The Structures of Cellulose - American Chemical Society

Chapter 11

The Structures of Cellulose Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 06/22/16. For personal use only.

Polymorphic and Morphological Aspects of Recrystallized Cellulose as a Function of Molecular Weight I. Quenin and H. Chanzy Centre de Recherches sur les Macromolécules Végétales, Centre National de la Recherche Scientifique, B.P. 68, 38402 Saint Martin d'Hères Cedex, France

The polymorphism and morphology of cellulose precipitated from solutions in amine oxide by the slow diffusion of water vapors, was investigated, as functions of the temperature of recrystallization and the degree of polymerization(DP) of the material to be recrystallized. At temperatures around 90°C, low DP cellulose crystallized almost exclusively as cellulose IV , whereas higher DP material was found in the form of cellulose II. Substantial differences were also found in the morphologies of the various samples: with cellulose II, rod-like crystals were obtained with low DP material while a crystalline f i b r i l l a r gel precipitated when high DP samples were recrystallized. In all cases, cellulose IV was obtained as a granular precipitate. II

II

The c r y s t a l l i z a t i o n behaviour o f polysaccharides i s influenced n o t o n l y by t h e i r c h a i n c o n f o r m a t i o n b u t a l s o by t h e m u l t i p l e p o s s i b i l i t i e s o f i n t e r and i n t r a m o l e c u l a r hydrogen bonding ( 1 , 2 ) . T h i s m u l t i p l i c i t y , i n p a r t i c u l a r , e x p l a i n s why s e v e r a l p o l y m o r p h i c forms c a n be o b t a i n e d when d i f f e r e n t c r y s t a l l i z a t i o n c o n d i t i o n s a r e s e l e c t e d f o r a g i v e n specimen. A s u r v e y o f t h e v a r i o u s p a r a m e t e r s which have a r o l e i n d i r e c t i n g t h e c r y s t a l l i z a t i o n o f a g i v e n p o l y s a c c h a r i d e toward one o r a n o t h e r polymorph, p r e s e n t s t h e temperature o f c r y s t a l l i z a t i o n as b e i n g t h e most i n f l u e n t i a l : p o l y s a c c h a r i d e s such as c e l l u l o s e ( 3 ) , mannan ( 4 ) , d e x t r a n ( 5 , 6 ) , e t c . a r e p a r t i c u l a r l y sensitive t o changes i n c r y s t a l l i z a t i o n t e m p e r a t u r e s as t h e y y i e l d c o m p l e t e l y d i f f e r e n t c r y s t a l s a t low o r h i g h t e m p e r a t u r e . In o t h e r i n s t a n c e s , i t i s t h e s o l v e n t o f c r y s t a l l i z a t i o n which p l a y s a d e c i s i v e r o l e i n o r i e n t i n g t h e c r y s t a l l i z a t i o n toward one o r t h e o t h e r polymorph. Such s o l v e n t dependant c r y s t a l l i z a t i o n i s w e l l documented i n t h e c a s e o f amy l o s e (7) where minute changes i n s o l v e n t / p r e c i p i t a n t r a t i o have a d r a m a t i c e f f e c t on s h i f t i n g t h e r e c r y s t a l l i z e d amylose among t h r e e p o l y m o r p s : amylose A , Β and V.

0097-6156/87/0340-0189$06.00/0 © 1987 American Chemical Society


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THE STRUCTURES OF CELLULOSE

A f i n a l and u n e x p e c t e d polymorphism parameter i s found i n the m o l e c u l a r w e i g h t o f the sample t o be c r y s t a l l i z e d . T h i s i s i n particular described i n the case o f mannan and glucomannan p o l y m e r s ( 4 , 8 ) where c r y s t a l s o f mannan I are u s u a l l y o b t a i n e d w i t h low degree o f p o l y m e r i z a t i o n (DP) m a t e r i a l , whereas o n l y mannan I I i s found when h i g h e r DP a r e r e c r y s t a l l i z e d . The p r e s e n t s t u d y d e a l s w i t h t h e polymorphism o f c e l l u l o s e recrystallized from solution. In p a r t i c u l a r , the interest is f o c u s s e d on the v a r i o u s p a r a m e t e r s which a r e c a p a b l e o f o r i e n t i n g the c r y s t a l l i z a t i o n toward c e l l u l o s e I I o r o s l l u l o s e I V J J / s o - c a l l e d low and h i g h temperature c e l l u l o s e polymorphs. These two polymorphs have not o n l y a d i f f e r e n t c r y s t a l l i n e arrangement b u t a l s o a d i f f e r e n t morphology as c e l l u l o s e I I o c c u r s as a g e l - l i k e precipitate whereas cellulose ^ rather more granular. R e c r y s t a l l i z e d c e l l u l o s e i s of great i n d u s t r i a l i n t e r e s t . For t h i s r e a s o n , i t seems i m p o r t a n t t o know whether o t h e r p a r a m e t e r s , such as the m o l e c u l a r weight o f the c r y s t a l l i z i n g c e l l u l o s e , may also p l a y a r o l e i n d i r e c t i n g the c r y s t a l l i z a t i o n toward oe l l u l o s e I I o r c e l l u l o s e I V ^ a t a g i v e n c r y s t a l l i z a t i o n t e m p e r a t u r e . With t h i s g o a l i n mind, a s e r i e s o f c e l l u l o s e samples o f v a r i o u s m o l e c u l a r w e i g h t s were c r y s t a l l i z e d from s o l u t i o n a t d i f f e r e n t t e m p e r a t u r e s . The morphology and s t r u c t u r e o f the c r y s t a l l i n e p r e c i p i t a t e were i n v e s t i g a t e d by X-ray d i f f r a c t i o n , e l e c t r o n m i c r o s c o p y and e l e c t r o n diffraction. T

ι

ν

N

E

s

Ι

Χ

Experimental C e l l u l o s e Samples. 4 d i f f e r e n t c e l l u l o s e samples were s e l e c t e d f o r the e x p e r i m e n t s : a) b l e a c h e d E g y p t i a n Menoufi c o t t o n (DP 2000), k i n d l y p r o v i d e d by Dr. R. Hagège, I n s t i t u t T e x t i l e de F r a n c e , b) C o t t o n l i n t e r s (DP 600), c) m i c r o c r y s t a l l i n e c e l l u l o s e A v i c e l pH 101 (DP 120), d) m i c r o c r y s t a l l i n e c e l l u l o s e from r a y o n (DP 3 4 ) , a g i f t from Dr. G. Raynor, FMC c o r p . Solutions Preparation. An amine o x i d e - b a s e d c e l l u l o s e solvent which was still liquid a t room temperature was selected. As d e s c r i b e d e a r l i e r (9) , i t c o n s i s t e d i n a m i x t u r e o f 22 % N-Methyl m o r p h o l i n e N-oxide (MMNO), 65 % N-N-dimethyl e t h a n o l a m i n e N-oxide (DMEAO) and 13 % H O (W/W) . 5 mgs o f c e l l u l o s e t o which were added 5 mgs o f η-propyl gallate, a cellulose stabilizer (10) were d i s s o l v e d i n l O c c o f c e l l u l o s e s o l v e n t a t 120°C w i t h s t i r r i n g . D i s s o l u t i o n took place i n 15 m i n u t e s , f o l l o w i n g which the s o l u t i o n s were a l l o w e d t o c o o l . They were t h e n s t o r e d i n a d e s s i c a t o r . Cellulose Crystallization. F o r c r y s t a l l i z a t i o n below 100°C, the s o l u t i o n s were p o u r e d i n t o p e t r i d i s h e s and p o s i t i o n e d i n s i d e a c l o s e d v e s s e l c o n t a i n i n g an e x c e s s o f water. The v e s s e l was t h e n f i t t e d i n t o a temperature c o n t r o l l e d o i l b a t h and b r o u g h t t o t h e c r y s t a l l i z a t i o n temperature. 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 r e s u l t e d from t h e d i f f u s i o n o f water i n t o the s o l u t i o n . C r y s t a l l i z a t i o n was complete w i t h i n a few m i n u t e s a t 90°C whereas s e v e r a l days were n e c e s s a r y a t room t e m p e r a t u r e . F o r c r y s t a l l i z a t i o n between 100°C and 120°C, a s i m i l a r p r i n c i p l e was used e x c e p t t h a t a t h i c k - w a l l s e a l e d g l a s s v e s s e l was u s e d t o p r e v e n t water v a p o r from e s c a p i n g .


11.

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Aspects of Recrystallized Cellulose

191

Sample P r e p a r a t i o n . After crystallization, t h e s o l u t i o n s were c o o l e d and t h e f l o c c u l e n t c r y s t a l s were washed and r e c o v e r e d by s u c c e s s i v e c e n t r i f u g a t i o n and r e d i s p e r s i o n i n water and f i n a l l y i n ethanol. P a r t o f t h e c r y s t a l s were a l l o w e d t o d r y and were t h e n i n s e r t e d i n t o t h i n w a l l c a p i l l a r i e s f o r X-ray d i f f r a c t i o n s t u d i e s which were made u s i n g a Wahrus f l a t - f i l m camera mounted on a Siemens K r i s t a l l o f l e x X-ray g e n e r a t o r . F o r e l e c t r o n m i c r o s c o p y , d r o p s o f c r y s t a l suspended i n e t h a n o l were a l l o w e d t o d r y on c a r b o n c o a t e d e l e c t r o n m i c r o s c o p y g r i d s . The g r i d s were e i t h e r used as such f o r e l e c t r o n d i f f r a c t i o n o r a f t e r W/Ta shadowing f o r i m a g i n g . The e l e c t r o n m i c r o s c o p y and e l e c t r o n d i f f r a c t i o n e x p e r i m e n t s were p e r f o r m e d w i t h a P h i l i p s EM 400 Τ e l e c t r o n m i c r o s c o p e o p e r a t e d a t 80 KV f o r imaging and 120 KV f o r diffraction. Results When t h e v a r i o u s c r y s t a l l i n e p r e c i p i t a t e s were examined by X-ray analysis, i t became evident that the ratio of cellulose I V j j / c e l l u l o s e I I c o u l d be c o r r e l a t e d n o t o n l y w i t h t h e temperature o f c r y s t a l l i z a t i o n , b u t a l s o w i t h t h e m o l e c u l a r weight o f t h e starting material. This i s p a r t i c u l a r l y well i l l u s t r a t e d i n Figure 1 by comparing t h e X-ray d i a g r a m o f two d i f f e r e n t specimens prepared a t 90 °C. In F i g u r e IA, c o r r e s p o n d i n g t o t h e c o t t o n c e l l u l o s e s o l u t i o n , t h e p r e c i p i t a t e has a w e l l d e f i n e d d i f f r a c t i o n l i n e a t 0.72 nm ( c e l l u l o s e I I ) b u t o n l y a v e r y weak one a t 0.56 nm ( c e l l u l o s e IV ) . On t h e o t h e r hand, as seen i n F i g u r e IB, when microcrystalline oe l l u l o s e from rayon i s recrystallized under i d e n t i c a l c o n d i t i o n s , the p r e c i p i t a t e d i s p l a y s a l a r g e excess o f c e l l u l o s e I V ^ and o n l y a minor c e l l u l o s e I I component. From t h e work o f S t i p a n o v i c and Sarko on c e l l u l o s e I I (11) and t h a t o f G a r d i n e r and Sarko (12) on c e l l u l o s e IV, i t c a n be deduced t h a t t h e i n t e n s i t y o f t h e l i n e s a t 0.72nm f o r œ l l u l o s e I I i s between 1/2 t o 1/3 o f t h a t a t 0.56 nm f o r c e l l u l o s e IV . By t a k i n g t h i s i n t o a c c o u n t , an e s t i m a t e o f t h e p e r c e n t a g e o f c e l l u l o s e I I and œ l l u l o s e IV c a n be e v a l u a t e d by r e c o r d i n g a r a d i a l t r a c i n g o f t h e X-ray p a t t e r n s and comparing t h e c o r r e c t i n t e n s i t i e s o f t h e l i n e s a t 0.56nm and 0.72 nm. Such e s t i m a t e d c e l l u l o s e I I / c e l l u l o s e II P° '- ^ summarized i n T a b l e I , as a f u n c t i o n o f t h e temperature o f c r y s t a l l i z a t i o n and f o r t h e 4 i n v e s t i g a t e d samples. These r e s u l t s i n d i c a t e a c l e a r - c u t tendency f o r low DP c e l l u l o s e t o y i e l d a s u b s t a n t i a l amount o f c e l l u l o s e I V ^ even a t t e m p e r a t u r e s as low as 50°C. F o r c e l l u l o s e o f h i g h e r DP, t h i s i s n o t t h e case and, i t i s o n l y a t 90°C and above t h a t c e l l u l o s e IV c a n be detected. A t t h e u l t r a s t r u c t u r a l l e v e l , s u b s t a n t i a l d i f f e r e n c e s c a n be r e c o r d e d when comparing t h e samples o f v a r i o u s m o l e c u l a r w e i g h t . As seen i n T a b l e I , when t e m p e r a t u r e s o f c r y s t a l l i z a t i o n below 50°C were u s e d , a l l t h e samples c r y s t a l l i z e d as t h e p u r e c e l l u l o s e I I polymorph. An e x a m i n a t i o n w i t h t h e e l e c t r o n m i c r o s c o p e r e v e a l s t h a t the low DP m a t e r i a l o c c u r r e d as an assembly o f r o d - l i k e e l e m e n t s , e a c h r o d h a v i n g a l e n g t h between a m i c r o n and h a l f m i c r o n and a w i d t h o f t h e o r d e r o f 100 nm. With h i g h e r DP samples, a g e l - l i k e I V

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s :

t

o n s

a

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e

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T a b l e I . Polymorph C o m p o s i t i o n as F u n c t i o n o f C e l l u l o s e and Temperature o f C r y s t a l l i z a t i o n Temperature o f crystallization

34

DP

120

DP

600

DP

1200

22 C

1004

cell

II

100%

cell

II

100%

cell

II

100%

cell

II

50°C

90% 10%

cell cell

II IV

100%

cell

II

100%

cell

II

100%

cell

II

90°C

25% 75%

c e l l II cellIV

70% 30%

cell cell

II IV

90% 10%

cell cell

II IV

90% 10%

c e l l II cellIV

50% 50%

c e l l II celllV

e


DP

DP

120°C

20% c e l l I I 80 % c e l l l V

i i

40% 60%

cell cell

τι

II IV*

II

cell cell

40% 60%

II IV

T T

p r e c i p i t a t e c o n s i s t i n g i n e n d l e s s and narrow i n t e r t w i n e d fibrils, was o b t a i n e d . These two m o r p h o l o g i e s are shown i n F i g u r e s 2A and 2B which are p r i n t e d a t the same m a g n i f i c a t i o n f o r comparison. W i t h X-ray a n a l y s i s , i t i s not possible to d i f f e r e n t i a t e betwen the samples i n F i g u r e s 2A and 2B. T h i s i s seen i n the i n s e r t i n 2C which c o u l d c o r r e s p o n d t o e i t h e r sample. With the electron d i f f r a c t i o n t e c h n i q u e , however, a s u b s t a n t i a l d i f f e r e n c e i s found when the d i f f r a c t i o n diagrams are analyzed i n term o f their o r i e n t a t i o n w i t h r e s p e c t t o o r i e n t e d fragments o f the p r e c i p i t a t e . T h i s p o i n t i s i l l u s t r a t e d by comparing F i g u r e s 3A and 3B. In F i g u r e 3A, a s e l e c t e d assembly o f p a r a l l e l r o d - l i k e e l e m e n t s , y i e l d s an oriented electron diffractogram where the (110) r e f l e c t i o n of c e l l u l o s e I I i s a l i g n e d w i t h the axes o f the r o d s . O t h e r f e a t u r e s o f the diagram i n d i c a t e t h a t the (110) interference i s directed p e r p e n d i c u l a r w i t h r e s p e c t t o the r o d d i r e c t i o n w h i l e (020) i s a t a s l i g h t a n g l e o f f the (110) r e f l e c t i o n . Such o r i e n t a t i o n s indicate t h a t i n the r o d s , the c h a i n axes are p e r p e n d i c u l a r t o the r o d a x i s and the r o d s c o n s i s t i n e l o n g a t e d c e l l u l o s e c r y s t a l s w i t h (110) as the growth p l a n e as o b s e r v e d b e f o r e i n a c o n n e c t e d s t u d y (14). In F i g u r e 3B the o r i e n t a t i o n i s d e c i s i v e l y d i f f e r e n t as a b u n d l e o f o r i e n t e d g e l g i v e s a c e l l u l o s e I I f i b e r diagram w i t h the c e l l u l o s e c h a i n a x i s a l i g n e d w i t h the a x i s o f the s t r a n d s c o n s t i t u t i n g the oriented structure. When the samples c r y s t a l l i z e d at higher temperature are examined, the p r e s e n c e o f c e l l u l o s e IV can be v i s u a l i z e d as i t o c c u r s i n the form o f a g r a n u l a r m a t e r i a l mixed o r not w i t h the c e l l u l o s e II p r e c i p i t a t e . This i s well i l l u s t r a t e d i n Figure 4A, c o r r e s p o n d i n g t o the specimen o f d i s s o l v e d A v i c e l , r e c r y s t a l l i z e d a t 90°C. In t h i s F i g u r e , the i n s e r t e d X-ray d i a g r a m d e n o t e s the p r e s e n c e o f about 30 % o f c e l l u l o s e ' located in several a g g r e g a t e s (see a r r o w s ) , whereas the main p a r t o f the p r e c i p i t a t e i s made o f i n t e r t w i n e d r o d - l i k e œllulose II c r y s t a l s s i m i l a r to what was seen i n F i g u r e 2A. Ι

ν

ι

τ

The i n d i c e s r e f e r t o the u n i t c e l l o f c e l l u l o s e I I , as d e f i n e d Kolpak and B l a c k w e l l (13).

by


11.

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F i g u r e 1. X-ray diagram o f c e l l u l o s e c r y s t a l s r e c r y s t a l l i z e d a t 9C°C. IA from c o t t o n c e l l u l o s e (DP 2000) s o l u t i o n . IB from m i c r o c r y s t a l l i n e c e l l u l o s e (DP 34) s o l u t i o n .

F i g u r e 2. E l e c t r o n micrographs o f c r y s t a l s o f c e l l u l o s e I I p r e p a r e d a t 50°C, a f t e r shadowing w i t h W/Ta. 2A Sample from microcrystalline cellulose (DP 34) s o l u t i o n . 2B Sample from cotton cellulose (DP 2000) solution. 2C X-ray diagram, i d e n t i c a l f o r e i t h e r samples.

193


194


F i g u r e 3. Low dose e l e c t r o n m i c r o g r a p h s and selected area e l e c t r o n d i f f r a c t i o n diagram o f 3A samples as i n F i g u r e 2A. 3B sample as i n F i g u r e 2B. In b o t h F i g u r e s , the diffraction diagrams are p r i n t e d w i t h c o r r e c t o r i e n t a t i o n w i t h r e s p e c t to the images.


11.

QUENIN AND CHANZY


F i g u r e 4. E l e c t r o n micrographs of crystals of cellulose p r e p a r e d a t 90°C a f t e r shadowing w i t h W/Ta. 4A from A v i c e l (DP 120) cellulose solution: the s t r u c t u r e c o n s i s t s of r o d - l i k e elements t o g e t h e r w i t h g r a n u l a r aggregates (arrows). I n s e r t : corresponding X-ray diagram. 4B from microcrystalline cellulose (DP 3 4 ) s o l u t i o n . Insert : corresponding electron d i f f r a c t i o n diagram.

195

196


In F i g u r e 4B, o b t a i n e d from DP 34 m a t e r i a l c r y s t a l l i z e d a l s o a t 90°C, t h e amount o f c e l l u l o s e IV i s much l a r g e r as d e s c r i b e d above i n T a b l e I . In f a c t t h e e l e c t r o n m i c r o s c o p y s t u d y r e v e a l s l a r g e a r e a s where o n l y a g r a n u l a r p r e c i p i t a t e i s p r e s e n t . These a r e a s c o n s i s t e x c l u s i v e l y o f c e l l u l o s e IV-J-J d e n o t e d by t h e i n s e r t e d e l e c t r o n d i f f r a c t i o n diagram i n F i g u r e 4B. a

s


Discussion T h i s study p r e s e n t s s e v e r a l aspects o f i n t e r e s t regarding the morphology o f r e c r y s t a l l i z e d c e l l u l o s e and i t s r e c r y s t a l l i z e d form. F i r s t , i n agreement w i t h e a r l i e r r e p o r t s by A t a l l a e t a l . (15, 1 6 ) , i t i s c o n f i r m e d t h a t t h e c r y s t a l l i n e polymorphism o f c e l l u l o s e may be i n f l u e n c e d by t h e m o l e c u l a r w e i g h t o f t h e r e c r y s t a l l i z e d sample. This i s particularly well demonstrated i n this system when t e m p e r a t u r e s o f r e c r y s t a l l i z a t i o n around 100°C a r e s e l e c t e d . A t t h o s e t e m p e r a t u r e s , t h e r a t i o o f c e l l u l o s e IV to cellulose II appears t o be i n v e r s e l y r e l a t e d t o t h e m o l e c u l a r w e i g h t o f t h e c e l l u l o s e samples under i n v e s t i g a t i o n . A t p r e s e n t , i t i s d i f f i c u l t t o come f o r w a r d w i t h a c o n c l u s i v e e x p l a n a t i o n f o r such b e h a v i o r . I t was shown by Maeda e t a l . (17) t h a t c h a i n a g g r e g a t e s formed i n s o l u t i o n p r i o r t o c r y s t a l l i z a t i o n when c e l l u l o s e c r y s t a l l i z e d i n the form o f œ l l u l o s e I I . The s t a b i l i t y o f such a p r e - a s s o c i a t e d s t r u c t u r e depends on the number o f hydrogen bonds c o n n e c t i n g t h e c e l l u l o s e c h a i n s t o g e t h e r . T h i s i s why, i n o u r o p i n i o n , such a g g r e g a t e s a r e more s t a b l e when h i g h e r m o l e c u l a r w e i g h t s a r e u s e d . On t h e o t h e r hand, when t h e temperature o f c r y s t a l l i z a t i o n i s h i g h o r when low DP m a t e r i a l i s choosen, t h e p r e - a s s o c i a t e d a g g r e g a t e s are l e s s t i g h t l y i n t e r l o c k e d and c o n s e q u e n t l y l e s s s t a b l e . T h i s should lead to a different crystallization pattern, where i n d i v i d u a l c h a i n s i n s t e a d o f a g g r e g a t e d b u n d l e s , would e n t e r t h e growing c r y s t a l s i n t h e c e l l u l o s e IV mode. Such a c o n c e p t , which needs t o be f u r t h e r s u b s t a n t i a t e d , c o u l d w e l l e x p l a i n t h e o c c u r e n c e o f c e l l u l o s e IV o r t h a t o f c e l l u l o s e I I when t h e c r y s t a l l i z a t i o n parameters are v a r i e d . A s i m i l a r i t y c a n be e s t a b l i s h e d between t h e polymorphism o f mannan and t h a t o f c e l l u l o s e , as w i t h b o t h p o l y s a c c h a r i d e s , one o b s e r v e s a m o l e c u l a r w e i g h t i n f l u e n c e on t h e polymorphism, even though t h i s i n f l u e n c e i s more pronounced i n t h e case o f mannan t h a n i n t h e case o f œ l l u l o s e . I t was shown t h a t w i t h mannan and glucomannan p o l y m e r s , t h e mannan I s t r u c t u r e was o b t a i n e d more easily w i t h low m o l e c u l a r w e i g h t m a t e r i a l , whereas mannan I I c o r r e s p o n d e d t o h i g h e r m o l e c u l a r w e i g h t (4,8). A c l o s e resemblance e x i s t s t h u s between 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 I V ^ and mannan I as w i t h b o t h p o l y m e r s , t h e c r y s t a l l i z a t i o n phenomena appear t o be d i r e c t e d by r e l a t e d parameters. S i m i l a r l y , the c r y s t a l l i z a t i o n b e h a v i o r o f mannan I I and c e l l u l o s e I I appear t o be also correlated. The resemblance between c e l l u l o s e and mannan c r y s t a l s i s n o t limited to their susceptibility toward molecular weight or crystallization temperature modifications. Even morphological f e a t u r e s a r e s i m i l a r : mannan I and c e l l u l o s e IV o c c u r e i t h e r as platelet c r y s t a l s ( 18,19) when f r a c t i o n a t e d polymers a r e u s e d , o r


11.

QUENIN AND

CHANZY


197

g r a n u l a r m a t e r i a l w i t h u n f r a c t i o n a t e d p o l y m e r ( see F i g u r e 4B and r e f 4 ) ; mannan I I and c e l l u l o s e I I take the shape o f e i t h e r a f i b r i l l a r g e l o r e l o n g a t e d r o d - l i k e c r y s t a l s , depending a l s o on t h e i r m o l e c u l a r w e i g h t ( F i g u r e 2 B and r e f . 4 ) . F o r b o t h p o l y m e r s , such a d i f f e r e n c e i n morphology r e f l e c t s c e r t a i n l y the dynamics o f the o r g a n i z a t i o n o f the inter-chain hydrogen bonds o u t s i d e or w i t h i n the growing c r y s t a l s . With mannan o r c e l l u l o s e , the d e t a i l s and the k i n e t i c s o f t h e i r c r y s t a l f o r m a t i o n are a t p r e s e n t unknown. I t s h o u l d however d e s e r v e f u r t h e r a t t e n t i o n as the a f o r e m e n t i o n e d shift from g r a n u l a r t o f i b r i l l a r g e l - l i k e s t r u c t u r e s can have a d r a m a t i c importance f o r mannan o r c e l l u l o s e - b a s e d p r o d u c t s . A f i n a l i n t e r e s t i n g a s p e c t o f the p r e s e n t s t u d y i s the change i n morphology o c c u r r i n g w i t h c e l l u l o s e I I when one g o e s from low t o h i g h DP m a t e r i a l . With low m o l e c u l a r w e i g h t c e l l u l o s e , the c r y s t a l s o c c u r as r o d - l i k e e l e m e n t s , where the c e l l u l o s e c h a i n s are a t 90° with respect to the rod axis. On the o t h e r hand, w i t h high m o l e c u l a r w e i g h t , the c e l l u l o s e I I c r y s t a l s are f i b r i l l a r i n n a t u r e as t h e y are e l o n g a t e d a l o n g the c e l l u l o s e c h a i n a x i s . Such changes i n morphology, a s s o c i a t e d w i t h a change i n m o l e c u l a r w e i g h t , i s remarkable and must c e r t a i n l y be c o n n e c t e d w i t h the impossibility f o r c e l l u l o s e t o c r y s t a l l i z e as r e g u l a r c h a i n f o l d e d c r y s t a l s (20). In an e a r l i e r s t u d y on c e l l u l o s e I I c r y s t a l s ( 1 4 ) , i t was shown t h a t the growth h a b i t f o r low DP c e l l u l o s e c r y s t a l s was i s the form o f long flat ribbons when the c r y s t a l l i z a t i o n conditions were o p t i m i z e d . In normal c a s e s , low DP c e l l u l o s e gave n e e d l e - l i k e or rod l i k e c r y s t a l l i n e e l e m e n t s . The p r e s e n t study confirms t h i s behavior which seems to be persistent with all cellulose crystallization systems. The case of high DP cellulose c r y s t a l l i z a t i o n y i e l d i n g a f i b r i l l a r g e l - l i k e p r e c i p i t a t e i s not new as i t was r e p o r t e d e a r l i e r by Manley (21). Here however, our e l e c t r o n d i f f r a c t i o n r e s u l t s p r o v e t h a t the c h a i n axes o f c e l l u l o s e are i n d e e d o r i e n t e d along the f i b r i l l a r a x i s o f the p r e c i p i t a t e . I f one a t t e m p t s t o r a t i o n a l i z e such d r a s t i c changes i n the g r o w t h h a b i t o f c e l l u l o s e I I c r y s t a l s , when the m o l e c u l a r w e i g h t i s i n c r e a s e d , i t i s p o s s i b l e t o b r i n g f o r w a r d the f o l l o w i n g arguments : i n a l l c a s e s , i t seems t h a t the c r y s t a l l i z a t i o n p a r a m e t e r s i n c e l l u l o s e are g o v e r n e d by the semi r i g i d i t y o f the c e l l u l o s e c h a i n and the strong influence p l a y e d by the i n t e r m o l e c u l a r hydrogen bonds. In p a r t i c u l a r , the f a c t t h a t the c e l l u l o s e c h a i n s are semi r i g i d i s l i k e l y t o h i n d e r the f o r m a t i o n o f r e g u l a r c h a i n folded c r y s t a l s (22). The growth o f the c e l l u l o s e c r y s t a l s i s e n v i s a g e d as b e i n g a two s t e p phenomenon(17). In the f i r s t s t e p , a b u n d l e o f c e l l u l o s e c h a i n i s formed w i t h the c h a i n s a s s o c i a t e d i n a p a r a l l e l fashion through multiple hydrogen bonds leading to the c r y s t a l l i z a t i o n n u c l e u s . I f o n l y s h o r t c h a i n s are u s e d , the n e x t step i s to i n c o r p o r a t e s i m i l a r elements i n a l a t e r a l f a s h i o n , l e a d i n g t o r i b b o n o r r o d - l i k e morphology growing p e r p e n d i c u l a r t o the c h a i n d i r e c t i o n i n a t y p i c a l polymer s i n g l e c r y s t a l f a s h i o n . When h i g h e r DP c e l l u l o s e i s u s e d , the b u n d l e o f c h a i n s c o n s t i t u t i n g the p r i m a r y c r y s t a l l i z a t i o n n u c l e u s w i l l t e n d t o a s s o c i a t e f u r t h e r as t h e y are k e p t i n c l o s e v i c i n i t y , due t o the i n h e r e n t semi r i g i d i t y o f the c e l l u l o s e c h a i n s . T h i s l e a d s t o a second s t e p where the growth i s l o n g i t u d i n a l instead of lateral. The structure becomes f i b r i l l a r and i s b a s e d on long and narrow e l e m e n t s h a v i n g

198

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OF C E L L U L O S E

r o u g h l y the d i a m e t e r o f the i n i t i a l n u c l e u s . Such c r y s t a l s a r e t y p i c a l polymer f i b r i l l a r c r y s t a l s , e x t e n d e d a l o n g the c h a i n a x i s and narrow i n a perpendi- cular direction. At p r e s e n t , the occurrence or not o f c h a i n f o l d i n g i n these f i b r i l l a r crystals remains t o be d e m o n s t r a t e d .


Acknowledgments The a u t h o r s acknowledge the a s s i s t a n c e o f Mr w i t h the e l e c t r o n m i c r o s c o p y .

R. Vuong

f o r h i s help

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1987

The Structures of Cellulose - American Chemical Society

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