irer . ,--,- n - T

SUPERDIELECTRICS POLYMERS

Herbert A. PohlFrancis Bitter National Magnet LaboratoryMassachusetts Institute of Technology

Cambridge, MA

and

Pohl Cancer Research LaboratoryStillwater, OK

ABSTRACT

A giant type of polarization appears in certain highlyaromatized polymeric solids. It arises when (a) the structurecan inherently dissociate to form electronic carriers, and (b)when there are long domains of equivalent sites for freed elec-tronic carriers to rove about in response to an external elec-tric field. It is referred to as 'nomadic' polarization, andjoins the familiar electronic, atomic, and dipolar orientationtypes as a fourth molecular mode of polarization.

Applications for the new super-dielectrics based upon theseconcepts will be many; including improved capacitor materialsfor better energy storage and control, low visibility coatings,obscuration, and EMP shielding, while in microcircuitry appli-cations are transducers and parametric circuits.

Progress in this promising new field is reviewed. A dis-cussion of structures essential for nomadic polarization isgiven, together with some new results exemplifying these con-cepts. Future work and trends are suggested.

INTRODUCTION

As so often occurs, the study of new chemical struc-tures reveals new and unexpected properties of matter.In the present instance, the study of organic polymershaving giant molecular orbitals (MO) available to theelectronic structure, revealed that the new polymerswere electro-active to an unexpected degree. In atleast the following attributes, the new "giant orbital'polymers could be made to be outstanding:

(1) polarizability [1-3,6-9,41,43,44,46,61-64,67,69-73, 75-80, 84],(2) inherent (i.e. undoped) conductivity [7,10-15,17-27,29,30,32,33,42,45-60,65,66,92-4,111-114,130],(3) piezo-resistivity [1,2,5,41,57,66,68,74,75],(4) piezo-polarizability [1,6,73], and(5) chemical and thermal stability [51,52].

Over the last two decades, the unusual behavior ofthese materials led to the discovery of a hitherto un-recognized giant type of electrical polarization,nomadic polarization. Moreover, analysis of the re-sults led to detailed theory and in turn further

experimental analysis. The theory proved quite success-ful in its many predictions, so that there is confidencein dealing with these new materials. The following re-marks highlight these findings, present new data, andpoint the way to further progress. Since these reviewremarks are addressed to a particularly diverse audience,including polymer chemists, quantum chemists, chemicalengineers, electrical engineers, electronic engineers,polymer physicists, and biophysicists, we have felt itnecessary to explain briefly certain basic physical andchemical concepts as we proceed with the discussions.Many scientists here and abroad have, of course, con-tributed to this huge field, as the frequent referencesand ample bibliography will- attest.

Interest in these new polymeric materials is sparkedby observing, for example, that one can tailor the"giant MO" polymers to produce pure and highly stablesuper-dielectrics with high relative dielectric per-mittivity, from 6 to 100000. Conventional polymers havepermittivities ranging from 1.8 to about 6. The highlypolarizable materials will be useful in energy storageand electric power control, as in large motor-startingcapacitors and in power-factor correction. They will

rllll --?~-7 ~ -' 11 cC---~ QI 2 T;~IE E

doubtless also be useful in low visibility coatings,in obscuration, in EMP shielding, in microcircuitry,in transducers, and in parametric circuits.

It remains to be shown, of course, whether or notthis recently uncovered molecular mode, nomadic polar-ization, can prove to be superior either in general orin particular cases, in energy storage and loss tothat of the conventional "dipole orientation" mode.Preliminary results indicate an advantage for the newgiant MO materials.

Studies of the conductive character of polymericspecies has been intense over the past 30 years, ashas that of the several monomer systems such as thecharge-transfer complexes formed with tetrathioful-valene (TTF), and tetracyanoquinodimethane (TCNQ) [11].Among the polymer systems studied [12-76] are poly-(acetylene) [17-21], substituted polyacetylenes [22,23], poly(p-phenylene) [24-25], poly(pyrrole) [26,27],poly(phenylene sulfide) [28-31,56,57], and poly-(phthaloacyanines) [32-37,46,48,49,64,65].

Again, the conductivity of conventional polymersusually ranges from about 10-18 to about 10-10 (Qcm)1l.On the other hand, that of the giant orbital materials,if they are doped with materials evoking an even higherconcentration of carriers, has been observed in pre-liminary studies, to go up to 50000 (Qcm)-1, i.e. aboutone half that of pure copper [7]. In the case of thecharge-transfer complexes, the giant inter-molecularorbital materials are generally quite unstable, evenat room temperature. The giant MO polymers, by con-trast, can be stable to over 10000C [28,51,52,125-132].

Since our emphasis here is upon the super-dielectrics,in the following discussion a brief overview of thevarious mechanisms of electrical polarization in matterwill be presented first. This will be followed by adiscussion of the chemical structures leading to giantpolarization and electroactivity, concluding with somenew results confirming these concepts.

POLARIZATION MECHANISMS

Broadly speaking, there are two main events, at leastin low field strenghts, that an experimenter sees uponplacing a potential across a material. There is a cur-rent whlich persists and enters through the material inand out of the contacting electrodes. This is referredto as conduction. There is also observed a currentwhich flows momentarily, then appears to be blocked.This is referred to as polarization. Both are conven-iently observed and quantified upon using sinusoidalpotentials at varied frequencies. There are a numberof mechanisms by which we understand electrical polar-ization to occur. At the molecular level, these arecalled electronic, atomic, orientational, and nomadicpolarizations. There are also numerous macroscopictypes of polarization.

When an external electric field is applied to matter,there appears the familiar "electronic" polarizationdue to the shift of electron density about the nucleiof atoms. There often also appears an "atomic" polar-ization due to the shift of differently charged atomsrelative to each other as the external field is applied.Application of an external field also helps realignmolecular or group dipoles, in an action called dipole"orientational" polarization. Finally, as the last ofthe recognized molecular polarization mechanisms, thereis "nomadic" polarization. It is due to the drift of

freed ("extra", arising from internal dissociation)carriers along molecularly large domains, such as thoseprovided by associated Tr orbitals in properly conjugated(i.e. "ekaconjugated", cf. below) polymers. Nomadicpolarization can result in huge or "giant" polarizations[1-3] for the resulting dipoles and dan be some 14orders of magnitude larger than those produced by elec-tronic polarization at the same field strength.

ELECTRONIC POLARIZATION

This is a universal polarization response. It arisesfrom the slight distortion of the electronic distributionabout the muclei of the atoms, and is therefore to beseen in compounds of all the elements, and in the ele-ments themselves. The distortion is usually quite small.This is to be expected since the electric fields withinthe atom are in the order of 1011 V/m; whereas man-madefields are normally in the order of 0 to 108 V/m. Ashift of the center of electronic charge relative to thenucleus of about 10-9 nm for a unit field of 1 V/cm isusual. Electronic polarization usually makes only amodest contribution to the polarizability of solids. Inmost organic solids, the electronic polarization leadsto a relative permittivity K, of about 1.8 to 4. In theinorganic compounds, especially those of the heavy ele-ments with higher electronic density, somewhat highervalues are experienced. For instance, the electronicpolarization contribution in elements such as silicon(K=12) and germanium (K=16) is rather higher than thatof elemental carbon as diamond (K=X5.5).

ATOMIC POLARIZATION

This type of polarization arises from the shifts ofdifferently charged atoms moving with respect to eachother as the external field is applied. A particularlyobvious example is that of sylvite, KCI, which is anionic solid. In this case the positive potassium ionsmove relative to the negatively charged chloride ionswhen subjected to an external electric field.

In most organic solids, where ions are absent, thistype of polarizations makes only a minor contributionto the total (electronic and atomic) polarization. Thiscontribution can be evaluated by comparing the permit-tivity obtained at low (i.e. "radio") frequencies withthose at very high (e.g. "optical") frequencies, wherethe inertia of the atoms does not permit their activeresponse. It is typical in organic solids that theatomic polarization contributes only about 15% to theoverall polarization. In the case of inorganic com-pounds, the atomic polarization can be quite sizable.That in perovskite structures ("Loose ion in a cage")such as BaTiO3 is the principal source of the remarkablyhigh, but highly temperature dependent, permittivity,about 3000.

DIPOLE ORIENTATION POLARIZATION

The presence of molecular groups having atoms of dif-fering electronegativity (electron affinity) leads tothe formation of dipoles. If these molecular groups aresomewhat free to rotate in the field, they will realignas the field is applied, thereby reducing their poten-tial energy. Examples of such dipolar molecules are HF,NO, HC1, H20, chlorobenzene, and poly(vinylchloride).Rotational polarization arises also from the rotation ofpendant polar groups such as -OH, or CO.OCH3, e.g. thistype of polarization response by intramolecular dipolesplays an important role in the low and medium range fre-quency dielectric behavior of macromolecular solids suchas commercial plastics.

, -c-;:- -r- M-- =. -- .'l -- 4- .- i - .R I ' - I - -; -.- E T -";- -, h,-. L-- i--.. -- C.-, -- '-' 1 `., . 5.- 1. = -- L - I -L! --.I. z 1 p-- Z.=. . I L-i t -.? . I .;-- ;. t -.7O. 5- !, Ocr-t---be.r - ,

Pnh1& -S --dieImc riC polivrmneC.

The actual contribution of dipole orientation to thepolarizability of either liquids or solids can bequite large. Liquid water has a permittivity of about78 at frequencies below about 19 GHz when at room tem-perature. The permittivity falls to about 1.8 at op-tical frequencies, where the molecular dipoles can nolonger "follow" the field reversals. Reviews of thethree earlier recognized types of polarization, elec-tronic, atomic, and orientational, for the case ofgases, liquids and solids, have been given by numerousstandard textbook writers over the past 40 years andneed not be repeated here.

In any case, the effective moment brought out by theaction of an external electric field can be describedby an effective polarizability, a- m or aind, wherefor the dipolar case, j and m refer to the rotationaland orientational quantum numbers; and the subscriptind refers to induced moments. The effective momentm is thus m=aj,mE for dipolar materials, and m=aindEfor field-induced dipoles.

The polarization evoked by dipole alignment or in-duction usually saturates at very high field strength,in the order of 105 V/cm. This is to be contrastedwith that for nomadic polarization, which typicallysaturates at much lower field strengths, in the orderof 102 to 103 V/cm.

NOMADIC POLARIZATION

As the fourth of the fundamental molecular polariza-tion mechanisms, we have "nomadic" polarization. Itis due to the drift of freed (locally superfluous) car-riers along molecularly large domains. Such domainscan be on long polymeric molecules or in certain cry-stal lattices. Super-dielectrics arise if thermallyexcited charges can drift through domains such as thoseprovided by associated 7r orbitals in properly conjugat-ed (i.e. "eka-conjugated", see below) polymers.

Nomadic polarization can result in huge or "giant"polarizations [1-3,73,79,80], for the resulting dipolesand can be some 14 orders of;magnitude larger thanthose produced by electronic polarization at the samefield strength. It can be of "electronic" or "pro-tonic" type, i.e. either protons or electronic carrierscan be the active carriers in the long domains. Theunusually high dielectric permittivity of Si (81) atlow frequencies is a case in point. Similarly, thevery high permittivity of certain hydrated lithiumsalts is assignable to protonic nomadic polarization[80,82,83].

6S5

.-te-;~~~~~~~~.J

...'::.: :': :::: . : .:~ ~ o

) e4

long< dipole >

Fig. 1: ExternaZ fieLd appZied. Note huge dipoZeby motion of the charge pairs. The charges moveZike "nomads" aZong the conjugation.

hole-type carriers. These may be created intrinsicallyby natural dissociation of the eka-conjugated polymer,or may be added by "doping", as by redox reactions suchas adding metals like Li, Na, K, etc.; or by use ofstrong electron acceptors such as I2 or Br2. The useof such redox agents is contraindicated for chemicalstability purposes, however. It is well known thatcharge transfer complexes are notoriously unstable.Those made by adding iodine etc. to polyacetylene orother non eka-conjugated hydrocarbons, for example,have half lives rarely exceeding hours or days. Second,there must be present the long MO domains available forelectron orbital delocalization, such that freed car-riers can move over long distances. It is importantthat the freed charges on each domain be "locally ex-cess" ones, i.e. not arising from the internal dis-sociation of the domain itself. In other words, oneprefers to have fully dissociated excitons, rather thanlocalized Frenkel-type excitons. With this in mind, letus compare normal electronic with normal nomadic polar-ization in a hypothetical eka-conjugated polymer.

When electronic poiarization is evoked in an organicsolid, the average displacement of the electrons rela-tive to the nucleus of the atoms is about 10-9 nm as afield of 1 V/cm is applied. On the other hand, thedomain available in long-range delocalized MO's inproperly eka-conjugated polymer molecules can be in theorder of 100 to 1000 nm.

The atomic or molecular dipole moment so generated bythe field is then

This giant electric response of matter is a revers-ible and usually hysteresis free, (i.e. not piezo-elec-tric or ferro'electric) electric polarization. It oc-curs in polymers containing giant electronic MO's andis due to the opportunity for long-range travel offreed charges within the MO. For this reason, it isgiven the name nomadic polarization. It can be under-stood as due to the presence of highly delocalizedelectron- or hole-type carriers as they move about,usually as small polarons, and possible occasionally asspinless bipolarons in favorable cases, over the longmolecular domains provided by contiguously associatedr orbitals of properly conjugated polymers. This isshown diagramatically in Fig. 1.

Let us make a simple calculation to indicate the po-tential strength of nomadic polarization. Recall thatit has several prerequisites. First, there must bepresent a number of freed carriers, e.g. electron- or

p = ed, (1)

where e is the electronic charge, and d is the averagedisplacement. The total polarization per unit volume Pis the number of such dipoles per unit volume Nj, timesthe size of the average moment.

Pi = Njedj (2)

We are now in position to compare the relative contri-butions to the polarizability of the electronic andnomadic modes. The concentration of electrons in organ-ic solids is about 1024 cm3, and we may assume a valueof nomadic charges of only say, 1017 cm3. Then for afield of 100 V/cm, the ratio of the polarizabilitiesPnomadic to Pe is 10000.

In other words, nomadic dipoles, despite beingfew in number, can, because of their great extent,

(771\ /

.......I..............

I

contribute many times the polarization available fromnormal electronic polarizability. It is just thisfeature we wish to apply in practice.

It is important to emphasize that, like the familiarelectronic, atomic, and dipole orientational polariza-tions, nomadic polarization is a phenomenon of mole-cular scale. In contrast, interfacial polarizationsin composite mixtures of materials are gross macro-scopic phenomena. These interfacial polarizations canalso be of numerous types, such as the bulk-bulk (Max-well-Wagner) polarization [135]. It occurs becausethe dielectric properties of the components of a mix-ture evoke charge accumulations (and therefore blockedcharge flow) at the interfaces. In addition, there areinterfacial polarization types involving ionic doublelayers at solid-liquid interfaces, at solid-solid in-terfaces, or in gel-like boundary layers (plasmoidalpolarization ) [134]. In the present discussion weshall focus on nomadic polarization because of itsevident applications, yet remain mindful of the numer-ous alternative ways in which matter can exhibit elec-trical polarization.

The theory of this giant form of polarization, nomad-ic polarization, was derived by M. Pollak in 1975 [79,80]. This theory has since been quite thoroughlychecked experimentally and found to describe matterswell [1].

ELECTROACTIVE STRUCTURES:EKA-CONJUGATION VS. CHARGE-TRANSFER

COMPLEXATION

We now turn to the chemical problem of the structuresrequired for obtaining super-dielectrics via eka-con-jugation. Organic giant orbital materials, those withlong-range electronic orbital delocalizatior turn outto have remarkable and unexpected electronic properties,as we have noted. There are, quantum chemicaliy speak-ing, two chemical types of organic solids having en-hanced electronic properties: Those with giant XT MO'svs. those with charge transfer complexes. The latteris based upon charge-transfer reactions; the former,and generally the superior one, is based upon polymerswith giant, long-ranging ("eka-conjugated") electronicmolecular X orbitals. Their superiority derives mainlyfrom their stability. Charge-transfer complexes arenotoriously unstable and are therefore generally unfitcandidates for practical use. A charge-transfer com-plex is in effect, a "chemical reaction waiting tohappen". On the other hand, the highly conjugated poly-mers with giant X MO's are generally very stable.Graphitization is a familiar and related concept. rrefers to orbital symmetry, to symmetric twofold reflex-ion across the plane of the molecular portion.

Many highly conjugated polymers having giant MO's canbe taken up to 5000C [51,52] and back, with littlechange, as compared to the room temperature instabilityof many charge-transfer complexes. Of particular in-terest here are the extraordinarily high stability, theelectronic conductivities, and the giant dielectricpermittivities, as well as to the high use temperatures,and the extreme pressure sensitivities attainable in thepure organic polymeric solids with giant MO's [1-3].Even remanent magnetism [4,85-91,96-110,115-120,121-124]should eventually be achieved with giant MO structures,though that will not be our emphasis here.

The search for conducting polymers has been widespread.Especially thorough has been the study of those polymersdoped by donors or acceptors of redox character. The

model systems formed by monomeric charge-transfer (CT)complexes have served as suggestive, although frus-tratingly unstable, guides. The monomeric CT systemsinvolving tetracyanoquinodimethane and analogues ofe.g. tetrathiofulvalene have been intensely studied.

The instability of organic CT complexes of high ox-idation-reduction susceptibility, whether of monomericor polymeric makeup, was recognized early. It hasbeen the aim of the present research to prepare highlystable electroactive polymers of such chemical archi-tectures that they can either conduct well inherently,i.e. without extensive and probably deleterious redoxdoping; or will have high inherent dielectric permit-tivities. Our model for this lay in the fact thatnature already provides examples of highly conductiveorganic materials in the form of charred carbonaceoussubstances. Early studies of such pyrolyzed polymershaving structural resemblances to graphite were doneby Winslow et al. [125], Mrozowski, [126,127] Oster etal. [128], and by Pohl et al. [8-10,53,57]. More re-cent studies, principally on carbon fibers and filmshave been carried out by Fitzer et al. [131], Helbergand Wartenberg [132], by Kaplan et al. [129], and byVogel [130]. Historically speaking, there was atfirst a strong reaction against including such amor-phous materials in solid state studies because of themistaken belief that only "single crystal" materialwas capable of high conductivity and related electro-activity. This opposition was later dispelled as theadvantages of amorphous, yet highly electro-activesolids were found. An early piece of evidence shakingthe "single crystal" advocates was the observation thatmolten (and hence amorphous) Li conducted much betterthan crystalline Li.

The giant molecular orbital materials are based uponselected types of stable molecular architecture ofhighly aromatized polymers. During two decades ofstudy of these features, we have learned that thereare at least three important aspects of molecularstructure which lead to electroactivity in undopedpolymers, i.e. those which have inherent ability toexhibit electro-activity such as Onhanced electronicconduction or dielectric permittivity. In this processthere are reduced energy differences between ionizedand unionized states, such that appreciable electro-activity is apparent at ambient temperatures. Thisdoes not preclude the later introduction of redox or"doping" to still further raise the electro-activity.These three aspects are described below.

ORBITAL DELOCALIZATION

First, there needs to be an extended long-range elec-tron orbital delocalization, usually of Tr-orbital type.This is responsible for the observed low activationenergies of carrier formation and transport.

CONFORMALI TY

Second, and somewhat surprising, but also greatlyeasing the job of synthesis, is that in such very largemolecular structures it is sufficient to have inter-molecular conformality or similitude rather than strictmolecule-to-molecule replication of orbital structuresto achieve enhanced electro-active character. Thisgreatly simplifies the synthetic tasks involved. Itprobably reflects the fact that at finite non-zero tem-peratures, the thermal librations, etc., continuallyalter the local structures and energies by the interplayof vibronic and other deformations. In molecular orbit-al terms for the past twenty years among quantum

nn.h Super-die I ectrr i c yIv e-

chemists, the highest occupied MO has been termed theHOMO; the lowest empty (or unoccupied and anti-bonding)orbital is referred to as the LEMO or occasionally asthe LUMO. In ensembles of sufficiently large eka-con-jugated MO structures where the HOMO-LEMO orbital dif-ferences are now small and commensurate with thermalenergies, it is probable that this occurs frequentlyenough so that, despite the strictly non-identicalcharacter of the statis ground-stage structures, iso-energetic states, even those of fleeting character, asrequired for active tunnelling, are occuring throughoutthe ensemble. This makes for thermally-assisted tun-nelling and other actions easing the creation and trans-port of carriers. Stated another way, and despite theearly expectations of the 1960's when it was claimed onthe basis of Si and Ge experience that only crystals,and single crystals at that, would do, researchersworking with the amorphous solids found that it is notabsolutely necessary to have crystal-like translationalsymmetry in order to have high degrees of electro-acti-vity in giant MO structures. Considerable amorphouscharacter can be present, yet permit high electro-acti-vity, as is becoming widely appreciated. Even super-conductivity has recently been observed in anorphousmetal alloys.

TRANSPORT

A third aspect found necessary for high electro-acti-vity in polymeric sQlids is that carriers once formedwould have an easy means of transport, such as by wave-packet drifting, by tunnelling, or. by a polaronic orbipolaronic hopping with low activation energy require-ments. To sum up, it is necessary to have (1) long-range MO delocalization, (2) molecular structure con-formality, and (3) easy transport.

In physico-chemical terms, the extent of either longrange or of restricted short range electron orbital de-localization is described in terms of the types of con-jugation present in macromolecular solids, in terms of"eka-conjugation" or "rubi-conjugation" which were pro-posed some years back and have become more and morewidely used [30,41]. They are a natural extension ofthe familiar ideas about conjugation in small molecules.An eka-conjugated structure is one in which, as thedegree of polymerization increases, the energy differ-ence (LEMO-HOMO) between the ground state and the firstexcited electronic ionized state approaches zero. Inthe rubi-conjugated structures, the difference in suchenergy states remains nonzero as the molecular size isincreased.

THE EKA-CONJUGATION INDEX

To characterize the degree of electronic orbital de-localization, one defines an eka-conjugation index EKas the ratio of the absorption frequencies,

EK = {fm - fp}/fm (3)

where fm and fp are the frequencies of absorptions cor-responding to the difference in energies of the lowestantibonding and highest bonding orbitals in the case ofmonomer and polymer. The eka-conjugation index can bethought of in terms of the electronic MO's describingthe lowest empty molecular orbital (LEMO) and the high-est occupied MO, (HOMO). I.e.

ER=

{E(LEMOJ-E(HOMO)}m -{E(LEM4O)-E(HOMO)J} (4)

E(LEMO)m -E(HOMO)p

where the indices m and p are for monomer and polymer,respectively.

The index EK approaches unity as the orbital delocal-ization becomes more and more extensive. Spectroscopicmeans can be employed to assay it.

In electronically (as opposed to ionically) conduc-tive substances, the carriers, electrons or holes, needpathways to travel as they move. In the case of syn-thetic macromolecular solids, the pathway can be thatprovided by the orbital delocalization among overlap-ping iT orbitals. Yet there must be a certain stabi-lization of those. For instance, polyacetylene, view-ed in a simplistic manner, is made up of chains with acarbon spine having alternating single and double bondswhich might permit such orbital delocalization to occur.That it does not in fact develop eka-conjugation orlong-range delocalization is probably due to orbitaldevelopment into Jahn-Teller-type state splitting(Peierl's instability), accompanied by and complicatedby the effects of local structural chain-foldingdefects. Instead, in order to obtain enhanced electro-active character, the materials must have conjugatedspines, accompanied by stabilized orbital delocaliza-tion. This can occur in polymer structures withorbital-stabilizing side groups along the chain of con-jugation, or by having ladder or sheet geometries,etc., so as to obtain the desired eka-conjugation.

As explicit examples of an eka-conjugated polymerseries, we have the "ladder polymer" series; naph-thalene, perylene, triterylene, quaterylene, etc.,which would be expected to, and do, develop an eka-conjugated series. As an example of the planar sheet-forming polymer series we have benzene, coronene, etc.,on to graphite. Here the EK index is known to reachunity. Other series among the ladder or sheet-typeshave been made, such as the poly(phthalocyanines) ofeither type [46,53]. The melanin polymers are verylikely examples of the sheet-type. The a, a' dehydro-genated poly(anthrone) polymer series is an example ofthe ladder type.

In rubi-conjugated polymers, by way of contrast,there are structures, defects, or kinks (solitons) inthe macromolecular structure causing breaks in the con-jugation of the system. All of these can interferewith the development of delocalized MO's. The inter-fering breaks in the conjugation can also be caused bycrosslinking, or simply by the presence of two con-secutive carbon-carbon (or other) single bonds in thechain. As simple, yet instructive examples of a rubi-conjugated structure we have poly(isoprene) (guttapercha), and poly(styrene). The former contains resi-dent double bonds, the latter contains conjugated ben-zene rings, but these are prevented from developinglong-range delocalization by the presence of sets ofsingle bonds between them. They therefore remain asinsulators and do not develop appreciable electro-activity.

We sum up the chemical aspects: In eka-conjugatedpolymers, as the molecular size increases, so do theelectroactivity, the free carrier concentration, thecarrier domain size, the conductivity, the dielectricpermittivity, and usually the free radical content;and with it increase the catalytic activity, the chemi-cal inertness, and the thermal stability. In a sensethey can become intrinsically electroactive, even in-trinsic semiconductors, for example. On the other hand,as the molecular size increases in the rubi-conjugated

EI-21 No.5. Oc'_ol- er t9l- 6

polymers, their electroactivity remains low and theirchemical behavior remain rather like that of the smalloligomers, because the giant orbital properties failto develop. These conclusions are by now supported on

a wide and firm base of experimental evidence whichhas been developed over several decades of researchhere and abroad by many workers [1,2,13,30,41,61,92-94].

The broad view is that in order to have stable poly-mers with enhanced electroactivity, one must have con-

tinuous intra-molecular overlap of X orbitals, one

that extends over long domains, i.e., in the word usedby the polymer chemists, eka-cQnjugation. To sum up,

eka-conjugation can be obtained by creating long con-

jugated, highly aromatized macromolecular chains withr-orbital stabilizing side groups, or ladder or sheetstructures containing little or no rubi-conjugation intheir molecular structure. Synthetic polymers design-ed upon these principles will provide the inherent car-

riers, and also the required long skeletal pathwaysupon which the electronic carriers can travel widely,nomadically, possibly as polarons, or bipolarons via a

network of overlapping MO's.

The quantitative theory for the behavior of eka-con-jugated polymers and nomadically polarizable solids hasbeen well developed, principally by M. Pollak [79,80].Experimental studies designed to test the numerous pre-

dictions of the theory have been carried out [1,76,78].The theory turns out to describe matters well so thatone feels confidence in the approach.

The concept of giant orbital materials opens up new

vistas of materials properties, and allows the prepara-

tion and study of several selectively designed eka-con-jugated, highly aromatized polymers.

EXPER I MENTAL

As described above, researchers have designed, madeand studied hundreds of new candidate electroactivepolymers based upon these giant MO concepts. To date,the emphasis was mainly upon obtaining those of highconductivity and/or dielectric permittivity. Guidingthe syntheses were the theoretical developments de-scribed earlier. Conductivity and dielectric propertieshad been measured as functions of frequency, fieldstrength, temperature, and pressure [1,76,77] to testthe theory and to provide feedback to guide the ongoingsynthesis research. Molecular lengths had been deter-mined by several new electronic [1,2] methods. Corre-lations were sought between the electronic propertiesand molecular properties, such as orbital delocaliza-tion, molecular length and structure.

In several studies, highly purified, and highly aro-

matized, eka-conjugated polymers of the polyacenequinone radical (PAQR) type were made by the step-growthcondensation polymerization of aromatic hydrocarbonderivatives with aromatic acid moieties. The details ofthe polymer syntheses and purifications were describedearlier; that for the anthraquinone and pyromelliticdianhydride (PMA) polymer by Vijayakumar and Pohl [1];that for pyrene and ortho iodobenzoic acid polymer byKho and Pohl [66]; that for the 2-chloroanthraquinoneand tetrachlorophthalic anhydride polymer by Dunn etal. [2].

Electrical measurements of specific conductivites,permittivities, and loss factors were made on void-free,pure polymer samples that were first compacted at ap-proximately 1 GPa, then examined at lower pressures with

the Bridgeman anvil technique using the methods de-scribed earlier [1]. The room temperature valuesfound are shown in Table 1.

Table 1.

The Electrical Properties of Several TypicalEka-conjugated PAQR Polymers

u

Polymer Pressure K' Conductivity tan6109 Pa (Qm)'I (estim.)

AnPMA[l] 2.0 1500 1.2X10-6 1.40.5 200 8xl0-7 7

ClAn/C14Pa[2] 2.0 240000 l.5xlO 4 1

Vio/Cl4Pa[3] 2.0 280 3xlO-6 20

Bianthrone/PMA 2.0 500 4X10-4 1400

Vio/PMA[4] 2.0 210 1.2xl0-10 Z0.001

Vio/PMA[5] 2.0 260 1.OX1O-7 0.6

Pyrene/o-iodoBA 0 4000 1X10-6 0.5

An = anthraquinone: PMA = pyromellitic dianhydride:ClAn = 2-chloroanthraquinone: C14Pa = tetrachloro-phthalic anhydride: Vio = violanthrone: o-iodoBA =o-iodobenzoic acid. The measurements are done at 23 C,and at 1 kHz with a p-p voltage of 0.1 V across thesample. Samples 1-4,6 were synthesized in the melt.No. 5 was polymerized in solution. It had been ob-served that the dc and ac conductivities in suchpolymers were roughly equal (to within a factor of twoat 1000 Hz) in prior measurements, hence here as arough guide, the tan6 was estimated from dc conduct-ivities and relative permittivity, using the relation

tan6 = 1.8x109 a at 1 kHz.K

We observe from the above results that these polymersare indeed electroactive. Much wider ranges can be ob-tained in the conductivities, for example [6,7,30,41,43,44,46,60,84] or permittivities [1,76,80,84]. Thepolarizabilities and permittivities of these particulareka-conjugated polymers are wide-ranging and can bealtered at will by controlling the molecular architec-ture. The enhanced electroactive properties of giantorbital polymers suggest applications in power capaci-tors, in transducers, and in parametric circuits andin obscuration, etc. The particular examples shownabove indicate that further research in this area ismerited in the search for high-quality dielectrics.

THE FUTURE OF SUPER-DIELECTRICS

Now that nomadic polarization phenomena have beenavailable for use, and this giant polarization has come

under control, we must ask as to its future. The basicphenomenon of nomadic polarization is now clear. It isup to us to use it wisely. Before it can be applied on

a significant scale in the very large market of powercontrol capacitors, for example, it will be necessaryto develop means to form and apply thin films of highmechanical and electrical quality. This has yet to bedone. It will call upon the best of our film-formingexpertise. Moreover, the present examples of super-dielectric polymers are sure to be accompanied by manyothers of wholly different chemical type, now that the

Polhl:S-S er-deletr p" 1 vner

general principles of chemical architecture requiredfor making the eka-conjugated and nomadically polar-izable structures are clear. To the present panoply ofthe hundreds of poly(acenequinoneradical) polymers [41,50-63,65-78] synthesizable from the step-condensationof a hydrocarbon derivative with an aromatic acid thereeventually will be added many other types. Among thesewill be the poly(phthalocyanine), the poly(anthrone),poly(perylene), poly(benzimidazole), poly(imidazole),and the melanine types, and many others. The presentgoals of obtaining materials with high permittivitywith low loss will be paralleled by those for obtainingtheir counterparts, i.e. materials with low permittivityand high loss. New, and highly conductive polymers willbe found, based upon the giant MO concept, surpassingthe present limit of only half the conductivity of cop-per. Another direction of development in the super-dielectrics will be the application of their enormouspressure sensitivity....about 10000 x that of presentmetallic strain gages. From these we will see newpressure sensors based upon piezo-resistive and otherchanges based upon the piezo-capacitive changes shownby these new giant MO materials. New parametric de-vices by the score will turn up, based upon the unusualproperties of these evocative substances and theirwidely ranging properties. There is much to do, andmuch to gain.

AC K4OWLEDGEME1NT

This research has been supported by the Pohl CancerResearch Laboratory, Inc. The author particularlythanks Drs. R. Hartman, J. R. Wyhof, P. S. Vijayakumar,James Mason, Lee Dunn, J. Kent Pollock and W. T. Ford,and also J. H. T. Koh, Mario Guterriez and Nabil Hilalfor their helpful collaboration in providing advice andassistance in the synthesis and measurement of samples.

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Manuscript was received 25 August 1986.

Editorial Note.

Due to his untimely death, Professor PohZ could notput the finishing touches on this paper.

AvR