Investigation of cross-linking of polyacrylontrile fibres by an isometric method

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  • INVEST IGAT ION OF CROSS-L INK ING OF POLYACRYLONTRILE

    F IBRES BY AN ISOMETRIC METHOD*

    M.A . Zharkova , G I . Kudryavtsev , I . F . Khudoshev , and T . A. Romanova

    UDC 677.494.745.32

    It is possible to change the major mechanical and thermomechanical propert ies of f ibres by means of s t ructura l -chemica l modification [1, 2]. Modifications leading to the production of chemical ly cross- l inked f ibres acquire a special significance.

    Previously the chemical cross- l inking of PAN f ibres with hydrazine hydrate has been investigated [3]. The present work studies the variat ion of certain deformation propert ies of chemical ly cross- l inked PAN f ibres by the isometr ic heating method. As has been shown, it is possible to determine the number of bonds in unit vol- ume of achainus ing the Mequilibrium contraction n curves of f ibres in ahighlyelast ic state. Hydrazine hydrate, semicarbaz ide and thiosemicarbazide were used as cross- l ink ing agents. The oriented f ibres based on PAN of 14.3 tex (N 70)t and 28.6 rex (N 35) were subjected to cross- l inking; the c ross- l ink ingwas control led by an iso- metr ic method since chemical methods give ambiguous results, owing to adequacy of formation of in t ra - and supermolecular structure.

    The curves on the isometr ic heating diagram (IHD) for the original and chemical ly cross- l inked f ibres are fundamentally different (Fig.l). The original PAN f ibre has a character ist ic bimodal curve, f i rst a maximum which corresponds approximately to the transit ion temperature in the viscoelast ic state, then a downward side character is ing the gradual transition f rom the highly elastic state to plastic flow. The ap- pearance of a second maximum at 280-300C is probably associated with the formation of intramolecular naphthyridine or s imi lar cyclic st ructures character ist ic of heat- t reated PAN.

    For chemical ly cross- l inked f ibres regard less of the nature of the cross- l ink ing agent the f i rst max- imum is displaced to the side of higher temperature (270-320C) then a second maximum appears at 380- 400C. It follows that structural processes are somehow involved here, which up to 270-320C impede fur - ther d isarrangement of the molecule. This impediment may be cross- l inkages, formed as the result of the processing of the f ibres with cross- l inking agents, while the position and magnitude of the maximum may vary depending on the nature and stabil ity of the cross- l inkages. The most stable cross- l inkages are formed, apparently, by the interaction of PAN with thiocarbazide (see Fig. 1, curve 4).

    The magnitudes of the maximum on the IHD curves can serve as a measure of the number of c ross - l inkages in the system. For example, chemical analysis data shows that PAN f ibres, cross- l inked with hy- drazine hydrate at various temperatures ,conta ind i f ferent quantities of combined hydrazine hydrate. F rom

    30 30 /50 2"/0 270 330

    Temp[ature,C

    Fig. 1. IH d iagram of c ross - linked PAN samples: 1) original; 2) cross- l inked byhydrazine hy- drate; 3) cross- l inked by semi - carbazide; 4) cross- l inked by thiosemicarbazide.

    Fig. 2, it is seen that the maximum tension ama x of all the c ross - linked f ibres at 280-290C is sharply decreased in compar ison with the tension of the original f ibre. The higher the reaction tempera- ture and, consequently, the more the cross- l inkage in the f ibre, the smal ler the maximum of the tension.

    Isometr ic heating d iagrams show not only the qualitative picture of the chemical cross- l inkages but also can be used for the determina- tion of the number of cross- l inkages (analogously to the determination of l inear oriented polymers [4]). If the kinetic nature of the force, originating at the stretching of the po lymers , is taken into considera- tion then in network po lymers it is possible to est imate the number of bonds by way of the formula of the kinetic theory of elast ic ity [5].

    * The second communication on the subject "Investigations of the structur isat ion propert ies of polyacrylonitr i le f ibres n. t Tex = g/1000 m, N = 1000/rex.

    (VNIIV) All-Union Scientific Research Institute of Synthetic F ibres. Translated f rom Khimicheskie Volokna, No. 2, pp. 21-23, March-Apr i l , 1969. Original art ic le submitted January 31, 1968.

    147

  • 30 90 tSg 2f0 270 330 390

    Temperature,C

    F ig. 2. IH d iagram of samples of PAN f ibre t reated with hydraz ine hydrate at var ious temperatures : 1) 1300; 2) 140C; 3) 150"0; 4) 160C; 5) or ig ina l .

    By invest igat ing po lar po lymers with chemica l c ross - l inkages it is poss ib le to es tab l i sh condit ions at which the react ions to a s t re tch ing fo rce show only nets of chemica l c ross - l inks .

    That is obtained when the po lymer is found in a highly e las t i c s tate . It is poss ib le to cons ider the temperature of the max imum on the i sometr i c heat ing d iagram as the t rans i t ion temperature in the bonds of h ighly e las t i c s ta te . The number of mo lecu lar network bonds N in unit vo lume are found by means of the fo rmula :

    ,\,, :=: ~m:Lx fo r /, . /'1

    where Xt is the length fo rmed af ter the appl icat ion of a s t re tch ing fo rce be fore IH, X2 is the same a f te r equ i l ib r ium cont ract ion at a temperature above the temperature of the max imum; K is the Bo l tz - mann constant ; T is the absolute temperature ; emax is the tension at the max imum on the curve of the d iagram.

    METHOD OF INVEST IGAT ION

    The spec imen is subjected to orientating stretching at a pressure of 400 kg /em 2, fixed with the c lamps of the isometr ic heating apparatus and gradual ly heated to a temperature cor respond ing to the max i - mum on the IH d iagram. After reaching this, max imum tens ionswere produced by the "equi l ibr ium contrac- tion" of the spec imen on gradual ly raising the temperature of the spec imen over a 20-30 range.

    An est imation of the number of fibre networks (cross-l inked with hydraz ine hydrate at 130, 150, and 160C for 2 h), cor respond ing to the arbitrary designations weak ly cross-l inked, optimally cross- l inked and strongly cross-l inked, is presented. In order to distinguish the intermolecular network, fo rmed by chemica l bonding (cross-links), f rom the networks existing in the fibres as the result of the interweaving ('over-lash") of macromolecu les , the propert ies of control fibres, heated at the same temperature but without cross- l inking agents, were determined. The number of cross- l inks was determined by the difference in the number of bonds in the molecu lar networks of the c ross - l inked f ib res and the contro l samples ca lcu - la ted by the fo rmula quoted above (see Table 1).

    F rom Table 1 it is seen that with increase of the heat ing temperature of the contro l spec imens the amount of in terweav ing fa l l s , s ince there is an increase in the mobi l i ty of the macromoleeu les and hence bet ter condit ions for re laxat ion are es tab l i shed . On heat ing the c ross - l inked f ib re the op- pos i te effect is observed-w i th increas ing temperature number of networks increases at the expense of c ross - l inkages . This is in good agreement with the phys icomechan ica l p roper t ies of c ross - l inked f ib res (Table 2).

    F rom Table 2 i t is seen that with increase of heat ing temperature the PAN f ibre obtained increases i ts thermostab i l i ty , par t i cu la r ly at 200C. The in i t ia l PAN at this temperature has pract i ca l ly no s tab i l i ty , that

    TABLE 1. Number of Cross -L inks in a Unit Vo lume of Di f ferent Samples of F ib re (Po ly - mer )

    Sample of PAN fibre

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

    a* at 280C, 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

    heated at 130C retains about 50% stabil ity, andat 150C ~79%. The breaking temperature of the f ibre also inc reases with the increase of heating temperature . Thus the more molecu lar bonds are fo rmed in the f ibre the h igher its thermal stabi l i ty.

    1.

    2.

    3o

    4.

    5.

    L ITERATURE C ITED

    A. A. Konkin and G. I. Kudryavtsev, Zhur. VKhO in D I. Mendeleeva, 1_1, No. 6, 637 (1966); A. B. Pakshver , Khim. Volokna, No.4, 54 (1967). S. Kamalov, B. E. Gol 'ts in, et al., Khim. Volokna, No. 2, 16 (1967); G. I. Kudryavtsev, T. A .Romanova, M, A. Zharkova, and V. S. Kl imenkov, Khim. Volokna, No. 5, 13 (1965}; L. G. Smol 'n ikovaand A. A. Konkin, Khim. Volokna, No. 2, 28 (1965). T. A. Romanova, M. A. Zharkova, G. I. Kudryavtsev, and V. S. Kl imenkov, Khim. Volokna, No. 5, 23 (1968). L. A. Laius and E.V. Kuvshinski i , F iz . Tverd. Tela, 5, 3113 (1963); M. V. Milagin, A. D. Gabaraeva, and N. I. Shishkin, F iz . Tverdo Tela, 6, 3636 (1964). L. A. Laius and E. V. Kuvshinski i , Mekhanika Po l imerov, 2 ,163 (1966).

    149

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