Chapter 16
Physical Structure and Alkaline Degradation of Hydrocellulose Victor M. Gentile, Leland R. Schroeder, and Rajai H. Atalla Institute of Paper Chemistry, Appleton, WI 54912
Degradations of fibrous cotton hydrocellulose and an amorphous hydrocellulose were conducted in oxygen-free 1.0M NaOH at 60 and 80°C. The physical structure of the fibrous hydrocellulose was not significantly altered, while the amorphous hydrocellulose underwent partial recrystallization into the cellulose II form and some loss of amorphous material through degradation. Endwise depolymerization (peeling) and formation of stable carboxylic acid endgroups (chemical stopping) were more rapid and extensive with the amorphous substrate. Both peeling and chemical stopping were inhibited by the more highly ordered physical structure of the fibrous hydrocellulose and the majority of degrading molecules terminated to stable inaccessible reducing endgroups, that is, by physical stopping. In contrast, chemical stopping was the dominant stabilization mechanism in the amorphous hydrocellulose. The rate of chemical stopping relative to peeling increased with temperature for both substrates. In addition, random chain cleavage, normally believed to be important only at much higher temperatures, was detected in the amorphous hydrocellulose.
A l k a l i n e d e g r a d a t i o n o f c e l l u l o s e o c c u r s by random c l e a v a g e o f g l y c o s i d i c l i n k a g e s and by s t e p w i s e e l i m i n a t i o n o f monomer u n i t s from the r e d u c i n g end ( p e e l i n g ) (_1_,2). These r e a c t i o n s o c c u r i n c o m p e t i t i o n w i t h a n o t h e r r e a c t i o n which s t a b i l i z e s c e l l u l o s e a g a i n s t a l k a l i n e d e g r a d a t i o n by c o n v e r t i n g the r e d u c i n g endgroup t o an a l k a l i - s t a b l e , c a r b o x y l i c a c i d endgroup ( c h e m i c a l s t o p p i n g ) . Though the major a l k a l i n e r e a c t i o n s o f c e l l u l o s e have been r e l a t i v e l y w e l l d e f i n e d , the r o l e o f c e l l u l o s e p h y s i c a l s t r u c t u r e i n those r e a c t i o n s has not been c l e a r l y e s t a b l i s h e d . C e l l u l o s e molec u l e s have been r e p o r t e d t o undergo p h y s i c a l s t o p p i n g o f the p e e l i n g r e a c t i o n when a m o l e c u l e i s p e e l e d back t o a c r y s t a l l i n e r e g i o n i n the c e l l u l o s e s t r u c t u r e , w i t h the r e s u l t t h a t the r e d u c i n g endgroup
0097-6156/87/0340-0272$06.00/0 © 1987 American Chemical Society
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becomes i n a c c e s s i b l e t o the a l k a l i n e medium (3^-5)· It i s a l s o r e p o r t e d t h a t f o r n a t i v e c e l l u l o s e the r a t e o f p e e l i n g r e l a t i v e to c h e m i c a l s t o p p i n g i s h i g h e r t h a n f o r m e r c e r i z e d c e l l u l o s e (3^_6-_8). F u r t h e r m o r e , random c h a i n c l e a v a g e o c c u r s more r a p i d l y i n m e r c e r i z e d c e l l u l o s e than i n n a t i v e c e l l u l o s e (6). These f i n d i n g s suggest t h a t b o t h m o l e c u l a r a c c e s s i b i l i t y and c o n f o r m a t i o n ( i . e . , c e l l u l o s e I o r I I ) i n f l u e n c e the s u s c e p t i b i l i t y o f c e l l u l o s e m o l e c u l e s t o a l k a l i n e reactions. However, s e p a r a t i n g the d i f f e r e n t e f f e c t s o f p h y s i c a l s t r u c t u r e from the i n h e r e n t r e a c t i v i t y o f the c e l l u l o s e m o l e c u l e ( i n an a l k a l i n e environment) i s made d i f f i c u l t by i t s l i m i t e d s o l u b i l i t y in alkaline solutions. In the p r e s e n t s t u d y , the r o l e o f c e l l u l o s e p h y s i c a l s t r u c t u r e i n a l k a l i n e r e a c t i o n s was i n v e s t i g a t e d by comparing the a l k a l i n e d e g r a d a t i o n o f h i g h l y c r y s t a l l i n e ( c e l l u l o s e I) f i b r o u s h y d r o c e i l u l o s e w i t h t h a t of amorphous ( n o n c r y s t a l l i n e ) h y d r o c e l l u l o s e . The amorphous s u b s t r a t e was t a k e n as a c e l l u l o s e model the r e a c t i v i t y o f which would most c l o s e l y approximate t h a t o f a l k a l i - s o l u b l e c e l l u lose. The a v a i l a b l i t y o f such an a p p r o x i m a t i o n to the i n h e r e n t r e a c t i v i t y o f c e l l u l o s e a l l o w e d e v a l u a t i o n o f the e f f e c t s o f the more h i g h l y o r d e r e d s t r u c t u r e o f the f i b r o u s h y d r o c e l l u l o s e . Results
and
Discussion
E x p e r i m e n t a l Approach. The e x p e r i m e n t a l study was a c o m p a r i s o n o f the a l k a l i n e d e g r a d a t i o n s o f f i b r o u s and amorphous h y d r o c e l l u l o s e s i n o x y g e n - f r e e 1.0M NaOH, at 60 and 80°C. The f i b r o u s h y d r o c e l l u l o s e was p r e d o m i n a n t l y c r y s t a l l i n e ( c e l l u l o s e I) and t h e r e f o r e s e r v e d as a s u b s t r a t e w h i c h would undergo a l k a l i n e r e a c t i o n s w i t h s i g n i f i c a n t p h y s i c a l s t r u c t u r e e f f e c t s . In c o n t r a s t , the amorphous h y d r o c e l l u l o s e was n o n c r y s t a l l i n e ( 9 , 1 0 ) . Thus, i t was a s u b s t r a t e which would e x p e r i e n c e s u b s t a n t i a l l y l e s s s t r u c t u r a l c o n s t r a i n t during i t s a l k a l i n e reactions. The f i b r o u s h y d r o c e l l u l o s e was p r e p a r e d by m i l d a c i d h y d r o l y s i s o f c o t t o n f i b e r s t o p r o v i d e s u f f i c i e n t numbers o f r e d u c i n g endgroups f o r p e e l i n g and s t o p p i n g t o o c c u r a t measurable r a t e s . The amorphous h y d r o c e l l u l o s e was p r e p a r e d by d i s s o l v i n g the f i b r o u s h y d r o c e l l u l o s e i n the d i m e t h y l s u l f o x i d e - p a r a f o r m - a l d e h y d e (DMSO-PF) s o l v e n t (9-12) and then r e g e n e r a t i n g the h y d r o c e l l u l o s e w i t h a sodium m e t h o x i d e - i s o p r o p o x i d e s o l u t i o n ( 9 , 1 0 ) . Both h y d r o c e l l u l o s e s were f r e e z e - d r i e d d u r i n g p r e p a r a t i o n and a f t e r d e g r a d a t i o n t o m i n i mize d r y i n g - i n d u c e d s t r u c t u r a l changes. Thus, s t r u c t u r a l changes c a u s e d by the a l k a l i n e medium and the d e g r a d a t i o n r e a c t i o n s c o u l d be d e t e c t e d more r e a d i l y . Data on endgroup c o n t e n t s and number-average d e g r e e s o f p o l y m e r i z a t i o n (DP ) f o r the h y d r o c e l l u l o s e s u b s t r a t e s are p r e s e n t e d i n T a b l e I. The h y d r o c e l l u l o s e s have s i m i l a r numbers o f c a r b o x y l i c a c i d endgroups formed d u r i n g p u r i f i c a t i o n o f the c o t t o n f i b e r s . But o n l y the amorphous h y d r o c e l l u l o s e c o n t a i n e d no i n a c c e s s i b l e r e d u c i n g endgroups, d e m o n s t r a t i n g the c a p a c i t y o f the d i s s o l u t i o n / r e g e n e r a t i o n p r o c e s s t o enhance a c c e s s i b i l i t y ( 9 , 1 0 ) . On the o t h e r hand, the t o t a l r e d u c i n g endgroup c o n t e n t o f the amorphous h y d r o c e l l u l o s e was g r e a t e r t h a n t h a t of the f i b r o u s h y d r o c e l l u l o s e . This, together w i t h the lower DP o f the amorphous s u b s t r a t e , i n d i c a t e s t h a t some chain cleavage occurred during regeneration. The c h a i n c l e a v a g e was n
n
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THE STRUCTURES OF CELLULOSE
a p p a r e n t l y r e l a t e d t o the scale-αρ o f the p r o c e s s , s i n c e no c l e a v a g e was d e t e c t e d when r e l a t i v e l y s m a l l samples (< 2 g) were r e g e n e r a t e d ($0. F o r t h i s r e a s o n , i n comparisons o f the p e e l i n g and s t o p p i n g r e a c t i o n s o f t h e two s u b s t r a t e s , r e a c t i o n r a t e s were c o r r e c t e d f o r the d i f f e r e n t a c c e s s i b l e ( r e a c t i v e ) r e d u c i n g endgroup c o n t e n t s .
Table
I.
Endgroup
a
and D P ^ Data f o r H y d r o c e l l u l o s e n
C a r b o x y l i c a c i d endgroups A c c e s s i b l e r e d u c i n g endgroups I n a c c e s s i b l e r e d u c i n g endgroups T o t a l r e d u c i n g endgroups DP n
Substrates
Fibrous Hydrocellulose
Amorphous Hydrocellulose
1.09 χ 10"^ 1.13 χ 10"^ 0.15 χ 10"^ 1.28 χ 10"" ^ 422
1.02 χ 10"^ 3.42 χ 10"^ 0 3.42 χ 10"^ 225
a
E n d g r o u p v a l u e s e x p r e s s e d as mole f r a c t i o n s o f t o t a l monomer u n i t s . ^ C a l c u l a t e d from the t o t a l endgroups c o n t e n t " * .
D u r i n g the c o u r s e o f t h e a l k a l i n e d e g r a d a t i o n s , b o t h p h y s i c a l and c h e m i c a l s t r u c t u r e s o f t h e h y d r o c e l l u l o s e s were m o n i t o r e d . H y d r o x y l a c c e s s i b i l i t y (13) was d e t e r m i n e d as a p r a c t i c a l measure o f the f r a c t i o n o f m o l e c u l e s a c c e s s i b l e t o t h e a l k a l i n e medium. The c r y s t a l l i n e s t r u c t u r e was c h a r a c t e r i z e d by x - r a y d i f f r a c t i o n ( 1 4 ) . In a d d i t i o n . Raman (15) and s o l i d - s t a t e carbon-13 n u c l e a r magnetic r e s o n a n c e (^C-NMR) (16,17) s p e c t r a were u t i l i z e d t o a s s e s s c o n f o r m a t i o n a l changes. Y i e l d l o s s was d e t e r m i n e d g r a v i m e t r i c a l l y and t a k e n as a measure o f a n h y d r o g l u c o s e u n i t s l o s t due t o p e e l i n g . The c h e m i c a l s t o p p i n g r e a c t i o n was m o n i t o r e d by m e a s u r i n g c a r b o x y l i c a c i d endgroup f o r m a t i o n , u s i n g methylene b l u e a b s o r p t i o n v a l u e s (10). The r e a c t i v e s p e c i e s f o r b o t h p e e l i n g and s t o p p i n g , t h a t i s , the a c c e s s i b l e r e d u c i n g endgroups, were d e t e c t e d by s e l e c t i v e r e d u c t i o n w i t h t r i t i u m - l a b e l e d sodium b o r o h y d r i d e ( 9 , 1 0 ) . Inaccessible ( n o n r e a c t i v e ) r e d u c i n g endgroups were a l s o d e t e c t e d by r e d u c t i o n w i t h sodium b o r o h y d r i d e - ^ H a f t e r they were made a c c e s s i b l e v i a the p r e v i o u s l y d i s c u s s e d r e g e n e r a t i o n technique (9^. It was t h e r e f o r e p o s s i b l e t o d e t e c t t h e s o - c a l l e d " p h y s i c a l s t o p p i n g ' o f the p e e l i n g r e a c t i o n as e v i d e n c e d i n t h e f o r m a t i o n o f i n a c c e s s i b l e o r u n r e a c t i v e r e d u c i n g endgroups. 1
The p h y s i c a l s t r u c t u r e d a t a t o g e t h e r w i t h the a l k a l i n e r e a c t i o n d a t a p e r m i t t e d e v a l u a t i o n o f the e f f e c t s o f p h y s i c a l s t r u c t u r e on a l k a l i n e degradation o f c e l l u l o s e . A l k a l i n e D e g r a d a t i o n s - Change i n P h y s i c a l S t r u c t u r e . The h y d r o x y l a c c e s s i b i l i t y o f the f i b r o u s h y d r o c e l l u l o s e was i n i t i a l l y 51.4 ± 0.8%. In c o n t r a s t , the amorphous s u b s t r a t e had an a c c e s s i b i l i t y o f 99.2 ± 1.0%. Exposure o f t h e f i b r o u s h y d r o c e l l u l o s e t o t h e a l k a l i n e media caused the a c c e s s i b i l i t y t o d e c r e a s e s l i g h t l y t o 50.7 ± 1.0% and 49.1 ± 1.2% a t 60 and 80°C, r e s p e c t i v e l y , but a c c e s s i b i l i t y d i d not change s i g n i f i c a n t l y d u r i n g t h e r e a c t i o n p e r i o d s (0-168 h r ) .
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The a c c e s s i b i l i t y o f t h e amorphous h y d r o c e l l u l o s e , however, d i d d e c l i n e , b o t h upon exposure t o t h e a l k a l i n e media and d u r i n g t h e r e a c t i o n periods (Figure 1). This i n d i c a t e s both r e c r y s t a l l i z a t i o n and s e l e c t i v e removal o f amorphous m a t e r i a l . X-ray d i f f r a c t o g r a m s o f t h e f i b r o u s h y d r o c e l l u l o s e ( F i g u r e 2) e x h i b i t t h e c h a r a c t e r i s t i c 002, 101, and 101 r e f l e c t i o n s o f t h e c e l l u l o s e I c r y s t a l l i n e l a t t i c e (14,18). The s h a r p l y d e f i n e d peaks i n d i c a t e a h i g h degree o f c r y s t a l l i n i t y . Although there appears t o be a s l i g h t i n c r e a s e i n peak i n t e n s i t y i n t h e d i f f r a c t o g r a m o f t h e z e r o - t i m e sample r e l a t i v e t o t h a t o f t h e i n i t i a l s u b s t r a t e , no f u r t h e r change i s e v i d e n t i n t h e d i f f r a c t o g r a m o f t h e 48-hour sample. Thus, x - r a y d i f f r a c t i o n c o n f i r m s t h a t t h e f i b r o u s h y d r o c e l l u l o s e does not undergo s i g n i f i c a n t change i n p h y s i c a l s t r u c t u r e d u r i n g degradation. The d i f f u s e d i f f r a c t o g r a m o f t h e i n i t i a l amorphous s u b s t r a t e ( F i g u r e 3) i s i n d i c a t i v e o f n o n c r y s t a l l i n e c e l l u l o s e ( 1 9 ) . The d i f f r a c t o g r a m o f t h e z e r o - t i m e sample e x h i b i t s _ j a s e t o f weak r e f l e c t i o n s c o r r e s p o n d i n g t o t h e 002, 101, and 101 p l a n e s o f t h e c e l l u l o s e I I c r y s t a l l i n e l a t t i c e (14,18). The p o o r l y d e f i n e d peaks i n d i c a t e a r e l a t i v e l y low degree o f c r y s t a l l i n i t y . Since the d i f f r a c t o g r a m o f the 48-hour sample d i s p l a y s s l i g h t l y more i n t e n s e r e f l e c t i o n s , a small increase i n the c e l l u l o s e I I content occurred during the react i o n period. This i s c o n s i s t e n t with the hydroxyl a c c e s s i b i l i t y data. Raman s p e c t r a o f t h e f i b r o u s h y d r o c e l l u l o s e i n t h e c o n f o r m a t i o n s e n s i t i v e 250 t o 650 cm"* r e g i o n have r e l a t i v e l y i n t e n s e c e l l u l o s e I bands ( F i g u r e 4 ) , i n d i c a t i n g t h a t t h e m o l e c u l e s a r e p r e d o m i n a n t l y i n the c e l l u l o s e I c o n f o r m a t i o n ( 1 5 ) . T h i s i s b e s t demonstrated by t h e i n t e n s e band a t 378 cm"*. The l a c k o f s i g i f i c a n t d i f f e r e n c e s i n t h e s p e c t r a o f t h e i n i t i a l s u b s t r a t e , z e r o - t i m e sample, and 48-hour sample c o n f i r m t h a t no s i g n i f i c a n t changes i n p h y s i c a l s t r u c t u r e occurred during degradation. In c o n t r a s t , the same r e g i o n i n t h e Raman spectrum o f t h e i n i t i a l amorphous s u b s t r a t e e x h i b i t s b r o a d bands ( F i g u r e 5) i n d i c a t i v e o f i r r e g u l a r sequences o f c o n f o r m a t i o n s a l o n g t h e c e l l u l o s e c h a i n s (15). The emergence o f a band a t 355 cm"* i n t h e spectrum o f t h e z e r o - t i m e sample i n d i c a t e s t h e p r e s e n c e o f t h e c e l l u l o s e I I a l l o morph. The a d d i t i o n a l , s m a l l i n c r e a s e i n band i n t e n s i t y i n t h e spectrum o f t h e 48-hour sample a g a i n d e m o n s t r a t e s a f u r t h e r s l i g h t increase i n c e l l u l o s e I I content during degradation. The s o l i d - s t a t e *^C-NMR s p e c t r a o f t h e f i b r o u s h y d r o c e l l u l o s e a l s o demonstrate t h e predominance o f t h e c e l l u l o s e I a l l o m o r p h ( F i g u r e 6). A l l three s p e c t r a c o n t a i n the sharp resonances assoc i a t e d w i t h t h e c e l l u l o s e I c o n f o r m a t i o n and t h e b r o a d e r C-4 and C-6 r e s o n a n c e s i n d i c a t i v e o f r e g i o n s o f t h r e e - d i m e n s i o n a l d i s o r d e r and c r y s t a l l i t e s u r f a c e s (16,17). The r e l a t i v e i n t e n s i t i e s o f t h e sharp and b r o a d resonances o f t h e t h r e e s p e c t r a a r e s i m i l a r , a g a i n demons t r a t i n g the l a c k o f change i n p h y s i c a l s t r u c t u r e d u r i n g d e g r a d a t ion. In comparison, t h e *^C-NMR spectrum o f t h e i n i t i a l amorphous s u b s r a t e e x h i b i t s o n l y b r o a d r e s o n a n c e s ( F i g u r e 7) c h a r a c t e r i s t i c o f r e g i o n s o f t h r e e - d i m e n s i o n a l d i s o r d e r (16,17). The p r o g r e s s i v e appearance o f s h a r p e r r e s o n a n c e s i n t h e s p e c t r a o f t h e z e r o - t i m e and 48-hour samples i n d i c a t e s i n c r e a s i n g c o n f o r m a t i o n a l order.
276
THE STRUCTURES OF CELLULOSE
100 r
Figure
1.
H y d r o x y l a c c e s s i b i l i t y o f the amorphous h y d r o c e l l u l o s e d u r i n g d e g r a d a t i o n i n 1.0M NaOH.
16.
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(002)1
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28
Figure
2.
1
26
1
24
1
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20 18 DEGREES 20
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X-ray d i f f r a c t o g r a m s o f t h e f i b r o u s h y d r o c e l l u l o s e d u r i n g d e g r a d a t i o n i n 1.0M NaOH a t 80°C.
Figure 3. X-ray diffractograms of the amorphous hydrocellulose during degradation i n 1.0M NaOH at 80°C.
Figure 4. Raman spectra of the fibrous hydrocellulose during degradation i n 1.0M NaOH a t 80°C.
F i g u r e 5. Raman s p e c t r a o f t h e amorphous h y d r o c e l l u l o s e d u r i n g d e g r a d a t i o n i n 1.0M NaOH a t 80°C.
13
F i g u r e 6. S o l i d - s t a t e C-NMR s p e c t r a o f t h e f i b r o u s h y d r o c e l l u l o s e d u r i n g d e g r a d a t i o n i n 1.0M NaOH a t 80°C.
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280
THE STRUCTURES OF CELLULOSE
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7.
S o l i d - s t a t e *^C-NMR s p e c t r a o f the amorphous h y d r o c e l l u l o s e d u r i n g d e g r a d a t i o n i n 1.0M NaOH a t 80°C.
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Alkaline Degradation of Hydrocellulose
281
Resonance l o c a t i o n s and m u l t i p l i c i t i e s a r e c h a r a c t e r i s t i c o f the c e l l u l o s e I I a l l o m o r p h , c o n f i r m i n g the r e s u l t s o f x - r a y d i f f r a c t i o n and Raman s p e c t r o s c o p y . The absence o f change i n the p h y s i c a l s t r u c t u r e o f the f i b r o u s h y d r o c e l l u l o s e d u r i n g d e g r a d a t i o n suggests three a l t e r n a t i v e hypotheses. F i r s t , s e l e c t i v e d e g r a d a t i o n o f amorphous c e l l u l o s e c o u l d have o c c u r r e d but t o an e x t e n t not d e t e c t a b l e by the methods applied. Second, removal o f amorphous m a t e r i a l c o u l d have been accompanied by a comparable amount o f d 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 domains. F i n a l l y , c e l l u l o s e removed d u r i n g d e g r a d a t i o n may have displayed p a r t i a l c e l l u l o s e I character. T h i s would i n v o l v e molec u l e s i n s l i g h t l y d i s t o r t e d c e l l u l o s e I domains ( t i l t e d o r t w i s t e d segments o f elementary f i b r i l ) and a t c r y s t a l l i t e s u r f a c e s ( 2 0 ) , s i n c e removal o f e i t h e r would not r e s u l t i n d e t e c t a b l e changes i n p h y s i c a l s t r u c t u r e . A l k a l i n e r e a c t i o n d a t a p r e s e n t e d i n the f o l l o w i n g s e c t i o n t e n d t o support the l a t t e r h y p o t h e s i s . The i n c r e a s e i n c e l l u l o s e I I c h a r a c t e r and d e c r e a s e i n a c c e s s i b i l i t y o f the amorphous h y d r o c e l l u l o s e upon exposure t o the a l k a l i n e medium and d u r i n g the r e a c t i o n i n t e r v a l d e f i n i t e l y i n d i c a t e p a r t i a l recrystallization. However, s e l e c t i v e removal o f amorphous c e l l u l o s e may have o c c u r r e d s i m u l t a n e o u s l y . This additional p o s s i b i l i t y i s c o n s i s t e n t w i t h the a l k a l i n e r e a c t i o n d a t a . P e e l i n g and S t o p p i n g R e a c t i o n s . The y i e l d l o s s d u r i n g a l k a l i n e d e g r a d a t i o n was more r a p i d and e x t e n s i v e f o r the amorphous h y d r o c e l l u l o s e than f o r the f i b r o u s h y d r o c e l l u l o s e , a t b o t h 60 and 80°C ( F i g u r e 8). However, the e v o l u t i o n o f y i e l d l o s s w i t h time was d i f f e r e n t at 60 and 80°C. While at 60°C y i e l d l o s s o c c u r r e d t h r o u g h o u t the time i n t e r v a l s t u d i e d (168 h r ) , the y i e l d o f b o t h s u b s t r a t e s l e v e l e d o f f a f t e r ca. 48 h o u r s a t 80°C. While s m a l l amounts o f p e c t i c m a t e r i a l a r e p r o b a b l y l o s t d u r i n g the d e g r a d a t i o n s , such l o s s e s are i n s i g n i f i c a n t r e l a t i v e t o y i e l d l o s s e s due t o p e e l i n g (10). D i r e c t c o m p a r i s o n o f y i e l d d a t a f o r the two s u b s t r a t e s i s not p o s s i b l e due t o the d i f f e r e n c e s i n i n i t i a l a c c e s s i b l e r e d u c i n g endgroup c o n t e n t s ( T a b l e I ) . The k i n e t i c model used by Haas, e t a l . (50 was t h e r e f o r e employed t o p r o v i d e a b a s i s f o r comparison o f r e a c t i o n r a t e s o f m o l e c u l e s w i t h i n the two s u b s t r a t e s . T h i s model i n corporates pseudo-first-order rate expressions for peeling (Equation 1), c h e m i c a l s t o p p i n g ( E q u a t i o n 2), and p h y s i c a l s t o p p i n g ( E q u a t i o n 4 ) ; our n o t a t i o n d i f f e r s from t h a t o f Haas, e t a l . In a l l t h r e e r a t e e x p r e s s i o n s , the r e a c t i o n r a t e s a r e r e l a t e d t o the number o f a c c e s s i b l e r e d u c i n g endgroups by p s e u d o - f i r s t - o r d e r r a t e c o e f f i cients. Thus, the r a t e c o e f f i c i e n t s r e f l e c t the r e a c t i v i t i e s o f a c c e s s i b l e r e d u c i n g endgroups o c c u p y i n g d i f f e r e n t s t r u c t u r a l environments. S i n c e the y i e l d l o s s e s were p r e d o m i n a n t l y due t o p e e l i n g ( 1 0 ) , the p s e u d o - f i r s t - o r d e r r a t e e x p r e s s i o n f o r p e e l i n g can be written: a[Y ]/dt x
where
= k [ARE ] p
t
[ Y \ ] = Y i e l d l o s s , as mole f r a c t i o n o f t o t a l monomer u n i t s at zero-time
(1)
282
THE STRUCTURES OF CELLULOSE
Figure
8.
Hydrocellulose y i e l d
during degradation
i n 1.0M
NaOH.
16.
GENTILE ET AL.
283
Alkaline Degradation of Hydrocellulose
t = Time, hr kp = Rate c o e f f i c i e n t f o r p e e l i n g , h r " * [ARE J = A c c e s s i b l e r e d u c i n g endgroup c o n t e n t at time " t , " as mole f r a c t i o n o f t o t a l monomer u n i t s a t z e r o - t i m e t
The d e r i v a t i v e i n E q u a t i o n 1 was e v a l u a t e d at s e l e c t e d r e a c t i o n t i m e s from the s l o p e s o f p l o t s o f y i e l d l o s s v e r s u s r e a c t i o n time. V a l u e s o f kp, c a l c u l a t e d from E q u a t i o n 1, are l i s t e d i n T a b l e I I .
Table I I . Reaction Time, h r 0 2 4 48 96 a
Rate C o e f f i c i e n t s f o r
Peeling
3
60° C Fibrous 4.13 3.28 2.74 0.48 0.38
80°C Amorphous 6.16 5.52 4.16 0.60 0.53
Fibrous 27.9 8.91 8.06 1.38 0.60
Amorphous 40.4 13.6 6.23 0.19 0.14
kp,hr"l.
In a l l c a s e s , kp d e c r e a s e d w i t h r e a c t i o n time. Thus, the a c c e s s i b l e r e d u c i n g endgroups i n b o t h h y d r o c e l l u l o s e s were more r e a c t i v e i n i t i a l l y , a p p a r e n t l y due to t h e i r l o c a t i o n i n l e s s o r d e r e d r e g i o n s o f the r e s p e c t i v e p h y s i c a l s t r u c t u r e s . As the l e s s o r d e r e d m a t e r i a l was removed, the a c c e s s i b l e r e d u c i n g endgroups o c c u p i e d i n c r e a s i n g l y o r d e r e d r e g i o n s o f the s t r u c t u r e s and were t h e r e f o r e l e s s r e a c t i v e . The h i g h e r kp v a l u e s f o r the amorphous h y d r o c e l l u l o s e t h r o u g h o u t the 60°C r e a c t i o n and d u r i n g the i n i t i a l p e r i o d o f the 80°C r e a c t i o n i n d i c a t e t h a t the a c c e s s i b l e r e d u c i n g endgroups were more r e a c t i v e t h a n t h o s e i n the f i b r o u s h y d r o c e l l u l o s e . T h i s c o i n c i d e s w i t h the p e r i o d s d u r i n g which the a c c e s s i b i l i t y d e c r e a s e d ( F i g u r e 1), s u g g e s t i n g t h a t s e l e c t i v e removal ( p e e l i n g ) o f amorphous m a t e r i a l did occur. Thus, the l e s s o r d e r e d environment o c c u p i e d by the d e g r a d i n g m o l e c u l e s i n the amorphous h y d r o c e l l u l o s e c l e a r l y r e n d e r e d them more s u s c e p t i b l e t o p e e l i n g . D u r i n g the l a t e r p e r i o d o f the 80°C r e a c t i o n , the amorphous h y d r o c e l l u l o s e e x h i b i t e d a lower v a l u e o f kp t h a n the f i b r o u s h y d r o cellulose. S i n c e t h i s c o r r e s p o n d s t o the p e r i o d d u r i n g which the h y d r o x y l a c c e s s i b i l i t y o f the amorphous h y d r o c e l l u l o s e l e v e l e d - o f f ( F i g u r e 1), i t appears t h a t the p o p u l a t i o n o f d e g r a d a b l e c h a i n s w i t h a c c e s s i b l e r e d u c i n g endgroups had been d e p l e t e d . Consequently, p e e l i n g was p r o b a b l y o c c u r r i n g c l o s e t o c e l l u l o s e I I domains where i t was s i g n i f i c a n t l y i n h i b i t e d . In c o n t r a s t , p e e l i n g p r o g r e s s e d more s l o w l y toward the c e l l u l o s e I domains o f the f i b r o u s h y d r o c e l l u l o s e , f o r example, i n s l i g h t l y d i s t o r t e d c e l l u l o s e I domains, but was a l s o s t r o n g l y i n h i b i t e d a t the f a c e s of more p e r f e c t c e l l u lose I c r y s t a l l i t e s . The degree o f i n h i b i t i o n o f p e e l i n g i s e v i denced by the convergence of 60 and 80°C kp v a l u e s f o r both s u b s t r a t e s at l o n g e r r e a c t i o n t i m e s .
284
THE STRUCTURES OF CELLULOSE
I n t h e case o f c h e m i c a l s t o p p i n g , t h e r a t e o f f o r m a t i o n o f c a r b o x y l i c a c i d endgroups i s a l s o p r o p o r t i o n a l t o the number o f a c c e s s i b l e r e d u c i n g endgroups. The p s e u d o - f i r s t - o r d e r r a t e e x p r e s s i o n i s g i v e n by: [AE]/dt = k
c s
[ARE ]
(2)
t
where [AE] = C a r b o x y l i c a c i d endgroup c o n t e n t , as mole f r a c t i o n o f t o t a l monomer u n i t s a t z e r o - t i m e k = Rate c o e f f i c i e n t f o r c h e m i c a l s t o p p i n g , h r " * c s
C a r b o x y l i c a c i d endgroup c o n t e n t s were f i r s t c o r r e c t e d f o r l o s s e s o f c a r b o x y l i c a c i d groups a s s o c i a t e d w i t h p e c t i c m a t e r i a l l o s t d u r i n g the r e a c t i o n s ( 1 0 ) . Values o f k were then d e t e r mined a t s p e c i f i c time i n t e r v a l s by t h e same g r a p h i c a l p r o c e d u r e as o u t l i n e d f o r k ; the values o f k are given i n Table I I I . c s
p
c s
Table I I I .
Rate C o e f f i c i e n t s
Reaction time, h r
Fibrous
Amorphous
Fibrous
0.0142 0.0112 0.0094 0.0015 0.0010
0.0302 0.0271 0.0204 0.0057 0.0055
0.106 0.0338 0.0305 0.0119 0.0073
0 2 4 48 96 *k
c s
f o r Chemical
Stopping
60°C
3
80°C Amorphous 0.262 0.107 0.0808 0.0225 0.0205
,hr-l.
The r a t e c o e f f i c i e n t s f o r c h e m i c a l s t o p p i n g d e c r e a s e d w i t h time f o r both s u b s t r a t e s i n a p a t t e r n s i m i l a r t o t h a t f o r p e e l i n g . Thus, as t h e a c c e s s i b l e r e d u c i n g endgroups o c c u p i e d p r o g r e s s i v e l y more o r d e r e d r e g i o n s o f the s t r u c t u r e s , t h e i r r e a c t i v i t y toward c h e m i c a l stopping also decreased. The amorphous h y d r o c e l l u l o s e e x h i b i t e d h i g h e r v a l u e s o f k throughout b o t h the 60 and 80°C r e a c t i o n s . D u r i n g t h e e a r l y p e r i o d o f t h e 80°C r e a c t i o n and throughout t h e 60°C r e a c t i o n , when t h e r a t e c o e f f i c i e n t f o r p e e l i n g was h i g h e r f o r t h e amorphous s u b s t r a t e (Table I I ) , i t s higher k v a l u e c a n be p r i m a r i l y a t t r i b u t e d t o t h e r e a c t i o n o c c u r r i n g i n l e s s o r d e r e d r e g i o n s o f the s t r u c t u r e . Howe v e r , t h i s c o u l d not account f o r t h e b e h a v i o r i n t h e l a t e r p e r i o d o f the 80°C r e a c t i o n s , where p e e l i n g was a p p a r e n t l y h i n d e r e d t o s i m i l a r e x t e n t s by t h e c r y s t a l l i n e r e g i o n s o f b o t h s t r u c t u r e s . Therefore, i t i s c o n c l u d e d t h a t the c e l l u l o s e I I domains i n the amorphous subs t r a t e d i d not i n h i b i t c h e m i c a l s t o p p i n g as d r a s t i c a l l y as the c e l l u l o s e I domains i n t h e f i b r o u s s u b s t r a t e . c s
c s
F u r t h e r c l a r i f i c a t i o n o f t h e s e d i f f e r e n c e s i s p r o v i d e d by comp a r i n g t h e r e l a t i v e r a t e s o f p e e l i n g and c h e m i c a l s t o p p i n g f o r t h e two s u b s t r a t e s . Average v a l u e s o f k p / k ( T a b l e IV) were c a l c u l a t e d u s i n g E q u a t i o n 3, d e r i v e d by d i v i d i n g E q u a t i o n 1 by E q u a t i o n 2. c s
16.
GENTILE ET AL.
285
Alkaline Degradation of Hydrocellulose d[Yi]/d[AE]
= k /k p
( )
c s
3
The r e l a t i v e r a t e s o f p e e l i n g and c h e m i c a l s t o p p i n g were h i g h e r f o r t h e f i b r o u s h y d r o c e l l u l o s e t h r o u g h o u t the r e a c t i o n s . F o r t h e amorphous s u b s t r a t e , t h e v a l u e s o f k p / k were lower a t t h e o u t s e t , and d e c r e a s e d s u b s t a n t i a l l y , a t l o n g e r r e a c t i o n t i m e s . Since k / k remained c o n s t a n t throughout t h e f i b r o u s h y d r o c e l l u l o s e r e a c t i o n s , the r e l a t i v e r e a c t i v i t y o f i t s a c c e s s i b l e r e d u c i n g endgroups toward b o t h r e a c t i o n s appears not t o change as t h e r e a c t i o n s p r o g r e s s from r e g i o n s o f lower o r d e r t o r e g i o n s o f h i g h e r o r d e r . This i s consist e n t w i t h the l e s s o r d e r e d r e g i o n s e x h i b i t i n g some c e l l u l o s e I c h a r a c t e r ; reference here i s to s l i g h t l y d i s t o r t e d c e l l u l o s e I domains and c r y s t a l l i t e s u r f a c e s ( 2 0 ) . In t h e l a t e r p e r i o d s o f the amorphous h y d r o c e l l u l o s e r e a c t i o n s , however, p e e l i n g was i n h i b i t e d much more than c h e m i c a l s t o p p i n g . T h i s i s c o n s i s t e n t w i t h the e a r l i e r p r o p o s a l t h a t c e l l u l o s e I I domains do not h i n d e r c h e m i c a l s t o p p i n g as e f f e c t i v e l y as c e l l u l o s e I domains, w h i l e b o t h c r y s t a l l i n e forms a r e h i g h l y r e s i s t a n t t o p e e l i n g . Thus, t h e p h y s i c a l p r e s e n c e o f the c r y s t a l l i n e domains appears t o d e t e r t h e p r o g r e s s i o n o f p e e l i n g a l o n g a c e l l u l o s e m o l e c u l e , w h i l e b o t h the degree o f s t r u c t u r a l o r d e r and the p a r t i c u l a r m o l e c u l a r c o n f o r m a t i o n d i c t a t e the r e a c t i v i t y o f an a c c e s s i b l e r e d u c i n g endgroup toward c h e m i c a l stopping. c s
p
Table
IV.
R e l a t i v e Rates o f P e e l i n g and C h e m i c a l
Reaction
c s
Stopping kp/k
F i b r o u s 60°C (0-168 h r ) F i b r o u s 80°C (0-96 h r ) Amorphous 60°C (0-48 h r ) (48-168 h r ) Amorphous 80°C (0-4 h r ) (4-96 h r )
c s
291 264 204 84 154 23
These f i n d i n g s a r e c o n s i s t e n t w i t h r e s u l t s o f c o m p a r a t i v e a l k a l i n e d e g r a d a t i o n s t u d i e s o f n a t i v e ( c e l l u l o s e I ) and m e r c e r i z e d ( c e l l u l o s e I I ) c e l l u l o s e (3^ 6^8). i n a d d i t i o n , decreases i n k p / k f o r b o t h s u b s t r a t e s w i t h i n c r e a s i n g temperature a r e i n agreement w i t h the h i g h e r a c t i v a t i o n energy r e p o r t e d f o r c h e m i c a l s t o p p i n g versus p e e l i n g i n h y d r o c e l l u l o s e ( 5 ) . In a d d i t i o n t o u n d e r g o i n g c h e m i c a l s t o p p i n g r e a c t i o n s , c e l l u l o s e m o l e c u l e s a l s o a r e thought t o t e r m i n a t e i n r e d u c i n g endgroups which a r e p h y s i c a l l y i n c a p a b l e o f r e a c t i n g due t o t h e i r i n a c c e s s b i l i t y t o the a l k a l i n e medium (3_-_5) · T h term " p h y s i c a l s t o p p i n g " has been used t o c h a r a c t e r i z e the f o r m a t i o n o f i n a c c e s s i b l e r e d u c i n g endgroups ( n o n r e a c t i v e ) on m o l e c u l e s which p r e v i o u s l y c o n t a i n e d a c c e s s i b l e ( r e a c t i v e ) r e d u c i n g endgroups. The p s e u d o - f i r s t - o r d e r rate expression f o r p h y s i c a l stopping i s written: c s
e
d[IRE]/dt
= k
p s
[ARE ] t
(4)
286
THE STRUCTURES OF CELLULOSE
where
[IRE] = I n a c c e s s i b l e r e d u c i n g endgroup c o n t e n t , as mole f r a c t i o n o f t o t a l monomer u n i t s a t z e r o - t i m e kp = Psudo-first-order rate c o e f f i c i e n t f o r physical stopping S
A l t h o u g h p h y s i c a l s t o p p i n g i s not a c h e m i c a l r e a c t i o n , per s e , kp v a l u e s d e t e r m i n e d u s i n g E q u a t i o n 4 may be compared t o k v a l u e s , p r o v i d i n g a measure o f t h e r e l a t i v e importance o f t h e two modes o f s t o p p i n g . F u r t h e r m o r e , comparison o f k p v a l u e s f o r two s u b s t r a t e s g i v e s an i n d i c a t i o n o f t h e r e l a t i v e e x t e n t o f s t r u c t u r a l hindrance to peeling. In b o t h the 60 and 80°C r e a c t i o n s , t h e f i b r o u s h y d r o c e l l u l o s e exhibited higher k v a l u e s t h a n t h e amorphous h y d r o c e l l u l o s e (Table V). T h i s appears t o be due t o t h e i n v o l v e m e n t o f more m o l e c u l e s i n c r y s t a l l i n e domains o f the f i b r o u s s u b s t r a t e . The g r e a t e r i n h i b i t i o n o f c h e m i c a l s t o p p i n g by c e l l u l o s e I than c e l l u l o s e I I domains may a l s o have c o n t r i b u t e d t o t h i s e f f e c t by a l l o w i n g more m o l e c u l e s i n t h e f i b r o u s h y d r o c e l l u l o s e t o p e e l t o a p o i n t where t h e r e d u c i n g endgroup would be i n a c c e s s i b l e . S
c s
S
p s
T a b l e V. Reaction Time, h r 0 2 4 48 96 3
k
p s
Rate C o e f f i c i e n t s f o r P h y s i c a l
Stopping
3
60° C
80° C
Fibrous
Amorphous
0.0410 0.0218 0.0157 0.0032 0.0028
0.0142 0.0137 0.0132 0.0021 0.0019
Fibrous 0.363 0.0812 0.0543 0 0
Amorphous 0.154 0.0700 0.0195 0 0
,hr-l.
At 80°C and f o r l o n g e r r e a c t i o n t i m e s , b o t h h y d r o c e l l u l o s e s ceased p h y s i c a l stopping. T h i s may be an i n d i c a t i o n t h a t each phys i c a l s t r u c t u r e has some maximum number o f p o t e n t i a l p h y s i c a l stopping s i t e s . As a consequence, i n a c c e s s i b l e r e d u c i n g endgroups c o u l d become a c c e s s i b l e as a d j a c e n t m o l e c u l e s a r e removed by p e e l i n g , g i v i n g r i s e t o a steady s t a t e d i s t r i b u t i o n o f a c c e s s i b l e and i n a c e s s i b l e r e d u c i n g endgroups. E x c e p t f o r t h e l a t e r p e r i o d o f t h e 80°C r e a c t i o n s , t h e f i b r o u s h y d r o c e l l u l o s e e x h i b i t e d a h i g h e r v a l u e o f k p ( T a b l e V) t h a n k (Table IV). C o n s e q u e n t l y t h e d e g r a d a t i o n o f a m a j o r i t y o f t h e molec u l e s i n the f i b r o u s h y d r o c e l l u l o s e was t e r m i n a t e d by p h y s i c a l r a t h e r than chemical stopping processes. In c o n t r a s t , c h e m i c a l s t o p p i n g was t h e dominant mechanism o f s t a b i l i z a t i o n i n t h e amorphous h y d r o c e l l u l o s e . S
c s
Random C h a i n C l e a v a g e R e a c t i o n . In a d d i t i o n t o p e e l i n g , c e l l u l o s e i s a l s o r e p o r t e d t o undergo random c l e a v a g e o f g l y c o s i d i c l i n k a g e s i n a l k a l i n e media (J^,2). T h i s r e a c t i o n r e s u l t s i n the f o r m a t i o n o f
16.
287
Alkaline Degradation of Hydrocellulose
GENTILE ET AL.
one r e d u c i n g and one n o n r e d u c i n g endgroup. S i n c e r e d u c i n g endgroups can a l s o be i n v o l v e d i n p e e l i n g and s t o p p i n g r e a c t i o n s , i t i s not p o s s i b l e to m o n i t o r d i r e c t l y t h e i r f o r m a t i o n due t o random c h a i n cleavage. However, the r a t e o f c h a i n c l e a v a g e can be c h a r a c t e r i z e d by m o n i t o r i n g i n c r e a s e s i n t h e t o t a l number o f endgroups. Accurate c h a r a c t e r i z a t i o n o f the r e a c t i o n does r e q u i r e t h a t no o t h e r changes i n the t o t a l number o f endgroups o c c u r , as f o r example, from l o s s o f m o l e c u l e s by complete p e e l i n g o r d i s s o l u t i o n . D u r i n g d e g r a d a t i o n o f the f i b r o u s h y d r o c e l l u l o s e , no changes i n t o t a l endgroup c o n t e n t were d e t e c t e d ( T a b l e V I ) . T h i s i s c o n s i s t e n t w i t h r e s u l t s o f p r e v i o u s s t u d i e s (6,7) i n which c h a i n c l e a v a g e was found to be i m p o r t a n t i n n a t i v e c e l l u l o s e o n l y above 100°C.
Table VI.
T o t a l Endgroup C o n t e n t s
Reaction Time, h r 0 2 4 8 24 48 96 168 3
E x p r e s s e d as t ime.
3
of
Hydrocelluloses 80°C
60°C Fibrous 1.94 1.94 1.92 2.04 1.95 1.92 1.86 1.89 10
3
Amorphous 4.13 3.57 3.44 3.52 3.59 3.65 3.76 4.09
χ mole
Fibrous
Amorphous
1.88 1.90 1.96 1.96 1.89 1.89 1.90
4.51 4.28 4.35 4.44 4.96 5.05 5.35
—
—
f r a c t i o n o f t o t a l monomer u n i t s at
zero-
In c o n t r a s t , t h e amorphous h y d r o c e l l u l o s e underwent i n i t i a l d e c l i n e i n t o t a l endgroup c o n t e n t ( T a b l e VI) which may be a t t r i b u t e d t o complete p e e l i n g and/or d i s s o l u t i o n o f low DP m o l e c u l e s . After the i n i t i a l p e r i o d s , t o t a l endgroup c o n t e n t s i n c r e a s e d g r a d u a l l y a t b o t h 60 and 80°C, i n d i c a t i n g t h a t random c h a i n c l e a v a g e o c c u r r e d . Random c h a i n c l e a v a g e must a l s o have o c c u r r e d d u r i n g t h e i n i t i a l p e r i o d s but was p r o b a b l y masked by the more s u b s t a n t i a l n e g a t i v e e f f e c t s o f complete p e e l i n g o r d i s s o l u t i o n on the t o t a l endgroup contents. The amorphous s u b s t r a t e s u f f e r e d the most r a p i d d e c l i n e i n h y d r o x y l a c c e s s i b i l i t y ( F i g u r e 1) d u r i n g the same p e r i o d s i n which t o t a l endgroup l o s s e s o c c u r r e d . T h i s i n d i c a t e s t h a t complete p e e l ing or d i s s o l u t i o n p r i m a r i l y involved molecules e x i s t i n g e n t i r e l y w i t h i n amorphous r e g i o n s and became i n s i g n i f i c a n t once the m a j o r i t y o f h i g h l y a c c e s s i b l e c h a i n s had been removed o r c h e m i c a l l y s t a b i lized. F u r t h e r s u p p o r t i s thus p r o v i d e d f o r the h y p o t h e s i s t h a t s e l e c t i v e p e e l i n g o f amorphous m a t e r i a l c o n t r i b u t e s t o the h i g h e r r a t e c o e f f i c i e n t o f p e e l i n g i n the case o f the amorphous h y d r o c e l l u l o s e ( T a b l e I I ) . The c o m p a r a t i v e l a c k o f s i m i l a r l o s s e s from t h e f i b r o u s s u b s t r a t e s u g g e s t s t h a t the l a r g e m a j o r i t y o f m o l e c u l e s were embedded t o some e x t e n t i n c r y s t a l l i n e r e g i o n s .
THE STRUCTURES OF CELLULOSE
288
Because endgroup l o s s e s o c c u r r e d s i m u l t a n e o u s l y w i t h random c h a i n c l e a v a g e d u r i n g the i n i t i a l p e r i o d s , a n a l y s i s o f the t o t a l endgroup d a t a f o r k i n e t i c s o f c h a i n c l e a v a g e was c o n f i n e d t o the l a t e r reaction periods. S i n c e the t o t a l number o f monomer u n i t s , o r y i e l d , i s e s s e n t i a l l y e q u a l t o the number o f g l y c o s i d i c l i n k a g e s , the p s e u d o - f i r s t - o r d e r r a t e e x p r e s s i o n f o r random c h a i n c l e a v a g e can be w r i t t e n as: d[TE]/dt where [TE] k [Y ] c c
t
= k
c c
[Y ]
(5)
t
= T o t a l endgroup c o n t e n t , as mole f r a c t i o n o f t o t a l monomer u n i t s a t z e r o - t i m e = Rate c o e f f i c i e n t f o r random c h a i n c l e a v a g e , h r " * = Y i e l d , as mole f r a c t i o n of t o t a l monomer u n i t s a t time " t "
Rate c o e f f i c i e n t s f o r random c h a i n c l e a v a g e i n the 60°C amor phous h y d r o c e l l u l o s e r e a c t i o n d e c r e a s e d g r a d u a l l y from 8 t o 168 hours ( T a b l e V I I ) . At 80°C, the d e c r e a s e i n k o c c u r r e d more r a p i d l y between 2 and 48 h o u r s , w i t h a more g r a d u a l d e c l i n e up t o 96 hours. T h i s r e f l e c t s the more r a p i d d e c l i n e i n a c c e s s i b i l i t y o f the amorphous h y d r o c e l l u l o s e at 80°C ( F i g u r e 1). Thus, random c h a i n c l e a v a g e appears t o be i n h i b i t e d by the l a r g e r c e l l u l o s e I I f r a c t i o n t h a t formed i n the amorphous s u b s t r a t e a t 80°C. c c
Table
VII.
Reaction Time, h r 0 2 8 48 96 168
3
Rate C o e f f i c i e n t s f o r Random C h a i n C l e a v a g e 60°C Fibrous 0 0 0 0 0 0
80° C Fibrous
Amorphous
8.42 8.38 8.34 7.53
ND ND χ 10" χ 10' χ 10" χ 1(T
6
6
6
6
ak ,hr-l. ND = Rate c o e f f i c i e n t s not d e t e r m i n e d due peeling or d i s s o l u t i o n
0 0 0 0 0
—
Amorphous
5.78 5.31 0.82 0.49
ND χ χ χ χ
ΙΟ" 10~ ΙΟ" ΙΟ"
—
c c
to s i m u l t a n e o u s
complete
The absence o f c h a i n c l e a v a g e i n the f i b r o u s h y d r o c e l l u l o s e s u g g e s t s t h a t i t s d i s o r d e r e d r e g i o n s were more h i g h l y s t r u c t u r e d t h a n the c o r r e s p o n d i n g r e g i o n s o f the amorphous h y d r o c e l l u l o s e . T h i s i s c o n s i s t e n t w i t h the r e s u l t s o f a p r e v i o u s study (6^ i n which m e r c e r i z e d c e l l u l o s e was found t o be more s u s c e p t i b l e t o random c h a i n c l e a v a g e t h a n n a t i v e c e l l u l o s e . Another i m p l i c a t i o n i s t h a t the d i s o r d e r e d r e g i o n s a s s o c i a t e d w i t h the two c r y s t a l l i n e polymorphs d i s p l a y d i f f e r e n t d e g r e e s o f s t r u c t u r a l o r d e r , g i v i n g r i s e to d i f f e r e n c e s i n r e a c t i v i t y . Thus, i n a d d i t i o n t o m o l e c u l a r m o b i l i t y and a c c e s s i b i l i t y , the p a r t i c u l a r m o l e c u l a r c o n f o r m a t i o n
5
5
5
5
16.
GENTILE ET AL.
289
Alkaline Degradation of Hydrocellulose
appears t o i n f l u e n c e s u s c e p t i b i l i t y reaction.
t o the random c h a i n
cleavage
Conclusions A l k a l i n e p e e l i n g and c h e m i c a l s t o p p i n g o c c u r more r a p i d l y i n t h e amorphous r e g i o n s o f amorphous h y d r o c e l l u l o s e t h a n i n t h e d i s o r d e r e d r e g i o n s o f f i b r o u s h y d r o c e l l u l o s e . In a d d i t i o n , random c h a i n c l e a v a g e a t 60 and 80°C o c c u r s o n l y i n amorphous h y d r o c e l l u l o s e . T h e r e f o r e , i t i s proposed t h a t the d i s o r d e r e d r e g i o n s o f the f i b r o u s hydrocelluose c o n s i s t o f less r e a c t i v e molecules at c r y s t a l l i t e s u r f a c e s and i n s l i g h t l y d i s t o r t e d c r y s t a l l i n e domains, as p r e v i o u s l y suggested ( 2 0 ) . P e e l i n g i s i n h i b i t e d t o s i m i l a r e x t e n t s by the c r y s t a l l i n e o r d e r o f b o t h c e l l u l o s e I and I I a l l o m o r p h s , w h i l e c h e m i c a l s t o p p i n g i s s i g n i f i c a n t l y more i n h i b i t e d i n t h e c e l l u l o s e I a l l o m o r p h . This i s c o n s i s t e n t w i t h the h i g h e r r a t i o o f the r a t e o f c h e m i c a l s t o p p i n g t o t h a t o f p e e l i n g t y p i c a l l y r e p o r t e d f o r m e r c e r i z e d c e l l u l o s e i n comp a r i s o n t o n a t i v e c e l l u l o s e (3^6--8)· Physical stopping, that i s , formation o f i n a c c e s s i b l e reducing endgroups, o c c u r s when p e e l i n g o f m o l e c u l a r c h a i n s r e a c h e s t h e c r y s t a l l i n e domains i n both c e l l u l o s e I and I I . The r e l a t i v e r a t e s o f p h y s i c a l and c h e m i c a l s t o p p i n g a r e d i c t a t e d by t h e number o f m o l e c u l e s i n v o l v e d i n c r y s t a l l i n e domains. In a p r e v i o u s study (5^), c e l l u l o s e m o l e c u l e s were r e p o r t e d t o m a i n t a i n c o n s t a n t r e a c t i v i t y toward p e e l i n g and c h e m i c a l s t o p p i n g u n l e s s p h y s i c a l s t o p p i n g occurred. However, the r e s u l t s o f t h e p r e s e n t study i n d i c a t e t h a t r e a c t i v i t y d i m i n i s h e s g r a d u a l l y as r e a c t i o n s approach more h i g h l y ordered regions of p h y s i c a l structure. S i m u l t a n e o u s l y , abrupt p h y s i c a l s t o p p i n g can o c c u r . The r a t e o f c h e m i c a l s t o p p i n g i n c r e a s e s w i t h temperature relat i v e t o p e e l i n g i n b o t h f i b r o u s and amorphous h y d r o c e l l u l o s e . T h i s o b s e r v a t i o n i s c o n s i s t e n t w i t h p r e v i o u s f i n d i n g s ( 5_). Experimental Cellulose Substrates. Raw c o t t o n f i b e r c u t i n c a . 0.25 i n c h l e n g t h s was p u r i f i e d by e x t r a c t i o n w i t h c h l o r o f o r m , 95% e t h a n o l , b o i l i n g 1% (w/w) sodium h y d r o x i d e ( o x y g e n f r e e ) , and d i e t h y l e n e t r i a m i n e p e n t a a c e t i c a c i d (0.15% w/v, pH 9) ( 1 0 ) . F i b r o u s h y d r o c e l l u l o s e was p r e p a r e d by t r e a t i n g the p u r i f i e d f i b e r s (60 g) w i t h 0.1M h y d r o c h l o r i c a c i d (6L) a t 40°C f o r 20 h o u r s , washing w i t h d i s t i l l e d water ( u n t i l n e u t r a l ) , and then f r e e z e - d r y i n g . Amorphous h y d r o c e l l u l o s e was p r e p a r e d by d r o p w i s e a d d i t i o n o f a DMSO-PF s o l u t i o n o f the f i b r o u s h y d r o c e l l u l o s e (0.2%, w/v, c e l l u l o s e / D M S O , 3.5L) t o 0.2M sodium m e t h o x i d e - i s o p r o p o x i d e s o l u t i o n (1:1, v / v , m e t h a n o l : i s o p r o p a n o l , 14L) ( 9 , 1 0 ) . The r e s u l t i n g p r e c i p i t a t e was washed w i t h 0.2M sodium m e t h o x i d e - i s o p r o p o x i d e , methanol ( u n t i l n e u t r a l ) , 0.1M h y d r o c h l o r i c a c i d , and d i s t i l l e d water ( u n t i l n e u t r a l ) , and then freeze-dried. Both the f i b r o u s and amorphous h y d r o c e l l u l o s e s were f u r t h e r d r i e d i n v a c u o o v e r phosphorus p e n t o x i d e t o c o n s t a n t weight. Degradation Procedure. A l k a l i n e d e g r a d a t i o n s were conducted i n 316 s t a i n l e s s s t e e l laboratory d i g e s t e r s (10). Hydrocellulose substrate
290
THE STRUCTURES OF CELLULOSE
(400 mg) and o x y g e n - f r e e 1.0M sodium h y d r o x i d e (40 mL) were s e a l e d i n t h e r e a c t i o n v e s s e l s under n i t r o g e n , and t h e v e s s e l s were r o t a t e d end-over-end a t c a . 3 rpm i n a c o n s t a n t temperature o i l b a t h . The r e a c t i o n m i x t u r e s were m a i n t a i n e d a t 60 o r 80°C f o r t h e s p e c i f i e d time i n t e r v a l , c o o l e d t o 20°C, and n e u t r a l i z e d w i t h 1.0M h y d r o chloric acid. Z e r o - t i m e samples were p r e p a r e d by l i m i t i n g t h e time a t t h e r e a c t i o n t e m p e r a t u r e t o c a . one minute. Degraded h y d r o c e l l u l o s e was washed w i t h 0.1M h y d r o c h l o r i c a c i d and d i s t i l l e d water ( u n t i l n e u t r a l ) , and then f r e e z e - d r i e d . Y i e l d was d e t e r m i n e d a f t e r f u r t h e r d r y i n g i n v a c u o o v e r phosphorus p e n t o x i d e t o c o n s t a n t weight. A n a l y t i c a l Methods. C a r b o x y l i c a c i d endgroup c o n t e n t s were d e t e r mined by methylene b l u e a b s o r p t i o n u s i n g TAPPI S t a n d a r d Method T237 su-63 w i t h minor m o d i f i c a t i o n s ( 1 0 ) . A c c e s s i b l e r e d u c i n g endgroups were d e t e c t e d by r e d u c t i o n w i t h sodium b o r o h y d r i d e - ^ H , and t o t a l r e d u c i n g endgroups were d e t e r m i n e d s i m i l a r l y a f t e r r e g e n e r a t i n g t h e c e l l u l o s e from t h e DMSO-PF s o l v e n t (9,10). Inaccessible reducing endgroup c o n t e n t s were c a l c u l a t e d as t o t a l l e s s a c c e s s i b l e r e d u c i n g endgroup c o n t e n t s . C e l l u l o s e h y d r o x y l a c c e s s i b i l i t y was measured by t h e d e u t e r a t i o n method o f R o u s e l l e and N e l s o n ( 1 3 ) , b u t t h e d e u t e r a t i o n time ( i n l i q u i d D2O) was extended t o 12 h o u r s ( 1 0 ) . X-ray d i f f r a c t o g r a m s were c o l l e c t e d on a N o r e l c o d i f f r a c t o m e t e r , u s i n g n i c k e l - f i l t e r e d , CuKct r a d i a t i o n . Raman s p e c t r a were a c q u i r e d w i t h a J o b i n Yvon Ramanor S p e c t r o m e t e r , u t i l i z i n g t h e 5145 Â l i n e o f an a r g o n l a s e r o p e r a t e d , a t 100 mw, as t h e e x c i t i n g s o u r c e . S o l i d - s t a t e l^C-NMR s p e c t r a were o b t a i n e d on a G e n e r a l E l e c t r i c S-100 i n s t r u m e n t employing t h e combined t e c h n i q u e s (16,17) o f p r o t o n - c a r b o n c r o s s p o l a r i z a t i o n , h i g h power p r o t o n d e c o u p l i n g , and m a g i c - a n g l e sample spinning. Acknowledgments The a u t h o r s pany, I n c . Woitkovich appreciates during this
w i s h t o thank Dr. T. E a r l y o f GE-NMR Instruments Comf o r o b t a i n i n g t h e s o l i d - s t a t e l^c-NMR s p e c t r a and Mr. C. f o r a c q u i r i n g t h e Raman s p e c t r a . V. M. G e n t i l e s i n c e r e l y f e l l o w s h i p s u p p o r t from The I n s t i t u t e o f Paper C h e m i s t r y work.
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16.
GENTILE ET AL.
Alkaline Degradation of Hydrocellulose
291
7. Franzon, O.; Samuelson, O. Svensk Papperstid. 1957, 60, 872. 8. Christofferson, K.; Samuelson, O. Svensk Papperstid. 1962, 65, 571. 9. Gentile, V. M.; Schroeder, L. R.; Atalla, R. H. J. Wood Chem. 1986, 6, 1. 10. Gentile, V. M. Doctoral Dissertation, The Institute of Paper Chemistry, Appleton, Wisconsin (1986). 11. Nicholson, M. D.; Johnson, D. C.; Haigh, F. C. Appl. Polymer Symp. 1976, 28, 931. 12. Baker, T. J.; Schroeder, L. R.; Johnson, D. C. Cellulose Chem. Technol. 1981, 15, 311. 13. Rouselle Μ. Α.; Nelson, M. L. Textile Res. J. 1971, 41, 599. 14. Wadsworth, L.C.; Cuculo, L. C. In Modified Cellulosics, Part III, p. 117, R. M. Rowell and R. A. Young (Eds.), Academic Press, New York, 1978. 15. Atalla, R. H. J. Appl. Polymer Sci. (Appl. Polymer Symp.) 1983, 37, 295. 16. Earl, W. L.; VanderHart, D. L. Macromolecules 1981, 14, 570. 17. VanderHart, D. L.; Atalla, R. H. Macromolecules 1984, 17, 1465. 18. Tripp, V. M. In Cellulose and Cellulose Derivatives, Part IV, p. 305, N. Bikales and L. Segal (Eds.), Wiley-Interscience, New York, 1981. 19. Howsmon, J. Α.; Sisson, W. A. In Cellulose, Part I, 2nd Ed., p. 231, E. Ott and H. M. Spurlin (Eds.), Interscience Publishers, New York, 1954. 20. Rowland, S. P. In Modified Cellulosics, Part III, p. 162, and references cited therein, R. M. Rowell and R. A. Young (Eds.), Academic Press, New York, 1978. RECEIVED March 5, 1987
