- Email: [email protected]
Phase separation in solutions of acrylic acid-2-methyl-5-vinylpyridine polyampholytes
Polymer Science U.S.S.R. Vol. 27, No. I0, pp. 2394-2401, 1985 Printed in Poland
0032-3950/85 $10.00+.00 © Pergamon Journals Ltd.
PHASE SEPARATION IN SOLUTIONS OF ACRYLIC ACID-2-METHYL-5-VINYLPYRIDINE POLYAMPHOLYTES* L. I. VEDIKHINA, A. I. KURMAYEVA and V. P. BARABANOV S. M. Kirov Chemico-Technological Institute, Kazan' (Received 11 March 1984)
The basic laws governing phase separation in solutions of the synthetic polyampholytes formed by acrylic acid and 2-methyl-5-vinylpyridine have been studied. A reversible equilibrium transition from true solution to a polymeric dispersion is observed when the pH of the medium or the polymer concentration is changed. The pH region for phase separation of solutions of the polyampholytes is determined by the composition and compositional non-uniformity of the copolymer but it does not depend on the molecular-mass distribution. Structureformation occurs above a critical polymer concentration, whose value depends on the pH of the medium and the copolymer's composition. THE PRESENCE in the macromolecular chain of polyampholytes with acidic and basic groups capable, under certain conditions, of being ionized imparts a special set of physico-chemical properties to solutions of these polymers. One of the most remarkable features of polyampholytes, caused by their structure, is the existence of an isoelectric state (IES), that is, a pH region in which the macromolecules exist as Zwitter ions. A large number of experimental and theoretical papers have been devoted to the investigation of the physico-chemical behaviour of polyampholyte macromolecules. Problems connected with the insolubility of amphoteric polyelectrolytes in the IES remain, however, the least studied because, in this case, the physico-chemical methods of investigation usually used for such polymers have only limited possibilities. Although the insolubility of synthetic polyampholytes in the IES is a trivial fact which is encountered every day in the study of these substances, investigations of the phenomenon have been limited to the simple statement of this fact [I-4]. Moreover, investigators always consciously attempt to avoid the insolubility of amphoteric polyelectrolytes by using water-alcohol [1, 5, 6] or salt [3, 7, 8] solutions. There are clearly insufficient experimental data for an understanding of the mechanism of formation of the new phase and the nature of the insolubility of polyampholytes in the IES. As yet, moreover, problems of phase separation in their solutions have hardly been subjected at all to physico-chemical analysis and the necessity for the present systematic investigation has therefore arisen. In connection with this purpose, the present work represents a study of the basic laws governing the formation of the new phase in solutions of polyampholytes formed by acrylic acid and 2-methyl-5-vinylpyridine existing in the isoelectric state. * Vysokomol. soyed. A27: No. 10, 2131-2136, 1985. 2394
Solutions of acrylic acid-2-methyl-5-vinylpyridine polyarnpholytes
2395
TABLE 1. PROPERTIES OF THE COPOLYMERS INVESTIGATED
Designation
SP-1 SP-2 SP-3 SP-4 SP-5
Composition of copolymer mole % AA
mole % MVP
21-3 27.1 43"0 53-5 66.0
78.7 72-9 57.0 46'5 34.0
[r/],
dl/g
~7/"w × 10 -3 [
200 111
1"30
0'73 0"90 0'38 0.28
IEP
6.0 5"9
P 93
5-4 5"0 4'35
Ccr,
G~
g/dl
J" m/kg
0"0008 0'002 0.02 0.002 0"04
21 "2 2'78 1-11 0'84 0.165
Note: The values of G and ccr are shown for the IEP. TABLE 2. PROPERTIES OF
Designation SP-F1 SP-F2 SP-F3 SP-F4
SP-3
Composition of copolymer mole ~o A A mole ~ M V P 44-3 42"8 43"l 44.2
55.7 57"2 56.9 55'8
FRACTIONS
[r/], dl/g
h4w x 10- 3
1 "25 0"89 0'66 0"38
254"4 134"3 116.9 55'1
IEP 5.35 5.42 5"40 5-35
Copolymers of acrylic acid (AA) with 2-methyl-5-vinylpyridine (MVP) were obtained by the radical copolymerization of the freshly prepared monomers in toluene (the total concentration of the comonorners was approximately 50 wt. Yo). The m o n o m e r mixtures were prepared on the basis of specified molar proportions. The reaction mixture was freed of oxygen by purging the ampoules with nitrogen, after which they were sealed. Polymerization was initiated by the thermal decomposition of A I B N (0.2 wt. ~o) at 60°C. The copolymers were purified by being precipitated twice from methanol into diethyl ether and by dialysis and were dried to constant weight in vacuum at 30-40°C AA, M V P and the solvents were purified by the usual methods [9, 10]. A I B N was repeatedly recrystallized from methyl alcohol and dried in vacuum to constant weight. The fractionation of the copolymer SP-3 (Table 1) was carried out by Meyerhoff's method [11] in the system methanol-toluene. Your of the twelve fractions obtained were used in the present work. The properties of the fractions are shown in Table 2. The average composition of the copolymers was calculated from the results of chemical analysis for nitrogen, from the concentration of pyridine groups determined spectrophotometrically in a 1 N solution of H2SO4 (using the absorption maximum in the U V region of the spectrum) and from the concentration of carboxyl groups determined by reverse titration [12]. The results obtained were in mutual agreement within the limits of error of the experiment. The compositions of the fractions were determined by spectrophotometry. The values of the 37/w of the copolymers a n d the fractions were obtained by the thin-layer equilibrium method using a Beckman ultracentrifuge. The molecular mass of the specimen SP-3, calculated from the formula [0] = 9.7 x 10-5 × ~ o . 7,, on the basis of results of measurement of the viscosity of solutions in 1 N NaCI at p H = l - l , was 1.66x 105 . The turbidity spectrum method was used to characterize quantitatively the parameters of the disperse phase formed in solutions of the poly-ampholytes. It is based on theoretical a n d numerical treatments of Mi's theory [13] of light scattering by spherically shaped particles. In the present work, we have used a version of the method, proposed by Klenin [14], based on the determination of the wave exponent n in the Angstrom equation z = c o n s t 2 -n. The value of n depends on the relative size 0t and the relative refractive index m=p/po of the scattering particles (p and IZo are the refractive indices of the particles and of the dispersion medium, namely water, respectively). The optical density D of the polymeric dispersions studied were measured with a Spekol-10 spectrophotometer with an EK-5 fixture in cells with a liquid layer of thickness, /, of 1 or 5 cm at a temperature o f
L. I. VEDIKHINAet al.
2396
25 + O-l°C. In order to exclude absorption effects, a region of the visible spectrum was established in which a plot with the coordinates log D against log 2 gave a straight line; this region was 420-580 rim. The value of n was determined as the slope of the line plotted with the coordinates log D and log 2. Tables of the characteristic light scattering functions were used in the calculations: these gave n, the coefficient of light-scattering K and the structure factor V as functions of ct and m [14]. These tables made it possible to determine the relative particle size ct from the value of n obtained for a selected relative refractive index of the particles, m; the mean particle size was correspondingly determined 74 = l~qv/2nao. The concentration of the precipitate polymer (the mass of the precipitated polymer) was calculated by taking account of the structure of the system (in terms of the structure factor V): M = SFr, where S is a constant depending on the physical properties o f the system under investigation, r=2.3 D/I is the turbidity of the system at 2av=500 nm. For aqueous dispersions of the copolymers under investigation, S = 3'07 x 10-3 g/100 cm 2. The mean particle radius, F;., and the concentration of the precipitated polymer, M, calculated for all the series of experiments for m = 1.1. The selection of this figure is reasonable. The particles of aqueous dispersions of polyampholytes clearly alter the degree of immobilization of the solvent, depending on the pH of the medium. It may be assumed that the degree of immobilization of the solvent by the particles decreases as the isoelectric point (IEP) is approached, since, in this case, the macromolecules with the most compact conformations will participate in the formation of the particles and the relative refractive index of the particles will correspondingly be increased. When the value of m is varied, the absolute values of the particle size alter (only for n < 2), as does the concentration of the precipitated polymer (for n > 2) [15-17], but the trends of the observed relationships are not altered. Experimental results have shown that, in the isoelectric state of the polyampholytes investigated, the values of n are generally less than 2, that is, the relationships found for the concentration of the precipitated polymer are fairly reliable and the maximum error in the determination of M will not exceed 5-10% for any value of m. An improved version of Wilhelm's method was used to obtain equilibrium values for the surface tension, tr, of the polymer solutions [18]. The depth of immersion of the plate in the liquid was recorded by using a VLA-200g-M analytical balance with an illuminated scale. All the measurements were made in a thermostatically controlled cell at 25°C. The value of ~r was calculated using the equation a=Kb(AM+L,dm) [18] where AM is the difference in weight between the dry plate and the plate partially immersed in the liquid being investigated, Am is the change in the intercept on the illuminated scale and L = 1.57 and Kb = 280"2 are constants for the balance. Values of the copolymers' surface activity, G = -c3tr/cge (Table 1) were determined from the slope of the initial section of the surface tension isotherm (Fig. 1), that is, from the dependence of ~r on concentration c. Investigations of the skrface-active properties of p o l y a m p h o l y t e s h a v e shown that limiting values of tr are characteristically established at values of p H c o r r e s p o n d i n g to the IES region (Fig. 1). The relationship, when plotted with the s e m i l o g a r i t h m i c coordinates tr a n d log c, has a clearly expressed change in slope at these values of pH. It clearly points to a critical c o n c e n t r a t i o n at which the adsorbed p o l y m e r layer at the l i q u i d - g a s b o u n d a r y becomes saturated a n d changes begin within the v o l u m e of the solution. Solutions of oplyampholytes are found, as a rule, to become t u r b i d at p o l y m e r c o n c e n t r a t i o n s above a critical c o n c e n t r a t i o n , which is evidence of their phase separation. The critical c o n c e n t r a t i o n , ccr, c o r r e s p o n d i n g to the establishment of the l i m i t i n g values of surface tension, becomes less a n d the surface activity becomes greater as the polya m p h o l y t e becomes closer to the IEP. The c o r r e s p o n d i n g results are shown below. pH co,, g]dl G, J.m/kg
5'1 0"09 0.045
5"2 0"065 0"18
5"4 0.02 1.11
5-5 0.065 0.205
5"75 0"09 0.052
Solutions of acrylic acid-2-methyl-5-vinylpyridine potyampholytes
2397
The way in which the critical concentration of the polymer depends on the pH of the medium may be explained by the fact that, as the IEP is approached, the thermodynamic quality of the solvent becomes impaired. Therefore, even in dilute solutions, the macromolecules can form supermolecular structures because of strong intermolecular interactions. In the region of the polyampholyte's iES at concentrations below the critical, the macromolecules clearly become coiled on themselves, forming intramolecular bonds, whereas, at higher concentrations, intermacromolecular interactions also occur. Two concentration regions must be distinguished: in the first (for polymer concentrations less than the critical concentration c , ) the solution has a molecular degree of dispersion but in the second (for c > ec~) aggregation of the macromolecules occurs with the formation of a disperse phase. The critical concentration depends not only on the pH of the medium but also on the composition of tile copolymer (Table 1). The greater the proportion of methyl pyridine groups in the composition, the lower is the value of e~, and the greater the surface activity, G, of the polyampholyte.
6#1 ~ nm
M ~ lOf,q/dl
CFO -I I
g'5
\
z ~1
-2
:
q
-f Zogc[g/dq 1
I 0.2
I Fm.
13 Oq 1
750 -
31
Z t c,#/dl
500 250-
Z
5.0
5.5
pH
Flq. 2
FIG. I. Dependence of surface tension on the SP-3 concentration at the following values of pH: •-5-2; 2-5.75 and 3-5.4. FIG. 2. Dependence of 1 - the particle size, -~a, 2-the concentration of the precipitated polymer M and 3-the turbidity, z, of the system on the pH of the medium for SP-3, c=0.1 g/dl. The phenomenon of phase separation in the IES region was observed for all the polyampholytes investigated. It was thus established that the intensity of formation of the disperse phase depends on the pH of the medium: the particle size, Fa, have maximum values at the 1EP as a rule (Fig. 2). The way in which ~z, M and r depend on the pH of the medium gives evidence that the equilibrium transition from a true solution to a polymeric dispersion is a consequence of a change in the charge of the macromolecules. Because of the intensification of electrostatic attraction between oppositely charged
2398
L.I. VEDIKHINAet aL
groups as the IES is approached, polymer-water contacts are replaced by polymer-polymer contacts and compact supermolecular structures are formed. The polymer-polymer contacts may be both intra- and also intermolecular. The new phase is formed initially as particles very small in size (nuclei) and then growth of the particles occurs. On going away from the IEP, the particles of the polymeric dispersion break up because of the fact that some of these macromolecules acquire a certain charge, which leads to the splitting-off of the macromolecules from the particles and their transition into solution. The concentration of the precipitated polymer consequently falls. Investigation of the changes in the parameters of the disperse system (its turbidity, the size of the polymeric particles, the number of them, and the concentration of the precipitated polymer) with time has shown that equilibrium between the phases is established in not more than 10-15 rain. After this induction period, the polymeric dispersions formed in the case of polyampholytes containing more than 25 mole ~ AA are stable and remain unchanged (the turbidity of the system, the particle size and the number of particles are constant) over a period of several days or even weeks in the case of initial polymer concentrations of up to 1 g/dl. Dispersions of copolymers containing less than 25 mole ~ of acidic groups, which can separate into two macrophases, are unstable. The separation process is connected with an increase in the dimensions of the particles being formed and with their settling under the action of gravity. It should be noted that the increase in the particle size and the separation of the system into two phases occur more intensively as the IEP is approached and as the polymer concentration is increased. In the investigation of SP-1 (for example, with a concentration of 0.2 g/dl) the transition of the polymer to the IES is thus accompanied by a marked increase in particle size and by the separation of the system into two macrophases (the separation of a precipitate is observed). Only the use of low concentrations of SP-I (0-02-0.05 g/dl) enabled equilibrium dispersions, stable with time, to be obtained. In order to compare the processes involved in the formation of the new phase in solutions of copolymers with various compositions, we introduced a parameter, the proportion of the polymer that was precipitated, calculated as the ratio of the concentration of the precipitated polymer, M, to the initial concentration of the polymer in solution, c: 0 = Mfc. The dependence of M and 0 on pH for all the copolymers investigated is shown in Fig. 3. It should be noted that all the curves have extreme values, and that the maxima on the curves correspond to the IEP of the polyampholyte, which is evidence of a marked change in the properties of the polyampholyte at this point and in the capacity for phase formation. It was established that the boundaries of the pH region in which stable polymeric dispersions exist are equal to IEP_+0.5 and depend on the copolymer's composition. An increase or decrease in the width of this region is evidently related to the compositional non-uniformity of the A A + M V P copolymers. The result~ of a study of the non-uniformity of our specimens has been presented [19]. A comparison of tbe curves for the pH dependence of the proportion of the polymer precipitated (Fig. 3) with the curves for the solubility of the copolymers in the system methanol-diethylether, which characterize the non-uniformity of the polymers with respect to composition (Fig, 4),
Solutions of acrylic acid-2-methyl-5-vinylpyridine polyampholytes
2399
indicates that the greater the non-uniformity with respect to composition, the broader is the pH region for phase separation. This effect appears with special clarity in the case of the copolymer SP-1 which has the greatest non-uniformity and for which the phaseseparation region is for p H values in the range 4.5-7.5 (Fig. 3). Attention should also be given to the fact that, in the case of SP~I, the proportion of the polymer precipitated is high. The effect of molecular mass and molecular mass distribution on the intensity of formation of the new phase in polyampholyte solutions was investigated. Figure 5 shows how the concentration and proportion of the polymer that is precipitated depends
e,lo z M.Td,g/dz
M,:,s/.: o
Lt!- 4
8
0.#
Z
0"4
1
O.Z
3-8 2-2 1-! 5
l 4
5
6
7 pH
FJC. 3. Dependence of the concentration, M, and the fraction, 0, of the precipitated polymer on the pH of the medium for: 1-SP-I; 2-SP-2; 3-SP-3; 4-SP-4 and 5-SP-5 for c=0.05 g/dl (1) and c=0-1 g/dl (2-5).
,\
s 1: •
-:
M,1o e/dl
Io
J
0"7
08 FIG. 4
&
8
2
2
I
I
t
I
5-Z5
~#0
I I
pH
FIG. 5
F1G. 4. Solubility curves for polymers in the system methanol-diethyl ether [19]: 1-MVP; 2-SP-2; 3-SP-1; 4-SP-3; 5-SP-4; 6 - P A A in a mixture with MVP; 7-PAA. FJC. 5. Dependence of M and 0 on the pH of the medium for: 1 - SP-F4; 2--SP-F3; 3-SP-F2; 4-SP-3; 5-SP-FI. c=0.1 g/dl.
2400
L . I . VEDIKH1NAet al.
on the p H of the medium for the unfractionated specimen SP-3 and four of its fractions (Table 2). It may be seen that non-uniformity with respect to molecular mass does not affect the p H region for phase separation. The position of the maxima for copolymers with various molecular masses but with the same compositions remains practically unchanged whereas, in the case of different*compositions, it depends on composition (Fig. 3). The molecular mass of the polyampholytes affects the concentration of the precipitate polymer so that, the higher the molecular mass, the greater is the value of M.
M'10 ,g/dl
8.7O'
b
3
3
72 4
6 t
02
0.4
L7.2
I
J
f
0"4 C, ~7/d!
FIG. 6. Concentration dependence of 0, M and b, 0, for SP-3 in solutions with the following values ofpH: 1-5.15; 2-5"2; 3-5.4; 4-5'5; 6-5.6 and 6-5-9. An increase in the total concentration of the copolymer, c, leads to an increase in the concentration of the precipitate polymer for an unchanged value of 0 (Fig. 6). It may be seen from the Figure that, as the IEP is approached, the new phase begins to f o r m at lower concentrations. In the case of SP-3, the values of concentration obtained by the turbidity spectrum method for the production of the new phase at various values of the medium's p H are in good agreement with the critical concentrations determined in the investigation of the copolymer's surface-active properties. The experimental data presented above have shown, in sum, that electrostatic intraand intermolecular interactions play the principal part in the formation of stable insoluble particles of the synthetic polyampholytes formed by AA and MVP. Translated by G. F. MODLEN REFERENCES
1. A. KATCHALSKY and J. MILLER, J. Polymer Sci., 13: 57, 1954 2. V. S. LEBEDEV and R. K. GAVURINA, Vysokomol. soyed. 6: 1353, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 5, 1493, 1964) 3. J. C. SALAMON, C. C. TSAI, A. P. OLSON and A. C. WATTERSON, Amer. Chem. Soc. Polymer Preprints, 19: 261, 1978 4. E. I. ABLYAKIMOV and R. K. GAVURINA, Vysokomol. soyed. A12: 1464, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 7, 1660, 1970) 5. N. A. KRUGLOVA and I. V. SAVINOVA, Vysokomol. soyed. A21: 282, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 2, 308, 1979)
Solutions of acrylic aeid-2-methyl-5-vinylpyridine polyampholytes
2401
6. L. F. MOSALOVA, N. A. KRUGLOVA, I. V. SAVINOVA, Ye. D. VORONTSOV and V. P. ¥EVDAKOV, Vysokomol. soyed. A24: 178, 1982 (Translated in Polymer Sci. U.S,S.R. 24: 1,205, 1982) 7. J. C. SALAMON, W. VOLKSEN, A. P. OLSON and S. C. ISRAEL, Polymer 19: 1157. 1978 8. S. MAXIM and M. DIMA, Revue Roumaine de Chimie 21: 631, 1976 9. Ye. V. KUZNETSOV, I. P. PROKHOROV and N. I. AVVAKUMOVA, (book) Tr. Kazan. khim.-tekhn, in-ta. Kazan', 36, 41 I, 1967 10. A. GORDON and R. FORD, Sputnik khin-~ika ('Chemist's Companion). 541 pp. Mir, Moscow, 1976 (Russian translation) 11. G. MEYERHOFF, Makromolek. Chem. 12: 45, 1954 12. N. I. AVVAKUMOVA, Dissertation for the degree of candidate of chemical sciences, 172 pp. Kazan' Chemico-Techn. Institute, Kazan', 1970 13. G. MI, Ann. Phys., 25: 377, 1908 14. V. I. KLENIN, S. Yu. SHCHEGOLEV and V. I. LAVRUSHIN, Kharakteristicheskiye funktsii svetorasseyaniya (Characteristic Functions of Lightscattering). 176 pp. Izd. Sarat. un-ta Saratov, 1977 15. V. I. KLENIN and S. Yu. SHCHEGOLEV, Vysokomol. soyed. A13: 1919, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 8, 2161, 1971) 16. S. Yu. SHCHEGOLEV, V. I. KLEN1N and B. I. SHVARTSBURD, Kolloidn. zhurn., 39: 324, 1977 17. O. V. KLENINA, Dissertation for the degree of candidate of chemical sciences. 181 pp. Saralo,, University, Saratov 1975 18. A. Ye. FAINERMAN, Yu. S. LIPATOV, V. M. KULIK and L. N. VOLOGINA, Kolloidn. zhurn. 32: 620, 1970 19. L. I. V E D I K H I N A , A. I. K U R M A Y E V A , N. I. AVVAKUMOVA and V. P. BARABONOV, Manuscript deposited in All-Union Institute for Scientific and Technical Information (VINITI). p. 5565, Dep. No. 1199, KhP-D-82. Published in RZhKhim., Moscow, 1983
0032-3950/85 $10.00+.00 © Pergamon Journals Ltd.
PHASE SEPARATION IN SOLUTIONS OF ACRYLIC ACID-2-METHYL-5-VINYLPYRIDINE POLYAMPHOLYTES* L. I. VEDIKHINA, A. I. KURMAYEVA and V. P. BARABANOV S. M. Kirov Chemico-Technological Institute, Kazan' (Received 11 March 1984)
The basic laws governing phase separation in solutions of the synthetic polyampholytes formed by acrylic acid and 2-methyl-5-vinylpyridine have been studied. A reversible equilibrium transition from true solution to a polymeric dispersion is observed when the pH of the medium or the polymer concentration is changed. The pH region for phase separation of solutions of the polyampholytes is determined by the composition and compositional non-uniformity of the copolymer but it does not depend on the molecular-mass distribution. Structureformation occurs above a critical polymer concentration, whose value depends on the pH of the medium and the copolymer's composition. THE PRESENCE in the macromolecular chain of polyampholytes with acidic and basic groups capable, under certain conditions, of being ionized imparts a special set of physico-chemical properties to solutions of these polymers. One of the most remarkable features of polyampholytes, caused by their structure, is the existence of an isoelectric state (IES), that is, a pH region in which the macromolecules exist as Zwitter ions. A large number of experimental and theoretical papers have been devoted to the investigation of the physico-chemical behaviour of polyampholyte macromolecules. Problems connected with the insolubility of amphoteric polyelectrolytes in the IES remain, however, the least studied because, in this case, the physico-chemical methods of investigation usually used for such polymers have only limited possibilities. Although the insolubility of synthetic polyampholytes in the IES is a trivial fact which is encountered every day in the study of these substances, investigations of the phenomenon have been limited to the simple statement of this fact [I-4]. Moreover, investigators always consciously attempt to avoid the insolubility of amphoteric polyelectrolytes by using water-alcohol [1, 5, 6] or salt [3, 7, 8] solutions. There are clearly insufficient experimental data for an understanding of the mechanism of formation of the new phase and the nature of the insolubility of polyampholytes in the IES. As yet, moreover, problems of phase separation in their solutions have hardly been subjected at all to physico-chemical analysis and the necessity for the present systematic investigation has therefore arisen. In connection with this purpose, the present work represents a study of the basic laws governing the formation of the new phase in solutions of polyampholytes formed by acrylic acid and 2-methyl-5-vinylpyridine existing in the isoelectric state. * Vysokomol. soyed. A27: No. 10, 2131-2136, 1985. 2394
Solutions of acrylic acid-2-methyl-5-vinylpyridine polyarnpholytes
2395
TABLE 1. PROPERTIES OF THE COPOLYMERS INVESTIGATED
Designation
SP-1 SP-2 SP-3 SP-4 SP-5
Composition of copolymer mole % AA
mole % MVP
21-3 27.1 43"0 53-5 66.0
78.7 72-9 57.0 46'5 34.0
[r/],
dl/g
~7/"w × 10 -3 [
200 111
1"30
0'73 0"90 0'38 0.28
IEP
6.0 5"9
P 93
5-4 5"0 4'35
Ccr,
G~
g/dl
J" m/kg
0"0008 0'002 0.02 0.002 0"04
21 "2 2'78 1-11 0'84 0.165
Note: The values of G and ccr are shown for the IEP. TABLE 2. PROPERTIES OF
Designation SP-F1 SP-F2 SP-F3 SP-F4
SP-3
Composition of copolymer mole ~o A A mole ~ M V P 44-3 42"8 43"l 44.2
55.7 57"2 56.9 55'8
FRACTIONS
[r/], dl/g
h4w x 10- 3
1 "25 0"89 0'66 0"38
254"4 134"3 116.9 55'1
IEP 5.35 5.42 5"40 5-35
Copolymers of acrylic acid (AA) with 2-methyl-5-vinylpyridine (MVP) were obtained by the radical copolymerization of the freshly prepared monomers in toluene (the total concentration of the comonorners was approximately 50 wt. Yo). The m o n o m e r mixtures were prepared on the basis of specified molar proportions. The reaction mixture was freed of oxygen by purging the ampoules with nitrogen, after which they were sealed. Polymerization was initiated by the thermal decomposition of A I B N (0.2 wt. ~o) at 60°C. The copolymers were purified by being precipitated twice from methanol into diethyl ether and by dialysis and were dried to constant weight in vacuum at 30-40°C AA, M V P and the solvents were purified by the usual methods [9, 10]. A I B N was repeatedly recrystallized from methyl alcohol and dried in vacuum to constant weight. The fractionation of the copolymer SP-3 (Table 1) was carried out by Meyerhoff's method [11] in the system methanol-toluene. Your of the twelve fractions obtained were used in the present work. The properties of the fractions are shown in Table 2. The average composition of the copolymers was calculated from the results of chemical analysis for nitrogen, from the concentration of pyridine groups determined spectrophotometrically in a 1 N solution of H2SO4 (using the absorption maximum in the U V region of the spectrum) and from the concentration of carboxyl groups determined by reverse titration [12]. The results obtained were in mutual agreement within the limits of error of the experiment. The compositions of the fractions were determined by spectrophotometry. The values of the 37/w of the copolymers a n d the fractions were obtained by the thin-layer equilibrium method using a Beckman ultracentrifuge. The molecular mass of the specimen SP-3, calculated from the formula [0] = 9.7 x 10-5 × ~ o . 7,, on the basis of results of measurement of the viscosity of solutions in 1 N NaCI at p H = l - l , was 1.66x 105 . The turbidity spectrum method was used to characterize quantitatively the parameters of the disperse phase formed in solutions of the poly-ampholytes. It is based on theoretical a n d numerical treatments of Mi's theory [13] of light scattering by spherically shaped particles. In the present work, we have used a version of the method, proposed by Klenin [14], based on the determination of the wave exponent n in the Angstrom equation z = c o n s t 2 -n. The value of n depends on the relative size 0t and the relative refractive index m=p/po of the scattering particles (p and IZo are the refractive indices of the particles and of the dispersion medium, namely water, respectively). The optical density D of the polymeric dispersions studied were measured with a Spekol-10 spectrophotometer with an EK-5 fixture in cells with a liquid layer of thickness, /, of 1 or 5 cm at a temperature o f
L. I. VEDIKHINAet al.
2396
25 + O-l°C. In order to exclude absorption effects, a region of the visible spectrum was established in which a plot with the coordinates log D against log 2 gave a straight line; this region was 420-580 rim. The value of n was determined as the slope of the line plotted with the coordinates log D and log 2. Tables of the characteristic light scattering functions were used in the calculations: these gave n, the coefficient of light-scattering K and the structure factor V as functions of ct and m [14]. These tables made it possible to determine the relative particle size ct from the value of n obtained for a selected relative refractive index of the particles, m; the mean particle size was correspondingly determined 74 = l~qv/2nao. The concentration of the precipitate polymer (the mass of the precipitated polymer) was calculated by taking account of the structure of the system (in terms of the structure factor V): M = SFr, where S is a constant depending on the physical properties o f the system under investigation, r=2.3 D/I is the turbidity of the system at 2av=500 nm. For aqueous dispersions of the copolymers under investigation, S = 3'07 x 10-3 g/100 cm 2. The mean particle radius, F;., and the concentration of the precipitated polymer, M, calculated for all the series of experiments for m = 1.1. The selection of this figure is reasonable. The particles of aqueous dispersions of polyampholytes clearly alter the degree of immobilization of the solvent, depending on the pH of the medium. It may be assumed that the degree of immobilization of the solvent by the particles decreases as the isoelectric point (IEP) is approached, since, in this case, the macromolecules with the most compact conformations will participate in the formation of the particles and the relative refractive index of the particles will correspondingly be increased. When the value of m is varied, the absolute values of the particle size alter (only for n < 2), as does the concentration of the precipitated polymer (for n > 2) [15-17], but the trends of the observed relationships are not altered. Experimental results have shown that, in the isoelectric state of the polyampholytes investigated, the values of n are generally less than 2, that is, the relationships found for the concentration of the precipitated polymer are fairly reliable and the maximum error in the determination of M will not exceed 5-10% for any value of m. An improved version of Wilhelm's method was used to obtain equilibrium values for the surface tension, tr, of the polymer solutions [18]. The depth of immersion of the plate in the liquid was recorded by using a VLA-200g-M analytical balance with an illuminated scale. All the measurements were made in a thermostatically controlled cell at 25°C. The value of ~r was calculated using the equation a=Kb(AM+L,dm) [18] where AM is the difference in weight between the dry plate and the plate partially immersed in the liquid being investigated, Am is the change in the intercept on the illuminated scale and L = 1.57 and Kb = 280"2 are constants for the balance. Values of the copolymers' surface activity, G = -c3tr/cge (Table 1) were determined from the slope of the initial section of the surface tension isotherm (Fig. 1), that is, from the dependence of ~r on concentration c. Investigations of the skrface-active properties of p o l y a m p h o l y t e s h a v e shown that limiting values of tr are characteristically established at values of p H c o r r e s p o n d i n g to the IES region (Fig. 1). The relationship, when plotted with the s e m i l o g a r i t h m i c coordinates tr a n d log c, has a clearly expressed change in slope at these values of pH. It clearly points to a critical c o n c e n t r a t i o n at which the adsorbed p o l y m e r layer at the l i q u i d - g a s b o u n d a r y becomes saturated a n d changes begin within the v o l u m e of the solution. Solutions of oplyampholytes are found, as a rule, to become t u r b i d at p o l y m e r c o n c e n t r a t i o n s above a critical c o n c e n t r a t i o n , which is evidence of their phase separation. The critical c o n c e n t r a t i o n , ccr, c o r r e s p o n d i n g to the establishment of the l i m i t i n g values of surface tension, becomes less a n d the surface activity becomes greater as the polya m p h o l y t e becomes closer to the IEP. The c o r r e s p o n d i n g results are shown below. pH co,, g]dl G, J.m/kg
5'1 0"09 0.045
5"2 0"065 0"18
5"4 0.02 1.11
5-5 0.065 0.205
5"75 0"09 0.052
Solutions of acrylic acid-2-methyl-5-vinylpyridine potyampholytes
2397
The way in which the critical concentration of the polymer depends on the pH of the medium may be explained by the fact that, as the IEP is approached, the thermodynamic quality of the solvent becomes impaired. Therefore, even in dilute solutions, the macromolecules can form supermolecular structures because of strong intermolecular interactions. In the region of the polyampholyte's iES at concentrations below the critical, the macromolecules clearly become coiled on themselves, forming intramolecular bonds, whereas, at higher concentrations, intermacromolecular interactions also occur. Two concentration regions must be distinguished: in the first (for polymer concentrations less than the critical concentration c , ) the solution has a molecular degree of dispersion but in the second (for c > ec~) aggregation of the macromolecules occurs with the formation of a disperse phase. The critical concentration depends not only on the pH of the medium but also on the composition of tile copolymer (Table 1). The greater the proportion of methyl pyridine groups in the composition, the lower is the value of e~, and the greater the surface activity, G, of the polyampholyte.
6#1 ~ nm
M ~ lOf,q/dl
CFO -I I
g'5
\
z ~1
-2
:
q
-f Zogc[g/dq 1
I 0.2
I Fm.
13 Oq 1
750 -
31
Z t c,#/dl
500 250-
Z
5.0
5.5
pH
Flq. 2
FIG. I. Dependence of surface tension on the SP-3 concentration at the following values of pH: •-5-2; 2-5.75 and 3-5.4. FIG. 2. Dependence of 1 - the particle size, -~a, 2-the concentration of the precipitated polymer M and 3-the turbidity, z, of the system on the pH of the medium for SP-3, c=0.1 g/dl. The phenomenon of phase separation in the IES region was observed for all the polyampholytes investigated. It was thus established that the intensity of formation of the disperse phase depends on the pH of the medium: the particle size, Fa, have maximum values at the 1EP as a rule (Fig. 2). The way in which ~z, M and r depend on the pH of the medium gives evidence that the equilibrium transition from a true solution to a polymeric dispersion is a consequence of a change in the charge of the macromolecules. Because of the intensification of electrostatic attraction between oppositely charged
2398
L.I. VEDIKHINAet aL
groups as the IES is approached, polymer-water contacts are replaced by polymer-polymer contacts and compact supermolecular structures are formed. The polymer-polymer contacts may be both intra- and also intermolecular. The new phase is formed initially as particles very small in size (nuclei) and then growth of the particles occurs. On going away from the IEP, the particles of the polymeric dispersion break up because of the fact that some of these macromolecules acquire a certain charge, which leads to the splitting-off of the macromolecules from the particles and their transition into solution. The concentration of the precipitated polymer consequently falls. Investigation of the changes in the parameters of the disperse system (its turbidity, the size of the polymeric particles, the number of them, and the concentration of the precipitated polymer) with time has shown that equilibrium between the phases is established in not more than 10-15 rain. After this induction period, the polymeric dispersions formed in the case of polyampholytes containing more than 25 mole ~ AA are stable and remain unchanged (the turbidity of the system, the particle size and the number of particles are constant) over a period of several days or even weeks in the case of initial polymer concentrations of up to 1 g/dl. Dispersions of copolymers containing less than 25 mole ~ of acidic groups, which can separate into two macrophases, are unstable. The separation process is connected with an increase in the dimensions of the particles being formed and with their settling under the action of gravity. It should be noted that the increase in the particle size and the separation of the system into two phases occur more intensively as the IEP is approached and as the polymer concentration is increased. In the investigation of SP-1 (for example, with a concentration of 0.2 g/dl) the transition of the polymer to the IES is thus accompanied by a marked increase in particle size and by the separation of the system into two macrophases (the separation of a precipitate is observed). Only the use of low concentrations of SP-I (0-02-0.05 g/dl) enabled equilibrium dispersions, stable with time, to be obtained. In order to compare the processes involved in the formation of the new phase in solutions of copolymers with various compositions, we introduced a parameter, the proportion of the polymer that was precipitated, calculated as the ratio of the concentration of the precipitated polymer, M, to the initial concentration of the polymer in solution, c: 0 = Mfc. The dependence of M and 0 on pH for all the copolymers investigated is shown in Fig. 3. It should be noted that all the curves have extreme values, and that the maxima on the curves correspond to the IEP of the polyampholyte, which is evidence of a marked change in the properties of the polyampholyte at this point and in the capacity for phase formation. It was established that the boundaries of the pH region in which stable polymeric dispersions exist are equal to IEP_+0.5 and depend on the copolymer's composition. An increase or decrease in the width of this region is evidently related to the compositional non-uniformity of the A A + M V P copolymers. The result~ of a study of the non-uniformity of our specimens has been presented [19]. A comparison of tbe curves for the pH dependence of the proportion of the polymer precipitated (Fig. 3) with the curves for the solubility of the copolymers in the system methanol-diethylether, which characterize the non-uniformity of the polymers with respect to composition (Fig, 4),
Solutions of acrylic acid-2-methyl-5-vinylpyridine polyampholytes
2399
indicates that the greater the non-uniformity with respect to composition, the broader is the pH region for phase separation. This effect appears with special clarity in the case of the copolymer SP-1 which has the greatest non-uniformity and for which the phaseseparation region is for p H values in the range 4.5-7.5 (Fig. 3). Attention should also be given to the fact that, in the case of SP~I, the proportion of the polymer precipitated is high. The effect of molecular mass and molecular mass distribution on the intensity of formation of the new phase in polyampholyte solutions was investigated. Figure 5 shows how the concentration and proportion of the polymer that is precipitated depends
e,lo z M.Td,g/dz
M,:,s/.: o
Lt!- 4
8
0.#
Z
0"4
1
O.Z
3-8 2-2 1-! 5
l 4
5
6
7 pH
FJC. 3. Dependence of the concentration, M, and the fraction, 0, of the precipitated polymer on the pH of the medium for: 1-SP-I; 2-SP-2; 3-SP-3; 4-SP-4 and 5-SP-5 for c=0.05 g/dl (1) and c=0-1 g/dl (2-5).
,\
s 1: •
-:
M,1o e/dl
Io
J
0"7
08 FIG. 4
&
8
2
2
I
I
t
I
5-Z5
~#0
I I
pH
FIG. 5
F1G. 4. Solubility curves for polymers in the system methanol-diethyl ether [19]: 1-MVP; 2-SP-2; 3-SP-1; 4-SP-3; 5-SP-4; 6 - P A A in a mixture with MVP; 7-PAA. FJC. 5. Dependence of M and 0 on the pH of the medium for: 1 - SP-F4; 2--SP-F3; 3-SP-F2; 4-SP-3; 5-SP-FI. c=0.1 g/dl.
2400
L . I . VEDIKH1NAet al.
on the p H of the medium for the unfractionated specimen SP-3 and four of its fractions (Table 2). It may be seen that non-uniformity with respect to molecular mass does not affect the p H region for phase separation. The position of the maxima for copolymers with various molecular masses but with the same compositions remains practically unchanged whereas, in the case of different*compositions, it depends on composition (Fig. 3). The molecular mass of the polyampholytes affects the concentration of the precipitate polymer so that, the higher the molecular mass, the greater is the value of M.
M'10 ,g/dl
8.7O'
b
3
3
72 4
6 t
02
0.4
L7.2
I
J
f
0"4 C, ~7/d!
FIG. 6. Concentration dependence of 0, M and b, 0, for SP-3 in solutions with the following values ofpH: 1-5.15; 2-5"2; 3-5.4; 4-5'5; 6-5.6 and 6-5-9. An increase in the total concentration of the copolymer, c, leads to an increase in the concentration of the precipitate polymer for an unchanged value of 0 (Fig. 6). It may be seen from the Figure that, as the IEP is approached, the new phase begins to f o r m at lower concentrations. In the case of SP-3, the values of concentration obtained by the turbidity spectrum method for the production of the new phase at various values of the medium's p H are in good agreement with the critical concentrations determined in the investigation of the copolymer's surface-active properties. The experimental data presented above have shown, in sum, that electrostatic intraand intermolecular interactions play the principal part in the formation of stable insoluble particles of the synthetic polyampholytes formed by AA and MVP. Translated by G. F. MODLEN REFERENCES
1. A. KATCHALSKY and J. MILLER, J. Polymer Sci., 13: 57, 1954 2. V. S. LEBEDEV and R. K. GAVURINA, Vysokomol. soyed. 6: 1353, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 5, 1493, 1964) 3. J. C. SALAMON, C. C. TSAI, A. P. OLSON and A. C. WATTERSON, Amer. Chem. Soc. Polymer Preprints, 19: 261, 1978 4. E. I. ABLYAKIMOV and R. K. GAVURINA, Vysokomol. soyed. A12: 1464, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 7, 1660, 1970) 5. N. A. KRUGLOVA and I. V. SAVINOVA, Vysokomol. soyed. A21: 282, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 2, 308, 1979)
Solutions of acrylic aeid-2-methyl-5-vinylpyridine polyampholytes
2401
6. L. F. MOSALOVA, N. A. KRUGLOVA, I. V. SAVINOVA, Ye. D. VORONTSOV and V. P. ¥EVDAKOV, Vysokomol. soyed. A24: 178, 1982 (Translated in Polymer Sci. U.S,S.R. 24: 1,205, 1982) 7. J. C. SALAMON, W. VOLKSEN, A. P. OLSON and S. C. ISRAEL, Polymer 19: 1157. 1978 8. S. MAXIM and M. DIMA, Revue Roumaine de Chimie 21: 631, 1976 9. Ye. V. KUZNETSOV, I. P. PROKHOROV and N. I. AVVAKUMOVA, (book) Tr. Kazan. khim.-tekhn, in-ta. Kazan', 36, 41 I, 1967 10. A. GORDON and R. FORD, Sputnik khin-~ika ('Chemist's Companion). 541 pp. Mir, Moscow, 1976 (Russian translation) 11. G. MEYERHOFF, Makromolek. Chem. 12: 45, 1954 12. N. I. AVVAKUMOVA, Dissertation for the degree of candidate of chemical sciences, 172 pp. Kazan' Chemico-Techn. Institute, Kazan', 1970 13. G. MI, Ann. Phys., 25: 377, 1908 14. V. I. KLENIN, S. Yu. SHCHEGOLEV and V. I. LAVRUSHIN, Kharakteristicheskiye funktsii svetorasseyaniya (Characteristic Functions of Lightscattering). 176 pp. Izd. Sarat. un-ta Saratov, 1977 15. V. I. KLENIN and S. Yu. SHCHEGOLEV, Vysokomol. soyed. A13: 1919, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 8, 2161, 1971) 16. S. Yu. SHCHEGOLEV, V. I. KLEN1N and B. I. SHVARTSBURD, Kolloidn. zhurn., 39: 324, 1977 17. O. V. KLENINA, Dissertation for the degree of candidate of chemical sciences. 181 pp. Saralo,, University, Saratov 1975 18. A. Ye. FAINERMAN, Yu. S. LIPATOV, V. M. KULIK and L. N. VOLOGINA, Kolloidn. zhurn. 32: 620, 1970 19. L. I. V E D I K H I N A , A. I. K U R M A Y E V A , N. I. AVVAKUMOVA and V. P. BARABONOV, Manuscript deposited in All-Union Institute for Scientific and Technical Information (VINITI). p. 5565, Dep. No. 1199, KhP-D-82. Published in RZhKhim., Moscow, 1983