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Thermostimulated switching in thin polymer films
Synthetic Metals, 59 (1993) 377-386
377
Thermostimulated switching in thin polymer films A. N. L a c h i n o v , A. Yu. Z h e r e b o v a n d M. G. Z o l o t u k h i n Physics Department, Bashkirian Research Centre, Russian Academy of Sciences, Ufa 450025 (Russian Federation) (Received December 3, 1992; in revised form January 25, 1993; accepted February 1, 1993)
Abstract The paper presents results of a study on a new kind of instability observed in thin poly(3,3'-phthalidylidene-4,4'-biphenylylene)films, namely, thermostimulated switching. This occurs at 265 K as a result of heating at a certain rate. After switching the sample transits to a metallic state. Investigations of the polymer dielectric loss and IR spectrum under similar heating regimes show the existence of relaxation excitations at this temperature. Another range of excitations is detected at 190 K. According to IR spectra, in the low temperature range, excitation of nonvalence aromatic distortion oscillations of the C--H bonds take place, whereas C=C double bonds are excited in the high temperature range. Dependence of the intensity of excitations on film thickness and heating regime allows us to suggest that they are associated with the thermostimulated ionization of trapping states. The model of polaron lattice formation is discussed as a possible explanation of the observed dielectric-metal transition.
Introduction T h e e x i s t e n c e of e l e c t r o n i c i n s t a b i l i t i e s in p o l y ( 3 , 3 ' - p h t h a l i d y l i d e n e - 4 , 4 ' b i p h e n y l y l e n e ) * ( P P B ) t h i n films in a n e l e c t r i c field a n d u n d e r u n i a x i a l p r e s s u r e w a s r e p o r t e d r e c e n t l y [ 2 - 4 ]. B o t h p h e n o m e n a r e s u l t in t h e t r a n s i t i o n o f t h e i n i t i a l l y d i e l e c t r i c s a m p l e s i n t o a m e t a l l i c s t a t e a n d in t h e a p p e a r a n c e o f h i g h l y c o n d u c t i n g c h a n n e l s . In t h i s s t a t e , i r r e s p e c t i v e o f t h e w a y it w a s induced, the e l e c t r o p h y s i c a l p o r p e r t i e s of the s a m p l e s are similar, namely, the negative t e m p e r a t u r e coefficient of conductivity, the linear c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s a n d t h e a n i s o t r o p y o f c o n d u c t i v i t y (r~/(rtl ~ 10 l°-tz, w h e r e a ± a n d cr~ a r e t h e c o n d u c t i v i t i e s a c r o s s a n d a l o n g t h e p o l y m e r film, r e s p e c t i v e l y , t h e c o n d u c t i v i t y o f a s i n g l e c h a n n e l b e i n g n o l e s s t h a n 105 ( o h m c m ) 'Switching-off" c a n b e p r o d u c e d b y s e v e r a l m e t h o d s , e.g., s u p p l y i n g s h o r t electric or mechanical pulses, passing an electric current which exceeds the critical value and movement of the upper electrode. T h e h e t e r o g e n e o u s s t r u c t u r e o f t h e s a m p l e s in t h e m e t a l l i c s t a t e w a s o b s e r v e d e x p e r i m e n t a l l y b y t w o m e t h o d s : u s i n g n e m a t i c l i q u i d c r y s t a l s [5] *A more precise name of a polymer initially called polydiphenylenephthalide (see, e.g., refs. 1 and 2).
0379-6779/93/$6.00
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378 and by means of electron microscopy [6]. The experiments displayed the existence of conductive domains of 5 0 - 5 0 0 nm in diameter with a concentration of more than 105 c m - 2. These effects attract a great deal of attention due to the extremely low threshold values and a considerably high conductivity in the metallic 'on' state. The mechanism of these p h e n o m e n a is not clear so far. Therefore, Complex investigations are needed to reveal the material's features responsible for the effects observed in the polymer. One of the curious features which may throw light on the possible mechanism of electronic instabilities in PPB was reported earlier [7]. This is the anomalous temperature dependence of conductivity, optical absorption and luminescence of pristine and iodine-doped PPB films. It was shown that there is a critical temperature Tc ~ 265 K which divides the a(T) dependence into two regions. Below T¢ the conductivity is almost independent of temperature, whereas above T¢ this dependence is extremely strong. It is obvious that the transition results from some intra- and intermolecular electron processes that are determined by a specific molecular structure. A suggestion was made that the lacton cycle in the phthalide group of the polymer molecule is the origin of the observed thermal instabilities. The mechanism is that of phenolphthalein affected by pressure [8 ], radical cations [9 ] and temperature, namely, the polarization or break of one of the C - O - C bonds of the lacton cycle in the side fragment of the PPB molecule caused the appearance of a positive charge on the quaternary carbon atom in the backbone. The appearance of this charge increases the conjugation between neighbouring biphenyl fragments and, hence, enhances conductivity. In this paper we present a more detailed analysis of the temperatureinduced transitions in PPB and associated modifications of its electron structure.
Objects and methods of investigations Poly(phthalidylidenearylene)s present a novel class of aromatic, high molecular, high-temperature, film-forming polymers synthesized in the last few years by Zolotukhin e t al. [10--12]. Their synthesis is based on the polycondensation of 3-chlorophthalides or phthaloyl dichloride with aromatic hydrocarbons, as well as on the homopolycondensation of 3-aryl-3-chlorophthalides via the Freidel-Crafts reaction. The general structure of the poly(phthalidylidenearylene)s is as follows:
379 where R is an aromatic fragment, for example, biphenyl, terphenyl, fluorene, etc. Poly(3,3'-phthalidylidene-4,4'-biphenylylene) is one of the prospective polymers of this class:
o PPB has a molecular weight of (50-80) × 103 depending on the conditions of synthesis. The softening temperature is 440 °C and the temperature of decomposition (TGA, 1% weight loss, air) is 450 °C. The polymer is soluble in conventional organic solvents, such as chloroform, methylenedichloride and 1,1,2,2-tetrachloroethane. The good solubility of PPB makes it possible to purify them effectively and to use them to obtain films and study various properties. PPB was synthesized according to a well-known method [1 ]. Viscosity was measured at 25 °C in an Ubbelohde-type viscosimeter. The intrinsic viscosity [~?]TCE Was calculated by extrapolating ~?sP/c for C--)0, where C is concentration of polymer solution, giving [~]TCE= 0.70 dl g - 1 Methods of polarization optical microscopy [13] were used to analyse the films thicker than 5 / z m (with smaller thicknesses the difference in optical path is too small to interpret the results convincingly). It was established that such films are characterized by the anisotropy of the refractive index. Their conoscopic interference patterns are those of plates of uniaxial crystals cut out normal to the optical axis. The conoscopic pattern is the shape of the Maltese cross and does not decompose into hyperbolas as the microscope table is rotated (Fig. 1). The evaluation of the refraction factor anisotropy yielded the following results: An=no-ne=O.O09-O.03 where no and ne are refractive indexes of the ordinary and extraordinary rays, respectively. The difference in the value of optical anisotropy is due to the difference in the conditions of film preparation at the solidification stage during the evaporation of the solvent. These data indicate that molecules in the polymer film are oriented in a certain way. This is confirmed by the evaluation of the crystalline fraction by means of X-ray diffraction which yielded about 25-30%. The electron structure of the molecules is such that, in the polymer backbone, the conjugation of ~--electrons is limited within the monomeric arylene segment. The quarter carbon separating these segments hinders the interference of the ~r-electron wave functions and creates a potential barrier for electrons in this part of the chain. This fact is well illustrated by the absorption spectra of a polymer and a m o n o m e r (Fig. 2). It can be seen that there is practically no difference between them in this spectral area. No hypsochromic shift of the low energy electron absorption band occurs
380
Fig. 1. Conoscopic figure of PPB film. A package of five plates each 50 ~ m thick.
I
? O
z
O
O 5~
0
-2
E, eV Fig. 2. Absorption spectra o f polymer and m o n o m e r films.
in t h e m a c r o m o l e c u l e . T h i s is, n o d o u b t , a n i n d i c a t i o n o f t h e s t r o n g localization o f ~r-electrons o f t h e a r o m a t i c s y s t e m o f t h e p o l y m e r b a c k b o n e . T h e electrical m e a s u r e m e n t s w e r e p e r f o r m e d in a ' s a n d w i c h ' - t y p e m e t a l / p o l y m e r / m e t a l e l e c t r o d e c o n f i g u r a t i o n a n d t h e electric field w a s a p p l i e d a c r o s s p o l y m e r film. F o r t h e dielectric m e a s u r e m e n t s , a ' p r o t e c t i v e ' r i n g e l e c t r o d e w a s a d d e d . The. m e t a l l i c (Cu, Cr, A1 o r Au) e l e c t r o d e s w e r e d e p o s i t e d o n t o t h e p o l y m e r film s u r f a c e s in v a c u u m or w e r e p r e s s e d . T h e s a m p l e s w e r e p l a c e d in a v a c u u m cell, w h i c h a l l o w e d us to p e r f o r m m e a s u r e m e n t s in t h e 77-430 K temperature range. T h e electrical m e a s u r e m e n t s w e r e c a r r i e d o u t u s i n g a n R 5 0 1 0 a.c. bridge, V7-34 v o l t m e t e r s a n d a B 5 - 5 0 d.c. s o u r c e . An E l e c t r o n i c a D 3 - 2 8 m i c r o c o m puter was used for data acquisition and processing.
381 F o r the IR m e a s u r e m e n t s , 1 /zm t h i c k PPB films w e r e c a s t o n t o a KBr plate. T h e s a m p l e s w e r e p u t into a v a c u u m cell, w h i c h a l l o w e d u s to v a r y t h e t e m p e r a t u r e f r o m 77 to 4 5 0 K. A U R - 2 0 Carl Zeiss J e n a s p e c t r o p h o t o m e t e r w a s u s e d in t h e s e e x p e r i m e n t s .
Results
F i g u r e 3 s h o w s t h e p l o t o f c u r r e n t v e r s u s t e m p e r a t u r e f o r P P B films o f different t h i c k n e s s e s . M e a s u r e m e n t s w e r e c a r r i e d o u t d u r i n g heating. T w o t e m p e r a t u r e r a n g e s c a n b e clearly d i s t i n g u i s h e d in t h e s e d e p e n d e n c e s . B e l o w T c 2 ~ 2 6 5 K t h e P P B is a dielectric, its c o n d u c t i v i t y b e i n g less t h a n 10 - ' 2 ( o h m c m ) - ' a n d a l m o s t i n d e p e n d e n t o f t e m p e r a t u r e ( e x c e p t n e a r T¢, ~ 190 K, w h e r e w e a k instabilities are o b s e r v e d in t h e thin films). A b o v e T¢2 the t e m p e r a t u r e d e p e n d e n c e o f c o n d u c t i v i t y b e c o m e s s t r o n g e r so t h a t a t r a n s i t i o n to a m e t a l l i c s t a t e o c c u r s in films l e s s t h a n 0.5 /zm t h i c k ( c u r v e 1 in Fig. 3). T h e c o n d u c t i v i t y of s u c h films r i s e s to 1 0 - a ( o h m c m ) - ' a n d its t e m p e r a t u r e d e p e n d e n c e b e c o m e s metallic, ~ ~ 1/7'. T h e metallic s t a t e o b t a i n e d p o s s e s s e s p r o p e r t i e s similar to t h o s e d e s c r i b e d in ref. 4, including t h e s a m e v a l u e o f a n i s o t r o p y o f c o n d u c t i v i t y . T h u s , t a k i n g into a c c o u n t the similarity of the s a m p l e s u s e d in e x p e r i m e n t s on s w i t c h i n g in a n electric field, u n d e r uniaxial p r e s s u r e a n d p r e s e n t e d in this p a p e r , it is also logical to s u g g e s t t h e f o r m a t i o n o f c o n d u c t i v e d o m a i n s in t h e c a s e o f t h e r m o s t i m u l a t e d switching. In t h i c k e r s a m p l e s the metallic s t a t e w a s n e v e r o b s e r v e d . Switching-off c a n b e p e r f o r m e d in the s a m e w a y as in t h e c a s e o f s w i t c h i n g in an electric field or u n d e r uniaxial p r e s s u r e [2]. In addition, the s a m p l e m a y r e t u r n to the initial s t a t e w h e n cooled. Such a t r a n s i t i o n c a n
Z
~
7
J
7 1 ,m I
I
180
I
I
•
230 280 330 T,K Fig. 3. Thermostimulated currents in PPB films of various thicknesses at a heating rate of 3 K min-~: 1, 10 /~m (left axis); 2, 1.5 /zm (right axis); 3, 0.4 /xm (right axis). 130
382 o c c u r b o t h n e a r T¢, a n d To2. In this c a s e it is interesting that, if the s a m p l e is in the 'off' state, it m a y transit into the 'on' state at T c l and vice versa. The r e p r o d u c i b i l i t y o f the t h e r m o s t i m u l a t e d switching is quite g o o d . W e o b s e r v e d m o r e t h a n 50 switching cycles o n the s a m e sample. Thick PPB films h a v e a v e r y low c o n d u c t i v i t y (less t h a n 10 -14 ( o h m c m ) - 1). T h e r e f o r e w e studied t h e dielectric loss in o r d e r to d e t e c t t e m p e r a t u r e peculiarities. As can be s e e n f r o m Fig. 4, t h e y are t h e s a m e as f o r thin films. B o t h real (tan 6) and imaginary (e) p a r t s o f the c o m p l e x dielectric p e r m i t t i v i t y o f P P B are practically i n d e p e n d e n t o f electric field f r e q u e n c y in the r a n g e f r o m 1 0 - 1 to 109 Hz. Such f r e q u e n c y d e p e n d e n c e o f e is specific for materials with a low loss and is unusually a s s o c i a t e d with the residual r e s p o n s e o f the dielectric lattice. As a rule, in this case, t h e r e is n o t e m p e r a t u r e d e p e n d e n c e either. T h e p e a k s , w h i c h m a y b e o b s e r v e d in this d e p e n d e n c e , are a s s o c i a t e d with s o m e s t r u c t u r a l modifications [14]. Thick P P B films b e h a v e in a similar w a y b e l o w To2. In this t e m p e r a t u r e r a n g e tan ~ is i n d e p e n d e n t o f t e m p e r a t u r e . A w e a k p e a k o b s e r v e d at T¢I is a p p a r e n t l y c a u s e d b y m o l e c u l a r excitations. A m o n o t o n i c i n c r e a s e in tan ~ with rising t e m p e r a t u r e , similar to t h a t o b s e r v e d in PPB films a b o v e T¢2, usually takes p l a c e in materials w h e r e the loss is c a u s e d primarily b y the o h m i c t h r o u g h c u r r e n t s [15]. S u c h i n c r e a s e in the p o l y m e r c o n d u c t i v i t y a b o v e T¢2 is r e f l e c t e d in Fig. 3. In b o t h c a s e s the p e a k at T¢2 is r e l a t e d to the t h e r m o s t i m u l a t e d c u r r e n t s . It is o b s e r v e d only w h e n the t e m p e r a t u r e i n c r e a s e s and its p o s i t i o n a n d intensity d e p e n d o n h e a t i n g rate. P P B IR s p e c t r a m e a s u r e d at different fixed t e m p e r a t u r e s in t h e r a n g e 7 7 - 4 5 0 K reveal n o differences, at least within t h e limits o f t h e s p e c t r o m e t e r resolution. The PPB IR s p e c t r u m at r o o m t e m p e r a t u r e is s h o w n in Fig. 5. C h a n g e s are d i s c o v e r e d w h e n the m e a s u r e m e n t s axe p e r f o r m e d d u r i n g h e a t i n g at a certain rate. T w o t e m p e r a t u r e intervals are d e t e c t e d in w h i c h c o n s i d e r a b l e c h a n g e s in the intensities of the IR lines are o b s e r v e d . T h e y c o i n c i d e with the r a n g e s o f the e x c i t a t i o n s o f e, tan 6 and t h e r m o s t i m u l a t e d c u r r e n t s . In -3.2
6-
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2 ~2.8 !i
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, 160
210
~f•
260
I ~2.6 310
T,K Fig. 4. Temperature dependence of (1) dielectric permittivity (¢) and (2) tan 6 of 100 ~m thick PPB films at a heating rate of 3 K rain-1, w= 1000 Hz.
383
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1800
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1600
1400
r
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17200 1000 ~,
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800
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660
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Fig. 5. IR s p e c t r u m o f 0.8 izm t h i c k P P B film a t r o o m t e m p e r a t u r e .
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Fig. 6. T e m p e r a t u r e d e p e n d e n c e of t h e PPB film IR a b s o r p t i o n at (1) 1 0 0 0 c m -~ a n d (2) 1 5 9 5 c m -~ a t a h e a t i n g r a t e of 5 K m i n - L
the f o r m e r , t h e i n c r e a s e o f d o u b l e a b s o r p t i o n line at 1 5 9 5 - 1 6 0 8 c m - 1 a n d a b s o r p t i o n in t h e r a n g e 8 0 0 - 1 0 0 0 c m -~ are o b s e r v e d (Fig. 6). A n o t h e r e x c i t a t i o n o f the 1 5 9 5 - 1 6 0 8 c m - ~ d o u b l e line o c c u r s in a t e m p e r a t u r e r a n g e of a b o u t To2. In addition, the e x c i t a t i o n s in the IR s p e c t r u m as well as t h e e x c i t a t i o n s o f e a n d t a n ~ are o f a r e l a x a t i o n c h a r a c t e r , i.e. t h e y a r e s h a p e d like p e a k s , e x i s t in a c e r t a i n t e m p e r a t u r e interval only a n d d i s a p p e a r w i t h f u r t h e r i n c r e a s e o f t e m p e r a t u r e . T h e p e a k h e i g h t d e p e n d s on t h e h e a t i n g rate: t h e f a s t e r the h e a t i n g t h e s t r o n g e r t h e excitation. No e x c i t a t i o n s are d e t e c t e d d u r i n g cooling. T h e IR s p e c t r u m r a n g e 8 0 0 - 1 1 0 0 c m -1 is usually a s s o c i a t e d with the n o n v a l e n c e a r o m a t i c d i s t o r t i o n o s c i l l a t i o n s o f the C - H b o n d s . T h e 1 5 9 5 - 1 6 0 8 c m - i d o u b l e line is t y p i c a l of t h e oscillations o f C = C quinoid d o u b l e b o n d s .
384
The observed changes in the IR spectrum are probably caused by the relaxation excitations of these bonds.
Discussion
Thus, in a~,,~lon to the instabilities in the electric field and under uniaxial pressure [2-4], another kind of instability was discovered in thin PPB films, namely, a thermostimulated instability. This can also result in the dielectric-metal transition and the final state possesses the same properties as in the case of switching in an electric field and under uniaxial pressure. As is usually the case in this polymer, the bistable switching into the metallic state can occur in thin (less than 0.5 Izm) films only. Summing up the presented results, we can point out that the thermostimulated switching in thermostable PPB occurs in temperature ranges where thermally induced instabilities are observed, both in electrical and optical properties of the polymer. Such correlation implies the c o m m o n nature of all the observed instabilities. The IR measurements allow us to single out the molecule fragments which are excited at these temperatures. At Tel the nonvalence aromatic distortion oscillations of the C - H bonds are excited. The excitations of C = C oscillations in the polymer backbone take place both at T¢1 and To2. The excitations at T¢~ are accompanied with thermostimulated currents. Results, presented above, together with those reported [2-4], allow us to suggest that the switching effect in thin PPB films is not of thermal, but rather of electronic nature. The appearance of metal-type conductivity implies the existence of a half-filled sub-band of delocalized states. The model of the creation of such a sub-band was proposed by Kivelson and Heeger [16] for polyacetylene and applied to polyaniline [17, 18]. The idea is that an increase in dopant concentration induces a decrease in energy difference between the soliton and polaron configurations in polyacetylene and bipolaron and polaron configurations in polyaniline. In contrast to solitons and bipolarons which form a completely filled sub-band, the energy per polaron decreases with increase in polaron concentration when the polaron sub-band is half-filled. A band diagram for a polaron lattice is sketched in Fig. 7. The creation of [
C.B.
I
"Y////////////////////~
~" "~ v/t//,~ " v/~ Vl/, 7,
Fig. 7. Band diagram for a p o l a r o n lattice (negative polaron). C.B. = c o n d u c t i o n band. T h e u p p e r polaron b a n d is h a l f filled.
V.B.=valence
band,
385 a half-filled polaron sub-band and delocalization of electron states in it due to their high concentration give rise to metal-type conductivity. Thus, the model of K i v e l s o n - H e e g e r assumes that the increasing concentration of the charged localized electron states in the bandgap can ultimately result in its delocalization into a polaron .sub-band. Let us consider the instabilities observed in PPB from this point of view. E x p e r i m e n t s on thermostimulated switching indicate that the transition to the metallic state occurs in thin pol ym er films when the intensity of thermostimulated currents b e c o m e s sufficiently high (no transition occurs if the heating is too slow), or, in other words, when a high concentration of charge is attained in the sample be c a us e thermostimulated currents are known to be caused by thermal ionization of trapping states. The e l e c t r o n - p h o n o n interaction in p o ly m e r s is ver y strong; hence the charges are stored in localized states such as polarons or bipolarons (the g round state in PPB is nondegenerate). If the p r oc e s s of ionization is sufficiently intensive, the concentration of charged localized states rapidly increases and can reach a critical value at which the delocalization into the polaron sub-band occurs. In order to induce this p h e n o m e n o n , certain conditions are required: (1) the concentration o f trapping states m us t be high; (2) the rate of ionization must greatly e x c e e d the rate of the s ec ond capture; (3) the net correlation effect p r o d u c e d by direct and indirect e l e c t r o n - e l e c t r o n interactions must stabilize the polaron lattice after its creation. In PPB, the first condition is met in thin films; the second one can be m e t by fitting the heating rate. For the third condition, theoretical investigations are required. However, no calculations are so far available. As an a r g u m e n t for the polaron model of switching in PPB, one can consider changes in the pol ym er IR s p e c t r u m observed near To2 in heating regimes similar to those in which transition to the metallic state occurs. Namely, this is the excitation of the 1 5 9 5 - 1 6 0 8 cm -1 double line in the PPB IR s p e c t r u m c or r e s pondi ng to the C = C oscillations indicating that the benzoid structure excites into the quinoid one. Such a process usually a c c o mp an ies the formation of charged localized defects in aromatic polymers [191. For the metallic conductivity to be displayed, the polaron lattice must be three dimensional. This requires an intermolecular structure to ensure the interaction between polarons placed in different molecules. This condition cannot be met t h r o u g h o u t such a disordered system as a polymer, but in some regions this can happen. Such regions possessing in addition a high concentration of trapping states are just channels. It may be regions with considerable local mechanical stress that are known to appear in the process of p o l y m e r film preparation. In the framework of the model in question the switching-off is either a total or a partial destruction of the pol a r on lattice. The m e t h o d s of switchingoff described in ref. 2 affect the molecular lattice of the polymer either directly (mechanically) or indirectly (through the Coulomb interaction). As indicated by the IR data, the switching-off at T~I is c o n n e c t e d with the
386
thermally induced excitations of the C-H bonds oscillations and, consequently, with the oscillations of the molecular lattice as well. The excitations of the molecular lattice destroy the correlation interaction which stabilizes the polaron lattice. This results in its destruction and, consequently, in the transition to the 'off' state. Switchings in an electric field and under uniaxial pressure [2-4], as well as the transition to the metallic state due to doping [20], observed in this polymer can also be explained in the framework of the polaron model. These processes differ only in the mechanism of supplying charge for the polaron lattice to be formed. In the electric field, this may be injection from an electrode or the Poole-Frenkel effect [21 ]. In the case of doping, a traditional chemical mechanism operates. It is possible that the instability of the lacton cycle under pressure [9] gives rise to the corresponding changes in conductivity. The existence of a three-dimensional polaron lattice probably cancels the necessity of conjugation along the whole polymer chain and allows us to obtain high conductivity in nonconjugated polymers. References 1 M. G. Zolotukhin, V. A. Kavardakov, S. N. Salazkin and S. R. Rafikov, Vysokomol. Soedi~, Set. A, 26 (1984) 1212. 2 A. N. Lachinov, A. Yu. Zherebov and V. M. Kornilov, Synth. Met., 44 (1991) 111. 3 A. Yu. Zherebov and A. N. Lachinov, Synth. Met., 44 (1991) 99. 4 A. Yu. Zherebov and A. N. Lachinov, Synth. Met., 46 (1992) 181. 5 0 . A. Scaldin, A. Yu. Zherebov, A. N. Lachinov, A. N. Chuvyrov and V. A. Delev, P / s ' m a Zh. Eksp. Teor. Fiz., 51 (1990) 141. 6 V. M. Komilov and A. N. Lachinov, Synth. Met., 53 (1992) 71-76. 7 A. N. Lachinov, M. G. Zolotukhin, A. Yu. Zherebov, S. N. Salazkin, A. N. Chuvyrov and I. L. Valeeva, Pis'ma Zh. Eksp. Teor. Fiz., 44 (1986) 272. 8 A. A. Petrov, M. G. Gonikberg, S. N. Salazkin, J. N. Aneli and Ya. S. Vygodskii, Izv. Akad. Nauk SSSR, Ser. Khim., 2 (1968) 279. 9 P. Ramart-Lucas, C.R., 213 (1941) 244. 10 M. G. Zolotukhin and S. N. Salazkin, Abstr. Int. Conf. Polycondensation and Related Reactions, Gargnano, Italy, 1000 p. 53. 11 M. G. Zolotukhin, Abstr. 2rid Fur. Tech. Symp. Polyimides and High-Temperature Polymers, MontpeUier, France, 1991, p. 14. 12 M. G. Zolotukhin, A. A. Panasenko, V. S. Sultanova, E. A. Sedova, L. V. Spirikhin, L. M. Kahlilov, S. N. Salazkin and S. R. Rafikov, Macromol. Chem., 186 (1985) 1747. 13 A. V. Shubnikov, Optitcheskaya KristaUografiya, Izdatelstvo Akad. Nauk SSSR, Moscow, 1950. 14 A. K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983. 15 I. S. Zheludev, Fizika KristaUitcheskih Dielektrikov, Nauka, Moscow, 1968. 16 S. Kivelson and A. J. Heeger, Phys. Rev. Lett., 55 (1985) 308. 17 S. StafstrSm, J. L. Br~das, A. J. Epstein, H. S. Woo, D. B. Tanner, W. S. Huang and A. G. MacDiarmid, Phys. Rev. Lett., 59 (1987) 1464. 18 J. M. Ginder, A. F. Richter, A. G. MacDiarmid and A. J. Epstein, Solid State Commun., 63 (1987) 97. 19 J. L. Br~das, B. Th~mans, J. G. Fripiat, J. M. Andr~ and R. R. Chance, Phys. Rev. B, 29 (1984) 6761. 20 A. N. Lachinov and M. G. Zolotukhin, Pis'ma Zh. Eksp. Teor. Fiz., 53 (1991) 297. 21 J. Frenkel, Phys. Rev., 54 (1938) 647.
377
Thermostimulated switching in thin polymer films A. N. L a c h i n o v , A. Yu. Z h e r e b o v a n d M. G. Z o l o t u k h i n Physics Department, Bashkirian Research Centre, Russian Academy of Sciences, Ufa 450025 (Russian Federation) (Received December 3, 1992; in revised form January 25, 1993; accepted February 1, 1993)
Abstract The paper presents results of a study on a new kind of instability observed in thin poly(3,3'-phthalidylidene-4,4'-biphenylylene)films, namely, thermostimulated switching. This occurs at 265 K as a result of heating at a certain rate. After switching the sample transits to a metallic state. Investigations of the polymer dielectric loss and IR spectrum under similar heating regimes show the existence of relaxation excitations at this temperature. Another range of excitations is detected at 190 K. According to IR spectra, in the low temperature range, excitation of nonvalence aromatic distortion oscillations of the C--H bonds take place, whereas C=C double bonds are excited in the high temperature range. Dependence of the intensity of excitations on film thickness and heating regime allows us to suggest that they are associated with the thermostimulated ionization of trapping states. The model of polaron lattice formation is discussed as a possible explanation of the observed dielectric-metal transition.
Introduction T h e e x i s t e n c e of e l e c t r o n i c i n s t a b i l i t i e s in p o l y ( 3 , 3 ' - p h t h a l i d y l i d e n e - 4 , 4 ' b i p h e n y l y l e n e ) * ( P P B ) t h i n films in a n e l e c t r i c field a n d u n d e r u n i a x i a l p r e s s u r e w a s r e p o r t e d r e c e n t l y [ 2 - 4 ]. B o t h p h e n o m e n a r e s u l t in t h e t r a n s i t i o n o f t h e i n i t i a l l y d i e l e c t r i c s a m p l e s i n t o a m e t a l l i c s t a t e a n d in t h e a p p e a r a n c e o f h i g h l y c o n d u c t i n g c h a n n e l s . In t h i s s t a t e , i r r e s p e c t i v e o f t h e w a y it w a s induced, the e l e c t r o p h y s i c a l p o r p e r t i e s of the s a m p l e s are similar, namely, the negative t e m p e r a t u r e coefficient of conductivity, the linear c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s a n d t h e a n i s o t r o p y o f c o n d u c t i v i t y (r~/(rtl ~ 10 l°-tz, w h e r e a ± a n d cr~ a r e t h e c o n d u c t i v i t i e s a c r o s s a n d a l o n g t h e p o l y m e r film, r e s p e c t i v e l y , t h e c o n d u c t i v i t y o f a s i n g l e c h a n n e l b e i n g n o l e s s t h a n 105 ( o h m c m ) 'Switching-off" c a n b e p r o d u c e d b y s e v e r a l m e t h o d s , e.g., s u p p l y i n g s h o r t electric or mechanical pulses, passing an electric current which exceeds the critical value and movement of the upper electrode. T h e h e t e r o g e n e o u s s t r u c t u r e o f t h e s a m p l e s in t h e m e t a l l i c s t a t e w a s o b s e r v e d e x p e r i m e n t a l l y b y t w o m e t h o d s : u s i n g n e m a t i c l i q u i d c r y s t a l s [5] *A more precise name of a polymer initially called polydiphenylenephthalide (see, e.g., refs. 1 and 2).
0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All fights reserved
378 and by means of electron microscopy [6]. The experiments displayed the existence of conductive domains of 5 0 - 5 0 0 nm in diameter with a concentration of more than 105 c m - 2. These effects attract a great deal of attention due to the extremely low threshold values and a considerably high conductivity in the metallic 'on' state. The mechanism of these p h e n o m e n a is not clear so far. Therefore, Complex investigations are needed to reveal the material's features responsible for the effects observed in the polymer. One of the curious features which may throw light on the possible mechanism of electronic instabilities in PPB was reported earlier [7]. This is the anomalous temperature dependence of conductivity, optical absorption and luminescence of pristine and iodine-doped PPB films. It was shown that there is a critical temperature Tc ~ 265 K which divides the a(T) dependence into two regions. Below T¢ the conductivity is almost independent of temperature, whereas above T¢ this dependence is extremely strong. It is obvious that the transition results from some intra- and intermolecular electron processes that are determined by a specific molecular structure. A suggestion was made that the lacton cycle in the phthalide group of the polymer molecule is the origin of the observed thermal instabilities. The mechanism is that of phenolphthalein affected by pressure [8 ], radical cations [9 ] and temperature, namely, the polarization or break of one of the C - O - C bonds of the lacton cycle in the side fragment of the PPB molecule caused the appearance of a positive charge on the quaternary carbon atom in the backbone. The appearance of this charge increases the conjugation between neighbouring biphenyl fragments and, hence, enhances conductivity. In this paper we present a more detailed analysis of the temperatureinduced transitions in PPB and associated modifications of its electron structure.
Objects and methods of investigations Poly(phthalidylidenearylene)s present a novel class of aromatic, high molecular, high-temperature, film-forming polymers synthesized in the last few years by Zolotukhin e t al. [10--12]. Their synthesis is based on the polycondensation of 3-chlorophthalides or phthaloyl dichloride with aromatic hydrocarbons, as well as on the homopolycondensation of 3-aryl-3-chlorophthalides via the Freidel-Crafts reaction. The general structure of the poly(phthalidylidenearylene)s is as follows:
379 where R is an aromatic fragment, for example, biphenyl, terphenyl, fluorene, etc. Poly(3,3'-phthalidylidene-4,4'-biphenylylene) is one of the prospective polymers of this class:
o PPB has a molecular weight of (50-80) × 103 depending on the conditions of synthesis. The softening temperature is 440 °C and the temperature of decomposition (TGA, 1% weight loss, air) is 450 °C. The polymer is soluble in conventional organic solvents, such as chloroform, methylenedichloride and 1,1,2,2-tetrachloroethane. The good solubility of PPB makes it possible to purify them effectively and to use them to obtain films and study various properties. PPB was synthesized according to a well-known method [1 ]. Viscosity was measured at 25 °C in an Ubbelohde-type viscosimeter. The intrinsic viscosity [~?]TCE Was calculated by extrapolating ~?sP/c for C--)0, where C is concentration of polymer solution, giving [~]TCE= 0.70 dl g - 1 Methods of polarization optical microscopy [13] were used to analyse the films thicker than 5 / z m (with smaller thicknesses the difference in optical path is too small to interpret the results convincingly). It was established that such films are characterized by the anisotropy of the refractive index. Their conoscopic interference patterns are those of plates of uniaxial crystals cut out normal to the optical axis. The conoscopic pattern is the shape of the Maltese cross and does not decompose into hyperbolas as the microscope table is rotated (Fig. 1). The evaluation of the refraction factor anisotropy yielded the following results: An=no-ne=O.O09-O.03 where no and ne are refractive indexes of the ordinary and extraordinary rays, respectively. The difference in the value of optical anisotropy is due to the difference in the conditions of film preparation at the solidification stage during the evaporation of the solvent. These data indicate that molecules in the polymer film are oriented in a certain way. This is confirmed by the evaluation of the crystalline fraction by means of X-ray diffraction which yielded about 25-30%. The electron structure of the molecules is such that, in the polymer backbone, the conjugation of ~--electrons is limited within the monomeric arylene segment. The quarter carbon separating these segments hinders the interference of the ~r-electron wave functions and creates a potential barrier for electrons in this part of the chain. This fact is well illustrated by the absorption spectra of a polymer and a m o n o m e r (Fig. 2). It can be seen that there is practically no difference between them in this spectral area. No hypsochromic shift of the low energy electron absorption band occurs
380
Fig. 1. Conoscopic figure of PPB film. A package of five plates each 50 ~ m thick.
I
? O
z
O
O 5~
0
-2
E, eV Fig. 2. Absorption spectra o f polymer and m o n o m e r films.
in t h e m a c r o m o l e c u l e . T h i s is, n o d o u b t , a n i n d i c a t i o n o f t h e s t r o n g localization o f ~r-electrons o f t h e a r o m a t i c s y s t e m o f t h e p o l y m e r b a c k b o n e . T h e electrical m e a s u r e m e n t s w e r e p e r f o r m e d in a ' s a n d w i c h ' - t y p e m e t a l / p o l y m e r / m e t a l e l e c t r o d e c o n f i g u r a t i o n a n d t h e electric field w a s a p p l i e d a c r o s s p o l y m e r film. F o r t h e dielectric m e a s u r e m e n t s , a ' p r o t e c t i v e ' r i n g e l e c t r o d e w a s a d d e d . The. m e t a l l i c (Cu, Cr, A1 o r Au) e l e c t r o d e s w e r e d e p o s i t e d o n t o t h e p o l y m e r film s u r f a c e s in v a c u u m or w e r e p r e s s e d . T h e s a m p l e s w e r e p l a c e d in a v a c u u m cell, w h i c h a l l o w e d us to p e r f o r m m e a s u r e m e n t s in t h e 77-430 K temperature range. T h e electrical m e a s u r e m e n t s w e r e c a r r i e d o u t u s i n g a n R 5 0 1 0 a.c. bridge, V7-34 v o l t m e t e r s a n d a B 5 - 5 0 d.c. s o u r c e . An E l e c t r o n i c a D 3 - 2 8 m i c r o c o m puter was used for data acquisition and processing.
381 F o r the IR m e a s u r e m e n t s , 1 /zm t h i c k PPB films w e r e c a s t o n t o a KBr plate. T h e s a m p l e s w e r e p u t into a v a c u u m cell, w h i c h a l l o w e d u s to v a r y t h e t e m p e r a t u r e f r o m 77 to 4 5 0 K. A U R - 2 0 Carl Zeiss J e n a s p e c t r o p h o t o m e t e r w a s u s e d in t h e s e e x p e r i m e n t s .
Results
F i g u r e 3 s h o w s t h e p l o t o f c u r r e n t v e r s u s t e m p e r a t u r e f o r P P B films o f different t h i c k n e s s e s . M e a s u r e m e n t s w e r e c a r r i e d o u t d u r i n g heating. T w o t e m p e r a t u r e r a n g e s c a n b e clearly d i s t i n g u i s h e d in t h e s e d e p e n d e n c e s . B e l o w T c 2 ~ 2 6 5 K t h e P P B is a dielectric, its c o n d u c t i v i t y b e i n g less t h a n 10 - ' 2 ( o h m c m ) - ' a n d a l m o s t i n d e p e n d e n t o f t e m p e r a t u r e ( e x c e p t n e a r T¢, ~ 190 K, w h e r e w e a k instabilities are o b s e r v e d in t h e thin films). A b o v e T¢2 the t e m p e r a t u r e d e p e n d e n c e o f c o n d u c t i v i t y b e c o m e s s t r o n g e r so t h a t a t r a n s i t i o n to a m e t a l l i c s t a t e o c c u r s in films l e s s t h a n 0.5 /zm t h i c k ( c u r v e 1 in Fig. 3). T h e c o n d u c t i v i t y of s u c h films r i s e s to 1 0 - a ( o h m c m ) - ' a n d its t e m p e r a t u r e d e p e n d e n c e b e c o m e s metallic, ~ ~ 1/7'. T h e metallic s t a t e o b t a i n e d p o s s e s s e s p r o p e r t i e s similar to t h o s e d e s c r i b e d in ref. 4, including t h e s a m e v a l u e o f a n i s o t r o p y o f c o n d u c t i v i t y . T h u s , t a k i n g into a c c o u n t the similarity of the s a m p l e s u s e d in e x p e r i m e n t s on s w i t c h i n g in a n electric field, u n d e r uniaxial p r e s s u r e a n d p r e s e n t e d in this p a p e r , it is also logical to s u g g e s t t h e f o r m a t i o n o f c o n d u c t i v e d o m a i n s in t h e c a s e o f t h e r m o s t i m u l a t e d switching. In t h i c k e r s a m p l e s the metallic s t a t e w a s n e v e r o b s e r v e d . Switching-off c a n b e p e r f o r m e d in the s a m e w a y as in t h e c a s e o f s w i t c h i n g in an electric field or u n d e r uniaxial p r e s s u r e [2]. In addition, the s a m p l e m a y r e t u r n to the initial s t a t e w h e n cooled. Such a t r a n s i t i o n c a n
Z
~
7
J
7 1 ,m I
I
180
I
I
•
230 280 330 T,K Fig. 3. Thermostimulated currents in PPB films of various thicknesses at a heating rate of 3 K min-~: 1, 10 /~m (left axis); 2, 1.5 /zm (right axis); 3, 0.4 /xm (right axis). 130
382 o c c u r b o t h n e a r T¢, a n d To2. In this c a s e it is interesting that, if the s a m p l e is in the 'off' state, it m a y transit into the 'on' state at T c l and vice versa. The r e p r o d u c i b i l i t y o f the t h e r m o s t i m u l a t e d switching is quite g o o d . W e o b s e r v e d m o r e t h a n 50 switching cycles o n the s a m e sample. Thick PPB films h a v e a v e r y low c o n d u c t i v i t y (less t h a n 10 -14 ( o h m c m ) - 1). T h e r e f o r e w e studied t h e dielectric loss in o r d e r to d e t e c t t e m p e r a t u r e peculiarities. As can be s e e n f r o m Fig. 4, t h e y are t h e s a m e as f o r thin films. B o t h real (tan 6) and imaginary (e) p a r t s o f the c o m p l e x dielectric p e r m i t t i v i t y o f P P B are practically i n d e p e n d e n t o f electric field f r e q u e n c y in the r a n g e f r o m 1 0 - 1 to 109 Hz. Such f r e q u e n c y d e p e n d e n c e o f e is specific for materials with a low loss and is unusually a s s o c i a t e d with the residual r e s p o n s e o f the dielectric lattice. As a rule, in this case, t h e r e is n o t e m p e r a t u r e d e p e n d e n c e either. T h e p e a k s , w h i c h m a y b e o b s e r v e d in this d e p e n d e n c e , are a s s o c i a t e d with s o m e s t r u c t u r a l modifications [14]. Thick P P B films b e h a v e in a similar w a y b e l o w To2. In this t e m p e r a t u r e r a n g e tan ~ is i n d e p e n d e n t o f t e m p e r a t u r e . A w e a k p e a k o b s e r v e d at T¢I is a p p a r e n t l y c a u s e d b y m o l e c u l a r excitations. A m o n o t o n i c i n c r e a s e in tan ~ with rising t e m p e r a t u r e , similar to t h a t o b s e r v e d in PPB films a b o v e T¢2, usually takes p l a c e in materials w h e r e the loss is c a u s e d primarily b y the o h m i c t h r o u g h c u r r e n t s [15]. S u c h i n c r e a s e in the p o l y m e r c o n d u c t i v i t y a b o v e T¢2 is r e f l e c t e d in Fig. 3. In b o t h c a s e s the p e a k at T¢2 is r e l a t e d to the t h e r m o s t i m u l a t e d c u r r e n t s . It is o b s e r v e d only w h e n the t e m p e r a t u r e i n c r e a s e s and its p o s i t i o n a n d intensity d e p e n d o n h e a t i n g rate. P P B IR s p e c t r a m e a s u r e d at different fixed t e m p e r a t u r e s in t h e r a n g e 7 7 - 4 5 0 K reveal n o differences, at least within t h e limits o f t h e s p e c t r o m e t e r resolution. The PPB IR s p e c t r u m at r o o m t e m p e r a t u r e is s h o w n in Fig. 5. C h a n g e s are d i s c o v e r e d w h e n the m e a s u r e m e n t s axe p e r f o r m e d d u r i n g h e a t i n g at a certain rate. T w o t e m p e r a t u r e intervals are d e t e c t e d in w h i c h c o n s i d e r a b l e c h a n g e s in the intensities of the IR lines are o b s e r v e d . T h e y c o i n c i d e with the r a n g e s o f the e x c i t a t i o n s o f e, tan 6 and t h e r m o s t i m u l a t e d c u r r e n t s . In -3.2
6-
~' O
5-
3.0
'O "~ 4 -
2 ~2.8 !i
3\4 2
j
, 160
210
~f•
260
I ~2.6 310
T,K Fig. 4. Temperature dependence of (1) dielectric permittivity (¢) and (2) tan 6 of 100 ~m thick PPB films at a heating rate of 3 K rain-1, w= 1000 Hz.
383
10 8 6I
4
J 0 ~
r
T
2000
•
1800
I
F
1600
1400
r
[
•
V
]
T
17200 1000 ~,
cIn
800
i
660
l
Fig. 5. IR s p e c t r u m o f 0.8 izm t h i c k P P B film a t r o o m t e m p e r a t u r e .
I F~
lOd
i
0
." o ~ <
/
.,
/
x,
\
: 4 ~ \ 9
0
]
t
80
I
T
120
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~
160
[
7200 T,K
T
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7240
:
;380
Fig. 6. T e m p e r a t u r e d e p e n d e n c e of t h e PPB film IR a b s o r p t i o n at (1) 1 0 0 0 c m -~ a n d (2) 1 5 9 5 c m -~ a t a h e a t i n g r a t e of 5 K m i n - L
the f o r m e r , t h e i n c r e a s e o f d o u b l e a b s o r p t i o n line at 1 5 9 5 - 1 6 0 8 c m - 1 a n d a b s o r p t i o n in t h e r a n g e 8 0 0 - 1 0 0 0 c m -~ are o b s e r v e d (Fig. 6). A n o t h e r e x c i t a t i o n o f the 1 5 9 5 - 1 6 0 8 c m - ~ d o u b l e line o c c u r s in a t e m p e r a t u r e r a n g e of a b o u t To2. In addition, the e x c i t a t i o n s in the IR s p e c t r u m as well as t h e e x c i t a t i o n s o f e a n d t a n ~ are o f a r e l a x a t i o n c h a r a c t e r , i.e. t h e y a r e s h a p e d like p e a k s , e x i s t in a c e r t a i n t e m p e r a t u r e interval only a n d d i s a p p e a r w i t h f u r t h e r i n c r e a s e o f t e m p e r a t u r e . T h e p e a k h e i g h t d e p e n d s on t h e h e a t i n g rate: t h e f a s t e r the h e a t i n g t h e s t r o n g e r t h e excitation. No e x c i t a t i o n s are d e t e c t e d d u r i n g cooling. T h e IR s p e c t r u m r a n g e 8 0 0 - 1 1 0 0 c m -1 is usually a s s o c i a t e d with the n o n v a l e n c e a r o m a t i c d i s t o r t i o n o s c i l l a t i o n s o f the C - H b o n d s . T h e 1 5 9 5 - 1 6 0 8 c m - i d o u b l e line is t y p i c a l of t h e oscillations o f C = C quinoid d o u b l e b o n d s .
384
The observed changes in the IR spectrum are probably caused by the relaxation excitations of these bonds.
Discussion
Thus, in a~,,~lon to the instabilities in the electric field and under uniaxial pressure [2-4], another kind of instability was discovered in thin PPB films, namely, a thermostimulated instability. This can also result in the dielectric-metal transition and the final state possesses the same properties as in the case of switching in an electric field and under uniaxial pressure. As is usually the case in this polymer, the bistable switching into the metallic state can occur in thin (less than 0.5 Izm) films only. Summing up the presented results, we can point out that the thermostimulated switching in thermostable PPB occurs in temperature ranges where thermally induced instabilities are observed, both in electrical and optical properties of the polymer. Such correlation implies the c o m m o n nature of all the observed instabilities. The IR measurements allow us to single out the molecule fragments which are excited at these temperatures. At Tel the nonvalence aromatic distortion oscillations of the C - H bonds are excited. The excitations of C = C oscillations in the polymer backbone take place both at T¢1 and To2. The excitations at T¢~ are accompanied with thermostimulated currents. Results, presented above, together with those reported [2-4], allow us to suggest that the switching effect in thin PPB films is not of thermal, but rather of electronic nature. The appearance of metal-type conductivity implies the existence of a half-filled sub-band of delocalized states. The model of the creation of such a sub-band was proposed by Kivelson and Heeger [16] for polyacetylene and applied to polyaniline [17, 18]. The idea is that an increase in dopant concentration induces a decrease in energy difference between the soliton and polaron configurations in polyacetylene and bipolaron and polaron configurations in polyaniline. In contrast to solitons and bipolarons which form a completely filled sub-band, the energy per polaron decreases with increase in polaron concentration when the polaron sub-band is half-filled. A band diagram for a polaron lattice is sketched in Fig. 7. The creation of [
C.B.
I
"Y////////////////////~
~" "~ v/t//,~ " v/~ Vl/, 7,
Fig. 7. Band diagram for a p o l a r o n lattice (negative polaron). C.B. = c o n d u c t i o n band. T h e u p p e r polaron b a n d is h a l f filled.
V.B.=valence
band,
385 a half-filled polaron sub-band and delocalization of electron states in it due to their high concentration give rise to metal-type conductivity. Thus, the model of K i v e l s o n - H e e g e r assumes that the increasing concentration of the charged localized electron states in the bandgap can ultimately result in its delocalization into a polaron .sub-band. Let us consider the instabilities observed in PPB from this point of view. E x p e r i m e n t s on thermostimulated switching indicate that the transition to the metallic state occurs in thin pol ym er films when the intensity of thermostimulated currents b e c o m e s sufficiently high (no transition occurs if the heating is too slow), or, in other words, when a high concentration of charge is attained in the sample be c a us e thermostimulated currents are known to be caused by thermal ionization of trapping states. The e l e c t r o n - p h o n o n interaction in p o ly m e r s is ver y strong; hence the charges are stored in localized states such as polarons or bipolarons (the g round state in PPB is nondegenerate). If the p r oc e s s of ionization is sufficiently intensive, the concentration of charged localized states rapidly increases and can reach a critical value at which the delocalization into the polaron sub-band occurs. In order to induce this p h e n o m e n o n , certain conditions are required: (1) the concentration o f trapping states m us t be high; (2) the rate of ionization must greatly e x c e e d the rate of the s ec ond capture; (3) the net correlation effect p r o d u c e d by direct and indirect e l e c t r o n - e l e c t r o n interactions must stabilize the polaron lattice after its creation. In PPB, the first condition is met in thin films; the second one can be m e t by fitting the heating rate. For the third condition, theoretical investigations are required. However, no calculations are so far available. As an a r g u m e n t for the polaron model of switching in PPB, one can consider changes in the pol ym er IR s p e c t r u m observed near To2 in heating regimes similar to those in which transition to the metallic state occurs. Namely, this is the excitation of the 1 5 9 5 - 1 6 0 8 cm -1 double line in the PPB IR s p e c t r u m c or r e s pondi ng to the C = C oscillations indicating that the benzoid structure excites into the quinoid one. Such a process usually a c c o mp an ies the formation of charged localized defects in aromatic polymers [191. For the metallic conductivity to be displayed, the polaron lattice must be three dimensional. This requires an intermolecular structure to ensure the interaction between polarons placed in different molecules. This condition cannot be met t h r o u g h o u t such a disordered system as a polymer, but in some regions this can happen. Such regions possessing in addition a high concentration of trapping states are just channels. It may be regions with considerable local mechanical stress that are known to appear in the process of p o l y m e r film preparation. In the framework of the model in question the switching-off is either a total or a partial destruction of the pol a r on lattice. The m e t h o d s of switchingoff described in ref. 2 affect the molecular lattice of the polymer either directly (mechanically) or indirectly (through the Coulomb interaction). As indicated by the IR data, the switching-off at T~I is c o n n e c t e d with the
386
thermally induced excitations of the C-H bonds oscillations and, consequently, with the oscillations of the molecular lattice as well. The excitations of the molecular lattice destroy the correlation interaction which stabilizes the polaron lattice. This results in its destruction and, consequently, in the transition to the 'off' state. Switchings in an electric field and under uniaxial pressure [2-4], as well as the transition to the metallic state due to doping [20], observed in this polymer can also be explained in the framework of the polaron model. These processes differ only in the mechanism of supplying charge for the polaron lattice to be formed. In the electric field, this may be injection from an electrode or the Poole-Frenkel effect [21 ]. In the case of doping, a traditional chemical mechanism operates. It is possible that the instability of the lacton cycle under pressure [9] gives rise to the corresponding changes in conductivity. The existence of a three-dimensional polaron lattice probably cancels the necessity of conjugation along the whole polymer chain and allows us to obtain high conductivity in nonconjugated polymers. References 1 M. G. Zolotukhin, V. A. Kavardakov, S. N. Salazkin and S. R. Rafikov, Vysokomol. Soedi~, Set. A, 26 (1984) 1212. 2 A. N. Lachinov, A. Yu. Zherebov and V. M. Kornilov, Synth. Met., 44 (1991) 111. 3 A. Yu. Zherebov and A. N. Lachinov, Synth. Met., 44 (1991) 99. 4 A. Yu. Zherebov and A. N. Lachinov, Synth. Met., 46 (1992) 181. 5 0 . A. Scaldin, A. Yu. Zherebov, A. N. Lachinov, A. N. Chuvyrov and V. A. Delev, P / s ' m a Zh. Eksp. Teor. Fiz., 51 (1990) 141. 6 V. M. Komilov and A. N. Lachinov, Synth. Met., 53 (1992) 71-76. 7 A. N. Lachinov, M. G. Zolotukhin, A. Yu. Zherebov, S. N. Salazkin, A. N. Chuvyrov and I. L. Valeeva, Pis'ma Zh. Eksp. Teor. Fiz., 44 (1986) 272. 8 A. A. Petrov, M. G. Gonikberg, S. N. Salazkin, J. N. Aneli and Ya. S. Vygodskii, Izv. Akad. Nauk SSSR, Ser. Khim., 2 (1968) 279. 9 P. Ramart-Lucas, C.R., 213 (1941) 244. 10 M. G. Zolotukhin and S. N. Salazkin, Abstr. Int. Conf. Polycondensation and Related Reactions, Gargnano, Italy, 1000 p. 53. 11 M. G. Zolotukhin, Abstr. 2rid Fur. Tech. Symp. Polyimides and High-Temperature Polymers, MontpeUier, France, 1991, p. 14. 12 M. G. Zolotukhin, A. A. Panasenko, V. S. Sultanova, E. A. Sedova, L. V. Spirikhin, L. M. Kahlilov, S. N. Salazkin and S. R. Rafikov, Macromol. Chem., 186 (1985) 1747. 13 A. V. Shubnikov, Optitcheskaya KristaUografiya, Izdatelstvo Akad. Nauk SSSR, Moscow, 1950. 14 A. K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983. 15 I. S. Zheludev, Fizika KristaUitcheskih Dielektrikov, Nauka, Moscow, 1968. 16 S. Kivelson and A. J. Heeger, Phys. Rev. Lett., 55 (1985) 308. 17 S. StafstrSm, J. L. Br~das, A. J. Epstein, H. S. Woo, D. B. Tanner, W. S. Huang and A. G. MacDiarmid, Phys. Rev. Lett., 59 (1987) 1464. 18 J. M. Ginder, A. F. Richter, A. G. MacDiarmid and A. J. Epstein, Solid State Commun., 63 (1987) 97. 19 J. L. Br~das, B. Th~mans, J. G. Fripiat, J. M. Andr~ and R. R. Chance, Phys. Rev. B, 29 (1984) 6761. 20 A. N. Lachinov and M. G. Zolotukhin, Pis'ma Zh. Eksp. Teor. Fiz., 53 (1991) 297. 21 J. Frenkel, Phys. Rev., 54 (1938) 647.