Modification of Lignin to Electrically Conducting Polymers - American

Chapter 16

Modification of Lignin to Electrically Conducting Polymers Tuula A . Kuusela , J. Johan Lindberg , Kiran Levon , and J. E. Österholm 1

1

1

2

1

Department of Wood and Polymer Chemistry, University of Helsinki, SF-00170 Helsinki, Finland Neste Oy, Research Centre, Kulloo, 06850 Finland

Downloaded by MONASH UNIV on February 28, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch016

2

Sodium lignosulfonate (NaLS) and sulfur lignin (SL) have been investigated as components in electrically conducting polymers. A thermostable and probably nontoxic polymer, sulfur lignin, has been obtained by modifying lignin-based sodium lignosulfonate through reactions with elemental sulfur in an autoclave at 473-513K. Sodium lignosulfonate and sulfur lignin are normally electrical insulators, but their conductivity can be increased for instance by compounding with graphite or doping with various electron acceptors or donors. We have investigated the combination of compounding with graphite followed by doping with bromine. The conducting properties were measured on samples ground and pressed to the size of IR-pellets. The measurements were made with standard four-point probe or two-point probe techniques. The doping was followed by infrared (IR) and electron spin resonance (ESR) spectroscopy. The percolation phenomenon in the combination of sulfur lignin and graphite was determined by measuring conductivities. The combination of sulfur lignin, graphite, and bromine was studied by measuring conductivities. Lignosulfonate, a water soluble p o l y m e r , is isolated f r o m sulfite spent l i q u o r , a b y - p r o d u c t f r o m the sulfite p u l p i n g process. It contains extraneous s u b stances, such as sugars, alcohols, terpenes, a n d sulfite or sulfate salts (1). T h e p o l y p h e n o l - l i k e m a t e r i a l is separated b y u l t r a f i l t r a t i o n t o remove these residues. T h e lignosulfonate contains such f u n c t i o n a l groups as m e t h o x y l , c a r b o n y l , phenolic a n d a l i p h a t i c h y d r o x y l , a n d sulfonic a c i d groups. T h e m e t h o x y l g r o u p is the m o s t characteristic g r o u p for a l l l i g n i n s . Since s u l fonate groups i n lignosulfonate macromolecules are i o n i z e d i n n e u t r a l sol u t i o n , lignosulfonates are a n i o n i c p o l y electrolytes (2). Lignosulfonates are 0097"6156/89A)397-0219$06.00/0 © 1989 American Chemical Society

In Lignin; Glasser, Wolfgang G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

LIGNIN: PROPERTIES AND MATERIALS

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n o n - l i n e a r , h i g h l y b r a n c h e d p o l y m e r s w i t h sulfonate groups a l o n g the b a c k bone as s h o w n i n F i g u r e 1. W e have investigated u l t r a f i l t r a t e d s o d i u m lignosulfonate w h i c h is a three d i m e n s i o n a l p o l y e l e c t r o l y t i c m a c r o m o l e c u l e . R a u m a - R e p o l a C o r p o r a t i o n , F i n l a n d , p r o d u c e d u l t r a f i l t r a t e d s o d i u m lignosulfonate i n 9 5 % p u rity, w h i c h c o n t a i n e d o n l y 5% i m p u r i t i e s , m o s t l y sugars. It was f o u n d (3) t h a t the b l a c k r e s i d u a l m a t e r i a l o n the s t r a i n e r of the sulfite digester was a p o l y m e r i c s u l f u r - c o n t a i n i n g substance. T h i s led us to investigate the reaction between lignosulfonate a n d s u l f u r , s i m u l a t i n g sulfite p u l p i n g process c o n d i t i o n s . U n d e r these c o n d i t i o n s , a s i m i l a r p o l y m e r i c m a t e r i a l was f o r m e d (3-5). Downloaded by MONASH UNIV on February 28, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch016

Preparation of Sulfur Lignin T h e synthesis of sulfur l i g n i n is r e l a t i v e l y s i m p l e , a n d o n a large scale i t s p r e p a r a t i o n becomes less expensive t h a n the p r e p a r a t i o n of s y n t h e t i c p o l y mers c u r r e n t l y used i n the semiconductor i n d u s t r y . T h e t h e r m a l d e g r a d a t i o n of l i g n i n occurs over a wide t e m p e r a t u r e range, f r o m about 420-773 K . Several p r a c t i c a l ways exist for the p r e p a r a t i o n of sulfur l i g n i n . These are s u m m a r i z e d i n T a b l e I. T a b l e I. Some of the methods to prepare sulfur l i g n i n at the t e m p e r a t u r e of 473-513 Κ S o d i u m lignosulfonate w i t h :

1 2 3 4

Ν,Ν-dimethyl acetamide a n d sulfur W a t e r a n d sulfur i n autoclave W a t e r a n d s o d i u m sulfide S o d i u m c a r b o n a t e , water, a n d sulfur

W e have used the first m e t h o d a n d got a raw m a t e r i a l , w h i c h was treated w i t h acetone a n d d i e t h y l ether i n a Soxhlet e x t r a c t o r for several h o u r s . A f t e r d r y i n g i n v a c u u m we d e t e r m i n e d some details of i t s s t r u c ture. T h e r e a c t i o n of s o d i u m lignosulfonate w i t h sulfur i n N , N - d i m e t h y l acetamide cannot be fully described. W e do not k n o w a l l the reactions w h i c h are proceeding s i m u l t a n e o u s l y i n the autoclave d u r i n g the process. A n y h o w , differential s c a n n i n g c a l o r i m e t r y ( D S C ) measurements of some m o d e l c o m p o u n d s have revealed t h a t sulfur p a r t i a l l y s u b l i m e s , a n d t h a t Ν,Ν-dimethyl acetamide evaporates. In a d d i t i o n , the r e a c t i o n is slow, a n d e n d o t h e r m i c r e a c t i o n is observed at 363-423 Κ i n d i c a t i n g the r e a c t i o n of sulfur w i t h the a l i p h a t i c chains. T h e e x o t h e r m i c r e a c t i o n at higher t e m peratures indicates the f o r m a t i o n of a r o m a t i c c a r b o n - s u l f u r linkages w h i l e s i m u l t a n e o u s reactions like desulfonation, d e m e t h y l a t i o n a n d c o n d e n s a t i o n m i g h t also o c c u r ( 6 , 7 ) . Samples of the final synthesis p r o d u c t s were investigated u s i n g the techniques l i s t e d i n T a b l e II. Synthesis c o n d i t i o n s a n d a n a l y t i c a l results ( T a b l e II) are consistent w i t h the h y p o t h e t i c a l s t r u c t u r e s of sulfur l i g n i n s h o w n i n F i g u r e 2. S u l fur l i g n i n is a n a r o m a t i c thioether a n d i t resembles p o l y p h e n y l e n e sulfide derivatives (8).


KUUSELA ET AL.

Modification to Electrically Conducting Polymers


16.

F i g u r e 2. T h e r e a c t i o n a n d possible s t r u c t u r e s of sulfur l i g n i n .


221


222

T a b l e I I . A n a l y s i s of the s t r u c t u r e of sulfur l i g n i n Methods

Results

X-ray

S u l f u r l i g n i n is a t o t a l l y a m o r p h o u s m a t e r i a l A m o r p h o u s s t r u c t u r e , p a r t i c l e sizes are between 1000-40000 n m S u l f u r content i n samples f r o m different e x p e r i m e n t s varies between 10-26 m a s s - % S t a b l e sulfur linkages between the a r o m a t i c u n i t s are f o r m e d

SEM-microscopy C o m b u s t i o n gases


S o l i d state N M R IR E l e m e n t a l analysis Conductivity ESR-spectroscopy

M a t e r i a l is n e a r l y completely desulfonated M a t e r i a l is d e m e t h o x y l a t e d a n d the p r o p y l group is s p l i t off. P o l y m e r is b r a n c h e d . S u l f u r l i g n i n can be m a d e c o n d u c t i n g b y compounding and by doping Paramagnetic structures and radicals are f o u n d

Conducting Polymers C o n d u c t i n g p o l y m e r i c m a t e r i a l s have become a n area of interest t o a c a demic a n d i n d u s t r i a l research groups, not o n l y due to their i n t e r e s t i n g properties, b u t also due to their technologically p r o m i s i n g future i n a w i d e range of a p p l i c a t i o n s . S u c h a p p l i c a t i o n s i n c l u d e , for instance, o r g a n i c b a t teries, p h o t o v o l t a i c devices, s w i t c h i n g a n d m e m o r y devices, e l e c t r o m a g netic interference s h i e l d i n g , a n d m a n y others w h i c h require c o n d u c t i n g or s e m i c o n d u c t i n g m a t e r i a l s (9). T h e r e is every prospect t h a t a m o r p h o u s , t h r e e - d i m e n s i o n a l m a t e r i a l s h o l d promise for future i n v e s t i g a t i o n s .

Future Prospects for Modified Lignin Materials S u l f u r l i g n i n ( S L ) is seemingly n o n t o x i c a n d v e r y stable i n a i r . P u r e s u l f u r l i g n i n is almost a n i n s u l a t o r . Its i n t r i n s i c c o n d u c t i v i t y varies between 2 a n d 200 p S / c m ( 2 - 2 0 0 x l 0 ~ S / c m ) d e p e n d i n g o n the m o i s t u r e content (10). T h e d r y m a t e r i a l has a c o n d u c t i v i t y of 2 p S / c m . S u l f u r l i g n i n seems to be a good adsorbent, too. W e have d e t e r m i n e d the m o i s t u r e content of o u r m a t e r i a l s , a n d they v a r y between 2 a n d 6 m a s s - % . D r i e d s t a r t i n g m a t e r i a l s were not doped e q u a l l y w e l l as those w i t h o r i g i n a l a t m o s p h e r i c m o i s t u r e . C o n d u c t i v i t i e s of sulfur l i g n i n ( S L ) , s o d i u m lignosulfonate ( N a L S ) , a n d some c o m m o n m a t e r i a l s are represented i n T a b l e I I I . P o l y a r o m a t i c s o d i u m lignosulfonate a n d sulfur l i g n i n resin c a n be m a d e c o n d u c t i n g b y d o p i n g . T h e c o n d u c t i v i t y increases b y several decades t h r o u g h d o p i n g w i t h electron acceptors a n d donors as s h o w n i n T a b l e I V . T h e m a j o r i t y of the c o m m o n p o l y a r o m a t i c c o m p o u n d s are donors. F o r these c o m p o u n d s d o p i n g w i t h acceptors is facile. W e have doped sulfur l i g n i n a n d s o d i u m lignosulfonate i n vapor-phase (iodine, b r o m i n e , a n d a m m o n i a ) a n d i n l i q u i d phase ( s o d i u m a n d f e r r i c h l o r i d e ) . T h e c o n d u c t i v i t y m a i n l y depends o n the n a t u r e of the d o p a n t i o n a n d the d o p i n g degree. D o p i n g can be m o n i t o r e d b y IR-spectroscopy. T h e intensities of the peaks decrease, a n d the fine s t r u c t u r e vanishes, w h e n the 1 2


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223

Table I I I . C o n d u c t i v i t i e s o f SL, NaLS, and some common m a t e r i a l s (S/cm) METALS

SEMICONDUCTORS ORGANIC


INORGANIC ~~6 4 exponent 10 Ag Cu

2

0

-2

-4

-6

INSULATORS

-8

ITO

IÏ2

7f6

(relative scale)

Bi

Ge Si

polyphenylenes SL

graphite doped SL doped NaLS

teflon quartz polystyrene polyethene anthracene polyethylene

Table I V . Conductivities of sodium lignosulfonate ( N a L S ) and sulfur lignin ( S L ) w i t h some dopants Material NaLS NaLS NaLS NaLS NaLS SL SL SL SL SL SL

Doping with

Conductivity

undoped iodine bromine ammonia sodium undoped iodine bromine ammonia ferrichloride sodium

140.0 p S / c m 2.0 u S / c m 0.3 m S / c m 1.0 m S / c m 40.1 m S / c m 120.0 p S / c m 280.0 u S / c m 340.0 u S / c m 5.4 m S / c m 6.0 m S / c m 10.0 m S / c m

Maximum Doping _ 52.0 49.3 32.1 21.0

mass-% mass-% mass-% mass-%

59.4 66.0 49.8 32.0 21.9

mass-% mass-% mass-% mass-% mass-%

-

Stability in A i r stable stable stable unstable unstable stable stable stable unstable stable unstable


224


d o p i n g degree increases. C o n d u c t i v i t y increases as w e l l . T h e E S R - s p e c t r a also i n d i c a t e the d o p i n g degree w h i c h coincides w i t h c o n d u c t i v i t y . T h e D y s o n i a n lineshape is assumed t h r o u g h o u t , a n d A / B - r a t i o s are m e a s u r e d ( T a b l e V ) (10-12). T a b l e V . A / B - r a t i o s of sulfur l i g n i n d o p e d w i t h b r o m i n e


Doping % 0 14 25 41 66

A/B

Conductivity

1.1 1.02 0.93 0.90 0.85

120.0 p S / c m 2.3 u S / c m 21.2 u S / c m 428.9 u S / c m 6.2 m S / c m

T h e effect of d o p i n g sulfur l i g n i n w i t h b r o m i n e o n the I R s p e c t r a is s h o w n i n F i g u r e 3 a a n d 3b.

T h e Compounding of Sulfur Lignin with Graphite W e have m a d e samples f r o m sulfur l i g n i n w i t h i n c r e a s i n g a m o u n t s of g r a p h i t e ( M e r c k , p a r t i c l e size < 50000 n m ) . T h i s s y s t e m produces increas i n g c o n d u c t i v i t y at a c e r t a i n t h r e s h o l d g r a p h i t e content. T h e c o n d u c t i o n of e l e c t r i c i t y t h r o u g h a sulfur l i g n i n - g r a p h i t e s y s t e m , where o n l y g r a p h i t e is c o n d u c t i n g , depends o n the c o n c e n t r a t i o n of the g r a p h i t e phase. A c u r rent m a y o n l y flow i f a g r a p h i t e p a t h exists t h r o u g h the s o l i d sulfur l i g n i n . W h e n a p a t h is f o r m e d , the c o n d u c t i v i t y of the s a m p l e shows a large i n crease. T h i s c r i t i c a l c o n c e n t r a t i o n is called the percolation threshold. The p e r c o l a t i o n occurs at a percentage value of about 5 m a s s - % g r a p h i t e i n sulfur l i g n i n as s h o w n i n F i g u r e 4 a . T h e curve of these samples has the c o m m o n p e r c o l a t i o n shape.

T h e Compounding with Bromine A new series of sulfur l i g n i n - g r a p h i t e samples w i t h k n o w n g r a p h i t e content were treated w i t h b r o m i n e i n the vapor phase ( d o p e d ) . A f t e r m e a s u r i n g the c o n d u c t i v i t i e s , we recorded the curve s h o w n i n F i g u r e 4b ( K u u s e l a , Τ . Α . , T h e U n i v e r s i t y of H e l s i n k i , F i n l a n d , to be p u b l i s h e d ) . It was a p r e v i o u s l y discovered p h e n o m e n o n t h a t g r a p h i t e c a n b i n d b r o m i n e i n t o i t s a r o m a t i c s t r u c t u r e (13). B o t h of these c o m p o n e n t s , g r a p h i t e ( c o n d u c t i v i t y 0.77 S / c m ) a n d sulfur l i g n i n ( c o n d u c t i v i t y 120 p S / c m ï , can be d o p e d s i m u l t a n e o u s l y w i t h b r o m i n e ( c o n d u c t i v i t y < 10 n S / c m ) . A f t e r b r o m i n e d o p i n g , measured c o n d u c t i v i t i e s have increased n o ticeably, a n d t h i s is a t t r i b u t e d to the change i n the percentage of g r a p h i t e i n the t o t a l s y s t e m . T h e c o n d u c t i v i t y increases up to 23 S / c m ( w h i c h is a n i n t e r c a l a t i o n c o m p o u n d of g r a p h i t e w i t h b r o m i n e w i t h o u t sulfur l i g n i n ) , as s h o w n i n F i g u r e 4 b . T h e difference between lower a n d higher c o n d u c t i v i t y values is s m a l l e r t h a n i n the sulfur l i g n i n - g r a p h i t e s y s t e m (14). B r o m i n e content is a b o u t 50 m a s s - % i n a l l sulfur l i g n i n - g r a p h i t e - b r o m i n e samples. P u r e g r a p h i t e c a n b i n d about 9 m a s s - % b r o m i n e i n t o i t s s t r u c t u r e .


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KUUSELA ET AL.


2 Ο


α. ι

ω xf /-->. α>

α

ε ο Β Ο

α.

& α ο

3 UH

Ο


225


226

G r a p h i t e s w i t h larger surface areas or greater porosities have a dis t i n c t l y lower p e r c o l a t i o n t h r e s h o l d . It is assumed t h a t the c o n d u c t i v i t y of a c o m p o u n d depends u p o n the s t r u c t u r e d agglomerates being sufficiently close to each other, or i n direct contact above the p e r c o l a t i o n p o i n t , a n d o n the continuous current p a t h w a y s created thereby (14-15).


Mechanism of Conductivity W h e n s o d i u m lignosulfonate or sulfur l i g n i n are c o m p o u n d e d , for instance, w i t h iodine or b r o m i n e , complexes supposedly f o r m (16-17). These systems are conductors w i t h m i x e d i o n i c a n d electronic n a t u r e . P r e s u m a b l y they are charge transfer complexes, since the electronic c o n d u c t i v i t y p r e d o m i n a t e s (18-19). These c o m p o u n d e d m a t e r i a l s f o r m charge transfer s t r u c t u r e s (20). W a t e r is supposed to i n t r o d u c e i o n i c c o n d u c t i v i t y to the s y s t e m . I m p u r i t i e s affect c o n d u c t i v i t y , too (21). I n any case, the m a i n m o d e l s of c o n d u c t i v i t y are p r o b a b l y based o n the band model a n d / o r the hopping model.

Lg(S/cm)

1

0

I

2

1

4

1

1

6

8

•

10

ι

12 Mass-?i

14

F i g u r e 4. C o m p o u n d i n g sulfur l i g n i n w i t h g r a p h i t e (a) a n d w i t h g r a p h i t e a n d b r o m i n e (b).


16.

KUUSELAETAL.

Modification to Electrically Conducting Polymers 227

Conclusions T h e r e has l o n g been a n interest i n u n d e r s t a n d i n g t h e m e c h a n i s m o f c o n d u c t i v i t y . I n g r a p h i t e , t h e electrons are t h e m a i n current c a r r i e r s , b u t i n t e r c a l a t i o n w i t h b r o m i n e increases the c o n d u c t i v i t y o f t h i s c o m b i n a t i o n . M o d i f i e d l i g n i n m a t e r i a l s m a y serve as components i n ( s e m i ) c o n d u c t i n g systems. A l l heterogeneous a n d a m o r p h o u s p o l y m e r i c complexes are n o w o f interest for possible future e m p l o y m e n t i n e l e c t r i c a l l y c o n d u c t i n g m a t e r i a l s .


Literature C i t e d 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

21.

Hrutfiord, B. F.; McCarthy, J. L . Tappi 1964, 47, 381. Rezanowich, Α.; Goring, D. A . I. J. Colloid Sci. 1960, 15, 452. Turunen, J. Ph.D. Thesis, University of Helsinki, Helsinki, 1963. Lindberg, J. J.; Turunen, J.; Hortling, B. Finnish Patent No. 50 998, 1982. Lindberg, J. J.; Turunen, J.; Hortling, B. U.S. Patent No. 410 711, 1982. Levon, K . Ph.D. Thesis, University of Tokyo and University of Helsinki, Tokyo, 1986. Hortling, B. Ph.D. Thesis, University of Helsinki, Helsinki, 1979. Laakso, J.; Hortling, B.; Levon, K.; Lindberg, J. J. Polymer Bull. 1985, 14, 138. Aldssi, M . Polym.-Plast. Technol. Eng. 1987, 26(1), 45. Kaila, E . ; Kinanen, Α.; Levon, K.; Turunen, J.; Osterholm, J.-E.; Lind berg, J. J. U.S. Patent No. 4 610 809, 1986. Lindberg, J. J.; Levon, K.; Kuusela, T . A . Acta Polymerica 1988, 39(1/2), 47. Levon, K.; Kinanen, Α.; Lindberg, J. J. Polym. Bull. 1986, 16, 433. Kagan, Η. B. Pure and Appl. Chem. 1976, 46, 177. Wessling, B.; Volk, H . Synthetic Metals 1986, 16, 127. Wessling, B. Macromol. Chem. 1984, 185, 1265. Neoh, K . G . ; Tan, T . C . ; Kang, Ε. T . Polymer 1988, 29, 553. Kang, E . T . ; Tan, T . C . ; Neoh, K. G . Eur. Polym. J. 1988, 24, 371. Ratner, Μ. Α.; Schriver, D. F . Chem. Rev. 1988, 88, 109. Hardy, L . C . ; Schriver, D. F . J. Am. Chem. Soc. 1985, 107, 3823. Stranks, D. R.; Heffernan, M . L . ; Lee Dow, K . C . ; McTigue, P. T . ; Withers, G . R. A . In Chemistry: A Structural View; Cambridge Uni versity Press, 1970; Chapter 25, p. 422. Jernigan, J. T . ; Chidsey, C . E . D.; Murray, R. W . J. Am. Chem. Soc. 1985, 107, 2824.

RECEIVED May 29,1989


Modification of Lignin to Electrically Conducting Polymers - American

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