Features of the behaviour of polyelectrolytes in concentrated salt solutions

0032-3950/89 $10.00+.00 © 1990 Pergamon Press plc

Polymer Science U.S.S.R. Vol. 31, No. 10, pp. 2217-2225, 1989 Printed in Poland

FEATURES OF THE BEHAVIOUR OF POLYELECTROLYTES IN CONCENTRATED SALT SOLUTIONS* V. YA. KABO, L. A. ]'TSKOVICHand V. P. BUDTOV State Institute for Design and Research Work in the Oil Industry (Received

18

March

1988)

Investigating phase separation and [q] in water-salt solutions of copolymers of sodium acrylamide and acrylate the authors explore aspects of the behaviour of the polyelectrolytes in concentrated salt solutions. A theoretical model is proposed qualitatively reflecting the character of the phase diagram of a polyelectrolyte and the dependence of [r/] on the salt content. IT IS known that the intrinsic viscosity of flexible chain polyelectrolytes (PELs) is well described in a wide concentration region of the salt c s by the linear function [r/],-,c] °'5. This is confi.rmed by a number of experimental studies and has found a theoretical explanation [1-5]. However, in references [6-9] using different PEL-solvent systems it is shown that in the region es>0.5 mote/l. (the concentration region of the salt depends on the specific PEL-solvent system) the dependence of [r/] on es passes through a minimum or is described by two branches, descending (cs<0.5 mole/l.) and ascending (c,>0.5 mole/1.) because of phase separation in the PEL solution occurring in the intermediate region of c s. The dependence of such a form was obtained in reference [10] during measurement by the method of light scatter of the dimensions of poly-l-(-hydroxyethyl)pyridinium benzosulphate methacrylate in aqueous KSCN solution. Common to all the cases considered is that the anomalous character of the behaviour of PE in salt solution is manifest at swelling coefficients of the macromolecular coil close to values characteristic of non-ionogenic polymers, ~ a <3. Increase in swelling of the macromolecular coils at cs>0.5 mole/l, is connected with change ia the thermodynamic quality of the solvent. This is indicated by the character of the phase separations-passage from a biphasic to a monophasic system [6, 7], change in the direction of modification of the value of the second virial coefficient-from fall to increase [10], the same for the magnitude a (exponent in the Mark-Kuhn-FIouwinck equation)-passage from increase to fall [11]. All this occm-s in PEL solutions in the region of high salt contents. Although this aspect was most obviously demonstrated by an experiment on different systems no attempt has been made at consistent study and interpretation of it. In the present work investigating phase separation and measuring [r/] of solutions of the * Vysokomol. soyed. A31: No. 10, 2019-2025, 1989. 2217

2218

v. YA. KABoet al.

copolymers of sodium acrylamide and acrylate (I) an attempt is made to analyze the patterns of swelling of the PEL molecules taking into account the extremal dependence of swelling on cs. We investigated laboratory samples* of solutions of I with a content of carboxyi groups = 0- I--0-5.The samples of I were obtained by alkaline hydrolysis of polyacrylamide with M.= 0. 6 x x 106. The content of the carboxyl groups in the samples of the solutions of I were determined by potentiometric titration.'Purification of the solutions of I and conversion of the polymer to the H form were undertaken on the KU-2-8 and AB-18 ion-exchange resins. The salt form of I was obtained by adding stoichiometric amounts of alkali. We used Ca(NOa)z of chemically pure grade. The measurements of [it] were made with the Viscomatic MS automatic capillary viscometer (France). The pH value of the polymer solution was before measurement adjusted with concentrated alkali to 9.0 corresponding to the completely ionized state of the carboxyl groups of the polymer. The viscometer was protected from atmospheric COz with soda lime which ensured ionization equilibrium in the PEL solution during measurement of [r/]. This is important in investigating the conformation of PEL which is sensitive to the state of the ionogenic groups. The magnitude ~r¢l lay within the range 1.2-1.8 allowing us to use the Huggins equation ~-~o - [v/]+ K1 c [t/] ~, r/o c where K~ is the Huggins constant. It is known [12] that KI for water-salt solutions of I is a function of the quality of the solvent and in the region of swelling coefficients a~ < 3 rises with worsening of the quality of the solvent but at ~ > 3 stays constant and equal to ~0.30. r/- ~/o In all cases the function -vs. c was linear. For the sample with ~=0. 52 in Ca(NO3)2 solur/o c

tions [rl] was calculated in line with the approach described in reference [13] from the equations [ttl =lnt/r*i

(at c,= ~0.0470.06 and 0.5-1 g-equiv./l.

C

1 /In t/,,l

x/2 /

[t/l=-~-~--~ +--~-~/t/,e1-1-Int/,®l)

(at c,= 1-4 g-equiv./I.)

Phase separation was investigated by determining the clouding temperature Tm of the polyme solution from the moment of sharp rise in the value of scattered light. The measurements were taken with the Specol ZV spectrocolorimeter with the Ti attachment, permitting measurement of the light scattered at an angle 90°, and supplemented with a device for heating the solution in the cuvette. As shown in reference [14] the system I solution-H20-salt possesses a LCTC. Figure 1 presents the dependences T,,--concentration o f polymer c. It will be seen that two concentration regions o f the salt exist. In one increase in the content of electrolyte shifts the curves Tin(c) to the region of lower temperatures and in the other to the region of higher ones. Thus, at a fixed temperature, increase in c, successively carries the system from monophasic to biphasic and again to monophasic. An illustration o f this is provided by Fig. 2. In all eases phase separation occurs with rise in temperature, i.e. the system possesses a LCTC. * The authors wish to thank V. A. Maslennikov for synthesizing and analyzing the polymer samples.

Behaviour of polyelectrolytes in concentrated salt solutions

2219

In connection with the feature of phase separation noted we investigated the function It/] (cs). Earlier this function, without reaching the region of high concentrations of the salt, was investigated in particular in solutions of I in presence of Ca(NO3)2 [15]. It was shown that the function It/] (cs) is satisfactorily described by a straight line in the coordinates [r/] : c~"°'s as predicted by theory.

330

320

o z_

31o

~

~

5

o

L_

290

'

;a t

0.05

~'

~

I

~ 2601 O.lO

I

J

0-05

0.;'0

c ,.q/d/

FIG. 1. Phase diagrams for solution of I (~=0.5) in the solvent water-Ca(NOa)2. Content of Ca(NO3)2 1.7 (1), 1.3 (2), 0.9 (3), 0.5 (4), 0.015 (5), 0.03 (6) and 0.06 (7) g-equiv./l. Arrow indicates direction of increase in salt concentration. The experimental data for It/] (cs) in the concentration region of the salts used in the experiment on phase sepal ation are given in Fig. 3. It will be seen that in the region of large es values a minimum is observed in the curves [r/] (c7°'5). For the polymer with ~=0.5 in the region c~=0.1-0.5 g-equiv./1, the system is biphasic and the function It/] (c~) is represented by two branches. At lower salt concentrations [r/]~c~-" and for higher ones [q]~ e~' (n, n' are empirical exponents). Figttre 4 presents the data on the dependence of the Hug,gins constant K1 on c,. It will be seen that the function K~(c~) is opposite to the function It/] (c,) and is described by a curve with a maximum or by two oppositely directed branches (in the case ~=0.5). &J, db(9 35J !

3 0

2

7 J

-2

0 lo9 cs

FIG. 2

I

2

I

¢, c J . s

FIo. 3

FIo. 2. Region of existence of monophasic (1) and biphasic (2) systems polymer I (a = 0-5)-solvent (aqueous solution of Ca(NO3)2). Fio. 3. Dependence of [t/] on concentration of Ca(NO3)z (25°C); pH 9. a = 0 . 1 (1), 0-2 (2), 0. 3 (3), 0.4 (4) and 0-5 (5).

V. YA. KABOet at.

2220

The position of the maximum in the curve Kl(cs) coincides with the position of the minimum in the curve [q] (cs). All the data on phase separation, the dependence of [q] and K~ on cs for solutions of I (Figs. 1--4)and the published data for different systems and also the data on the dependence of the second virial coefficient ,42 and the magnitude a on G show that increase

tft O'L 0,4 ~ 0,2 I

0.2 6'.#"

o

0.2~'~-oO~O4 o.# 0.2 -2

5

I

~ 0 0

I 0

lo9 cs ,9-equiv./l FIG. 4

Fro. 5

FIG. 4. Dependence of the Huggins constant for solutions of I on the concentration of Ca(NO3)2 at 25°C. 6=0.4 (1), 0.3 (2), 0.2 (3), 0.1 (4) and 0 (5). Fi6. 5. Dependence oft, on es. Curve 1 is calculated from relation (9); curves 2 and 3 are the second and third terms of relation (9); region of biphasic state is shaded. in the salt concentration to high values in the PEL solution changes the direction of the action of the salt and further increase in the salt content improves the quality of the solvent and, as a result, leads to swelling of the macromolecular coil (rise in [r/I). As is known the dependence of [~/] on the quality of the solvent may be described by using the modified Stockmayer-Fixman equation [16]

4#where K 0 = ~

'

; ~ is the Flory constant; h~ is the square of the unperturbed dimen-

sions of the macromolecule; Z is a parameter of the excluded volume theory. The magnitude Z is connected with the reduced excluded volume e [17]:

Z= -crux/Me

(2)

Behaviour of polyelectrolytes in concentrated salt solutions

(3

Here, cm= \ ~ ]

2221

M3/2

V1 NA ho3 ' where v is the specific partial volume; 1,"1 is the molar

volume of the solvent. In the simplest case for solutions of non-electrolytes ~--2)~-1 =a 1 - -f-+ 1

, wherexis the thermodynamic Flory-Huggins parameter; at is a con-

stant; 02 is the Flory 0 temperature close to the LCTC. A similar dependence of e on T is characteristic of the LCTC and for the HCTC ~=~ul(O1/T- 1) [16]. For polyelectrolyte solutions for the magnitude Z we have [4] A

Z=Zo + Ze=Zo +-==,

(3)

where Zo is the magnitude Z for c, ~ co (polyelectrolyte effects suppressed); A is a value very weakly depending on c, [4]. As may be seen from the comparisons of eqn. (1) and (3) and the experimenta dependences of [t/] on e~ the theoretical dependences do not describe the experimenta data. At the same time the data on the phase separation of the system with variation in c (Fig. 1) and the dependences of [r/] and KI on cs (Figs. 3 and 4) iesemble the experimenta data obtained with variation in the quality of the solvent close to the conditions of phase separation. The phase separation theory in PEL solutions has not been developed. Below we give a qualitative picture of the possible influence of Cs or~ phase separation in PEL solutions based on analogy with phase separation in polymer solutions (non-electrolytes) in a mixture of two liquids [18]. In tact the PEL-water-salt system may be regarded as a three-component system: a polymer in a mixture of low molar mass liquids, namely, in the mixture water+ concentrated salt ~olution. Let us apply the results for such systems to the case discussed. We shall consider the calculation of the spinodal for a solution of a monodisperse polymer in a solvent (one may also likewise calculate the binodal although for the spinodal analytical expressions may be obtained). The equation for the spinodal ~0~has the form (p~ 1 --+ -s=0, (4) 1 - % m ~0~ where m is the relative MM. The s value at the critical point is equal to e* =z/x/m. In the case e>e* the system may be in the biphasic state and for e
in the monophasic state [19]. Since m>>l then ~0~ \ / m <<1 and relation (4) is simplified. Using formulae (2), (3) and (4) we get 1

Z

. . . . . .+. .c,,x/ .. m=0 %+m~o

(5)

At the critical point for the usual polymers Z* ~ - 2. It will be seen that with fall in c

2222

V. YA. KABOet aL

the magnitude Z may change from Z < 0 to Z > 0 and so on which leads to increase in the solubility of the macromolecules and increase in Jr/]. Thus, the usual (with replacement of Z by Zo + Ze) extension of the theory of phase separation in polymei solutions does not lead to phase separation in PEL solutions and does not give an explanation of the data in Fig. 3. Let us consider a solution of a low molar mass salt as a mixture of two liquids: water and a concentrated salt solution, for example, with the concentration c2. The interaction with the polymer of these liquids is characterized by ~x and e2. Then the theory of phase separation of the polymer solution in a mixture of two liquids may be applied. The equation for the spinodal is described by the relation (5), the magnitude ec being given by the expression [18] Fztsz 2 ] ~'c = 81 + X2 Z~8 "~ X 1 X 2 L--~-- "[- ~1 2 -- 2X12 (81 + t2) (6)

where x2 is the fraction of the second liquid in the mixture (x~ +x2 = 1); Ztz is the interaction parameter of the first and second liquids. For our case (solution of low molecular weight salt) Z~z ~<0. For solution of I both water and the concentrated salt solutions are good solvents, i.e. e~ "82
,

(7)

where the constant b may be calculated trom relation (6) although we shall regard it as an empirical positive parameter ( b ~ 2~2--2x12~. \ c2 / Taking all this into account the equation for the spinodal of the PEL solution will have the form 1 Zo+Z+ { ~ + - - ÷ - - - ~ - - - c + / 1 - c" / b = 0 m ¢ c,~/M \ c2 }

(8)

The relation obtained describes (at least qualitatively) all the experimental dependences presented here. From the relations (3) and (8) it is easy to obtain the dependence of ep on c, for the PEL solution

c.4 4c,

<9)

Figure 5 qualitatively shows the character of the dependence tp and its components on c, (taking 81 =0). Having regard to the relations (1), (3) and (9) for Jr/] we get [~] - K o 4- 1 "06 Z o - l - - - A r:-cmbc ~M-[ ~/c,

c+ ( -~-2)1

s 1

(10)

Behaviour of polyelectrolytes in concentrated salt solutions

2223

Qualitatively the dependence of It/] on cs obtained from relation (10) describes the data in Figs. 1 and 4. In fact, the usual term (~c~ -°'s) leads to fall in [r/] with rise in Cm (suppression of PEL swelling), the last term leads to fall in (c,0.5 e2) of the value [r/]. The phase diagram in the coordinates T , , - cs may be obtained from the relation

~,

2

-,/;-a,,,

(

02_l']+befl_Cs" ]

:

A

,,

(ll)

Here it was assumed that the second and third teIms do not depend on T a n d the first has a very simple form: al is a numerical coefficient.

I [+Y":

,a4

-, 1

~

3

6 , 7-/'-1-- -~.~.1) . ~i0

[-t

I

:

q

12

(--~-)"10~

FIG. 6. Temperature dependence of [v/]: a - i n terms of the relation (13); 1, 3-1eft; 2, 4-right branches of the function Tm (c,) in Fig. 2; b-modified coordinates of eqn. (13) for the case 02 = Tm= T® for M~ or. Let us introduce Too= T,, for Cs= c2(Ze(c2),~ 0). Then from eqn. (11) follows

b

11 -

Zm

+

Too

( c,

alO 2

c,'~ 1 - - -

A -

_

~

,

(12)

O:

e2J

i.e. the dependence of Tm on cs is described by a non-symmetrical curve with a minimum. Thus, relation (12) qualitatively describes the data in Fig. 2. Substituting the expression (12) in formula (10) we obtain [7] _Ko+l.O6Koax

-i

(13)

Figure 6 presents the dependence of [r/] on T- 1 _ T~, 1 for solutions of I in Ca(NO3)2 for the left and right branches of Fig. 2. It will be seen that in line with relation (13) the experimental data are generalized by a single dependence for all salt concentrations and compositions of the copolymer. For our case (very large M,,, 106) relation (13) may be simplified. In fact, 02T,,," Too

for M -~ m. Then [q](e,)~(T~(TC')- l ) . After constructing the corresponding dependence we also obtain a single straight line for solutions in different solvents (Fig. 6). From the fact that extreme changes in It/] with rise in e, are observed in different

2224

V. YA. KABOet al.

systems it may be asserted that the model phenomenological approach considered correctly conveys the general tendencies in the behaviour of the water-salt-PEL solutions. However, it should be noted that the PELs studied in this work are copolymers containing not only ionogenic C O O - groups but also hydrophilic amide CONH2 groups. Therefore, the molecalar interpretation of the effects discussed here (because of the local interactions of the components of the system) is complex. In fact, in the region of low cs (cs < 0.1 mole/l.) change in the configuration o f the coil is associated with change in the electrostatic potential of the charged macromolecule. The behaviour of the solution of I in this region was studied in greater detail in reference [20] where it was shown that ~ i s a linear function of c7°'5i%. Here i is the charge density; e is the dielectric constant; n is an empirical constant (n < 1) and depends on the type of counter-ion. In reference [14] it is shown that the loss Of solubility o f sample I in the interval 0.1 < c,<0,5 is connected with dehydration of the chain as a result of binding of the ions o f the alkaline earth metals to the carboxylate groups. This effect (loss of solubility) is most clearly observed for a 40 mole % content of the carboxylate groups in the chain which is determined by the specific interaction of the amide groups. The features of the manifestation of the solvation of the amide groups of polyacrylamide was studied in reference [21]. With rise in the concentration of the salt of alkaline earth metals, swelling of the coils is observed which is determined by the binding of the metal cations to the amide groups. Thus, in the region of high salt concentrations the local interactions of the salts with the amide groups raise the solubility. Translated by A. CROZY REFERENCES

1. I. NODA, T. TSUGE and M. NAGASAWA, J. Phys. Chem. 74: 710, 1970 2. R. YEH and A. TSIHARA, J. Polymer Sci. A-2: 9, 373, 1971 3. H. CHION and A. TSIHARA, J. Polymer Sci. Polymer Phys. Ed. 4: 1015, 1976 4. M. FIXMAN and I. SKOLNICK, Macromolecules 11: 863, 1978 5. N. IMAI, Rpts. Progr. Polymer Phys. Japan 23: 95, 1980 6. I. KAGAMI and R. M. FUOSS, J. Polymer Sci. 18: 535, 1955 7. G. MULLER, J. P. JAINE and J. C. FENYO, J. Polymer Sci. Polymer Chem. Ed. 17: 659, 1979 8. K. J. LINOW, J. HOLRAPIEL and B. PHILIPP, Acta Polymerica 33: 616, 1982 9. Ye. A. BYEKTUROV, S. KUDAIBYERGENOV and R. E. KHAMZAMULINA, Kationnye polimery (Cationic Polymers). p. 45, Alma Ata, 1986 10. J. STEISKAL, M. I. BENES and P. KRATOCHVIL, J. Polymer Sci. Polymer Phys. Ed. 12: 1941, 1974 11. T. DZHUMADILOV, Trud. Nauch. konf. molodykh uchenykh inst. khim. nauk Akad. Nauk KazSSR (Proceedings of the Scientific Conference of Young Scientists of the Institute of Chemical Sciences, Kaz.S.S.R. Academy of Sciences). p. 136, Ala Ata, 1982 12. L. A. ITSKOVICH, V. Ya. KABO and V. P. BUDTOV, Vysokomol. soyed. ][328: 610, 1986 (Not translated in Polymer Sci. U.S.S.R.) 13. V. Ya. KABO and L. A. ITSKOVICH, Ibid. A29: 2009, 1987 (Translated in Polymer Sci. U.S.S.R. 29: 9, 2207, 1987) 14. V. Ya. KABO, Dissert. Cand. Chem. Sci. (in Russian) 12 pp., Inst. khim. BF Akad. Nauk SSSR, Ufa, 1984-

Polymerization and copolymerization of (phenyl) (m-tolyl)cyclotrisiloxanes

2225

15. V. Ya. KABO, V. A. MASLENNIKOV and L. A. ITSKOVICH, XXII Konf. po vysokomoleksoyed. (Twenty-Second Conference on High Molecular Weight Compounds) p. 147, Chernogolovka, 1986 16. S. R. RAFIKOV,'V. P. BUDTOV and Yu. B. MONAKOV, Vvedeniye v fizikokhimiyu rastvoroy polimerov (Introduction to the Physicochemistry of Polymer Solutions). p. 177, Moscow, 1978 17. V. P. BUDTOV, Vysokomol. soyed. A27: 951, 1985 (Translated in Polymer Sci. U.S.S.R. A27: 5, 1066, 1985) 18. Idem, Ibid. A29: 1897, 1987 (Translated in Polymer Sci. U.S.S.R. 29: 9, 2083, 1987) 19. V. P. BUDTOV and V. V. KONSETOV, Teplomassoperenos v polimerizatsionnykh protsessakh (Heat-Mass Transfer in Polymerization Processes). p. 256, Moscow, 1983 20. L. A. ITSKOVICH, V. Ya. KABO and V. P. BUDTOV, III Konf. "Vodorastvorimye polimery i ikh primenenye (Third Conference "Water Soluble Polymers and their Uses"). p. 89, Irkutsk, 1987 21. V. Ya. KABO, V. A. MASLENNIKOV and V. P. GORODNOV, Vysokomol. soyed. B24: 326, 1982 (Not translated in Polymer Sci. U.S.S.R.')

PolymerScienceU.S.S.R.Vol. 31, No. 10, pp. 2225-2231, 1989 Printed in Poland

0032-3950189 $10.00+.00 1990PergamonPressplc

POLYMERIZATION AND COPOLYMERIZATION OF (PHENYL)(m-TOLYL)CYCLOTRISILOXANES* N. G. VASILENKO, k. M. TARTAKOVSKAYA, B. D. LAVRUKHIN and A. A. ZHDANOV Nesmeyanov Institute of Elemento-Organic Compounds, U.S.S.R. Academy of Sciences (Received 21 March 1988) The polymerization of (phenyl) (m-tolyl) cyclotcisiloxanes with a different number of phenyl and m-tolyl groups in the cycle and also the copolymerization of hexa(phenyl)- and hexa(m-tolyl) cyclotrisiloxanes have been investigated in presence of ~,co-di-(potassoxy)poly(phenyl) (p-tolyl) siloxane. Starting from the early stages of polymerization, depolymerization occurs at an appreciable rate and the equilibrium yield of the polymers tends to zero. The 1H and laC N M R method has established that the formation of polymers is accompanied by disturbance of the ratio of the di(phenyl)- and di(tolyl) siloxane units in relation to the starting monomers and disturbance of the order of alternation of the siloxane units set by the structure of the cyclotrisiloxanes. IT WAS earlier shown t h a t on p o l y m e r i z a t i o n o f ( p h e n y l ) ( t o l y l ) cyclotrisiloxanes, p o l y m e r s m a y be o b t a i n e d c a p a b l e o f f o r m i n g o r i e n t e d films [1]. It was f o u n d t h a t the p r o p e r t i e s o f the p o l y m e r s d e p e n d not only o n the n u m b e r a n d n a t u r e o f the (p- o r m-) * Vysokomol. soyed. A31: No. 10, 2026-2030, 1989.