Properties of Polyelectrolytes: Maleic Acid-vinylpyrrolidone Copolymers. I.

Viscometric Behavior in Dilute Solution

HERNAN RIOS, LIGIA GARGALLO, and DEODATO RADIC, Facultad de Quimica, Pontifkia Universidud Catblica de Chile,

Campus Sun Joaquin, Casillu 61 77, Santiago, Chile

Synopsis

The viscometric behavior of maleic acid-vinylpyrrolidone copolymer systems p(MA-co-VP) were studied in dilute solution. The weight-average molecular weights of fractionated copolymers were determined by gel permeation chromatography. Viscometric relations were determined in aqueous salt solutions and in organic solvents. Two theta-conditions were found for p(MA-co-VP): in 0.968 M Na,SO,, and in a DMSO/acetone mixture, 35.6/56.4 (u/u) a t 25°C. The conformational and thermodynamic parameters K , and B were evaluated from viscosity data in good solvents using the Stockmayer-Fixman and Berry extrapolations. A linear variation of B with the salt concentration was observed. The electrostatic ( B e ) and nonelectrostatic (B , ) contributions to B were estimated. It was found that Be > B,, and this was attributed to the hydrophylic nature of p(MA-co-VP) copolymers. The latter would reflect a greater influence of the electrostatic interaction on the hydrodynamic behavior of this polymer. The electrostatic (a&), and the nonelectrostatic limc,,,(a3 - 1) contributions to the expansion factor a: were determined. It was found that a& is a function of.(aw/C5)’’2, as is predicted by the Fixman theory.

INTRODUCTION

Macromolecules carrying dissociable groups, e.g., COOH, S03H, NH; , etc., “polyelectrolytes” display two types of behavior.’,2 In a medium with a high-dielectric constant, polyelectrolytes are ionized, and the macromolecules assume extended conformations due to repulsive electrostatic interactions between charged groups. If the ionization is suppressed, the molecules assume the random-coil conformation typical of nonionic polymers. The compact form in the uncharged state transforms to an expanded coil in the fully charged state. The size and the shape of these macromolecules are conditioned by polymer-solvent interactions, short-range interactions, steric and van der Waals interactions.’

Viscometric measurements have always been an important tool in the characterization of polyelectrolytes in solution since the results depend di- rectly on macroion dimensions. In the last few years considerable attention has been paid to the experimental study of copolymers containing ionogenic groups. Copolymers of maleic acid with alkyl vinyl ethers have been investi- gated.3-5 The copolymer with ethyl vinyl ether behaves as a normal polyacid, but the butyl and hexyl copolymers undergo a conformational transition3. The conformational transition of maleic acid-styrene copolymer has been studied by Ohno et ~ 1 . ~ 3 ~ Strauss et a1.’ have pointed out that copolymers with larger

Journal of Polymer Science: Part B: Polymer Physics, Vol. 24, 2421-2431 (1986) 0 1986 John Wiley & Sons, Inc. CCC 0098-1273/86/112421-11$04.00

2422 RIOS, GARGALLO, AND RADIC

alkyl side groups show different thermodynamic behavior from those with smaller alkyl residues. It is of interest to ascertain the polymer conformation and viscometric behavior of polyelectrolytes containing hydrophilic instead of hydrophobic groups in order to compare these systems with those of hydro- phobic character.

The aims of the present work include the study of solvent effects on chain dimensions and the contribution of ionogenic groups and the nature of the side-groups to the conformational behavior exhibited by p(MA-co-VP) in dilute solutions.

The literature contains numerous reports on alternating copolymers of maleic acid and vinylpyrr~lidone.~-'~ It is an excellent system for our objec- tives, since the copolymer composition is very well The acid form dissolves in both organic solvents and water, owing to the presence of vinylpyrrolidone as the comonomer. In addition, both maleic acid and vinyl- pyrrolidone are more hydrophilic than hydrophobic. This copolymer system can be used to study properties in solution in relation to those of the maleic acid copolymers reported previously.

EXPERIMENTAL

Copolymer Preparation

Alternating (1 : 1) copolymer of maleic anhydride-vinyl-pyrrolidone p(MAn-co-VP) was synthesized according to the method described in a patent13 with only slight modifications. Polymerizations were carried out under vacuum in anhydrous benzene using AIBN as initiator. The mixture was agitated at 60°C for 48 h. The copolymer which had precipitated out of benzene solution during the course of polymerization was washed several times with pure benzene and dried under vacuum a t room temperature (conversion, 72%). This copolymer was designated CP,.

Hydrolysis

A sample of CP, was hydrolyzed in water a t 60°C for 24 h. The solution was freeze dried and gave a white spongy copolymer which was designated CP,H.

Characterization

The IR(KBr) spectrum of CP, shows strong absorption bands a t 1850 and 1770 cm-' (due to carbonyl in the anhydride moiety) and 1680 cm-' (due to the five-membered lactam ring). The IR spectrum of the CP,H copolymer shows a strong absorption band a t 1720 cm-' (the C = 0 stretching vibration in the carboxyl moiety) and the bands due to the carbonyl in the anhydride disappear. The results of the microanalysis of the copolymers are summarized in Table I.

Fractionation

Fractionation of CP,H was carried out by slow addition of diethyl ether to a 2% (w/v) solution of p(MA-co-VP) in methanol. This process gave ten fractions (designated F - 1 to F - 10).

PROPERTIES OF POLYELECTROLYTES 2423

TABLE I Elemental analysis of CP, and CP, H

Calculated ( W ) Found (SR)

Copolymer C H N C H N

CP, 57.41 5.26 6.69 56.48 5.26 6.03 CP, H 52.86 5.73 6.17 52.08 5.58 5.87

Viscosity Measurements

Viscosities of CP,H fractions in aqueous and organic solvents were de- termined at 25°C with capillary type Desre~x-Bischoff'~ viscometers. Correc- tions for kinetic energy and shear rate were found to be negligible. Intrinsic viscosities [ q ] were estimated according to classical relations of Huggins, Kraemer,15* l6 and Solomon-G~tesrnan.'~

Reagents

Maleic anhydride (Hoechst Co. Ltd.) was recrystallized from hot chloroform solution, and dried under vacuum. N-vinyl-2-pyrrolidone (Aldrich Chem. Co.) was distilled under reduced pressure before use. All the solvents used were analytical grade, these were dried over metallic sodium and distilled twice.

Molecular Characterization

Weight average molecular weights ( Hw) were determined by gel permeation chromatography (GPC) with a Perkin-Elmer high-performance liquid chro- matograph (HPLC) equipped with a 6OOO-psi pump, a Perkin-Elmer differen- tial refractometer model LC-25, a 175-pl injector and a Shodex Ion pak S-804/S column. Samples were eluted with 0.1 M NaNO,. A universal calibration curve was obtained using a set of Shodex Standard P-82 Pullulans (molecular weights 7.58 x lo3 to 5.30 x lo3: Mw/Mn 1.14 to 1.07) which is a polymaltotriose (a linear macromolecular polysaccharide). The flow rate was 1 ml/min and the volume of the injected polymer solutions was always 5 pl. The analysis of the elution data was performed according to the Rabek18 treatment of the data.

RESULTS AND DISCUSSION

Figures 1 and 2 show, respectively, the reduced viscosity qsp/C as a function of polymer concentration and the variation of the reduced viscosity with the degree of neutralization aN, for one fraction of poly(MA-co-VP). I t is evident that qsp/C increases with dilution and with neutralization up to aN 0.7, then decreases slowly due probably to the increasing screening of the charges of the polyion by counter ions as neutralization increases (see Fig. 2). On the other hand, the polyelectrolyte behavior of p(MA-co-VP) in pure water vanishes in the presence of a simple electrolyte (Na,SO,). The reduced viscosity decreases as the salt concentration C, is increased (see Table 111). The overall conformation of p(MA-co-VP) chains is thus dependent on electrostatic repulsions as normally found for weak polyelectrolytes. The

2424

I

t

RIOS, GARGALLO, A N D RADIC

0.0 I I I I I I I I I I I e 0.0 0.2 0.4 0.6 0.8 1.0

Plot of qsp/c versus c for p(MA-co-VP) fraction in water at 25°C. C ( g I d [ )

Fig. 1.

behavior shown in Figure 2 is quite different from that of copolymers with long hydrophobic side groups. The coil dimensions of the latter copolymer will not increase unless the electrostatic repulsive forces reach a certain critical v a l ~ e . ' ~ - ~ ~ This anomalous behavior is due to the hydrophobic interaction of alkyl side groups, which results in a compact structure of the polymer chain.

I- I /

0.0 0.0 0.2 0.4 0.6 0.8 1.0

dN Fig. 2. Variation of reduced viscosity with the degree of neutralization aN for a fraction of

p(MA-CO-VP).

PROPERTIES OF POLYELECTROLYTES 2425

TABLE I1 Intrinsic viscosity [q], weight average molecular weights (from GPC), and

polydispersity index Mw/Mn of p(MA-co-VP) fractions

F [?lzs.c(dl g- ' )" .Gw x 5Tw/Mnb

1 0.265 10.00 1.18 2 0.196 7.47 1.12 3 0.160 5.90 1.08 4 0.130 4.95 1.06 5 0.108 4.14 1.04 6 0.088 3.47 1.04 7 0.080 3.08 1.02 8 0.067 2.74 1.06 9 0.061 2.29 1.04 10 0.057 1.70 1.06

'Calculated according to Solomon and Gotesmad7 in 0,025 M Na,SO, at 25°C. bFrom GPC

In this work the viscometric study of p(MA-co-VP) was carried out with varying copolymer chain length and concentration of added salt. The degree of neutralization aN was kept the same in all cases (aN = 0.1) and was estimated according to the classical treatment of titration data.,

In order to analyze the molecular weight dependence of the intrinsic viscosity of p(MA-co-VP), we worked at low and constant charge density (ion-binding).,' We assumed that the degree of ion binding is independent of both added salt and polymer concentration. This assumption appears to be justified according to previous studies.21-26

Table I1 summarizes the molecular weight, and the intrinsic viscosity [q] of the fracltions studied in 0.025 M Na,SO,. The Mark-Houwink-Sakurada relations, [q] = K , a E in organic and aqueous media were established. These relations were derived from least-squares fits of double logarithmic plots of the intrinsic viscosity versus aW. Figures 3 and 4 show the viscometric relationships in organic and aqueous solvents respectively. Table I11 shows the Mark-Houwink-Sakurada parameters K , and a, for this copolymer in all the media studied. Table I11 also shows that both DMSO/acetone (34.6/65.4 u / u ) and 0.968 M Na2S0, are theta solvents for these copolymers (a = 0.50). For this copolymer K e is (39.2 f 0.5) X lob5 in an organic medium and (32.3 0.5) - in 0.968 M aqueous Na,SO,. Slight differences in the K e values obtained under theta conditions do not seem large enough to indicate specific solvent effects.

In order to separate the types of interactions involved we have used two excluded volume theories to determine the conformational parameter K e and the thermodynamic parameter B. The data obtained in good solvents were treated by the St~ckmayer-Fixman~' and Berry 28,29 extrapolation methods.

Berry:

where @o is Flory's universal hydrodynamic constant. Stockmayer-Fixman

2426

- A F - -0.5- 0 0 1

-0.7

-0.9

-1.1

RIOS, GARGALLO, AND RADIC

-

-

-

-

-

-

-

I I I I I I I I I I I I , 4.3 4.3 4.5 4.7 4.9 5.1 -1.31

Log Kiw Fig. 3. Intrinsic viscosity-molecular weight relation for p(MA-co-VP). (0) DMSO; (A)

DMSO/acetone 35/65 (u/u); (0) DMSO/acetone 34.6/65.4 ( u / u ) (theta-condition) at 25°C.

-1 .o

-1.4

0 -1.2

-1.2

-1.4 ' ' l.0 4.2 4.4 4.6 4.8 50 5.2 Log iiw

- Fig. 4. Intrinsic viscosity-molecular weight relation for p(MA-co-VP) (m) 0.025 M Na,S04;

(0) 0.05 M Na,SO,; (A) 0.075 M Na,S04; (V) 0.968 M Na,S04 (theta-condition) at 25°C.

PROPERTIES OF POLYELECTROLYTES 2427

TABLE 111

various media Mark-Houwink-Sakurada constant K, and a for (maleic acid-vinylpyrrolidone) copolymer in

~~ ~

Solvent K, x lo5 a

DMSO 4.79 0.74 DMSO/Acetona (35,455 v/v) 24.60 0.56 DMSO/Acetona (34.6/65.4 V/V) ((3) 39.20 0.50 Na,S04 0.025 M 0.48 0.93 Na,S04 0.050 M 2.05 0.81 Na,S04 0.075 M 8.48 0.69 Na,S04 0.968 M ((3) 32.30 0.50

plots for p(MA-co-VP) in the two theta-solvents and in the good solvent DMSO are shown in Figure 5.

The results obtained by measurements in theta-solvents and in good solvents, organic, and aqueous media, using Berry's equation are shown in Figure 6. The results are in good agreement. The hydrodynamic behavior of this copolymer in the condition of the lowest charge density can be described by the classical theories for the nonionic polymers in dilute solution. The theories of the expansion factor of nonionic polymers, such as those of Flory30 or of Stockmayer and F i ~ m a n ~ ~ seems applicable to polyelectrolytes if the molecular weights are low and if the expansion factor a3 is not high. The experimental data appear to be in good agreement with the Stockmayer- Fixman theory for both nonionic and ionic polymer.31

All data obtained in this work show that a good linear relationship holds between [ v ] / M ' / ~ and M'/' in the molecular weight range 1.7 x lo4 to 10.0 X lo4. At the degree of neutralization aN = 0.10, we may conclude that

I I I I I * 1 2 3

( i7J2 x

Fig. 5. Stockmayer-Rxman plot for p(MA-co-VP) in various media: (0) DMSO; (0) DMSO/acetone 34.6/65.4 ( U/D) (theta-condition); (v) 0.968 M Na,SO, (theta-condition) at 25OC.

RIOS, GARGALLO, AND RADIC

4

Fig. 6. Berry plot for p(MA-co-VP) in organic and aqueous solvents: (0) DMSO (good solvent); (0 ) DMSO/Acetone 34.6/65.4 (u/u) (theta-solvent); (A) 0.075 M Na,S04 and (v) 0.968 M Na,S04 (theta-solvent) at 25°C.

the long-range interaction parameter B may be estimated from the slope of the Stockmayer-Fixman plots. The excluded volume parameter B as a func- tion of added salt for aN = 0.1, decreases with increasing added salt con- centration. These results could be described by two different interactions: nonelectrostatic (B,) and electrostatic (Be) which can contribute to the B value 21

B = B, + Be (3)

B = B, + B’f( i)l/Ci’2. (4)

B‘ is a numerical constant and f ( i) should be an increasing function of charge density i according to Noda et aL21 The variation of B with 1/ as is shown in Figure 7. The linear relationship between B and 1/ fls is due to the fact that [ q ] is linearly correlated with 1/ fls for our fractions, and the value of B, must be independent of the charge density. The nonelectrostatic contribu- tion to the excluded volume parameter B is -5.0 X lop2* (cc mol gp2). These results are in good agreement with that of the literature2 in the sense that electrostatic and nonelectrostatic contributions to B are of opposite sign. Taking for B,, the value extrapolated to infinite salt concentration, the Be value can be estimated. The B, B,, and Be values calculated from Figure 7 are summarized in Table IV. It is interesting to note that in this case the electrostatic interaction is the most important contribution to the chain

PROPERTIES OF POLYELECTROLYTES 2429

2 0 t

V V v

m 0 '1 0 5

0

I I I I I I 1 2 3 4 5 6

-101

1 / Es (mol Fig. 7. Variation of long-range interaction parameter B with 1/ fl for at aN = 0.1.

expansion. This is not a surprising result since it is known that long-range attractive interactions of hydrophobic groups attached to polyions may pro- vide powerful resistance to chain expansion.2 In the system studied by us, the hydrophobic interaction is weak, because of the hydrophilic nature of the copolymer. These results agree with Figure 2.

The theta condition was reached at a high salt concentration (0.968 M Na,SO,). This theta state of the polymeric chain in its nonionized form is reflected by the zero slope of the Stockmayer-Fixman, plot at 1/ eS = 1.016 (mole/l)-'/2. This value is found to fit well in the plot shown in Figure 7. The validity of the Stockmayer-Fixman theory was confirmed experimentally, since the values of K , thus determined, agree with the values determined in other theta solvents. The electrostatic contribution to the linear expansion coefficient (Y is a function of the molecular weight of the fractions and the salt concentration. This relation is expressed by the equation:

B ' f ( i ) M ll2 ( Y ; e = l + - -

K, ( r ) ( cs) + - * * .

TABLE IV

from Fig. 7 Electrostatic and nonelectrastatic contributions to the thermodynamic parameter B estimated

0.025 0.050 0.075 0.968

16.10 12.47 3.47 0

-5 -5 -5

0

21.10 17.47 8.74 0

2430 RIOS, GARGALLO, AND RADIC

1 0.0

0.4 0.8 1.2 1.6 2.0 1/2. ,()-3

( r n W / C S )

Fig. 8. Plot*of electrostatic expansion factor a& versus (a/CS)'" for five fractions. (0) Fl; (v) 4; (0) F3; ('4 F4, and (A) 4.

The nonelectrostatic part of the expansion factor (lime, - 1) can be estimated by extrapolating'[[q]/[q], - 1) to C, + 00 according to Noda et aL21 Figure 8 shows this behavior when a:e is plotted against (aw/C,) for different fractions. We should find a unique extrapolation to infinite salt concentration. In fact the points are rather scattered (Figure €9, but their tendency show that a& would be a function of ( ~ w / C s ) " 2 as predicted by the b a n theory.25 However it is clear that a:e cannot be expressed by a unique function of the molecular weight and the salt concentration. The most scattered experimental points, are probably those for low salt concentration, where the electrostatic interaction could play an important role.

The maleic-vinylpirrolidone copolymer has recently been studied by Csiikvari et U Z . ' ~ - ' ~ They studied this copolymer system over a narrow molecular weight range (1.10 x lo4 to 3.08 X lo4) and reported an intrinsic viscosity-molecular weight relationship: [ q ] = 3.25 X Mo.62 in water at 25°C and pH = 2.1. Obviously, the experimental conditions used by these authors in relation to the degree of neutralization and also to the molecular weight range used, does not allow comparison with the results of this study.

Support for this research was provided by DIUC from Pontificia Universidad Cat6lica de Chile.

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(1977).

PROPERTIES OF POLYELECTROLYTES 2431

7. N. Ohno, K. Nitta, S. Makino, and S. Sugai, J. Polym. Scz., Polym. Phys. Ed., 11, 413

8. P. J. Martin and V. P. Straws, Biophys. Chem., 11, 397 (1980). 9. J. Pat6, M. Azori, and F. Tiid&, Makromol. Chem. Rapid Commun., 3, 643 (1982).

10. E. GAkvM, M. Azori, and F. Tiidijs, Polym. Brcll., 5, 413 (1981). 11. E. Cskkvkri, M. Azori, and F. Tiidijs, Polym. BUZZ., 6, 673 (1981). 12. E. Cshkvilri, M. Azori, and F. Tiidijs, Polym. B d . , 5,437 (1984). 13. U.S. Patent. 2 676 949 (1954), Chem. A M . , 48, 9OOOf (1954). 14. V. Desreux and F. Bischoff, BUU. SOC. Chim. Belg., S9,93 (1950). 15. M. L. Huggins, J. Am. Chem. Soc., 64,2716 (1942). 16. E. 0. Kraemer, Znd. Eng. Chem., 30, 1200 (1938). 17. 0. F. Solomon and B. S. Gotesman, Mukromol. Chem., 104, 177 (1967). 18. J. F. Rabek, Experimental Methods in Polymer Chemistry. 19. R. M. Fuoss and U. P. Straws, Ann. of the New York Acud. of Sci., 51,836 (1949). 20. S. Miyamoto, Y. Ishii, and H. Ohnuma, Makromol. Chern., 182, 483 (1981). 21. J. Noda, T. Tsuge, and M. Nagasawa, J. Phys. Chem., 74, 710 (1970). 22. M. Fixman, J. Chem. Phys., 41, 3772 (1964). 23. R. Mock and C. A. Marshall, J. Polym. Sci., 13, 263 (1954). 24. M. Nagasawa, M. Izumi, and I. Kagawa, J. Polym. Sci., 37, 375 (1959). 25. M. Nagasawa, A. Takahashi, M. Izumi, and I. Kagawa, J. Polym. Scz., 38, 213 (1959). 26. Z. Alexandrowicz, J. Polym. Sci., 43,337 (1960). 27. W. H. Stockmayer and H. Fixman, J. Polym. Sci., Pt. C . 1, 137 (1963). 28. G. C. Berry, J. Chem. Phys., 44, 4550 (1966). 29. G. C. Berry, J. Chem. Phys., 46, 1338 (1967). 30. P. J. Flory, Principles of Polymer Chemistry.

31. M. Kurata and W. H. Stockmayer, Forstschr. Hochpolym. Forsch., 3, 196 (1963).

(1973).

New York: Wiley, (1983).

Ithaca, NY: Cornell University Press, 1953, ch. XIV.

Received April 1, 1985 Revised January 24,1986 Accepted March 31, 1986