Graft Copolymerization of Lignocellulosic Fibers - American Chemical

11 Graft Copolymer of Cellulose Nitrate K. S. V. SRINIVASAN and D. N.-S. HON Virginia Polytechnic Institute and State University, Department of Forest Products, Blacksburg, V A 24061-0299

Downloaded by CORNELL UNIV on May 11, 2017 | http://pubs.acs.org Publication Date: June 18, 1982 | doi: 10.1021/bk-1982-0187.ch011

M. SANTAPPA Central Leather Research Institute, Adyar, Madras, India

Grafting of vinyl monomers onto cellulose nitrate in heterogeneous and homogeneous media initiated by ceric ions, and benzoyl peroxide and azobisisobutyronitrile, respectively, have been studied. Isolation of graft copolymers from homopolymers and unreacted cellulose nitrate was conducted by using selective solvent extraction technique. The graft copolymers were characterized by infra red spectroscopy, gel permeation chromatography, viscosity measurements, and nuclear magnetic resonance. The effects of various grafting para meters, v i z . , i n i t i a t o r concentration, monomer concentration, reaction time and the substrate concentration on grafting efficiency and degree of grafting were examined. Probable grafting reaction mechanisms for heterogeneous and homo geneous media are proposed. Methods f o r g r a f t c o p o l y m e r i z a t i o n of v i n y l monomers onto c e l l u l o s e i n i t i a t e d by u l t r a v i o l e t i r r a d i a t i o n (1-3), h i g h energy r a d i a t i o n (4-9), redox and o x i d a t i o n r e a c t i o n s (10-12), and mechanoreactions (13-15) have been e x t e n s i v e l y i n v e s t i g a t e d i n d i f f e r e n t p a r t s o f the w o r l d . However, g r a f t i n g of v i n y l monomers onto c e l l u l o s e d e r i v a t i v e s as a means of a l t e r i n g p r o p e r t i e s of the base polymer has a t t r a c t e d l i t t l e a t t e n t i o n u n t i l r e c e n t l y . The g r a f t c o p o l y m e r i z a t i o n of methyl methacrylate and a c r y l o n i t r i l e onto methyl c e l l u l o s e i n the absence of r a d i c a l i n h i b i t o r s (16) and by the persulphate i o n (17), and of s t y r e n e onto c e l l u l o s e a c e t a t e (18), has been reported. M o d i f i c a t i o n o f c e l l u l o s e n i t r a t e has not been reported u n t i l r e c e n t l y i n the l i t e r a t u r e d e s p i t e i t s v e r s a t i l e a p p l i c a t i o n i n surface c o a t i n g and l e a t h e r i n d u s t r i e s (19-22). C e l l u l o s e n i t r a t e f i l m cast from s o l u t i o n i s h i g h l y b r i t t l e and i s made f l e x i b l e f o r s u r f a c e c o a t i n g a p p l i c a t i o n s by the a d d i t i o n of p l a s t i c i z e r s l i k e d i b u t y l p h t h a l a t e , d i o c t y l p h t h a l a t e ,

©

0097-6156/82/0187-0155$7.00/0 1982 American Chemical Society

Hon; Graft Copolymerization of Lignocellulosic Fibers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

GRAFT COPOLYMERIZATION OF


156

LIGNOCELLULOSIC FIBERS

e t c . (23,24). The drawback w i t h these p l a s t i c i z e r s i s that they tend to migrate on aging, r e s u l t i n g i n b r i t t l e n e s s of the f i l m . We have envisaged that one of the best means of a l t e r i n g the p r o p e r t i e s of c e l l u l o s e n i t r a t e could be by g r a f t i n g of v i n y l monomers onto c e l l u l o s e n i t r a t e . By proper choice of v i n y l monomers, the d e s i r e d f l e x i b i l i t y of the c e l l u l o s e n i t r a t e f i l m could be achieved and m i g r a t i o n of molecules could be prevented because the copolymers are c h e m i c a l l y g r a f t e d to the c e l l u l o s e n i t r a t e . In a d d i t i o n to t h i s p r o p e r t y , g r a f t i n g of proper v i n y l monomers could impart h i g h g l o s s , water r e p e l l e n c y , and degradat i o n from UV l i g h t to c e l l u l o s e n i t r a t e f i l m . This paper deals w i t h the g r a f t i n g of monomers to c e l l u l o s e n i t r a t e i n i t i a t e d by e e r i e ammonium n i t r a t e i n heterogeneous medium and by benzoyl peroxide and a z o b i s i s o b u t y r o n i t r i l e i n homogeneous media and the c h a r a c t e r i z a t i o n of the g r a f t e d products. Experimental Materials. Commercially a v a i l a b l e c e l l u l o s e n i t r a t e (h sec)(CN) w i t h 11.8-12.2% n i t r o g e n content s u p p l i e d by Ashahi Chemicals and Hercules, Inc., was p u r i f i e d by washing w i t h water and d r y i n g under vacuum. Methyl methacrylate monomer s u p p l i e d by Rohm and Haas was freed from i n h i b i t o r bv washing w i t h sodium hydroxide s o l u t i o n , d r y i n g over anhydrous sodium s u l p h a t e , and d i s t i l l i n g under vacuum.Ceric ammonium n i t r a t e (CAN) reagent grade was used without p u r i f i c a t i o n . A stock s o l u t i o n of O.1N c e r i c s o l u t i o n i n IN n i t r i c a c i d was prepared and s t o r e d i n a r e f r i g e r a t o r p r i o r to the experiment. Benzoyl peroxide (BPO), a z o b i s i s o b u t y r o n i t r i l e (AIBN), methanol, benzene, petroleum e t h e r , and methyl i s o b u t y l ketone were A.R. grade and used without purification. Methods C e r i c I o n - I n i t i a t e d G r a f t Copolymerization. The r e q u i s i t e amount of c e l l u l o s e n i t r a t e was d i s p e r s e d i n 300 ml of water i n a Waring blender a f t e r a l l o w i n g the polymer to s w e l l f o r 30 minutes. The h e t e r o d i s p e r s e d s o l u t i o n was then t r a n s f e r r e d to a three-necked f l a s k equipped w i t h a g l a s s s t i r r e r and a n i t r o g e n i n l e t . The r e q u i r e d q u a n t i t y of monomer and the i n i t i a t o r were added, and then the p u r i f i e d n i t r o g e n was purged i n t o the s o l u t i o n f o r 30 minutes. The p o l y m e r i z a t i o n was allowed to proceed f o r a s p e c i f i e d l e n g t h of time at 30°C. Hydroquinone was added at the end of the r e a c t i o n time to a r r e s t the p o l y m e r i z a t i o n . The g r a f t e d products were separated by f i l t r a t i o n and were washed w i t h d i s t i l l e d water r e p e a t e d l y . Benzoyl Peroxide and A z o b i s i s o b u t y r o n i t r i l e - I n i t i a t e d G r a f t Copolymerization. C e l l u l o s e n i t r a t e (CN) (10 g) was d i s solved i n methyl i s o b u t y l ketone (90 ml) i n a three-necked f l a s k and to t h i s s o l u t i o n methyl methacrylate (10 ml) and benzoyl


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peroxide (1 g) were added; dry n i t r o g e n was passed through the s o l u t i o n f o r 30 minutes. The n i t r o g e n was then stopped and the r e a c t i o n v e s s e l was immersed i n a thermostatic bath maintained at 70°C., and the p o l y m e r i z a t i o n was c a r r i e d out w i t h g e n t l e s t i r r i n g . A f t e r 3 hours, the contents were poured i n t o an excess of petroleum ether (b.p. 60-80°C) t o p r e c i p i t a t e the copolymer.


V i s c o s i t y . The v i s c o s i t y measurements of the PMMA g r a f t chains obtained by h y d r o l y s i s o f g r a f t copolymer were c a r r i e d out i n benzene a t 30°C using an Ubbelhode viscometer, and the number average molecular weight of the g r a f t e d chain was c a l c u l a t e d u s i n g the equation(Z5): 5

0

[η] = 8.96 χ 1 0 - Μ . n

76

The d e f i n i t i o n s o f g r a f t i n g parameters a r e summarized as f o l l o w s : τ η Wt. o f (CN-g-PMMA) - wt. of CN Percent G r a f t i n g = * fχ 100 Wt. of NC 1 Λ Π

G r a f t i n g E f f i c i e n c y (%) = Wt. o f PMMA g r a f t e d Wt. of PMMA g r a f t e d + Wt. of homopolymer (PMMA)

1 Q Q X

Number of G r a f t i n g S i t e s = Wt. of PMMA grafted/Mol, wt. of g r a f t e d PMMA Wt. of CN used/Mol. wt. of CN I n f r a r e d Spectra (IR) - I n f r a r e d s p e c t r a of c e l l u l o s e n i t r a t e , poly (methyl methacrylate) and c e l l u l o s e n i t r a t e - g - p o l y ( m e t h y l methacrylate) f i l m s were recorded using a Perkin-Elmer 337 g r a t ing IR spectrophotometer. Nuclear Magnetic Resonance (NMR) - NMR s p e c t r a of c e l l u l o s e n i t r a t e - g - p o l y ( m e t h y l methacrylate) and c e l l u l o s e n i t r a t e were recorded i n deuterated acetone using Perkin-Elmer-R-32 (90 MHz). Gel Permeation Chromatography (GPC): GPC of all samples were recorded w i t h a Waters A s s o c i a t e s Model 244 high-pressure l i q u i d chromatograph u s i n g a d i l u t e s o l u t i o n (O.02% i n t e t r a h y d r o f u r a n and a flow r a t e o f 5 ml/min). The chromatograph was connected w i t h four m i c r o s t y r a g e l columns i n s e r i e s (pore s i z e 1 0 , 1 0 , ιο\ ίο3 I). 6


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GRAFT COPOLYMERIZATION OF LIGNOCELLULOSIC FIBERS


R e s u l t s and D i s c u s s i o n An i n i t i a l t r i a l run of experiments was c a r r i e d out to g r a f t v a r i o u s monomers ( s t y r e n e , v i n y l a c e t a t e , a c r y l i c a c i d , a c r y l a mide, methyl methacrylate, methyl a c r y l a t e ) onto c e l l u l o s e n i t r a t e i n i t i a t e d by c e r i c i o n s . No g r a f t c o p o l y m e r i z a t i o n was found to occur w i t h s t y r e n e , v i n y l a c e t a t e , acrylamide and a c r y l i c a c i d , though g r a f t i n g d i d occur w i t h methyl methacrylate and methyl acrylate. Between these monomers, higher g r a f t i n g percentage and g r a f t i n g e f f i c i e n c y were observed f o r methyl methacrylate than methyl a c r y l a t e . Brauer and Termini (26) have a l s o observed the absence of g r a f t i n g w i t h many monomers onto c o l l a g e n (protein) when c e r i c ions were used as the i n i t i a t o r . This could be due to the s p e c i f i c r e a c t i v i t y of the monomer towards the s p e c i f i c backbone. I s o l a t i o n and C h a r a c t e r i z a t i o n of Grafted Products. The product obtained by the above c o p o l y m e r i z a t i o n c o n t a i n s the backbone polymer ( c e l l u l o s e n i t r a t e ) , g r a f t copolymer, and homopolymer. Solvent e x t r a c t i o n procedure was employed f o r i s o l a t i o n of the g r a f t copolymer. The products separated by f i l t r a t i o n were soxhl e t e x t r a c t e d f o r 72 hours w i t h benzene to remove poly(methyl methacrylate) homopolymer. The remaining products were again soxhlet e x t r a c t e d w i t h methanol f o r 72 hours to remove unreacted c e l l u l o s e n i t r a t e . The remaining product i s hence a t r u e g r a f t copolymer. This i s one of the few i n s t a n c e s i n g r a f t copolymeriz a t i o n where complete s e p a r a t i o n of the i n d i v i d u a l polymer can be achieved. The i n f r a r e d s p e c t r a of c e l l u l o s e n i t r a t e and the c e l l u l o s e n i t r a t e - g - p o l y ( m e t h y l methacrylate) are shown i n F i g u r e 1. F i g u r e l a shows the i n f r a r e d spectrum of c e l l u l o s e n i t r a t e chara c t e r i z e d by a broad medium band at 3400 cm" due to the h y d r o x y l group of c e l l u l o s e n i t r a t e and perhaps a t r a c e amount of moisture i n the sample, a weak a b s o r p t i o n at 2900 cm i n d i c a t i n g a small number of C-H l i n k a g e s , and 1280 cm and 1655 cm"" characterist i c of covalent -ONO2 asymétrie and symmetric s t r e t c h i n g v i b r a tions, respectively. F i g u r e l b shows the IR spectrum of pure g r a f t copolymer completely separated from poly(methyl methcrylate) and c e l l u l o s e n i t r a t e backbone. I t shows a s m a l l peak f o r the C=0 group at 1740 cm i n d i c a t i n g t h a t attachment of poly(methyl methacrylate) homopolymer to n i t r o c e l l u l o s e backbone took p l a c e . This g r a f t copolymer was hydrolyzed w i t h 6N HC1 at 115°C f o r 48 hrs to separate poly(methyl m e t h a c r y l a t e ) - g r a f t e d chains from c e l l u l o s e n i t r a t e . The IR spectrum of t h i s poly(methyl methac r y l a t e ) gave a more pronounced C=0 peak at 1740 cm (Figure 2). A p h y s i c a l blend of c e l l u l o s e n i t r a t e and poly(methyl methacrylate) a f t e r soxhlet extraction with appropriate solvent f o r poly(methyl m e t h a c r y l a t e ) , however, shows no peak at 1740 cm"" due to C=0 group. This f u r t h e r s u b s t a n t i a t e s the formation of a true g r a f t copolymer. 1

-1

-1

1

-1

-1

1


Hon; Graft Copolymerization of Lignocellulosic Fibers ACS Symposium Series; American Chemical Society: Washington, DC, 1982. )

Figure 1. IR spectra of a, cellulose nitrate and b, cellulose nitrate-g-poly(methyl methacrylate).

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Figure 2. IR spectra of a, cellulose nitrate-g-poly(methyl methacrylate) and b, cellulose nitrate-gpoly(methyl methacrylate) after hydrolysis. (Reprinted, with permission, from Ref. 20. Copyright 1978, Wiley.)

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The n u c l e a r magnetic resonance s p e c t r a of c e l l u l o s e n i t r a t e and o f c e l l u l o s e - g - p o l y ( m e t h y l methacrylate) a f t e r s e p a r a t i o n from c e l l u l o s e n i t r a t e and poly(methyl m e t h a c r y l a t e ) , are shown i n F i g u r e s 3 and 4, r e s p e c t i v e l y . The g r a f t copolymer shows the presence of (XCH3 proton a t 1.06, i n d i c a t i n g the presence of poly(methyl methacrylate) chains attached t o c e l l u l o s e n i t r a t e . Gel permeation chromatography (GPC) of poly(methyl methacry l a t e ) and c e l l u l o s e n i t r a t e showed e l u t i o n volume peaks a t 62.5 ml f o r PMMA and a t 87.5 f o r c e l l u l o s e n i t r a t e (Figure 5 ) , due t o t h e i r d i f f e r e n c e i n molecular weight. A mixture of poly(methyl methacrylate) and c e l l u l o s e n i t r a t e of the same r a t i o as t h a t of the g r a f t copolymer was recorded and two peaks i n e l u t i o n volume at almost i d e n t i c a l p o s i t i o n s were observed. This shows that the c o n s t i t u e n t homopolymers r e t a i n t h e i r i d e n t i t y i n a p h y s i c a l mixture. The i s o l a t e d g r a f t copolymer showed a s i n g l e peak i n e l u t i o n volume a t 80.0 ml. The second peak i n e l u t i o n volume i s absent i n s p i t e of poly(methyl methacrylate) attached t o c e l l u l o s e n i t r a t e as revealed by i n f r a r e d spectrum. Hence, these r e s u l t s i n d i c a t e that GPC can be used as a technique t o d i f f e r e n t i a t e between homopolymer, p h y s i c a l mixture, and g r a f t copolymer. G r a f t i n g Parameters E f f e c t o f V a r i a b l e s on G r a f t i n g I n i t i a t e d by C e r i c Ions E f f e c t of G r a f t i n g Time. The e f f e c t of g r a f t i n g time on molecular weight of the g r a f t e d chain and g r a f t i n g per centage i s given i n Table I and F i g u r e 6. I n the stages o f g r a f t c o p o l y m e r i z a t i o n (up t o 75 min), the c e r i c i o n i n i t i a t e s a l a r g e number of growing PMMA branches. This i s r e f l e c t e d by an i n c r e a s e i n the g r a f t i n g percentage up t o 75 min; w i t h g r a f t i n g time greater than t h i s , the g r a f t i n g percentage remains more or l e s s constant. The number average molecular weight of t h e g r a f t e d chains a l s o increased up t o 75 min and then remained constant. E f f e c t of I n i t i a t o r Concentration. The e f f e c t of i n i t i a t o r c o n c e n t r a t i o n on the g r a f t i n g percentage and on the number average molecular weight of the g r a f t e d chains i s shown i n Table I I and F i g u r e 7. A s i g n i f i c a n t i n c r e a s e i n g r a f t i n g per centage was achieved when CAN c o n c e n t r a t i o n was 1.7 χ 10~ mol/1 and a t higher c o n c e n t r a t i o n the degree of g r a f t i n g decreased s l i g h t l y . The number average molecular weight of the g r a f t e d chains a l s o i n c r e a s e d up t o t h i s c o n c e n t r a t i o n and f u r t h e r i n crease i n c o n c e n t r a t i o n caused a decrease i n number average molecular weight o f the g r a f t e d c h a i n s . The probable reason f o r t h i s i s that a greater number of g r a f t i n g s i t e s were created by i n c r e a s e i n the c e r i c i o n c o n c e n t r a t i o n . 3


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3 Figure 3.

5

4

2

1

NMR spectrum of cellulose nitrate.

3

2

1

0 δ Scale

Figure 4.

NMR spectrum of cellulose nitrate-g-poly(methyl methacrylate).



CURVES

Figure 5. GPC curves of 1, cellulose nitrate; 2, polyfmethyl methacrylate); 3, physical mixture of cellulose nitrate and polyfmethyl methacrylate); and 4, graft copolymer of cellulose nitrate-g-poly(methyl methacrylate). (Reprinted, with permission, from Ref. 21. Copyright 1980, Springer-Verlag.)

GPC


164



Table I .

E f f e c t of G r a f t i n g Time on Percentage G r a f t i n g and Molecular Weight

Time

Grafting %

Mol. Wt. of Grafted Chains M χ 10

30

10.0

O.7

O.019

75

25.0

1.5

O.021

120

21.7

1.7

O.017

180

22.0

1.7

O.017

300

25.0

1.8

O.017

5

n

No. of G r a f t i n g S i t e s mole/mole

G r a f t i n g Conditions: C e l l u l o s e n i t r a t e - 6 g; [MMA] - O.25 mol/l; [CAN] - 1.7 χ 10" m o l / l ; Temperature - 30°C.; T o t a l volume 300 ml. (Reprinted w i t h permission from reference 19.) 3

Table I I .

E f f e c t of I n i t i a t o r Concentration on Percent G r a f t i n g and Molecular Weight

[CAN]

No.

m o l / l χ 10

3

Grafting %

Molecular Weight M χ 10"

No. of G r a f t i n g S i t e s mole/mole

5

n

1

1.0

10.0

O.8

O.016

2

1.7

25.0

1.5

O.021

3

2.7

23.3

1.3

O.023

4

3.3

23.3

1.2

O.025

G r a f t i n g Conditions: C e l l u l o s e n i t r a t e - 6 g; Time - 75 min.; [MMA] - O.25 m o l / l ; T o t a l volume - 300 ml; G r a f t i n g temperature 30°C. (Reprinted w i t h permission from reference 19.)


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ϋ Ζ


ο

TIME, min

Figure 6. Effect of grafting time on the graft copolymerization of methyl meth acrylate onto cellulose nitrate with ceric ammonium nitrate as initiator. (Reprinted, with permission, from Ref. 19. Copyright 1979, Wiley.)

3

[CAN] χ ΙΟ" mol L

- 1

Figure 7. Effect of ceric ammonium nitrate concentration on the graft copolymeri zation of methyl methacrylate onto cellulose nitrate. (Reprinted, with permission, from Ref. 19. Copyright 1979, Wiley.)


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E f f e c t of V a r i a t i o n of Monomer and C e l l u l o s e N i t r a t e . The e f f e c t of v a r y i n g monomer c o n c e n t r a t i o n and c e l l u l o s e n i t r a t e on the g r a f t i n g percentage i s shown i n Figures 8 and 9 and Tables I I I and IV. As the monomer c o n c e n t r a t i o n i n c r e a s e d , the g r a f t i n g percentage and the number average molecular weight increased. I t reached a maximum at a c o n c e n t r a t i o n of O.22 m o l / l and then decreased. The a d d i t i o n of higher monomer content caused agglomeration of c e l l u l o s e n i t r a t e r e s u l t i n g i n the forma t i o n of lumps. The g r a f t i n g percentage increased up to 6 g of c e l l u l o s e n i t r a t e and then decreased f u r t h e r w i t h i n c r e a s e i n c e l l u l o s e n i t r a t e . This i s due to the i n c r e a s e i n the r e l a t i v e r a t i o of c e l l u l o s e n i t r a t e to c e r i c i o n . G r a f t i n g of V i n y l Monomers to C e l l u l o s e N i t r a t e i n Homogeneous Phase I n i t i a t e d by Benzoyl Peroxide and A z o b i s i s o b u tyronitrile. One of the c r i t i c a l problems i n g r a f t i n g r e a c t i o n s i s the formation of homopolymer and i t s subsequent e f f e c t s on g r a f t i n g e f f i c i e n c y . I f the homopolymer formed i s i n s o l u b l e i n monomer s o l u t i o n or medium i n which the g r a f t i n g r e a c t i o n i s c a r r i e d out, t u r b i d i t y occurs and the g r a f t i n g becomes e r r a t i c or even causes premature t e r m i n a t i o n . Hence, the y i e l d of homo polymer, which i s always a competing r e a c t i o n to g r a f t i n g , i s an extremely important f a c t o r i n the g r a f t i n g process. For indus t r i a l a p p l i c a t i o n s , i f the homopolymer formed i s i n s o l u t i o n and does not i n t e r f e r e w i t h the p r o p e r t i e s of the g r a f t e d product, i t can be used as such i n surface c o a t i n g and other a p p l i c a t i o n s . Hence, the g r a f t i n g of v i n y l monomers i n homogeneous medium i s p r e f e r r e d to the heterogeneous phase to achieve g r e a t e r e f f i ciency of g r a f t i n g and f o r d i r e c t a p p l i c a t i o n s . E f f e c t of V a r i a b l e s on G r a f t i n g Reaction i n Homogeneous Medium. Tables V and VI and F i g u r e 10 show the e f f e c t of benzoyl peroxide (BPO) and a z o b i s i s o b u t y r o n i t r i l e (AIBN) c o n c e n t r a t i o n s on the percent g r a f t i n g , g r a f t i n g e f f i c i e n c y , and number average molecular weights of the g r a f t e d PMMA chains. The maximum i n percent g r a f t i n g was obtained at BPO = 3.46 χ 10~ m o l / l and then decreased w i t h f u r t h e r i n c r e a s e i n c o n c e n t r a t i o n . I t i s presumed that up to t h i s c r i t i c a l i n i t i a t o r c o n c e n t r a t i o n , all the i n i t i a t o r r a d i c a l produced by decomposition of the i n i t i a t o r i s u t i l i z e d i n producing r a d i c a l s on the backbone polymer and/or homopolymer r a d i c a l s . A f t e r the excess c o n c e n t r a t i o n of BPO = 1.77 χ 10~ m o l / l and AIBN = 5.11 χ 10~ m o l / l , i n i t i a t o r r a d i c a l s are mostly involved i n a c c e l e r a t i n g homopolymer formation. Consequently, the molecular weights of the g r a f t e d chains decrease w i t h increase i n the i n i t i a t o r c o n c e n t r a t i o n . Tables V I I and V I I I and F i g u r e 11 show the e f f e c t of reac t i o n time on the percent g r a f t i n g , g r a f t i n g e f f i c i e n c y , and molecular weight of the g r a f t e d chains f o r BPO and AIBN systems. 2

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Graft Copolymerization of Lignocellulosic Fibers - American Chemical

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