The Structures of Cellulose - American Chemical Society

Chapter 9

Crystalline Alkali-Cellulose Complexes as Intermediates During Mercerization 1

2

Anatole Sarko, Hisao Nishimura , and Takeshi Okano

Downloaded by PENNSYLVANIA STATE UNIV on February 14, 2016 | http://pubs.acs.org Publication Date: June 22, 1987 | doi: 10.1021/bk-1987-0340.ch009

Department of Chemistry and Cellulose Research Institute, College of Environmental Science and Forestry, State University of New York, Syracuse, NY 13210

During a controlled mercerization of ramie cellulose, the cellulose I crystal structure i s irreversibly converted to cellulose II through several crystalline alkali-cellulose complexes. The crystal structures of three of the complexes -- Na-celluloses I, IIB, and IV -- are providing information on the characteristics of the interactions between cellulose and the Na ions, on the forces operating in the formation of these structures, and on the likely mechanism of the conversion. Although the formation of secondary bonds between Na ions and the hydroxyl groups of cellulose must be an important driving force in the formation of crystalline complexes, the hydrophobic attractions between cellulose chains appear to be at least as important. The transformation of the parallel-chain structure of cellulose I to an antiparallel one takes place already during the i n i t i a l conversion step, from cellulose I to Na-cellulose I. +

+

I t was o b s e r v e d i n e a r l i e r s t u d i e s o f c o n t r o l l e d alkali-mercerization of ramie c e l l u l o s e t h a t t h e c r y s t a l s t r u c t u r e of n a t i v e c e l l u l o s e i s t r a n s f o r m e d t o c e l l u l o s e I I through a s e r i e s o f c r y s t a l l i n e a l k a l i c e l l u l o s e complexes (1,2). The r e l a t i o n s h i p s between t h e s e "Nac e l l u l o s e s " and t h e i r pathways o f t r a n s f o r m a t i o n a r e i l l u s t r a t e d i n F i g . 1. I t has f u r t h e r been o b s e r v e d t h a t a l l o f t h e t r a n s f o r m a t i o n s are c r y s t a l - t o - c r y s t a l phase changes, not i n v o l v i n g intermediate amorphous phases. A l l o f t h e e x p e r i m e n t a l e v i d e n c e has s u g g e s t e d

1

Current address: Research Center, Daicel Chemical Industries, Ltd., Himeji, Japan Current address: Department of Forest Products, Faculty of Agriculture, University of Tokyo, Bunkyu, Tokyo, Japan

2

0097-6156/87/0340-0169$06.00/0 © 1987 American Chemical Society

In The Structures of Cellulose; Atalla, Rajai H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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THE STRUCTURES OF CELLULOSE


t h a t the f i r s t c o n v e r s i o n s t e p — from c e l l u l o s e I to N a - c e l l u l o s e I — i s a p p a r e n t l y the s t e p i n which a t r a n s f o r m a t i o n o f the p a r a l l e l c h a i n p o l a r i t y to the a n t i p a r a l l e l one takes p l a c e . As shown by the x - r a y d i f f r a c t i o n diagrams r e p r o d u c e d i n F i g . 2, the N a - c e l l u l o s e s e x h i b i t a r e l a t i v e l y h i g h degree of c r y s t a l l i n i t y and excellent crystalline orientation. In view of t h i s , further d e l i n e a t i o n of the t r a n s f o r m a t i o n s and the mechanism of m e r c e r i z a t i o n were attempted through the c r y s t a l s t r u c t u r e a n a l y s i s of t h r e e of the complexes: N a - c e l l u l o s e s I, I I B , and IV. A l l a n a l y s e s have now been n e a r l y completed, and a p r e l i m i n a r y a c c o u n t o f the r e s u l t s i s g i v e n below. The d e t a i l e d d e s c r i p t i o n s of the c r y s t a l s t r u c t u r e s w i l l be p u b l i s h e d s e p a r a t e l y a f t e r the c o m p l e t i o n o f the s t u d i e s . Experimental The methods of sample p r e p a r a t i o n , the c h a r a c t e r i s t i c s and the p r o b a b l e composition of a l l of the complexes, and the p r o c e d u r e s f o r obtaining x-ray fiber diffraction diagrams have been p r e v i o u s l y described i n d e t a i l (1,2). The p r o c e d u r e s of c r y s t a l s t r u c t u r e a n a l y s i s f o l l o w e d i n t h e s e s t u d i e s a r e i d e n t i c a l to t h o s e used i n p r e v i ous a n a l y s e s c o n c e r n e d w i t h the s t r u c t u r e s of c e l l u l o s e s and other p o l y s a c c h a r i d e s ( c f . , i n p a r t i c u l a r , r e f s . 3-5). In a l l c a s e s , b o t h stereochemical and c r y s t a l l o g r a p h i c s t r u c t u r e r e f i n e m e n t s were c a r r i e d out i n p a r a l l e l . The r e f i n e m e n t of b o t h the c h a i n c o n f o r m a t i o n and the c h a i n p a c k i n g were c o n d u c t e d w i t h c o m p l e t e l y f l e x i b l e c h a i n models, u s i n g c o m p u t a t i o n a l p r o c e d u r e s t h a t a l l o w any d e s i r e d s t r u c t u r a l parameter to be made a r e f i n a b l e v a r i a b l e ( 3 ) . The p o s i t i o n s of the s o l v e n t and the c o m p l e x i n g m o l e c u l e s i n the u n i t c e l l were e x p l i c i t l y c o n s i d e r e d , whenever w a r r a n t e d ( 5 ) . F u r t h e r d e t a i l s of the a n a l y s i s and the r e f i n e m e n t p r o c e d u r e s w i l l be g i v e n i n r e p o r t s d e a l i n g w i t h the i n d i v i d u a l c r y s t a l s t r u c t u r e s . Results Na-cellulose I. The structure of the Na-cellulose I complex, a l t h o u g h not as c r y s t a l l i n e as t h a t of c e l l u l o s e I, o b v i o u s l y shows an e q u a l l y good f i b r o u s o r i e n t a t i o n ( c f . F i g . 2A). The crystal s t r u c t u r e i s d e s c r i b e d by a l a r g e , f o u r - c h a i n u n i t c e l l , shown i n F i g . 3. I t c o n t a i n s 8 Na and 0H~ p a i r s of i o n s and p r o b a b l y 16 mole c u l e s of w a t e r . The c h a i n c o n f o r m a t i o n i s marked by f e a t u r e s common to a l l c r y s t a l l i n e c e l l u l o s e polymorphs: an a p p r o x i m a t e l y 10.3 Â f i b e r r e p e a t , a r i b b o n - l i k e , t w o f o l d h e l i c a l m o l e c u l a r shape, and the f a m i l i a r 0 ( 3 ) — 0 ( 5 ' ) and 0 ( 6 ) — 0 ( 2 ' ) i n t r a m o l e c u l a r hydrogen bonds. The c h a r a c t e r i s t i c s of the c h a i n p a c k i n g are i n accord with this c h a i n c o n f o r m a t i o n , showing a s t a c k i n g i n t o s h e e t s a l o n g two d i r e c tions. The p r e s e n c e o f NaOH and water i n the c r y s t a l s t r u c t u r e , however, o b v i o u s l y c o n t r i b u t e s to c o n s i d e r a b l e d i f f e r e n c e s between the s t r u c t u r e s o f N a - c e l l u l o s e I and c e l l u l o s e I . The major d i f f e r e n c e between these two crystal structures r e s i d e s i n the c h a i n p a c k i n g p o l a r i t y . As e x p e c t e d from the c o n v e r s i o n s t u d i e s and the irreversibility of the c e l l u l o s e I to Nac e l l u l o s e I transformation, the c r y s t a l s t r u c t u r e of N a - c e l l u l o s e I i s based on a n t i p a r a l l e l c h a i n s ( c f . F i g . 3 ) . Because of the p r e s ence of N a i o n s , which a p p a r e n t l y form s e c o n d a r y bonds w i t h the e e l +


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Crystalline Alkali-Cellulose Complexes

Να-CELL. Ill

Γ

Na-CELL Na-CELL. I I CELLULOSE IV

CELLULOSEMNo-CELL

I


NaOH

*\l Na-CELL. IIB NaOH Wash

Dry

F i g . 1. T r a n s f o r m a t i o n pathways between c e l l u l o s e and Nacellulose crystal structures. (Reproduced w i t h p e r m i s s i o n from réf. 1. C o p y r i g h t 1986 John W i l e y & Sons, I n c . )

F i g . 2. X-ray f i b e r d i f f r a c t i o n diagrams o f : (A) N a - c e l l u l o s e I , (B) N a - c e l l u l o s e I I B , and (C) N a - c e l l u l o s e IV. (Fiber axis i s vertical).

Να-Cell I F i g . 3. The u n i t c e l l o f N a - c e l l u l o s e I i n x-y p r o j e c t i o n : a_ = 8.83, b = 25.28, c ( f i b e r a x i s ) = 10.29 %. The c e l l u l o s e c h a i n s a r e shown i n o u t l i n e o n l y , and f i l l e d c i r c l e s i n d i c a t e the p o s i t i o n s o f Na ions. Secondary and hydrogen bonds a r e shown by dashed l i n e s . +


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l u l o s e h y d r o x y l groups, a l l o f the i n t e r c h a i n hydrogen bonds t h a t o r d i n a r i l y s t a b i l i z e the c e l l u l o s e I s t r u c t u r e have been broken. As a r e s u l t , d i s t a n c e s between c h a i n s i n the t> d i r e c t i o n o f the u n i t c e l l have i n c r e a s e d and i n t e r m o l e c u l a r hydrogen bonds a r e not p r e s ent. N o n e t h e l e s s , the c h a i n s a r e s t i l l a r r a n g e d i n s h e e t - l i k e s t r u c t u r e s , b o t h a l o n g the a_ and b dimensions of t h e u n i t c e l l . It appears t h a t t h e s e s h e e t - l i k e f o r m a t i o n s r e s u l t from the r i b b o n l i k e conformation of the c e l l u l o s e m o l e c u l e and, as d i s c u s s e d later, hydrophobic a t t r a c t i o n s . Na-cellulose IIB. When N a - c e l l u l o s e I i s a l l o w e d t o a b s o r b more NaOH, a c o n s i d e r a b l y d i f f e r e n t c r y s t a l s t r u c t u r e r e s u l t s ( c f . F i g . 4). The c h a i n c o n f o r m a t i o n d e p a r t s from 2^ symmetry and forms, instead, a threefold helix. The h e l i c e s pack a n t i p a r a l l e l i n a hexagonal f a s h i o n , with a r e l a t i v e l y l a r g e s e p a r a t i o n d i s t a n c e . The u n i t c e l l c o n t a i n s more t h a n 60% of n o n - c e l l u l o s e c o n s t i t u e n t s — NaOH and water — s u r r o u n d i n g each h e l i x w i t h a l i q u i d - l i k e s t r u c ture. The p r e s e n c e of a l a r g e number of N a ions quite l i k e l y r e s u l t s i n the f o r m a t i o n o f many s e c o n d a r y bonds between the c e l l u l o s e h y d r o x y I s and the i o n s , f o r c i n g a s c i s s i o n of the r e m a i n i n g i n t r a m o l e c u l a r hydrogen bonds t h a t a r e p r e s e n t i n the N a - c e l l u l o s e I structure. The t h r e e f o l d h e l i c e s of c e l l u l o s e a r e c h i r a l , i . e . , t h e i r l e f t and r i g h t h a n d e d c o n f o r m a t i o n s a r e not i d e n t i c a l . I t i s not y e t known whether the s t r u c t u r e o f N a - c e l l u l o s e I I B i s c h a r a c t e r i z e d by one p a r t i c u l a r h e l i x handedness, as b o t h c o n f o r m a t i o n s a r e s t a b l e and of not v e r y d i f f e r e n t c o n f o r m a t i o n a l energy. The x - r a y d i f f r a c t i o n d i a gram ( c f . F i g . 2B) i s r i c h i n d e t a i l and i t s h o u l d be p o s s i b l e t o determine the handedness o f the N a - c e l l u l o s e I I B h e l i x from a d e t a i l e d x-ray refinement. +

N a - c e l l u l o s e IV. A f t e r a l l o f the a l k a l i has been washed from the N a - c e l l u l o s e I I B complex, but p r i o r to i t s d r y i n g , an x - r a y d i f f r a c t i o n diagram v e r y s i m i l a r t o t h a t o f c e l l u l o s e I I i s o b t a i n e d ( c f . F i g . 2C). The c r y s t a l s t r u c t u r e of t h i s i n t e r m e d i a t e — N a - c e l l u l o s e IV — i s based on a two-chain, m o n o c l i n i c u n i t c e l l t h a t i s i n d e e d v e r y s i m i l a r to t h a t of c e l l u l o s e I I ( c f . F i g . 5 ) . The s i m i l a r i t i e s extend t o an a n t i p a r a l l e l c h a i n p a c k i n g and a hydrogen-bonded s h e e t s t r u c t u r e ( 6 ) ; the d i f f e r e n c e s a r i s e from the p r e s e n c e o f two water m o l e c u l e s i n the u n i t c e l l . The water m o l e c u l e s a r e s i t u a t e d i n c r y s t a l l o g r a p h i c a l l y d e f i n e d p o s i t i o n s , w i t h i n the s h e e t s composed of c o r n e r c h a i n s , i . e . , between c h a i n s o f l i k e p o l a r i t y . As a c o n s e quence, they p a r t i c i p a t e i n the hydrogen bonding l i n k i n g the c h a i n s i n the b d i r e c t i o n o f the u n i t c e l l . I n so d o i n g , they l e n g t h e n the b-axis r e l a t i v e to c e l l u l o s e I I . A l t h o u g h the o v e r a l l p a t t e r n of hydrogen bonds i n N a - c e l l u l o s e IV d i f f e r s l i t t l e from t h a t i n c e l l u l o s e I I , t h e r e a r e some s i g n i f i c a n t d i f f e r e n c e s ( 6 ) . F o r example, because the water m o l e c u l e s d i s r u p t the 0 ( 3 ) — 0 ( 6 ) i n t e r m o l e c u l a r hydrogen bonds between the c o r n e r c h a i n s , the n o r m a l l y t g c o n f o r m a t i o n of the c o r n e r c h a i n 0(6) h y d r o x y l s i s changed to gt_ i n Nac e l l u l o s e IV. T h i s e v i d e n t l y a l l o w s the f o r m a t i o n of a maximum numb e r o f hydrogen bonds, as each water m o l e c u l e t a k e s p a r t i n f o u r hydrogen bonds. The c e n t e r c h a i n s , not h a v i n g any water m o l e c u l e s p r e s e n t w i t h i n the s h e e t , r e t a i n the gt_ 0(6) c o n f o r m a t i o n s and the


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b

F i g . 4. The u n i t c e l l o f N a - c e l l u l o s e I I B i n x-y p r o j e c t i o n : a_ = b = 14.94, c ( f i b e r a x i s ) = 15.39 Α, γ = 1 2 0 ° . The u n i t c e l l i s assumed t o be f i l l e d w i t h NaOH and w a t e r . ( A l s o see c a p t i o n o f F i g . 3 ) . ( R e p r o d u c e d w i t h p e r m i s s i o n from r e f . 13. C o p y r i g h t 1985 Gordon & Breach.)

Ma-Ce11 IU F i g . 5. The u n i t c e l l o f N a - c e l l u l o s e IV i n x-y p r o j e c t i o n : a_ 9.57, b = 8.72, c ( f i b e r a x i s ) = 10.35 Α, γ = 1 2 2 ° . The p o s i t i o n s o f water m o l e c u l e s a r e i n d i c a t e d by f i l l e d c i r c l e s . (Also see c a p t i o n o f F i g . 3 ) .


174


intermolecular II.

hydrogen

bonds

that

are

characteristic

of

cellulose


Discussion From the p o i n t of view of the mechanism of m e r c e r i z a t i o n , the f e a t u r e s of t h e s e c r y s t a l s t r u c t u r e s and t h e i r i n t e r l i n k i n g transformat i o n s s u p p o r t our p r e s e n t u n d e r s t a n d i n g of the p r o c e s s . F o r example, i t i s known from p r e v i o u s s t u d i e s t h a t the c o n v e r s i o n of c e l l u l o s e I t o N a - c e l l u l o s e I b e g i n s i n amorphous r e g i o n s of the former, and p r o ceeds i n i t i a l l y by c o n v e r t i n g both such r e g i o n s as w e l l as the s m a l l c r y s t a l l i t e s (7,8). The amorphous o r p o o r l y c r y s t a l l i n e r e g i o n s o f c e l l u l o s e I a r e of the o r d e r o f 30-40 % i n l a t e r a l d i m e n s i o n s , as i n d i c a t e d by c r y s t a l l i t e s i z e measurements ( 7 ) . T h e r e f o r e , a c o n s i d e r a b l e amount of cellulose I material can be converted to Nac e l l u l o s e I b e f o r e the l a r g e r c r y s t a l l i t e s a r e a t t a c k e d . The c o n v e r s i o n thus proceeds f o r the most p a r t i n the p r e s e n c e of c r y s t a l l i t e s of c e l l u l o s e I t h a t may e x e r t a d i r e c t i n g i n f l u e n c e toward the p r o d u c t t h a t forms. The t h r e e f o l d h e l i c a l N a - c e l l u l o s e IIB i s l i k e l y to be a more s t a b l e s t r u c t u r e r e l a t i v e to N a - c e l l u l o s e I , but i t a p p a r e n t l y i s not formed i n the p r e s e n c e of u n c o n v e r t e d c e l l u l o s e I . The i n i t i a l c o n v e r s i o n to an a l k a l i - c o m p l e x e d c e l l u l o s e may, consequentl y , be c o n t r o l l e d by some f e a t u r e s of the s h e e t - o r i e n t e d crystalline celluloses. The a n t i p a r a l l e l s t r u c t u r e of N a - c e l l u l o s e I i s a l s o not surprising. I t i s now w e l l u n d e r s t o o d t h a t a c e l l u l o s e f i b e r i s composed of a l a r g e number of m i c r o f i b r i l s t h a t a r e e s s e n t i a l l y s i n g l e c r y s t a l s i n cross section. The m i c r o f i b r i l s of c e l l u l o s e I a r e p a r a l l e l - c h a i n s i n g l e c r y s t a l s whose f o r m a t i o n i s d i r e c t e d by b i o l o g i c a l processes ( 9 ) . The a g g r e g a t i o n of m i c r o f i b r i l s i n t o a f i b e r , however, i s most l i k e l y a s t a t i s t i c a l l y random p r o c e s s , r e s u l t i n g i n a f i b e r morphology t h a t i s marked by r o u g h l y e q u a l numbers of "up" and "down" pointing microfibrils. The majority of the nonc r y s t a l l i n e or amorphous r e g i o n s i n a c e l l u l o s e I f i b e r may, theref o r e , be thought o f as i n t e r f a c i a l r e g i o n s between m i c r o f i b r i l s t h a t a r e randomly p o i n t i n g i n two d i r e c t i o n s ( c f . F i g . 4 i n r e f . 2 ) . A supply of a n t i p a r a l l e l - o r i e n t e d c h a i n s i s thus r e a d i l y a v a i l a b l e , l e a d i n g to an a n t i p a r a l l e l - c h a i n c r y s t a l s t r u c t u r e w i t h l i t t l e e f f o r t i n l a t e r a l rearrangement of c h a i n s . The p r e s e n c e of hydrogen-bond b r e a k i n g NaOH i n c o n s i d e r a b l e q u a n t i t y c e r t a i n l y f a c i l i t a t e s l a t e r a l segmental m o t i o n and the r e s u l t i n g t r a n s f o r m a t i o n to N a - c e l l u l o s e I . These p r o c e s s e s and the a b o v e - d e s c r i b e d f i b e r morphology a r e schemati c a l l y i l l u s t r a t e d i n F i g . 6. Once a l l v e s t i g e s of an i n t e r c h a i n hydrogen-bonded c e l l u l o s e s t r u c t u r e have d i s a p p e a r e d and t h e NaOH s u p p l y i s s u f f i c i e n t l y l a r g e , the more s t a b l e t h r e e f o l d h e l i c a l N a - c e l l u l o s e I I B s t r u c t u r e forms q u i c k l y and e a s i l y . As r e f e r e n c e to any c o n f o r m a t i o n a l energy map of c e l l u l o s e shows ( c f . , f o r example, F i g . 2 o f r e f . 10), both l e f t - and righthanded t h r e e f o l d h e l i c a l c o n f o r m a t i o n s of an i s o l a t e d c e l l u l o s e c h a i n a r e w i t h i n the a l l o w e d r e g i o n of c e l l u l o s e c o n f o r m a t i o n s . They a r e not w i t h i n the minimum energy r e g i o n s surrounding the twofold helical c h a i n because of the absence o f intramolecular hydrogen bonds. By p r o v i d i n g a f i e l d of e l e c t r o s t a t i c a t t r a c t i o n from the surrounding Na i o n s and the p r o b a b l e f o r m a t i o n o f many s e c o n d a r y +


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Small and disordered crystals

Large and well ordered crystal Up crystal

Down crystal

Θ

Θ Θ Θ © Θ \


^

NaOH

Up crystal

Down crystal

Θ

Θ No-cellulose I

F i g . 6. P r o b a b l e c o n v e r s i o n o f t h e c e l l u l o s e I c r y s t a l s t r u c t u r e to t h a t o f N a - c e l l u l o s e I by t h e a c t i o n o f NaOH. C r y s t a l l i t e s a r e i n d i c a t e d by b o x e d - i n a r e a s and c h a i n s by c i r c l e s ; + i n d i c a t e s "up" and - i n d i c a t e s "down" c h a i n d i r e c t i o n s . The N a i o n s a r e denoted by f i l l e d c i r c l e s . (Reproduced by p e r m i s s i o n from r e f . 8. C o p y r i g h t 1987 John W i l e y & Sons, I n c . ) +

Cell I

Να-Cell I I

Μα-Cell I

Μα-Cell I U

Cell I I

Fig. 7. A comparison o f t h e u n i t c e l l s o f c e l l u l o s e s I and I I , and N a - c e l l u l o s e s I , I I B , a n d IV, drawn r o u g h l y t o s c a l e . Arrows indicate the probable d i r e c t i o n s of hydrophobic attractions. F i l l e d c i r c l e s i n d i c a t e t h e p o s i t i o n s o f N a i o n s o r water mol ecules. Secondary and hydrogen bonds a r e shown by dashed l i n e s . +


176


bonds between t h e l a t t e r and t h e h y d r o x y l groups o f each g l u c o s e r e s i d u e , such a c o n f o r m a t i o n c o u l d become a v e r y s t a b l e one- I t s s t a b i l i t y i s p r o b a b l y n o t d e c r e a s e d s i g n i f i c a n t l y by t h e l i q u i d - l i k e surroundings of the c e l l u l o s e h e l i x i n N a - c e l l u l o s e IIB r e l a t i v e to a s t r u c t u r e i n which a l l N a i o n s would be i n c r y s t a l l o g r a p h i c a l l y defined positions.


+

Removing t h e NaOH from t h e s t r u c t u r e through washing w i t h w a t e r removes t h e e n e r g y - l o w e r i n g e l e c t r o s t a t i c f i e l d . This results i n a c o n v e r s i o n o f t h e s t r u c t u r e t o t h e o n l y e n e r g y - l o w e r i n g one t h a t i s a v a i l a b l e to i t — a t w o f o l d h e l i c a l , i n t e r c h a i n hydrogen-bonded sheet s t r u c t u r e . Because c e l l u l o s e I I i s t h e most s t a b l e c e l l u l o s e polymorph ( 1 0 ) , i t i s n o t s u r p r i s i n g t h a t t h e c o n v e r s i o n p r o d u c t o f N a - c e l l u l o s e I I B approaches i t a f t e r washing. I t i s somewhat s u r p r i s i n g t h a t a h y d r a t e d s t r u c t u r e forms a t a l l , as i t i s u n s t a b l e and c o n v e r t s r e a d i l y t o c e l l u l o s e I I upon d r y i n g . N o n e t h e l e s s , i t does form and i t s s t r u c t u r a l f e a t u r e s s u g g e s t t h e p r e s e n c e o f h y d r o p h o b i c a t t r a c t i o n s t h a t may have a b e a r i n g on a l l t w o f o l d h e l i c a l c e l l u l o s e structures. F o r example, i n t e r c h a i n hydrogen bonds c o u l d be thought o f a s the s i n g l e dominant f o r c e i n t h e c r y s t a l l i z a t i o n o f c e l l u l o s e s and Na-celluloses. T h e r e f o r e , i t might be e x p e c t e d t h a t i n N a - c e l l u l o s e s I and IV t h e N a i o n s and t h e water m o l e c u l e s , r e s p e c t i v e l y , would occupy p o s i t i o n s between t h e hydrogen-bonded s h e e t s . I n s t e a d , they d i s r u p t t h e hydrogen bonds w i t h i n t h e s h e e t s , l e a v i n g inter-sheet c o n t a c t s a l o n g t h e 020 (and 110, r e s p e c t i v e l y ) d i r e c t i o n s unchanged. Because t h e r e a r e no hydrogen bonds p r e s e n t i n t h e s e p l a n e s , i t i s very probable that hydrophobic a t t r a c t i o n s operate along these d i r e c t i o n s , between t h e hydrogen-bonded s h e e t s . Comparing t h e s t r u c t u r e s of c e l l u l o s e s I and I I , and N a - c e l l u l o s e s I and I V , a s shown i n F i g . 7, r e v e a l s a common form o f s t a c k i n g o f c h a i n s i n a l l o f t h e s e s t r u c tures — s t r o n g l y suggestive o f hydrophobic a t t r a c t i o n s . Other c e l l u l o s e polymorphs, e.g., c e l l u l o s e s I I I - p I V j , and I V J J ( n o t shown h e r e ) , a l s o conform t o such c h a i n s t a c k i n g (11,12). Therefore, i t i s very probable that the aggregation of c e l l u l o s e chains i n t o v a r i o u s crystalline structures may p r i m a r i l y be governed by h y d r o p h o b i c attractive forces. The o n l y e x c e p t i o n seems t o be N a - c e l l u l o s e I I B where t h e s t r o n g i n t e r a c t i o n between c e l l u l o s e and t h e Na ions appears t o o v e r r i d e any o t h e r f o r c e s , w i t h t h e consequence t h a t t h e c e l l u l o s e c h a i n a d o p t s an u n u s u a l c o n f o r m a t i o n . +

Acknowledgment The s t u d i e s r e p o r t e d h e r e i n have been s u p p o r t e d by t h e N a t i o n a l S c i ence F o u n d a t i o n , under g r a n t s CHE7727749, CHE8107534, and PCM8320548.

Literature Cited Okano, T.; Sarko, A. J. Appl. Polym. Sci. 1984, 29, 4175. Okano, T.; Sarko, A. J. Appl. Polym. Sci. 1985, 30, 325. Zugenmaier, P.; Sarko, A. In Fiber Diffraction Methods; French, A. D.; Gardner, Κ. H., Eds.; ACS Symposium Series No. 141; American Chemical Society: Washington, DC, 1980; p 225. 4. Woodcock, C.; Sarko, A. Macromolecules 1980, 13, 1183. 5. Sarko, Α.; Biloski, A. Carbohydr. Res. 1980, 79, 11. 6. Stipanovic, A. J.; Sarko, A. Macromolecules 1976, 9, 851.

1. 2. 3.


9.

S A R K O ET

7. 8. 9. 10. 11. 12. Downloaded by PENNSYLVANIA STATE UNIV on February 14, 2016 | http://pubs.acs.org Publication Date: June 22, 1987 | doi: 10.1021/bk-1987-0340.ch009

13.

AL.


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Nishimura, H.; Sarko, A. J. Appl. Polym. Sci. 1987, 29, (in press). Nishimura, H.; Sarko, A. J. Appl. Polym. Sci. 1987, 29, (in press). Haigler, C. H.; Brown, R. M., Jr.; Benziman, M. Science 1980, 210, 903. Sarko, A. Appl. Polym. Symp. 1976, 28, 729. Sarko, Α.; Southwick, J.; Hayashi, J. Macromolecules 1976, 9, 857. Gardiner, E. S.; Sarko, A. Can. J. Chem. 1985, 63, 173. Sarko, A. In New Developments in Industrial Polysaccharides; Crescenzl, V.; Dea, I. C. M.; Stivala, S. S., Eds.; Gordon & Breach: New York, 1985; p 100.

RECEIVED March 5, 1987


The Structures of Cellulose - American Chemical Society

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