© Copyright 1997 American Chemical Society
MAY 28, 1997 VOLUME 13, NUMBER 11
Articles Structural Analysis in Interpolyelectrolyte Complex Formation of Sodium Poly(styrenesulfonate) and Diallyldimethylammonium Chloride-Acrylamide Copolymers by Viscometry Fabien Brand† and Herbert Dautzenberg* Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Teltow, Germany Received June 17, 1996. In Final Form: February 28, 1997X A viscometric analysis for studying the polyelectrolyte complexation mechanism of sodium poly(styrenesulfonate) (NaPSS) by addition of diallyldimethylammonium chloride-acrylamide copolymers (DADMAC-AAM copolymers) was developed. The viscosity of the reacting mixture was studied as a function of the mixing ratio (ratio of the polycationic to polyanionic groups). A minimum viscosity (point of equivalence) was observed for an equimolar mixing ratio, thus indicating a 1:1 stoichiometry of the polyelectrolyte complexes (PECs) at this point. With an increase of the cationic charge density (35-100%) of the copolymer, the packing density of the complexes increased (8-60 mg/mL). For a charge density below 35% of the copolymer, flocculation occurred before the equimolar mixing ratio was reached. This was due to the larger size of the formed aggregates. To obtain additional structural information (stoichiometry, density) on the PECs before or after the point of equivalence, the contributions of the free polyelectrolyte in excess and of the PEC to the specific viscosity of the reacting mixture were estimated. Nonstoichiometric PECs were shown to be formed before the point of equivalence. Factors of stoichiometry (ratio of polyanionic to polycationic groups in the PECs) were evaluated to 1.4-1.6. The polyanionic excess tended to be lower at high cationic charge density and disappeared with increasing the mixing ratio until an equimolar ratio was reached. After the point of equivalence, the swelling of the PEC dominated.
Introduction In recent years polyelectrolyte complexes (PECs) have acquired a permanently growing significance.1 Colloidal interpolyelectrolyte complexes have demonstrated an enormous potential in the design of composite structures (i.e., microcapsules,2 surface modification by nonstoichiometric PECs3 , ...). The aggregation tendency of oppositely charged polyelectrolytes results in insoluble flocs settling † Schlumberger Dowell, SRPC-D, 26, rue de la Cavee, B.P. 202, 92142 Clamart Cedex, France. X Abstract published in Advance ACS Abstracts, May 1, 1997.
(1) Philipp, B.; Dautzenberg, H.; Linow, K.-J.; Ko¨tz, J.; Dawydoff, W. Prog. Polym. Sci. 1989, 14, 91. (2) Vogt, W. Macromol. Chem. Phys. 1994, 195, 1557. (3) Buchhammer, H. M.; Kramer, G.; Lunkwitz, K. Colloids Surf, A 1995, 95, 299.
S0743-7463(96)00592-6 CCC: $14.00
out of the solution. This has been found to be useful in separation or purification processes (i.e., lignin separation in pulping4 ...). The strong structural changes occurring during PEC formation from extended polymer chains to compact PEC microgel aggregates also have constituted the basis for several applications (i.e., the control of transport properties of polyelectrolytes in biological systems5 ). To contribute to a better understanding of the mechanism of PEC formation, particularly of the influence of charge density on PEC formation, we studied in the present work the structural changes during complex formation between sodium poly(styrenesulfonate) (NaPSS) (4) Chen, Y.; Kholodenko, A. L. J. Chem. Phys. 1987, 86, 1540. Muthukumar, M. J. Chem. Phys. 1987, 86, 7230. (5) Kabanov, V. A. Vysokomol. Soedin. 1994, 36 (2), 198.
© 1997 American Chemical Society
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and diallyldimethylammonium chloride-acrylamide copolymers (DADMAC-AAM copolymers) by viscometric measurements. From the estimations of the contributions of the PEC particles and of the polyelectrolyte in excess to the specific viscosity of the reacting mixture, the stoichiometry and density of the PEC particles were evaluated as a function of the mixing ratio. Experimental Section Materials. The synthesis of DADMAC-AAM copolymers by radical copolymerization in aqueous solution with feeding of the more reactive AAM monomer has been described.6 The samples were linear cationic components with strong ionic groups and various charge densities depending on their chemical compositions but with broad molecular weight distributions. From 13C spectroscopy, the monomer residues were shown to follow a Markov distribution of first order (r1 ) 0.35, r2 ) 5.0). A NaPSS standard from Polymer Standard Service, Mainz, Germany, was used as a polyanion with strong ionic groups and narrow molecular weight distribution. The polyanion had a hydrophobic backbone. The molecular parameters of both compounds are summarized in Table 1.
Figure 1. Specific viscosities of NaPSS solutions in dependence on salt content and molar concentration [P-]0 of the anionic charges at 20 °C. Table 1. Molecular Parameters of the Polyelectrolyte Components
Polyelectrolyte Solutions. Ultrapure water obtained by ion-exchange and filtration steps (Elgastat UHQ-PS) was used as the solvent. The specific resistance was greater than 18.2 MΩ cm, with the total organic content less than 10 ppb according to the manufacturer specifications. All solutions were prepared under mechanical agitation during an 8 h period at room temperature and filtered through Schott glas filters, G1. The copolymer concentrations were controlled by chloride potentiometry of the DADMAC chloride counterions. The polyanionic concentration was controlled by UV-vis spectroscopy of NaPSS after adequate dilution. The concentration uncertainty was less than 1% for both components. Viscometric Analysis. Polyelectrolyte Components. The viscosity data of aqueous solutions of DADMAC-AAM copolymer and NaPSS were investigated as a function of polymer and salt concentration, using the Viscoboy 2 from Lauda and an Ubbelohde-Viscometer type 531, capillary I, from Schott. For the polyanion, dilution series were prepared at a constant salt concentration. A concentrated stock solution with the desired salt concentration was mixed in the viscometer to the solvent containing the same salt concentration with the electronic Dosing System EDOS 5221 from Eppendorf. The specific viscosities of NaPSS in dependence on polymer and salt concentration are represented in Figure 1. The intrinsic viscosities and Huggins constants of the DADMAC-AAM copolymers obtained previously6 by isoionic dilutions are given in Table 2. PEC Measurements. PEC formation between components of comparable high molecular weight and strong ionic groups led to highly aggregated systems. At higher concentrations (>0.1 g/mL) macroscopic flocculation occurred. However, in highly diluted solutions (10-5-10-3 g/mL) “quasi-soluble” PECs on a colloidal level were formed,7 which could be studied by viscometry. A solution of DADMAC-AAM copolymer in water of concentration [P+]0 (molar concentration of the cationic groups) was added to the Ubbelohde tank containing v0 ) 20 mL of a solution of NaPSS in water of concentration [P-]0 (molar concentration of anionic groups) with [P-]0 ) 1/2[P+]0. The specific viscosities ηsp (to water) were determined as a function of mixing ratio. In the concentration range used, the low molecular salt released during (6) Brand, F.; Dautzenberg, He.; Jaeger, W.; Hahn, M. Appl. Macromol. Chem. Phys., in press. (7) Dautzenberg, He.; Rother, G.; Hartmann, J. In Macroion Characterization; ACS Symposium Series 548; Schmitz, K. S., Ed.; American Chemical Society: Washington, DC, 1994; p 210.
polycations (DADMAC content of the copolymers (mol %))
polyanion (NaPSS)
8 21 35 47 58 73 100
Mn(OS) 10-6 g/mol
Mw(LS) 10-6 g/mol
Mw/Mn
0.585 0.532 0.487 0.538 0.326 0.210 0.160
4.22 3.96 2.31 1.44 1.23 0.58 0.26 0.350
7.2 7.4 4.7 2.7 3.8 2.8 1.6 0.9). Obviously, flocculation is favored for the copolymers of lower charge densities and higher molecular weights, because of a higher aggregation level of the primary particles.10 (8) Veis, A. J. Phys. Chem. 1960, 64, 1203; 1961, 65, 1798; 1963, 67, 1960. (9) Ko¨tz, J. Nord. Pulp. Paper Res. J. 1993, 11, 1. (10) Dautzenberg, He.; Hartmann, H.; Grunewald, S.; Brand, F. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1024.
(1)
The contribution of the polyelectrolyte can be determined from the calibration measurements of the specific viscosity of the polyelectyrolytes after the calculation of the concentration of noncomplexed polyanionic [P-] and polycationic [P+] groups as well as of the released low molecular salt [NaCl] and resulting ionic strength I in dependence on the mixing ratio u. Previous work10 on the NaPSS/DADMAC-AAM copolymer systems identified that a quantitative release of salt can be determined by measuring the activity of the chloride ions during PEC formation using a Cl- selective electrode. The findings have confirmed the 1:1 stoichiometry at the point of charge equivalence and the 1:1 charge compensation between polyanionic and polycationic groups in the whole mixing range. Taking into account these conclusions, i.e., assuming a 1:1 stoichiometry, the concentrations of all components can be described by the equations given in Table 3, as a function of conversion. To determine the ionic strength, the contributions of the polyelectrolytes are considered as well.11 The theoretical curves obtained are represented in Figure 3. Before the point of equivalence, an increase of salt concentration, an increase of ionic strength depending on the effective charge density of the NaPSS, and a decrease of the NaPSS concentration take place. After the point of equivalence, a slight decrease of ionic strength, depending on the effective charge density of the copolymers, an increase of the cationic component in excess, and a decrease of salt concentration by dilution are described. Using these data, the specific viscosities of the free components in excess, as a function of the mixing ratio, can be evaluated from the previously measured viscosity data of the NaPSS solutions as a function of salt concentration (Figure 1) or from the experimental viscosity data of the DADMAC-AAM copolymer solutions as a function of ionic strength (Table 2). The results are represented in Figure 4, depicting a similar behavior as the measured curves. However, a quantitative analysis reveals significant deviations. The difference between the experimental specific viscosity and the theoretical contribution of the polyionic components in excess should give the contribution of the PEC (Figure 5). An increasing positive contribution to the viscosity until the point of equivalence and a slight decrease by dilution after the point of (11) Dautzenberg, He.; Jaeger, W.; Koetz, J.; Philipp, B.; Seidel, Ch.; Stscherbina, D. PolyelectrolytessFormation, Characterization and Application; Hanser Verlag: Muenchen, 1994; Chapters 3 and 5.
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Table 2. Intrinsic Viscosities and Huggins Constants of DADMAC-AAM Copolymers in Isoionic Dilutions 10-3[η] (mL/g)
kH
DADMACp (mol %)
I) 10-4 M
I) 10-3 M
I) 10-2 M
I) 0.5 M
I) 10-4 M
I) 10-3 M
I) 10-2 M
I) 0.5 M
8 21 35 47 58 73 100
8.55 11.74 10.7 8.73 8.55 5.67 4.48
4.32 6.74 6.57 5.92 5.22 4.08 2.75
1.99 2.95 3.04 2.67 2.46 1.78 1.15
0.726 0.727 0.654 0.495 0.440 0.311 0.202
0.466 0.857 2.31 5.21 5.19 15.7 22.9
0.532 0.459 0.792 0.928 1.18 1.73 2.51
0.180 0.233 0.270 0.312 0.204 0.330 0.394
0.377 0.361 0.300 0.318 0.300 0.300 0.285
Figure 3. Theoretical changes of the concentrations of polyionic groups [P+] and [P-], of the released low molecular salt [NaCl] and of the ionic strength I during PEC formation of NaPSS by addition of DADMAC-AAM copolymers. [P-]0 ) 1/2 [P+]0 ) 4.85 × 10-4 mol/L.
type” (polyelectrolytes with ionic groups located on side groups of the chain backbone). Whereas investigations of Tsuchida14 for polyions of the “pendant type” have suggested a 1:1 stoichiometry of the PEC at different mixing ratios, our results are rather consistent with systematic investigations of Philipp et al.1 in which the 1:1 stoichiometry before the point of equivalence has turned out to be an exception. After the point of equivalence, the strong increase in viscosity may be attributed to the swelling of the PEC. The stabilization of the values does not give indications for further strong structural changes. 3. Contribution of the PEC to Viscosity. To evaluate the stoichiometry of the PEC in dependence on the mixing ratio, their contribution to the specific viscosity of the reacting mixture is estimated by the following considerations. For small deviations from stoichiometry, the PECs can be considered as weakly charged and the electroviscous interactions between charged particles can be neglected. Due to the compact structure of the PEC, the contribution of the complexes to the specific viscosity of the reacting mixture is small and can be described by the following expression assuming spherical complexes.15
ηsp ) 2.5φ
(2)
where φ ) F-1 c, the volume fraction of the particles, F is density of a particle, and c is weight concentration of the particles. At the conversion u ) 1 no free polyelectrolyte exists and a 1:1 stoichiometry is found so that the density of the complex particles can directly be determined from the specific viscosity, corresponding to the expression:
ηPEC(1) ) 2.5
[P+]0v1 (m3′ + me′) v0 + v1 F(1)
(3)
equivalence were expected. However, negative values before the point of equivalence are found. After the point of equivalence, a strong increase of the difference is followed by a weaker dependence. The negative values before the point of equivalence indicate the formation of nonstoichiometric PEC, as suggested in previous investigations.12,13 PDADMAC should be considered as a polyelectrolyte of the “pendant
where me′ ) m1′ + (1 - f1)/f1m2, equivalent molecular weight of a positive charge of the copolymer without counterion, m1′ ) 126.2 g/mol, molecular weight of the DADMAC without counterion, m2 ) 70.08 g/mol, molecular weight of AAM, m3′ ) 183.2 g/mol, molecular weight of the NaSS without counterion, and F(u) is the density of the PEC at mixing ratio u. The concentration of the PEC particles is calculated according to Table 3. The resulting density values are summarized in Table 4. For comparable charge densities of both components, relatively high packing densities are found. The complete charge compensation must be accompanied by strong conformational changes of the chains. With decreasing charge density, the charge compensation causes the inclusion of a still higher number of chains. Due to the quite different charge densities of both components, the ion pairs are expected to be weaker.
(12) Kabanov, V. A.; Zezin, A. B. Pure Appl. Chem. 1984, 56, 343. (13) Schindler, T.; Nordmeier, E. Polym. J. 1994, 26, 1124.
(14) Tsuchida, E. J. Macromol. Sci., Pure Appl. Chem. 1994, A31, 1. (15) Einstein, A. Ann. Phys. 1906, 19, 289; 1911, 34, 591.
Figure 4. Calculated specific viscosity of the polyionic excess assuming stoichiometric PEC formation. [P-]0 ) 1/2[P+]0 ) 4.85 × 10-4 mol/L.
Interpolyelectrolyte Complex Formation
Langmuir, Vol. 13, No. 11, 1997 2909
Table 3. Basic Equations Describing the Concentration of Free Polyelectrolyte Groups, [P+], [P-], with an Effective Contribution to Viscosity, the Concentration of the Released Salt [NaCl], and the Ionic Strength I in Dependence on the Mixing Ratio before the equivalent point
after the equivalent point
[P+]
0
[P+]0v [P-]0v0 v0 + v v0 + v
[P-]
[P-]0v0 [P+]0v K(u) v0 + v v0 + v
0
[NaCl]
[P+]0v v0 + v
[P+]0v1 [P-]0v0 ) v0 + v v0 + v 1 [NaCl] + ξe-1 [P+] 2
I
1 [NaCl] + ξ3-1 [P-] 2
u ) [P+]0v/[P-]0v0, mixing ratio v, total volume of the cationic additions (DADMAC-AAM copolymer) v0 ) 20 mL, initial volume of the polyanion solution (NaPSS) [P+]0, molar concentration of cationic groups in the polycationic solution added [P-]0, initial molar concentration of anionic groups in the polyanionic solution v1 ) ([P-]0/[P+]0)v0, total volume of copolymer solution added at the equivalent point ξe-1 ) 1 for be > λb and ξe-1 ) be/λb for be < λb, correction taking in account the Manning condensation of counterions for the copolymers be ) b1 ((1 - f1)/f1)b2, average charge spacing of the copolymer b1 ) 0.55 nm, segment length of the DADMAC unit b2 ) 0.25 nm, segment length of the AAM unit f1, molar content of DADMAC in the copolymers ξ3-1 ) b3/λb for b3 < λb and ξ3-1 ) 1 for b3 > λb, correction taking in account the Manning condensation of counterions for NaPSS b3 ) 0.25 nm, segment length of the NaSS unit λb ) 0.7 nm, Bjerrum length in water at 25 °C K(u), factor of stoichiometry at mixing ratio u (K1) ) 1)
Figure 5. Difference between the experimental specific viscosity of the reacting mixture (Figure 2) and the calculated contribution of the polyionic excess (Figure 4). [P-]0 ) 1/2[P+]0 ) 4.85 × 10-4 mol/L.
Therefore, a less dense structure of the PEC is formed which is additionally promoted by the presence of the hydrophilic acrylamide component. However, the copolymer with a content of DADMAC 73% does not follow the tendency. The measurement is reproducible. The low density of the PEC may be understood by a hindered accessibility of the charged ionic groups. As previously observed for the system NaPSS/PDADMAC,7 the structural parameters of the “primary aggregates” formed do not change significantly with increasing conversion, an almost constant density is obtained by light scattering. However, the densities of the “primary” particles are larger. Nevertheless, we set F(u) ) F(1) to estimate the maximum value of the contribution
of the PEC to the specific viscosity before and after the point of equivalence. This seems to be justified, especially in the face of the low contribution of the PEC at higher contents of free polyelectrolytes. The calculated values, given in Figure 6, are remarkably different from the data in Figure 5. Before the 1:1 mixing ratio this may be explained by an excess binding of the major component NaPSS. The general idea is that mainly on the surface of the PEC a certain rate of the major component is bound with partial charge compensation by the low molecular counterions. This layer protects the particles via electrostatic stabilization against further coagulation. The conformation of the chains incorporated in this layer may vary from more compact to highly swollen coils. An exact description of this complicated system on the basis of viscometric data cannot be given. We introduced a correction factor K(u) for the concentration of the PEC and also the consumption of the free polyelectrolyte in the expressions of Table 3 and eqs 1-3. This factor of nonstoichiometry is determined by fitting the experimental data using eq 1, where the contribution of the polyelectrolyte in excess is obtained from the calibration measurements of the specific viscosity of the free polyelectyrolyte and where the contribution of the PEC is calculated in a good approximation from the expression
[P+]0v (K(u) m3′ + me′) c(u) ηPEC(u) ) 2.5 ) 2.5 v0 + v F(1) F(1)
(4)
neglecting the contribution of the counterions of the polyelectrolyte bound in excess. After the point of equivalence, only the further dilution is taken into account for the change of the contribution of the PEC. The factor K(u) is given in Figure 7. At low mixing ratio, relatively high factors between 1.4 and 1.6, a nearly linear decrease
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Table 4. Equivalent Molecular Weight per Charge (m3, me), Charge Spacing (b3, be), and Densities G of the Polyelectrolyte Components (Evaluated at Ionic Strength [I] ) 10-4 mol/L Using G ) 2.5/[η]), Specific Viscosity (ηPEK(1)), and Packing Densities (G(1)) of the PEC, [P-]0 ) 1/2 [P+]0 ) 4.85 × 10-4 mol/L NaPSS
DADMAC-AA copolymer
m3 (g/mol)
b3 (nm)
F (g/mL)
206
0.25
10-4-10-3
DADMACp (mol %)
me (g/mol)
be (nm)
8 21 35 47 58 73 100
945 429 292 241 213 188 162
3.30 1.50 1.00 0.83 0.73 0.64 0.55
Figure 6. Specific viscosity of the PEC during PEC formation calculated from expression 4.
of this factor with increasing mixing ratio, and a stronger decrease shortly before the point of equivalence are determined. For copolymers with low charge densities, somewhat higher factors of stoichiometry are observed. However, we must be aware that we ascribe to the nonstoichiometric layer an average density which corresponds to the complex. In this way we include the chains which are forced to stronger conformational changes, while loosely attached molecules are not monitored. Despite simplifying assumptions, the quantitative analysis of the viscometric data gives a deeper insight into the structural changes during the complex formation and a more realistic picture of the structure of the PEC particles. The obtained K values could recently be confirmed by determination of the NaPSS content in the supernatant of PEC solutions after separation of the complex particles by ultracentrifugation, leading to somewhat higher nonstoichiometry. After the point of equivalence, the viscosity data in Figure 5 are significantly higher than the values obtained under the assumption of an unchanged structure of the complex. This may be understood by a swelling of the complex, due to an interaction with the polycations in excess.
PEC F (g/mL)
102ηPEC(1)
102F(1) (g/mL)
4.50 1.18 0.60 2.25 0.40
0.79 2.66 4,86 1.21 6.25
10-5-10-4
Figure 7. PEC stoichiometry (ratio of polyanionic to polycationic groups in the PEC) in dependence on the mixing ratio.
Conclusions Besides a qualitative description of the processes taking place during interpolyelectrolyte complex formation of NaPSS and DADMAC-AAM copolymers, a quantitative description of the viscometric data could be given, introducing a factor K(u) of nonstoichiometry. At the point of equivalence, stoichiometric complexes were formed; an increasing packing density (8-60 mg/mL) with increasing cationic charge density (35-100%) of the copolymer was evaluated. At low mixing ratios, however, nonstoichiometric PECs were formed; the factors of nonstoichiometry (ratios of anionic to cationic groups), evaluated to 1.41.6, decreased with increasing cationic charge density and approached 1 with increasing mixing ratio. The accessibility of the ionic groups during charge compensation appeared to be hindered for a cationic content of 73% of the copolymer. After the point of equivalence, a swelling of the complex particles is observed. LA960592S
