STRUCTURAL STUDY OF THE CIS-TO-TRANS THERMAL ISOMERIZATION IN POLYACETYLENE

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STRUCTURAL STUDY OF THE CIS-TO-TRANS

THERMAL ISOMERIZATION IN POLYACETYLENE

P. Robin, Jean Pouget, R. Comès, H. Gibson, A. Epstein

To cite this version:

C3-77

STRUCTURAL STUDY O F THE CIS-TO-TRANS THERMAL ISOMERIZATION IN POLYACETY LENE

P . RobinW, J . P . pougetxW, R. ~ o m ~ s * " , H.W. ~ibson***and A.J. ~pstein**" *Laboratoire Central de Recherches, fiomson-CSF, Domaine de C o r b e v i l l e , 91401 Orsay, France

*

Laboratoire de Physique des S o l i d e s and Laboratoire pour 2 ' u t i l i s a t i o n du Rayonnement EZectromagn&tique, Universite' Paris-Sud, 81405 Orsay, France

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Xerox Webster Research Center, 800 P h i l l i p s , Road-W-111, Webster, New York 14580, U.S.A.

Resume : Les r e s u l t a t s d'une etude par rayons X de 1'isomGrisatio thermique

7

c i s - t r a n s dans l e polyacetylene sont presentes. 11s montrent q e l e proces- sus diisomGrisation e s t homogene, sans formation d'amas de tranJdans l e reseau hote c i s .

A b s t r ~ c t : An X-ray stud of the c i s t o trans therr-a1 isorerizp#ion in poly- acety ene i s 91ven. I t sXows t h a t the isoneriration rrocess i s 'h~mo~eneous without clustering of trans regions within the c i s host l a t t i g e .

I - INTRODUCTION

Polyacetylene, (CH), has been subject t o intense theoretical and e x p e r i ~ e n t a l s t u - dies due i n part t o the f a c t t h a t i t i s the simplest conjugated polymer. I t s f i - b r i l l a r morpholo~y with a g r e a t s p e c i f i c area has allowed several technological applications l i k e b a t t e r i e s . Polyacetylene can be synthetized in e i t h e r a c i s or a thermodynamically more s t a b l e trans isomer. There are two processes t o obtain a trans film. In the f i r s t process a c i s film i s synthetized aD

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78OC and then an- nealed a t high temperature (200°C during 20 mn f o r example). In the second process the synthesis i s performed a t high temperature (lOO°C).

One mustnotice t h a t from a s t r u c t u r a l point of view these two processes are not r e a l l y equivalent. In the f i r s t process the c r y s t a l l i s a t i o n of the c i s form i s f i r s t performed, then the trans form i s obtained l a t e r by thermal isomerization. In the second process, on the contrary, the c r y s t a l l i z a t i o n and the i s o ~ e r i z a t i o n occur a t the same time.

The use of the f i r s t process allows a study, s t e p by s t e p , of tbe isomerization Re- chanism. A number of experimental and theoretical studies of t h i s process have been reported(1). However, as c i s ( 2 ) and trans ( 3 ) ( 4 ) isomers have d i f f e r e n t c r y s t a l - l i n e s t r u c t u r e s , a crystallographic study of p a r t l y isomerized films requires spe- c i a l a t t e n t i o n . In p a r t i c u l a r i t i s not c l e a r i f these samples show segregated c i s and trans l a t t i c e s or i f the c i s and t r a n s forms coexist i n a unique l a t t i c e . Such

a s t r u c t u r a l analysis i s the puroose cf the o r e 5 w t paper.

I1

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EXPERIIIENTAL CONDITIONS

Polyacetylene was prepared by the Shirakawa technique a t

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78OC ( 5 ) . Samples were stored a t 1 iquid nitrogen temperature in vacuum until studied. Scanning electron

JOURNAL DE PHYSIQUE

micrographi6 s t u d i e s confirmed a f i b r i l l a r morphology with a t y p i c a l f i b r i l diame- t e r of 500 A ( 6 ) . Except t o obtain nearPy completely converted t r a n s (CH),, t h e i s o - merizations were c a r r i e d out a t 100°C i n an evacuatedyglass tube. The r a t l o of c i s t o t r a n s isomer i n a given f i l m was determined through e x t e n s i v e c o r r e l a t e d i n f a r e d and s o l i d NVR a n a l y s i s ( 1 ) . X-ray data were c o l l e c t e d on photographic f i l m s f i x e d on a c y l i n d r i c a l camera with t h e sample placed a t t h e c e n t e r of t h e chamber. During a l l t h e exposures t h e samples were kept under vacuum. In o r d e r t o have l i m i t e d expo- s u r e time a t ambiant temperature, data were obtained with t h e X-ray s nchrotron r a - d i a t i o n a t t h e s t a t i o n D 16 a t LURE (Orsay) ( I = 1.34 A and h = 1.25

r\).

Cornplemen- t a r y experiments on r o l l e d samples were a l s o performed with t h e conventional CuKa (1.542 A) r a d i a t i o n .

rIf

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RESULTS AND DISCUSSION

A preliminary s t r u c t u r a l study of t h e c i s t o t r a n s thermal isomerization mechanism, being under p r e s s ( 7 ) , we s h a l l summarize here only t h e main r e s u l t s obtained :

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f o r a l l degree of isomerization, o n l y one s e t of Debye S c h e r r e r r e f l e c t i o n s i s observed and not a superposition of c i s and t r a n s r e f l e c t i o n s .

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no n o t i c e a b l e change of t h e X-ray c r y s t a l l i n i t y can be d e t e c t e d during t h e isomerization process. In c i s and t r a n s (CH), forms ( 8 ) , a s well a s f o r i n t e r m e d i a t e compositions, t h e X-ray c r y s t a l l i n i t y i s estimated a t about 90 %, a value c l o s e t o t h a t a l r e a d y reported i n e a r l i e r s t u d i e s of c i s and t r a n s (CH)x(9).

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using t h e S c h e r r e r f o r m ~ l a , ~ w i t h (h01) r e f l e c t i o n s ( s e e below), a l a t e r a l X-ray coherence length of 150

+

30 A , independant of t h e degree of i s o m e r i z a t i o n , i s found.

These r e s u l t s mean t h a t t h e thermal isomerization process i s homogeneous within each c r y s t a l , i n o r d e r t o keep i t s i n t e g r i t y . I f t h e process was heterogeneous, i . e with nucleation of t r a n s i n a c i s m a t r i x , a s u p e r p o s i t i o n of t h e X-ray Oebye Scherrer P a t t e r n s of pure t r a n s (CH), should be observed. This i s not t h e c a s e .

We s h a l l now d e s c r i b e more completely t h e change of t h e Debye Scherrer p a t t e r n upon isomerization.

The nebye S c h e r r e r X-ray p a t t e r n observed i n c i s (CH), i s i d e n t i c a l t o those a l r e a - dy published i n r e f ( 2 ) ,

(9).

Using r o l l e d samples i t i s easy t o s e p a r a t e t r a n s v e r s e (h01) r e f l e c t i o n s from those containing t h e chain index k ( i n t h e study we choose b as t h e chain d i r e c t i o n ) . Transverse r e f l e c t i o n s used during t h i s study a r e given i n t a b l e I , with t h e i r d spacings. One n o t i c e s , i n p a r t i c u l a r , t h a t t h e couples of t r a n s v e r s e r e f l e c t i o n s (200) and (101), (301) and (002J, (400) and (202 cannot be separated i n c i s (CH),. The r e f l e c t i o n s ( 119 a t 3.12 A , (211) a t 2.44

1

,

a s well a s t h e i n chain (020) r e f l e c t i o n a t 2.18 !.clearly observed i n r o l l e d c i s (CH),, have been a l s o followed with t h e degree of isomerization.

Upon annealing, t h e t r a n s v e r s e (h01) r e f l e c t i o n s s h i f t continuously with t h e degree of isomerization. The e v o l u t i o n of t h r e e of them : t h e couples of r e f l e c t i o n s (200) and (101), (301) and (002) a s well a s t h e s i n g l e (201) r e f l e c t i o n , has already been r e p o r t e d i n r e f e r e n c e ( 7 ) . Using a l l t h e r e f l e c t i o n s indexed i n t a b l e I , i t i s then p o s s i b l e t o obtain a and c l a t e r a l parameters a s a f u n c t i o n of t h e amount of t r a n s ( f i g u r e 17 assuming an orthorhonbic l a t t i c e ( 1 4 ) . Two regimes can be c l e a r l y obser- r e a :

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below 80 % t r a n s , a and c t r a n s v e r s e parameters f o l l o w , within experimental e r r o r s , t h e l i n e a r laws :

parameters given i n reference ( 4 ) f o r t r a n s (tRf:s(7.32 dtb8aS4 .24 A r e s p e c t i v e l y ) .

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above 80 % a s l i g h t increase o f t h e parameter a and a more r a p i d decrease o f t h e parameter c, w i t h r e s p e c t t o t h e previous l i n e a r laws

,

i s observed. I n n e a r l y completely i omerized samples ( - 98 % tgans) t h e l a t t i c e parameters found, a 7.46 c 0.02 and c

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= 4.08 i 0.02 A, are v e r y d i f f e r e n t from those o f r k f i f ~ ~ n c e ( 4 ) b u t they agree &r?fisthose r e p o r t e d i n an e a r l i e r study o f t r a n s (CH) ( 3 ) . Table

I shows t h a t i n our t r a n s (CH)

,

t h e (hO1) r e f l e c t i o n s can be separated?In p a r t i - c u l a r t h e s p l i t t i n g between (206) and (101) r e f l e c t i o n s can be observed, as i n data shown i n references ( 5 ) and ( 9 ) . Such an e f f e c t cannot be obtained w i t h l a t t i c e pa- rameters o f reference ( 4 ) . F i n a l l y , t h e u n i t c e l l t h a t we found i s more compact than t h a t o f reference ( 4 ) . Conditions under which two d i f f e r e n t c r y s t a l l i n e forms o f t r a n s (CH) can be obtained need f u r t h e r s t u d i e s , due t o t h e l a r g e number o f expe- r i m e n t a l a#d t h e o r e t i c a l works devoted t o t r a n s (CH),.

The (011) and (211) r e f l e c t i o n s o f c i s (CH) are no longer observablc f o r samples annealed f o r more than 3 hours a t 100°C (= $0 % t r a n s ) . A s i m i l a r observation can be done w i t h t h e (020) i n c h a i n r e f l e c t i o n . W i t h i n experimental e r r o r , t h e c i s chain parameter b (4.36

+

0.02

a)

deduced from t h e p o s i t i o n o f these r e f l e c t i o n s , does n o t change w i t h x. Prelimary experilrents performed on r o l l e d samples seem t o show t h a t the (020) r e f l e c t i o n disappears by broadening toward t h e l a r 9 e Eragg angles. T h i s a i g h t i n d i c a t e t h a t i n chain Bragg r e f l e c t i o n s t r a n s f o r m i n t o d i f f u s e sheets perpendicular t o t h e c h a i n a x i s , as expected by a l o s s o f l a t e r a l s o r r e l a t i o n s bet- ween c i s units.The sharp (020) r e f l e c t i o n o f t r a n s (CH) a t 1.23 A begins t o be observed f o r samples annealed f o r more than 1 day a t 1 0 0 ~ ~ ( = 80 % t r a n s ) . I t i s i n t e r e s t i n g t o remark t h a t t h i s r e f l e c t i o n i s observed a t about t h e same value of x

a t which t h e a and c l a t e r a l parameters present a d i f f e r e n t law o f e v o l u t i o n as a f u n c t i o n o f t h e t r a n s c o n t e n t ( f i g u r e 1 ) . The vanishing o f i n c h a i n r e f l e c t i o n s , f o r mixed isomer composition, ay be due $0 t h e l a r g e d i f f e r e n c e between t h e c i s and t r a n s p e r i o d i c i t i e s (4.38

1

and 4.92 A f o r 4 CH u n i t s . r e s p e c t i v e l y ) : f o r a p o l y - mer chain composed o f random segments o f c i s and t r a n s , t h e r e i s no long range cohe- rence along t h e i n d i v i d u a l chain ( d i s o r d e r o f 2nd k i n d according t o G u i n i e r ( 1 0 ) ) .

I V

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CONCLUSION

X-ray r e s u l t s obtained i n t h i s study show t h a t t h e thermal i s o m e r i z a t i o n process i n (CH) i s homogeneous, w i t h o u t c l u s t e r i n g o f t r a n s i n a c i s m a t r i x . From t h e s t r u c - t u r a ? p o i n t o f view, t h i s r e c a l l t h e homogeneous p o l y m e r i z a t i o n o f diacetylenes (11) (12). A continuous change o f the transverse parameters a and c, d e f i n i n g an average s t r u c t u r e , i s observed w i t h t h e degree o f i s o m e r i z a t i o n (13). However, c o n t r a r y t o diacetylene, a commun i c chain p e r i o d i c i t y b cannot be obtained f o r mixed c r y s t a l , due t o t h e very d i f f e r e n t c i s and t r a n s p e r i o d i c i t i e s . Study o f t h e d i s t r i b u t i o n of c i s and t r a n s u n i t s along b, w i t h t h e degree o f i s o m e r i z a t i o n , needs f u r t h e r works. S t r u c t u r a l r e s u l t s demonstrate t h a t i n d i v i d u a l polyacetylene chains a r e r e s t r a i n e d w i t h i n an o v e r a l l l a t t i c e s t r u c t u r e . T h i s must be taken i n t o account i n any model of

i s o m e r i z a t i o n

.

For example, i n order t o keep t h e d i r e c t i o n o f t h e chain, a t l e a s t two simultaneous r o t a t i o n o f bonds must occur, which i n p l i e s t h a t t h e i s o ~ e r i z a t i o n k i n e t i c s a r e a t l e a s t o f second order. T h i s makes a s u b s t a n t i a l d i f f e r e n c e w i t h dia- cetylenes, which show lSt order p o l y m e r i z a t i o n k i n e t i c s (11)

JOURNAL DE PHYSIQUE CIS

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4.O0

25

50

75

100

O/o

TRANS

FIGURE 1 V a r i a t i o n o f t h e t r a n s v e r s e parameters a and c as a f u n c t i o n o f i s o m e r i z a t i o n . TRANS

Table I Some d s p a c i n g between r e f l e c t i o n planes c o n t a i n i n q t h e c h a i n a x i s b, f o r

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