Wolfgang Kumstler, Hans von Berlepsch, Armin. Wedel, Rudi Danz and Detlev Geiss. Institute of Polymer Chemistry "Erich Corrms". Academy of 5cisnccss of the ...
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PIEZOELECTRICITY A N D POLARIZATION IN ACRYLONITRILE/METHYLACRYLATE COPOLYMER Wolfgang K u m s t l e r , H a n s von Berlepsch, Wedel, Rudi Danz and Detlev G e i s s
Armin
Institute o f Polymer Chemistry "Erich C o r r m s " Academy of 5cisnccss of t h e GDR, Kantstr. 33, 1530 Teltew, German Democratic Republic ABSTRACT T h e piezoelectric strain coefficient d 3 1 of stretched and unrtretched acrylonitrile/ methylacrylate copolymer films ha6 been investigated in dependence o n poling conditione. T h e maximum v a l u e of d-1-3 pC/N i s obtained far stretched films. T h e t i m e rtability of piezoelectricity i s relatively low. A hysteresis loop of electric displacement vs. electric field h a s been observed for the first time indicating of a ferraelectriclikc behaviour. I NTRODUCTXQCJ
T h e polymer w i t h t h e s t r m g e s t piaroeioctric activity arising from d i p o l e orientrtiarr Sr poly(viny1idena fluoride) (PVDF) C A I . Ubt-ear the -CF,g r o u p has a d i p o l e moment o f 2.1 D, t h e nitril s i d e group of poly(acrylonikri1e) ( P A N ) has a d i p o l e moment of about 3.5 D. If these dipoles can be oriented by a n applied field, a piezoelectric activity c a n be cxpected. Indeed, it could b e observed C2-43. T h e aim of the present study w a s t o gain m o r e insight into t h e n a t u r e of piezoelectricity in P A N . Owing t o t h e better processability instead ) of AN homopolymer a copolymer of AN ( ~ 9 3 % and methylacrylate (6-7%) (P(CSN-MA)) w a s used. EXPERIMENTAL Films
o f t h e copolymer were c a s t from a
CH2593-2/88/ oooo-o3e4$01.00Copyright 1988 IEEE
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thylformamide s o l u t i o n , d r i e d i n an evacuated oven, u n i a x i a l l y s t r e t c h e d (draw r a t i o about 41, e l e c t r o d e d w i t h aluminium and p o l e d [SI. The sample t h i c k n e s s v a r i e d from 10 t o 15 F m . The measurement o f dmz was c a r r i e d o u t a t room temperature two days a f t e r p o l i n g . The h y s t e r e sis loops were measured by t h e c o n v e n t i o n a l Sawyer-Tower method u s i n g a t r i a n g u l a r e l e c t r i c at various field o f 0.2 Hz ( s t a n d a r d regime) temperatures u p t o 9 0 ° C .
Fig. 1 r e p r e s e n t s t h e p o l i n g f i e l d s t r e n g t h E, dwandence o f d f x f o r s t r e t c h e d and u n s t r e t c h e d films. The p o l i n g t i m e t, was chosam as 10 s. I n t h e case o f s t r e t c h e d f i l m s two different 1409C, p o l i n g temperatures o f T,=9OoC and r e s p e c t i v e l y were used. The same s a t u r a t i o n v a l u e o f d S z s3 pC/N i s reached i n both cases. Secmdly t h e p o l i n g t i m e was v a r i e d between 10 s and 25 m i n . F o r s t r e t c h e d samples p o l e d at 90°C under E , = 8 ~ 1 0 ' ~ V / m a v a l u e o f d s z = 3 pC/N was a l s o found independent o f t,. The p i e z o e l e c t r i c a c t i v i t y r t r o n g l y dopends on t h e p o l i n g temperature as shown i n F i g . 2. Vary ir g r e a t e r l i t t l e a c t i v i t y i s found u n l e s s T, a c r i t i c a l v a l u e o f about 90°C. Clt than Ep=Sx107 V / m daz increaoeo s h a r p l y w h e n t h e 3
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69°C and reaches poling temperature exceeds i t s s a t u r a t i o n v a l u e around 80°C. This behaviour i s related w i t h the glass transition. By was found t o dynamic mechanical measurements T, The temperature r e g i o n above 50-C be - 9 5 O C . coincides closely w i t h the loss of elastic modu1us. To o b t a i n i n f o r m a t i o n on t h e s t a b i l i t y o f the piezoelectric p r o p e r t i e s t h e s t o r a g e t i m e deinvespendence o f d:sL a t room temperature was tigated. The p i e z o e l e c t r i c decay f o l l o w s logar i t h m i c k i n e t i c s [4]. The i s o t h e r m a l i n v e s t i g a t i o n s were supplemented by t h e r m a l l y s t i m u l a t e d measurements. For this reason an initially p o l e d sample was annealed s t e p w i s e a t rising f o r 30 min and q u i c k l y c o o l e d temperatures T, down t o room temperature. A f t e r each a n n e a l i n g step the r e s i d u a l dZ1 c o e f f i c i e n t was e s t i mated. T h i s procedure was c a r r i e d o u t on t h r e e samples which were p r e v i o u s l y s t o r e d d i f f e r e n t t i m e s (Zd, 7d, 200d) a t room temperature. F i g . 3 We n o t e a n e a r l y g i v e s t h e p l o t s o f d3= vs. .T, linear decay o f dS1. a f t e r an i n i t i a l c o n s t a n t plateau. F o r aged samples t h e l o s s o f piezoin at higher temperatures e l e c t r i c i t y sets compared w i t h t h e f r e s h sample. However, the initial v a l u e o f dZ1 is s m a l l e r o r i g i n a t i n g i n t h e f o r e g o i n g i s o t h e r m a l aging. The most con-
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s p i c u o u s f a c t i s t h e sample h i s t o r y independent linear l o s s o f p i e z o e l e c t r i c i t y a t h i g h e r temperatures. In F i g . 4 a c h a r a c t e r i s t i c hysteresis loop f o r a f i e l d a m p l i t u d e o f 1 . 0 5 ~ 1 0V~/ m i s p l o t t e d . The l o o p s a f t e r two c y c l e s were found to be i s nearly a l m o s t s t a t i o n a r y and t h e i r shape independent o f t h e frequency ( r a n g i n g from 0.03 t o 0.2 H z ) . F o r h i g h e r f i e l d s and temperatures t h e l o o p s a r e d i s t o r t e d by t h e e f f e c t due t o dc conduction.
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CONCLUSIONS CIS demonstrated, both homo C21 and copolymers o f CIN possess appreciable piezoelectric activity in s p i t e of their assumed helical conformaT h i s i s in contrast t o literature tion C51. [&I where it i s suggested that PCIN should not show any piezoelectricity as t h e result o f the cancellation of dipoles. T h e present investigat i o n s indicate that t h e polarization a r i s e s at least partially from a preferential orientation of dipoles. T h e hysteresis phenomenon observed s u g g e s t s t h e copolymer t o be quasi ferroelectric. Only quasi ferroelectric, because t h e piezoelectric s t a t e i s obviously metastable as i t s low time stability shows. In order t o clain more rify the origin of piezoelectricity detail investigations in relation t o t h e structure a r e necessary.
REFERENCES
CllT.Furukawa,"Piezoel~ctricity and Pyroelectricity in Ferroelectric PolymersN,Proc. 5th Intern.Symp.Electrets,Heidelberg 1 9 8 5 (IEEE,New York,1985),pp.883-888. [Z]H.lJeda and S.H.Carr,"Piezoelectricity in Polyacrylonitrile~~,Polymcr J.,Vo1.16,pp.661667,1984. [3]H.v.Berleprch,W.Kuenstler and R.Danz,"Piezoelcctricty in CIcrylonitrilc/Methylacrylate Copolymer",Fcrroelectricr, in press. C4lH.v.Berlepsch and W.Kuenstler,"Piezoelectricity in f A c r y l o n i t r i l e / M a t h y l a c r y l a t e Copolymer" ,submitted t o Polymer By1 1.. C5lB.Hanrici-01iv~ and S.Olive,"Molecular Interactions and Macroscopic Properties o f Polyacrylonitrile and Model Substances", Cldv. Polym. Sci., Vo1.32,pp.123-152,1979. C6lH.K.Hall jr., R.J.H.Chan, J.Oku, O.R.Hugher, and B. Newman, "Piezoelec tric J Sc hein beim activity in f i l m s o f poly(1-bicyclobutanecarbonitrile)",Polymer Bull.,Vol. 17,pp.135136,1987.
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