Extraordinary Behavior of Salt-Free Solutions of Strongly Charged

Chapter 26 Extraordinary Behavior of Salt-Free Solutions of Strongly Charged Polyelectrolytes

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on February 21, 2016 | http://pubs.acs.org Publication Date: December 13, 1993 | doi: 10.1021/bk-1994-0548.ch026

Marián Sedlák Institute of Experimental Physics, Slovak Academy of Sciences, 043 53 Košice, Slovakia

The extraordinary behavior of salt-free polyelectrolyte solutions as observed by light scattering, i.e. existence of the slow mode and angular dependencies of scattering intensity, is discussed in light of three series of experiments. Results on binary mixtures of NaPSS with different molecular weights indicate that the slow mode originates as a result of intermolecular interactions and cannot be explained on the basis of purely intramolecular processes or stable particles/impurities in solution. A comparison between data obtained on samples filtered through filters with different pore sizes demonstrates thatresultsare influenced by the filtration procedure, however, the origin of the slow mode is in electrostatic interactions, not in filterable aggregates. The third series of experiments deals with a possible role of non-electrostatic interactions in the mechanism of the slow mode due to the hydrophobicity of a polyion backbone. Results are not in favor of this assumption.

The issue of the extraordinary behavior of salt-free solutions of strongly charged polyelectrolytes has been rather controversial forrecentyears. Taking into account the complexity of the problem, this situation is not so surprising. The multicomponent nature of polyelectrolyte solutions isreflectedin many types of interactions: intramolecular interactions between segments of the same macromolecule, intermolecular interactions between different macromolecules, interactions between polyions and counterions, electrostatic screening by counterions, co-ions and even by polyions themselves, polymer-solvent interactions, and entropie forces (mainly due to the presence of counterions). This complexity becomes "more visible" in the limit of low salt concentrations, where unscreened interactions are strong and longrange. Experimental data obtained by numerous techniques is at variance with what is typical for neutral macromolecules and even for polyelectrolytes at high salt concentrations. Typical examples are peaks in the concentration dependence of reduced viscosity (7,2) and angular maxima in experiments of small-angle X-ray or small-angle neutron scattering(3,4). Other less frequent techniques show also rather unusualresults(5).Static and dynamic light scattering, which were used as main experimental techniques in our investigation, belong to the most sensitive tools for the study of these systems. A characteristic feature of static light scattering data is a 0097-6156/94/0548-0337$06.00/0 © 1994 American Chemical Society

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

338

MACRO-ION CHARACTERIZATION

pronounced angular dependence of scattered light, even for very small molecules (e.g. Mw = 5000 (6)). The occurrence of two dynamic modes with widely separated characteristic times is typical for dynamic light scattering (7,8,9 ). The relatively fast one is on the order of several microseconds and the relatively slow one is on the order of 10 to 10 microseconds. The slow mode appears when the salt concentration decreases below some critical value. This was referred to as an ordinary-extraordinary transition (10). The existence of the slow mode and pronounced angular dependencies of scattering intensity was interpreted as a result of formation of multichain domains (clusters, temporal aggregates) in solution (7-9). Results of various theoretical treatments of salt-free polyelectrolyte solutions differ qualitatively according to accepted approximations (11-13). Most of general theoretical calculations for polyelectrolyte systems do not predict any experimentally observed "extraordinary behavior" and are thus restricted to low charge densities and/or higher salt concentrations. The aim of the present paper is to contribute to the understanding of the extraordinary phenomena which are currently a subject of frequent scientific discussions.


3

5

Experimental Sodium poly(styrenesulfonate) (NaPSS) with M = 5 000, 47 000, 100 000 and 1 200 000 was from Pressure Chemical (Pittsburgh, PA). Poly(acrylic acid) (PAA) with M = 90 000 and poly(methacrylic acid) (PMA) with M = 30 000 were prepared by radical polymerization. All samples were thoroughly purified by mixedbed ion exchange resins. According to our previous experience (6), even dialyzed grades of commercially available NaPSS contain several percent of Na2S04 as an impurity. Consequently, rather different results are obtained without and with an ion exchange purification. The acidic form of poly(styrenesulfonate) was converted after the purification back to the sodium salt bytitrationwith sodium hydroxide. Polyacids were neutralized by NaOH and LiOH to various degrees of ionization. Deionized water with resistivity of 15 ΜΩαη was used as a solvent Binary mixtures with given compositions χ = c(P2)/(c(Pl)+c(P2)) where c(Pl) and c(P2) are concentrations of polymer 1 and polymer 2, respectively, were prepared by mixing of binary solutions. Samples were gently shaken and allowed to stand and mix sufficiently. Solutions werefilteredthrough 0.1- and 0.2- μπι polycarbonate membrane filters from Nuclepore. These filters were chosen because the pores are well-defined and relatively monodispersed. This was important especially for the investigation of the influence of the filter pore size on the solution behavior. The light scattering measurements were made using a Carl Zeiss Jena ILA 120-1 argon laser with 200 mW at 514.5 nm. A laboratory made goniometer was used to collect data from 30° to 150° for both static and dynamic light scattering experiments. The scattering cell was thermostated in the temperature range 10 °C to 80 °C with a precission of ± 0.1 °C. Scattering intensities were measured by photon counting and normalized using a doubly distilled and filtered benzene as a standard. Photon correlation measurements used an ALV5000 correlator with a transputer board for numerical analysis of correlation curves by CONTIN program. Correlation curves were in most cases multi-exponential. Characteristic decay times τ\ and their relative amplitudes Α(τΟ were evaluated through the moments of distribution functions of decay times Α(τ). Diffusion coefficients were then calculated as Di = (l/xi)q"2 where q is the scattering vector defined as q = (47cnAo)sin(0/2), with η the solution refractive index, λο the laser wavelength, and θ the scattering angle. w

w

w


26. SEDLAK

Salt-Free Solutions of Strongly Charged Pofyelectrofytes 339

Results and Discussion Light Scattering from Binary Mixtures of Polyelectrolytes. The slow diffusion coefficient D at relatively high polymer concentrations (-10 g/L) strongly depends on molecular weight (in the range of two orders of magnitude)(6). Hence upon mixing of two samples with appreciably different molecular weights, two peaks in the spectrum of relaxation times corresponding to two slow diffusion coefficients could be (in principle) separated, even in the case of some shifts with respect to positions from binary solutions. The main stimulus for performing experiments on binary mixtures of polyelectrolytes with different molecular weights was to prove or rather disprove this possibility. NaPSS was chosen for these experiments because of the availability of welldefined monodispersed samples and very detailed experimental data on molecular weight dependencies(tf). Fractions with M = 5 000, 100 000, and 1 200 000 were used. A set of solutions with a total polymer concentration c = 25 g/L and various mixture compositions χ = c(P2)/(c(Pl)+c(P2)) was prepared [c(Pl) and c(P2) are concentrations of polymer 1 with M = 5 000 and polymer 2 with M = 1 200 000, respectively]. Static and dynamic light scattering measurements were performed on these samples. Distribution functions of characteristic times Α(τ) obtained from correlation curves for selected values of χ are presented in Figure 1. As can be seen, the peak corresponding to the fast mode (Df) does not change it's position with x, although the relative amplitude slightly increases with x. More importantly, only one peak corresponding to the slow mode (Ds) was observed in all cases. The position of the peak shifts towards longer times with χ going from 0 to 1, while the amplitude remains high and the slow mode dominates the spectrum. In addition to the fast and slow modes, a third mode clearly developes upon mixing of two polyelectrolyte samples. Therelativeamplitude of this modereachesa maximum around χ = 0.5 and decreases to zero in the limits χ — > 0 and χ -» 1. As will be discussed later, this mode is diffusive in nature and hence it is marked as D in Figure 1. Figure 2 shows the dependence of the slow diffusion coefficient D on the mixture composition x. D gradually decreases with increasing x. The fast diffusion coefficient Df is independent of χ (not shown). Static light scattering measurements yielded linear dependencies of the normalized inverse scattering intensity Ι(0)/Ι(θ) on sin (8/2), where θ is the scattering angle. Apparent radii of gyration were calculated according to the formula


s

w

w

w

m

s

s

2

2

2

2

Ι(0)/Ι(θ) = 1 + Csin (0/2)

1/2

θ -> 0

2

(1) 2

1/2

with C = 1/3(4πη/λο) , and are summarized in Table I. No dependence of on χ was observed. Our interpretation of the existence of the slow diffusive mode and angular dependencies of scattering intensity is based on an idea of formation of multichain domains, where the slow diffusive mode corresponds to the dynamics of creation and annihilation of domains (slow, long-range fluctuations in refractive index), and the apparent radius of gyration obtained from angular dependencies corresponds roughly to the size of domains. Domains are not considered to be stable particles, but rather temporal objects. Detailed discussions supporting this temporal nature of domains goes beyond the scope of this presentation. Briefly it can be noted that while D$ is strongly dependent on molecular weight (two orders in magnitude), the apparent size of a domain is independent of molecular weight A different relationship between the size and the diffusion coefficient holds for stable particles.


340


[

ίο Ε0.5 0.0


0.5 F 0.0 1.0

Δ

0.5 0.0 1.0 0.5 0.0 1.0 0.5 0.0 1.0 0.5 0.0 ίο-·

10

Ι Ο 10»

e

1

10»

10*

10»

τ, MS

Figure 1. Spectra of relaxation times for binary mixtures of two polyelectrolyte samples in water: PI = NaPSS, M = 5 000, P2 = NaPSS, M = 1 200 000. The total polymer concentration was c = 25 g/L. The mixture composition χ = c(P2)/(c(Pl)+c(P2)) was from top to bottom χ = 0,0.046, 0.188, 0.488,0.824, 1.0. All measurements were performed at θ = 90°. Three diffusive modes present for 0 < χ < 1 are marked as Df, D , and D . w

m

w

s

Figure 2. Dependence of slow diffusion coefficient D on mixture composition χ of the binary mixture of NaPSS samples with M = 5 000 and M = 1 200 000 in water. s

w

w


26. SEDLAK

Salt-Free Solutions of Strongly Charged Polyelectrolytes 341

In the case of a low molecular weight sample (M = 5 000), domains could be visualized as temporal particles consisted of many chains (on the order of hundreds or more), surrounded by solvent with single polyions. In the case of the high molecular weight sample (M = 1 200 000), the overlap is highly probable and domains could be visualized as regions of higher density and/or higher order in the net. The process of creation and annihilation of domains is much slower in the net where the motion of chains is more restricted than in a not-entangled solution of very short and mobile chains. Regardless this hypothesis, two general conclusions can be made from the present data: (1) the slow mode is not caused by any stable species (particles) in solution; and (2) it does not correspond to an intramolecular (or purely intramolecular) process. In both cases two peaks would be expected. The existence of only one fairly narrow peak oppositely confirms the intermolecular interaction character of the slow mode. The independence of Df and on the mixture composition χ is consistent with the independence of these quantities on molecular weight (6). Admitting that the model of the visualization of domains is correct, we can further speculate on what happens upon mixing of two solutions. The addition of a small amount of the high molecular weight sample into a solution of the low molecular weight sample (χ ~ 0) causes incorporation of long chains into domains and a relatively rapid slowing-down of the process of creation of domains as longer chains are less mobile than short ones. Oppositely, addition of a small amount of the low molecular weight sample into a net in die solution of the high molecular weight sample (χ ~ 1) accelerates the process. The change in the rate of the process is, however, not as pronounced as for χ - 0. It seems that the slower motion (i.e. the motion of longer chains) is crucial for thefinaleffect. Table Π shows results for mixtures of three different samples. Again, it can be seen that Df and are independent of the combination of polymer samples. D of the mixture appears to be a geometrical average of D values from corresponding binary solutions. More combinations would be helpful to prove exactly this conclusion, but still, geometrical averages are much closer to real data than arithmetic averages. The geometrical average means that lower values of the diffusion coefficient are more weighted. This is consistent with the above mentioned statement that the slower motion is crucial for thefinaleffect in the domain formation. Another set of experiments was performed on the total polymer concentration dependence. The results will be published in a separate paper (14). Here we can briefly mention that the concentration dependence of quantities characterizing the extraordinary phenomena resembles those from binary solutions. The "medium" diffusive mode marked as D in Figure 1 gets quickly in influence with decreasing the total polymer concentration (Figure 2). This new mode will be also discussed in detail (14). According to our experimental data, it was interpreted as a polyelectrolyte interdiffusion. It was proved that the main effect of the interdiffusion is caused by electrostatic interactions (14 ). w


w

2

2

1/2

1/2

s

s

m

On the possible role of non-electrostatic interactions in the mechanism of the slow mode. Although there is an increasing number of experimental data on the slow mode, the mechanism of this mode is not clearly understood yet. The question is what type of interactions play a key role in the process of domain formation. As light scattering is sensitive to fluctuations in solution refractive index, domains can be visualized as regions with a different refractive index than the rest of the solution. This can be caused by a different density and/or different degree of order inside the domain. The question is how these regions arise. An attractive interaction between charged particles through the intermediary of counterfoils was proposed by Ise (13). This idea was created on the basis of a very extensive experimental work, mainly naked eye observations of charged latex


342


Table I. Dependence of the Apparent Radius of Gyration of Domains (nm) on Mixture Composition χ χ 2

1/2

0.000 86.6

0.0005 84.5

0.085 83.6

0.217 90.0

0.488 84.0

0.766 93.7

1.000 109.8

The mixture composition is χ = c(P2)/(c(Pl)+c(P2)), and the molecular weight ( M ) of the NaPSS were PI = 5 000 and P2 = 1 200 000. The total polymer concentration was c = 25 g/L. Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on February 21, 2016 | http://pubs.acs.org Publication Date: December 13, 1993 | doi: 10.1021/bk-1994-0548.ch026

w

Table II. Values of Fast (Df) and Slow (D ) Diffusion Coefficients and Apparent Radius of Gyration of Domains for Mixtures of Three Different NaPSS Samples s

2

Sample

M

w

Df (10-6 m2/s) C

PI 5000 P2 100 000 P3 1 200 000 P1+P2 P1+P3 P2+ P3

5.6 5.7 5.5 5.7 5.7 5.5

M

(I0" cm2 /s)

l/2 (nm)

44.0 20.6 4.15 30.7 15.8 8.3

62.4 59.4 87.9 65.6 65.1 84.3

9

.

(10" cm2/s) 9

30.1 13.5 9.3

The mixture composition was χ = 0.5 with a total polymer concentration of c = 5 g/L. is the geometrical averages defined as = (Dj D p where Dj and Dj are values of the slow diffusion coefficient for samples Pi and Pj, respectively. 1/2

10-'

10»

10·

10*

10»

10·

10»

τ, με

Figure 3. Spectra of relaxation times for binary mixtures of two polyelectrolyte samples in water: PI = NaPSS, M = 5 000, P2 = NaPSS, M = 1 200 000. The mixture composition χ = c(P2)/(c(Pl)+c(P2)) was 0.488. The total polymer concentration in g/L was from top to bottom c = 25.0, 15.0, 3.8, 1.7, and 0.58. All measurements were performed at θ = 90°. w

w



26. SEDLAK

Salt-Free Solutions of Strongly Charged Polyelectrolytes 343

particles. Authors believe that the mechanism could be the same for linear polyelectrolytes, where individual chains have a compact conformation due to intramolecular attractive interactions and the system resembles a solution of charged spheres. Schmitz (7) explained the existence of "temporal aggregates" as a result of fluctuating forces created by counterion dynamics. The instantaneous distribution of counterions around a charged chain is assymmetrical and hence dipole attractive interactions are possible. Recently an issue of the hydrophobicity of a polyelectrolyte chain backbone in a polar solvent was discussed (75-77). Theory was developed for weakly charged polyelectrolytes (75,76) and it was shown that formation of mesophases (microscopic dense regions) or micelles originating as a result of aggregation of hydrophobic parts of chains between charges, is possible. Thus a question arose if this mechanism could play some role in the domain formation in solutions of strong polyelectrolytes. Combinations of two polymers [poly(acrylic acid) and poly(methacrylic acid)] and two solvents (water and methanol) were taken into consideration. Water is a better solvent for PAA than methanol. In the case of PMA, the situation is opposite, i.e. methanol is a better solvent than water (7S,79). It has been shown (9,20) that while domains are very pronounced in aqueous PMA solutions, they do not form in methanolic PMA solutions. This could be explained according to above mentioned ideas by a weaker or no aggregation of chain backbones in a better solvent (methanol). In this case, results for PAA should be opposite, i.e. the tendency to form aggregates should be higher in methanol than in water. Therefore measurements were performed on methanolic and aqueous PAA solutions. Samples were neutralized by lithium hydroxide in water and by lithium methoxide in methanol. Figure 4 shows spectra of relaxation times for PAA solutions neutralized to degree of neutralization α = 0.2 in methanol (a) and water (b). The polymer concentration was c = 20 g/L in both cases. As can be seen, there is a pronounced difference in these spectra. While the situation in water is typical for strongly charged salt-free polyelectrolytes, (the slow and fast modes are widely separated and the slow mode dominates), the methanolic sample shows a relatively broad spectrum with less pronounced slow mode. Quantitatively, the slow diffusion coefficient in the methanolic solution is "less slow" and the ratio Ag/Af of amplitudes corresponding to slow and fast modes, respectively, is much lower (Table ΙΠ). Clearly, the extraordinary behavior is less pronounced in methanol. This can be also supported by results of static light scattering obtained on the same samples. Figure 5 shows angular dependencies of normalized reciprocal excess scattering intensities for both samples. In the case of the aqueous solution, a pronounced dependence yields apparent radius of gyration of 64 nm, while in the case of the methanolic solution, there is a very slight angular dependence at the experimental recognition threshold ( ~ 26 nm). These results clearly do not support an assumption that the hydrophobic aggregation due to the hydrophobic nature of the polymer backbone is the main factor in domain formation. Another way to verify the possible role of hydrophobic interactions in domain formation is to change solvent quality by a change in temperature. The tendency to form domains should be accordingly temperature-dependent. However, results of temperature dependencies also contradict the idea of the hydrophobic aggregation. Table IV shows results obtained by static and dynamic light scattering on an aqueous solution of NaPSS (M = 47 000) at different temperatures. Dfc and D are values of diffusion coefficients corrected for a change in solvent viscosity and kinetic effects according to the formula 2

w

1/2

s c

Die = Di(T) (η(Τ)Τ /ησο)Τ) 0


(2)


344


Extraordinary Behavior of Salt-Free Solutions of Strongly Charged

Recommend Documents