Interaction between Polyelectrolytes and Latices with Covalently

Chapter 12

Interaction between Polyelectrolyte s and Latices with Covalently Bound Ionic Groups

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H. Dautzenberg, J. Hartmann, G. Rother, Κ. Tauer, and K.-H. Goebel Max-Planck-Institute of Colloid and Interface Research, O-1530 Teltow-Seehof, Germany

The interaction between latex particles with covalently bound ionic groups and polyelectrolytes was studied by dynamic and static light scattering. In the systems investigated two effects occurred: a coating of the latex particles with a closed-fitting polyelectrolyte layer and coagulation, caused by interparticle bridging. The aggregation effects could be suppressed by a large excess of the polycation in highly diluted systems. The amount of bound polyelectrolyte was determined via a subsequent polyelectrolyte complex formation. Possible determinants of the structure of the polyelectrolyte layer are discussed.

It is a well-known fact that in highly diluted systems latex particles may be covered with a layer of water-soluble polymers, preventing coagulation. This makes it possible to study the structural parameters of the polymer layer by static and dynamic light scattering. Killmann et al.(7) could show that the thickness of the adsorbed layer of poly(ethyleneoxide)s on polystyrene latexes depends strongly on the amount of the adsorbed polymer and still more on the molecular weight of the polymer. The strong Coulombic interaction between a charged latex surface and an oppositely charged polyelectrolyte impedes the suppression of coagulation effects. Main objectives of this paper are : - coating of a charged latex with a polyelectrolyte without coagulation - study of the structural parameters of the polymer layer , using static and dynamic light scattering - determination of the amount of bound polyelectrolyte by subsequent polyelectrolyte complex formation 0097-6156/93/0532-0138S06.00/0 © 1993 American Chemical Society

Dubin and Tong; Colloid-Polymer Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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DAUTZENBERG ET AL.

Interaction between Polyelectrolytes and Latices

- modification of the structure of the polymer layer by variation of the charge density of the latex. Materials To exclude effects of desorption of the ionic groups on the latex surface during the interaction with an oppositely charged polyelectrolyte we used a latex with covalently bound ionic groups. The latex was synthesized by emulsion polymerization employing a non-ionic initiator and a monomer emulsifier with sulfonate groups (2,3), which is primarily incorporated at high degrees of conversion (Figure 1), leading to high charge density on the surface of the latex particles. Conditions of polymerization see Table 1. The latex sample was characterized by turbidimetry, light scattering, electron microscopy and conductometric titration. The polycation used to cover the latex was poly(diallyl-dimethylammonium chloride). As polyanion in PEC formation Na-poly(styrene sulfonate) was employed. To vary the charge density by blocking of the anionic charge of the latex we applied the cationic surfactant Hyamine 1622. The characteristics of all samples used in this work are given in Table 2. Methods of investigation Static and dynamic light scattering as well as electron microscopy were used for structural investigations. By a combination of these methods one may determine the mass, the radius of gyration, the hydrodynamic radius and the polydispersity of a particle system. By a comparison of the results obtained for the original and the polymer covered latex the structural parameters of the polymer layer can be assessed. Ultracentrifugation was used for latex separation. A subsequent determination of the polycation concentration via PEC-formation in the supernatant yields exact information on the amount of polycation, which was bound on the latex. Experimental All experiments were carried out by mixing of the latex and the polymer solutions directly in the scattering cell of the light scattering instrument, a Sofica 42 000 (Wippler and Scheibling, Strasbourg, France) under gentle stirring, with continuous registration of the light scattering intensity at fixed angle (Θ = 45°). As solvent saltfree water, purified by repeated filtration


139

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COLLOID-POLYMER INTERACTIONS

ΙΚ·/·3

Figure 1. Amount of polymerized monomer emulsifiers in dependence on the degree of conversion. Styrene with Na-sulfopropyl-octadecyl-maleate (o) Na-sulfopropyl-tetradecyl-maleate (x). Table 1; Condition of synthesis of latex: Emulsion polymerization: - 10 h at t = 60° C under stirring Composition: water: 250 g styrene: 50 g Na-sulfopropyle-tetradecyl-maleate: 6.85 g (monomer emulsifier), M = 456.6 Azoisobutyronitrile: 436 mg (non-ionic initiator)


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DAUTZENBERG ET AL.


and ion exchange, was used. The solutions were made dust-free by filtration through 0.2 μιη-membrane filters. Addition of solutions into the scattering cell was carried out via an immersed capillary with very slow dosage rates. Periodic interruptions of the dosage were made so that the complete angular dependence of the scattering intensity could be obtained in various intermediate states. Dynamic light scattering experiments were carried out with the instrument SIMULTAN (ALV, Langen, Germany). The correla tion functions were analyzed by a third order cumulant fit. Latex separation was performed by 4 hour centrifugation at 50 000 revol./min with a preparative ultracentrifuge (Janetzki, Germany). Results Latex coating. The first objective was to cover the latex with the polycations without coagulation. Cationic surfactants as well aspolycations are normally used as titrants for the determination of the charge density of anionically stabilized latexes (4). In an appropriate range of concentration, the point of charge compensation is indicated by clearly pronounced flocculation. In a first experiment we added to 10 mL of a latex solution of concentration c = 8.15 10 g cm" the polycation (PC) solution of c = 1 10' g cm* with a dosage rate v = 2 mL/70 min. Fig. 2 shows the dependence of the reduced scattering intensity I /c (normalized to 1 for the pure latex) on the added volume of the PC-solution. After the addition of 1.8 mL PC-solution, flocculation occurred. This corresponds exactly to a 1:1 charge stoichiometry in the system, because the number of charges N of the latex and the polycation respectively is given by N = N c V m , where N - Avogadro's number , c - concentration, V - volume, m molar mass per charge unit (see Table 2). The increase of the l4 /c -values indicates a coagulation of the latex already at small contents of polycations. The scattering curves, registered after stepwise addition of the polycation solution are given in Figure 3 as the Guinier plot ln (R(h)/(K c)) vs. sin (0/2), where R(h) - Rayleigh ratio of the scattering intensity, Κ - contrast factor, c - mass concentration of the scattering species, h = 4π/λ sin(0/2), λ - wave length in the medium, θ -scattering angle. The increasing level and curvature of the scattering curves confirm the increasing coagulation with addition of polycation. The addition of the latex into the PC solution should provide a better means of covering the latex completely with polycation without coagulation. Therefore, we added to 10 mL PC solution of concentration c = 2 10' g c m a latex solution of c = 9.35 10~ g cm" with a dosage rate v = r5

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Table 2: Materials Latex: Polystyrene with covalently bound Na-sulfonate groups Radius: a = 40 ± 2 nm Polydispersity: relative standard deviation σ = 0.1 Surface charge density: ρ = 18 μΟ cm" =1.12 10 e cm" Molar mass per charge unit: m^ = 7350 Refractive index increment: dn/dc = 0.243 cm g" Polycation: Poly(diallyl-dimethyl-ammonium chloride) (PDADMAC) M = 1.4 10 g mol" , M /M = 2 m = 161.5 , dn/dc = 0.193 cm g Polyanion: Na-Poly(styrene sulfonate) (NaPSS) 2

14

3

5

2

1

1

w

w

n

c

3

l

3

1

c

M

5

1

= 2.0 10 g mol ,

w

MJU

= 1.1

n

A

m = 204 , dn/dc = 0.192 cm g Cationic surfactant: Hyamine 1622, Switzerland ( C H CI N 0 . H 0 ) m = 466 , dn/dc = 0.192 cm g c

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4 2

2

3

2

1

c

interruption

v CmU c

Figure 2. L ^ L of the system latex/polycation (normalized to 1 for the pure latex) in dependence on the volume of the added PC-solution.


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DAUTZENBERG ET AL.

4 mL/70 min. However, the results obtained for the reduced scattering intensity I /c (Figure 4) again show coagulation and flocculation, also indicated by the curvature of the scattering curves (Figure 5) at intermediate states. Macroscopic flocculation occurred before charge compensation was reached (theoretically at V = 9.7 mL). On the other hand, the extrapolation of the curve in Figure 4 to V 0 shows that I /c tends to the value of the non-coagulated latex. Consequently, it may be expected that at very low concentration of the latex and very high excess of polycations coagulation can be suppressed. To check this, we investigated several systems , obtained by slow addition of highly diluted latex to PC solutions of differently high excess concentrations of polycation. It must be notized here that "excess of polycations" means an excess of the cationic charges in the system and not of the mass concentration. Table 3 gives the results of static and dynamic light scattering measurements on such systems. With increasing PC concen tration a decrease of the radius of gyration and the hydrodynamic radius was observed, while the M -value remained nearly constant, i.e. in all systems coagulation plays a negligible role. The increase of the mass of the latex particles by covering with polycations corresponding to a 1:1 stoichiometry is in the range of two percent (see the different m -values), which is in the limit of the accuracy of preparation and measurement. At very high excess of polycations the radius of gyration and the hydrodynamic radius approach to the values of the pure latex. This result can only be explained by a coating of the latex with a compact layer of polycations. The scattering curves of the systems 1, 2 and 6 of Table 3 show that coagulation is completely excluded only in the system with the highest PC concentration (Figure 6a). The angular dependence of the apparent diffusion coefficients reflects the same result (Figure 6b), but the sensitivity of the shape of the static scattering curve to coagulation is higher. In principle also the ratio of the second cumulant to the square of the first one (μ /Γ\ - relative quadratic standard deviation of the size distribution) should give an information about changes of the polydispersity due to coagu lation. However, at the scattering angle 90°, μ ^\ l î hi the range of 0.03 ± 0.02 and no systematic alterations could be detected. In the small angle range the fluctuations of the μ /ΐΥ - values are too large to draw reliable conclusions about coagulation. 45

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Determination of the bound amount of polycation. Since the amount of bound polycations could not be calculated from the increase of the mass of the latex particles, we tried to determine it from the concentration change of free polycations in the original solution and after the mixing with latex. PEC formation offers a very sensitive method for a quantitative analysis


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180

1

1

1

0.25

1

0.50

1

0.75 1.00 sin ( Θ / 2 ) 2

Figure 3. Guinier plot of the scattering curves at different stages of PC-solution dosage (compare Figure 2).

2.5

v [mU L

Figure 4.1 /c of the system polycation/latex in dependence on the volume of the added latex. 45

L


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DAUTZENBERG ET AL.


1 - + 2 - χ

In R(h)l Κ c

3 5 -o 6

20.0-

it

ΠΙΗ»

19.0-

18.0

' 0.25

1

« 0.50

1

'

0.75 1.00 s i n (Θ/2) 2

Figure 5. Guinier plot of the scattering curves at different stages of latex addition (compare Figure 4). Table 3: Results of light scattering measurements on coated latex at high excess of cationic charges. Addition of 8 mL latex (c = 1 10' g cm" ) to 8 mL PC-solutions of different concentrations (v = 4 mL/70 min), a, = < a > = (5/3 ) 5

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1/2

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1/2

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z

6

c 10 [g cm" ] c

3

0 1 2.5 5 10 25

Μ „ ΙΟ"

8

1

[g mol ]

1.7 ± 0.1

RH

[nm]

[nm]

43.1 56.0 52.0 48.7 46.4 43.0

42.2 46.7 43.8 43.5 42.1 42.2


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(5,d)« During the mixing of strong oppositely charged polyelectrolytes the growth of PEC particles started immediately with the mixing process. The structural parameters of the PEC particles do not change significantly with rising degree of conversion (X) up to about X = 0.9. In many cases flocculation occurs at the 1:1 charge stoichiometry of the polyelectrolyte components. This is demonstrated by the scattering curves of PEC particles between PDADMAC and NaPSS in highly diluted systems (c ° = 2.5 I0r g cm" , c ° = 5 10~ g cm' ) at different degrees of conversion (Figure 7). While the scattering curves up to X = 0.9 are quite similar, a sharp transition to a highly aggregated system occurs at full conversion, which is indicated by a change of the scattering intensity of about a factor 50 and a strong curvature of the scattering curve. To determine the fraction of polycations bound by the latex, we added to 8mL PC solution of a concentration c = 5 10' g cm" , 8 mL latex with concentrations of c = 5 10' g cm' and 1 10" g cm' respectively, corresponding to 22% and 44% of the charges of polycations. After separation of the coated latex by ultracentrifugation, the PC concentration was determined by PEC formation with NaPSS (c = 5 10" g cm' ) in comparison with a pure PC solution. The titration curves are given in Figure 8. Curve 1 corresponds to the titration of 8mL pure PC solution of a concentration c = 2.5 10" g cm . The full lines mark the points of 1:1 stoichiometry, taking into account the binding of polycations on the separated latex. Flocculation occurs exactly at these points, indicating a covering of the latex with a PC layer according to the anionic groups on the latex surface. The reliability of the results obtained may be assessed by a comparison with the flocculation point, which should be expected for the supernatants without binding of the polycations on the latex particles (dashed line). This line is somewhat shifted in regard to the pure PC solution, because only 7.5 mL of the supernatants were titrated. PEC formation was also carried out in the presence of coated latex. Three systems were prepared by mixing lOmL PC solution (c = 1 10" g cm ) with 0.5, 1.0 and 2.0 mL latex (c = 1 10" g cm" ) respectively, corresponding to 11, 22 and 44% binding of the polycation. 8 mL of the mixed systems were then titrated with NaPSS solution (c = 2.5 10" g/cm' , v = 4 mL/70 min). Even in this case the coagulation points are clearly pronounced (Figure 9), although the scattering level of the latex particles is much higher. The dashed lines in Figure 9 mark the flocculation points corresponding to the total amount of polycation in the systems and the full lines correspond to the flocculation for the amount of free PC. The experimentally observed flocculation points are in very good agreement with the amount of free PC, suggesting that the polycations bound on the latex do not take part in PEC formation. 5

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Interaction between Polyelectrolytes and Latices 147

1-χ 2- • 3- +

In R(h) Κ c

19.0

18.0 ι—1

«|! 6.00 «fc 4.00

• d

π α

α

0.25

0.50

075

1.00

s i n (Θ/2) 2

Figure 6. Scattering curves (a) and diffusion coefficients (b) of the systems 1 (1), 2 (2) and 6 (3) of Table 3. Ln R(h)

Κ c 20.0|

1 2 3 4 5 M =10-10 gmol6 2> i/2-300nm 7 8 * X * Χ Χχχ»** 9 10 -

+ * ° • ° • * · + χ

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Interaction between Polyelectrolytes and Latices with Covalently

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