Raman Spectra of Celluloses - ACS Publications - American Chemical

Chapter 8

Raman Spectra of Celluloses James H. Wiley and Rajai H. Atalla

Downloaded by PENNSYLVANIA STATE UNIV on September 10, 2013 | http://pubs.acs.org Publication Date: June 22, 1987 | doi: 10.1021/bk-1987-0340.ch008

Institute of Paper Chemistry, Appleton, WI 54912

An investigation to study molecular orientation and polymorphy in cellulose fibers, and to further assign the bands in the vibrational spectrum of cellulose was conducted utilizing the Raman microprobe. The microprobe allows spectra to be recorded from domains as small as 1 micrometer and thereby greatly increases the potential of Raman spectroscopy as a tool for studying the structure of cellulose fibers. In the band assignment work, spectra were recorded from oriented fibers by varying the polarization of the incident light relative to the fiber axis. Analysis of the band intensities revealed new information about the directional character of the vibrational displacements. This information was used in conjunction with the spectra of deuterated celluloses and normal coordinate analyses of cellulose model compounds to make assignments. Cellulose polymorphy was studied by comparing the spectra of Valonia, ramie, and mercerized ramie. It appears that the conformation of the cellulose backbone is the same in Valonia and ramie celluloses, but that the hydrogen bonding patterns are different. Mercerized cellulose and native celluloses differ in both their backbone conformations and hydrogen bonding patterns. Cellulose orientation in the plane perpendicular to the chain axis was studied by recording spectra of ramie cross sections with different polarizations of the incident light relative to the cell wall surface. The intensities are consistent with random cellulose orientation. It appears, however, that the sample preparation techniques can influence the cellulose orientation. Therefore, further studies will be necessary to understand the molecular orientation in cellulose fibers. Many q u e s t i o n s about t h e m o l e c u l a r o r g a n i z a t i o n i n p l a n t c e l l w a l l s r e m a i n unanswered. A new i n s t r u m e n t , t h e Raman m i c r o p r o b e , has

0097-6156/87/0340-0151 $06.00/0 © 1987 American Chemical Society

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

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p r o v i d e d a n o v e l method f o r i n v e s t i g a t i n g p l a n t c e l l w a l l s t r u c t u r e . The o b j e c t i v e i n t h i s work was t o study t h e s t r u c t u r e o f the p r i n c i p a l c e l l w a l l component, c e l l u l o s e , u s i n g the Raman m i c r o p r o b e s unique c a p a b i l i t i e s . The i n v e s t i g a t i o n had two phases. In t h e f i r s t phase, t h e i n f o r m a t i o n made a c e s s i b l e by t h e m i c r o p r o b e was used t o advance t h e assignment o f the Raman spectrum o f c e l l u l o s e . In t h e second phase, t h e m i c r o p r o b e was used t o study m o l e c u l a r o r i e n t a t i o n and polymorphy i n c e l l u l o s e f i b e r s . The major r e s u l t s from b o t h phases o f t h i s i n v e s t i g a t i o n w i l l be d i s c u s s e d i n t h i s report. 1


Background Raman S p e c t r a o f C e l l u l o s e . In l a s e r e x c i t e d Raman s p e c t r o s c o p y , a sample i s exposed t o monochromatic l i g h t , and t h e s c a t t e r e d l i g h t i s analyzed. The frequency o f a s m a l l f r a c t i o n o f t h e s c a t t e r e d l i g h t i s s h i f t e d r e l a t i v e t o the e x c i t i n g l i g h t . The magnitude o f t h e frequency s h i f t corresponds t o the v i b r a t i o n a l f r e q u e n c i e s o f the m o l e c u l e s i n t h e sample. T h e r e f o r e , Raman s p e c t r o s c o p y p r o v i d e s i n f o r m a t i o n s i m i l a r t o t h a t p r o v i d e d by i n f r a r e d s p e c t r o s c o p y . Both Raman and i n f r a r e d s p e c t r o s c o p y y i e l d i n f o r m a t i o n about c h e m i c a l f u n c t i o n a l i t y , m o l e c u l a r c o n f o r m a t i o n , and hydrogen bonding. Raman s p e c t r o s c o p y , however, h a s some i m p o r t a n t advantages. H i g h l y p o l a r bond systems, w h i c h r e s u l t i n i n t e n s e i n f r a r e d bands, have r e l a t i v e l y low p o l a r i z a b i l i t i e s and, hence, weak Raman i n t e n sities. Water, t h e r e f o r e has v e r y weak Raman bands and does n o t i n t e r f e r e w i t h the spectrum o f c e l l u l o s e . In t h e Raman t e c h n i q u e , c o n t r o l o f t h e p o l a r i z a t i o n o f the e x c i t i n g l i g h t c o u p l e d w i t h a n a l y s i s o f t h e p o l a r i z a t i o n o f the s c a t t e r e d l i g h t c a n f a c i l i t a t e a s signment o f t h e s p e c t r a and p r o v i d e i n f o r m a t i o n about m o l e c u l a r orientation. In i n f r a r e d s p e c t r a , t h e a t t e n u a t i o n o f the i n c i d e n t beam i s measured. T h i s means t h a t any p r o c e s s e s o t h e r than a b s o r p t i o n which cause a t t e n u a t i o n o f t h e i n c i d e n t beam a r e p r o b l e m a t i c . S i n c e t h e r e f r a c t i v e i n d e x o f the sample w i l l o f t e n go through l a r g e e x c u r s i o n s i n t h e n e i g h b o r h o o d o f a b s o r p t i o n bands, t h e s c a t t e r i n g l o s s e s w i l l vary g r e a t l y w i t h frequency over the i n f r a r e d r e g i o n . The v a r i a t i o n s i n t h e r e f r a c t i v e i n d e x c a n cause anomalous f e a t u r e s i n i n f r a r e d s p e c t r a . In Raman s p e c t r o s c o p y , r e f r a c t i v e index v a r i a t i o n s a r e n o t a problem, s i n c e t h e e x c i t a t i o n f r e q u e n c y i s f a r removed from any a b s o r p t i o n bands. Therefore, i t i s easier to r e c o r d Raman r a t h e r t h a n i n f r a r e d s p e c t r a from samples such as c e l l u l o s e which s c a t t e r l i g h t s t r o n g l y . Raman M i c r o p r o b e . A r e c e n t i n n o v a t i o n i n Raman s p e c t r o s c o p y was the development o f the Raman m i c r o p r o b e . The m i c r o p r o b e i s a s p e c i a l l y designed o p t i c a l microscope coupled with a c o n v e n t i o n a l Raman s p e c t r o m e t e r . The m i c r o s c o p e performs two key f u n c t i o n s . I t f o c u s e s the e x c i t i n g l i g h t on t h e sample down t o a d i a m e t e r o f one m i c r o m e t e r ; t h e n i t g a t h e r s t h e s c a t t e r e d l i g h t and t r a n s m i t s i t t o t h e e n t r a n c e s l i t o f the s p e c t r o m e t e r . Since the microprobe a c q u i r e s s p e c t r a from such s m a l l domains, t h e s t r u c t u r a l h e t e r o g e n e i t y o f the domains i s g r e a t l y r e d u c e d r e l a t i v e t o t h e domains examined i n c o n v e n t i o n a l Raman s p e c t r o s c o p y . The m i c r o p r o b e makes i t p o s s i b l e t o i d e n t i f y t h e m o r p h o l o g i c a l f e a t u r e s from which


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s p e c t r a are r e c o r d e d so t h a t o r i e n t a t i o n , c o m p o s i t i o n , and structure can be s t u d i e d as a f u n c t i o n o f morphology. The s p e c i a l a t t r i b u t e s o f the m i c r o p r o b e make new information available. T h i s i n v e s t i g a t i o n u t i l i z e d the a b i l i t y o f the m i c r o probe to r e c o r d s p e c t r a from m o r p h o l o g i c a l l y homogeneous domains, u s i n g p o l a r i z e d l i g h t t o d e r i v e new i n f o r m a t i o n f o r a s s i g n i n g the Raman spectrum o f c e l l u l o s e . These a s s i g n m e n t s a l o n g w i t h o t h e r s p e c t r a l i n f o r m a t i o n o b t a i n e d w i t h the m i c r o p r o b e were then used t o study polymorphy w i t h i n the c e l l u l o s e I f a m i l y and c e l l u l o s e o r i e n t a t i o n as a f u n c t i o n o f morphology. Band A s s i g n m e n t s . In o r d e r f o r Raman s p e c t r o s c o p y t o p r o v i d e s t r u c t u r a l i n f o r m a t i o n , the assignment o f the s p e c t r a must be u n d e r s t o o d . The l a r g e number o f v i b r a t i o n a l d e g r e e s o f freedom and low symmetry p o s s e s s e d by the c e l l u l o s e m o l e c u l e have made i n t e r p r e t a t i o n o f the spectrum d i f f i c u l t . G i v e n the c o m p l e x i t y o f the problem, a s e r i e s o f d e t a i l e d normal c o o r d i n a t e a n a l y s e s o f model compounds were cond u c t e d i n our l a b o r a t o r y to p r o v i d e a b a s i s f o r i n t e r p r e t i n g the v i b r a t i o n a l spectrum o f c e l l u l o s e ( 1 - 8 ) . The c a l c u l a t i o n s showed t h a t , except f o r the i n t e r n a l v i b r a t i o n s o f the methylene groups, the modes below 1500 cm""* are de l o c a l i z e d motions w h i c h are not adeq u a t e l y d e s c r i b e d by the group f r e q u e n c y a p p r o x i m a t i o n i n which modes are assumed t o be l o c a l i z e d w i t h i n c e r t a i n c h e m i c a l groups i n the m o l e c u l e . Polymorphy. The two major a l l o m o r p h s o f c e l l u l o s e , c e l l u l o s e I and c e l l u l o s e I I , have been s t u d i e d e x t e n s i v e l y . The q u e s t i o n o f p o l y morphy w i t h i n the c e l l u l o s e I ( n a t i v e c e l l u l o s e ) f a m i l y has not r e c e i v e d as much a t t e n t i o n . E v i d e n c e has been r e p o r t e d , however, t h a t the s t r u c t u r e o f n a t i v e c e l l u l o s e v a r i e s d e p e n d i n g on the source. E a r l y x - r a y s t u d i e s o f c o t t o n , ramie, l i n e n , a l g a l c e l l u l o s e , and b a c t e r i a l c e l l u l o s e d e t e c t e d s i g n i f i c a n t d i f f e r e n c e s i n the u n i t c e l l parameters (9_). Based on e l e c t r o n d i f f r a c t o g r a m s , Honjo and Watanabe (10) and o t h e r s (11-13) c o n c l u d e d t h a t the u n i t c e l l o f a l g a l c e l l u l o s e i s l a r g e r and has lower symmetry t h a n the commonly a c c e p t e d c e l l f o r c e l l u l o s e I . M a r r i n a n and Mann (14-15) and l a t e r L i a n g and M a r c h e s s a u l t (16) o b s e r v e d t h a t the i n f r a r e d s p e c t r a o f a l g a l and b a c t e r i a l c e l l u l o s e d i f f e r from the s p e c t r a o f ramie. More r e c e n t l y , A t a l l a and VanderHart (17-18) have s t u d i e d the s o l i d - s t a t e l^C ^MR s p e c t r a o f s e v e r a l forms o f n a t i v e c e l l u l o s e . They c o n c l u d e d t h a t n a t i v e c e l l u l o s e s appear t o be composites o f two d i s t i n c t c r y s t a l l i n e forms o f c e l l u l o s e c a l l e d I and Ig. The proportions of I and Ig i n the composite v a r i e s depending on the s o u r c e o f the c e l l u l o s e . In a l g a l and b a c t e r i a l c e l l u l o s e , the I form dominates, whereas the Ig form dominates i n ramie, c o t t o n , and wood p u l p . These r e s u l t s a r e c o n s i s t e n t w i t h the d a t a from d i f f r a c tometry and i n f r a r e d s p e c t r o s c o p y i n t h a t a l g a l and b a c t e r i a l c e l l u l o s e are s i m i l a r t o each o t h e r but d i f f e r e n t from o t h e r n a t i v e celluloses. a

a

a

The s t r u c t u r a l d i f f e r e n c e s between I and Ig are not u n d e r s t o o d yet. A t a l l a (18) compared the Raman s p e c t r a o f v a r i o u s n a t i v e c e l luloses with d i f f e r e n t I to Ig r a t i o s . He a l s o compared the n a t i v e c e l l u l o s e s p e c t r a w i t h the spectrum o f c e l l u l o s e I I . The s p e c t r a o f a

a



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the n a t i v e c e l l u l o s e s are a l l s i m i l a r to each o t h e r i n the r e g i o n most s e n s i t i v e to c e l l u l o s e c o n f o r m a t i o n . The spectrum o f c e l l u l o s e II i s quite d i f f e r e n t i n t h i s region. Therefore A t a l l a concluded that c e l l u l o s e s I and Ig have s i m i l a r c o n f o r m a t i o n s but are packed in different lattices. In c e l l u l o s e I I , both the c o n f o r m a t i o n and l a t t i c e are d i f f e r e n t from t h a t o f c e l l u l o s e I. Other workers (19-20) have i n t e r p r e t e d t h e s e d i f f e r e n c e s i n the NMR s p e c t r a and o t h e r d a t a i n a l t e r n a t i v e ways. They b e l i e v e t h a t c e l l u l o s e s I and I I have the same s k e l e t a l c o n f o r m a t i o n but are packed i n d i f f e r e n t l a t t i c e s . In t h i s t h e o r y , the differences w i t h i n the c e l l u l o s e I f a m i l y a r e d e r i v e d from the s i z e o f the u n i t cells. V a l o n i a c o n t a i n s a l a r g e r 8 c h a i n u n i t c e l l , whereas ramie c o n t a i n s a m i x t u r e o f the 8 c h a i n u n i t c e l l and the s m a l l e r Meyer and M i s c h u n i t c e l l . T h e r e f o r e the i n t e r p r e t a t i o n o f the NMR s p e c t r a remains c o n t r o v e r s i a l .


a

Orientation. The o r i e n t a t i o n o f the c e l l u l o s e c h a i n a x i s i n a number o f d i f f e r e n t f i b e r s has been s t u d i e d i n d e t a i l ( 2 1 - 2 2 ) . Much l e s s i s known about the c e l l u l o s e o r i e n t a t i o n i n the p l a n e p e r p e n d i c u l a r to the c h a i n a x i s . The o r i e n t a t i o n i n t h i s p l a n e i s d e t e r mined by the l a t e r a l arrangement o f the m i c r o f i b r i l s r e l a t i v e t o each o t h e r . In a l g a l c e l l u l o s e s , the e v i d e n c e from x - r a y and e l e c t r o n d i f f r a c t i o n i n d i c a t e s t h a t the m i c r o f i b r i l s are a r r a n g e d nonrandomly i n the p l a n e p e r p e n d i c u l a r t o the c h a i n a x i s (21-29). P r e s t o n (22) proposed the model shown i n F i g u r e 1 t o e x p l a i n h i s xray data. There are two d i f f e r e n t o r i e n t a t i o n s o f the m i c r o f i b r i l s . The 002 p l a n e s i n one set o f m i c r o f i b r i l s are a p p r o x i m a t e l y p e r p e n d i c u l a r t o the 002 p l a n e s i n the second s e t . In b o t h s e t s o f m i c r o f i b r i l s , the 101 p l a n e s are o r i e n t e d p a r a l l e l t o the c e l l w a l l s u r f a c e ( r e f e r to F i g u r e 1). P r e s t o n ' s model has been c o n f i r m e d i n more r e c e n t s t u d i e s ( 2 9 ) . In the remainder o f t h i s r e p o r t , the t y p e o f o r i e n t a t i o n shown i n F i g u r e 1 w i l l be r e f e r r e d to as a l t e r n a t i n g orientation. D i r e c t measurements o f c e l l u l o s e o r i e n t a t i o n i n f i b e r s have yielded conflicting results. E v i d e n c e from x - r a y d i f f r a c t i o n s t u d i e s o f wood samples s u g g e s t e d t h a t the c e l l u l o s e c r y s t a l l i t e s a r e a r r a n g e d randomly i n the p l a n e p e r p e n d i c u l a r t o the f i b e r a x i s (30-33). Raman s p e c t r o s c o p i c s t u d i e s o f c o t t o n f i b e r s d r i e d under t e n s i o n , however, d e m o n s t r a t e d t h a t the methine C-H bonds are o r i e n t e d p r e f e r e n t i a l l y p e r p e n d i c u l a r t o the s u r f a c e o f the c e l l w a l l (34). S i n c e the C-H bonds a r e p e r p e n d i c u l a r t o the 002 p l a n e , the o r i e n t a t i o n s u g g e s t e d by the Raman e v i d e n c e d i f f e r s from the alternating orientation in algal celluloses. Overview o f E x p e r i m e n t a l Method In the s p e c t r a o f nonrandomly o r i e n t e d polymers, the i n t e n s i t y and p o l a r i z a t i o n o f the Raman s c a t t e r e d l i g h t are dependent on the p o l a r i z a t i o n o f the e x c i t i n g l i g h t r e l a t i v e to the o r i e n t a t i o n o f the m o l e c u l e s ( 3 5 ) . I f the o r i e n t a t i o n o f the m o l e c u l e s i s known, comparison o f s p e c t r a r e c o r d e d w i t h d i f f e r e n t p o l a r i z a t i o n s o f the e x c i t i n g l i g h t y i e l d s u s e f u l i n f o r m a t i o n about the d i r e c t i o n a l i t y o f the v i b r a t i o n a l m o t i o n s . C o n v e r s e l y , i n f o r m a t i o n about m o l e c u l a r o r i e n t a t i o n can be g a i n e d i f the d i r e c t i o n a l i t y of the v i b r a t i o n s i s


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known. In the p r e s e n t work, the major t e c h n i q u e employed i n the e x p e r i m e n t s was t o compare s p e c t r a r e c o r d e d w i t h d i f f e r e n t p o l a r i z a t i o n s o f the i n c i d e n t l a s e r beam r e l a t i v e t o the morphology o f the samples. In a l l s p e c t r a r e c o r d e d , p r e c a u t i o n s were t a k e n t o a v o i d c o m p l i c a t i o n s a s s o c i a t e d w i t h the d i c h r o i s m i n h e r e n t i n the o p t i c s o f the m i c r o s c o p e and the monochromator. F i r s t , a p o l a r i z a t i o n scrambler was i n s e r t e d i n the path o f the Raman s c a t t e r e d l i g h t a t the c o u p l i n g between the m i c r o s c o p e and the monochromator. Second, we d i d not change the p o l a r i z a t i o n o f the i n c i d e n t l i g h t d i r e c t l y but i n s t e a d used a r o t a t i n g s t a g e t o r o t a t e the sample r e l a t i v e t o the p l a n e o f p o l a r i z a t i o n o f the i n c i d e n t l i g h t . Two c l a s s e s o f e x p e r i m e n t s were conducted. In b o t h s e t s o f e x p e r i m e n t s , f i b e r s i n which the c e l l u l o s e c h a i n s a r e o r i e n t e d p a r a l l e l t o the f i b e r a x i s were used. In t h e f i r s t c l a s s o f e x p e r i ments, the p l a n e of p o l a r i z a t i o n o f the i n c i d e n t l i g h t was changed r e l a t i v e t o the a x i s o f the f i b e r s by r o t a t i n g the f i b e r s around the o p t i c a l a x i s o f the m i c r o s c o p e (see F i g u r e 2a). The dependence o f the band i n t e n s i t i e s on the p o l a r i z a t i o n o f the i n c i d e n t l i g h t was s t u d i e d t o d e t e r m i n e the d i r e c t i o n a l c h a r a c t e r o f the v i b r a t i o n a l motions. T h i s i n f o r m a t i o n was used t o advance the assignment o f the Raman spectrum o f c e l l u l o s e . S p e c t r a from V a l o n i a , ramie, and mer c e r i z e d ramie f i b e r s , which have d i f f e r e n t a l l o m o r p h i c c o m p o s i t i o n s , were compared t o study the s t r u c t u r a l d i f f e r e n c e s between the a l l o morphs. In the second c l a s s o f e x p e r i m e n t s , s p e c t r a were r e c o r d e d from ramie f i b e r c r o s s - s e c t i o n s . The p l a n e o f p o l a r i z a t i o n o f the i n c i dent l i g h t was changed r e l a t i v e t o p l a n e o f the c e l l w a l l by r o t a t i n g the c r o s s - s e c t i o n s around the o p t i c a l a x i s o f the m i c r o s c o p e a x i s (see F i g u r e 2b). The i n f o r m a t i o n from t h e s e s p e c t r a was used t o study the o r i e n t a t i o n o f the c e l l u l o s e i n the p l a n e p e r p e n d i c u l a r t o the c h a i n a x i s . R e s u l t s and

Discussion

Assignment o f t h e V i b r a t i o n a l Spectrum o f C e l l u l o s e . Sets o f s p e c t r a i n which the o r i e n t a t i o n o f the e l e c t r i c v e c t o r o f the i n c i dent l i g h t was v a r i e d i n 15 degree i n c r e m e n t s r e l a t i v e t o the f i b e r a x i s were r e c o r d e d . F i g u r e 3 shows the s e t r e c o r d e d from a f i b r i l l a r a g g r e g a t e o f V a l o n i a macrophysa c e l l u l o s e . Scanning e l e c t r o n m i c r o g r a p h s showed t h a t the a g g r e g a t e s a r e h i g h l y o r i e n t e d b u n d l e s of f i b r i l s . T h e r e f o r e , the c e l l u l o s e c h a i n s are p a r a l l e l t o the a x i s of the b u n d l e . A s e t o f s p e c t r a r e c o r d e d from a ramie f i b e r i s shown i n F i g u r e 4. In ramie, the c e l l u l o s e c h a i n s a r e a l s o a p p r o x i m a t e l y p a r a l l e l t o the f i b e r a x i s (21-22). From the f i g u r e s i t i s c l e a r t h a t the band i n t e n s i t i e s a r e s t r o n g l y dependent on the p o l a r i z a t i o n o f the l i g h t . The i n t e n s i t i e s are r e l a t e d t o the o r i e n t a t i o n o f the e l e c t r i c v e c t o r by t h e following equation: 2

4

I = a + b (cos 0) + c (cos e)

(1)

where a, b, and c a r e c o n s t a n t s r e l a t e d t o s c a t t e r i n g a c t i v i t i e s , and θ i s the a n g l e between the e l e c t r i c v e c t o r o f the i n c i d e n t l i g h t



156


Figure

1.

The c e l l u l o s e o r i e n t a t i o n i n the p l a n e p e r p e n d i c u l a r t o the c h a i n a x i s found i n a l g a l c e l l u l o s e s .

(a)

F i g u r e 2.

M i c r o p r o b e e x p e r i m e n t s i n which the p o l a r i z a t i o n o f the e x c i t i n g l i g h t was v a r i e d r e l a t i v e t o the geometry o f the samples.



8.

WILEY A N D ATALLA

Figure

3.


P o l a r i z e d Raman s p e c t r a from a f i b r i l l a r a g g r e g a t e o f Valonia cellulose. The a n g l e between the e l e c t r i c v e c t o r and the c h a i n a x i s was v a r i e d from 0° t o 9 0 ° .


157


158


and the f i b e r a x i s . E q u a t i o n 1 was d e r i v e d by f o l l o w i n g Snyder's t r e a t m e n t (35) and assuming t h a t the c e l l u l o s e c h a i n s a r e p a r a l l e l t o the f i b e r a x i s , and t h a t the c h a i n s a r e o r i e n t e d randomly around t h e i r axes. E q u a t i o n 1 was f i t t e d t o the d a t a i n F i g u r e 3 and 4 by a l i n e a r r e g r e s s i o n technique. E q u a t i o n 1 a d e q u a t e l y d e s c r i b e d the o b s e r v e d d a t a f o r bands which were w e l l r e s o l v e d . An example o f the f i t which c o u l d be a c h i e v e d by t h i s a n a l y s i s i s shown i n F i g u r e 5, where the o b s e r v e d and p r e d i c t e d peak h e i g h t s are p l o t t e d a g a i n s t θ f o r the i n t e n s e band a t 1095 cm"" 1. Bands which were weak and/or p o o r l y r e s o l v e d c o u l d not be f i t t e d as w e l l . Based on the number and l o c a t i o n o f the maxima and minima i n the i n t e n s i t y v s . θ c u r v e s , the bands i n the Raman spectrum o f n a t i v e c e l l u l o s e were d i v i d e d i n t o f o u r c a t e g o r i e s . T a b l e 1 summarizes the band c l a s s i f i c a t i o n s f o r t h o s e bands which were r e s o l v e d w e l l enough t o be a n a l y z e d . The c l a s s i f i c a t i o n s p r o v i d e i n f o r m a t i o n about the d i r e c t i o n a l c h a r a c t e r o f the v i b r a t i o n s . The f o u r c a t e g o r i e s a r e d e s c r i b e d as f o l l o w s : 1)

2)

3)

4)

A Q bands a r e most i n t e n s e when the i n c i d e n t e l e c t r i c v e c t o r i s p a r a l l e l t o the c e l l u l o s e c h a i n a x i s . There f o r e , the maximum change i n p o l a r i z a b i l i t y a s s o c i a t e d w i t h the v i b r a t i o n s i s p a r a l l e l t o the c h a i n a x i s . The i n t e n s i t y v s . θ c u r v e s c o n t a i n a s i n g l e maximum and minimum. A 9 0 bands a r e most i n t e n s e when the i n c i d e n t e l e c t r i c v e c t o r i s p e r p e n d i c u l a r t o the c h a i n a x i s . The maximum change p o l a r i z a b i l i t y i s p e r p e n d i c u l a r t o the c h a i n axis. These bands a l s o e x h i b i t a s i n g l e maximum and minimum i n the i n t e n s i t y v s . θ c u r v e s . Bg bands a r e most i n t e n s e when the i n c i d e n t e l e c t r i c v e c t o r i s p a r a l l e l t o the c h a i n a x i s . As was the case w i t h A Q bands, the maximum change i n p o l a r i z a b i l i t y i s p a r a l l e l t o the c h a i n a x i s . In the i n t e n s i t y v s . θ c u r v e s , however, t h e s e bands e x h i b i t two maxima and a s i n g l e minimum. The m u l t i p l e maxima may r e s u l t from a c c i d e n t a l l y d e g e n e r a t e modes which have maxima a t 0 and 90° o r from modes i n which some o f elements o f the p o l a r i z a b i l i t y d e c r e a s e d u r i n g the v i b r a t i o n . B 9 Q bands are most i n t e n s e when the i n c i d e n t e l e c t r i c v e c t o r i s p e r p e n d i c u l a r t o the c h a i n a x i s . The maximum change i n p o l a r i z a b i l i t y i s p e r p e n d i c u l a r t o the c h a i n axis. These bands a l s o e x h i b i t two maxima and a s i n g l e minima i n the i n t e n s i t y v s . p o l a r i z a t i o n c u r v e s .

As a supplement t o the i n t e n s i t y work, the n a t u r e o f the v i b r a t i o n s was a l s o s t u d i e d by r e c o r d i n g s p e c t r a from d e u t e r a t e d c e l l u loses. By comparing the spectrum o f f u l l y d e u t e r a t e d c e l l u l o s e w i t h t h a t o f normal c e l l u l o s e , the v i b r a t i o n s i n v o l v i n g the h y d r o g e n atoms can be s e p a r a t e d from the pure s k e l e t a l m o t i o n s . Figure 6 i s the spectrum o f c a r b o n - d e u t e r a t e d bacterial cellulose. T h i s sample was k i n d l y p r o v i d e d by Dr. H. L. C r e s p i . It was p r e p a r e d by growing A c e t o b a c t e r x y l i n u m i n d e u t e r a t e d growth media ( 3 6 ) . The r e s i d u a l i n t e n s i t y i n the 0-H and C-H s t r e t c h i n g r e g i o n s i n d i c a t e s t h a t the c e l l u l o s e i s not f u l l y d e u t e r a t e d . We a l s o r e c o r d e d Raman s p e c t r a


WILEY AND ATALLA



8.

Figure

5.

The dependence o f i n t e n s i t y on the p o l a r i z a t i o n o f the i n c i d e n t l i g h t f o r t h e band a t 1095 cnf * i n the spectra of Valonia.


159

160

Table


1.

Summary o f i n t e n s i t y maxima, d e u t e r a t i o n s e n s i t i v i t i e s , and band a s s i g n m e n t s f o r t h e Raman s p e c t r a o f V a l o n i a and ramie

Band Frequency (cm" ) Valonia Ramie 3


1

a

Intensity Classification

AQ B?

Deuteration Sensitivity

331 344 381 437 459 520 913 968 997 1034 1057 1095 1118 1123

331 344 380 437 458 519 910 969 995 1035 1057 1095 1117 1121

weak

AQ

"

1152

1151

B?

?

1279 1292 1334 1337 1378 1406 1455 1477 2868 2885 2941 2965 3291 3334 3261 3395

1275 1291 1331 1337 1378 1407 1456 1475 2866 2889 2943 2963 3286 3335 3363 3402

AQ ? AQ AQ

11

Assignment

heavy atom b e n d i n g , some heavy atom s t r e t c h i n g

1 1

B?

"

B? BQ A

9

"

0

B B AQ

? ? ?

0

9

0

AQ

?

AQ

?

AQ B

"

HCC and HCO b e n d i n g a t C6 heavy atom (CC and CO) stretching

weak

0

? ? strong "

"

heavy atom s t r e t c h i n g p l u s HCC and HCO b e n d i n g HCC and HCO b e n d i n g

HCC, HCO,

and HOC

bending

B? A^

B

" 9

A

9

HCH and HOC

bending

11

C-H and CH2

stretching

0

B B B B BQ ? ? BQ 9

9

"

11 0

0

0

?

"

"

0

0

11

"

0-H

stretching "

0

O n l y the bands r e s o l v e d i n the t a b l e .

i n b o t h the V a l o n i a and ramie a r e i n c l u d e d



8.

WILEY AND

ATALLA

161


o f o r i e n t e d samples o f p a r t i a l l y d e u t e r a t e d c e l l u l o s e w i t h the e l e c t r i c v e c t o r b o t h p a r a l l e l and p e r p e n d i c u l a r to the c h a i n a x i s . Based on t h e s e s t u d i e s , the d e u t e r a t i o n s e n s i t i v i t y f o r s e v e r a l o f the bands was d e t e r m i n e d . This information i s also l i s t e d i n Table I. Based on the i n t e n s i t y s t u d i e s , d e u t e r a t e d c e l l u l o s e s p e c t r a , and normal c o o r d i n a t e a n a l y s e s o f model compounds (7_-&), the a s s i g n ment o f the bands i n the v i b r a t i o n a l spectrum o f c e l l u l o s e was advanced. The i n f o r m a t i o n i s summarized i n T a b l e 1. A detailed d i s c u s s i o n o f the band a s s i g n m e n t s i s beyond the scope o f t h i s r e p o r t and w i l l be g i v e n e l s e w h e r e ( 3 7 ) . A b r i e f o v e r v i e w o f the a s s i g n m e n t s w i l l be g i v e n h e r e . The f r e q u e n c y r e g i o n between 600 and 250 cm"" i s dominated by b e n d i n g m o t i o n s o f the c e l l u l o s e skeleton. These a r e complex modes w h i c h a r e v e r y d e l o c a l i z e d and o f t e n i n v o l v e m o t i o n a t the g l y c o s i d i c l i n k a g e . The f r e q u e n c i e s , e s p e c i a l l y between 400 and 300 cm" , a r e s e n s i t i v e t o the c o n f o r m a t i o n o f the a n h y d r o g l u c o s e r e s i d u e s about the l i n k a g e (8). In t h e modes between 900 and 1200 cm" , CC and CO s t r e t c h i n g m o t i o n s a r e dominant. T h i s r e g i o n c o n t a i n s v e r y i n t e n s e bands. The modes b e t ween 1200 and 1500 cm" i n v o l v e methylene, methine, and h y d r o x y l bending motions. A l t h o u g h the modes i n t h i s r e g i o n are g e n e r a l l y d e l o c a l i z e d m o t i o n s , the HCH b e n d i n g m o t i o n i s i s o l a t e d and behaves as a group mode. In the r e g i o n s between 2700 and 3000 cm" and 3200 and 3500 cm" , the C-H and 0-H s t r e t c h i n g motions o c c u r . These m o t i o n s behave as pure group v i b r a t i o n s . A l t h o u g h the a s s i g n m e n t s do not p r o v i d e a complete d e s c r i p t i o n o f the v i b r a t i o n a l m o t i o n s , they s e r v e t o i n c r e a s e our u n d e r s t a n d i n g o f the c e l l u l o s e v i b r a t i o n a l spectrum. 1

1

1

1

1

1

Polymorphy. C e l l u l o s e polymorphy w i t h i n the c e l l u l o s e I f a m i l y was s t u d i e d by comparing the Raman s p e c t r a o f V a l o n i a and ramie c e l l u lose. S o l i d s t a t e NMR s p e c t r a i n d i c a t e t h a t the I form predomin a t e s i n V a l o n i a w h i l e the Ig form p r e d o m i n a t e s i n ramie ( 1 7 - 1 8 ) . The c e l l u l o s e I s p e c t r a were a l s o compared w i t h s p e c t r a o f c e l l u l o s e I I r e c o r d e d from a m e r c e r i z e d ramie f i b e r . F i g u r e s 7 and 8 show the Raman s p e c t r a o f t h e s e t h r e e c e l l u l o s e s . S p e c t r a were r e c o r d e d w i t h the e l e c t r i c v e c t o r o f the i n c i d e n t l i g h t p a r a l l e l and perpendicular t o the c h a i n a x i s . These s p e c t r a can be d i v i d e d i n t o two r e g i o n s . The r e g i o n below 1600 cm" ( F i g u r e 7) i s most s e n s i t i v e t o the c o n f o r m a t i o n o f the c e l l u l o s e backbone ( e s p e c i a l l y below 700 c m " ) . The h i g h e r f r e q u e n c y r e g i o n , above 2700 cm" ( F i g u r e 8 ) , i s more s e n s i t i v e t o hydrogen b o n d i n g . In the low f r e q u e n c y r e g i o n ( F i g u r e 7), t h e r e a r e o n l y m i n o r d i f f e r e n c e s between the s p e c t r a of n a t i v e ramie and V a l o n i a . The peaks i n the V a l o n i a s p e c t r a a r e narrower and b e t t e r r e s o l v e d . The r e a s o n f o r t h i s i s p r o b a b l y the l a r g e r s i z e o f the c r y s t a l l i t e s i n V a l o n i a c e l l u l o s e (38-39). When the c r y s t a l l i t e s a r e l a r g e r , the environment o f the m o l e c u l e s i s more homogeneous. T h e r e f o r e , the v i b r a t i o n a l energy o f the m o l e c u l e s i s more u n i f o r m , r e s u l t i n g i n narrower bands. The most s i g n i f i c a n t d i f f e r e n c e between the two n a t i v e c e l l u l o s e s p e c t r a i n the low f r e q u e n c y r e g i o n i s t h a t the i n t e n s i t y o f the peak at 913 cm" i s g r e a t e r i n the ramie s p e c t r a . T h i s peak i s a l s o more i n t e n s e i n the spectrum o f b a c t e r i a l c e l l u l o s e t h a n i n the a

1

1

1

1




300

700

Figure

6.

1100

1500

1900

Raman spectrum

2300

2700

o f deuterated b a c t e r i a l

3100

3500

cellulose.

Valonia

mercerized ramie 300

500

700

900

1100 cm'

Figure

7.

1300

1500

1

Comparison o f t h e Raman s p e c t r a from V a l o n i a , ramie, and m e r c e r i z e d ramie (low f r e q u e n c y r e g i o n ) . S p e c t r a were r e c o r d e d w i t h t h e e l e c t r i c v e c t o r a t b o t h 0° and 90°.


WILEY AND ATALLA



8.


163

164


Valonia spectra. S i n c e b a c t e r i a l c e l l u l o s e has a p p r o x i m a t e l y the same I to Ig r a t i o as V a l o n i a (17-18), the i n t e n s i t y o f t h i s peak does not appear t o be r e l a t e d t o ~ ~ s t r u c t u r a l d i f f e r e n c e s between I and I g . I n s t e a d , the i n t e n s i t y o f t h i s peak appears t o be i n v e r s e l y c o r r e l a t e d w i t h the s i z e o f the c r y s t a l l i t e s . It i s v e r y weak i n the s p e c t r a o f V a l o n i a which has v e r y l a r g e c r y s t a l l i t e s , but i t i s s t r o n g e r i n the s p e c t r a o f ramie and b a c t e r i a l c e l l u l o s e w h i c h b o t h have s m a l l e r c r y s t a l l i t e s (38-39). a


a

The d i f f e r e n c e s between the s p e c t r a o f ramie and V a l o n i a a r e q u i t e s m a l l compared t o the d i f f e r e n c e s between n a t i v e c e l l u l o s e and c e l l u l o s e I I (see F i g u r e 7 ) . In the s p e c t r a o f ramie and V a l o n i a , the d i f f e r e n t peak w i d t h s and r e l a t i v e i n t e n s i t i e s can be a t t r i b u t e d t o the d i f f e r e n c e i n the c r y s t a l l i t e s i z e s . In the spectrum o f c e l l u l o s e I I , however, the f r e q u e n c y and number o f peaks i s s i g n i f i cantly different. In p r e v i o u s p u b l i c a t i o n s , the d i f f e r e n c e s between the s p e c t r a of c e l l u l o s e s I and I I have been i n t e r p r e t e d as e v i d e n c e f o r d i f f e r e n t c o n f o r m a t i o n s i n c e l l u l o s e s I and I I (40-41). The s p e c t r a l d i f f e r e n c e s which are i n d i c a t i v e o f c o n f o r m a t i o n a l change a r e not o b s e r v e d i n the s p e c t r a o f ramie and V a l o n i a . S i n c e ramie and V a l o n i a have d i f f e r e n t I t o Ιβ r a t i o s , i t would appear t h a t c e l l u l o s e s I and Ig must have s i m i l a r m o l e c u l a r c o n f o r m a t i o n s . a

a

1

In the C-H s t r e t c h i n g r e g i o n ( F i g u r e 8, 2700-3000 cm"* ), t h e p r i m a r y d i f f e r e n c e between the s p e c t r a o f ramie and Va I o n i a i s the b r o a d n e s s o f the peaks. The peaks i n the ramie s p e c t r a a r e b r o a d e r as was the case i n the low f r e q u e n c y r e g i o n presumably due t o the smaller c r y s t a l l i t e size. In the s p e c t r a o f m e r c e r i z e d ramie, the C-H s t r e t c h i n g r e g i o n d i f f e r s s l i g h t l y from t h a t i n the n a t i v e c e l l u l o s e s but the d i f f e r e n c e s a r e not as l a r g e as t h o s e i n the low frequency r e g i o n . 1

In the 0-H s t r e t c h i n g r e g i o n (3200-3600 c m " ) , however, s i g n i f i c a n t d i f f e r e n c e s are o b s e r v e d between a l l t h r e e c e l l u l o s e s . These d i f f e r e n c e s are most prominent i n the s p e c t r a r e c o r d e d w i t h the e l e c t r i c v e c t o r p a r a l l e l t o the f i b e r a x i s ( F i g u r e 8 a - c ) . The f r e quency as w e l l as the b r o a d n e s s o f the peaks v a r i e s i n t h i s r e g i o n . The s p e c t r a o f V a l o n i a c e l l u l o s e have a peak a t 3231 cm" that i s not o b s e r v e d i n the ramie s p e c t r a . The s p e c t r a o f n a t i v e ramie on the o t h e r hand, have a peak a t 3429 cm" t h a t i s not o b s e r v e d i n Valonia. The spectrum o f m e r c e r i z e d ramie r e c o r d e d w i t h the e l e c t r i c v e c t o r p a r a l l e l has two sharp peaks a t f r e q u e n c i e s above t h o s e o b s e r v e d i n the n a t i v e c e l l u l o s e s . The d i f f e r e n c e s i n the 0-H r e g i o n between V a l o n i a , ramie, and m e r c e r i z e d ramie suggest t h a t the h y d r o g e n b o n d i n g p a t t e r n s a r e d i f f e r e n t i n each o f t h e s e c e l l u l o s e s . In summary, the Raman s p e c t r a i n d i c a t e t h a t c e l l u l o s e s I and Ig e x h i b i t d i f f e r e n t hydrogen b o n d i n g p a t t e r n s but have s i m i l a r m o l e c u l a r conformations. C e l l u l o s e s I and I I have d i f f e r e n t m o l e c u l a r c o n f o r m a t i o n s as w e l l as d i f f e r e n t hydrogen b o n d i n g p a t t e r n s . 1

1

a

Cellulose Orientation. The o r i e n t a t i o n o f the c e l l u l o s e m o l e c u l e s i n the plane p e r p e n d i c u l a r t o t h e c h a i n a x i s was s t u d i e d by u s i n g the m i c r o p r o b e t o r e c o r d s p e c t r a o f ramie f i b e r c r o s s s e c t i o n s . F i g u r e 9 shows s p e c t r a r e c o r d e d w i t h the e l e c t r i c v e c t o r o f the e x c i t i n g l i g h t t a n g e n t i a l , p e r p e n d i c u l a r , and at 45° t o the c e l l wall surface. I f the c e l l u l o s e o r i e n t a t i o n i n the p l a n e p e r p e n d i c u l a r t o the f i b e r a x i s i s a n i s o t r o p i c , t h e n the i n t e n s i t i e s



8.

WILEY AND ATALLA

Figure

9.


165

P o l a r i z e d Raman s p e c t r a o f a ramie c r o s s s e c t i o n . The a n g l e between t h e e l e c t r i c v e c t o r and t h e c e l l w a l l s u r f a c e was v a r i e d from 0° t o 9 0 ° .


166


s h o u l d d i f f e r i n the c r o s s - s e c t i o n s p e c t r a r e c o r d e d w i t h d i f f e r e n t o r i e n t a t i o n s o f the i n c i d e n t e l e c t r i c v e c t o r . With the e x c e p t i o n o f the peaks at 1095 and 1123 cm"" , the r e l a t i v e peak i n t e n s i t i e s i n the c r o s s - s e c t i o n s p e c t r a d i d not vary n o t i c e a b l y as the p o l a r i z a t i o n o f the i n c i d e n t l i g h t was changed (see F i g u r e 9 ) . The i n s e n s i t i v i t y o f the m a j o r i t y o f the bands t o the o r i e n t a t i o n o f i n c i d e n t e l e c t r i c v e c t o r i s not c o n s i s t e n t w i t h e i t h e r a p r e f e r e n t i a l o r i e n t a t i o n of the methines p e r p e n d i c u l a r t o the c e l l w a l l s u r f a c e o r the a l t e r n a t i n g type o f o r i e n t a t i o n found i n a l g a l c e l l u l o s e s . The v a r i a t i o n i n the r e l a t i v e i n t e n s i t i e s o f the 1095 and 1123 cm" bands between the 0° and 45° s p e c t r a s u g g e s t s a n i s o t r o p y i n the c e l l u l o s e o r i e n t a t i o n . T a b l e I shows t h a t t h e s e peaks a r e s k e l e t a l s t r e t c h i n g modes t h a t are most i n t e n s e when the e l e c t r i c v e c t o r o f the i n c i d e n t l i g h t i s p a r a l l e l t o the c h a i n a x i s . S i n c e the 1095 cm" peak i s very s e n s i t i v e t o the o r i e n t a t i o n o f the i n c i d e n t e l e c t r i c v e c t o r r e l a t i v e t o the c h a i n a x i s , the i n t e n s i t y v a r i a t i o n s u g g e s t s t h a t the p l a n e o f s e c t i o n i n g was not e x a c t l y p e r p e n d i c u l a r t o the c e l l u l o s e c h a i n axes so t h a t the c h a i n s are t i l t e d r e l a t i v e t o the p l a n e o f s e c t i o n i n g . I f the c e l l u l o s e i s o r i e n t e d randomly i n the p l a n e p e r p e n d i c u l a r t o the c h a i n a x i s , then the band i n t e n s i t i e s would be the same r e g a r d l e s s o f whether the i n c i d e n t e l e c t r i c v e c t o r was p a r a l l e l , p e r p e n d i c u l a r , o r 45° t o the c e l l w a l l s u r f a c e . The c r o s s - s e c t i o n s p e c t r a , t h e r e f o r e , a r e c o n s i s t e n t w i t h random c e l l u l o s e o r i e n t a t i o n i n the p l a n e p e r p e n d i c u l a r t o the c h a i n a x i s . These r e s u l t s conf l i c t w i t h our e a r l i e r s p e c t r a o f t e n s i o n d r i e d c o t t o n f i b e r s (34) t h a t i n d i c a t e d the methines were o r i e n t e d p r e f e r e n t i a l l y p e r p e n d i c u l a r t o the c e l l w a l l s u r f a c e . More r e c e n t s p e c t r a o f c o t t o n f i b e r s have shown t h a t i f the f i b e r s a r e not d r i e d under t e n s i o n , the methine o r i e n t a t i o n i s random i n the p l a n e p e r p e n d i c u l a r t o the chain axis. T h e r e f o r e , i t appears the c e l l u l o s e o r i e n t a t i o n can be i n f l u e n c e d by the sample p r e p a r a t i o n methods. Since microtoming e x e r t s l a r g e f o r c e s on the f i b e r s , i t i s a l s o p o s s i b l e t h a t the c e l l u l o s e o r i e n t a t i o n c o u l d have been d i s r u p t e d d u r i n g the p r e p a r a t i o n of the c r o s s - s e c t i o n s . F u r t h e r e x p e r i m e n t s w i l l be n e c e s s a r y t o u n d e r s t a n d the f a c t o r s which i n f l u e n c e the c e l l u l o s e o r i e n t a t i o n . 1


1

1

Conclusions Based on the number and l o c a t i o n o f the maxima and minima i n the r e l a t i o n s h i p between the band i n t e n s i t i e s and the p o l a r i z a t i o n o f the i n c i d e n t l i g h t r e l a t i v e t o the c h a i n a x i s , the bands i n the Raman spectrum o f c e l l u l o s e c o u l d be d i v i d e d i n t o f o u r g r o u p s . The about the d i r e c t i o n o f the v i b r a t i o n a l m o t i o n s i n c e l l u l o s e . The d i r e c t i o n s o f the v i b r a t i o n s are such t h a t the major change i n p o l a r i z a b i l i t y a s s o c i a t e d w i t h the m o t i o n s i s e i t h e r p a r a l l e l o r p e r p e n d i c u l a r t o the c h a i n a x i s . Raman s p e c t r a r e c o r d e d from deut e r a t e d c e l l u l o s e s a l l o w e d the v i b r a t i o n a l modes i n v o l v i n g C-H and 0-H m o t i o n s t o be i d e n t i f i e d . These s p e c t r a demonstrated t h a t most o f the modes are complex c o u p l e d v i b r a t i o n s . Normal c o o r d i n a t e a n a l y s e s o f c e l l u l o s e model compounds were done t o d e t e r m i n e the t y p e s o f m o t i o n most l i k e l y t o o c c u r i n each r e g i o n o f the spectrum. The c a l c u l a t i o n s a l s o s u g g e s t e d t h a t the v i b r a t i o n a l m o t i o n s are v e r y complex. The i n f o r m a t i o n from the normal c o o r d i n a t e c a l c u l a t i o n s ,



8.

WILEY AND ATALLA


167

i n t e n s i t y s t u d i e s , and s p e c t r a o f d e u t e r a t e d c e l l u l o s e s was used t o advance the assignment o f the c e l l u l o s e v i b r a t i o n a l spectrum. Comparison o f the Raman s p e c t r a o f V a l o n i a , ramie, and mer c e r i z e d ramie i n d i c a t e s t h a t t h e c o n f o r m a t i o n o f the c e l l u l o s e b a c k bone i s s i m i l a r i n Va I o n i a and n a t i v e ramie but d i f f e r e n t i n m e r c e r i z e d ramie. The h y d r o g e n b o n d i n g p a t t e r n s , however, a r e d i f f e r e n t i n V a l o n i a and n a t i v e ramie as w e l l as i n m e r c e r i z e d ramie. S p e c t r a r e c o r d e d from ramie c r o s s - s e c t i o n s suggest t h a t t h e c e l l u l o s e i s o r i e n t e d randomly i n t h e p l a n e p e r p e n d i c u l a r t o the chain axis. I t a p p e a r s , however, t h a t t h e sample p r e p a r a t i o n methods c a n i n f l u e n c e t h e c e l l u l o s e o r i e n t a t i o n . Therefore, further s t u d i e s w i l l be n e c e s s a r y t o c h a r a c t e r i z e t h e m o l e c u l a r o r i e n t a t i o n in cellulose fibers. Acknowledgment The a u t h o r s w i s h t o thank Dr. H. L. C r e s p i f o r f u r n i s h i n g t h e sample o f d e u t e r a t e d b a c t e r i a l c e l l u l o s e and f o r h i s a d v i c e c o n c e r n i n g t h e growth o f a l g a e i n heavy water, and Dr. U. P. Agarwal f o r t h e many h e l p f u l d i s c u s s i o n s he c o n t r i b u t e d t o t h i s work. Portions o f t h i s work were used by J . H. W i l e y as p a r t i a l f u l f i l l m e n t o f t h e r e q u i r e ments f o r t h e Ph.D. degree a t The I n s t i t u t e o f Paper C h e m i s t r y .

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RECEIVED March 5, 1987


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