DTIC ADA035789: Role of Mobile Ionic Species in Polymer Electrets.

northwestern UNIV EVANSTON ILL DEPT OF MATERIALS SCI— ETC F/6 7/3
ROLE OF MOBILE IONIC SPECIlS IN POLYMER ELECTRETS. (U)

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

AOA035789

f

OFFICE OF NAVAL RESEARCH

Contract No. N00014-75-C-0963

Project No. NR 051-599

TECHNICAL REPORT NO. 4

"ROLE OF MOBILE IONIC SPECIES IN POLYMER ELECTRETS"

S. H. Carr

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

D D C

January 20, 1977

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j^OLE OF iJOBILE JONIC JSPECIES IN

POLYMER ELECTRETS^

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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

POLYMER ELECTRETS

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

i

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.

PEBSISTEMT ELECTRICAL POLARIZATION IN PAN

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 .

i

1

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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Properties of Electrets . U.S. National Bureau of Standards
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ACKNOWLEDGEMENT

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

FIGURE CAPTIONS

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.

INHOMOGENIFTIF.S IM PAN

annealed

DCt) w a

stretched

stretched -polarized

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