Mechanochemically Initiated Copolymerization Reactions in Cotton

20 Mechanochemically Initiated Copolymerization Reactions in Cotton Cellulose

Downloaded by GEORGE MASON UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch020

DAVID N.-S. HON Virginia Polytechnic Institute and State University, Department of Forest Products, Blacksburg, VA 24061 The potentiality of using mechanical stress to i n i tiate graft copolymerization onto cotton cellulose was investigated. A Norton ball mill and a Wiley mill were used. The absorption of mechanical energy by cellulose molecules during milling and its consequence on cellulose properties were taken into consideration. Decreases in degree of polymerization and crystallinity, and increases in accessibility and copper number of milled cellulose were observed. Free radicals formed in the interim were detected by electron spin resonance (ESR) techniques. Three types of mechanoradicals contributing singlet, doublet and triplet ESR signals were identified. ESR studies also revealed that cellulose mechanoradicals were capable of initiating graft polymerization. Methylmethacrylate propagating radicals were identified when the monomer was in contact with cellulose mechanoradicals.. High grafting efficiency was obtained for ball milled and cut fibers, but a higher degree of grafting was obtained from the ball milled fiber.

Cotton i s a major world f i b e r and c e l l u l o s e resource, cont r i b u t i n g to the h e a l t h , s a f e t y , and w e l l being of a l l people. And even more s i g n i f i c a n t , i t i s a renewable organic raw m a t e r i a l by the f i x a t i o n of s o l a r energy by green p l a n t s . Cotton c e l l u l o s e f i b e r (gossypiwn spp.) i s the seed f i b e r o f cotton p l a n t which normally has a higher p u r i t y and a higher molecular weight than other c e l l u l o s e s such as those i s o l a t e d from wood (1). Cotton c e l l u l o s e provides high s t r e n g t h , d u r a b i l i t y and thermal s t a b i l i t y , a b i l i t y to absorb moisture, easy d y e a b i l i t y and wearing comfort. In defiance of these s e r v i c e a b l e p r o p e r t i e s , c o t t o n c e l l u l o s e d i s a l l o w s i t s e l f f o r wider commercial a p p l i c a t i o n s due to poor s o l u b i l i t y i n common inexpensive s o l v e n t s , l a c k of t h e r 0097-6156/83/0212-0259$06.25/0 © 1983 American Chemical Society Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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m o p l a s t i c i t y , low dimensional s t a b i l i t y and poor crease r e s i s tance. Many s c i e n t i s t s with r e s t l e s s and probing minds have sought ways to improve cotton c e l l u l o s e p r o p e r t i e s f o r a long time. Many techniques have been developed i n the past decades; of these, g r a f t copolymerization r e a c t i o n s appear to be a p r o p i t i o u s one to synthesize c e l l u l o s e copolymers with unique and usef u l p r o p e r t i e s (_2, 3_). G r a f t copolymers can be synthesized by v a r i o u s i n i t i a t i o n methods such as using u l t r a v i o l e t and i o n i z i n g r a d i a t i o n s , and thermal and chemical r e a c t i o n s (2, 4_) . Despite considerable r e search i n the f i e l d , u n f o r t u n a t e l y , due to the low g r a f t i n g e f f i ciency, high degree of homopolymer formation, c e l l u l o s e copolymers are unable to reach commercial successes (2). In order to e s t a b l i s h a g r a f t i n g technique to achieve c e l l u l o s e copolymers with high g r a f t i n g e f f i c i e n c y , i t has been recognized that mechan i c a l l y generated f r e e r a d i c a l s , i . e . , mechanoradicals, are capable of i n i t i a t i n g g r a f t copolymerization with high g r a f t i n g e f f i c i e n c y (5). Although the s y n t h e s i s of b l o c k - and g r a f t - c o p o l y mers by mechanical f o r c e s has been studied e x t e n s i v e l y f o r synt h e t i c polymers (6, 7), very l i t t l e work has been performed on c e l l u l o s i c m a t e r i a l s . W h i s t l e r and Goatley (8) had i n v e s t i g a t e d the p o s s i b i l i t y of using f r e e r a d i c a l s generated by b a l l - m i l l i n g of corn s t a r c h f o r acrylamide p o l y m e r i z a t i o n . Deter and Huang (9) had attempted to g r a f t a c r y l o n i t r i l e , methyl methacrylate and v i n y l acetate by a v i b r o m i l l . Various degrees of g r a f t i n g were obtained, but v i n y l c h l o r i d e d i d not g r a f t w e l l onto c e l l u l o s e . Hon (_5, _10, 11) had demonstrated that mechanoradicals generated i n wood, high y i e l d pulps, c e l l u l o s e and l i g n i n by a glass-bead m i l l are capable of i n i t i a t i n g g r a f t copolymerization, and a higher degree of g r a f t i n g e f f i c i e n c y was obtained from mechanic a l l y i n i t i a t e d g r a f t i n g systems than from those i n i t i a t e d by ultraviolet light irradiation. O r d i n a r i l y , the disadvantages of using mechanical s t r e s s f o r chemical r e a c t i o n s are a r e l a t i v e l y high energy consumption and equipment complexity. Fortunately, i t has been recognized that g r a f t i n g r e a c t i o n s can be c a r r i e d out d i r e c t l y during polymer p r o c e s s i n g and i n standard equipment, such as i n c u t t e r s and g r i n d e r s , without adding e x t r a energy and production c o s t . D e t a i l s are reported i n t h i s paper. The manufacture of cotton c e l l u l o s e products i n v o l v e s complex conversion methods. In order to convert cotton c e l l u l o s e f i b e r i n t o u s e f u l consumer and i n d u s t r i a l products, mechanical processings, such as g r i n d i n g , crushing, c u t t i n g , e t c . , are i n e v i t a b l y c a r r i e d out at g i n s , t e x t i l e and paper m i l l s , as to render the cotton f i b e r p r o c e s s i b l e and commercially u s e f u l . As a consequence, i t i s opportune i f the e x i s t i n g mechanical o p e r a t i o n energy can be u t i l i z e d to i n i t i a t e g r a f t i n g r e a c t i o n . Accordi n g l y , the p o s s i b i l i t y of u t i l i z a t i o n of t h i s energy f o r i n i t i a t ing g r a f t i n g r e a c t i o n was experimented using a Wiley m i l l and a Norton b a l l m i l l as to simulate a c u t t i n g and a m i l l i n g o p e r a t i o n i n cotton m i l l s . In a d d i t i o n to the e v a l u a t i o n of g r a f t copoly-

Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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mers, the absorption of mechanical energy generated by these m i l l s by c e l l u l o s e molecules and i t s consequence on c e l l u l o s e p r o p e r t i e s were taken i n t o c o n s i d e r a t i o n . Experimental r e s u l t s revealed that a high degree of g r a f t i n g e f f i c i e n c y can be ob tained during c u t t i n g or g r i n d i n g process. Only a very short c u t t i n g time i s required f o r the former process to achieve high g r a f t i n g e f f i c i e n c y , whereas a longer m i l l i n g time i s required f o r the l a t t e r process.


Experimental M a t e r i a l s . P u r i f i e d acetate-grade cotton f i b e r i n a sheet form was used. Methyl methacrylate was used as the monomer a f t e r p u r i f i c a t i o n by a l k a l i n e e x t r a c t i o n and followed by d i s t i l l a t i o n under reduced pressure. Procedures. Cotton c e l l u l o s e was e i t h e r cut i n a Wiley m i l l or m i l l e d i n a Norton b a l l m i l l (Roalox p o r c e l a i n j a r ) of 1.3 g a l l o n c a p a c i t y . The charge always c o n s i s t s of 3500 grams of high carbon chrome s t e e l b a l l s (3/8" diameter) and 25 grams of cotton f i b e r . The m i l l was r o t a t e d at a constant speed of 60 rpm The change i n degree of p o l y m e r i z a t i o n before and a f t e r mechani c a l treatments was determined from l i m i t i n g v i s c o s i t y number ob tained by using a c a p i l l a r y viscometer. The measurements were c a r r i e d out i n a thermostat at 298.00 ± 0.05°K i n c u p r i e t h y l e n e diamine s o l u t i o n and converted to degree of p o l y m e r i z a t i o n using the f o l l o w i n g equation (12): DP = 190

[η]

C r y s t a l l i n i t y of cotton c e l l u l o s e was measured using a den s i t y method by means of a d e n s i t y gradient column (Techne Inc., Model DC-2). Xylene and carbon t e t r a c h l o r i d e were used to make up the s o l u t i o n . Based on the d e n s i t y data, c r y s t a l l i n i t y of c e l l u l o s e can be c a l c u l a t e d from the f o l l o w i n g equation (13): Crystallinity =

V

Va - V ^ ; a

V q

1 Density = -

where Va, Vc and V are s p e c i f i c volume of amorphous p o r t i o n , c r y s t a l l i n e p o r t i o n and unknown sample, r e s p e c t i v e l y . Due to Kast (14), the values of Va and Vc are 0.680 and 0.628, r e s p e c t i v e l y . A c c e s s i b i l i t y of c e l l u l o s e was determined by an i o d i n e ab s o r p t i o n method described by H e s s l e r and Power (15) using the f o l l o w i n g equation: mg of i o d i n e _ (a-b) χ 2.04 g of sample 0.3

χ

2.54

where a i s the volume of 0.02N t h i o s u l f a t e f o r the blank and b i s


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the corresponding volume f o r determination, a r a t i o of the m i l l i grams of i o d i n e absorbed per gram of c e l l u l o s e to 412 (the mg of i o d i n e adsorbed per gram of methocel) gives a value f o r the amor phous f r a c t i o n . The percentage of c r y s t a l l i n i t y i s thus equalled to 100 minus percentage of amorphous p o r t i o n . Copper number, a method of determination of reducing end groups i n c e l l u l o s e , was measured using a standard method de s c r i b e d by Earland and Raven (16). The average number of chain s c i s s i o n per chain u n i t (S) and the degree of degradation (a) were c a l c u l a t e d based on the f o l lowing equations:


ξ

α

=

D =

P

initial _ DP cut

1 DP cut

1 DP i n i t i a l

G r a f t Copolymerization by Norton B a l l M i l l i n g : Cotton f i b e r sheets were t o r n i n t o lengths of 1-2 cm and soaked with methyl methacrylate monomer and water (9:1 r a t i o i n volume) f o r 24 h r s p r i o r to m i l l i n g i n n i t r o g e n atmosphere. G r a f t Copolymerization by Wiley M i l l C u t t i n g : Cotton f i b e r sheets (2 inches i n width) were soaked with methyl methacrylate and water (9:1 r a t i o i n volume) f o r 24 h r s p r i o r to c u t t i n g i n n i t r o g e n atmosphere. The p o l y m e r i z a t i o n was f i n a l l y terminated by the a d d i t i o n of hydroquinone. The copolymerization products were c o l l e c t e d and extracted with benzene to remove the homopolymers. The ungrafted f i b e r under i d e n t i c a l m i l l i n g or c u t t i n g c o n d i t i o n s were a l s o ex t r a c t e d with benzene i n c o n s i d e r a t i o n of the p o s s i b l e l o s s of f i ber bundles during e x t r a c t i o n . Degree of g r a f t i n g and g r a f t i n g e f f i c i e n c y were c a l c u l a t e d as f o l l o w s : Degree of g r a f t i n g

Grafting efficiency

A-B (%) = (-=-) x 100 Β (%) = (^f) G—Β

χ 100

where A i s the weight of c e l l u l o s e a f t e r copolymerization and e x t r a c t i o n , Β i s the weight of o r i g i n a l c e l l u l o s e , and C i s the t o t a l weight of products a f t e r copolymerization. E l e c t r o n Spin Resonance (ESR) Studies: ESR s p e c t r a were mea sured with an X-band ESR spectrometer (Varian E-12, 100 KHz f i e l d modulation). To avoid d i s t o r t i o n of the s p e c t r a by a power satu r a t i o n , the ESR measurements were c a r r i e d out a t a microwave of 3mW. The g-value was measured by comparison with the strong p i t c h provided by V a r i a n A s s o c i a t e s . In a l l cases, ESR s p e c t r a were recorded at 77°K.


20.

Mechanochemically Initiated Reactions in Cotton Cellulose

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263

For ESR measurements, the b a l l m i l l e d or cut f i b e r s were t r a n s f e r r e d to a Dewar f l a s k f i l l e d with l i q u i d n i t r o g e n immed i a t e l y a f t e r mechanical treatments i n order to avoid s i g n i f i c a n t decay of unstable f r e e r a d i c a l s . The f i b e r s were then t r a n s f e r r e d slowly to ESR sample tubes together with l i q u i d n i t r o g e n , which was removed by a vacuum pump afterward. Subsequently, the ESR tube was sealed i n vacuum f o r ESR measurements.


Results and

Discussion

Mechanical E f f e c t on Cotton F i b e r P r o p e r t i e s . I t has been reported e a r l i e r (5, J 7 , 18) that i n the mechanical processing, wood, c e l l u l o s e , and l i g n i n are inescapably absorbing mechanical s t r e s s , i . e . , shear f o r c e s . The consequence of t h i s energy uptake normally leads to the slippage of secondary bonds due to the i n t r a - and i n t e r m o l e c u l a r hydrogen bonds, and the rupture of cov a l e n t bonds, which leads to the shortening of f i b e r length. Mechanoradicals are formed i n the i n t e r i m . The r e d u c t i o n of degree of polymerization of cotton f i b e r was studied a f t e r c u t t i n g and m i l l i n g . Results are shown i n Figures 1 and 2. When cotton sheets were cut through a 6 mm s i e v e of the Wiley m i l l f o r s e v e r a l consecutive c y c l e s , the cotton sheet was disaggregated i n t o f i b r i l l a r bundles as mechanical processing progressed. The f i b e r l o s t i t s DP s i g n i f i c a n t l y at the i n i t i a l s t a t e of c u t t i n g , i . e . , the f i r s t c u t t i n g c y c l e , followed by a slow drop of DP. C a l c u l a t i o n of chain s c i s s i o n showed that 0.19 cleavage takes place per molecular chain at the f i r s t c u t t i n g c y c l e . No s i g n i f i c a n t increment of the reducing end group of c e l l u l o s e , i . e . , aldehyde group, as determined by copper number, was observed (Figure 1). When cotton f i b e r was m i l l e d i n the Norton b a l l m i l l , however, the d r a s t i c change i n DP was observed (•Figure 2). I t was n o t i c e d that a f t e r 50 hrs of m i l l i n g , a blend of two f r a c t i o n s of c e l l u l o s e was formed: one f r a c t i o n of deformed f i b e r r e t a i n e d most of i t s f i b r o u s s t r u c t u r e , and one form of c e l l u l o s e powder has l o s t i t s f i b r o u s s t r u c t u r e completely. For the f i b e r f r a c t i o n , the l o s s of DP was l e s s severe than the powder f r a c t i o n . The l o s s of DP was 17, 31.5, 45.5, 49.5, and 50.5% of t h e i r o r i g i n a l value a f t e r 50, 100, 198, 336 and 400 hours of m i l l i n g , r e s p e c t i v e l y . For the powder f r a c t i o n , the l o s s of DP was s i g n i f i c a n t . Cotton c e l l u l o s e l o s t 50.1, 66.4, 72.3, 77.3 and 77.6% of i t s o r i g i n a l DP value a f t e r the same periods of m i l l i n g , i n d i c a t i n g that mechanochemical chain s c i s s i o n was ext e n s i v e . The r a t e of chain s c i s s i o n and degree of degradation as a f u n c t i o n of m i l l i n g time f o r the f i b e r and powder f r a c t i o n s are shown i n Table I. I t i s c l e a r l y evident that severe degradat i o n of f i b e r took place during m i l l i n g . The increment of reducing end group due to the cleavage of main chains was a l s o observed i n terms of copper number study. Results are a l s o shown i n Figure 2. I t i s obvious that the r a t e of producing reducing end group i n m i l l e d powder was much f a s t e r


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INITIATION OF POLYMERIZATION


3500

10.8

3000h

10

20 Cutting

30 Time

40 (sees)

50

60

Figure 1. Changes in degree of polymerization and copper number of cotton cellulose during cutting with a Wiley mill.


HON

Mechanochemically

Initiated Reactions

in Cotton

Cellulose


20.

Figure 2. Changes in degree of polymerization and copper number of cotton cellulose during milling with a Norton ball mill. Key: ψ , · , milled fiber; V , O , milled powder.


265

Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 1.02

1750 1622 1593

198

336

400

Average number of chain s c i s s i o n s

Degree of degradation

a:

0.45

0.20

S:

Degree o f p o l y m e r i z a t i o n .

3. 16

0.98

2209

100

DP:

3.05

0.83

2673

50

0

3211

Control

4

4

6.12 1.96 2.60

1083 892

1.41 2.60

720

729

3.67

1.18

1475

0. 63

10.06 10.77

3.40 3.46

8.10

0

âdo- ) 0

DP

Powder P o r t i o n

3211

0

adO- )

DP

M i l l i n g Time (hrs)

Fiber Portion

Chain S c i s s i o n and Degree of Degradation o f Cotton F i b e r A f t e r M i l l i n g i n a Norton B a l l M i l l

TABLE I


δ 2

H

H

Ο

δ ο

H

>

H

2

Ν) ON ON


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than i n the m i l l e d f i b e r . I t i s i n agreement with the severe r e duction of DP f o r m i l l e d powder. Moreover, the d i r e c t evidence of the rupture of primary bonds was o r i g i n a t e d from ESR s t u d i e s . When cotton f i b e r was cut i n the Wiley m i l l f o r 60 sec a t 298°K i n n i t r o g e n , a s i n g l e t s i g n a l with a l i n e width of 18 gauss, and a g-value of 2.003, due to the alkoxy r a d i c a l s (5), was observed (Figure 3a). This i n d i c a ted that mechanoradicals were generated i n the cut f i b e r due to a chain s c i s s i o n r e a c t i o n . When the cotton f i b e r was m i l l e d i n the Norton b a l l m i l l , an i l l - d e f i n e d f i v e - l i n e s i g n a l was detected (Figure 3b). This spectrum i s very s i m i l a r to those observed from wood c e l l u l o s e m i l l e d a t 77°K (5), which was a s u p e r p o s i t i o n of a s i n g l e t , a doublet and a t r i p l e t s i g n a l (5). Accordingly, three types o f mechanoradicals were produced i n c e l l u l o s e during ball-milling. The assignment of these r a d i c a l s t r u c t u r e s c o n t r i buting to the s i g n a l s has been discussed elsewhere (5). Comparison between the mechanically cut and the m i l l e d f i b e r s c l e a r l y i n d i c a t e d that b a l l - m i l l e d f i b e r e x h i b i t e d more intense ESR s i g n a l than the cut f i b e r , implying that a higher amount o f mechanor a d i c a l s was generated i n b a l l - m i l l e d f i b e r . I t a l s o suggested that the degree of degradation o f the b a l l - m i l l e d f i b e r was much more c r i t i c a l than that o f the cut f i b e r . Much o f the chemical behavior o f c e l l u l o s e f i b e r can be a t t r i b u t e d to c e l l u l o s e s t r u c t u r e . Since c e l l u l o s e i s a h i g h l y c r y s t a l l i n e polymer, i t can absorb mechanical energy e f f i c i e n t l y f o r mechanical s t r e s s r e a c t i o n (5, 19). The mechanically a c t i vated thermal energy, i n a d d i t i o n to rupture o f main chains, may a l t e r morphology o r m i c r o s t r u c t u r e o f cotton c e l l u l o s e . Accordi n g l y , the c r y s t a l l i n i t y and a c c e s s i b i l i t y o f cotton f i b e r may be influenced. When cotton f i b e r was cut through the Wiley m i l l through a 6 mm s i e v e f o r d i f f e r e n t c y c l e s , i . e . , f o r d i f f e r e n t c u t t i n g times, the change o f c r y s t a l l i n i t y was not observed (Figure 4 ) . When cotton f i b e r was m i l l e d f o r 10 h r s , the change of c r y s t a l l i n i t y was h a r d l y recognized, although the increment o f a c c e s s i b i l i t y was observed (see Figure 5). The n o t i c e a b l e change i n c r y s t a l l i n i t y was observed only a f t e r 20 h r s of m i l l i n g . I t should be noted here that the d e s t r u c t i o n o f c r y s t a l l i n i t y h e a v i l y depended upon the m i l l i n g equipment and operation c o n d i t i o n s . Howsmon and Marchessault (19) destroyed the c r y s t a l l i n i t y o f wood c e l l u l o s e i n a r e l a t i v e l y short b a l l - m i l l i n g p e r i o d . F o r z a i t i et a l . (20) had reported that by using a v i b r a t o r y b a l l m i l l , the cotton c e l l u l o s e was converted almost completely to the amorphous form i n 30 mins. As mentioned e a r l i e r , a f i b e r f r a c t i o n and a powder f r a c t i o n of cotton c e l l u l o s e were formed during m i l l i n g ; the changes i n c r y s t a l l i n i t y f o r these two f r a c t i o n s a r e d i f f e r e n t . For the f i b e r f r a c t i o n , the l o s s of c r y s t a l l i n i t y was c r i t i c a l f o r the i n i t i a l 100 h r s of m i l l i n g , as shown i n Figure 4. The r a t e of change i n c r y s t a l l i n i t y was then l e v e l l e d o f f a f t e r 100 h r s o f



Figure 3. ESR spectra of cotton cellulose cut in a Wiley mill for 60 s at 298 Κ in nitrogen (a) and cotton cellulose milled in a Norton ball mill for 4 h at 298 Κ in nitrogen (b).



Figure 4.

Change in crystallinity of cotton cellulose during cutting and milling. Key: V , cut fiber; ·, milled fiber; O , milled powder.



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milling. A f t e r 72 and 200 hrs of m i l l i n g , the l o s s of c r y s t a l l i n i t y was 29.7 and 40.7% of t h e i r o r i g i n a l c r y s t a l l i n i t y v a l u e s , r e s p e c t i v e l y . A d d i t i o n a l s e v e r a l percentage of c r y s t a l l i n i t y may be l o s t i f the m i l l i n g time i s prolonged. But i t i s b e l i e v e d that i f the m i l l i n g time i s prolonged, the powder f r a c t i o n of the c o t ton would probably be increased, which a c t u a l l y has a lower c r y s t a l l i n i t y than the f i b e r f r a c t i o n . For the powder f r a c t i o n , at the i n i t i a l 22 hrs of m i l l i n g , the c r y s t a l l i n i t y was only 34%, which was 46.9% r e d u c t i o n of c r y s t a l l i n i t y of i t s o r i g i n a l value, i . e . , 64.04%. A f t e r 200 hrs of m i l l i n g , about 67% of the c r y s t a l l i n i t y was l o s t . The r e d u c t i o n of c r y s t a l l i n i t y was continuously observed i f the m i l l i n g time was prolonged. Since the consequences of mechanical energy uptake were d i s aggregation of f i b e r bundles, shortening of f i b e r l e n g t h and r e duction of DP, i t i s p l a u s i b l e to consider that new surface areas were a l s o created. A c c o r d i n g l y , the change of a c c e s s i b i l i t y of mechanically t r e a t e d f i b e r s may be observed. The a c c e s s i b i l i t y of mechanically t r e a t e d f i b e r s was evaluated based on the i o d i n e abs o r p t i o n techniques. Results are shown i n F i g u r e 5. Although the c r y s t a l l i n i t y of cotton f i b e r was not i n f l u e n c e d i n the c u t t i n g process, a s l i g h t increase i n a c c e s s i b i l i t y was observed f o r the cut f i b e r . As shown i n Figure 5, the a c c e s s i b i l i t y increased from 10.06% to 17.51% a f t e r 60 sec of c u t t i n g . The i n crease i n a c c e s s i b i l i t y was enhanced when cotton f i b e r was b a l l m i l l e d . During the f i r s t 72 hrs of m i l l i n g , the a c c e s s i b i l i t y of f i b e r f r a c t i o n increased to 32.12% (from 10.06%), whereas f o r the powder f r a c t i o n , i t even increased to 50%. The r a t e of increment of a c c e s s i b i l i t y was slowed down f o r the f i b e r f r a c t i o n even i f the m i l l i n g time was prolonged, but f o r the powder f r a c t i o n , the a c c e s s i b i l i t y was continuously increased. A f t e r 400 hrs of m i l l ing, 72% of a c c e s s i b i l i t y was achieved. Based on the experimental data of c r y s t a l l i n i t y and a c c e s s i b i l i t y , i t i s revealed that the mechanical shear f o r c e s involved i n the c u t t i n g process were able to open up the u n a c c e s s i b l e r e gions i n c e l l u l o s e by c r e a t i n g new surfaces without damaging the c r y s t a l l i n e s t r u c t u r e . I t i s l i k e l y that the surfaces of c r y s t a l l i t e s are made more a c c e s s i b l e during c u t t i n g . The mechanical shear f o r c e s involved i n the m i l l i n g process are able to i n c r e a s e a c c e s s i b i l i t y p e r s p i c u o u s l y i n accompanying with d e s t r u c t i o n of c r y s t a l l i n e regions. The increment of a c c e s s i b i l i t y u s u a l l y set forward the p e n e t r a t i o n of monomer i n t o channels and pores of f i ber to achieve high degree of r e a c t i o n . However, i t should be borne i n mind that the l o s s of c r y s t a l l i n i t y a l s o i s an i n d i c a t i o n of the l o s s of p h y s i c a l p r o p e r t i e s of cotton f i b e r . Hence, the proper c o n t r o l of the mechanical process to achieve high a c c e s s i b i l i t y with l e s s d e s t r u c t i o n of c r y s t a l l i n e s t r u c t u r e s of f i b e r i s b e n e f i c i a l to accomplish the purpose f o r which a chemical m o d i f i c a t i o n i s designed.


20.

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200

300

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ο

I

ι

ι

ι

ι

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20

30

40

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Cutting Figure 5.

Time

1

(sees)

Change in accessibility of cotton cellulose during cutting and milling. Key: O, powder; ·, fiber.



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Mechanochemically I n i t i a t e d Graft Copolymerization. According to the ESR study, i t i s c l e a r that mechanoradicals were generated i n c e l l u l o s e e i t h e r by means of mechanical c u t t i n g or b a l l milling. These mechanoradicals may be u t i l i z e d as r e a c t i o n s i t e s f o r the i n i t i a t i o n of v i n y l p o l y m e r i z a t i o n which would r e s u l t i n g r a f t copolymerization of c e l l u l o s e . Based on t h i s p r i n c i p l e , the a b i l i t y of c e l l u l o s e mechanoradicals to i n i t i a t e c o p o l y m e r i z a t i o n was pursued. When cotton f i b e r was b a l l - m i l l e d f o r 4 hrs at 298°K and t r a n s f e r r e d immediately to a sample tube f o r ESR measurement at 77°K, an i l l - d e f i n e d f i v e - l i n e spectrum with a g-value of 2.003 was detected (Figure 6a). When cotton f i b e r was m i l l e d i n the presence of MMA, only a s i n g l e t s i g n a l with a l i n e width of 18 gauss was observed (Figure 6b). This implied that other f r e e r a d i c a l s which generated s i g n a l s other than the s i n g l e t component were i n t e r a c t e d with MMA during m i l l i n g . No MMA-propagating r a d i c a l s were observed. Moreover, immediately f o l l o w i n g the m i l l i n g , MMA was introduced i n t o the mechanically t r e a t e d c e l l u l o s e f o r 2 mins at 298°K and recorded i t s ESR s i g n a l at 77°K, the f i v e - l i n e ESR s i g n a l of c e l l u l o s i c mechanoradicals was converted to an asymmetrical m u l t i p l e t spectrum (Figure 7b), and these s i g n a l s were f u r t h e r i n t e n s i f i e d when the sample was warmed at 298°K f o r 10 mins, the ESR spectrum observed at 77°K was a n i n e - l i n e spectrum, as shown i n F i g u r e 7d. The n i n e - l i n e spectrum o r i g i n a t e d from the c h a r a c t e r i s t i c propagating r a d i c a l s of MMA f o r p o l y m e r i z a t i o n (2_1) . S i m i l a r r e s u l t s were observed when wood c e l l u l o s e was m i l l e d with g l a s s beads at 77°K (5). MMApropagating r a d i c a l s were a l s o detected from cut f i b e r under i d e n t i c a l warm-up treatment, with the exception that the s i g n a l i n t e n s i t y was very weak. This could be due to the low f r e e r a d i c a l c o n c e n t r a t i o n being generated by c u t t i n g . The propagating r a d i c a l s of MMA were not detected when monomer was added to the untreated c e l l u l o s e f i b e r s i n i d e n t i c a l experiments. This c l e a r l y i n d i c a t e d that propagating r a d i c a l s of MMA were created by contact of MMA with c e l l u l o s i c mechanoradicals. Subsequently, i t i s evident that c e l l u l o s i c mechanoradicals are capable of i n i t i a t i n g v i n y l p o l y m e r i z a t i o n . In l i g h t of these f i n d i n g s , g r a f t copolymerization of MMA onto c e l l u l o s e f i b e r was c a r r i e d out at ambient temperature by impregnating monomer i n t o f i b e r p r i o r to c u t t i n g or m i l l i n g . R e s u l t s are shown i n F i g u r e s 8 and 9. For the v i n y l g r a f t copolymerization i n i t i a t e d by the c u t t i n g process, the degree of g r a f t i n g was approached at 50% a f t e r 60 sec of c u t t i n g . The g r a f t i n g e f f i c i e n c y reached i t s maximum of 89% between 20 and 30 sec of c u t t i n g . The i n i t i a t i n g r e a c t i o n f o r the b a l l m i l l system took place s t e a d i l y . A longer i n d u c t i o n p e r i o d seemed to be needed to reach the maximum degree of g r a f t i n g of 196% f o r 5 hrs of m i l l i n g , and the g r a f t i n g e f f i c i e n c y reached i t s maximum a f t e r 2-3 hrs of m i l l i n g . When the g r a f t i n g r e a c t i o n time was prolonged, the degree of g r a f t i n g as w e l l as g r a f t i n g


Mechanochemically

Initiated Reactions in Cotton

Cellulose


HON

Figure 6. ESR spectra of cotton cellulose milled in absence (a) and presence (b) of methyl methacrylate for 4 h at 298 K. ESR spectra were recorded at 77 K.


INITIATION OF

POLYMERIZATION


274

Figure 7. Changes in ESR spectra of cotton cellulose mechanoradicals. Key: a, initial spectrum observed at 77 Κ immediately after milling for 4 h at 298 K; b, cotton cellulose after milling was contacted with methyl methacrylate and warmed at 298 Κ for 2 min; c, 5 min; d, 10 min; recorded at 77 K.


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Mechanochemically



20.


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276

Milling

Time

(hrs)

Figure 9. Graft copolymerization of methyl methacrylate onto cotton cellulose induced by mechanical milling with a Norton ball mill from 1 to 4 h.


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Mechanochemically



20.

Figure 10. Graft copolymerization of methyl methacrylate onto cotton cellulose induced by mechanical milling with a Norton ball mill from 1 to 5 h.


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e f f i c i e n c y reduced sharply a f t e r 10 hrs of m i l l i n g (Figure 10). I t i s p o s s i b l e that i f the m i l l i n g was continued, the g r a f t e d PMMA chains might w e l l have a l s o s u f f e r e d cleavage of covalent bonds by mechanical s t r e s s which l e d to low degree of g r a f t i n g . In comparison, a higher degree of g r a f t i n g was achieved f o r the m i l l i n g system than f o r the c u t t i n g system. I t i s p l a u s i b l e that higher mechanical energy was s u p p l i e d f o r the former system, i n which c e l l u l o s e absorbed more energy to produce mechanoradic a l s f o r g r a f t i n g r e a c t i o n . The increment of a c c e s s i b i l i t y and destroying of c r y s t a l l i n e s t r u c t u r e of cotton f i b e r during the m i l l i n g process could a l s o be the f a c t o r s c o n t r i b u t i n g to the high degree of g r a f t i n g . On the other hand, the change i n p h y s i c a l and chemical p r o p e r t i e s of m i l l e d c o t t o n f i b e r were more c r i t i c a l than those of the cut cotton f i b e r . Conclusions On the b a s i s of the experimental f i n d i n g s , the f o l l o w i n g conclusions may be drawn: 1. Cotton c e l l u l o s e i s s u s c e p t i b l e to degradation by mechanical shear f o r c e s supplied by e i t h e r a Wiley m i l l or a Norton b a l l m i l l . Mechanoradicals are produced i n the i n t e r i m . 2. The consequences of the energy absorption by cotton c e l l u l o s e molecules are r e d u c t i o n of degree of p o l y m e r i z a t i o n and i n crement of a c c e s s i b i l i t y and reducing end group. Cotton f i ber s u f f e r e d l e s s degradation and i t s c r y s t a l l i n e s t r u c t u r e was not i n f l u e n c e d when i t was t r e a t e d with a Wiley m i l l . When cotton f i b e r was t r e a t e d with a Norton b a l l m i l l , a deformed f i b e r f r a c t i o n and a powder c e l l u l o s e f r a c t i o n were formed. D r a s t i c changes i n c r y s t a l l i n i t y and a c c e s s i b i l i t y were observed from both f r a c t i o n s . A c c o r d i n g l y , i t i s important to c o n t r o l the mechanical processing p r o p e r l y i n order to avoid l o s i n g p h y s i c a l and chemical p r o p e r t i e s of c o t t o n f i b e r s . 3. Mechanoradicals are capable of i n i t i a t i n g g r a f t copolymerizat i o n . A high degree of g r a f t i n g e f f i c i e n c y and a low degree of homopolymer formation were obtained from the f i b e r s t r e a t e d with the Wiley m i l l and the Norton b a l l m i l l , but a higher degree of g r a f t i n g was obtained from the b a l l - m i l l e d f i b e r . These a u s p i c i o u s f i n d i n g s w i l l c e r t a i n l y shed l i g h t on the development of g r a f t copolymerization techniques w i t h high g r a f t ing e f f i c i e n c y . And the u t i l i z a t i o n of e x i s t i n g mechanical processing energy f o r g r a f t copolymerization r e a c t i o n i s c e r t a i n l y an a t t r a c t i v e mode of o p e r a t i o n f o r commercial a p p l i c a t i o n s .

Literature Cited 1.

Hon, D. N.-S. "Yellowing of Modern Papers" in "Preservation of Paper and Textiles of Historic and Artistic Value II" (Williams, J. C., ed.), Advances in Chemistry Series, 1981, 193, 119.


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2.

Stannett, V. "Some Challenges in Grafting to Cellulose and Cellulose Derivatives" in "Graft Copolymerization of Ligno cellulosic Fibers" (Hon, D. N.-S., ed.), ACS Symposium Series, in press. Hebeish, A., and Guthrie, J . T. "The Chemistry and Technology of Cellulosic Copolymers", Springer-Verlag, Berlin-Heidelberg -New York, 1981, p 351. Hon, D. N.-S. "Graft Copolymerization of Lignocellulosic F i bers", ACS Symposium Series, in press. Hon, D. N.-S. J . Appl. Polym. Sci., 1979, 23, 1487. Casale, A., and Porter, R. S. "Polymer Stress Reactions", Academic Press, New York, Volume 1, 1978, Chapter V. Ceresa, R. J . J . Polym. Sci., 1961, 53, 9. Whistler, R. L., and Goatley, J . L. J . Polym. Sci., 1962, 62, 5123. Deter, W., and Huang, D. C. Faserforsch. Textil. Tech., 1963, 14, 58. Hon, D. N.-S. J . Polym. Sci. Polym. Chem. Ed., 1980, 18, 1957. Hon, D. N.-S. "Modification of Lignocellulosic Fibers." Pa per presented at the Second Chemical Congress of the North Americal Continent, San Francisco, CA., Aug., 24-29, 1980, Abstracts of Papers, Part 1, cell-39. Browning, B. L. "Methods of Wood Chenistry", Academic Press, New York, 1967, Vol. 2, p 519-529. Migita, Ν., Yonezawa, Η., and Kondo, T. "Wood Chemistry", Kyoritsu, Tokyo, 1968, Vol. 1, p 102. Kast, Κ. Z. Elektrochemie, 1953, 57, 525. Kessler, L. Ε . , and Power, R. E. Textile Res. J., 1954, 18 (9), p 822-827. Earland, C., and Raven, D. J . "Experiments in Textile and Fiber Chemistry", Butterworth, London, 1971, p 134. Hon, D. N.-S., and Glasser, W. G. TAPPI, 1979, 62, 107. Hon, D. N.-S., and Srinivasan, K. S. V. J . Appl. Polym. Sci., in press. Howsmon, J . Α., and Marchessault, R. H. J . Appl. Polym. Sci., 1959, 1, 313. Forziati, F. Η., Stone, W. Κ., Rowen, J . W., and Appel, W. D. J. Res. of Natl. Bur. of Stand., 1950, 45, 2116. Campbell, D. Macromol. Rev., 1970, 91.

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5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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Mechanochemically Initiated Reactions in Cotton Cellulose

November 5, 1982


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Mechanochemically Initiated Copolymerization Reactions in Cotton

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