I N V E S T I G A T I O N O F C R O S S - L I N K I N G O F P O L Y A C R Y L O N T R I L E

F I B R E S B Y AN I S O M E T R I C M E T H O D *

M . A . Z h a r k o v a , G I . K u d r y a v t s e v , I . F . K h u d o s h e v , a n d T . A. R o m a n o v a

UDC 677.494.745.32

It is poss ib le to change the ma jo r mechanical and the rmomechan ica l p rope r t i e s of f i b re s by means of s t r u c t u r a l - c h e m i c a l modificat ion [1, 2]. Modifications leading to the product ion of chemica l ly c r o s s - l i n k e d f ib res acquire a special s ignif icance.

P rev ious ly the chemica l c ros s - l i nk ing of PAN f ib res with hydrazine hydrate has been invest igated [3]. The p re sen t work studies the var ia t ion of ce r t a in deformat ion p r o p e r t i e s of chemica l ly c r o s s - l i n k e d PAN f ib r e s by the i s o m e t r i c heating method. As has been shown, it i s poss ib le to de te rmine the number of bonds in unit vo l - ume of a c h a i n u s i n g the Mequilibrium contrac t ion n cu rves of f i b re s in ah igh lye l a s t i c s ta te . Hydrazine hydra te , s e m i c a r b a z i d e and th iosemica rbaz ide were used as c ro s s - l i nk ing agents . The or iented f ib re s ba sed on PAN of 14.3 tex (N 70)t and 28.6 rex (N 35) were subjected to c ross - l ink ing ; the c r o s s - l i n k i n g w a s control led by an i so - m e t r i c method since chemica l methods give ambiguous resu l t s , owing to adequacy of fo rmat ion of i n t r a - and s u p e r m o l e c u l a r s t ruc tu re .

The cu rves on the i some t r i c heating d iag ram (IHD) for the original and chemica l ly c r o s s - l i n k e d f ib re s a r e fundamental ly different (Fig. l) . The original PAN f ibre has a cha r ac t e r i s t i c b imodal curve , f i r s t a m a x i m u m which co r r e sponds approximate ly to the t ransi t ion t e m p e r a t u r e in the v i scoe las t i c s ta te , then a downward side cha r ac t e r i s i ng the gradual t ransi t ion f r o m the highly e las t ic s tate to p las t ic flow. The ap- pea rance of a second m ax i m um at 280-300°C is probably assoc ia ted with the fo rmat ion of i n t r amolecu la r naphthyridine or s i m i l a r cyclic s t r u c t u r e s c h a r a c t e r i s t i c of hea t - t r ea t ed PAN.

For chemica l ly c r o s s - l i n k e d f ib res r e g a r d l e s s of the nature of the c ros s - l i nk ing agent the f i r s t m a x - imum is displaced to the side of higher t e m p e r a t u r e (270-320°C) then a second max imum appea r s at 380- 400°C. It follows that s t ruc tu ra l p r o c e s s e s a r e somehow involved here , which up to 270-320°C impede f u r - ther d i s a r r a n g e m e n t of the molecule . This imped iment may be c r o s s - l i n k a g e s , f o r m e d as the r e su l t of the p roces s ing of the f ib re s with c ros s - l i nk ing agents , while the posit ion and magnitude of the m a x i m u m may v a r y depending on the nature and stabi l i ty of the c r o s s - l i n k a g e s . The mos t s table c r o s s - l i n k a g e s a re f o r med , apparent ly , by the in terac t ion of PAN with th iocarbazide (see Fig. 1, curve 4).

The magni tudes of the m ax i m um on the IHD cu rves can s e r v e as a m e a s u r e of the number of c r o s s - l inkages in the sys t em. For example , chemical ana lys is data shows that PAN f ib res , c r o s s - l i n k e d with hy- draz ine hydra te at var ious t e m p e r a t u r e s , c o n t a i n d i f f e r e n t quantit ies of combined hydrazine hydra te . F r o m

30 30 /50 2"/0 270 330

Temp¢[ature,°C

Fig. 1. IH d i a g r a m of c r o s s - linked PAN sample s : 1) original ; 2) c r o s s - l i n k e d byhydraz ine hy- d ra te ; 3) c r o s s - l i n k e d by s e m i - ca rbaz ide ; 4) c r o s s - l i n k e d by th iosemica rbaz ide .

Fig. 2, it is seen that the m a x i m u m tension ama x of all the c r o s s - linked f ib res at 280-290°C is sharp ly d e c r e a s e d in compar i son with the tension of the original f ibre . The higher the reac t ion t e m p e r a - ture and, consequently, the more the c r o s s - l i n k a g e in the f ibre , the s m a l l e r the m a x i m u m of the tension.

I some t r i c heat ing d i ag rams show not only the qualitative p ic ture of the chemica l c r o s s - l i n k a g e s but a l so can be used fo r the d e t e r m i n a - tion of the number of c r o s s - l i n k a g e s (analogously to the de te rmina t ion of l inear or iented p o l y m e r s [4]). If the kinet ic na ture of the fo rce , originat ing at the s t re tch ing of the p o l y m e r s , is taken into cons ide ra - tion then in network p o l y m e r s it is poss ib le to e s t ima te the number of bonds by way of the fo rmula of the kinetic theory of e las t ic i ty [5].

* The second communica t ion on the subject "Invest igat ions of the s t ruc tu r i sa t ion p r o p e r t i e s of po lyacry lon i t r i l e f ib res n. t Tex = g/1000 m, N = 1000/rex.

(VNIIV) All-Union Scientific R e s e a r c h Inst i tute of Synthetic F ib re s . T rans la t ed f r o m Khimichesk ie Volokna, No. 2, pp. 21-23, March -Apr i l , 1969. Original a r t i c l e submit ted January 31, 1968.

147

30 90 tSg 2f0 270 330 390

Temperature,°C

F ig . 2. IH d i a g r a m of s a m p l e s of PAN f i b r e t r e a t e d with h y d r a z i n e h y d r a t e at v a r i o u s t e m p e r a t u r e s : 1) 130°0; 2) 140°C; 3) 150"0; 4) 160°C; 5) o r i g i n a l .

By i n v e s t i g a t i n g p o l a r p o l y m e r s wi th c h e m i c a l c r o s s - l i n k a g e s i t i s p o s s i b l e to e s t a b l i s h cond i t i ons at which the r e a c t i o n s to a s t r e t c h i n g f o r c e show only ne t s of c h e m i c a l c r o s s - l i n k s .

Tha t i s ob ta ined when the p o l y m e r i s found in a h igh ly e l a s t i c s t a t e . It i s p o s s i b l e to c o n s i d e r the t e m p e r a t u r e of the m a x i m u m on the i s o m e t r i c h e a t i n g d i a g r a m as the t r a n s i t i o n t e m p e r a t u r e in the b o n d s of h igh ly e l a s t i c s t a t e . The n u m b e r of m o l e c u l a r n e t w o r k

b o n d s N in uni t v o l u m e a r e found by m e a n s of the f o r m u l a :

,\,, :=: ~ m : L x f o r / , . /'1

w h e r e Xt i s the length f o r m e d a f t e r the a p p l i c a t i o n of a s t r e t c h i n g f o r c e b e f o r e IH, X2 i s the s a m e a f t e r e q u i l i b r i u m c o n t r a c t i o n at a t e m p e r a t u r e above the t e m p e r a t u r e of the m a x i m u m ; K i s the B o l t z - m a n n c o n s t a n t ; T i s the a b s o l u t e t e m p e r a t u r e ; e m a x i s the t ens ion at the m a x i m u m on the c u r v e of the d i a g r a m .

METHOD OF INVESTIGATION

The specimen is subjected to orientating stretching at a pressure of 400 kg/em 2, fixed with the clamps of the isometric heating apparatus and gradually heated to a temperature corresponding to the maxi- mum on the IH diagram. After reaching this, maximum tensionswere produced by the "equilibrium contrac- tion" of the specimen on gradually raising the temperature of the specimen over a 20-30 ° range.

An estimation of the number of fibre networks (cross-linked with hydrazine hydrate at 130, 150, and 160°C for 2 h), corresponding to the arbitrary designations weakly cross-linked, optimally cross-linked and strongly cross-linked, is presented. In order to distinguish the intermolecular network, formed by chemical bonding (cross-links), from the networks existing in the fibres as the result of the interweaving ('over-lash") of macromolecules, the properties of control fibres, heated at the same temperature but without cross-linking agents, were determined. The number of cross-links was determined by the difference in the n u m b e r of b o n d s in the m o l e c u l a r n e t w o r k s of the c r o s s - l i n k e d f i b r e s and the c o n t r o l s a m p l e s c a l c u - l a t e d b y the f o r m u l a quoted above (see T a b l e 1).

F r o m T a b l e 1 i t i s s e e n that wi th i n c r e a s e of the hea t i ng t e m p e r a t u r e of the c o n t r o l s p e c i m e n s the amoun t of i n t e r w e a v i n g f a l l s , s i n c e t h e r e i s an i n c r e a s e in the m o b i l i t y of the m a c r o m o l e e u l e s and h e n c e b e t t e r cond i t i ons f o r r e l a x a t i o n a r e e s t a b l i s h e d . On hea t i ng the c r o s s - l i n k e d f i b r e the o p - p o s i t e e f fec t i s o b s e r v e d - w i t h i n c r e a s i n g t e m p e r a t u r e n u m b e r of n e t w o r k s i n c r e a s e s at the e x p e n s e of c r o s s - l i n k a g e s . Th i s i s in good a g r e e m e n t wi th the p h y s i c o m e c h a n i c a l p r o p e r t i e s of c r o s s - l i n k e d f i b r e s (Tab le 2).

F r o m T a b l e 2 i t i s s e e n tha t wi th i n c r e a s e of h e a t i n g t e m p e r a t u r e the PAN f i b r e ob ta ined i n c r e a s e s i t s t h e r m o s t a b i l i t y , p a r t i c u l a r l y at 200°C. The i n i t i a l PAN at th i s t e m p e r a t u r e h a s p r a c t i c a l l y no s t a b i l i t y , that

T A B L E 1. N u m b e r of C r o s s - L i n k s in a Unit V o l u m e of D i f f e r e n t S a m p l e s of F i b r e ( P o l y - m e r )

Sample of PAN fibre

Control at 130"C . . . . . . . . . . . . Cross-linked at 130"C . . . . . . . . Control at 150°C . . . . . . . . . . . . Cross-linked at 150"C . . . . . . . . Control at 160"C . . . . . . . . . . . . Cross-linked at 160"C . . . . . . . . * For o0= 400 kg/em 2.

a* at 280°C, kg/em 2

148 194 128.5 225 108 263

1.15 1.11 1.14 1.12 1.16 1.12

No. (.102°) in 1 cm ~

of bonds in the molecular network

41.5 75.5 37.6 79.5 28.6 92.5

of cro~s-links

34.0

41.6

63.9

No. of cross-links in 100 monomer units

26

32

47

148

TABLE 2. Dependence of Physicomechanical Properties of PAN Fibre on the Temperature of Heating

PAN fibre

Original . . . . . . . . . . . . Processed with hydrazine

hydrate . . . . . . . . . . . The same . . . . . . . . . .

. . . . . . . . . .

• t t

N n , o . o . . , o . .

Temp.. °C

130 140 150 160

Physicomechanieal properties at temperatures, ° C

20

strength, km 1 breaking / leng,h___

45.5 I

extension,%

14.4

46.4 18,4 40.0 21,9 35.9 23.0 35.5 24,2 14.3 13.1

150 200

degree of re- tention of strength, %

56.6

54.4 60.6 64.5 85.5 93.5

degree ofre- tention of ex- tensivity, %

143

136 115 106 98 95.5

degree of re- degree of re- tention of tention ofex- strength, % tensivity,%

6.3 186

19.3 133 49.0 108 48.0 91 79.5 1o0 72.0 88

* At 100*C in concentrated H2SO 4 over a period of 30 rain.

Shrinkage *, %

Dissolves

Swells 70 46 36 14

hea ted at 130°C r e t a in s about 50% s tabi l i ty , anda t 150°C ~79%. The b r e a k i n g t e m p e r a t u r e of the f ib re a l so i n c r e a s e s with the i n c r e a s e of heat ing t e m p e r a t u r e . Thus the m o r e m o l e c u l a r bonds a r e f o r m e d in the f ib re the h ighe r i ts t h e r m a l s tab i l i ty .

1.

2.

3o

4.

5.

L I T E R A T U R E C I T E D

A. A. Konkin and G. I. K u d r y a v t s e v , Zhur . VKhO in D I. Mendeleeva, 1_1, No. 6, 637 (1966); A. B. P a k s h v e r , Khim. Volokna, No .4 , 54 (1967). S. Kama lov , B. E. Gol ' t s in , et al . , Khim. Volokna, No. 2, 16 (1967); G. I . K u d r y a v t s e v , T. A . R o m a n o v a , M, A. Zharkova , and V. S. Kl imenkov, Khim. Volokna, No. 5, 13 (1965}; L. G. S m o l ' n i k o v a a n d A. A. Konkin, Khim. Volokna, No. 2, 28 (1965). T. A. Romanova , M. A. Zharkova , G. I. Kud ryav t s ev , and V. S. Kl imenkov, Khim. Volokna, No. 5, 23 (1968). L. A. La ius and E.V. Kuvshinski i , F iz . Tverd . Te la , 5, 3113 (1963); M. V. Milagin, A. D. Gaba raeva , and N. I. Shishkin, F i z . Tverdo Tela , 6, 3636 (1964). L. A. Laius and E. V. Kuvshinski i , Mekhanika P o l i m e r o v , 2 , 1 6 3 (1966).

149