Thickening of electrostatically stabilized latices by ethyl acrylate-methacrylic acid copolymers with various molecular weights

COLLOIDS

AND Colloids and Surfaces A: Physicochemicaland EnginccringAspects113 (1996)I-I0

ELSEVIER

A

SURFACES

Thickening of electrostatically stabilized latices by ethyl acrylatemethacrylic acid copolymers with various molecular weights P. Bradna a,,, p. Stern b, O. Quadrat a, J. Sfiupfirek c a Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, I62 06 Prague 6, Czech Republic b Institute of Hydrodynamics, Academy of Sciences of the Czech Republic, 166 12 Prague 6, Czech Republic c Department of Polymers, University of Pardubice, 532 10 Pardubice, Czech Republic

Received 20 April 1995; accepted 13 March 1996

Abstract

Electrostatically stabilized latices having various particle sizes were used to study the thickening effect of alkalisoluble ethyl acrylate - - methacrylic acid copolymer (50 wt.% of the acid; ~Qw=0.78 × 105, 4.36 x 105 and 11.6 x 105). It was found that the effect increases with the molecular weight of the copolymers and with decreasing size of the latex particles. The pronounced tendency towards non-bridging flocculation and phase separation suggests that the thickening effect of the materials results from depletion (volume restriction) flocculation rather than depletion stabilization. The role of the effective volume fraction of the dissolved copolymer as a variable which controls the rheological properties of the thickened latices was tested experimentally. Keywords: Electrostatically stabilized latices; Ethyl acrylate-methacrylic acid copolymers; Flocculation; Rheological properties; Thickening

1. Introduction

It was reported [ 1] that the thickening of electrostatically stabilized latices by dispersions of ethyl acrylate-methacrylic acid copolymers, i.e. a pronounced increase in the viscosity, yield stress and dynamic moduli of the system after alkali addition, strongly depends on the copolymer composition. With increasing content of the acid, the effect increased but, at the same time, flocculation and phase separation occurred. A more detailed study of the structure of the copolymers has shown differences in their behaviour during alkali addition. At a methacrylic acid content of 15-20 wt.%,

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the copolymers prepared by semicontinuous emulsion polymerization were branched or cross-linked, and thus the dispersion particles swelled on neutralization [2,3]. On the other hand, the copolymers containing more than 40 wt.% of the acid dissolved molecularly on alkali addition to a polyelectrolyte solution [2]. These results suggested that differences in the thickening effect of the copolymers might result from different mechanisms of the interactions of the particles of the latices with swollen dispersion particles or dissolved macromolecules. The swelling of dispersion particles increases the volume occupied by the dispersed phase, which leads to stronger interparticle interactions [4]. It was concluded that electrostatic repulsions between the copolymer microgels and the latex

2

P. Bradna et al./Colloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 1-10

particles stabilize the system, forming a kinetically stable pseudocrystalline lattice with pronounced elastic properties [ 1]. On the other hand, the mechanism of the thickening effect of molecularly dissolved polymers is less clear. This is due to the fact that interactions between colloidal particles and polymer molecules may lead to various physical phenomena. The adsorption of macromolecules on particles, for example, increases their stability due to repulsions between attached polymer chains. However, the adsorption of a single polymer chain on several particles can induce flocculation through the formation of bridges between individual particles. The flocculation of latices might also be caused by the addition of free (non-adsorbing) polymer as proposed in the theory of depletion (volume restriction) flocculation [5]. When two particles approach each other at a distance smaller than the diameter of a free macromolecule, the macromolecules are expelled from the space between the particles due to the loss of configurational entropy of the polymer chains. A difference in the osmotic pressure between the polymer-free zone and the bulk solution produces an attractive force which induces flocculation of the particles. On the other hand, pushing the macromolecules against the concentration gradient in good solvents increases the Gibbs free energy of the system, which produces repulsion between the particles and, hence, might stabilize the system [6]. In our previous study [ 1 ] it was speculated that the thickening and phase separation of electrostatically stabilized latices induced by alkali-soluble ethyl acrylate-methacrylic acid copolymers might arise from depletion phenomena. It is obvious that the molecular weight of the copolymers and the size of the latex particles should play an important role in these effects. A detailed investigation of the influence of these factors on the thickening effect of the copolymers is the object of the present study.

on alkali addition, and electrostatically stabilized latices of ethyl acrylate- 1 wt.% acrylic acid copolymers (EA latices) were prepared by semicontinuous emulsion copolymerization with a monomer emulsion feed. Polymerization was carried out in a 3 1 stainless steel reactor under a nitrogen atmosphere at 80°C using the ammonium peroxodisulfatesodium metabisulfite initiator system and Disponil AES 60 (sodium alkylaryl polyglycol ether sulfate; Henkel) as emulsifier. In the preparation of the EM copolymers, dodecyl mercaptan was used as a chain transfer agent (Table 1). This procedure ensured a statistical composition and different molecular weights of the copolymers [7,8]. Emulsions of technical grade ethyl acrylate (Chemical Works, Sokolov, Czech Republic) with methacrylic acid (Roehm, Darmstadt, Germany) or acrylic acid (Chemical Works, Sokolov, Czech Republic) were fed into a reactor at a constant rate during 3 h. After polymerization, the solid contents of the EM copolymers and the EA latices were 25 and 50 wt.% respectively; the pH was 1-2. 2.2. EM copolymers

2. Experimental

The molecular weight of the EM copolymers was determined in 2-ethoxyethanol by static light scattering as described in Ref. [3]. Prior to the measurements, the solutions were optically cleaned by centrifugation using a Beckman L8-55 ultracentrifuge (1 h; 15000 rev min-1; rotor E). The intensities of the scattered light (unpolarized primary beam of wavelength 546 nm) were measured with a Fica 50000 apparatus. The refractive index increment, dn/dc=O.1277 cm 3 g-l, was estimated as a linear combination of the increments of the corresponding homopolymers. The light scattering results were evaluated by the Zimm method. The molecular weights of the individual copolymers are listed in Table 2. Standard samples of the EM copolymers were obtained by dialysis (Dialysierschlauch Kalle AG, Wiesbaden, Germany) against twice-distilled water until constant conductivity was attained.

2.1. Emulsion polymerization

2.3. EAlatices

Dispersions of ethyl acrylate -50 wt.% methacrylic acid copolymers (EM copolymers), soluble

The EA latices denoted L 105, L 157 and L 221 were polymerized according to recipes which lead

P. Bradna et al./Colloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 1-10

3

Table 1 Polymerization recipes (in grams) for the ethyl acrylate-methacrylic acid copolymers EM 1, EM 4 and EM 12 and the ethyl acrylate-acrylic acid latices L 105, L 157 and L 221 Ingredient

EM 1

EM 4

EM 12

L 105

L 157

L 221

Reactor charge Water Disponil AES 60 Ammonium peroxodisulfate -sodium metabisulfite

400 2.6 4 0

400 2.6 4 0

400 2.6 1 0

400 26 0 1

200 13 0 1

200 0 0 1

Monomer emulsion feed Water Disponil AES 60 Ammonium peroxodisulfate Ethyl acrylate Methacrylic acid Acrylic acid Dodecyl mercaptan

800 26 0 200 200 0 3

800 26 0 200 200 0 1

800 26 0 200 200 0 0

360 26 6 792 0 8 0

560 39 6 792 0 8 0

560 52 6 792 0 8 0

2.4. Sample preparation

Table 2 Characteristics of the materials used EM copolymer Characteristic

EM 1

EM 4

0.78 49.4

4.46 49.6

11.6 46.6

Characteristic

L 105

L 157

L 221

d a (nm)

105 8.51 1.77

157 7.35 1.51

221 6.25 1.31

10 5 . M w Methacrylic acid content (wt.%)

EM 12

EA latex

[?/]L b

Acrylic acid content (wt.%)

a The z-average of the hydrodynamic particle diameter. b The intrinsic viscosity of the EA latices after ammonia addition (pH 9.5). The theoretical value for a hard sphere is 2.5.

to various particle sizes (Table 1). In their preparation, the concentration of low-molecular-weight substances was controlled to reduce differences in the ionic strength of the dispersion medium between individual materials. The z-averages of the hydrodynamic diameters of the latex particles determined by autocorrelation spectroscopy of scattered light (Nano-Sizer, Coulter Electronics Ltd., UK) are presented in Table 2.

Samples for the rheological measurements were prepared by the addition of a standard NH4OH solution to mixtures of EM copolymers and EA latices which were originally acid. An excess amount of the alkali was added so that the molar ratio of NH4OH to acid groups was 1.6. The pH of the samples (PHM64 digital pH-meter; combined G 2321 electrode, Radiometer, Copenhagen) was 9.4-9.6. The content of acid groups in both materials (Table 2) was determined by conductimetric titration with NaOH (OK 104 conductoscope, Radelkis, Hungary). The ionic strength was not adjusted in order to avoid the coagulation of the EA latices by excess electrolyte. All measurements were performed at 25°C, 24 h after sample preparation.

2.5. Analysis of supernatant The extent of adsorption of EM copolymer on the latex particles was estimated from a decrease in the concentration of the EM copolymers in the dispersion medium after the latex particles were added. In these experiments, various amounts of the EM 12 copolymer were mixed with the latex L 221 and the alkalinity was adjusted to pH 9.5.

4

P. Bradna et al./Colloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 1-10

The amount of EM copolymer in the dispersion medium was determined viscometrically after separation of the latex particles by centrifugation (15000 rev rain -1 for 2 h). Differences in the ionic strength between individual samples and those used for calibration were compensated by the addition of a standard amount of 1 M NaC1 solution (pH 9.5), which guaranteed measurements at practically constant ionic strength.

was 4 ° . Dynamic measurements were carried out in the linear viscoelastic region (shear strain, 0.03-0.08; amplitude, 0.3 °). Flow curves were measured at a continuously increasing shear rate up to 200 s -1 during 2 min followed by its decrease to zero in the same time interval. The experimental data were analyzed using the Herschel-Bulkley equation [ 10] T = T0

2.6. Viscometry The intrinsic viscosity [t/] of the EM copolymers after alkali addition was obtained by extrapolation of the qsp/C values measured in an Ubbelohde dilution viscometer to zero concentration of the copolymer (c=0). The specific viscosity is t/sp= ( t l - tlo)/tlo, where t/is the viscosity of the solution and t/o is that of the solvent. Aqueous solutions of NH4OH (pH 9.5) were employed for dilution. The influence of electrostatic interactions between ionized carboxylic groups on the expansion of the copolymer molecules was reduced by NaC1 addition. The effective hydrodynamic volume of the particles of the EA latices was characterized by their intrinsic viscosities [t/] L obtained by linear extrapolation of ~b/ln t/r to zero volume fraction ~b of the EA latices according to the Mooney equation [9] t/r = exp {[-t/JL~b/( 1 - ~b/~bc)}

( 1)

where $c is the volume fraction at which the viscosity reaches an infinite value (the volume fraction at maximum packing). The relative viscosity t/, = q/qo was measured at volume fractions in the range 0.03-0.30 using an O stwald capillary viscometer. The same type of viscometer was used to measure the viscosity of less-concentrated mixtures of the EM copolymers with the latices. The densities of the dispersions were determined pycnometrically.

2.7. Rheological measurements The rheological experiments were performed using a cone-plate rotational rheometer, Haake CV 20 N (Haake, Germany). The diameters of the cone and plate were 19.6 mm, and the gap angle

-I'-

t/a]) n

(2)

where z is the shear stress, % is the yield stress, q, is the plastic viscosity and n characterizes the pseudoplastic decrease in the viscosity of the system. The parameters were obtained by the best fit of the data using the software provided with the rheometer. The measurements were performed 24 h after sample preparation. Due to the flocculation of the latex particles during this period, the rheological parameters were determined after preshearing the samples during the first run.

3. Results and discussion

3.1. Viscosity at lower latex concentrations With increasing concentration of the EM copolymers, the viscosity of the 5 wt.% EA latices increased almost linearly and did not differ significantly from the dependences found for pure copolymers (Figs. la and 2a). The higher the molecular weight of the EM copolymers, the more pronounced an increase in the viscosity was observed. At higher latex contents, the different shape of the dependences showed a more complicated behaviour of the systems. As depicted in Fig. lb, a rapid increase in the viscosity occurred above a certain critical concentration of the EM 12 copolymer, which indicated a thickening effect of the material. At the highest copolymer concentration, however, a viscosity plateau appeared (Fig. lb). At the same time, a marked sedimentation of the latex particles suggested their flocculation and, hence, the colloidal instability of the system. A similar behaviour was found at a latex concentration of 15 wt.%, where a rapid viscosity increase as well as a viscosity plateau were observed even with the E M 4 copolymer, (Fig. lc). As can be seen in

P. Bradna et al./Colloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 1-10

30 a

o~

20

01

__~

7-o

40 b 30 20 10

30 20

~.,o-

f_10

o~

5

'~ 10

E

O Q,.

%o 80

b

60

e

4O

~:

20

0

0 50 4O 30 20

o

o'.2

o'.4

d.6

o'.8

c [g/dr]

Fig. 1. Dependences of the viscosity t/ of latex L 105 on the concentration of the copolymer (0) EM 1, (O) EM 4 and (©) EM 12. Latex concentrations (wt.%): (a) 5; (b) 10; (c) 15. The broken lines indicate pure copolymers.The arrows indicate the critical concentration of EM copolymer for a rapid viscosity increase (pH 9.4-9.6). Fig. lc, with increasing molecular weight of the EM copolymer, both thickening and flocculation started at lower concentrations. The thickening effect of the EM copolymers was smaller when large latex particles were used. Although the critical concentrations at the same latex content (15 wt.%) were slightly smaller with the latex L 221 than with the latex L 105 (Figs. lc and 2b), the viscosity of the latex L 221 increased with the concentration of the E M copolymers much more slowly than that of the latex L 105, which contained the smallest particles used. On the other hand, a more intensive thickening observed at a still higher content (25 wt.%) of latex L 221 (Fig. 2c) was accompanied by an irreversible aggregation of the particles at concentrations

0

0.2

0.4

0.6 0.8 c [g/dt ]

Fig. 2. Dependences of the viscosity r/ of latex L 221 on the concentration of the EM copolymers. Latex concentrations (wt.%): (a) 5; (b) 15; (c) 25. (pH 9.4-9.6). The data points are denoted as in Fig. 1.

above 0.8g dl - t or 0.4g d1-1 with E M 1 or 4, respectively, and already at the lowest concentration of the EM 12 copolymer. This revealed an increased instability of the systems when the free space for the accommodation of the polymer chains was highly suppressed. The influence of the sizes of the latex particles on the viscosity of the thickened latices is illustrated in Fig. 3. At a lower copolymer concentration (Fig. 3a), the viscosity only decreased with the latex particle size. This behaviour obviously results from a decrease in the surface area of the latex particles and a lower content of surface carboxylic groups when the diameter of the particles is increased (Table 2). Due to a smaller contribution of the electric double layers to the volume of large latex particles, as indicated by decreasing values of

6

P. Bradna et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 1-10

than weakly flocculated structures produced at lower molecular weights of the copolymer.

100

3.2. Flow and dynamic behaviour at a higher latex concentration

At a 35 wt.% latex concentration, even a small amount of the EM copolymers caused not only thickening o f the latices, as reflected in a steep increase in several rheological quantities (the yield stress, the plastic viscosity and the dynamic moduli) with the EM copolymer concentration (Figs. 4 and 5), but also induced phase separation. This behaviour is not surprising as the values of Zo and the dynamic moduli for pure EA lattices (Figs. 4 and 5), are non-zero and indicate pronounced interparticle interactions resulting from double layer overlap and, hence, a filling-up of the space by latex particles prior to EM copolymer

50 O 13-

E

0

100

50 50 0

0

~ 100

I 150

o_ 40

t 200 [nm]

30

Fig. 3. Dependences of the viscosity ~/ of EA latices (15 wt.%) thickened by (O) EM 1, (O) EM 4 and (©) EM 12 copolymers

20

on the latex particle diameter tL The concentrationsof the EM copolymers were (a) 0.45 g dl 1; (b) 0.80g d1-1. (pH 9.4-9.6.)

10

the intrinsic viscosity (Table 2), the interparticle interactions decreased, bringing about a decrease in the viscosity of the system. At a higher concentration of the EM copolymers (Fig. 3b), a similar decrease in viscosity was found for the EM 1 copolymer. On the other hand, the behaviour of latices thickened by the EM 4 and especially by the EM 12 copolymers was different. In this case, an increase in viscosity with latex particle size and a lower viscosity of the latices L 105 and L 157 thickened by EM 12 than that of those thickened by EM 4 confirmed an increased ability of highmolecular-weight copolymers to induce flocculation rather than stabilization of the latices. We suppose that with increasing molecular weight of the EM copolymers, more compact flocs are formed which contribute less to energy dissipation

0.6

b

o

O 12.

%

0 0

0.4

0.2 0.4

'

0'8.

I

I

I

I

1.2 1.6 c [g/dl]

Fig. 4. Dependencesof (a) the yield stress ~o and (b) the plastic viscosity qa (7=20s-1) on the concentration c of the EM copolymers. Data points are denoted as in Fig. 1. (Latex concentration, 35 wt.%; pH 9.4-9.6.)

P. Bradnaet aL/Colloids Surfaces A: Physicochem.Eng. Aspects 113 (1996) 1-10

~60

6o

Cl

7

0 t

n

4oJ

k~ 40

20 n I.-,

20

,--~3o

0

b

2

Q.

20

e

o

o

~

613 40

10

20

o

0.4

0.8

1.2

1.6 c [g/d[]

Fig. 5. Dependences of (a) the loss modulus G' and (b) the storage modulus G" (o9=9.1 s -1) on the concentration c of the EM copolymers.Data points are denoted as in Fig. 1. (Latex concentration, 35 wt.%; pH 9.4-9.6.) addition. It is obvious that under these conditions, the addition of EM copolymers induces aggregation of the particles. The presence of aggregates, which can be broken in the shear field [ 11 ] and may be restored when shear forces acting in the system decrease, was also reflected in a pronounced pseudoplastic behaviour of the thickened latices and marked differences between the flow curves obtained during the first and following runs (Fig. 6). 3.3. The role of depletion flocculation in the thickening effect of the E M copolymers Electrostatic repulsion between ionized carboxylic groups on the surface of the latex particles and the groups attached to the EM copolymer chain prevents the adsorption of the copolymers on the latex particles, as reflected in small changes

I

I

I

40

80

120

I

I

160 200 [s-~l

Fig. 6. Dependenceof the shear stress T on increasing (--*)and subsequently decreasing(*--)shear rates ~,. Concentration of L 157 latex 35 wt.%; concentration of EM 4 copolymer,(a) 0,30g dl-1; (b) 0.80g dl-~; (c) 1.20g d1-1. (pH 9.4-9.6.) in the concentration of the EM copolymer in the dispersion medium after the latex particles were added. Similarly to sterically stabilized polystyrene latices in the presence of charged poly(acrylic acid) (pH above 5) [12], these results indicate that bridging flocculation can be ruled out in the system studied. It was reported that depletion flocculation occurs at a polymer concentration in the bulk solution near the polymer overlap concentration [13,14]. Using intrinsic viscosities of the EM copolymers (Table 3), it was found that the critical concentrations, ccr, in the bulk solution at which the viscosity rapidly increased (Figs. 1 and 2) corresponded to ccr[r/]~1.84-0.30 (Table4), which is near the theoretical value c [r/] = 1 for the chain overlap [ 15]. These results suggest that the viscosity increase and thus the thickening effect of the EM copolymers might result from aggregation of the latex particles induced by the depletion

8

P. Bradna et al./Colloids Surfaces A." Physicochem. Eng. Aspects 113 (1996) 1-10

Table 3 Intrinsic viscosities [~/] (dl g-1) and reduced intrinsic viscosities [q]/[q]o of EM copolymers at various NaC1 concentrations after ammonia addition (pH 9.5)

4O O 13..

30 EM 0.01 M NaC1 copolymer ['1] [~]/[~]o

0.04 M NaCI

0.80 M NaCI

[q]

[q]/Eq]o

[~]

[~]/[q]o

20

EM 1 EM 4 EM 12

1.44 3.73 6.18

1 2.69 4.30

0.64 1.49 2.35

1 2.34 3.70

10

2.58 1 7.53 2.92 12.50 4.84

0

1o~@

100

mechanism on exceeding the critical concentrations of the EM copolymers. A more detailed analysis of the viscosity data is extremely difficult. However, the fact that flocculation in various latex-polymer systems occurred near the polymer overlap concentration suggests that the effective hydrodynamic volume of the polymer molecules proportional to c[tl] might be a universal variable enabling a comparison of data obtained at different molecular weights of the copolymers. However, the intrinsic viscosities of the copolymers and the viscosities of the thickened EA latices were measured at different ionic strengths, and this fact should be taken into account when applying the product c [q]. To overcome this difficulty, we replace c [7] by a new variable ~rea defined as m,o~ = c [r/]/[r/]o

eO

e •

0

b e o

80

0

60 0

40 20

O

°°~.o° 0

I

0

I

1

I

~

i

I

3 ~red [ g / d r ]

Fig. 7. Dependences of the viscosity q of latex L 105 on the reduced volume concentration ~r¢d =c[q]/[q]o of the EM copolymers at latex concentrations (a) 10 wt.%; (b) 15 wt.%. Data points are denoted as in Fig. 1. (pH 9.4-9.6.)

(3)

Table 4 Critical polymer concentrations ccr (g dl-1) in the bulk polymer solution for the onset of a viscosity increase in various EA latex-EM copolymer systems, and product cot[q] at 0.04 and 0.80 M NaCI EM copolymer

Latex L 105 EM 12 Latex L 105 EM 4 EM 12 Latex L 221 EM 4 EM 12 Latex L 221 EM 1 EM 4

ccr (g d1-1)

c~r[q], 0.04 M NaCI

ccr[r/], 0.80 M NaC1

0.25

1.55

0.59

0.48 0.29

1.79 1.79

0.72 0.68

0.28 0.20

1.05 1.26

0.42 0.47

0.53 0.20

0.76 0.75

0.34 0.30

(10 wt.%) (15 wt.%)

(15 wt.%)

(25 wt.%)

P. Bradna et al./Colloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 1-10

where [q] is the intrinsic viscosity of the copolymers EM 4 or EM 12, c is their concentration and [q]o is the intrinsic viscosity of the EM 1 copolymer, chosen as a reference material. The ~red value gives the concentration of the reference copolymer that would produce the same rheological effect as the E M 4 or EM 12 copolymers at a concentration c. The ratio [r/]/[r/] o depends on molecular weight and ionic strength. The values of [r/]/[q]0 at a constant molecular weight vary within a narrow range (Table 3), although the salt concentrations vary between 0.01 and 0.80 M NaC1. The arithmetic means are very close to [q]/[~/]o for 0.04 M NaC1 and may be used as a constant independent of the ionic strength and dependent only on the copolymer molecular weight. With the values of [r/I/It/] o at 0.04 M NaC1, a satisfactory superposition of the viscosity data at latex contents 10-25 wt.% was achieved (Figs. 7

9

and 8). The same holds true for more concentrated systems (35 wt.% of the EA latices) characterized by other rheological properties (Figs. 9 and 10). This shows that the hydrodynamic volume of the EM copolymer molecules controls the rheological properties even for strongly flocculated systems.

4. Conclusion The results of the study suggest that thickening of the electrostatically stabilized latices by the linear ethyl acrylate - - methacrylic acid copolymer arises from flocculation rather than stabilization phenomena. It can be supposed that an increase in the rheological parameters of the latices is caused by the presence of large aggregated structures such as clusters or flocs of latex particles formed by the depletion flocculation mechanism. 50

8O ~ 4O @ O

~o

6o

O

30

t9

0

O

4O

•

20 @O

20

0

•

10

O

e

0

O

m

b

e

b ~ 0.6 if) 0

50

0 g

%

40

@O

0.4 •

o

"o@

30 O

0.2

O

20 @

I

0

I

I

I

I

2 9red

I

I

3

[g/d[ ]

Fig. 8. Dependences of the viscosity q of latex L 221 on the reduced volume concentration ~r.a = c[-~/]/[r/]o. Latex concentration: (a) 15 wt.%; (b) 25 wt.%. Data points are denoted as in Fig. 1. (pH 9.4-9.6.)

I

0

4

I

I

6 (~red

8

[g/dll

Fig. 9. Dependences of (a) the yield stress z0 and (b) the plastic viscosity r/a at ~ = 20 s - 1 on the reduced volume concentration ~rCa =c[-~/]/[q]0 of the EM copolymers. Data points are denoted as in Fig. 1. (Latex concentration, 35 wt.%; pH 9.4-9.6.)

10

P. Bradna et al./Colloids Surfaces A." Physicochem. Eng. Aspects 113 (1996) 1-10

polyelectrolytes as non-adsorbing polymers where the osmotic pressure in the bulk of the solution is much higher than in the case of non-charged macromolecules I-5].

60

g. O

0

40 0

Acknowledgement

0 Q610

20

°~

The grant agency of the Academy of Sciences of the Czech Republic is gratefully acknowledged for supporting this work by grant no. 45043.

-3o

g_

References O

2C

0 6~

10

.¢

df III

0

i

e

0

0 0

I

2

I

I

4

I

I

I

6 ~red [ g / d r

8 ]

Fig. 10. Dependences of (a) the loss modulus G' and (b) the storage modulus G" on the reduced volume concentration ~red=C[q]/[q]o of the EM copolymers. Data points are denoted as in Fig. 1. (Latex concentration, 35 wt.%; pH 9.4-9.6.)

A strong flocculation or even an irreversible aggregation of the latices instead of their stabilization observed at higher concentrations of the copolymers 1-6] might reflect the fact that electrostatically stabilized latices are prone to flocculation in the region of the secondary or primary energetic minima. This behaviour could be expected with

[1] P. Stern, P. Bradna and O. Quadrat Rheol. Acta., 31 (1992) 361. [2] O. Quadrat, L. Mrkvi6kov~, E. Jasn~i and J. Sfiup~trek, J. Colloid Polym. Sci., 268 (1990) 493. [3] J. Stejskal, O. Quadrat, P. Bradna and J. Sfiup~irek, Colloids Surfaces, 69 (1992) 31. [4] P. Bradna, P. Stern, O. Quadrat and J. Sfiup~trek, Colloid Polym. Sci., 273 (1995) 324. [5] S. Asakura and F. Oosawa, J. Chem. Phys., 22 (1954) 1255, J. Polym. Sci., 33 (1958) 183. [6] R.I. Feigin and D.H. Napper, J. Colloid Interface. Sci., 74 (1980) 567. [7] J. Sfiup~trek, Makromol. Chem. Suppl., 10/11 (1985) 129. [8] L.W. Morgan and D.P. Jensen, Makromol. Chem. Suppl., 10/11 (1985) 59. [9] M. Mooney, J. Colloid Sci., 6 (1951) 162. [10] W.H. Herschel and R. Bulkley, Kolloid-Z., 39 (1926) 291. [11] P.R. Sperry, H.B. Hopfenberg and N.L. Thomas, J. Colloid Interface. Sci., 82 (1981) 62. [12] W. Liang and T.F. Tadros, Langmuir, 10 (1994) 441. [13] D. Heath and T.F. Tadros, Faraday Discuss. Chem. Soc., 76 (1983) 203. [14] T.F. Tadros, Prog. Colloid. Polym. Sci., 83 (1990) 36. [15] S.G. Weissberg, R. Simha and S. Rothman, J. Res. Natl. Bur. Stand., 47 (1951) 298.