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JAN 77 S H CARR N00014-75-C-0963 




Contract No. N00014-75-C-0963 

Project No. NR 051-599 



S. H. Carr 

Depackinent of Materials Science and Engineering 
Northwestern University 
Evanston, Illinois 60201 

D D C 

January 20, 1977 


FEB « tgn 

Reproduction In whole or In part Is permitted for 
any purpose of the United States Government 

IT Lilt 


Approved for Public Release; Distribution Unlimited 



Technical Report No. 4 

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I S. H.yCarr < 'j 


Department of Materials Science and Engineering 
Northwestern University 
Evanston, Illinois 60201 


ONR Branch Office 
536 S. Clark Street 
Chicago. Illinois 60605 

14. MONITORING AGENCY NAME A ADORESSCIf dIUtrtnl frooi Conirolling Olllc*) 

N)3rp014-75-C-0963 j 

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Project No. NR 051-599 

12. report date //) 

— Janiiaiy 1977 ~ 

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This paper will also appear in the proceedings of this conference. 

It. KEY WORDS (Continuo on rororoo oldo II noooooonr ond Idonllty by block ntmbor) 

Review; Electrical polarization; Thermally stimulated discharge; 
Electrets; Polyacrylonitrile. 

„,^^RACT IConrinuo on fororoo oldo II noeoooorr tdonllly by bimtk mmbot) 

This paper is a preliminary, abridged review of current ideas regarding 
mobile ionic species in polymer electrets . The chief concept is that much 
of the persistent electrical polarization, developed in polymers prepared 
for piezo— or pyroelectric applications. Is due to displaced ions. It is 
recognized that l)preferentlal orientation of dipolar parts of the macro- 
molecules (e.g., side groups) and 2) fluctuations in supermolecular order 
(including crystalline regions) lead to inhomogenieties in internal elec- 
trical fields which affect where such mobile ions become stabilized 


An exciting new category of polymer solids, polymer electrets, 
has been the subject of Intense research and development ac~ 
tlvltles during the past eight , or so , years Cl~3) . The pri- 
mary properties possessed by these materials are piezoelec- 
tricity and pyroelectricity, and the kinds of devices that can 
exploit these properties Include microphones and heat detectors. 
Polymer electrets also can serve as radiation dosimeters or 
Imaging devices C^). In addition, devices which can exploit the 
ferroelectric behavior of certain polymer electrets can be de- 
signed to store Information which Is In the form of electrical 
signals. Some of the polymers used In these applications are 
commonly available solids, such as polyvinylchloride, poly- 
methylmethacrylate, and especially polyvlnylldene fluoride 
(PVF 2 ) . Other polymer electrets contain fillers which them- 
selves may be piezoelectric materials. However, In all cases, 

It is common for the pol 3 rmer to have been subjected to a thermo- 
electric treatment which Imparts a persistent electrical polar- 
ization to the solid. Some of these materials may actually 
exhibit a spontaneous polarization behavior, but It Is usually 
regarded as necessary for the sake of reproducibility that the 
polarization process mtist be applied to each of these kinds of 
solids In order to get the appropriate performance desired. 

This persistent electrical polarization arises from a combi- 
nation of Individual effects (5-7) . One such contribution to 
this polarization Is preferential orientation of groups of atoms 
having permanent dipole moments. This polarization requires, 
of course, that the macromolecules In question actually possess 
dipolar moieties, but even In the case of polyethylene there Is 
the possibility that the very dilute concentration of carbonyl 
groups can contribute some degree of persistent electrical 
polarization. The other kind of polarization arises from the 
asymmetric displacement of Ionic species. This can be achieved 
either by permitting Ions already present In the solids to be 
rearranged during the electrical polarization process, or they 
can be Injected from the surroundings during polarization 
(8-14) . Both origins of a persistent electrical polarization 
give rise to what Is called a heterocharge, because the sur- 
faces of the dielectric have a polarity In opposition to that 
of the polarizing electrode adjacent to that surface. 

Persistent electrical polarization due to preferential orien- 
tation of dipoles Is known to give predictable levels of plezo- 
and pyroelectricity (2). Although this work by Broadhurst, et 
al. (1,2) has rather successfully characterized the case for 
noncrystalllne polymers, the situation with the semicrystalline 
polymers has been a bit more complicated (15) . For example. In 
the case of PVF. (see ref. 2), It Is absolutely necessary to 
utilize copolymers In which the phase II . crystal polymorph Is 


favored. In both, nylons C15) and PVF^ Che crystal polymorph 
necessarily must have a permanent dipole moment to each unfr 
cell In order for there to be appreciable piezoelectricity dent- 
onstrated by their respective solid. Stretching PVF^ (16,17) 
has been shown to be an effective way to vary piezoelectric 
activity. Likewise, stretching polyacrylonitrile (PAN) leads 
to improvements In electrical polarization by factors ranging 
from 3- to 10 (18) . 

Homocharges, l.e. surface charge whose sign Is the same as that 
of the polarizing electrode next to It, can be Induced In a 
number of ways, one Important one of which Is by exposure to a 
corona discharge (19-25) . Analysis of electrets made by the 
corona discharge technique has revealed several Interesting 
features of such solids. The first Is that the homocharges are 
trapped In the solids at different energy levels. The second 
Is that the heterocharge, to whatever extent It exists. Inter- 
acts with the homocharge, with the possibility of cancelling 
the overall polarization. Often when the physical micros truc- 
ture of the solid, e.g. polyethylene terephthalate. Is taken 
Into account, one notices that heterocharge and homocharge can 
actually stabilize each other at the surface layers. Another 
way to create homocharges Is by direct Ion Injection from the 
surfaces (25). In this work by Osaki and Ishlda (25), It Is 
seen that It Is possible to put Na, Ca, Al, and Cl Into PVF. 
films. The resulting nonuniform charge distribution can be^ 
analyzed to get Information on the diffusion of such Ionic Im- 
purities under the Influence of electrical fields. Of course, 
polyelectrolytes also can act as strong electrets (26-29). 

Here, variation of the composition of the solid (as for example 
the formation of polymer blends) or the degree of saponifica- 
tion of polyelectrolytes will produce very different kinds of 
electret properties. It Is likely that there Is further ex- 
ploitation of this line of polymers that needs to be pursued. 
Finally, the direct Injection of electrons from the surround- 
ings has also been Investigated for Its ability to make polymer 
electrets (29-32). Perhaps the most effective way to achieve 
a persistent electrical polarization for most of the technical 
applications exploited so far Is the corona discharge In com- 
bination with direct electron Injection. Certainly the extent 
to which any chemical reaction can occur at surfaces being ex- 
posed to Incoming ionizing particles will determine the effect 
that results and the piezoelectric or pyroelectric activity 
that is achieved. 


As inferred from the thermally stimulated discharge current 
analysis of PAN electrets (18,33-36), this polymer can achieve 
very high levels of persistent electrical polarization. How- 
ever, unpublished results from experiments measuring the piezo- 

3 . 



and pyroelectric properties of polarized PAN have revealed only 
a nodest level of performance. This puzzle Is just now being 
unraveled, and It appears that much of the charge stored In PAN 
Is In the form of a monopole C2) electret. There Is clear evi- 
dence from birefringence measurements. Infrared dlchroslm 
studies, and x-ray diffraction analysis that a preferred orien- 
tation of the nitrile side groups does exist In these films. 

In fact, even films which have never been subjected to the 
thermoelectric polarizing treatment exhibit substantial levels 
of persistent electrical polarization themselves. This kind 
of spontaneous polarization behavior Is unusual but has been 
attributed to what takes place during the casting of films of 
dimethyl formamide (DMF) solutions. 

Figure 1 gives a thermally stimulated discharge (TSD) spectrum 
(5) revealing three temperature ranges over which there are 
discharge current maxima. The one above 90°C, labeled y, has 
been attributed (18,34-36) to the randomization of nitrile side 
groups which have adopted a preferred orientation. The peak 
labeled 8, which occurs in the temperature range of 140 to 
160®C, appears (Fig. 2) to be due to the onset of mobility for 
sorbed Ionic species. The peak at the temperature range 180 to 
205°C, labeled the a peak, has recently been attributed to the 
earliest stages of thermodegradaclon of PAN (36) . The final 
current seen in the temperature range above 215°C appears to 
represent wholesale degradation of this polymer. The impor- 
tance of this study Is that it provides some understanding of 
the various contributions to polarization in PAN, and it is 
clear that a variety of phenomena are operating at the same 
time. One notes from Fig. 1 that the y peak Is of polarity 
opposite to that of the a and 3 peaks; this is seen only In the 
case of unpolarlzed PAN films. What Is thought to be occurring 
here Is that during the process by which solvent evaporates 
through the top surface of a PAN film, the ionic impurities be- 
come redistributed and create such an internal electrical field 
that the nitrile side groups adopt a preferential orientation 
in opposition to this ion-induced elect ’’leal asymmetry. 

When an electrical field is applied to the PAN films at eleva- 
ted temperatures, several things happen at once. Firstly, the 
polymer undergoes a molecular reorganization which leads to the 
creation of Inherent crystalline domains (18). Secondly, the 
nitrile side groups undergo a reorientation In response to the 
externally applied field and therefore have a polarization ^ 

which Is the same sign as that of the ions which depolarize in ; 

the temperature range of the 8 peak. In some cases, the y peak 
Is actually reduced In intensity on polarized films from what 
is observed with impolarlzed films. This is thought to be due 
to the masking of the external field by the Internally sorbed 
Impurity Ions. The a peak also responds to the Imposition of 
an external electric field, but the reason for this is not 

4 . 

quite so clear. It Is currently advanced C36) that DMF mole- 
cules residual In these films also undergo some diffusion under 
the Influence of an externally applied field, and therefore 
become asymmetrically distributed, jxist as the other sorbed 
Ions do. It has been observed from this study that DHF mole- 
cules actually accelerate the thermal degradation of PAN, and 
so one would expect the side of a film to i^ch the had 
diffused as being the side on which the Initial stages of 
thermal degradation (and attending production of Ions) occurs. 
Figure 2 supports some of these assertions. It can be seen 
that when nitrate Ions are doped (38) In PAN films, the en- 
hancement of concentration (as seen by ATR Infrared absorption 
spectroscopy) reveals the onset of mobility at the upper range 
of the 8 peak.. Likewise, enhanced residual DMF concentration 
Is seen to decay over the same temperature as the 8 peak. 
Diffusion of such small molectiiar or Ionic species is control- 
led largely by the onset of segmental mobility In such dis- 
ordered polymers, and so one infers that the kind of chain 
mobility required for these redistributions Is available only 
in these higher temperature ranges. One would similarly infer 
that the y peak corresponds to the onset of motions that relate 
only to the environment of nitrile groups themselves and not 
of the chains to vhich they are attached. 

In summary for polarized PAN, one can see In Figure 3 a 
schematic representation of a film seen In cross section. The 
eliptlcal domains are intended to be ordered regions in which 
a permanent dipole is created by virtue of an asymmetric place- 
ment of the nitrile groups (18) , and surrounding these domains 
are Ionic species which give rise to a kind of Maxwell-tfagner 
(39,40) polarization. Depending upon the relative concentra- 
tions of negative and positive ions, there may be a complete 
masking of polarizations contributed by these domains; in ad- 
dition, if negative Ions are more abundant on one surface than 
the other (or conversely with positive ions) , then there can be 
a persistent polarization that is present in these films even 
though they are unpolarlzed. When such films are stretched 
uniaxlally. It Is assumed that the ordered regions will adopt 
some sort of order coherence with regard to their neighboring 
domains. It would be Imagined furthermore that counterions 
would reorganize such that they preserve the Maxwell-Wagner 
polarizations that they created earlier. Such stretched, but 
as yet unpolarized, films still would lack any polarization 
other than that which occurred during the initial formation of 
the films. However, If these films are subsequently subjected 
to a polarizing thermoelectric treatment, then it is imagined 
that nitrile groups within these ordered regions will undergo 
a slight rearrangement permitting the resulting dipole moments 
of these domains to align In a single direction. Diffusion of 
the counterions can begin to occur at the same time, with the 

bl.. • 

result thst sone of the shielding of the polarization arising 
from these domains Is now lost, and at the same time buildup 
of some charged species can begin to occur In the region of the 
surfaces against electrodes of the opposite polarity. Diffus- 
ion of the species Is expected to be Impeded by the presence 
of these ordered domains, as they should have higher masked 
density than the surrounding matrix. One may speculate, in 
view of all of this, that the electrical polarization possible 
in such polymers as polyacrylonitrile might be enhanced further 
by the deliberate Incorporation of Ionic species such as those 
found in polyacrylonitrile. 


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and Pyroelectric Symposium-Workshop , U.S. National Bureau 
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2. M.G. Broadhurst and G.T. Davis, Piezo- and Pyroelectric 

Properties of Electrets . U.S. National Bureau of Standards 
publication, NBSIR 75->87, 1975. , 

3. N. Murayama, T. Olkawa, T. Katto, and K. Nakamura, J. 
Polymer Scl., Polym. Phys. Ed., 1033 (1975). 

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Research support for this work has been generously provided by 
the Office of Naval Research. 


Fig. 1. Thermally stimulated discharge (TSD) current spectrum 
for PAN that had never been subjected to a thermoelectric polar- 
izing treatment. Data from ref. 35. 

Fig. 2. TSD from polarized PAN, along with data for correspond- 
ing surface concentrations of residual DMF or doped nitrate. 

Fig. 3. Schematic representation of Internal electrostatic ! 

non-unlformltles In PAN. 



DCt) w a 


stretched -polarized 


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