Molecular Weight Distribution Studies Using Lignin Model Compounds

Chapter 8

Molecular Weight Distribution Studies Using Lignin Model Compounds D. K. Johnson , Helena L i Chum , and John A. Hyatt 1

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Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401 Research Laboratories, Eastman Chemicals Division, Eastman Kodak Company, Kingsport, TN 37662

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 4, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch008

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High performance size exclusion chromatography (HPSEC) has been extensively employed in studies of the molecular weight distributions of lignins. Tetrahydrofuran (THF) is the most commonly used mobile phase with polystyrene-divinyl benzene H P S E C columns. Under these conditions the apparent molecular weight distributions of lignins may be determined relative to the standards employed, usually polystyrenes. There are, however, many lignins that are partially or totally insoluble in T H F that require a solvent of greater solvating power. Many problems exist in using such eluants, e.g., dimethyl formamide alone or in the presence of added salts, including solute-solute association, and interactions between solute and solvent, column gel and solvent, and calibration standards and column gel. These interactions will be reported in light of their effect on the elution of lignin model compounds obtained from stepwise syntheses (molecular weights of 200-1100) and quinonemethide polymerizations (apparent weight-average molecular weights of 2000-7000). The H P S E C of homogeneous linear polymers using polystyrene-divinyl benzene gels and relatively non-polar solvents such as tetrahydrofuran (THF) is well understood and can be used for the determination of molecular weight distributions (MWD) of similar polymers (1,2). The estimation of lignin M W D is, however, complicated by the interaction of several components in the H P S E C system (3). Lignins are irregular, multiply branched polymers containing various polar functionalities and a number of different interunit linkages which are a marked function of the method of lignin isolation. Thus, lignin solubility in T H F varies from zero to 100%. To increase 0097-6156/89A)397-0109$06.00A) © 1989 American Chemical Society

Glasser and Sarkanen; Lignin ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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LIGNIN: PROPERTIES AND MATERIALS

s o l u b i l i t y a n d prevent i n t e r a c t i o n s a r i s i n g f r o m the presence o f h y d r o x y 1 (alcoholic a n d phenolic) groups, l i g n i n s are often d e r i v a t i z e d . L i g n i n s are m o s t l y acetylated (4,5), b u t have also been m e t h y l a t e d (6) a n d s i l y l a t e d (7) p r i o r to H P S E C . T h i s s t u d y focuses o n a c e t y l a t e d l i g n i n s a n d m o d e l c o m p o u n d s b u t also includes u n d e r i v a t i z e d l i g n i n m o d e l c o m p o u n d s a n d some other derivatives. C o n n o r s et al (8) e x a m i n e d the use o f solvents of higher s o l v a t i n g power for the S E C of lignins, e.g., d i m e t h y l f o r m a m i d e ( D M F ) a n d d i m e t h y l sulfoxide. However, the observation of m u l t i m o d a l e l u t i o n profiles i n these solvents, even w h e n u s i n g d e r i v a t i z e d l i g n i n s where h y d r o g e n b o n d i n g between solute a n d solvent molecules s h o u l d n o t o c c u r , complicates the use of these solvents. E l u t i o n of t h i s t y p e has been asc r i b e d to solute-solute association. T o overcome these associative effects, salts such as L i C l a n d L i B r were added to give more u n i m o d a l e l u t i o n p r o files. T h e m e c h a n i s m of association of l i g n i n molecules a n d effects of a d d i n g electrolytes has been discussed b y Sarkanen et al (9). C h a n g e s i n solvent can also alter the size a n d shape of molecules w h i c h must s t r o n g l y influence a technique r e l y i n g o n size e x c l u s i o n for measurem e n t o f m o l e c u l a r weight. M o l e c u l a r size m a y also be altered b y h y d r o g e n b o n d i n g of solvent molecules to solute molecules. T h i s has been observed i n the H P S E C of u n d e r i v a t i z e d phenols w i t h T H F as solvent (10,11). P e l l i nen a n d S a l k i n o j a - S a l o n e n (7) have e x a m i n e d the H P S E C of a n u m b e r of l i g n i n m o d e l c o m p o u n d s a n d their acetylated a n d s i l y l a t e d derivatives using T H F as e l u a n t . T h e y f o u n d no clear evidence of association or a d s o r p t i o n of these c o m p o u n d s to the c o l u m n gel; however, the r e l a t i o n s h i p between m o l e c u l a r weight a n d e l u t i o n v o l u m e , w h i l e s i m i l a r for b o t h sets o f derivatives, was different f r o m t h a t of the u n d e r i v a t i z e d m o d e l c o m p o u n d s w h i c h eluted earlier b y c o m p a r i s o n . T h e y p o s t u l a t e d t h a t t h i s was due t o s t r o n g s o l v a t i o n of the u n d e r i v a t i z e d m o d e l c o m p o u n d s . It w o u l d appear, t h e n , t h a t l i g n i n s s h o u l d be d e r i v a t i z e d before H P S E C a n a l y s i s ; however, the e l u t i o n of the d e r i v a t i z e d m o d e l c o m p o u n d s d i d not coincide w i t h t h a t of the polystyrene s t a n d a r d s . H P S E C as a technique relies o n t r a n s f o r m a t i o n of e l u t i o n t i m e or v o l ume i n t o a scale of m o l e c u l a r weight, b y observing the e l u t i o n of p o l y m e r s t a n d a r d s of k n o w n m o l e c u l a r weight or M W D . T h i s i m p l i e s t h a t the r e l a t i o n s h i p between size a n d m o l e c u l a r weight of the s t a n d a r d s a n d p o l y m e r s under i n v e s t i g a t i o n s h o u l d be at least s i m i l a r . W i t h l i n e a r h o m o p o l y m e r s , s t a n d a r d s can u s u a l l y be f o u n d so t h a t g o o d c a l i b r a t i o n s can be m a d e . W i t h more c o m p l i c a t e d p o l y m e r s , e.g., b r a n c h e d p o l y m e r s a n d c o p o l y m e r s , c a l i b r a t i o n becomes more i n e x a c t . T h e r e are o n l y a few l i g n i n m o d e l c o m pounds of low m o l e c u l a r weight available for c a l i b r a t i o n a n d so other p o l y mers such as polystyrenes have been used. M o r e p o l a r solvents such as D M F further c o m p l i c a t e the use of chemically different s t a n d a r d s as they m a y w e l l behave differently t h a n the l i g n i n s b e i n g s t u d i e d , t h r o u g h changes i n m o l e c u l a r size a n d f r o m i n t e r a c t i o n s w i t h the c o l u m n gel. T h e use of more p o l a r solvents can also affect the c o l u m n gel c a u s i n g s w e l l i n g of the gel beads a n d excessive back pressure. A n y changes i n pore size of the gel w i l l o b v i o u s l y have a large effect o n S E C a l t h o u g h the current gel p o l y m e r s are h i g h l y cross-linked to m i n i m i z e t h i s effect.



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Recently, efforts have been m a d e to p r o d u c e c a l i b r a t i o n s t a n d a r d s of higher m o l e c u l a r weight t h a t are c h e m i c a l l y s i m i l a r to l i g n i n s , b y step-wise syntheses (12), a n i o n - i n i t i a t e d p o l y m e r i z a t i o n of quinonemethides (13), a n d p r e p a r a t i v e H P S E C of acetylated lignins (14). K n o w l e d g e of the m o l e c u l a r weights of these m a t e r i a l s is either b u i l t i n t o the m e t h o d of p r e p a r a t i o n or d e t e r m i n e d b y absolute methods such as s e d i m e n t a t i o n e q u i l i b r i u m m e a surements. T h i s s t u d y compares e l u t i o n of l i g n i n m o d e l c o m p o u n d s a n d m o d e l p o l y m e r s i n two solvents of s i m i l a r l y h i g h s o l u b i l i t y p a r a m e t e r (9.9 for T H F a n d 12.1 for D M F ) , b u t w i t h quite different f r a c t i o n a l p o l a r i t i e s (0.075 for T H F a n d 0.77 for D M F ) (15). E l u t i o n of m o d e l c o m p o u n d s is c o m p a r e d to t h a t of acetylated lignins so t h a t inferences m a y be m a d e concerning m o l e c u l a r s t r u c t u r e a n d the various i n t e r a c t i o n s t h a t occur between l i g n i n s a n d the H P S E C s y s t e m . A n y evidence of association of the m o d e l c o m p o u n d s a n d m o d e l p o l y m e r s is of p a r t i c u l a r interest. T h e effects of m o d i f y i n g D M F b y a d d i t i o n of L i B r a n d f o r m i c a c i d are also assessed p a r t i c u l a r l y t o over come associative effects. N a r r o w M W D s t a n d a r d s are i n c l u d e d to s t u d y the difference i n their e l u t i o n c o m p a r e d to the l i g n i n m o d e l c o m p o u n d s as solvent c o m p o s i t i o n is changed.

Experimental H i g h performance size exclusion c h r o m a t o g r a p h y was p e r f o r m e d u s i n g a H e w l e t t - P a c k a r d H P 1090 l i q u i d c h r o m a t o g r a p h c o n t a i n i n g a n H P 1 0 4 0 A u l traviolet diode a r r a y ( U V ) detector after w h i c h was connected a n H P 1 0 3 7 A refractive i n d e x ( R I ) detector. T w o c o l u m n s ( 3 0 x 0.8 c m each) c o n t a i n i n g p o l y s t y r e n e - d i v i n y l benzene gel beads (ΙΟμπι diameter) w i t h n o m i n a l pore diameters of 500Â ( A l t e x , μ-Spherogel) a n d 100Â ( P o l y m e r L a b o r a t o r i e s , P L - G e l ) were used c o n nected i n series. These c o l u m n s were chosen to give m a x i m u m r e s o l u t i o n i n the m o l e c u l a r weight range of the m o d e l c o m p o u n d s to be s t u d i e d . T h e c o l u m n s were m a i n t a i n e d at 28°C a n d injections (5-10 μ L ) were m a d e w i t h a n a u t o m a t i c injector of samples (1-2 m g / m L ) t h a t were dissolved i n the e l u a n t b e i n g used. T h e solvents used were c h r o m a t o g r a p h i c grade T H F a n d D M F ( B u r d i c k a n d J a c k s o n or s i m i l a r q u a l i t y ) . F o r m i c a c i d ( A l d r i c h , 9 6 % A C S reagent grade) a n d l i t h i u m b r o m i d e ( A l d r i c h , 9 9 + % , a n h y d r o u s ) were used w i t h o u t further p u r i f i c a t i o n for the higher i o n i c s t r e n g t h D M F e l u a n t s . T h e five eluants s t u d i e d were pure T H F a n d D M F , a m i x t u r e of T H F a n d D M F (1:1 o n a v o l u m e basis), D M F c o n t a i n i n g formic a c i d (5 w t % ) a n d D M F c o n t a i n i n g l i t h i u m b r o m i d e (0.1 M ) . A flow rate of 1.0 m L / m i n was used t h r o u g h o u t t h i s study. T h e r e p r o d u c i b i l i t y of r e t e n t i o n v o l u m e was measured b y f o l l o w i n g the e l u t i o n of toluene i n c l u d e d i n m a n y samples as an i n t e r n a l s t a n d a r d . T h e polystyrene ( P S ) a n d p o l y m e t h y l m e t h a c r y l a t e ( P M M A ) n a r r o w m o l e c u l a r weight s t a n d a r d s were o b t a i n e d f r o m three k i t s ( P o l y m e r L a b oratories, S - L - 1 0 , S - M - 1 0 a n d M - M - 1 0 ) covering the range f r o m 3,000,000 to 500. F i v e n o n y l p h e n y l - t e r m i n a t e d polyethylene oxides ( A l d r i c h , Igepals)




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w i t h m o l e c u l a r weights of 4600 to 300 were also s t u d i e d . T h e m o n o d i s perse l i g n i n m o d e l c o m p o u n d s were p r e p a r e d b y a m o d i f i e d enolate a d d i t i o n m e t h o d (12). O t h e r monodisperse l i g n i n m o d e l c o m p o u n d s were p r e p a r e d as described by H i m m e l et ai (16). T h e s y n t h e t i c l i g n i n m o d e l p o l y m e r s ( a , / ? - b i s ( 0 - 4 - a r y l ) ether b o n d e d ) were prepared b y a n i o n - i n i t i a t e d p o l y m e r i z a t i o n of a q u i n o n e m e t h i d e a c c o r d i n g to the procedure o f C h u m et ai (13). T h e s t r u c t u r e s of the l i g n i n m o d e l c o m p o u n d s a n d p o l y m e r s are s h o w n i n F i g u r e 1. T h e l i g n i n samples were p r e p a r e d b y organosolv p u l p i n g of aspen w o o d i n aqueous m e t h a n o l (70 v o l % ) w i t h s u l f u r i c a c i d c a t a l y s t (0.05 M ) as described b y C h u m et ai (17). Before a n a l y s i s the l i g n i n s were q u a n t i t a t i v e l y acetylated u s i n g a m e t h o d developed b y G i e r e r a n d L i n d e b e r g (18).

Results and Discussion W i t h T H F as the solvent the retention times o f the P S , P M M A a n d Igepal s t a n d a r d s were o b t a i n e d . F i g u r e 2 compares the e l u t i o n of these three sets of s t a n d a r d s o n the c o l u m n set used, a n d shows a curve o b t a i n e d b y a n o n - l i n e a r least squares f i t t i n g of this d a t a . C o n s i d e r i n g the differences i n c h e m i c a l s t r u c t u r e of these p o l y m e r s , i t is s u r p r i s i n g t h a t such a g o o d fit was f o u n d . T h i s suggests t h a t the change i n h y d r o d y n a m i c v o l u m e w i t h m o l e c u l a r weight of these p o l y m e r s is s i m i l a r . F i g u r e 3 compares the e l u t i o n i n T H F of the l i g n i n m o d e l c o m p o u n d s h a v i n g free phenolic groups (squares) to those i n w h i c h the phenolic groups were d e r i v a t i z e d (stars). T h e s o l i d line t h a t is s h o w n is the c a l i b r a t i o n curve o b t a i n e d b y n o n - l i n e a r least squares fitting of the r e t e n t i o n t i m e s of the s t a n d a r d P S , P M M A a n d Igepal p o l y m e r s . T h e m o d e l c o m p o u n d s w i t h d e r i v a t i z e d phenolic groups eluted later t h a n p r e d i c t e d b y t h i s c u r v e . P e l l i n e n a n d S a l k i n o j a - S a l o n e n (7) observed a s i m i l a r result u s i n g a different set of l i g n i n m o d e l c o m p o u n d s w h i c h they acetylated a n d s i l y l a t e d . A best fit of the d e r i v a t i z e d models is s h o w n as the dashed line i n F i g u r e 3 a n d appears to p a r a l l e l the c a l i b r a t i o n curve. T h e reason for the later e l u t i o n o f the d e r i v a t i z e d models relative to the p o l y m e r s t a n d a r d s is m o s t likely t h a t they have a r e l a t i v e l y smaller h y d r o d y n a m i c v o l u m e per u n i t m o l e c u l a r weight t h a n the p o l y m e r s t a n d a r d s . A l t e r n a t i v e l y , e l u t i o n o f the d e r i v a t i z e d l i g n i n models m a y have been delayed b y a d s o r p t i o n t o the c o l u m n gel. W h e n u s i n g T H F as solvent, a d s o r p t i o n has been r e p o r t e d for h i g h l y condensed a r o m a t i c c o m p o u n d s (10,11). T h e dashed line c o u l d be used as a c a l i b r a t i o n for acetylated lignins b u t covers too s m a l l a range of m o l e c u l a r weights even for low-molecular-weight lignins. A p a r t f r o m one c o m p o u n d (II), the l i g n i n m o d e l c o m p o u n d s t h a t h a d free phenolic groups eluted at close to the r e t e n t i o n t i m e s p r e d i c t e d b y the c a l i b r a t i o n curve f r o m the p o l y m e r s t a n d a r d s a n d not f r o m the d e r i v a t i z e d m o d e l c o m p o u n d s . T h i s c o u l d s i m p l y be a result o f the u n d e r i v a t i z e d models h a v i n g a s i m i l a r v a r i a t i o n i n h y d r o d y n a m i c v o l u m e w i t h m o l e c u l a r weight as the p o l y m e r s t a n d a r d s . However, i t is to be expected t h a t s o l v a t i o n of the u n d e r i v a t i z e d m o d e l c o m p o u n d s s h o u l d o c c u r w i t h T H F as solvent (10), w i t h hydrogen b o n d i n g of one T H F molecule t o each u n d e r -



JOHNSON ET AL.


•Me

Lignin Model Polymers •H X R « -ΠΜβ η - " 2 0 XI R - - O H η - "15 XII R= - O H n= - 4

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F i g u r e 1. S t r u c t u r e s of l i g n i n m o d e l c o m p o u n d s used i n t h i s s t u d y .


113

114



or

RETENTION VOLUME

(mL)

F i g u r e 2. C a l i b r a t i o n of c o l u m n set w i t h T H F as eluant u s i n g polystyrenes (*), p o l y m e t h y l m e t h a c r y l a t e s ( • ) a n d Igepals ( Q ) -

F i g u r e 3. E l u t i o n of derivatized (•) a n d u n d e r i v a t i z e d ( • c o m p o u n d s w i t h T H F as eluant.

) lignin model


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i v a t i z e d O H g r o u p , the degree of hydrogen b o n d i n g b e i n g higher the more acidic the O H g r o u p . T h i s w o u l d make the u n d e r i v a t i z e d models large for their m o l e c u l a r weight relative to the d e r i v a t i z e d m o d e l c o m p o u n d s a n d e x p l a i n w h y they elute earlier t h a n the d e r i v a t i z e d models. T h e b e h a v i o r of c o m p o u n d I I I , the b i p h e n y l tetramer h e x a o l , appears a n o m a l o u s . T h e effects o n the c o l u m n p a r a m e t e r s , v o i d v o l u m e ( V ) a n d t o t a l p e r m e a t i o n v o l u m e ( V t ) , of changing the eluant were also e x a m i n e d . T h e v o i d v o l u m e was d e t e r m i n e d b y following the r e t e n t i o n volumes of the largest P S ( 3 0 0 0 K ) a n d P M M A ( 1 3 0 0 K ) p o l y m e r s . A s can be seen f r o m T a b l e I, the v o i d v o l u m e appeared to increase s l i g h t l y as the p o l a r i t y of the solvent was increased, a l t h o u g h the change i n r e t e n t i o n volumes c o u l d have other causes, especially as the P S seemed to increase more t h a n the P M M A .


0

T a b l e I. Investigation of C h a n g e s i n V t a n d V

0

Retention Volume (mL) THF Hexane Toluene Dioxane Acetone Me T H F THF Me Formate DMA DMF DEF V, 3000K P S 1300K P M M A Vo

17.45 18.61 18.91 18.25 18.41 18.61 18.74 18.88 18.30 8.99 9.13 8.99

THF/DMF 20.07 19.04 18.22 18.24 18.19 18.25 18.36 18.28 18.30 8.99 9.11 8.99

DMF

DMF/LiBr

DMF/HC0 H

26.74 23.63 20.90 19.48 23.07 22.91 19.34 19.30

28.08 24.20 21.23 19.65 23.67 23.35 19.50 19.26

24.36 21.19 19.65 23.59 23.32 19.52 19.19

19.40 9.01 9.19 9.00

19.44 19.40 9.26 9.22 9.07

19.43 19.40 9.26 9.26 9.11

2

M e T H F = methyl tetrahydrofuran; M e Formate = m e t h y l formate; D M A = d i m e t h y l acetamide; D E F = d i e t h y l f o r m a m i d e . A p p r o x i m a t e values for Y were d e t e r m i n e d u s i n g 16 c o m p o u n d s , some of w h i c h are i n c l u d e d i n T a b l e I, w i t h m o l e c u l a r weights of about 100 or less a n d c o n t a i n i n g a variety of f u n c t i o n a l groups. In T H F the r e t e n t i o n volumes of these c o m p o u n d s fell w i t h i n a f a i r l y n a r r o w range w i t h i n a b o u t ± 5 % of V t . B y c o m p a r i s o n , the r e p r o d u c i b i l i t y of r e t e n t i o n v o l u m e for toluene i n T H F was 0.01 m L a n d i n D M F was 0.14 m L . A s e x e m p l i f i e d b y the r e t e n t i o n times of hexane, toluene a n d dioxane i n T a b l e I, increasi n g the solvent p o l a r i t y s u b s t a n t i a l l y increased the r e t e n t i o n volumes of the more n o n - p o l a r compounds to as m u c h as 5 0 % more t h a n the e s t i m a t e d \ T h i s is almost c e r t a i n l y an a d s o r p t i o n effect, w i t h the r e l a t i v e l y n o n - p o l a r c o l u m n p a c k i n g increasingly r e t a i n i n g the n o n - p o l a r solutes as the solvent t

t


116


p o l a r i t y increases. T h e best estimate o f V t was o b t a i n e d b y f o l l o w i n g the re t e n t i o n volumes of c o m p o u n d s c h e m i c a l l y s i m i l a r to the solvent b e i n g used. These were detected u s i n g the refractive i n d e x detector. F o r T H F , V t was e s t i m a t e d u s i n g m e t h y l t e t r a h y d r o f u r a n . F o r the D M F c o n t a i n i n g e l u a n t s , d i m e t h y l acetamide, d i e t h y l f o r m a m i d e , etc., were used. T h e n a r r o w range of e l u t i o n volumes for a l l the T H F a n d D M F analogues w h e n u s i n g the m i x e d T H F / D M F solvent appears to j u s t i f y t h i s a p p r o a c h . T h e increase i n V t of about 1 m L w i t h the more p o l a r solvents represents about a 1 0 % i n crease i n the pore v o l u m e of the p a c k i n g m a t e r i a l . T h e s m a l l increase i n V a n d larger increase i n V m a y or m a y not be consistent w i t h s w e l l i n g of the p o l y m e r beads. Because o f the change i n c o l u m n p a r a m e t e r s , c o m p a r i s o n of solute e l u t i o n w i t h change i n solvent was subsequently m a d e after c a l c u l a t i o n of solute d i s t r i b u t i o n coefficients (Κχ> = ( V — V ) / ( V t — V ) where V is the solute r e t e n t i o n volume) (19).


0

t

r

0

0

r

F i g u r e 4 shows the change i n e l u t i o n of the polystyrene s t a n d a r d s w i t h a change i n solvent. T h e r e t e n t i o n volumes of the P S p o l y m e r s s u b s t a n t i a l l y increased as the solvent p o l a r i t y increased such t h a t i n D M F a n d D M F m o d i f i e d w i t h L i B r a n d f o r m i c a c i d , e l u t i o n b e y o n d V was observed. S i m i l a r l y , b u t t o a lesser extent, the e l u t i o n volumes of the other p o l y m e r s t a n d a r d s ( T a b l e II) also increased. A n increase i n K& m a y be e x p l a i n e d b y either a solute-solvent or a s o l u t e - c o l u m n gel i n t e r a c t i o n . It is c e r t a i n l y conceivable t h a t a change i n solvent c o u l d produce a change i n the h y d r o d y n a m i c v o l u m e of a solute. L a r g e molecules are considered to be p r i m a r i l y r a n d o m coils i n s o l u t i o n . T h e coiled o r i e n t a t i o n of the molecule, a n d there fore its h y d r o d y n a m i c v o l u m e , is likely to be s t r o n g l y affected b y the solvent b e i n g used. S m a l l molecules have less flexibility so t h i s m e c h a n i s m o f size change s h o u l d be less i m p o r t a n t for s m a l l molecules. T h e f o r m a t i o n o f solute-solvent complexes v i a m e c h a n i s m s such as hydrogen b o n d i n g w o u l d increase m o l e c u l a r size r e s u l t i n g i n s m a l l e r K # . A decrease i n m o l e c u l a r size w i l l increase the Kf> of a solute; however, K # s h o u l d not be greater t h a n 1 unless solute a d s o r p t i o n to the c o l u m n p a c k i n g takes place. t

In t h i s s t u d y the lower m o l e c u l a r weight P S were m u c h more s t r o n g l y affected b y the changes i n solvent w i t h K p values u p to 1.5 b e i n g observed. T h e P M M A were affected to a m u c h lesser extent a n d the Igepals the least of a l l b y the solvent changes. T h e P S are clearly b e i n g r e t a i n e d b y the c o l u m n gel b y a d s o r p t i o n as solvent p o l a r i t y is increased. Since the c o l u m n gel is a c o p o l y m e r of styrene a n d d i v i n y l benzene a n d is c h e m i c a l l y s i m i l a r to the P S s t a n d a r d s , affinity of the P S for the p a c k i n g m a t e r i a l s h o u l d increase as solvent p o l a r i t y increases. T h e lower m o l e c u l a r weight P S are more s t r o n g l y affected p r o b a b l y because they permeate a larger f r a c t i o n of the t o t a l pore v o l u m e . A n y effect due to change i n size o f the P S p o l y m e r s is obscured b y the s t r o n g a d s o r p t i o n effect. T h e s m a l l e r increases i n for the P M M A a n d Igepals are p r o b a b l y the result of a lower affinity of these p o l y m e r s for the p a c k i n g m a t e r i a l a l t h o u g h the influence of increased m o l e c u l a r size as a counterbalance to a d s o r p t i o n m a y also be i m p o r t a n t . A s has been noted before (3), P S are very p o o r s t a n d a r d s for H P S E C w h e n u s i n g D M F - b a s e d solvents, as they e x h i b i t s u b s t a n t i a l non-size e x c l u s i o n



8.

JOHNSON ET AL.

MOLECULAR

o10

2

117


10

s

10

4

10

WEIGHT e

10*

10

7

F i g u r e 4. E l u t i o n o f p o l y s t y r e n e s t a n d a r d s w i t h different eluants. * = T H F ; + = T H F / D M F ; • = D M F ;Q = DMF/LiBr; • = D M F / H C 0 H . 2



118

T a b l e II. Effect of solvent changes o n e l u t i o n o f P M M A a n d Igepals Solute D i s t r i b u t i o n Coefficient


Mol. Wt. PMMA 1300000 330000 107000 60000 27000 10300 3000 Igepals 4626 1983 749 441 308

(Κ#)

THF

THF/DMF

DMF

DMF/LiBr

DMF/HC0 H

0.02 0.03 0.04

0.02 0.04 0.06

0.06 0.15 0.30

0.01 0.04 0.04 0.05 0.07 0.16 0.32

0.08 0.18 0.39

0.02 0.03 0.04 0.05 0.09 0.20 0.47

0.02 0.03 0.04 0.05 0.09 0.20 0.44

0.19 0.31 0.47 0.58 0.68

0.20 0.33 0.52 0.65 0.76

0.22 0.37 0.59 0.77 0.94

0.23 0.39 0.64 0.81 0.96

0.23 0.39 0.66 0.84 1.03

2

b e h a v i o r . T h e other p o l y m e r s t a n d a r d s c o u l d possibly be used, b u t o n l y i f the p o l y m e r s being a n a l y z e d experienced very s i m i l a r non-size e x c l u s i o n effects. F i g u r e 5 shows a t y p i c a l e x a m p l e of H P S E C of a n a c e t y l a t e d l i g n i n i n the various solvent systems. A d d i t i o n of D M F to the m o b i l e phase re sulted i n a m u l t i m o d a l c h r o m a t o g r a m w i t h a s u b s t a n t i a l f r a c t i o n of the l i g n i n p o l y m e r being excluded f r o m the pores of the c o l u m n gel. I n fact, e l u t i o n of m a t e r i a l u p to 1 m l before V was observed. T h i s t y p e of b e h a v ior has been r e p o r t e d b y several researchers a n d is generally a t t r i b u t e d to a s s o c i a t i o n o f l i g n i n molecules. C o n n o r s et al (8), u s i n g d i m e t h y l sulfox ide a n d μ-styragel c o l u m n s , observed association of b o t h u n d e r i v a t i z e d a n d a c e t y l a t e d kraft lignins, i n d i c a t i n g t h a t the m e c h a n i s m of a s s o c i a t i o n was not t h r o u g h hydrogen b o n d i n g . In non-aqueous solvents, E k m a n a n d L i n d berg (6), s t u d y i n g e x h a u s t i v e l y m e t h y l a t e d l i g n i n s , a n d C h u m et al. (3), w i t h a c e t y l a t e d l i g n i n s a n d D M F as solvent, also f o u n d association t a k i n g place. S a r k a n e n (9) r e p o r t e d t h a t u n d e r i v a t i z e d organosolv l i g n i n s also ex h i b i t e d association d u r i n g gel p e r m e a t i o n c h r o m a t o g r a p h y o n d e x t r a n gels u s i n g aqueous s o d i u m h y d r o x i d e as solvent. H e proposed t h a t interactions of the highest o c c u p i e d m o l e c u l a r o r b i t a l - lowest u n o c c u p i e d m o l e c u l a r o r b i t a l ( H O M O - L U M O ) t y p e c o u l d be governing these associative processes. A n u m b e r of researchers (3,8) have s h o w n t h a t a d d i t i o n of a n electrolyte ( L i C l , L i B r , etc.) to the solvent d i s r u p t s m o l e c u l a r association so t h a t u n i m o d a l e l u t i o n of l i g n i n s is again o b t a i n e d . F r o m F i g u r e 5 i t appears t h a t neither f o r m i c a c i d (5 w t % ) nor L i B r (0.1 M ) c o u l d break a l l the associative i n t e r a c t i o n s , w i t h f o r m i c a c i d b e i n g s l i g h t l y less effective. These associa tive i n t e r a c t i o n s have been observed w i t h other p o l y m e r s a n d it has been 0



8.

JOHNSON ET AL.

119


suggested (3) t h a t a d d i t i o n of L i B r c o u l d e l i m i n a t e association b y s h i e l d i n g the dipoles of i n d i v i d u a l molecules. B y u s i n g m o d e l c o m p o u n d s a better u n d e r s t a n d i n g o f these i n t e r a c t i o n s s h o u l d be o b t a i n e d . T a b l e I I I describes the effect of solvent change o n the l i g n i n m o d e l c o m p o u n d s . N o n e of the m o d e l c o m p o u n d s e x h i b i t e d evidence of associa t i o n ; a l l h a d u n i m o d a l e l u t i o n i n the different solvents. T h e KD of the f u l l y d e r i v a t i z e d m o d e l c o m p o u n d s tended to increase as solvent p o l a r i t y was increased, as h a d t h a t of the p o l y m e r s t a n d a r d s . A s these are f u l l y d e r i v a t i z e d , r e l a t i v e l y s m a l l molecules, the p o s s i b i l i t y for size change t h r o u g h i n t e r a c t i o n w i t h the solvents is s m a l l . Increasing affinity for the c o l u m n gel as the solvent p o l a r i t y increased is the most p r o b a b l e e x p l a n a t i o n for t h e i r greater r e t e n t i o n . T a b l e I I I . Effect of Solvent C h a n g e s on E l u t i o n of L i g n i n M o d e l C o m p o u n d s and Polymers Solute D i s t r i b u t i o n Coefficients Mol.Wt.

THF

THF/DMF

DMF

DMF/LiBr

DMF/HC0 H

0.51 0.58 0.68 0.75 0.91

0.54 0.62 0.71 0.77 0.94

0.54 0.62 0.72 0.81 0.97

0.45 0.67 0.59 0.66 0.75 0.79

0.42 0.65 0.57 0.65 0.76 0.77

0.46 0.70 0.61 0.69 0.77 0.82

E t h e r B o n d e d M o d e l P o l y m e r s2 0.22 0.18 0.21 0.19 0.24 0.26 0.21 0.22 0.46 0.40 0.45 0.40

0.23 0.28 0.49

Fully Derivatized Models IA 1078 0.46 IIA 1034 0.49 ΠΙΑ 446 0.64 VI 378 0.70 VII 256 0.80 Partially I V III IV VIII IX

1

2

2

1

0.47 0.52 0.64 0.68 0.80

or U n d e r i v a t i z e d M o d e l s 784 0.49 0.44 408 0.66 0.62 320 0.66 0.59 304 0.64 0.68 0.82 0.74 196 166 0.83 0.76

a,/?-bis(0-4-aryl) X 6100 XI 4500 XII 1200

(KD)

T h e A indicates peracetylated derivatives of m o d e l c o m p o u n d s I, II and III. A p p a r e n t molecular weights for the l i g n i n m o d e l p o l y m e r s s h o w n here were d e t e r m i n e d b y H P S E C .

T h e p a r t i a l l y d e r i v a t i z e d a n d u n d e r i v a t i z e d m o d e l c o m p o u n d s h a d KD t h a t appeared to v a r y quite differently. T h e KD for these c o m p o u n d s were generally lower i n the m i x e d T H F / D M F solvent t h a n i n the pure solvents. A d d i t i o n of L i B r i n D M F generally resulted i n lower K p , whereas a d d i t i o n of f o r m i c a c i d resulted i n higher KD t h a n were observed u s i n g the


120



pure solvent. A s these c o m p o u n d s have free phenolic a n d alcoholic h y d r o x y groups, i t is likely t h a t their size was s u b s t a n t i a l l y affected because of hydrogen b o n d i n g w i t h the solvents. However, the c o m p l e x v a r i a b i l i t y of the b e h a v i o r of these u n d e r i v a t i z e d m o d e l c o m p o u n d s is p r o b a b l y due to the c o m b i n a t i o n of several i n t e r a c t i o n s w i t h the H P S E C s y s t e m . T h e effect o f solvent change o n the e l u t i o n of the l i g n i n m o d e l p o l y mers is also s h o w n i n T a b l e I I I . A l l the p o l y m e r s were i n c r e a s i n g l y r e t a i n e d b y the c o l u m n gel as eluant p o l a r i t y was increased. However, as c a n be seen i n the e x a m p l e i n F i g u r e 6, a d d i t i o n o f D M F to the solvent p r o d u c e d m u l t i m o d e l e l u t i o n s i m i l a r t o t h a t observed w i t h a c e t y l a t e d l i g n i n s w h i c h has been a t t r i b u t e d to a s s o c i a t i o n . A s observed w i t h l i g n i n s , a d d i t i o n of L i B r or f o r m i c a c i d prevented association w i t h o n l y a s l i g h t shoulder i n the c h r o m a t o g r a m o b t a i n e d i n D M F / f o r m i c a c i d i n d i c a t i n g t h a t any associa t i o n r e m a i n e d . C o m p a r i n g the c h r o m a t o g r a m s of the a c e t y l a t e d l i g n i n s a n d l i g n i n m o d e l p o l y m e r s , it appears t h a t the l i g n i n s associated to a greater degree t h a n d i d the m o d e l p o l y m e r s . T h e question posed b y these results is: W h y do the m o d e l p o l y m e r s a p pear t o behave like l i g n i n s w h i l e the m o d e l c o m p o u n d s do n o t ? C h e m i c a l l y the m o d e l c o m p o u n d s (e.g., I A a n d II A ) are more s i m i l a r to the a c e t y l a t e d lignins t h a n are the m o d e l p o l y m e r s . T h e linkages between Co, u n i t s i n the m o d e l c o m p o u n d s are either /?-0-4 or b i p h e n y l w h i c h are t w o i m p o r t a n t i n t e r u n i t linkages f o u n d i n organosolv l i g n i n s . T h e linkages between C9 u n i t s i n the m o d e l p o l y m e r s are a - 0 - 4 , b u t they do have a g u a i a c y l g r o u p b o n d e d to the β- c a r b o n o f the p r o p y l s i d e c h a i n . W h i l e the m o d e l p o l y mers are u n a c e t y l a t e d , they have very few free h y d r o x y 1 groups. T h e m o d e l p o l y m e r s are p r o d u c e d b y a n i o n i n i t i a t e d p o l y m e r i z a t i o n of a q u i n o n e m e t h i d e u s i n g h y d r o x i d e or m e t h o x i d e . C h a i n p r o p a g a t i o n occurs b y r e a c t i o n of the r e s u l t i n g phenoxide w i t h the u n s a t u r a t e d ct-C of the q u i n o n e m e t h i d e , w i t h t e r m i n a t i o n o c c u r r i n g when the phenoxide reacts w i t h a p r o t o n i n s t e a d . C o n s e q u e n t l y , the m o d e l p o l y m e r s s h o u l d o n l y have one free phe n o l i c h y d r o x y l a n d one free a l i p h a t i c h y d r o x y l (or a m e t h o x y l ) per molecule independent of their m o l e c u l a r weight. Despite the differences between the a c e t y l a t e d l i g n i n s a n d the m o d e l p o l y m e r s , their H P S E C b e h a v i o r i n the various solvents is s i m i l a r a n d different f r o m t h a t of the a c e t y l a t e d m o d e l c o m p o u n d s . T h e r e is one p r o p e r t y i n w h i c h the m o d e l p o l y m e r s are s i m i l a r a n d the m o d e l c o m p o u n d s are d i s s i m i l a r to lignins, a n d t h a t is t h e i r d i s persity. T h e m o d e l p o l y m e r s a n d lignins are polydisperse, w h i l e the m o d e l c o m p o u n d s are monodisperse. C o n n o r s et al. (3) observed m u l t i m o d a l e l u t i o n f r o m gel p e r m e a t i o n c h r o m a t o g r a p h y of a s y n t h e t i c D H P l i g n i n w h e n using D M F as solvent. S i m i l a r behavior was f o u n d for a k r a f t l i g n i n i n the same s y s t e m . T e n t a t i v e conclusions t h a t m a y be d r a w n f r o m these results are t h a t h i g h a n d low m o l e c u l a r weight c o m p o n e n t s must be present for a s s o c i a t i o n to occur w i t h l i g n i n - l i k e molecules, such t h a t complexes suffi c i e n t l y large to be e x c l u d e d f r o m the pores o f the c o l u m n gel are p r o d u c e d , or t h a t the c o m p l e x t h r e e - d i m e n s i o n a l s t r u c t u r e s of these molecules some how enhance associative i n t e r a c t i o n s . E x p e r i m e n t s to o b t a i n evidence of associative i n t e r a c t i o n s between low



Figure 5. H P S E C of an acetylated organosolv aspen lignin in the various eluants.

Figure 6. H P S E C of an a , /?-bis(o4-aryl) ether bonded model polymer (XI) in the various eluants.



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a n d h i g h m o l e c u l a r weight l i g n i n - l i k e components have been a t t e m p t e d . H P S E C of m i x t u r e s of acetylated m o d e l c o m p o u n d s (ΙΑ, I I A a n d Π Ι Α ) w i t h a n a c e t y l a t e d organosolv l i g n i n were o b t a i n e d u s i n g T H F a n d D M F as eluants. In a l l cases, no a d d i t i o n a l associative behavior was observed. T h e H P S E C of the m i x t u r e s appeared to be s i m p l y the a d d i t i o n of the H P S E C of the m o d e l c o m p o u n d s to t h a t of the l i g n i n . In T H F no e x c l u d e d peaks were observed a n d i n D M F the excluded peaks of the l i g n i n were r e p r o d u c e d w i t h o u t m o d i f i c a t i o n . T h e r e was no clearly discernible i n t e r a c t i o n between these l i g n i n m o d e l tetramers a n d d i m e r a n d the l i g n i n molecules.

Conclusions 1. A p h y s i c a l change i n the c o l u m n gel was f o u n d w h e n u s i n g the h i g h f r a c t i o n a l p o l a r i t y eluants m a d e w i t h D M F . T h e r e was a p p a r e n t l y a s m a l l increase i n the v o i d v o l u m e a n d a p p r o x i m a t e l y a 1 0 % increase i n the t o t a l pore v o l u m e . T h e observed changes were r e p r o d u c i b l e as the c o l u m n set was cycled t h r o u g h eluants of h i g h a n d low f r a c t i o n a l polarity. 2. A s s h o w n p r e v i o u s l y i n the l i t e r a t u r e , polystyrenes are s t r o n g l y affected b y a d s o r p t i o n when u s i n g solvents c o n t a i n i n g D M F a n d p o l y s t y r e n e d i v i n y l b e n z e n e gels. T h e P M M A , Igepals, d e r i v a t i z e d l i g n i n m o d e l c o m p o u n d s , a n d l i g n i n m o d e l p o l y m e r s also appear to be affected b y a d s o r p t i o n b u t to a lesser extent. Unless a different c o l u m n gel is used for c h r o m a t o g r a p h y w i t h D M F , this non-size e x c l u s i o n effect is always likely to be a p r o b l e m . 3. T h e a , / ? - b i s ( 0 - 4 - a r y l ) ether b o n d e d l i g n i n m o d e l p o l y m e r s e x h i b i t e d associative b e h a v i o r o n g o i n g to the h i g h f r a c t i o n a l p o l a r i t y solvents such as D M F / T H F a n d D M F . T h e association was decreased b y a d d i t i o n of 0.1 M L i B r more t h a n by a d d i t i o n of 5 w t % f o r m i c a c i d . T h i s behavior is very s i m i l a r to t h a t observed for d e r i v a t i z e d a n d u n d e r i v a t i z e d organosolv a n d other lignins. These s i m p l e linear p o l y m e r s are therefore g o o d m o d e l polymers for lignins a n d s h o u l d receive more a t t e n t i o n as c a l i b r a t i o n s t a n d a r d s for the H P S E C of lignins. 4. N o n e of the low-molecular-weight l i g n i n m o d e l c o m p o u n d s used i n this s t u d y , d e r i v a t i z e d or n o t , clearly e x h i b i t e d associative b e h a v i o r i n the presence of higher f r a c t i o n a l p o l a r i t y eluants. T h i s indicates t h a t the self-association constants for these c o m p o u n d s i n these m e d i a are s m a l l , at least for the c o n c e n t r a t i o n range s t u d i e d . 5. T h e c h r o m a t o g r a m s of m i x t u r e s of well-defined l o w - m o l e c u l a r - w e i g h t l i g n i n m o d e l c o m p o u n d s a n d a , /?-bis(0-4-aryl) ether b o n d e d l i g n i n m o d e l p o l y m e r s a n d acetylated lignins i n h i g h f r a c t i o n a l p o l a r i t y s o l vents appeared to be merely a d d i t i v e . In the c o n c e n t r a t i o n range inves t i g a t e d , these m i x t u r e s d i d not e x h i b i t evidence of association between the l o w - m o l e c u l a r - w e i g h t c o m p o u n d s a n d the p o l y m e r s , i n a d d i t i o n to t h a t already e x h i b i t e d by the p o l y m e r s themselves w i t h i n the e x p e r i m e n t a l errors of these measurements.


8.

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A cknowledgment s T h e authors w i s h t o t h a n k D r . M . E . H i m m e l a n d D r . S. S a r k a n e n for m a n y profitable discussions a n d suggestions. T h e s u p p o r t o f the B i o m a s s E n e r g y Technology D i v i s i o n o f the U . S . D e p a r t m e n t o f E n e r g y t h r o u g h F W P B F 8 2 is gratefully acknowledged.


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Molecular Weight Distribution Studies Using Lignin Model Compounds

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