Physical Structure and Alkaline Degradation of Hydrocellulose

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

16.

Alkaline Degradation of Hydrocellulose

GENTILE ET AL.

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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

274

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 ) .

16.

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275

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


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.

277


GENTILE ET AL.

(002)1

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Figure

2.

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24

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22

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20 18 DEGREES 20

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14

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10

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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Figure

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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GENTILE ET AL.


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


Figure

8.

Hydrocellulose y i e l d

during degradation

i n 1.0M

NaOH.

16.

GENTILE ET AL.

283


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


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


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


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 .


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


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


(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.

References 1.

Meller, A. Holzforschung 1960, 14, 78 and references cited therein. 2. Richards, G. N. In Cellulose and Cellulose Derivatives; Part V, p. 1007 and references cited therein, N. Bikales and L. Segal (Eds.), Wiley-Interscience, New York, 1971. 3. Machell, G.; Richards, G. N. Tappi 1958, 41, 12. 4. Colbran, R. L.; Davidson, G. F. J. Textile Inst. 1961, 52, T73. 5. Haas, D. W.; Hrutfiord, B. F.; Sarkanen, Κ. V. Appl. Polymer Sci. 1967,11,587. 6. Lai, Y.-Z.; Sarkanen, Κ. V. Cellulose Chem. Technol. 1967, 1, 517.

16.

GENTILE ET AL.


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

Physical Structure and Alkaline Degradation of Hydrocellulose

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