Langmuir 1994,10, 441-446
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Flocculation of Sterically Stabilized Polystyrene Latex Particles by Adsorbing and Nonadsorbing Poly(acrylic acid) W. Liang and Th. F. Tadros* Zeneca Agrochemicals (Formerly Part of The ICI Group), Jealott's Hill Research Station, Bracknell, Berkshire RG12 6EY, U.K.
P. F. Luckham Department of Chemical Engineering and Chemical Technology, Imperial College, London SW7 2AZ, U.K. Received July 22, 1993. In Final Form: November 3 , 1 9 9 9 The stability of sterically stabilized polystyrene latex has been investigated in the presence of poly(acrylic acid) (PAA)at various pH values. Flocculation (aggregation)has been monitored by microscopy for a series of samples with various PAA concentrationsat fiied latex volume fraction and pH values. It was found that at low pH values (7, three different phase regions could be established on the phase diagram, namely, depletion flocculation at lower PAA concentrations (W,+< W < W,++where W,is the weight fraction of PAA), depletion stabilizationat higher PAA concentration (b < W, < W,m) and a secondary unstable region ( W > W,m), where phase separation occurs, at very higL PAA concentrations. The molecular weight has littfe influence on depletion restabilization at higher pH. However, it influenced the critical bridging flocculationconcentration(CBFC)and critical depletion flocculationconcentration(CDFC),i.e. the larger the PAA molecular weight the lower the critical flocculation concentration of polymer. Rheological data showed the same CFC as the microscopy results at the secondary unstable region. The effects of particle size, latex volume fraction, PAA molecular weight, and PAA concentration on viscoelastic properties of the suspensions at pH = 8.0 and 9.0 were also studied. ++
Introduction The effect of addition of free (nonadsorbing) nonionic polymers on the rheology/flocculation of sterically stabilized dispersions has been a subject of many investigators.*-' Several systems have been studied in our laboratories using polystyrene latex containing grafted poly(ethy1ene oxide) and hydraxyethyl cellulose (HEC) or poly(ethy1ene oxide) (PEO)as the free polymer.Concentrated dispersions were used for such studies and the interaction between the particles was studied using rheological techniques. In all these systems there was always a general trend of the'effec't of molecular weight of free polymer, particle size df the dispersions, a d the volume fraction of the particles on the rheological properties. A scaling analysis wa8 used to analyze the rheological results. The power law relationship between the yield stress and the particle volume fraction and the elastic modulus and volume fraction of free polymer could be represented by
G' = K, 4," where K1 and K Z are constants, 4, is the free polymer volume fraction, 4,+ is the critical flocculation volume fraction of free polymer, &is the particle volume fraction, and n is the power exponent in 4pwhich may be related to the particle size. The rheological results were also used to calculate the energy of separation between the particles in a flocculated structure. E- could be related to the various parameters of a system by the following equation'
(3) where a is the particle radius and n is the average number of contacte between particles in a floc, i.e. the coordination number. From the equation (31, one can easy find that the E,, increases with the increase in polymer volume fraction and molecular weight as expected.' All the above investigations were carried out using uncharged polymers and we thought that introducing a charge in the polymer chain (Le. a polyelectrolyte) may not change the general picture of depletion flocculation, if an allowance is made for the increase in polymer coil dimensions as a result of electiostatic repulsion. To test this hypothesis we have carried out systematic studies using the stericallystabiliged latex dispersionwith grafted PEO and poly(acry1icacid) (PAA)as the free polymer. At pH < 5, the PAA chains would predominately consist of uncharged COOH group. Above pH 5, dissociation of COOH groups occurs and this increases as the pH further increased. In this paper, we report results on the stability/ flocculation of latex dispersions as a function of PAA
* To whom correspondence should be addressed.
Abatract Dubliehedin Advance ACSAbstracts. Januarvl. 1994. (1) Napper,-D. H.Polymeric Stabilization of Colloidal DGpekons; Academic h. London, 1983. (2)Tadroe, Th.F. The Effect of.~Polymers on Dispersion Properties; Academic Pr&: London, 1982. (9) Vinceat, B.; Luckham, P. F.; Wait%,F. A. J. Colloid lnterfoce Sci. 1980,73, 608. (4) Fleer, G. J.; Scheutjena, J. H. M. H.; Vincent, B. ACS Symp. Ser. 1984,No. 240,246. (6) Preatidge, C.; T a b , Th.F. J. Colloid Interface Sci. 1988,124, 660. (6) Heath,D.;Tadrw. Th.F. Faraday Discuss. Chem. SOC.1983,76, 2nx
(7) Liang,W.;Tadroe,Th.F.; Luckham, P. F. J. Colloid Znterfoce Sci.
iooa, is,i~.
(8)Liang, W.; Tadrqe,Th.F.; Luckham, P. F. J. Colloid Interface Sci. 1993,168,162. (9) Liang, W.;Tadros,Th.F.; Luckham,P. F. J. Colloid Znterjace Sei. 1993,160,183.
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1994 American Chemical Society
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Figure 1. Plot of pH as a function of PAA (6oooO)concentration for latex (D= 435 nm) at 4B= 0.02,CN.CI= 0.OOO.
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Results and Discussion Figure 1shows the phase diagram of PAA(60000)as a function of pH for latex (D= 435 nm) suspensions (& = 0.02). These resulb were obtained by using microscopic investigations. The polymer concentration was expressed aa a weight fraction instead of a volume fraction, since it (10) Liang, W.;Tadros, Th. F.; Luckham, P. F. J. Colloid Interface Sci. 1992, 153, 131. (11)Bromley, C. Colloids Surf. 1986, 17, 1.
0.001 Weight 0.003 fraction 0.01of
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Figure 2. Transmission of M(EO)+ (10%)+ PAA(6oooO) at pH = 9.0 as a function of PAA concentration.
concentration and pH of the systems. Microscopic observations were carried out in the dilute dispersions, whereas at high concentration rheological measurements were used as a tool to study the flocculation. Experimental Section Polystyrene Latex Dispersions. Polystyrene latex grafted with poly(ethy1ene oxide) was prepared using the method described by Liang et al.10 and Bromley." Basically, styrene is polymerized in a solvent mixture of water and ethanol in the presence of methoxy poly(ethy1ene oxide) methacrylate, using azoisobutyronitrile (AIBN) as the initiator and terminator to reduce excess monomer. The average molecular weight of the poly(ethy1eneoxide)was 2000. The z-averageparticle diameters were found to be 147,435,and 915 nm as determined by photon correlation spectroscopy (PCS).lO The results showed that the latices had a narrow particle size distribution.1° Sodium polyacrylate (average molecular weights of 20 OOO, 60000,and 170000) were Fluka materials and used without further purification. The other materials used were described previously.7JO Microscopy Investigations. A Zeiss Universal light microscopy was used to observe the stability of latex plus PAA suspensionsat variousweightfractions of the addedfree polymers of PAA (20OOO, 60 OOO, and 170 OOO). Samples of the suspension were placed on a microscope slide while directly observing the stability/flocculation under the cover slip. The volume fraction of the latex was 0.02 and the particle size of latex was 435 nm in diameter. PAA concentrations are expressed as weight fractions Wp. The amount of standard NaOH or HC1solution needed was calibrated for each PAA samples before making samples. In a given experiment a series of suspensions were prepared containingthe latex dispersion, PAAsolution, standard NaOH/HCl solution, and water for which all parameters were kept constant, expect that the PAA concentration was systematically increased. Visual observation of the structure of the suspensions changing from single particles to flocculation with increasing PAA concentration was made and the appearance of aggregation was taken to be an indication of the onset of flocculation. Rheological Measurements. Twomethods were usedin this study namely steady-state shear stress (+shear rate (7) and oscillation measurements and these were described in detail before.'
Phase separation
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Figure 3. Plot of pH as a h c t i o n of PAA(6oooO)concentration for latex (D= 435 nm) at 4. = 0.02, CN.CI= 0.001.
is difficult to estimate the change in degsity on increasing concentration due to liberation of Na+ ions. One may consider four regions in the phase behaviour of this system: pH 0.09, phase separation clearly occurs with the formation of two liquid layers. This concentration is comparable to that obtained in the phase diagram (Figure 1)namely -0.1. Figures 3-5 show the influence of addition of electrolyte 1P2,and 10-l mol dm-9 on the phase (NaC1 a t diagram. In the region below pH 7, the trends are similar to that obtained in the absence of electrolyte. However, in the region pH 7-12, there is a significant effect of addition of electrolyte on the phase behavior. The "depletion stabilization" region decreased dramatically with increase in electrolyte concentration and it disappeared altogether when the NaCl concentration was increased to 0.1 mol dm-3 (see Figure 5). Addition of electrolyte decreases the charges on the PAA chains, which becomes significantly less expanded. At sufficiently high NaCl(O.1 mol dm3) the volume occupied by the polymer chains is significantly reduced and, hence, entropic effects become much weaker. This explains the reason why "restabilization", an entropic effect, is not observed in this case. Figure 5, 6, and 7 show a comparison of the effect of changingthe PAA molecular weight on the phase behavior. Below pH 5, there is a small effect of the molecular weight on the phase behavior. Above pH 5, there is a significant effect of molecular weight on the critical flocculation concentration caused by depletion. The higher the molecular weight, the lower the value of Wp+,as expected for depletion effects ( Wp+ = 7.6 X 10-4,5.0X 10-4, and 2.9
444 Langmuir, Vol. 10, No. 2, 1994
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Figure 11. Plot of yield value as a function of latex (D 435 nm)volume fraction in the presence of three PAA polymers at pH = 9.0 and W,= 0.16. 1,000
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Volume fraction of PAA(60000) d, Figure 10. Plot of yield value aa a function of PAA(6oooO)weight fraction at various latex (D= 436 nm) volume fraction at pH = 9.0. X l W for PAA 2oo00, 6oo00, and 17oo00,respectively). However, in the region of "depletion restabilization", increasing the molecular weight of PAa causes a shift oE
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Volume fraction of latex (D=435nm) 9, Figure 12. Plot of storege modulus as a function of latex (D= 435 nm)volume fraction in the presence of three PAA polymers at p H = 9.0 and W,= 0.16. the "depletion restabilization loop" to lower pH values. This is shown more clearly in Figure 8 which gives an
Flocculation of Polystyrene Particles
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Figure 13. Plot of yield value as a function of particle volume fraction with PAA(17oooO)for three latex particle sizes at pH = 9.0 and W,= 0.16.
expanded scale for this region. This figure shows that by increasing the molecular weight, the onset of “depletion restabilization” occurs at a higher weight fraction of PAA. This behavior is difficult to explain since a number of simultaneouschanges may occur as one changes the system. At present, we cannot offer any explanation for this effect. Rheological experiments, whereby the volume fraction of the latex could be varied, could only be carried out at certain PAA concentrations. This was due to the difficulty in concentrating the latex by centrifugation when the densities of both latex and the medium are very close to each other. Results could be obtained in the depletion flocculation, “depletion restabilization” zone, as well as the upper unstable region. In the latter case, the latex was lower in density than the medium, and the concentration could be achieved by “creaming”. Figure 9 shows plots of Bingham yield stress 78 versus volume fraction of latex at various PAA concentrations. In the absence of PAA, 70 was almost zero a t #a < 0.55 after which 78 shows a rapid increase at > 0.58. At a PAA weight fraction of 0.08, the dispersions show Newtonian behavior up to & = 0.5. This implies a stable dispersion as shown for the dilute system in Figure 1.No measurement could be obtained above this value since the latex could not be concentrated further by centrifugation. At #, 0.14-0.20,there is a clear trend in the results obtained, namely, rapid increase in 78 with #a when ##was increased above 0.1. The higher the PAA concentration, the higher the value of TO a t any given qha value. This is consistent with the increase in flocculation with increase in PAA concentration. Figure 10 shows 78 versus weight fraction of PAA a t various latex volume fractions at pH = 9. All dispersions
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Figure 14. Plot of storagemodulus as a function of latex volume fraction with PAA(17oooO) for three latex particle sizes at pH = 9.0 and W,= 0.16.
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Figure 16. Plot of yield stress as a function of free polymer (PAA(17oooO))at latex volume fraction 0.530 and pH = 8.0.
show the same critical flocculation concentration (-0.12 weight fraction) which agrees with the phase diagram results (0.104). At W,> Wp+,there is a linear increase in TO with W,. A scaling law may be used for the region obtained in Figures 9 and 10 r8 = k &m (4) The value of the exponent m a t different PAA concentrations is 2.0 f 0.2. This exponent is consistent with previous investigations for weakly aggregated systems.719 Figure 11shows the effect of PAA molecular weight on the yield value a t pH 9 and W,= 0.16, i.e. a t concentrations corresponding to the upper unstable region. For PAA 2oooO and 6oo00, the yield stress showed clearly the same value, whereas those for PAA 170000 are considerably
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Figure 16. Plot of complexmodulusas a functionof free polymer (PAA(17oooO))at latex volume fraction 0.530 and pH = 8.0.
higher. Since the flocculation occurs, in this case, as a result of phase separation, one would expect a higher degree of flocculation for the higher molecular weight polymer, as found experimentally. Figure 12 shows the storage modulus as a function of latex (D = 435 nm) volume fraction at pH 9 and W,, = 0.16. It can be seen that there is a regular trend in increase G’ at any given &value with the increase in molecular weight of the PAA. This systematic increase in the storage modulus reflects the increase in flocculation extent and strength with the higher molecular weight polymers. Figures 13 and 14 show the effect of particle size on the ~ rand$ G’+,~relationships. The reduction in particle size causes an increase in all rheological parameters
(particularly when considering the results for G’) which is due to the increase in number of contact points in the flocculated structure. As discussed before,1° it is a surface area-to-volume effect. As discussed previouslp the storage modulus is dominated by the number of bonds in a flocculated system, and with decrease in particle size the number of such bonds increases resulting in a higher modulus. Figures 15 and 16 show plots of yield stress and complex modulus as a function of PAA weight volume fraction at pH 8. These results clearly show the phase change that occurs as a result of increasing PAA concentration (compare Figure 6). The same results were obtained for these systems at pH 9 and 10. The rheology data show the same trend as obtained for dilute dispersions (using microscopy) and provide more evidence for the complex behavior of this system and the potential for rheological methods to monitor phase changes in concentrated systems.
Conclusions The addition of poly(acry1ic acid) into sterically stabilized polystyrene latex dispersions may induce flocculation depending on the concentration of PAA, pH, and electrolyte concentration. Phase diagrams can be established by determining the critical flocculation concentration of PAA at varioius pH and NaCl concentrations. Rheological investigations of concentrated sterically stabilized polystyrene latex dispersion in the presence of PAA demonstrate the phase behavior of the suspension at various concentrations.
