Graft Copolymerization of Lignocellulosic Fibers - American Chemical

13 Cross-linking in Saponified Starch-gPolyacrylonitrile

Downloaded by UCSF LIB CKM RSCS MGMT on November 30, 2014 | http://pubs.acs.org Publication Date: June 18, 1982 | doi: 10.1021/bk-1982-0187.ch013

GEORGE F. FANTA, ROBERT C. BURR, and WILLIAM M . DOANE Northern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Peoria, IL 61604

Evidence, based on solubility data i n DMSO, was obtained for crosslinking during the graft polymerization of a c r y l o n i t r i l e onto starch, and this crosslinking apparently occurs by way of chain combination of growing PAN macroradicals. Crosslinking also occurs between starch and PAN during alkaline saponification. Soluble (and thus uncrosslinked) starch-g-PAN polymers were rendered partially insoluble by alkaline saponification, as was a synthetic mixture of starch and PAN. Although PAN w i l l crosslink with i t s e l f if saponifications are carried out i n ethanol-water systems containing predominantly ethanol, PAN:PAN crosslinking does not take place during aqueous saponifications. Since the individual grafted granules of starch are crosslinked, these granules maintain their integrity after alkaline saponification and exist i n the saponificate as highly swollen gel particles. When the saponificate i s dried, these gel particles coalesce into either films or macroparticles which w i l l not redisperse back into gel when placed i n water. These properties are analogous to those of crosslinked rubber latexes and may be similarly explained by assuming interdiffusion of polymer chain ends on the surfaces of individual gel particles, followed by hydrogen bonding between these polymer chains. If dry saponified starch-gPAN i s heated, i t s a b i l i t y to absorb water i s reduced; however, this absorbency loss i s not related to loss of hydrogen-bonded water from the graft copolymer.

A c r y l o n i t r i l e g r a f t polymerizes r e a d i l y w i t h e i t h e r granular o r pasted s t a r c h t o y i e l d s t a r c h - g - p o l y a c r y l o n i t r i l e (PAN), and r e a c t i o n s are g e n e r a l l y c a r r i e d out i n water w i t h e e r i e ammonium n i t r a t e as the i n i t i a t i n g system ( 1 ) . Reaction of starch-j*-PAN w i t h a l k a l i a t e l e v a t e d temperatures converts the n i t r i l e s u b s t i t u e n t s o f PAN t o a mixture o f carboxamide

©

0097-6156/82/0187-0195$6.25/0 1982 American Chemical Society

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


196

GRAFT COPOLYMERIZATION OF LIGNOCELLULOSIC FIBERS

and a l k a l i metal carboxylate and y i e l d s a f a m i l y of f i n a l products that are u s e f u l as t h i c k e n e r s (2) and as water absorbents (3, 4 ) . Although s a p o n i f i e d (hydrolyzed) s t a r c h - g PAN (HSPAN) i s h y d r o p h i l i c and e x h i b i t s extensive s w e l l i n g i n aqueous systems, T a y l o r and Bagley (5) have shown that i t e x i s t s l a r g e l y as i n s o l u b l e g e l p a r t i c l e s . We l a t e r showed by scanning e l e c t r o n microscopy (6) that these p a r t i c l e s maintain the outward appearance of the o r i g i n a l PAN-grafted s t a r c h granules. Since both s t a r c h and s a p o n i f i e d PAN are s o l u b l e i n aqueous a l k a l i , the f a c t that a copolymer o f the two components i s not only i n s o l u b l e but still bears some p h y s i c a l resemblance t o the o r i g i n a l starch-g-PAN p r e c u r s o r , as i t s w e l l s and imbibes water, i s i n d i c a t i v e of c r o s s l i n k i n g i n the f i n a l s a p o n i f i e d product. The purpose of t h i s research was t o gain some i n s i g h t as t o how t h i s c r o s s l i n k i n g might take p l a c e , and a l s o t o e x p l a i n some of the p r o p e r t i e s of HSPAN f i l m s and p a r t i c l e s . Although g e l a t i n i z e d s t a r c h gives t h i c k e n i n g agents and water absorbents w i t h s u p e r i o r p r o p e r t i e s , we used g r a n u l a r , unpasted s t a r c h i n n e a r l y all of t h i s study because f i n a l products are e a s i e r to handle. Any conclusions based on r e s u l t s obtained w i t h granular s t a r c h would c e r t a i n l y be a p p l i c a b l e t o g e l a t i n i z e d s t a r c h . R e s u l t s and D i s c u s s i o n C r o s s l i n k i n g During G r a f t P o l y m e r i z a t i o n . Perhaps the most obvious r e a c t i o n path l e a d i n g t o c r o s s l i n k i n g i n HSPAN i s the combination of two growing PAN macroradicals during the g r a f t p o l y m e r i z a t i o n step. Combination of a PAN and a s t a r c h macroradical would g i v e the same f i n a l r e s u l t s . Since the h i g h degree of s w e l l i n g of HSPAN i n water i n d i c a t e s a low c r o s s l i n k d e n s i t y , only a few such chain combinations would be s u f f i c i e n t t o produce the observed p h y s i c a l p r o p e r t i e s . C e r i c i n i t i a t e d aqueous p o l y m e r i z a t i o n s of a c r y l o n i t r i l e i n the presence of p o l y o l s have been s t u d i e d , and evidence has been presented f o r t e r m i n a t i o n by chain combination a t low e e r i e i o n concentrations (7 8 ) . To g a i n some i n s i g h t i n t o whether c r o s s l i n k i n g by f r e e r a d i c a l combination indeed takes place during g r a f t polymeriz a t i o n r e a c t i o n s w i t h s t a r c h , a number of these p o l y m e r i z a t i o n s were r u n under c a r e f u l l y c o n t r o l l e d c o n d i t i o n s , and the s o l u b i l i t i e s i n d i m e t h y l s u l f o x i d e (DMSO) of the r e s u l t i n g s t a r c h g r a f t copolymers were determined a t 75°C. Since s t a r c h and PAN a r e i n d i v i d u a l l y s o l u b l e under these c o n d i t i o n s , we assumed t h a t a t o t a l l y u n c r o s s l i n k e d g r a f t copolymer of the two components would be s o l u b l e a l s o , and that the s o l u b i l i t y of a p a r t i c u l a r polymer would thus decrease as i t s c r o s s l i n k w i t h e e r i e ammonium d e n s i t y increased. n i t r a t e i n i t i a t i o n (Ce -.starch anhydroglucose u n i t [AGU] molar r a t i o of 1:100) and t h e i r r e s p e c t i v e s o l u b i l i t i e s i n DMSO are shown i n Table I . When corn s t a r c h was allowed to r e a c t w i t h 9

k


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

b

17,000

83,000

PAN

grafted

Anhydroglucose u n i t .

C

100

473

AGU /graft

frequency,

28

34

8

36

100

Solubility, %

Grafting

of

Μ V

Ceric I n i t i a t i o n

a t 75°C.

C a l c u l a t e d from i n t r i n s i c v i s c o s i t y i n dimethylformamide.

Determined by weight l o s s on a c i d h y d r o l y s i s .

--

10

+4 Ce -Treated s t a r c h (C.H SH) ο 1j

5

13

50

6

52

--

Starch-g-PAN ( C H S H )

-Treated s t a r c h

4

+4

--

%

Starch-g-PAN

Ce

Starch

Polymer

3

2

1

No.

Add-on,

Polymer S o l u b i l i t i e s i n D i m e t h y l s u l f o x i d e

TABLE I



198


e e r i e ammonium n i t r a t e i n the absence of a c r y l o n i t r i l e (No. 2 ) , i t s s o l u b i l i t y was reduced from 100% t o 36%: a r e s u l t which p a r a l l e l s an e a r l i e r study w i t h wheat s t a r c h (9). Although s t a r c h can thus be rendered p a r t i a l l y i n s o l u b l e by r e a c t i o n w i t h e e r i e i o n , we w i l l l a t e r show t h a t by a l l o w i n g c e r i c t r e a t e d s t a r c h t o r e a c t w i t h a l k a l i under the c o n d i t i o n s of s a p o n i f i c a t i o n , we o b t a i n a product that i s h i g h l y s o l u b l e i n wa|er. Therefore, any c r o s s l i n k s introduced by r e a c t i o n w i t h Ce apparently have no l a r g e bearing on the f i n a l p r o p e r t i e s of HSPAN. When s t a r c h was g r a f t polymerized w i t h a c r y l o n i t r i l e t o g i v e a product c o n t a i n i n g 52% g r a f t e d PAN, the s o l u b i l i t y of the r e s u l t i n g starch-£-PAN i n DMSO was only 8% (No. 3, Table I ) , a r e s u l t which immediately suggests c r o s s l i n k i n g during the p o l y m e r i z a t i o n r e a c t i o n . I f t h i s c r o s s l i n k i n g i s indeed due t o t e r m i n a t i o n o f growing PAN macroradicals by chain combinat i o n , g r a f t p o l y m e r i z a t i o n i n the presence of a chain t r a n s f e r agent should not only lower the molecular weight o f g r a f t e d PAN but should i n c r e a s e s o l u b i l i t y i n DMSO as w e l l . E x a c t l y t h i s e f f e c t was observed when the g r a f t p o l y m e r i z a t i o n was repeated i n the presence o f 1-hexanethiol (No. 4 ) . G r a f t molecular weight was decreased by about a f a c t o r of 5, w h i l e s o l u b i l i t y i n DMSO i n c r e a s e d approximately f o u r f o l d . When 1-hexanethiol was i n c l u d e d iiji the r e a c t i o n mixture when s t a r c h was allowed t o r e a c t w i t h Ce i n the absence of a c r y l o n i t r i l e , we obtained a s t a r c h product t h a t was only 28% s o l u b l e i n DMSO (No. 5 ) . Since the p a r t i a l i n s o l u b i l i t y of the s t a r c h - c e r i c ammonium n i t r a t e r e a c t i o n product complicates the i n t e r p r e t a t i o n of s o l u b i l i t y d a t a , we r a n a second s e r i e s of g r a f t p o l y m e r i z a t i o n s u s i n g cobalt-60 as an i n i t i a t o r i n an attempt to remove t h i s v a r i a b l e (Table I I ) . I f combination of PAN macroradicals i s o c c u r i n g during g r a f t p o l y m e r i z a t i o n , i t should occur d u r i n g cobalt-60 i n i t i a t e d p o l y m e r i z a t i o n s as w e l l as i n those 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 the f i r s t four r e a c t i o n s of Table I I , s t a r c h was i r r a d i a t e d as a water s l u r r y under g r a f t p o l y m e r i z a t i o n c o n d i t i o n s , but i n the absence o f a c r y l o n i t r i l e , t o determine the i n f l u e n c e of d i f f e r e n t doses o f i r r a d i a t i o n on s t a r c h s o l u b i l i t y . An e f f e c t of t o t a l dose on s t a r c h s o l u b i l i t y was indeed observed, and t h i s can perhaps be a s c r i b e d t o competing r a d i a t i o n - i n d u c e d degradation and c r o s s l i n k i n g (10, 11). One path l e a d i n g t o c r o s s l i n k i n g might be a c e t a l and hemiacetal formation through carbonyl groups formed by i r r a d i a t i o n (12). Since maximum s o l u b i l i t y (95%) was observed a t O.1 Mrad, t h i s t o t a l dose was used i n subsequent g r a f t p o l y m e r i z a t i o n s w i t h acrylonitrile. 4



Starch-g-PAN (HOCH CH SH)

9

15

15

Determined by weight l o s s on a c i d h y d r o l y s i s .

AN = a c r y l o n i t r i l e .

O.1

Starch-£-PAN ((^H^SH)

8 2

Starch-g-PAN

7

2

O.1

Starch-g-PAN

6

5

15

O.1

Starch-£-PAN

5

O.1

O.2

Starch

4

10

O.1

Starch

3

0

480 870

29,000

1,800

1,800

1,200

d

AGU /graft

frequency,

Grafting

30,000

180,000

110,000

30,000

---

PAN

grafted

V

M of

53

45

26

29

34

80

95

62

100

%

Solubility,

Cobalt-60 I n i t i a t i o n

C a l c u l a t e d from i n t r i n s i c v i s c o s i t y i n ^ dimethylformamide. Anhydroglucose u n i t .

17

28

38

27

13

--

—

---

b

---

on,

%

a

g

AN,

O.1

O.05

Starch

2

0

Mrad

Starch

Polymer

1

No.

Dose,

Add-

Polymer S o l u b i l i t i e s i n D i m e t h y l s u l f o x i d e a t 75°C.

TABLE I I



200


In numbers 5-7 of Table I I , starch-g-PAN copolymers c o n t a i n i n g d i f f e r e n t amounts o f g r a f t e d PAN were prepared by the mutual i r r a d i a t i o n of s t a r c h and v a r y i n g amounts of a c r y l o n i t r i l e i n water. Although v a l u e s f o r % add-on i n these three r e a c t i o n s ranged from 13-38%, and g r a f t molecular weights v a r i e d from 30,000 t o 180,000, s o l u b i l i t i e s i n DMSO were not g r e a t l y d i f f e r e n t (26-34%). The roughly t h r e e f o l d r e d u c t i o n i n s o l u b i l i t y observed by the i n c o r p o r a t i o n o f as l i t t l e as 13% PAN i n t o the s t a r c h m a t r i x i s a good i n d i c a t i o n that g r a f t p o l y m e r i z a t i o n had l e d t o c r o s s l i n k i n g . Although complete DMSO s o l u b i l i t y was not achieved when g r a f t p o l y m e r i z a t i o n s were run i n the presence o f 1-hexanethiol (No. 8) o r 2mercaptoethanol (No. 9 ) , both of these chain t r a n s f e r agents enhanced s o l u b i l i t y . Numbers 5 and 9 o f Table I I make an e s p e c i a l l y good comparison t o show the e f f e c t of a chain t r a n s f e r agent on s o l u b i l i t y , s i n c e both PAN content and g r a f t M i n the two polymers are s i m i l a r . v

C r o s s l i n k i n g During A l k a l i n e S a p o n i f i c a t i o n . Although granules of starch-g-PAN are a p p a r e n t l y c r o s s l i n k e d , the PAN moiety i s not i t s e l f a network polymer, s i n c e removal of the s t a r c h component gives a m a t e r i a l that i s r e a d i l y s o l u b l e (e.g., i n DMF, f o r M d e t e r m i n a t i o n ) . We, t h e r e f o r e , needed t o know i n i t i a l l y whether the s a p o n i f i e d PAN moiety was l i k e w i s e s o l u b l e or whether aqueous a l k a l i n e s a p o n i f i c a t i o n causes the PAN p o r t i o n o f starch-g-PAN t o c r o s s l i n k w i t h i t s e l f . A c e r i c - i n i t i a t e d starch-g-PAN c o n t a i n i n g 50% PAN (M :64,000) was thus prepared and s a p o n i f i e d i n aqueous a l k a l i , and the r e s u l t i n g HSPAN was i s o l a t e d by methanol p r e c i p i t a t i o n and d r i e d . Enzymatic h y d r o l y s i s o f t h i s product removed the s t a r c h component and a f f o r d e d s a p o n i f i e d PAN w i t h M = 44,000, as determined by membrane osmometry. Since s a p o n i f i c a t i o n of the PAN moiety g i v e s s o l u b l e polymer w i t h a molecular weight not g r e a t l y d i f f e r e n t from i t s PAN p r e c u r s o r , PAN:PAN c r o s s l i n k i n g i s a p p a r e n t l y not o c c u r r i n g t o a l a r g e extent d u r i n g a l k a l i n e s a p o n i f i c a t i o n i n water. We next examined the e f f e c t o f running the s a p o n f i c a t i o n s i n aqueous e t h a n o l i n s t e a d o f i n water, s i n c e a l c o h o l c o n t a i n i n g s o l v e n t systems are a l s o being used f o r some s a p o n i f i e d starch-g-PAN p r e p a r a t i o n s (13). Contrary t o aqueous s a p o n f i c a t i o n s , we found evidence that c r o s s l i n k i n g o f the PAN moiety does take p l a c e i n t h i s s o l v e n t system, although i t i s h i g h l y dependent on the ethanol:water r a t i o . I n i t i a l l y , we examined the s a p o n i f i c a t i o n of PAN homopolymer i n solvent combinations ranging from 95% e t h a n o l t o a 1:1 mixture o f 95% e t h a n o l and water (Table I I I ) . C r o s s l i n k i n g i n the d i f f e r e n t s o l v e n t s was compared by determining percent s o l u b l e polymer i n the undried s a p o n i f i c a t i o n r e a c t i o n mixtures a f t e r exhaustive d i a l y s i s a g a i n s t d i s t i l l e d water. B r o o k f i e l d v i s c o s i t i e s of these water d i s p e r s i o n s a l s o served as an


13.

FANTA E T AL.

Saponified Starch-g-Polyacrylonitrile

201

Table I I I . I n f l u e n c e of Ethanol:Water R a t i o on A l k a l i n e Saponification a


95% EtOH (ml) 8.9 8.0 7.56 4.44

S a p o n i f i e d PANb S a p o n i f i e d starch-g-PAN^ dispersion dispersion H 0 (ml) S o l u b l e s Viscosity S o l u b l e s Absorbency of (%) cpc (% s o l i d s ) (%) insoluble g e l 2

0 O.88 1.32 4.44

64 84 71 100

4,600 (O.25) 300 (O.27) 200 (O.26) 19 (O.24)

31 27

150 140

28

310

a

2.00 g of 50% NaOH added to each s o l v e n t system. O.50 g p o l y a c r y l o n i t r i l (PAN) or 1.00 g starch-g-PAN added, and d i s p e r s i o n heated under r e f l u x f o r 3 h. S a p o n i f i e d d i s p e r s i o n s were d i a l y z e d a g a i n s t d i s t i l l e d water. hi of PAN: 49,000. B r o o k f i e l d v i s c o s i t y a t 30 rpm. No. 3, Table I . E x p r e s s e d as grams of water-swollen g e l per gram of dry polymer. c

d

e

i n d i c a t o r f o r c r o s s l i n k i n g ; s i n c e the v i s c o s i t y of a c r o s s l i n k e d , p a r t i a l l y s o l u b l e , swollen g e l network would be higher than t h e corresponding u n c r o s s l i n k e d polymer s o l u t i o n . Although c r o s s l i n k i n g c l e a r l y occurs i n 95% e t h a n o l , as i n d i c a t e d by a water s o l u b i l i t y of only 64% f o r s a p o n i f i e d PAN and a B r o o k f i e l d v i s c o s i t y of 4600 cp f o r the aqueous r e a c t i o n mixture a tO.25%s o l i d s , s o l u b i l i t i e s p r o g r e s s i v e l y i n c r e a s e and B r o o k f i e l d v i s c o s i t i e s decrease w i t h i n c r e a s i n g water contents of the s o l v e n t systems. When s a p o n i f i e d i n a 1:1 s o l u t i o n of 95% e t h a n o l and water, PAN apparently does not c r o s s l i n k t o a s i g n i f i c a n t e x t e n t , s i n c e the r e s u l t i n g s a p o n i f i e d PAN i s 100% water s o l u b l e , and the B r o o k f i e l d v i s c o s i t y of the s o l u t i o n i s only 19 cp. The s a p o n i f i c a t i o n of a e e r i e - i n i t i a t e d starch-g-PAN was l i k w i s e examined a t the d i f f e r e n t ethanol-water r a t i o s , and these r e s u l t s a r e a l s o shown i n Table I I I . Once a g a i n , s o l u b i l i t i e s were determined f o r undried water d i s p e r s i o n s a f t e r exhaustive d i a l y s i s . B r o o k f i e l d v i s c o s i t y was not used as a measure o f c r o s s l i n k i n g f o r these HSPAN p r e p a r a t i o n s , because i n d i v i d u a l g e l p a r t i c l e s could not absorb all of the a v a i l a b l e water a tO.22%s o l i d s and thus could not e x i s t i n


e


202


the c l o s e l y packed s t a t e which i s necessary f o r the attainment of meaningful v i s c o s i t i e s Ç5). To more r e l i a b l y estimate t h e extent o f c r o s s l i n k i n g , the i n s o l u b l e g e l was separated from excess s o l u t i o n by f i l t r a t i o n and f r e e z e d r i e d . The weight of f r e e z e - d r i e d s o l i d was determined, and water absorbency was then expressed as grams o f swollen g e l per gram o f dry polymer. Low c r o s s l i n k d e n s i t i e s would thus g i v e h i g h l y water-swollen g e l s and high absorbency numbers. Although the percent s o l u b l e s d i d not vary g r e a t l y w i t h the ethanol content o f the s a p o n i f i c a t i o n m i x t u r e , 1:1 ethanol (95%)-water produced an i n s o l u b l e HSPAN g e l f r a c t i o n w i t h about twice the absorbency o f those prepared at higher ethanol contents. We can t h e r e f o r e assume that some PAN:PAN c r o s s l i n k i n g l i k e w i s e has taken p l a c e a t h i g h ethanol c o n c e n t r a t i o n s , as was observed f o r PAN homopolymer. Although the formation o f PAN:PAN c r o s s l i n k s during a l k a l i n e s a p o n i f i c a t i o n of starch-g-PAN takes p l a c e t o a s i g n i f i c a n t extent only when a l c o h o l i s the major component of the s a p o n i f i c a t i o n s o l v e n t system, some c r o s s l i n k i n g between s t a r c h and PAN occurs even when s a p o n i f i c a t i o n s are c a r r i e d out i n water o r i n 1:1 ethanol (95%)-water (Table I V ) . Evidence f o r t h i s starch:PAN c r o s s l i n k i n g was obtained by s a p o n i f y i n g two t o t a l l y DMSO-soluble (and thus not c r o s s l i n k e d ) starch-g-PAN polymers and then showing t h a t : (1) The r e s u l t i n g s a p o n i f i e d polymers had only a p a r t i a l s o l u b i l i t y i n water, and (2) they gave v i s c o u s aqueous d i s p e r s i o n s o f g e l a t low concentrations (about O.25% s o l i d s ) . The f i r s t s o l u b l e starch-£-PAN (No. 2, Table IV) was prepared by d i s p e r s i n g s t a r c h g r a f t copolymer No. 1 i n hot DMSO and then s u b j e c t i n g the d i s p e r s i o n t o treatment w i t h u l t r a s o u n d to m e c h a n i c a l l y degrade the polymer. R e s i d u a l g e l was removed from the DMSO s o l u t i o n by f i l t r a t i o n , and the s o l u b l e polymer was then i s o l a t e d by a l c o h o l p r e c i p i t a t i o n . The second s o l u b l e starch-g-PAN (No. 3) was prepared by s u b j e c t i n g another s t a r c h g r a f t copolymer t o a m i l d a c i d h y d r o l y s i s t o p a r t i a l l y depolymerize the s t a r c h moiety. The ethanol-water s o l v e n t system apparently promotes a greater number o f starch:PAN c r o s s l i n k s than water, as evidenced by the lower s o l u b i l i t i e s (45 vs. 71% and 42 vs. 65%) and the higher v i s c o s i t i e s f o r water d i s p e r s i o n s (2400 vs. 920 cp and 690 vs. 220 cp). The next l o g i c a l step to prove c r o s s l i n k i n g between s t a r c h and PAN was t o saponify a 1:1 p h y s i c a l mixture o f s t a r c h and PAN and t o show that the r e s u l t i n g s a p o n i f i c a t e had s o l u b i l i t y and v i s c o s i t y p r o p e r t i e s d i f f e r e n t than those of the s a p o n i f i c a t e of the i n d i v i d u a l components. Numbers 4, 5, and 6 (Table IV) show the r e s u l t s o f s a p o n i f y i n g s t a r c h , c e r i c - t r e a t e d s t a r c h , and PAN homopolymer. S a p o n i f i c a t i o n s i n water and i n e t h a n o l water gave complete o r n e a r l y complete s o l u b i l i t i e s as w e l l as low v i s c o s i t i e s f o r the d i a l y z e d s o l u t i o n s . Although s a p o n i f i c a t i o n o f the p h y s i c a l mixture (No. 7) i n water gave about the same r e s u l t s as the i n d i v i d u a l components,



+ 4

V

Polymer

+ 4

e

h

^Polymers (same q u a n t i t i e s as f o o t n o t e a) s a p o n i f i e d f o r 3 h a t r e f l u x i n a s o l u t i o n o f 2.0 g 50% NaOH, 4.44 ml water, and 4.44 ml EtOH.

11

V

v

of PAN: ^Polyaerylamide content: 35%.

^TAN content: 37%; M

7, Table I I .

v

of PAN:

v

(O.25)

PAN content: 53%;M of PAN: PAN content: 46%; M V).

g

^ B r o o k f i e l d v i s c o s i t y a t 30 rpm.

°Expressed as g water-swollen g e l / g d r y polymer.

100

3.0 (O.28) 2. 8 (O.25) 19 (O.24) 1000 (O.25) 3900 (O.23) 2900 (O.25)

690 (O.25)

42 100 94 100 85 28 51

2400 (O.25)

45

28

c

2

i n EtOH-H 0

480

340

310

100,000.

35,000.

36,000.

Prepared under the same c o n d i t i o n s as No. 3, Table I , PAN content: 51%; M of PAN: 64,000.

270

f

2.4 (O.25) 3.0 (O.25) 16 (O.25) 22 (O.31)

220 (O.23)

65

99 100 100 100

920 (O.25)

71

26

(%)

Saponified

Dispersions

Solubles, V i s c o s i t y , Absorbency of cp (% solids) insoluble g e l (%)

of S a p o n i f i e d

Viscosity« Absorbency of cp (% s o l i d s ) i n s o l u b l e g e l

i n water

Properties

^ E i t h e r 1.0 g s t a r c h g r a f t copolymer or O.5 g homopolymer ( s t a r c h or PAN) s a p o n i f i e d i n 9.0 ml O.7 Ν NaOH f o r 3 h a t 95-100°C.

6 0

10 S o n i f i e d starch-g-polyacrylamide ( C o ) j

6 0

Starch-g-PAN ( C e ) S o n i f i e d starch-g-PAN (Ce+*)f A c i d - t r e a t e d starch-g-PAN (Ce+4) g Starch Ce -treated starch PAN (5T : 49,000) Starch-PAN phys. mixture 8 Starch-g-PAN ( C o ) 9 S o n i f i e d starch-g-PAN

No.

Solubles,

Saponified

Table IV. A l k a l i n e S a p o n i f i c a t i o n s .


c


204


s a p o n i f i c a t i o n i n ethanol-water gave a product that was 85% s o l u b l e and that had a B r o o k f i e l d v i s c o s i t y of 1000 cp a t O.25% s o l i d s i n water. Although the number of starch:PAN c r o s s l i n k s i n HSPAN i s too low t o permit t h e i r i d e n t i f i c a t i o n by spectroscopy, we can propose a reasonable c r o s s l i n k i n g mechanism based on the known r e a c t i o n s of PAN and s t a r c h w i t h a l k a l i . When PAN i s heated w i t h a l k a l i , a deep red-brown c o l o r i s produced due t o n u c l e o p h i l i c - i n i t i a t e d p o l y m e r i z a t i o n through the n i t r i l e groups t o y i e l d a polyimine type of conjugated r i n g s t r u c t u r e (14, 15). T h i s conjugated ladder-type polymer i s then opened i n a second r e a c t i o n t o g i v e a p o l y a c r y l a m i d e , which i s then f u r t h e r hydrolyzed t o the observed p o l y ( a c r y l i c a c i d s a l t - c o a c r y l a m i d e ) . Under the c o n d i t i o n s o f Table IV, a c r o s s l i n k e d g i n a l product i s not observed^in the absence of s t a r c h , s i n c e OH (and t o a l e s s e r e x t e n t , OC2H5) i s the n u c l e o p h i l e that i n i t i a t e s the n i t r i l e p o l y m e r i z a t i o n and then r e a c t s t o open the conjugated r i n g s t r u c t u r e . I n the presence of s t a r c h , however, s t a r c h a l k o x i d e can a l s o serve as the n u c l e o p h i l e , thus l e a d i n g t o the observed c r o s s l i n k i n g . The c l o s e p r o x i m i t y of s t a r c h and PAN, when the two polymers are c h e m i c a l l y bonded through g r a f t p o l y m e r i z a t i o n , would favor these r e a c t i o n s . The r o l e of the conjugated polyimine intermediate i n the observed c r o s s l i n k i n g can be seen by comparing the s a p o n i f i c a t i o n of a starch-g-PAN (No. 9, Table IV) w i t h that of a s t a r c h - g polyacrylamide prepared under the same c o n d i t i o n s (No. 10). Although polyacrylamide i s an intermediate i n the conversion of PAN t o p o l y ( a c r y l i c a c i d s a l t - c o - a c r y l a m i d e ) , the polyimine s t r u c t u r e i s of course not formed during i t s s a p o n i f i c a t i o n . Both starch-g-PAN and starch-g-polyacrylamide were prepared by cobalt-60 i n i t i a t i o n , s i n c e acrylamide does not g r a f t e f f i c i e n t l y i n the presence of e e r i e ammonium n i t r a t e (16). Both g r a f t copolymers were i r r a d i a t e d w i t h u l t r a s o u n d t o g i v e t o t a l l y s o l u b l e products; the u l t r a s o n i c treatment of starch-g-PAN was c a r r i e d out i n DMSO, w h i l e t h a t of starch-g-polyacrylamide was run i n water. S a p o n i f i c a t i o n s of these g r a f t copolymers were c a r r i e d out i n 1:1 ethanol (95%)-water (Table I V ) . Reaction No. 8 shows the s a p o n i f i c a t i o n r e s u l t s f o r starch-g-PAN before treatment w i t h u l t r a s o u n d ; and although the i n s o l u b l e g e l f r a c t i o n was more h i g h l y s w o l l e n , the % s o l u b l e s was the same as f o r s t a r c h g-PAN prepared w i t h e e r i e i n i t i a t i o n (No. 1 ) . A f t e r s o l u b i l i z a t i o n by treatment w i t h u l t r a s o u n d , starch-g-PAN gave a s a p o n i f i c a t i o n product t h a t was only 51% w a t e r - s o l u b l e and t h a t showed a B r o o k f i e l d v i s c o s i t y of 2900 cp a t O.25% s o l i d s i n water. The corresponding starch-g-polyacrylamide, however, gave a t o t a l l y w a t e r - s o l u b l e s a p o n i f i c a t i o n product w i t h low v i s c o s i t y , i n d i c a t i n g t h a t c r o s s l i n k i n g d i d not occur.



13.

FANTA ET AL.


205

F i l m Formation. From our f i r s t s t u d i e s of HSPAN, i t was apparent t h a t continuous f i l m s can be formed by simply a l l o w i n g water d i s p e r s i o n s of the polymer t o evaporate t o dryness (17). These f i l m s a r e unique i n that they do not r e v e r t back t o the o r i g i n a l g e l p a r t i c l e s when p l a c e d i n water. Rather, the e n t i r e f i l m s w e l l s as a continuous e n t i t y as i t imbibes water; and w i t h i n a few minutes, a h i g h l y s w o l l e n sheet of g e l i s formed that has enough t e n s i l e s t r e n g t h t o a l l o w i t t o be c a r e f u l l y manipulated without breaking. I f these f i l m s are r e d i s p e r s e d i n water w i t h r a p i d s t i r r i n g (e.g., Waring Blendor f o r short p e r i o d s of t i m e ) , the r e s u l t i n g d i s p e r s i o n w i l l again form a f i l m w i t h s i m i l a r p r o p e r t i e s if i t i s allowed t o evaporate t o dryness. Swollen f i l m s d e r i v e d from g e l a t i n i z e d s t a r c h g r a f t copolymers possess more s t r e n g t h and c o n t i n u i t y than s i m i l a r f i l m s prepared from g r a n u l a r s t a r c h . Bonding of i n d i v i d u a l g e l p a r t i c l e s w i t h each other to g i v e l a r g e r conglomerates of h i g h l y absorbent polymer i s a l s o observed when other i s o l a t i o n techniques are used, e.g., a l c o h o l p r e c i p i t a t i o n or drum d r y i n g ( 4 ) . However, if a l c o h o l p r e c i p i t a t i o n i s c a r r i e d out by slow a d d i t i o n of a l c o h o l t o the water d i s p e r s i o n , HSPAN tends t o p r e c i p i t a t e as i n d i v i d u a l g e l p a r t i c l e s r a t h e r than as l a r g e r chunks, and the r e s u l t i n g d r i e d polymer t h e r e f o r e g i v e s very smooth water d i s p e r s i o n s when hydrated (18). To v e r i f y that these f i l m - and conglomerate-forming p r o p e r t i e s of HSPAN were not r e l a t e d t o the s t i c k i n g together of i n d i v i d u a l g e l p a r t i c l e s by the minor amount of s o l u b l e polymer present i n s a p o n i f i c a t i o n m i x t u r e s , f i l m s were c a s t from water d i s p e r s i o n s of HSPAN polymers, prepared from e i t h e r g r a n u l a r o r g e l a t i n i z e d s t a r c h , from which the s o l u b l e s had been e x t r a c t e d w i t h water. These f i l m s were indeed continuous, and they absorbed water t o form continuous g e l sheets. We a l s o observed that a d d i t i o n of 10% of e i t h e r s a p o n i f i e d PAN or s o l u b l e s t a r c h t o HSPAN prepared from g r a n u l a r s t a r c h d i d not outwardly a l t e r f i l m p r o p e r t i e s . To e x p l a i n the f a c t that HSPAN s w e l l s i n water to form g e l sheets o r m a c r o p a r t i c l e s r a t h e r than d i s i n t e g r a t i n g i n t o a g e l d i s p e r s i o n , we i n i t i a l l y f e l t that chemical bonding must take p l a c e between i n d i v i d u a l p a r t i c l e s of water-swollen g e l as water evaporates. Although we cannot t o t a l l y e l i m i n a t e t h i s p o s s i b i l i t y , the p r o p o s a l of primary chemical bonding i s not necessary t o e x p l a i n the behavior of these f i l m s and conglomerates. For example, V o y u t s k i i (19) has reviewed the formation of f i l m s from v u l c a n i z e d rubber l a t e x e s and concludes t h a t f i l m formation i n these systems i s observed because of i n t e r d i f f u s i o n of ends of i n d i v i d u a l macromolecules i n adjacent l a t e x p a r t i c l e s . T h i s d i f f u s i o n can take p l a c e even though i n d i v i d u a l l a t e x p a r t i c l e s a r e c r o s s l i n k e d , 3-dimensional networks; and the c o n t i n u i t y of the r e s u l t i n g f i l m s , even when



206


p a r t i a l l y s w o l l e n i n a hydrocarbon s o l v e n t , can be a s c r i b e d to Van der Waals f o r c e s between interwoven polymer ends. Bradford and Vanderhoff (20) have a l s o prepared f i l m s from c r o s s l i n k e d l a t e x p a r t i c l e s . These authors s t u d i e d a 65:35 styrene-butadiene copolymer c r o s s l i n k e d w i t h v a r y i n g amounts of d i v i n y l b e n z e n e and found that although the i n c o r p o r a t i o n of d i v i n y l b e n z e n e r e t a r d e d the coalescence of l a t e x p a r t i c l e s , these p a r t i c l e s d i d indeed c o a l e s c e , presumably due to a s i m i l a r i n t e r d i f f u s i o n of polymer c h a i n ends. The analogy between c r o s s l i n k e d l a t e x p a r t i c l e s and the p a r t i c l e s of water-swollen HSPAN i s obvious. I n a d d i t i o n t o Van der Waals f o r c e s , hydrogen bonding between the h y d r o p h i l i c polymer chains can a l s o take p l a c e , and some of these hydrogen bonds could be q u i t e s t r o n g , as evidenced by the w e l l known phenomenon of amylose rétrogradation (21) and the r e s i s t a n c e o f retrograded amylose t o water s o l u t i o n . An analogy to the v u l c a n i z e d rubber l a t e x e s can a l s o be seen i n the f a c i l e conversion of HSPAN f i l m t o g e l p a r t i c l e s by a short p e r i o d of high-speed s t i r r i n g i n water, f o l l o w e d by the r e f o r m a t i o n of a f i l m s i m i l a r t o the o r i g i n a l by simply d r y i n g the water d i s p e r s i o n . V o y u t s k i i (22) found that a f i l m prepared from v u l c a n i z e d l a t e x gave a continuous rubber sheet on c o l d m i l l i n g , presumably because t h e absence o f primary chemical bonding between i n d i v i d u a l p a r t i c l e s a l l o w s them t o e a s i l y s l i d e w i t h respect t o each o t h e r ; the same type of secondary (Van der Waals) bonds can then r e a d i l y reform. I n c o n t r a s t , a rubber f i l m t h a t was v u l c a n i z e d a f t e r i t had been prepared, and was thus c r o s s l i n k e d throughout the e n t i r e polymer m a t r i x , was converted t o a rubber crumb on m i l l i n g . I f the formation of continuous f i l m s from HSPAN g e l i s indeed l a r g e l y due t o hydrogen bonding and Van der Waals f o r c e s between i n t e r d i f f u s e d polymer c h a i n s , r a t h e r than t o the formation of primary chemical bonds, t h i s property should not be unique t o the s t a r c h g r a f t copolymers but should be e x h i b i t e d , a t l e a s t t o some e x t e n t , by other w a t e r - s w o l l e n , h y d r o p h i l i c g e l s as w e l l . T h i s p r o b a b i l i t y was v e r i f i e d e x p e r i m e n t a l l y f o r a commercial t h i c k e n i n g agent (Carbopol 941) and a commercial water absorbent (Permasorb 10). Water d i s p e r s i o n s of both of these polymers ( n e i t h e r of which c o n t a i n s t a r c h ) formed f i l m s when d r i e d ; and although the water-swollen f i l m s were more e a s i l y d i s i n t e g r a t e d than those from HSPAN, they d i d remain i n t a c t if they were not a g i t a t e d . Carbopol i s a p a r t i c u l a r l y good polymer f o r comparison, s i n c e T a y l o r and Bagley Ç5, 23) have s t u d i e d i t s rheology and g e l content and i n these r e s p e c t s have found i t t o be q u i t e s i m i l a r t o HSPAN. Heat-Induced C r o s s l i n k i n g of Dry Polymer. When a d r y sample of HSPAN i s heated, the polymer g r a d u a l l y l o s e s i t s c a p a c i t y t o absorb water (24), and t h i s absorbency l o s s


13.

FANTA ET AL.

207



i n c r e a s e s w i t h both temperature and the d u r a t i o n of the heat treatment. P h y s i c a l mixtures of s t a r c h and s a p o n i f i e d PAN a l s o c r o s s l i n k when heated (25). An i n c r e a s e i n hydrogen bonding between polymer chains (e.g., through s t a r c h h y d r o x y l s ) due t o removal o f bound water molecules at the higher temperatures could e x p l a i n these f i n d i n g s ; however, if t h i s mechanism were o p e r a t i n g , l o s s of absorbency should p a r a l l e l the l o s s of water when HSPAN i s heated. T h i s i s not what we observed e x p e r i m e n t a l l y (Table V ) . When samples of polymer,

Table V. I n f l u e n c e of Water Loss on Absorbency of S a p o n i f i e d Starch-g-PAN a

Heating c o n d i t i o n s Q

Temp., C

Time, h

25 ( c o n t r o l ) 60 80 100 120 140 100 100 (vacuum)

Weight L o s s , %

= 1 1 1 1 1 3 3

2.4 6.8 10.4 12.8 14.8 12.4 14.1

b

Absorbency , g/g 730 680 700 680 680 480 700 680

a

A commercial sample prepared from g e l a t i n i n z e d s t a r c h (SGP 502S, Henkel Corp., screened t o 100-200 mesh, water content: 14.2%. b

D i s t i l l e d water.

c o n t a i n i n g 14.2% water, were heated i n an oven f o r 1 h r a t temperatures ranging from 60 t o 120°C., absorbencies of the heated polymers were not g r e a t l y a f f e c t e d , even though n e a r l y all of the water had been d r i v e n o f f a t 100-120°C., as evidenced by weight l o s s of the samples. The f a c t t h a t , even at these temperatures, a s m a l l amount of water was still r e t a i n e d by the polymer was apparently not the reason that absorbency d i d not decrease. When two separate polymer samples were heated i n the oven f o r 3 h r a t 100°C., the f i r s t at atmospheric pressure and the second under a vacuum of about 3 mm Hg, both polymers showed about the same absorbency, even



208


though the vacuum-heated polymer had v i r t u a l l y all of i t s water removed. When a sample o f HSPAN was heated f o r 1 h r a t 140°C (Table V ) , i t s weight l o s s was a c t u a l l y s l i g h t l y g r e a t e r than the o r i g i n a l water content of 14.2%, and there was a l s o a s i g n i f i c a n t decrease i n water absorbency. A condensation type of c r o s s l i n k i n g r e a c t i o n w i t h l o s s of v o l a t i l e s might account f o r these o b s e r v a t i o n s , and e i t h e r h y d r o x y l , carboxamide, or c a r b o x y l s u b s t i t u e n t s could p a r t i c i p a t e i n such r e a c t i o n s at t h i s h i g h temperature. A c h o i c e among the s e v e r a l m e c h a n i s t i c pathways must await f u r t h e r research. Conclusions There are a number of f a c t o r s which s i m u l t a n e o u s l y c o n t r i b u t e t o g i v e HSPAN i t s unique p r o p e r t i e s . F i r s t of all, c r o s s l i n k i n g occurs d u r i n g the g r a f t p o l y m e r i z a t i o n r e a c t i o n of a c r y l o n i t r i l e w i t h s t a r c h , probably by combination of PAN m a c r o r a d i c a l s . F u r t h e r c r o s s l i n k i n g w i t h i n the granule s t r u c t u r e takes p l a c e during a l k a l i n e s a p o n i f i c a t i o n . C r o s s l i n k i n g of PAN w i t h i t s e l f can occur when s a p o n i f i c a t i o n s are c a r r i e d out i n systems composed predominantly of a l c o h o l ; however, starch:PAN c r o s s l i n k i n g i s the major r e a c t i o n i n s o l v e n t s which c o n t a i n mainly water. As a r e s u l t of these c r o s s l i n k i n g r e a c t i o n s , i n d i v i d u a l granules of starch-£-PAN do not d i s i n t e g r a t e when s a p o n i f i e d w i t h hot a l k a l i , but r e t a i n t h e i r i n t e g r i t y and e x i s t i n water as h i g h l y swollen g e l p a r t i c l e s . Aqueous d i s p e r s i o n s of these g e l s thus have h i g h v i s c o s i t y , when all the f r e e water has been absorbed and the g e l p a r t i c l e s a r e i n a c l o s e l y packed s t a t e . When v i s c o u s aqueous d i s p e r s i o n s of HSPAN are spread onto a T e f l o n - c o a t e d t r a y and allowed t o dry, i n d i v i d u a l g e l p a r t i c l e s k n i t together t o form continuous f i l m s . G e l p a r t i c l e s a l s o c o a l e s c e t o form l a r g e r conglomerates when the polymer i s i s o l a t e d from water by other techniques, such as a l c o h o l p r e c i p i t a t i o n or drum d r y i n g . Dry HSPAN does not r e d i s p e r s e back i n t o g e l p a r t i c l e s when p l a c e d i n water but s w e l l s as e i t h e r a continuous f i l m or as m a c r o p a r t i c l e s as i t imbibes water. The f o r m a t i o n of primary chemical bonds between i n d i v i d u a l g e l p a r t i c l e s need not be proposed t o account f o r these p r o p e r t i e s . The behavior of HSPAN i s s i m i l a r i n many respects t o the p u b l i s h e d behavior of c r o s s l i n k e d rubber l a t e x e s , and HSPAN p r o p e r t i e s may be s i m i l a r l y explained by assuming i n t e r d i f f u s i o n of polymer chain ends on the surfaces of g e l p a r t i c l e s , followed by hydrogen bonding between these polymer chains. When d r y HSPAN i s heated, i t s a b i l i t y t o absorb water i s d i m i n i s h e d . T h i s l o s s of absorbency i s not r e l a t e d t o l o s s of bound water and t h e r e f o r e cannot be due t o a d d i t i o n a l


13.

FANTA ET AL.


209

hydrogen bonding. A condensation type of c r o s s l i n k i n g r e a c t i o n i s probably o c c u r r i n g between h y d r o x y l , carboxamide, or c a r b o x y l s u b s t i t u e n t s at h i g h temperatures.


Experimental M a t e r i a l s . Granular Globe P e a r l corn s t a r c h (Globe 3005; water content about 10%) was from CPC I n t e r n a t i o n a l . A c r y l o n i t r i l e (Eastman p r a c t i c a l grade) was f r a c t i o n a t e d at atmospheric p r e s s u r e through a 1 5 - i n . Vigreux column, and a center cut was c o l l e c t e d . C e r i c ammonium n i t r a t e was F i s h e r c e r t i f i e d ACS grade. PAN homopolymer was Polymeric A c r y l o n i t r i l e Type A from DuPont. Graft Polymerization. Ceric I n i t i a t i o n . A s t i r r e d s l u r r y of 10 g of s t a r c h i n 200 ml of water was sparged w i t h a slow stream of n i t r o g e n f o r 1 hr at room temperature; 15 g of a c r y l o n i t r i l e was then added, f o l l o w e d a f t e r 5 min by a s o l u t i o n of O.338 g of c e r i c ammonium n i t r a t e i n 3 ml of IN n i t r i c a c i d . In the r e a c t i o n run w i t h 1-hexanethiol, c e r i c ammonium n i t r a t e s o l u t i o n was added f i r s t , f o l l o w e d a f t e r 5 min by a s o l u t i o n of 1.0 g of mercaptan i n 15 g of a c r y l o n i t r i l e . The r e s u l t i n g mixture was allowed t o s t i r f o r 2 hr a t 25-27°C (temperature maintained w i t h i c e - w a t e r ) and then was d i l u t e d w i t h 200 ml of e t h a n o l . The pH was adjusted to 6-7 w i t h sodium hydroxide s o l u t i o n , and the polymer was i s o l a t e d by f i l t r a t i o n , washed w i t h water and e t h a n o l , and vacuum d r i e d a t 60°C. Ungrafted PAN was removed by repeated e x t r a c t i o n of the polymer w i t h dimethylformamide (DMF) at room temperature. G r a f t P o l y m e r i z a t i o n . Cobalt-60 I n i t i a t i o n . The c o b a l t 60 source was a Gammacel 200 u n i t from Atomic Energy of Canada, L t d . Dose r a t e at the center of the chamber was O.44O.42 Mrad/hr, as c a l c u l a t e d from the o r i g i n a l dosimetry data provided by the manufacturer and the decay r a t e of c o b a l t - 6 0 . A s t i r r e d s l u r r y of 10 g of s t a r c h i n 100 ml of water was sparged w i t h a slow stream of n i t r o g e n f o r 30 min at room temperature, and a c r y l o n i t r i l e (5, 10, or 15 g) was added. In r e a c t i o n s run w i t h mercaptans, e i t h e r 1.0 g of 1-hexanethiol or O.1 g of 2-mercaptoethanol was d i s s o l v e d i n 15 g of a c r y l o n i t r i l e . The r e a c t i o n mixture was s t i r r e d m a g n e t i c a l l y and i r r a d i a t e d w i t h cobalt-60 to a t o t a l dose of O.1 Mrad. The r e a c t i o n mass thickened w i t h the onset of p o l y m e r i z a t i o n , and most r e a c t i o n mixtures were too t h i c k to s t i r by the end of the i r r a d i a t i o n p e r i o d . R e a c t i o n temperature reached 4050°C due to the exothermic p o l y m e r i z a t i o n s . The r e a c t i o n mass was allowed to stand f o r 2 hr at ambient temperature, and the polymer was i s o l a t e d by f i l t r a t i o n , washed w i t h water and w i t h e t h a n o l , and vacuum d r i e d at 60°C. Ungrafted PAN was



210


removed from the polymer by repeated e x t r a c t i o n w i t h DMF at room temperature. A s i m i l a r procedure was used t o prepare s t a r c h - g p o l y a c r y l a m i d e , and 15 g o f acrylamide (97%, P o l y s c i e n c e s ) was used i n the p o l y m e r i z a t i o n . A f t e r i r r a d i a t i o n , the r e a c t i o n mass was e x t r a c t e d s e v e r a l times w i t h water t o remove ungrafted p o l y a c r y l a m i d e , and the g r a f t copolymer was washed w i t h e t h a n o l and vacuum d r i e d a t 60°C. Weight percent p o l y a c r y l a m i d e i n the g r a f t copolymer (% add-on) was 35%, based on the weight gain of starch. C h a r a c t e r i z a t i o n o f G r a f t Copolymers. S t a r c h was removed from starch-g-PAN by h e a t i n g 2 g o f g r a f t copolymer i n 150 ml of O.5N h y d r o c h l o r i c a c i d f o r 1.5 h r under r e f l u x . The r e s i d u a l PAN was separated by f i l t r a t i o n , washed w i t h water and w i t h e t h a n o l , and d r i e d . I n f r a r e d a n a l y s i s showed the amount o f carbohydrate remaining t o be near the l i m i t of d e t e c t a b i l i t y of about 5%. Percent add-on was c a l c u l a t e d by weight l o s s on a c i d h y d r o l y s i s , and the molecular weight of PAN (M ) was determined from i t s i n t r i n s i c v i s c o s i t y i n DMF s o l u t i o n (26). G r a f t i n g frequency, expressed as the average number o f anhydroglucose u n i t s (AGU) per g r a f t e d PAN c h a i n , was c a l c u l a t e d from % add-on and M of PAN. ν

S o l u b i l i t y Determination i n D i m e t h y l s u l f o x i d e (DMSO). About 2-2.5 g o f polymer, which had been p r e v i o u s l y oven d r i e d , was allowed t o stand overnight a t room temperature over water i n a c l o s e d c o n t a i n e r . Water contents of these moistened polymers ranged from 13-22%, depending on PAN content. About 1.2 g ( a c c u r a t e l y weighed) of moistened polymer was d i s p e r s e d i n 200.0 g o f DMSO, and the r e s u l t i n g mixture was s t i r r e d and heated t o 75°C. The f l a s k was stoppered, and the d i s p e r s i o n was heated w i t h o c c a s i o n a l s t i r r i n g i n a 75°C oven f o r 22 h r and then was cooled t o room temperature. The mixture was c e n t r i f u g e d , and the supernatant was g r a v i t y f i l t e r e d through f l u t e d Whatman 54 paper. A weighed p o r t i o n of f i l t r a t e was evaporated t o near dryness, and the polymer was p r e c i p i t a t e d w i t h 95% e t h a n o l , washed w i t h 95% e t h a n o l , and d r i e d . Percent s o l u b i l i t y was c a l c u l a t e d from the weight o f dry s o l u b l e polymer and the i n i t i a l weight o f polymer used, a f t e r c o r r e c t i n g f o r % moisture. P r e p a r a t i o n o f DMSO-Soluble Starch-g-PAN Samples and Water-Soluble Starch-g-Polyacrylamide. F i v e grams of s t a r c h g-PAN (prepared by e i t h e r c e r i c - o r c o b a l t - 6 0 i n i t i a t i o n ) was d i s p e r s e d i n 333 g o f DMSO, and the d i s p e r s i o n was heated f o r 2 h r a t 115°C. The cooled d i s p e r s i o n was then t r e a t e d w i t h u l t r a s o u n d f o r 10 min t o m e c h a n i c a l l y r u p t u r e g e l p a r t i c l e s and to render the polymer s o l u b l e (Branson S o n i f i e r , Model S 125;


13.

FANTA ET AL.


211

set f o r maximum output) the t r e a t e d d i s p e r s i o n then was allowed to f i l t e r through f l u t e d Whatman 54 paper. The f i l t r a t e was concentrated, and the polymer was p r e c i p i t a t e d w i t h 95% ethanol. The polymer was removed by f i l t r a t i o n , washed w i t h 95% e t h a n o l , and d r i e d under vacuum at 60°C. A p o r t i o n of s o n i f i e d s t a r c h g-PAN w a s s u b j e c t e d t o a c i d h y d r o l y s i s , and % add-on by weight l o s s and M of g r a f t e d PAN were determined. T o t a l s o l u b i l i t y i n DMSO a t 75°C was confirmed f o r the c e r i c - i n i t i a t e d starch-&PAN p r e p a r a t i o n . Starch-g-PAN w i t h 40% add-on (M of PAN:35,000) was prepared by c e r i c i n i t i a t i o n as described above, w i t h the exception t h a t only 10 g o f a c r y l o n i t r i l e was used. F i v e grams of t h i s g r a f t copolymer was dispersed i n 200 ml of water, and concentrated h y d r o c h l o r i c a c i d was added t o give a pH of 2.O. The mixture was heated under r e f l u x f o r 15 min, and the polymer was i s o l a t e d by f i l t r a t i o n . The polymer was suspended i n water, the pH was adjusted t o 6.5 w i t h sodium hydroxide s o l u t i o n , and the polymer was again separated by f i l t r a t i o n . The polymer was washed w i t h water and w i t h ethanol and d r i e d under vacuum at 60°C. The y i e l d was 4.357 g of starch-g-PAN, which was 99% s o l u b l e i n DMSO a t 75°C. The % add-on was 46%, as c a l c u l a t e d from the o r i g i n a l 40% add-on and the l o s s o f O.643 g of s t a r c h by a c i d h y d r o l y s i s . F i v e grams of starch-g-polyacrylamide was dispersed i n 333 ml of water and the mixture was heated f o r 4.5 h r a t 95100°C. The cooled d i s p e r s i o n was then t r e a t e d w i t h u l t r a s o u n d , as described f o r starch-g-PAN i n DMSO. The r e s u l t i n g s o l u t i o n was g r a v i t y - f i l t e r e d through f l u t e d Whatman 54 paper, and the f i l t r a t e was f r e e z e - d r i e d t o g i v e 4.4 g of polymer. To g i v e a denser, more compact product, which might more c l o s e l y resemble s o l u b l e starch-g-PAN, the f r e e z e - d r i e d polymer was d i s p e r s e d i n 20 ml of water, and the polymer was p r e c i p i t a t e d from the t h i c k paste by a d d i t i o n of ethanol. The polymer was separated by f i l t r a t i o n , washed w i t h e t h a n o l , and vacuum d r i e d a t 60°C.


v

P r e p a r a t i o n of Starch-PAN S y n t h e t i c M i x t u r e . S o l u t i o n s of both s t a r c h and PAN homopolymer were prepared i n DMSO a t 1% s o l i d s . Equal weights of the two s o l u t i o n s were combined and then concentrated under vacuum. The polymer mixture was p r e c i p i t a t e d w i t h e t h a n o l , and the p r e c i p i t a t e d s o l i d was washed w i t h ethanol and d r i e d under vacuum at 60°C. PAN content of the mixture was 45%, as determined by weight l o s s on acid hydrolysis. A l k a l i n e S a p o n i f i c a t i o n s i n Water. A suspension of 1.0 g of starch-g-PAN o r starch-PAN p h y s i c a l mixture i n 9 ml o f O.7N sodium hydroxide ( i n a 25 ml Erlenmeyer f l a s k ) was heated on a steam bath f o r 5-10 min u n t i l the mixture thickened s u f f i c i e n t l y to preclude s e t t l i n g . Reactions w i t h e i t h e r s t a r c h or PAN



212


homopolymer were run w i t h O.5 g of polymer. The f l a s k was l o o s e l y stoppered ( t o permit escape of ammonia) and heated f o r 3 hr i n a 95-100°C oven. The r e a c t i o n mass was then d i l u t e d w i t h water (75 or 150 ml, depending on the weight of polymer s a p o n i f i e d ) and d i a l y z e d against d i s t i l l e d water. A known weight of the d i a l y z e d d i s p e r s i o n (pH about 7) was freeze d r i e d to determine % s o l i d s . The d i s p e r s i o n was d i l u t e d w i t h water to O.25% s o l i d s , and the B r o o k f i e l d v i s c o s i t y was obtained at 30 rpm. With some d i s p e r s i o n s , a w e l l - d r a i n e d i n s o l u b l e g e l f r a c t i o n could be i s o l a t e d by g r a v i t y f i l t r a t i o n through f l u t e d Whatman 54 paper. A weighed p o r t i o n of t h i s i n s o l u b l e g e l was f r e e z e - d r i e d , and the absorbency (expressed as grams of water-swollen g e l per gram of dry polymer) was c a l c u l a t e d from the weight of f r e e z e - d r i e d s o l i d . To reduce v i s c o s i t y and polymer s w e l l i n g , the d i s p e r s i o n was a c i d i f i e d w i t h h y d r o c h l o r i c a c i d to pH 4.5, and a p o r t i o n of t h i s d i s p e r s i o n was allowed to g r a v i t y f i l t e r through f l u t e d Whatman 54 paper. The percent s o l u b l e polymer was c a l c u l a t e d from the % s o l i d s i n both the f i l t r a t e and the u n f i l t e r e d d i s p e r s i o n , as determined by f r e e z e - d r y i n g . A l k a l i n e S a p o n i f i c a t i o n s i n Aqueous Ethanol. A s o l u t i o n of 2 g of 50% sodium hydroxide i n 4.44 ml of water and 4.44 ml of 95% ethanol was prepared, and 1 g of e i t h e r starch-g-PAN or starch-PAN p h y s i c a l mixture was added. Reactions w i t h s t a r c h or PAN homopolymer were run w i t h O.5 g of polymer. The mixture was heated under r e f l u x f o r 3 h r , and the r e s u l t i n g r e a c t i o n mass was dispersed i n water, d i a l y z e d , and analyzed as described f o r s a p o n i f i c a t i o n s i n water. S a p o n i f i c a t i o n s run at higher ethanol:water r a t i o s were c a r r i e d out i n a s i m i l a r manner. Enzymatic H y d r o l y s i s of HSPAN. HSPAN f o r t h i s p a r t i c u l a r experiment was prepared as f o l l o w s . F i v e grams of starch-g-PAN (prepared from granular s t a r c h , 50% add-on, M of PAN:64,000) i n 45 ml of O.7N sodium hydroxide s o l u t i o n was heated f o r 3 hr at 95-100°C. The r e a c t i o n mass was added to 500 ml of methanol, and the mixture was subjected t o high-shear s t i r r i n g (Waring Blendor) f o r 1 min. HSPAN was separated by f i l t r a t i o n , washed w i t h methanol, and vacuum d r i e d at 60°C. Two grams of HSPAN was added t o 500 ml of water, the pH was adjusted from 9 t o 6.5 w i t h d i l u t e h y d r o c h l o r i c a c i d , and O.1 ml of Thermamyl 60L enzyme s o l u t i o n (Novo Enzyme Corp.) was added. The r e s u l t i n g mixture was heated at 95°C f o r 21 h r , and the c l e a r y e l l o w s o l u t i o n was e x h a u s t i v e l y d i a l y z e d against d i s t i l l e d water. Freeze d r y i n g y i e l d e d 1.235 g of polymer, which contained only about 5% r e s i d u a l carbohydrate (by i n f r a r e d a n a l y s i s ) . The number average molecular weight of t h i s polymer was 44,000, as determined i n O.15N sodium c h l o r i d e s o l u t i o n on a Melabs Model CSM-2 membrane osmometer equipped w i t h a B-19 membrane ( S c h l e i c h e r and S c h u e l l Co.).



13.

FANTA ET AL.

Saponified Starch-g-Poly acrylonitrile

213

F i l m Formation. Starch-g-PAN samples used f o r t h i s study were prepared as described above, except that the s m a l l amounts of PAN homopolymer were not removed by DMF e x t r a c t i o n . For the g r a f t p o l y m e r i z a t i o n w i t h g e l a t i n i z e d s t a r c h , the starch-water s l u r r y was heated f o r 30 min at 85°C b e f o r e r e a c t i o n at room temperature w i t h a c r y l o n i t r i l e and c e r i c ammonium n i t r a t e . S a p o n i f i c a t i o n s i n these experiments were c a r r i e d out by s t i r r i n g . 5 5 g of g r a f t copolymer w i t h 450 ml of O.7N sodium hydroxide s o l u t i o n i n a sigma mixer f o r 2 hr at 90-100°C. The r e a c t i o n mass was s t i r r e d w i t h 3 1. of methanol, and HSPAN was i s o l a t e d by f i l t r a t i o n . The polymer was washed w i t h 90:10 methanol-water, d r i e d under vacuum at 40°C., and ground t o pass 40 mesh ( y i e l d : 63-64 g ) . E i t h e r 1.0 g of HSPAN from g r a n u l a r s t a r c h or O.5 g of the polymer d e r i v e d from g e l a t i n i z e d s t a r c h was added to 800 ml of water, and the d i s p e r s i o n was s l o w l y s t i r r e d overnight at room temperature. The mixture was c e n t r i f u g e d (1500 X g) and the supernatant decanted. The g e l f r a c t i o n was subjected t o three more water e x t r a c t i o n s and was then spread onto a T e f l o n - c o a t e d t r a y and d r i e d i n a forced a i r oven a t about 40°C. HSPAN f i l m s c o n t a i n i n g 10% by weight of e i t h e r s a p o n i f i e d PAN o r s o l u b l e s t a r c h were prepared by s e p a r a t e l y s a p o n i f y i n g starch-g-PAN (prepared from granular s t a r c h by c e r i c - i n i t i a t i o n ) and PAN homopolymer and then d i a l y z i n g the s a p o n i f i c a t e s . S t a r c h s o l u t i o n was prepared by d i s s o l v i n g an a c i d - m o d i f i e d s t a r c h ( C l i n t o n 290B) i n water. S o l u t i o n s / d i s p e r s i o n s were then combined i n the d e s i r e d r a t i o s ( a f t e r determining c o n c e n t r a t i o n s by f r e e z e d r y i n g ) and were d r i e d on T e f l o n - c o a t e d t r a y s . A f i l m from Carbopol 941 (B. F. Goodrich) was prepared by d i s p e r s i n g O.5 g of polymer i n 200 ml of water. Sodium hydroxide s o l u t i o n was added t o adjust the pH t o 7.5, and the d i s p e r s i o n was then d r i e d on a Teflon-coated t r a y . A f i l m from Permasorb 10 ( N a t i o n a l Starch & Chemical Corp.) was prepared by d i s p e r s i n g O.50 g of polymer i n 200 ml of water and d r y i n g the d i s p e r s i o n on a T e f l o n - c o a t e d t r a y . I n f l u e n c e of Heating on Absorbency and M o i s t u r e Loss. The HSPAN used i n t h i s study was a commercial sample prepared from g e l a t i n i z e d s t a r c h (SGP 502 S, Henkel Corp.), which we screened to g i v e a f r a c t i o n that passed 100 mesh but was r e t a i n e d by 200 mesh. The water content was 14.2%, as determined by d r y i n g i n an Abderhalden d r y i n g apparatus f o r 4 hr under vacuum (1 mm Hg) a t 100°C over Ρ 2 ° 5 . D u p l i c a t e O.5 g samples of HSPAN were a c c u r a t e l y weighed i n t o s m a l l g l a s s - s t o p p e r e d v i a l s , and the open v i a l s were kept f o r 1 h r i n an oven heated to the d e s i r e d temperature. The v i a l s were stoppered, allowed to c o o l i n a d e s i c c a t o r , and then weighed t o determine the percentage of the o r i g i n a l sample weight l o s t through v o l a t i l i z a t i o n . Percent weight




214

l o s s of two d u p l i c a t e samples g e n e r a l l y d i d not d i f f e r by more than O.2. To determine water absorbency, an a c c u r a t e l y weighed 5 mg sample of HSPAN was allowed t o soak f o r 30 min i n 50 ml of d i s t i l l e d water. The swollen polymer was then separated from unabsorbed water by screening through a tared 280 mesh s i e v e , which was 4.8 cm i n diameter. The polymer on the s i e v e was allowed t o d r a i n f o r 20 min, and the s i e v e was then weighed t o determine the weight of water-swollen g e l . Absorbency was c a l c u l a t e d as grams of water per gram of polymer. No c o r r e c t i o n f o r r e s i d u a l moisture i n the polymer was a p p l i e d . Absorbencies were run i n d u p l i c a t e , and r e s u l t s agreed t o w i t h i n 10%. Literature Cited 1.

2. C. 3. 4.

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

12. 13. 14.

15.

Fanta, G. F.; Bagley, Ε. B. Encyclopedia of Polymer Science and Technology, Supplement, Vol. 2, Ν. M. Bikales, Ed., Wiley, New York, 1977, p. 669. Gugliemelli, L. Α.; Weaver, M. O.; Russell, C. R.; Rist, E. J. Appl. Polym. Sci. 1969, 13, 2007. Fanta, G. F.; Weaver, M. O.; Doane, W. M. Chem. Technol. 1974, 4, 675. Weaver, M. O.; Montgomery, R. R., M i l l e r , L. D.; Sohns, V. E.; Fanta, G. F.; Doane, W. M. Staerke, 1977, 29, 413. Taylor, N. W.; Bagley, Ε. B. J . Appl. Polym. Sci. 1974, 18, 2747. Fanta, G. F.; Baker, F. L.; Burr, R. C.; Doane, W. M.; Russell, C. R. Staerke 1977, 29, 386. Rout, Α.; Rout, S. P.; Singh, B. C.; Santappa, M. Makromol. Chem. 1977, 178, 639. Mohanty, N.; Pradhan, B.; Mahanta, M. C. Eur. Polym. J. 1980, 16, 451. Fanta, G. F.; Burr, R. C.; Russell, C. R.; Rist, C. E. Cereal Chem. 1970, 47, 85. Hofreiter, B. T. J . Polym. Sci., Polym. Chem. Ed. 1974, 12, 2755. Whistler, R. L.; Ingle, T. R. Starch: Chemistry and Technology, Vol. 1, R. L. Whistler and E. F. Paschall, Eds., Academic Press, New York, 1965, p. 414. Hofreiter, B. T.; Russell, C. R. Staerke 1974, 26, 18. Smith, T. U.S. Patent 3,661,815, May 9, 1972. Peebles, L. Η., J r . Encyclopedia of Polymer Science and Technology, Supplement, Vol. 1, H. F. Mark and Ν. M. Bikales, Eds., Wiley, New York, 1977, p. 2. Takata, T.; H i r o i , I.; Taniyama, M. J . Polym. Sci. A, 1964, 2, 1567.


13.

FANTA ET AL.

16.

Fanta, G. F.; Burr, R. C.; Doane, W. M.; Russell, C. R. J. Appl. Polym. Sci. 1971, 15, 2651. Weaver, M. O.; Bagley, E. B.; Fanta, G. F.; Doane, W. M. Appl. Polym. Symp. 1974, No. 25, 97. Antholz, P.; Swarthout, E. J . U.S. Patent 4,221,684, Sept. 9, 1980. Voyutskii, S. S. Autohesion and Adhesion of High Polymers, Polymer Reviews, Vol. 4, H. F. Mark and Ε. H. Immergut, Eds., Wiley, New York, 1963, p. 80. Bradford, Ε. B.; Vanderhoff, J . W. J. Macromol. Chem. 1966, 1, 335. Foster, J . F., Starch, Chemistry and Technology, Vol. 1, R. L. Whistler and E. F. Paschall, Eds., Academic Press, New York, 1965, p. 350. Voyutskii, S. S. Autohesion and Adhesion of High Polymers, Polymer Reviews, Vol. 4, H. F. Mark and Ε. H. Immergut, Eds., Wiley, New York, 1963, p. 87. Taylor, N. W.; Bagley, Ε. B. J. Polym. Sci., Polym. Phys. Ed. 1975, 13, 1133. Bagley, Ε. B.; Taylor, N. W. Ind. Eng. Chem., Prod. Res. Dev. 1975, 14, 105. Fanta, G. F.; Doane, W. M. U.S. Patent 4,116,899, Sept. 26, 1978. Onyon, P. F. J. Polym. S c i . 1959, 37, 315.

17. 18.


19.

20. 21.

22.

23. 24. 25. 26.

RECEIVED December 17,


1981.


Graft Copolymerization of Lignocellulosic Fibers - American Chemical

Recommend Documents