Investigation of microphase separation occurring in interpenetrating polymeric networks prepared from polyurethane and styrene-divinylbenzene copolymer

1492

Y~. S. LIPATOV et al.

3. P. I. ZUBOV, D o k t ~ r s k a y a dissertatsiya (Doctorate Dissertation). N I P h C h I (L. Ya. K a r p o v Institute), Moscow, 1949 4. B. A. DOGADKIN and V. Ire. GUL', Dokl. A N SSSR 70: 1017, 1950 5. V. Ye. GUL', Kolloidn. zh. 15: 170, 1953 6. P. I. ZUB0V and M. Ts. ZBEREV, Kolloidn. zh. 18: 679, 1956 7. A. N. TIKHONO¥ and A. A. SAMARSKII, U r a v n e n i y a matematicheskoi fiziki (Mathematical Physics Equations). Izd. " N a u k a " , 1966 8. A. EINSHTEIN, Sbornik nauchnykh t r u d o v (Collection of Research Transactions). Izd. " N a u k a " , 1966 9. L. D. LANDAU and Ye. M. LIFSHITS, Mekhanika sploshnykh sred (Mechanics of Continua) State Press for Techn. a n d Theoret. Lit., 1954 10. A . A . ASKADS]KII, Deformatsiya polimerov (Deformation of Polymers). Izd. " K k i m i y a " , 1973

l~olymer Science U.S.S.R. Vol. 22, No. 6, pp. 1492-1500, 1980

0032-8950/80/061492-09507.50[0

Pflnted In Poland

© 1981 Pergamon Press Ltd.

INVESTIGATION OF MICROPHASE SEPARATION OCCURRING IN INTERPENETRATING POLYMERIC NETWORKS PREPARED FROM POLYURETHANE AND STYRENE-DIVINYLBENZENE COPOLYMER* YU. S. LIPATOV, V. V. Smmov, V. A.

BOGDANOVICH, L. V. KARABANOVA

and L. M. SERG~.Y~.VA High P o l y m e r Institute, U.S.S.R. Academy of Sciences

(Received 23 M a y 1979)

I n t e r p e n e t r a t i n g polymeric networks based on polyurethane and s t y r e n e divinylbenzene copolymer have been prepared. The heterogeneous structure of the networks has been investigated b y the methods of small- a n d wide-angle X - r a y scattering. The e x t e n t of heterogeneity domains has been calculated and the transitional layer thickness obtained, using the experimental data, and in addition the diffusivity of phase boundaries and the degree of segregation of the components have been determined as well as the molecular level of mi~ing of the components. I t is shown t h a t the degree of microphase separation of components in the interp e n e t r a t i n g polymeric networks depends on the amount of the second component in the system.

* V y s o k o m o l . s o y e d . A$2: N o . 6, 1359-1365, 1980.

:Interpenetrating polymeric networks

1493

THE results o f an i n v e s t i g a t i o n of t h e p h y s i c o - c h e m i c a l p r o p e r t i e s of interp e n e t r a t i n g p o l y m e r i c n e t w o r k s show t h e p r o p e r t i e s of the~se s y s t e m s to I)e n o n m o n o t o n i c f u n c t i o n s o f c o m p o s i t i o n [1, 2]. Since i n t e r p e n e t r a t i n g polymt~H,, n e t w o r k c o m p o n e n t s are usually t h e r m o d y n a m i c a l l y i n c o m p a t i h l e [3], t h e m a i n role in the d e v e l o p m e n t of a set of p r o p e r t i c s is playc(I b y p h e n o m e n a of for(.e,] c o m p a t i b i l i t y resulting f r o m kinetic factors o f n e t w o r k s t r u c t u r e ibrmati,,:~. C o n s e q u e n t l y , special c h a r a c t e r i s t i c s of t h e s t r u c t u r a l s t a t e of these s y s t e m s ~',~ e x c e p t i o n a l l y i m p o r t a n t f r o m t h e p o i n t of view of u n d e r s t a n d i n g a n d forecasting p h y s i c o - c h e m i c a l p r o p e r t i e s of i n t e r p e n e t r a t i n g p o l y m e r i c n e t ~ o r k s . I t was desired to e s t i m a t e t h e degree of h e t e r o g e n e i t y of i n t e r p e n e t r a t i n g p o l y m e r i c n e t w o r k s a n d to m o d i f y t h e l a t t e r b y c o m p o s i t i o n changes. U s i n g t h e m e t h o d o f small-angle X - r a y s c a t t e r i n g for n e t w o r k s b a s e d on polyureti~ane a n d p o l y u r e t h a n e a c r y l a t e [4], we o b t a i n e d i n f o r m a t i o n on the e x t e n t of heter,)g e n e i t y d o m a i n s a n d o n t h e t h i c k n e s s of t r a n s i t i o n a l layers. I n t h e p r e s e n t i n s t a n c e we carried o u t a detailed s t r u c t u r a l analysis of interp e n e t r a t i n g p o l y m e r i c n e t w o r k s based on p o l y u r e t h a n e a n d s t y r e n e - d i v i n y l b e n z ene c o p o l y m e r , using t h e m e t h o d s of small- a n d wide-angle X - r a y s c a t t e r i n g to o b t a i n d a t a n o t o n l y on the size of t h e h e t e r o g e n e i t y d o m a i n s , b u t also on p a r a m e t e r s c h a r a c t e r i z i n g m i c r o p h a s e s e p a r a t i o n , so t h a t it would be possible to e s t i m a t e t h e degree of m i e r o p h a s e s e p a r a t i o n o f n e t w o r k c o m p o n e n t s w}~i!c v a r y i n g t h e r a t i o of c o m p o n e n t s in t h e n e t w o r k s . Preparation of the interpenetrating polymeric networks started with the formation ,,f ~t polvllrethane network based on an adduct of trimethylolpropane and toluenediisocyanato and oligoethylcne glycol adipate with a molecular weight of 2000. A mixture of the initial components in methylene chloride solution was coated on a teflon support, vacuum dried to completely eliminate solvent and hardened for 48 hr at 80 °. The resulting network wa.q extracted in a Soxhlet apparatus to remove the soluble fraction. The extracted and dried polyurethane was subjected to swelling in a styrene-divinylbenzene (DVB) mixture (30o DVB) followed by hardening of the monomers at 80 ° in the presence of benzoyl peroxide. The copolymer concentration in the system was controlled by varying the swelling tim- fi,r polyurethane in the monomer mixture. Wide-angle diffraction patterns were obtained with a DRON-2.0 difraetometor (Cuanode, nickel filter, proportional detector, automatic step-by-step scatming). These diffraction curves were reduced to a single primary beam intensity and to :~ single thickness of specimens. Small-angle diffraction patterns were obtained in line with the procedure described in [4, 6], and were reduced to absolute units by the Kratky method [5]. The values obtained ,,r~ analyzing the small-angle diffraction patterns were as follows: the Pored parameters l~ and l~ [7]; the former characterizes the integral size of heterogeneity domains, and the latter, the average sizes of small domains; (l~/(1--p) is the size of the latter domains, and Ip/p the distance between their boundaries [7], while p is the volume fraction of the component). :In addition, the thickness of the intercomponent layer B, calculated on the basis of deviations from Porod's principle, was obtained [8, 9]. Quantities characterizing microphase separation were: the degree of segregation of components Ap'/A~¢~, where J ~ is the mean square fluctuation in electron density for a system with smooth phase borders of thickness B [10], Ap~=p(1--p) (pa_p,)a (p~ and p, l)eing electron densities of the components).

1494

Yu. S. LiPA~OV eta/.

There is also the diffusivity of phase borders 1--zl~¢/zt~ ='', where A~2'" is the m e a n square fluctuation in electron density for an idealized structure with clear-cut phase borders

[10]. Finally, there is the measure of the molecular level of mixing of components 1 - - , ~ " ]

/z~. Displayed in Fig. 1 are the wide-angle diffractograms of the individual networks and of interpenetrating polymeric networks based on the latter. A feature of the polyurethane diffractogram (curve 1) is the intense maximum at a scattering angle of 20.2 °. This maximum reflects the presence of short-range ordering of polyurethane network fragments. The diffractogram of the styrenedivinylbenzene copolymer (curve 6) is practically identical to scattering curves for amorphous polystyrene based on data in the literature [11]. The latter diffrac-

I,

imp

2o,0ool

~o,ooo

x

;'(

I

Zg

X'xw.x="

~

~X~x

I

z~o

I

20

FIG. 1. Wide-anglo diffractograms for polyurethane (PU) (1), and for the copolymer of ~ y r o n o (St) with divinylbonzone (DVB) (6), a n d for interpenetrating polymeric ne~works based on thoso polymers a n d cont~inin~ 3.6 (2), 3'6 (3), 31.4 (4) a n d 35"4~o eopolymer (5).

Interpenetrating polymeric networks

1495

t o , a m is characterized b y two intense maxima. The first of these is at an angle of 10% a n d is duo to intermolecular interference effects resulting from characteristic short-range ordering of the macromolecules. The second m a x i m u , n is at an ~ngle of 19.2 ° a n d is due to intramolecular illterference [11] (periodicity in the po.~itioning of rings). Utilizing the position characteristic of the first m a x i m u m for the copolymcr ,,ne m a y analyze features of diffraction curves for the interpenetrating polymeric n(~tworks (curves 2-5). On the curves corresponding to a large concentn~tion ~. e 2. mo/eZ/'cm z

I% x

102u

A

3 1

10

z~ 5

2

5 10 X,mn7 FIG. 2. Low-anglo scattering curves for polyurethane (1), and for the styrcno--divinylbenzeno e..opolymer (6), and for interpenetrating polymeric networks containing 3.6 (2), 8.6 (3), 31.1 (4) and 35"4~o copolymer (5).

1496

Yu. S. LIpATOV e$ a/.

of the second component (curves 4, 5) there is the first maximum for the copolymer and a m a x i m u m for the polyurethane. This is direct evidence t h a t in the i~lterpenetrating polymeric networks there are domains having the structures of the pure components. The maximum characteristic of the copolymer is not directly in evidence in diffractograms of the interpenetrating polymeric networks containing small additions of the second network (curves 2 and 3). How* ever, in the vicinity of 10° the shape of the diffraction curves for the latter specimens differs slightly from that observed for the pure polyurethane. Figure 2 shows the small-angle scattering curves for the individual networks and interpenetrating polymeric networks. I t can be seen that a minimal degree of scattering is typical for the pure components (curves 1 and 6). This is normal for all amorphous single-component polymers [12]. Starting at minimal penetrating network concentrations, the emergence of interpenetrating polymeric networks leads to increased low-angle scattering intensity compared with the pure components. I t is known that this phenomenon can be accounted for solely through the presence in the polymer volume of microregions differing in respect to their electron density. This means that low-angle scattering curves for interpenetrating polymeric networks, being a sensitive test of the heterogeneity of a system, evidence the onset of microphase separation even in the case of small additions of the penetrating network. I t can be seen from Fig. 2 that as the concentration of the second component (the copolymer)increases, the low-angle scattering intensity is gradually increased. This is attributable to two factors: to a higher degree of phase separation in the system, or to an increased concentration of the second component. A quantitative evaluation and separation of these two factors m a y be carried out in the light of the results of an analysis of the low-angle diffractograms. The analysis was carried out in line with the procedure which Bonart developed [10] to characterize micro separation in segmentized polyurethanes. Since there are no real differences from the point of view of structural heterogeneity between interpenetrating polymeric networks and segmen'tized polyurethanes, we used the method of Bonart as a means of analyzing the low-angl e data for the networks under study. The results obtained in this way for the lowangle diffractograms are given in the Table and in Figs. 3, 4. It can be seen from the Table t h a t the values of A~~' and zt~z'' increase as the amount of the second network increases. This reflects a general tendency for the heterogeneity of a system to increase as the composition range approaches middle values. A marked contrast in the foregoing characteristics is observed in the case of small additions of the copolymer. Since ~ z ' and zl~2'' are associated with the occurrence of smooth and abrupt density changes at inteffacial boundaries, it m a y be said that if the copolymer addition is small, the amount of the latter component in transitional layer compositions will be relatively large. In contradistinction for small additions, it is seen that for interpenetrating polymeric networks with a copolymer concentration exceeding 30~/o there is a similarity

Interpenetrating polymeric networks

1497

between the d~ ~' and d~ ~'' values. This means that the amount of eopolymer in transitional layers is quite small in the case of polymeric network compositions containing more than 30 % of the copolymer. Figure 3 shows the concentration dependence of quantitative characteristics of the degree of microphase separation. It can be seen from curve 1 that the degree of segreg~dion of the components is m~rkc(lly increased on going from small additions to medium concentrations of coI)olymers. The diffusivitv of the phase boundaries and the (legree of mixing of the ~'omponents arc re(tuoe(I as the ~mount of the second network is increased (curves 2 and .3).

I-0 -(~)

(z)

U) , Lp Lp

Lc , T-, l_--p-, 3#0 ff~) (Z) (3) 0.8

,5

0.6

7'

220 0.~ 100

0.2

I0

30

so

8t-DVB, wt%

FIG. 3

S f - D V B 3 ~4/.~ ox ,o

FIG. 4

F i e . 3. Concentration dependences of the degree of segregation of component,~ in interpenetrating polymeric networks (1), and for the diffusivity of phase bomldaries 1 --Jp~'/A~ ~'" (2) and for the degree of mixing of components 1--A~"/,dp~ (3). Fro. 4. Concentration dependences of integral dimensions of heterogeneity doma.ins l~ (1), for distances between the domain botmdaries l~/p (2) and for dimensions of the smallest heterogeneity domains l~/( l - - p ) (3).

Figure 4 shows the concentration dependence of heterogeneous structural dimensional parameters lc, lv[p, lv/(1--p). In the range of small additions of copolymer Ic (curve 1) and l v / ( 1 - - p ) ( c u r v e 3) are characterized by major differences, b u t the difference between their values lessens as the copolymer concentration increases. Taking these differences as a qualitative indicator of polydispersity according to the size of phase particles, one m a y speak of a narrowing

1498

Yu. S. LIPATOVet ed.

of size distribution of the particles as the middle range of compositions is approached. On increasing the copolymer concentration it is seen that the distance between heterogeneity domain boundaries (curve 2) is reduced. With the aid of heterogeneous structure characteristics obtained b y expeririment, a study may be made of possible models of the structural state of interpenetrating polymeric networks, while varying the penetrating network content. On comparing scattering curves in the range of low angles for the pure polyurethane and for the interpenetrating polymeric network containing 3.6% copolymer, it appears that. contres of heterogeneous structure appear in the latter network. The nature of the heterogeneity domains m a y b~ associated with th e onset of eopolymer separation to form a separate phase. C H A R A C T E R I S T I C S OF T H E HETEROGE~N-EOUS S T R U C T U R E OF I N T E R P E N E T R A T I N G . P O L Y M E R I C .N-ET~'ORKS B A S E D O1~ T H E .POLYURETHA-N-E A N D T H E S T Y R E N E - D V B

Amount of copolymer, % 3.6 8.6 31.4 35.4

o'. mole s crfl s

0.307 × 10-5 0.576 × 10-~ 0.664 × 10 - s 0.767 × 10-s

A~a,, os" moloZ cm 6 0.111 × 10-s 0.179 × 10-s 0.735 × 10-s 0.781 × 10-3

lp, A

20 24 72 80

COPOLYMER

B,A 37 30 23 21

On the basis of the experimental data it can be said that the size of inclusions of this type is of the order of 20 A, with a distance of approximately 270 A between their boundaries (Fig. 4). In view of the considerable thickness of the transitional layer observed for the specimen under study (37 A; see Table), it appears that b y far the largest part of the copolymer goes to make up the composition of the transitional layer which is formed at the inteffacial boundary. Apparently what we are observing in the case under consideration is b u t the initial stage of phase separation in the system. The marked diifusivity of the phase boundaries (0.97), the slight degree of segregation of the components (0.013) and the relatively large proportion of "dissolved" macromolecules (0.538) support a n d corroborate this pattern. I t is noteworthy that a major role is played b y the transitional layer in phase separation of the interpenetrating polymeric network containing 3-6 % copolymer. According to present-day thinkiug the transitional layer is an interracial layer containing a variable concentration of the components [1]. These layers surround domains of relatively constant density, i.e. of constant composition. The quant i t y known as the level of mixing of components is based on an evaluation of differences between densities in these domains and in the matrices. The latter measure in the case of the interpenetrating network containing 3.6~/o copolymer shows t h a t second network inclusions formed during polymerization do not, however, constitute a pure phase, containing as they do a large amount of incorporated polyurethane fragments.

Interpenetrating polymeric networks

1~99

It is interesting to compare the va.lues of lp/p, l~/(1--p) and Ic tbr the interpenetrating network containing 3.6o/o copolymer. As may. be seen fl'om Fig. 4. the latter quantity, being an integral measure, is much larger than lp/(l--p). The fact that relatively extensive heterogeneity domains (of the order of sev(:ral hundred angstroms) appear a.long with the smaller oncs appears to corroborate the fact that the system is now at the very start of a I)hase separation process, whi(.'h is characterized by considerable tluctnations in the size of copolymer in(.lusi,),,..,. The heterogeneous structm'e of the interpenetrating networks containing large amounts of the penetrating network (the copolymer) has characteristics differing markedly from those of the nctworks containing small amounts. This is apparent, in the first place, from the widc-angle diffractograms (Fig. ]) co:~raining maxima for amorphous structure of the copolymer. Secondly, further evi(tencc appears at, a qualitative level in the shape of the low-angle s(.atlerin~ curves. The ('urve for the intcri)enetrating networks with mnall additions oI" the second network (Fig. 2, curv(~ 2) features a marked reduction in the (~as,- of low scattering anglcs, whereas a. smoother reduction in intensity appears in this range of ~tngles for the curvcs for medium compositions. This mcans that maximum dimensions of the heterogeneity domains are reduced in the case (mdcr (.(msideration. I t can bc seen (Fig. 4) that for the maximum copolymer concentration (35.4O/o) the size of heterogeneity domains is stabilized within the limits of 120-190 A. Apparently the system represents, in this composition range, a relatively uniform distributionof practically pure copolymer inclusions in a t,olyurcthane matrix. Both the matrix and the inclusions contaill very small amounts of dissolved copolymer, phase boundaries between I)olyurethane and copolymer are sufficiently clear-cut, i.e. in many respects polymeric networks with a coml,osition of this type are like highly filled systems based on clastomers and rigid fillers. However, the main distinctive tbature of interpenetrating polymevi(networks is a high degree of dispersity that is practically unol)tainable in fill~,d systems. The network containing 8.6~o copolymer is a transitional typc system. A(.e,,r(ling to the thermo(13manfic data in [l] the enthalpy of mixing beeome~ i,:ss negative for the networks at or around this comt)osition region; afterwards, with a 30~o eopolymer conecntrat.ion it goes over to positive values, cvidcncinu thermodynami(" incompatibility of the eomt)onents. At the same time the integral size of heterogeneity domains is redu('ed (130 ~) compared wi~h the interl)enf> trating networks having small additions of the copolymer, although the iucr~.J.se. i,1 lp/(1--p) is negligible (26 .~_). The degree of segregation of (~onlponents in ghe polymeric networks rises sharply in the ease of lhe latter corn position, although the degree of mixing is reduced less abruptly. It appears that with this amountof eopolymer the sizes of the inclusions havc not as yet. become stabilized, and particular regions of the latter contain a large amount of "dissolved" polyurethane fragmeItts.

1500

Yu'. S. LIPATOVe$ aZ.

T h u s t h e e x p e r i m e n t a l results show t h a t t h e h e t e r o g e n e o u s s t r u c t u r e o f t h e i n t e r p e n e t r a t i n g p o l y m e r i c n e t w o r k s v a r i e s according to the r a t i o o f the c o m p o nents. I f t h e a m o u n t o f s t y r e n e - D V B c o p o l y m e r a d d e d is small, p r a c t i c a l l y all o f t h e c o p o l y m e r will be in t r a n s i t i o n a l layers; as t h e a m o u n t of c o p o l y m e r is increased, s e p a r a t i o n of c o p o l y m e r p h a s e f o r m a t i o n s s u r r o u n d e d b y a t r a n s i t i o n a l l a y e r t a k e s place. I t is i n t e r e s t i n g to find t h a t t h e t r a n s i t i o n a l l a y e r t h i c k n e s s is a t t h e s a m e t i m e reduced, a l t h o u g h t h e overall size o f t h e t r a n s i t i o n a l region increases for t h e i n t e r p e n e t r a t i n g p o l y m e r i c n e t w o r k as t h e r a t i o of c o m p o n e n t s in t h e l a t t e r rises [1]. All this m e a n s t h a t t h e s e p a r a t i o n of c o m p o n e n t s d u r i n g t h e f o r m a t i o n o f i n t e r p e n e t r a t i n g n e t w o r k s t a k e s place largely on a p p r o a c h i n g a m e d i u m r a n g e of compositions. This is p r o b a b l y a t t r i b u t a b l e to different r a t e s o f f o r m a t i o n o f c o n s t i t u e n t n e t w o r k s a c c o m p a n y i n g differences in t h e r a t i o s o f t h e ilfitial c o m p o n e n t s .

Translated by R. J. A. HE~DRY REFERENCES 1. Yu. S. LIPATOV, A. Ye. NESTEROV, L. M. SERGEYEVA, L. V. KABABANOVA and T. D. IGNATOVA, Dokl. AN SSSR 220: 637, 1975 2. Yu. S. LIPATOV, T. S. KHRAMOVA, L. M. SERGEYEVA and L. V. ~ A N O V A , Dokl. AN SSSR 226: 1360, 1976 3. Yu. S. LIPATOV, L. V. KARABANOVA, T. S. KHRAMOVA and L. M. SERGEYEVA, Vysokomol. soyed. A20~ 46, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 1, 51, 1978) 4. Yu. S. LIPATOV, V. V. SHILOV, L. V. KARABANOVA and L. M. SERGEYEVA, Vysokotool. soyed. A20: 643, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 3, 725, 1978) 5. J. PILZ and O. KRATKY, J. Colloid Interface Sci. 24: 211, 1967 6. T. Ye. LIPATOVA, V. V. SHILOV, N. P. BASILEVSKAYA and Yu. S. LIPATOV, Brit. Polymer J. 9: 159, 1977 7. R. PERRET and W. RULAND, J. Appl. Crystallogr. 5: 183, 1972 8. W. R ~ , J. Appl. Crystallogr. 4: 70, 1971 9. C. VONK, J. Appl. Crystallogr. 6: 81, 1973 I0. R. BONART and E. H. MULLER, J. Macromolec. Sci. BI0: 177, 1974 11. H.-G. KILIAN and K. BONEKE, J. Polymer Sci. 58: 311, 1962 12. J. H. WENDORF and E. W. FISHER, Kolloid-Z. trod Z. fdr Polymere 251: 884, 1973