interchain contribution is predominant for polyaniline, so some degree of control over the conductivity may be achieved by controlling the morphology. The morphology of a conducting polymer depends on a number of factors, such as the oxidation state, degree of doping, nature of the dopant (size and shape), and the presence of any solvent or additives.

Another important property when considering potential RAM is the permittivity, or dielectric constant (ε), of the material. When an electrically conductive material interacts with electromagnetic radiation, the permittivity is described by a complex number, in which the


5

imaginary component represents the loss, or absorption of energy by the material. The electromagnetic radiation is effectively converted to heat.

ε = ε’ - jε” (3) The magnitude of the loss is often expressed as the loss tangent, tan δe (where the subscript e denotes electric loss, as opposed to magnetic), and is frequency dependent. At high frequencies, ωε” >> σ, thus the numerator (see equation 4) is reduced to ωε”, and the loss tangent is reduced to the ratio of the imaginary component, ε”, to the real component, ε’.

tan = +eδ σ ωε

ωεεε

"'

"'

≈ (4)

In this paper, the results of preliminary experiments of the preparation of polyaniline under a variety of conditions are reported. The major objective was to determine whether polyaniline would absorb EM radiation in the microwave region, and the degree to which synthetic and processing parameters affect the dielectric loss.


6

Experimental

Materials

Aniline (>99.5 %, Aldrich), ammonium persulfate (APS) (Fisher Scientific, 99%), N-phenyl-1,4-phenylene diamine (dianiline) (Aldrich, 98%), p-toluenesulfonic acid (PTSA) (Aldrich, 98%), dodecylbenzenesulfonic acid (DBSA) (Aldrich, 70 wt% in isopropanol), camphorsulfonic acid (CSA) (Aldrich, 98 %), and hydrochloric acid (Anachemia, 37 wt%), were used as received.

Synthetic Conditions

The reactions were carried out in aqueous acidic medium (1 M, pH ~ 0), in which the source of the acid was also the dopant (see Figure 5 for dopant structure). Typically, the reactions were usually allowed to run for 24 hours, the oxidant/monomer ratio was 0.75, and the reactions were normally carried out at room temperature – unless, of course these were parameters under investigation. In most cases dianiline (N-phenyl-1,4-phenylene diamine), which is simply the dimer of aniline, was used as an initiator at a dianiline:aniline ratio of 1:38. The purpose of the initiator, which is more easily oxidized than aniline and thus provides a starting point for the polymer chain, is to exert some control over the distribution of polymer chain length.

HO3S CH3(CH2)11

HO3S CH3

HO3S O

CH3CH3

p-toluenesulfonic acid (PTSA)

dodecylbenzenesulfonic acid (DBSA)

camphorsulfonic acid (CSA)

ClH hydrochloric acid (HCl)

Figure 5. Dopant Structures

Plaque Preparation

Conducting polymers are brittle solids thus must be processed into a film or a composite in order to measure the permittivity. Acrylic plaques were prepared by mixing the polyaniline, polymethyl methacrylate (PMMA), and plasticizer (hydroquinone), using a coffee grinder or


7

ball mill, then compression-molding the mixture at high pressure (80 psi) and high temperature (150 °C or 180 °C). The plaque was then milled to size.

Conductivity Measurements

Conductivity measurements were performed on pressed pellets or acrylic plaques using the four-point probe technique. In this method, four equally spaced probes are placed in contact with the sample. A current is applied via the outer probes, and the voltage drop is measured between the inner probes. Current-voltage data are collected and the conductivity is calculated using these data and correction factors associated with the size and shape of the specimen.

Figure 6. Conductivity Measurements

Permittivity Measurements

For the permittivity measurements, the source of the EM radiation was a vector network analyzer (VNA) (Hewlett-Packard 8720C) operating in the X band (8.2 to 12.4 GHz). The milled specimens were placed in a waveguide perpendicular to the direction of propagation, and irradiated at X-band frequencies. The complex permittivity was determined from the measured transmission and reflection coefficients using a genetic algorithm, and the loss tangent was calculated at 10 GHz.


8

Results and Discussion

A number of synthetic parameters (oxidant/monomer ratio, nature of the dopant, and reaction temperature) have been varied, and the effect of these parameters have been quantified in terms of the conductivity. In addition to the synthetic parameters, a number of parameters associated with the production of acrylic plaques, (processing parameters), have been varied, and their effect on conductivity and permittivity of the composites determined.

Effect of Oxidant/Monomer Ratio

The synthesis of HCl doped polyaniline (PAni/HCl) was carried out under variable oxidant/monomer conditions. Figure 7 illustrates the relationship between oxidant/monomer ratio and the yield, and conductivity of the product. It was found that as the oxidant/monomer ratio increased, the yield increased, and the conductivity increased to a maximum at a ratio of 0.75, then dropped dramatically beyond that. At a ratio of 1, the polymer has been oxidized beyond emeraldine to pernigraniline, which is not very conductive or stable under atmospheric conditions. According to Beadle et al,[16] the oxidant:monomer stoichiometry is 1:4, and the excess aniline serves to reduce the growing polymer chain that is formed in the pernigraniline oxidation state. Based on this, one would expect that an oxidant:monomer ratio greater than 1:4 should result in a polymer having an average oxidation state less than y = 0.5 (more quinoid units) (see Figure 2), thus lower conductivity. The experimentally observed increase in conductivity beyond the theoretical optimum ratio of 1:4 may be due to segregation of conducting sections along the chain. In this case, the polymer would effectively be a block copolymer with conducting and insulating regions along the chain. The high conductivity would then be attributed to the higher contribution of interchain conductivity to the overall conductivity. The concept of polyaniline consisting of segregated conducting and insulating phases, has been introduced by Shimano and Heeger,[17] in the context of the emeraldine base in N-methyl pyrrolidinone (NMP) solution existing as a fluxional block copolymer.

Effect of Dopant

Figure 8 illustrates the effect of the nature of the dopant on conductivity. The results are not surprising, as the conductivity of the doped polymers appears to be inversely proportional to the size of the dopant. Hydrochloric acid (HCl), which is a relatively small dopant, yields a highly conducting polymer, although its solubility is low, and therefore processibility is very poor. The use of organic sulfonic acids results in a polymer having lower conductivity, however, the doped polymers have enhanced solubility; this phenomenon is termed “counter-ion induced processibility”.[18]


9

0

0.2

0.4

0.6

0.8

1

1.2

0 0.25 0.5 0.75 1 1.25

Oxidant:Monomer Ratio

Con

duct

ivity

(S/c

m)

0

0.5

1

1.5

2

2.5

Yiel

d (g

)

conductivityyield

Figure 7. Plot of Conductivity versus Oxidant/Monomer Ratio

02468

10121416

PTSA DBSA CSA HCl

Con

duct

ivity

(S/c

m)

Figure 8. Effect of Dopant on Conductivity


10

Effect of Temperature

This plot illustrates the effect of the synthesis temperature on conductivity for PTSA and CSA doped polyaniline. Not surprisingly, the conductivity increases with decreasing temperature – the lower temperature would minimize side reactions and defects in the chain, which would lower the conductivity. The magnitude of the effect is fairly low, supporting the conclusion that the intrachain contribution is minor. The origin of the higher conductivity for the PTSA doped polyaniline is unconfirmed, but may be due to the shape of the PTSA (less bulky) resulting in a more favourable morphology.

Stability of Conductivity

Although the resulting conductivity is lower for the organic dopants, the dopant size and lack of volatility, serve to stabilize the conductivity. After monitoring the conductivity of both PAni/HCl and PAni/PTSA over a period of three months, it is evident that the PTSA doped polyaniline is far more stable than that doped with HCl. Initially the HCl doped polyaniline exhibits higher conductivity than the PTSA doped polyaniline, however, even over a month, the conductivity diminishes fairly quickly. In contrast, the conductivity of the PTSA doped polyaniline remains stable over at least 3 months. As previously mentioned the improvement in stability may be attributed to the size of the counter-ion, which limits its mobility within the polymer, and its lack of volatility. In addition, the PTSA doped polyaniline is more soluble in organic solvents, improving its processibility.


11

0

2

4

6

8

10

-10 0 10 20 30

Temperature (°C)

Con

duct

ivity

(S/c

m) PAni/PTSA

PAni/CSA

Figure 9. Plot of Conductivity versus Synthesis Temperature

0

5

10

15

20

0 20 40 60 80 100Age (days)

Con

duct

ivity

(S/c

m) PAni/HCl

PAni/PTSA

Figure 10. Conductivity as a Function of Sample Age


12

Permittivity

Figure 11, below, is a plot of permittivity versus frequency for a sample containing 30 % by wt. PAni/CSA. The blue line represents the real and the pink line represents the imaginary data; the green lines represent the uncertainty bounds. This sample is fairly lossy, that is, it absorbs radiation, as evidenced by the loss tangent of 1.8 at 10 GHz. The measured dc conductivity of this particular sample is 0.14 S/cm, and the red line corresponds to the theoretical ε” calculated from the measured conductivity. What is interesting about this sample is the fact that the measured ε” is much greater than the theoretical ε”. This indicates that there is additional loss over and above that expected due strictly to the dc conductivity. Overall, this sample appears to be a good candidate for RAM, although we need to perform some free space measurements on such samples, to fully assess their viability.

Figure 11. Sample Permittivity Data


13

Processing Conditions

There are a number of parameters associated with the preparation of composite plaques, which have an effect on the conductivity and permittivity. The processing affects the conductive network within the composite, thus the electrical properties. Different methods of mixing the solids (conducting polymer, plasticizer, and acrylic) together were investigated, such as ball milling and using a coffee grinder. The latter method appears to give more conductive samples, however this is under further study.

After compression molding, which simply melts the solids under high pressure, the samples are cooled to room temperature. The effect of cooling rate was investigated but didn’t seem to yield any clear correlation between cooling rate and loss tangent, although the faster cooled samples exhibited less phase separation. Two heating temperatures were studied, ~150°C and ~180°C, and it was found that the lower temperature yielded a more conductive sample.

The next three figures (Figures 12, 13, and 14) have the same format and colour scheme in an effort to minimize confusion. The left hand y-axis corresponds to the real (black) and imaginary (blue) permittivity components, whereas the right hand y-axis corresponds to the conductivity (green) and loss tangent (red).

The effect of polyaniline loading on the conductivity and permittivity was investigated and the data are presented in Figure 12. As the polyaniline content increases, ε’ increases and levels off at ~ 25 wt %. The ε” (and therefore the loss tangent) increases with loading, and appears to reach a maximum at ~ 35 %. The conductivity increases with increased loading, as expected. With increasing loading, the loss (ε”) appears to reach a maximum despite the continued increase in conductivity. The high loading behaviour represents a deviation from theory, and has been noted in other conducting polymer systems that have been studied. The origin of this deviation is not yet known, but further studies are underway. Also, these data were acquired prior to optimizing the processing conditions, and will be investigated further.

Figure 13 illustrates the effect of plasticizer on the electrical properties. The real component is fairly constant over the range of plasticizer studied, and the imaginary data reach a maximum at ~ 5% by wt. In the absence of plasticizer, the loss tangent is less than one, and increases to > 2.5 ~ 5%. The conductivity also reaches a maximum in the range of 5-10% by wt. After ~ 5% both the loss tangent and the conductivity drop off dramatically. The decrease in conductivity and loss tangent with increased plasticizer content might indicate an increase in phase separation (anti-plasticization effect). In fact an increase in phase separation is observed visually in the specimens as the plasticizer content increases.


14

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60% PAni/PTSA

Perm

ittiv

ity

0

0.5

1

1.5

2

2.5

3

Loss

e'e"loss tangentconductivity

Figure 12. Permittivity, Loss Tangent, and Conductivity as a Function of PAni/PTSA Loading

0

10

20

30

40

50

0 5 10 15% Plasticizer

Perm

ittiv

ity

0

0.5

1

1.5

2

2.5

3

Loss

Tan

gent

/Con

duct

ivity

(S

/cm

)



15

Figure 13. Permittivity, Loss Tangent, and Conductivity as a Function of Plasticizer Content

The compression molding heating time has a definite effect on the electrical properties, as illustrated in Figure 14. Based on the time range studied, the optimal heating time is the shortest (30 minutes). A heating time of 45 minutes yielded results similar to that of 30 minutes, however with heating times longer than 45 minutes, the imaginary permittivity decreased dramatically. In general, as the heating time increased, the ε” (and therefore the loss tangent) decreased.

0

20

40

60

80

100

20 40 60 80 100

Time (min)

Perm

ittiv

ity

0

0.5

1

1.5

2

2.5

3

3.5

4

Loss

ta

ngen

t/Con

duct

ivity

(S

/cm

)


Figure 14. Permittivity, Loss Tangent, and Conductivity as a Function of Heating Time

To summarize the processing studies thus far, ε” (and thus the loss tangent) increases with lower heating temperature, lower plasticizer content, and shorter heating time. All of these conditions appear to minimize the opportunity for phase separation, thereby improving the


16

conductive network. This is further supported by the observation that more homogeneous samples are achieved with faster cooling.


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Conclusion

As previously mentioned, these results are preliminary, and research is ongoing on a number of fronts, including improvement in processibility and stability of the polymer itself, as well as optimizing the conductive network in the composites. Processibility can be improved by the introduction of solubilizing substituents on the aniline ring, and stability can be improved by introducing substituents capable of doping the polymer, producing self-doped polymers, or by using large polymeric dopants. Optimization of the conductive network by controlling the particle size and shape, or by optimizing plasticizer and matrix polymer, one can minimize the percolation threshold (the minimum loading required to produce a conductive network).

Based on these preliminary results, conducting polymers look promising: they are lossy, lightweight, and corrosion resistant. In addition, they are extremely versatile and may be made into films, composites, or coated onto fabric, so there is great potential for fabricating a multiple layer device. The electrical properties are tailorable and there is scope for improvement in solubility, enabling ink-jet printing of patterns, which can improve radar absorption.


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References

1. Vinoy, K.J., Jha, R.M., Radar Absorbing Materials: from theory to design and characterization. 1996, Boston: Kluwer Academic Publishers.

2. Heeger, A.J., Semiconducting and Metallic Polymers: The Fourth Generation of Polymeric Materials (Nobel Lecture). Angew. Chem. Int. Ed. Engl., 2001, 40, p. 2591-2611.

3. Jagur-Grodzinski, Electronically Conducting Polymers. Polym. Adv. Technol., 2002, 13, p. 615-625.

4. Gregory, R.V., Kimbrell, W. C., Kuhn, H. H., Conductive textiles. Synth. Met., 1989, 28, p. C823-C835.

5. Kuhn, H.H., Kimbrell, W.C., Electrically conductive textile materials and method for making same, 1989, 4,803,096.

6. Kuhn, H.H., Kimbrell, W.C., Method for making electrically conductive textile materials, 1991, 5,030,508.

7. Kim, M.S., Kim, H.K., Byun, S.W., Jeong, S.H., Hong, Y.K., Joo, J.S., Song, K.T., Kim, J.K., Lee, C.J., Lee, J.Y., PET Fabric/Polypyrrole Composite with High Electrical Conductivity for EMI Shielding. Synthetic Metals, 2002, 126, p. 233-239.

8. Oh, K.W., Hong, K.H., Kim, S.H., Electrically Conductive Textiles by in situ Polymerization of Aniline. J. Appl. Poly. Sci., 1999, 74, p. 2094-2101.

9. Oh, E.J., Jang, K.S., Synthesis and characterization of high molecular weight, highly soluble polypyrrole in organic solvents. Synth. Met, 2001, 119, p. 109-110.

10. Angelopoulos, M., Gelorme, J.D., Newman, T.H., Patel, N.M., Seeger, D.E., Water-Soluble Electrically Conducting Polymers, Their Synthesis and Use, 2000, 6,010,645: U.S.

11. Ahn, S.H., Czae, M., Kim, E.R., Lee, H., Han, S.H., Noh, J., Hara, M., Synthesis and Characterization of soluble Polythiophene Derivatives Containing Electron-Transporting Moiety. Macromolecules, 2001, 34, p. 2522 - 2527.

12. Kinlen, P.J., Liu, J., Ding, Y., Graham, C.R., Remsen, E.E., Emulsion Polymerization Process for Organically Soluble and Electrically Conducting Polyaniline. Macromolecules, 1998, 31, p. 1735 - 1744.

13. Lin, H.-K., Chen, S.-A., Synthesis of New Water-Soluble Self-Doped Polyaniline. Macromolecules, 2000, 33, p. 8117 - 8118.

14. MacDiarmid, A.G., "Synthetic Metals": A Novel Role for Organic Polymers (Nobel Lecture). Angew. Chem. Int. Ed. Engl., 2001, 40, p. 2581-2590.

15. Ray, A., Asturias, G.E., Kershner, D.L., Richter, A.F., MacDiarmid, A.G., Epstein, A.J., Polyaniline: Doping, Structure and Derivatives. Synth. Met, 1989, 29, p. E141-E150.


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16. Beadle, P.M., Nicolau, Y.F., Banka, E., Rannou, P., Djurado, D., Controlled polymerization of aniline at sub-zero temperatures. Synth. Met, 1998, 95, p. 29-45.

17. Shimano, J.Y., MacDiarmid, Phase segregation in polyaniline: a dynamic block copolymer. Synth. Met, 2001, 119, p. 365-366.

18. Cao, Y., Smith, P., Heeger, A.J., Counter-ion induced processibility of conducting polyaniline and of conducting polyblends of polyaniline in bulk polymers. Synth. Met., 1992, 48, p. 91 - 97.


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List of symbols/abbreviations/acronyms/initialisms

APS ammonium persulfate

CSA camphorsulfonic acid

°C Celsius

cm centimeter

δe dielectric loss tangent

DND Department of National Defence

DBSA dodecylbenzensulfonic acid

ε permittivity

ε’ real permittivity

ε” imaginary permittivity

EM electromagnetic

GHz gigahertz

HCl hydrochloric acid

M molar (moles/liter)

µ charge mobility

n number

NMP N-methylpyrrolidinone

PAni polyaniline

PMMA polymethylmethacrylate

PTSA p-toluenesulfonic acid

q charge


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RAM radar absorbing material

RCS radar cross section

σ conductivity

S siemens

VNA vector network analyzer

ω angular frequency


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Polyaniline as a Potential Radar Absorbing Material:Preliminary Experiments

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September 2003

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13. ABSTRACT (a brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is

highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual). Conducting polymers offer many advantages over traditional radar absorbing materials (RAM), such as ferrites and carbon black. Conducting polymers are lightweight, corrosion-resistant, and relatively inexpensive. In addition, conducting polymers exhibit a high degree of tailorability and good matrix adhesion. The morphology of a polymer, and thus the conductivity, is highly dependent on reaction conditions, such as temperature, dopant, and oxidant/monomer ratio. Polyaniline has been synthesized under a variety of reaction conditions, including varying dopant, temperature, and oxidant:monomer ratio. The results of the synthetic experiments will be presented. In addition, the polymer has been incorporated into an acrylic matrix under a variety of conditions. The effect of these processing conditions on the conductivity and permittivity has been investigated, and the results will be discussed.

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document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus. e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus-identified. If it not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title). Radar Absorbing Materials, RAM, Polyaniline, microwave


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