Structure of Polyelectrolyte Solutions at Intermediate Charge Densities

Chapter 21

Structure of Polyelectrolyte Solutions at Intermediate Charge Densities 1

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W. Essafi , F.Lafuma ,and C. E. Williams Laboratoire pour l'Utilisation du Rayonnement Electromagnétique, Centre National de la Recherche Scientifique, Commisariat à l'Energie Atomique, Ministère de l'Education Nationale, University of Paris-Sud, F91405 Orsay, France Laboratory of Macromolecular Physical Chemistry, Ecole Supérieure de Physique et de Chimie Industrielle, 10, rue Vauquelin, F75231 Paris, France

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 21, 2016 | http://pubs.acs.org Publication Date: December 13, 1993 | doi: 10.1021/bk-1994-0548.ch021

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We report a small angle X-ray scattering (SAXS) study of the structural properties of semi-dilute solution in water of poly(styrene-co-cesium styrene sulfonate) of various compositions. It is found that there is a striking continuity of all structural properties, e. g. scattering peak position and intensity, as a function of polymer concentration and of ion content, from 100% to 30%, the limit of solubility. The more intense peaks all have a hint of an associated second order peak, an observation that indicates that the correlations are more pronounced than predicted by the simple correlation hole model for highly charged polyelectrolytes. However there is no evidence of the microphase separation of weakly charged polymers. Finally, the effect on the structure of added salt is reported. It is suggested that the hydrophobicity of the polystyrene backbone could have a definite influence on the structure. In the last few years, there has been a substantial theoretical and experimental effort to understand the structure of semi-dilute solutions of highly charged polyelectrolytes in water. In salt free solutions, the highly charged chains are usually described as forming a transient network, similar to that of semi-dilute solutions of neutral polymers, except that each macromolecule is surrounded by a depletion volume or "correlation hole" from which other chains are excluded by the screened repulsive electrostatic interactions (7) This model relies on the fact that each individual chain, that was highly stretched when dilute, becomes more flexible as the concentration increases and remains stiff at the local scale only. How rigid is the chain is still a much debated question, outside the scope of this note. Most structural investigations have been focused on polystyrene sulfonate, considered as a model polyelectrolyte. Extensive neutron (2, 3) and light scattering measurements (4,5) have shown qualitative agreement with the theoretical predictions of this isotropic model without long range order. For instance for all polymer concentrations, Cp, above that of chain overlap, a broad halo is visible whose position, q*, scales as Cp^2 ver many orders of magnitude in concentration. However, no really satisfactory quantitative agreement has been found and a detailed model is still lacking. In the light of new puzzling experimental data (6, 7) it has even been suggested more recently that the solution might not be homogeneous and that ordered and disordered regions could coexist in 0

0097-6156y94/0548-0278$06.00/0 © 1994 American Chemical Society

Schmitz; Macro-ion Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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ESSAFI E T A L

Structure of Polyelectrolyte Solutions

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the solution (two-state model). All theoretical models for highly charged polyelectrolytes assume that the solutions properties are dominated by long range coulombic interaction between charges and the hydrophobicity of the backbone is not taken into account On the other hand, for weakly charged polymers in a poor solvent, the delicate balance between hydrophobic and electrostatic interactions may lead to a phase separation, at a mesoscopic scale, into oppositely charged polymer-rich and polymerpoor domains. The theoretical models (#, 9) have been found adequate to describe the properties of semi-dilute solutions of poly(acrylic acid) and poly(methacrylic acid) (70). For both systems the hydrophobic effects are rather small and the degrees of ionization were kept very low. Here also, the scattering pattern is characterized by a rather broad single peak, related to pretransitional concentration fluctuations and whose position varies with charge content The microphase transition itself has not been observed. It should be noted that no theory exists, up to now, that would be able to describe the behaviour of polyelectrolytes in the whole range of charge content In order to gain insight into the effect of hydrophobicity and to attempt to bridge the gap between these two extreme models, the properties of poly(styrenesulfonateco-styrene) have been investigated in the intermediate range of charge content, between 100% and 30%, the limit of solubility in water. This polymer has a very hydrophobic backbone as shown by the fact that partially sulfonated polystyrene of lower charge content (< 20%), is only soluble in organic solvents and behaves as an ionomer; its properties are then dominated by the dipolar attraction between undissociated ion pairs. Herein, small angle x-ray scattering is used to provide an experimental description of the structural characteristics of semi-dilute solutions of this polymer as a function of charge content, polymer concentration and added salt concentration. Materials and Methods Samples of poly(styrene-c0-styrene sulfonate) of various composition have been prepared by postsulfonation of polystyrene (PS) according to a procedure described elsewhere (77). The precursor polymer was a commercial polystyrene (CdF-Chimie) of molecular weight 250,000 and polydispersity of 2. It was first purified by centrifugation of a solution (90g l' ) in 1,2 dichloroethane (DCE) followed by precipitation in ethanol of the clear supernatant. For sulfonation, 5g of PS were dissolved in 70ml of DCE and the requisite amounts of acetic anhydride and sulfuric acid were added to the solution at 50°C; the mixture was stirred for 60 minutes to obtain the polystyrene sulfonate acid which was subsequently converted into a sodium salt by addition of a sodium hydroxide solution. The mixture was then dialysed until its conductivity was close to that of distilled water. Finally, the concentrated solution was freeze dried to obtain poly(styrene-c0-sodium styrene sulfonate), abbreviated NaPSS. The samples were characterized by elemental analysis in order to determine the degree of sulfonation, or equivalently, the charge content f, defined as the ratio of the number of sulfonated monomers to the total number of monomers; it varies between 0.27 and 0.9 for this investigation. Poly(styrene-c0cesium styrene sulfonate), CsPSS, was obtained by ion exchange of the corresponding NaPSS on a DOWEX H resin followed by neutralisation with cesium hydroxide. Salt-free semi-dilute solutions were prepared by dissolving the polyelectrolyte in deionized H2O at 60°C for 30 minutes, then let to rest for two days prior to the measurements. Small angle X-ray scattering (SAXS) experiments were performed on beam line D22, at LURE, using the DCI synchrotron radiation source. Data were obtained with pinhole collimation and recorded with a linear detector of 512 cells. The scattering vector q varied from 0.008 to 0.2 Â" [q = (4π/λ) sin(0/2), where θ is the observation angle and λ, the wavelength was 1.37 Â]. 1

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The scattering data were normalized to constant beam intensity and corrected for transmission, sample thickness, parasitic and background scattering, according to a standard procedure. The resulting scattering profiles are obtained as normalized intensities I vs. scattering vector q. Conversion to absolute scattering units involves a simple multiplicative factor.


Results Effect of Charge Content and Polymer Concentration. Figure 1 shows the evolution of the scattering profiles as a function of charge content, f, for salt-free solutions at a constant monomer concentration cp = 0.32 monomol 1"! of NaPSS (72). For all charge contents, a maximum appears in the scattering profile at a finite scattering vector q* whose position shifts continuously to lower values as f decreases; concomitantly, the intensity at q* increases and the peak narrows, i.e. Aq/q* decreases. 1(0), the intensity extrapolated to q=0, increases as f decreases, as expected for samples containing fewer charges, since it is related to the osmotic pressure π in the solution and is proportional to Cn(3cp/d7c). At large q's, all scattering profiles coincide indicating that at small spatial scales the monomer-monomer correlations are indépendant of f. For each charge content, the polymer concentration was varied in a range between 0.03 and 0.6 monomol 1*1. A typical set of scattering profiles is shown on figure 2 for a CsPSS sample with f = 0.35. The same evolution is observed at each f: I(q)/c extrapolated at q = 0 increases as c decreases ; the peak is visible even at the lowest concentrations and its position q* shifts to larger q's as Cp increases; the intensity scattered per monomer also decreases as Cp increases and the peak broadens. One striking feature of this set of data is the appearance of a very weak associated second order scattering shoulder at a position qi of the order of 1.8q*, best seen in a I(q)q^ vs q representation (figure 3). It is clearly visible for small f ( < 0.5), when the total scattered intensity is maximum and the main peak is situated at low q's. It is impossible to know whether it is absent at high f (f > 0.7) or too small to be visible above the background. Both CsPSS and NaPSS show the same trend. For each f the position of the intensity maximum is found to vary as C p with a between 0.4 and 0.45 for both NaPSS and CsPSS (figure 4). Because of the limited concentration range that could be investigated and of a possible systematic error in the position of the peak at low angle when 1(0) is more intense, this result is compatible with the Cp^^ dependency usually found for charged polyelectrolytes. The f-dependance of q*, for samples containing a constant number of chains, scales like fb but, unlike the preceding case, the exponent is dependent on the cation type: b equals 0.9 for NaPSS and 0.7 for CsPSS. a

Effect of Added Salt. So far the reported data have been obtained in "salt-free" conditions, i.e. where only the counterions contribute to the ionic strength of the solution. By adding a simple electrolyte, it is possible to increase the ionic strength and considerably screen out the electrostatic interactions. A typical evolution of the structure factor is shown in figure 5 for NaPSS (f =0.67) in the presence of an increasing amount of NaCl, Cs, at a fixed polymer concentration of 0.32 mol l" . As Cs increases the maximum in the structure factor shifts slightly to smaller qvalues while the intensity at q* increases; the peak is wiped out for a ratio cp/c which is a strong function of f. The maximum of intensity is then centered on q = 0 and simultaneously, its intensity 1(0) increases, q* varies approximately as Cs~*. This behaviour appears to be quite general for all cp and f. A more systematic study is in progress. 1

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0.055

0.11

f=0.27

0.165

0.22

1

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Figure 1. Normalized SAXS profiles for NaPSS at various charge contents f between 0.27 and 0.8. The monomer concentration is constant at 0.32 mol H . ι

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0.20

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Figure 2. Normalized scattered intensities per monomer for CsPSS at f=0.35 as a function of polymer concentration. Concentrations are noted on the figure in monomol l" . 1


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1.9 10

1.3 10'

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Figure 3. Kratky plot of the intensity scattered by a solution of CsPSS at f = 0.35 and Cp = 0.16 monomol H showing the existence of two peaks in the structure factor.

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Figure 4. Scattering vector corresponding to the peak position plotted as a function of polymer concentration for f = 0.27, 0.35, 0.41 and 0.69 (log-log representation). The full straight lines correspond to the best fits with slopes of 0.4, 0.42 and 0.43.



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ESSAFI E T A L


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7

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0.1

0.2

0.3

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Figure 5. Evolution of the scattering profiles as a function of increasing salt concentration for a sample of NaPSS at f=0.67 and c =0.32 monomol l* . The concentrations of increase from (1) to (7) and correspond to 0,10' ,6 ΙΟ" ,10" , 10- ,3 10-1 and 1 mol of Nad. 1

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3

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Effect of Counterfoils. The effect of charge content and polymer concentration have been investigated on both NaPSS and CsPSS samples. The same general qualitative behaviour is observed, although the absolute value of q* depends on the type of cation. As noted earlier, q* scales as cp^ f both ions but at the same concentration the peak for the sodium salt is narrower and displaced to higher q's. It is also less intense, as might be expected since the counterions are likely to contribute to the scattering and sodium is a lighter element than cesium. Because the precise origin of the X-ray contrast for polyelectrolytes is difficult to ascertain in the absence of a detailed model, we have compared X-ray and neutron scattering data of the same sample of protonated NaPSS dissolved in D2O. With the latter technique, the chain is visible whereas the counterions are not. The peak was found at exactly the same position, indicating that the counterions do "decorate" the chain and thus a appreciable number of them are in the close vicinity of the chain. However the scattered intensity was so much weaker that it was not possible to be sure of the presence of the second order shoulder. 2


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Discussion A striking feature of these SAXS data is the continuity of all structural properties as the proportion of charged monomers f varies from 0.27 to 1. This implies that a unique model should be able to describe the semi-dilute solutions of this polyelectrolyte when it is fully charged as well as when one in every three monomers is charged. The structure of semi-dilute solutions of fully charged PSS, as revealed by previous neutron scattering experiments (2,3) had usually been described in terms of the isotropic model first introduced by de Gennes et al. (1). However, if a qualitative agreement exist here as well for the fully charged PSS, important discrepancies have been found here for the less charged samples which are discussed in what follows. The shape and the width of the peak and the hint of a second order are indicative of stronger correlations than predicted by a correlation hole effect. In the original model, the total structure factor S(q) reaches a maximum value at q*, of the order of the inverse of the Debye-Huckel screening length κ* , where κ =4π1β(fcp+2c ) and IB is the Bjerrum length; for q > q*, S(q) decreases as q" , then as q' as q increases. This behaviour reflects the flexible character of the chain at large scale and its rigidity at short scale. The presence of a maximum in intensity is not due to some order in the solution but rather to the fact that no two monomers belonging to different chains can approach each other closer than K"l. Hence the term "correlation hole effect". Experimentally, it is found that the peak can befittedreasonably well with a lorentzian line shape; a q~ dependence as been found (within experimental error) only in the tail of the peak and, surprisingly, no q'* dependence could be detected. At low f this behaviour could be masked by the second order shoulder but it is not visible either at high f when no shoulder is seen. Therefore it appears that the observed SAXS structure factor does not show the monomer-monomer correlations characteristic of arigidchain (rodlike) at short length. It is interesting to note that no rodlike behaviour has been found either in a recent molecular dynamics simulation of the structure factor of polyelectrolyte solutions above c* (13). However a q~l behaviour has been reported in a neutron scattering investigation of quaternized poly(2-vinylpyridine) (14). Clearly more experimental and theoretical effort has to be devoted to this question. Although both cp and f change the ionic strength of the solution and are equivalent as far as the electrostatic interactions are concerned, they do not have the same effect on the structure of the solution. The observed variation of q* with C p ^ is predicted whenever the characteristic length associated with the peak is related to the screening length K"l and is generally found for all charged polyelectrolytes. It is 1

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21. ESSAFIETAL.


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not therefore a very severe criterion for a specific model. The fact that the effect of f is counterion dependent, at least for Na and Cs, is puzzling; it could reflect a difference in condensation for these ions. Indeed condensation should take place for most of our samples: for the fully charged chain, the average distance between charges is of the order of a monomer size (2.5Â) and much smaller than the Bjerrum length of 7.14Â in water at 25°C, and a considerable fraction of the counterions should be condensed. For the lowest f, one in every three monomers is charged; since the charges are spaced from the backbone and can alternate along the chain, their average distance is more than three bond lengths and slightly larger than 1β. In the absence of a model that takes explicitely into account the effect of f, it is tentatively proposed that two characteristic distances appear in this problem, the electrostatic length K~1 for the solution and a length related to the chain itself that would also depend on the presence of condensed counterions. Although the charge content of these polymers is too high for them to qualify as weakly charged polyelectrolytes, we did compare our data to the predictions of the mesophase model (#, 9,70). In this model the perturbation introduced by the charged monomers is small, the chain is assumed to remain almost gaussian and the structure factor can be calculated exactly. A broad peak appears which reflects the fact that concentration fluctuations of characteristic length ίπ/q* are favoured in the system; at the microphase transition it would transform into a Bragg peak corresponding to the periodicity of polymer-rich and polymer-poor regions. The relevant parameter, q*2 + K2, is predicted to vary as f C p l ^ N h relation is found for any of our data. The hydrophobicity of tne polystyrene backbone is likely to play an important role in the organization of the solution. Indeed fluorescence measurements of a small amount of pyrene molecules dissolved in the solution show that its environment is rather hydrophobic even when the chain is fully charged; the ratio of the intensities of the first to third peak of the fluorescence emission spectrum which is sensitive to the polarity of the pyrene local environment, is 1.46 for a cp of 0.038 mol l" compared to 1.9 for pure water. Furthermore there is no discontinuity in the intensity ratio as a function of f which would show the existence of a structural transition; it decreases smoothly as f decreases (to be published). If the local dielectric constant close to the backbone is low, some ion-pairs can remain associated as dipoles and further increase the chain insolubility. 0

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Conclusion This SAXS investigation of semi-dilute solutions in water of poly(styrenesulfonate) at intermediate charge densities has shown a striking continuity of all structural properties from the fully charged polyelectrolyte to the chain where 30% of the monomers only are charged. This implies that a single model should describe the solution properties for all charge contents. No adequate model seems to exist for the moment. It has been found that the interchain monomer-monomer correlations are stronger than those due to a simple correlation hole effect and that the intrachain ones do not show rodlike behaviour even when highly charged. No transition to a microphase separated structure has been found, although the hydrophobic backbone of polystyrene seems to have a definite influence on the structure of die solution. In order to have a deeper understanding of the structure of these polyelectrolytes, complementary experiments are in progress to test the single chain configuration by small angle neutron scattering and the counterion environment by nuclear magnetic resonance.


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References and Notes 1. de Gennes, P.G.; Pincus, P.; Velasco, R. M; Brochard, F.,J..Physique, 1976, 37, 1461. 2. Nierlich, M.; Williams, C. E.; Boué, F.; Cotton, J. P.; Daoud M.; Farnoux, B.; Jannink, G.; Picot, C.; Moan, M.; Wolf, C.; Rinaudo, M.; de Gennes, P. G., J. Physique, 1979, 40, 701. 3. Jannink, G.; Makromol. Chem., Macromol. Symp., 1986, 67, 1; and references

therein Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 21, 2016 | http://pubs.acs.org Publication Date: December 13, 1993 | doi: 10.1021/bk-1994-0548.ch021

4. Drifford, M.; Dalbiez, J. P., J. Phys. Chem.. 1984, 88, 5368; J. Phys. Lett. (France), 1985, 46, L311.

5. Sedlák, M.; Amis, Ε. J, J. Chem. Phys., 1992, 96, 826. 6. Ise, N., Angew. Chem., Int. Ed. Engl., 1986, 25, 323.

7. Sedlák, M., Macromolecules, 1993, 26, 1158. 8. Borue, V.; Erukhimovich, I., Macromolecules , 1988, 21, 3240. 9. Joanny, J. F.; Leibler, L.,J. Physique , 1990, 51, 545. 10. Moussaid, Α.; Schosseler, F.; Munch, J. P.; Candau, S.J.,J. Phys II France, 1991, 1, 637. 11. Makowski, H. S.; Lundberg, R. D.; Singhal, G. S., US Patent 3870841, 1975, to EXXON Research and Engineering Company. 12. The weight of dissolved polymer has been calculated so that the number of chains is kept constant for all charge contents 13. Stevens, M.J.; Kremer, K., to be published 14. Förster, S.; Schmidt, M.; Antonietti, M., Polymer, 1990, 31, 781. RECEIVED August 22, 1993


Structure of Polyelectrolyte Solutions at Intermediate Charge Densities

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