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Stability of Dispersions in the Presence of Graft Copolymer (II) Adsorption of Graft Copolymers on Titanium Dioxide and the Stability and Rheology of the Resulting Dispersions W. Liang,† G. Bognolo,‡ and Th. F. Tadros*,† Zeneca Agrochemicals, Jealott’s Hill Research Station, Bracknell, Berkshire, RG12 6EY, U.K., and UNIQEMA, Everslaan 45, B-3078 Kortenberg, Belgium Received June 23, 1999. In Final Form: September 28, 1999 The adsorption of two graft copolymers (Atlox 4913 and Hypermer CG-6 consisting of poly(methyl methacrylate) methacrylic acid backbone and polyethlene oxide side chains) on titanium dioxide dispersions have been investigated. Hypermer CG-6 contains more polymethcrylic acid groups in the backbone. The influence of copolymer structure, temperature, and electrolyte concentration on the stability of titanium dioxide dispersions was studied using rheological measurements and microscopy observation. The adsorbed layer thickness of copolymer on the titanium dioxide particle surface at saturation adsorption was evaluated by measuring the rheological properties of the concentrated dispersions using shear stress-shear rate and oscillatory measurements. The results showed that the adsorption behavior of copolymer on TiO2 is different from polystyrene latex which has a hydrophobic surface, especially for Atlox 4913. The dispersions showed weak flocculation when using Atlox 4913 but stable dispersions for Hypermer CG-6. For the stable dispersions using Hypermer CG-6, the adsorbed layer thickness decreased with increase in the volume fraction of the dispersion. Increasing temperature showed little effect on the viscoelastic properties. However, with the increase of electrolyte concentration, moduli increased sharply indicating flocculation of the dispersions.
Introduction The effect of graft copolymers on the stability and rheology of polystyrene lattices has been recently investigated in our laboratory.1 Two graft copolymers were used based on a poly(methyl methacrylate-methacrylic acid) backbone with poly(ethylene oxide) (PEO) side chains. However, one of the graft copolymers contains a larger amount of poly(methacrylic acid) in the backbone, which gives it more flexibility. The results showed that the structure of the particle surface and the copolymer play a crucial role in adsorption behavior and thus affect the stability and rheological properties of the dispersions. The adsorbed amount of the two copolymers (mg m-2) was determined for latex particles of various sizes. The results showed no dependence on particle size within the range studied (D ) 426 nm to 867 nm). Results of adsorption were also obtained as a function of temperature and this showed the expected increase in adsorption amount with increase of temperature, as a result of reduction of solvency of the PEO chains with increase of temperature. The rheological results were used to obtain the adsorbed layer thickness as a function of the volume fraction of the dispersion φ, as a result of interpenetration and/or compression of the chains with increase of φ. In the present investigation, we have extended the above research work for a different dispersion, namely titanium dioxide (TiO2), which is extensively used in many industrial applications such as paints, ceramics, and cosmetics. In most of these applications, a highly stable dispersion is required and it is interesting to find whether the graft copolymers used for polystyrene latex dispersions can also be applied to stabilize titanium dioxide dispersions. * To whom correspondence should be addressed. † Zeneca Agrochemicals. ‡ UNIQEMA. (1) Liang W.; Bognolo, G.; Tadros, T. F. Langmuir 1995, 11, 2899.
Clearly, the mechanism of adsorption of the graft copolymers on the hydrophillic TiO2 surface would be different. The hydrophobic backbone of the above-mentioned graft copolymers may not be a good “anchoring” chain, although introduction of a poly(methacrylic acid) group may help in enhancing the adsorption of the backbone. Clearly to obtain a stable dispersion one needs the PEO chains to be oriented toward the bulk solution, rather than become attached to the TiO2 surface. We will see in the present investigation this requires a modification of the backbone to give it a more polar character thus allowing it to become preferentially attached to the TiO2 surface. To answer some of the above questions, we have measured the adsorption of the two graft copolymers used before for polystyrene latex dispersions1 as a function of temperature. The stability of the resulting dispersions was investigated using rheological measurements as a function of the volume fraction of the dispersions. Optical micrographs were also obtained using the two graft copolymers in order to confirm the stability/instability of the dispersions. Results were also obtained as a function of electrolyte (Na2SO4) concentration. The structure of the graft copolymer plays a crucial role to the adsorption behavior on colloidal particles and thus affects the stability and rheological properties of the dispersions. The adsorption of surfactant or polymer or a mixture of both surfactant and polymer on TiO2 has been studied by several investigators.2-7 However, there is lack of systematic investigation on the adsorption, (2) Gargallo, L.; Cid, E. Colloid Polym. Sci. 1977, 225, 556. (3) Cohen Stuart, M. A.; Fleer, G. J.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1982, 90, 117. (4) Ma, C. Colloids Surf. 1985, 16, 185. (5) van der Beek, G. P.; Cohen Stuart, M. A.; Cosgrove, T. Langmuir 1991, 7, 327. (6) Sato, T.; Kohnosu, S. Colloids Surf. A 1994, 88, 197. (7) Kipling, J. J.; Wilson, R. B. J Appl. Chem. 1960, 10, 109.
10.1021/la990811k CCC: $19.00 © 2000 American Chemical Society Published on Web 12/01/1999
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stability, and rheology of a TiO2 suspension in the presence of graft copolymers. Experimental Section Materials. Titanium dioxide (from Tioxide Ltd) was supplied by ICI Surfactants and used as received. The specific surface area of titanium dioxide was measured using methylene blue adsorption7,8 and was found to be 4.95 m2/g. From PCS measurement, the z-average diameter of particle was found to be 316 nm with polydispersity of 0.29 indicating the particles are polydisperse. The specific surface area estimated from PCS result is 4.46 m2/g (assuming that TiO2 particles are spherical) which is close to the result determined by methylene blue. Two different graft copolymers, supplied by ICI Surfactants, namely Atlox 4913 and Hypermer CG-6 (a), were used as received. As mentioned in the Introduction, the two graft copolymers are based on polymethyl methacrylate-methacrylic acid backbone and PEO (Mw ) 750) side chains. The Hypermer CG-6 (a) contains a higher proportion of methacrylic acid, and its exact composition is not known. Adsorption Isotherms. Titanium dioxide (0.100 g) was equilibrated with polymer solutions (30.00 mL) of different concentrations for more than 16 h at fixed temperature (20 or 40 °C). The particles then were removed by centrifugation at 20 000 rpm(25 000 g) for 30 min which was sufficient to sediment all the particles, leaving a clear supernatant at the top. The concentration of the polymer in the supernatant was determined colorimetrically using the method originally developed by Balleux9 and later adapted10,11 for other nonionic surfactants of the ethoxylate type. The specific surface area of TiO2 was determined using Methlyene Blue (M. B.) method.9,10 Basically one measures the adsorption amount of M. B. on TiO2 and assumes that the M. B. molecules are adsorbed in a flat configuration on the TiO2 surfaces and this allows one to estimate the specific surface area of TiO2. Adsorbed Layer Thickness Measurements. The adsorbed layer thickness was determined by rheological measurements. Details of this method has been described previously.12 Preparation of Concentrated Latex Dispersions for Rheological Measurements. Copolymer solutions were added to the titanium dioxide dispersion (∼30 wt %) at concentrations corresponding to the plateau of the adsorption isotherm. After equilibration (>16 h), the TiO2 dispersion was concentrated by centrifugation at 10 000 rpm for 30 min and the concentrates were diluted with the supernatant liquid in order to cover a wide range of φs values. Rheological Measurements. A Bohlin VOR rheometer was used for both oscillatory and steady-state measurements at 25 °C. Details of measurements were described previously.1,12 Microscopy Observation. A Leitz Diaplan scientific and clinical microscope (Germany) linked with a COHU high performance CCD camera and a color video copy processor (Mitsubishi, Japan) was used to obtain optical micrographs for TiO2 suspensions stabilized by Atlox 4913 and Hypermer CG-6 (a). Samples were placed on a microscope slide and diluted in situ with the supernatant under a microscope cover slip. The micrographs were taken using the video copy processor.
Results and Discussions Adsorption Isotherms. As an illustration, Figure 1 shows the adsorption isotherms (Γ mg m-2 vs equilibrium concentration C) for the two copolymers Atlox 4913 and CG-6 (a) on TiO2 particles at 20 °C. The plateau value of the adsorption amount Γ is higher for Atlox 4913 than that of the modified Hypermer CG-6 (a). As discussed (8) Ardizzone, S.; Gabrielli, G.; Lazzari, P. Colloids Surf. A 1993, 76, 149. (9) Baleux, B. C. R. Acad. Sci. Ser. C 1992, 274, 1617. (10) Tadros, T. F.; Vincent, B. J. Phys. Chem. 1980, 84, 1575. (11) Taylor, P.; Liang, W.; Bognolo, G.; Tadros, T. F. Colloids Surf. 1991, 61, 147. (12) Liang, W.; Tadros, T. F.; Luckham, P. F. J. Colloid Interface Sci. 1992, 153, 131.
Figure 1. Adsorption isotherms for two copolymers adsorbed on TiO2 at 20 °C.
Figure 2. Plot of yield stress as a function of TiO2 (stabilized by copolymers) volume fraction. Table 1. Plateau Adsorption Values, Γsat, for Atlox 4913 and CG-6 on TiO2 at Two Temperatures Γsat/mg m-2 temperature/°C
Atlox 4913
CG-6
20 40
2.0 2.3
1.4 2.1
before1 this is due to the fact that copolymer PEO side chain density is lower with Hypermer CG-6 (a) when compared to that of Atlox 4913. The plateau values of copolymer adsorbed on TiO2 are slightly higher than those obtained with latex dispersions. The effect of temperature on adsorption isotherms for Atlox 4913 is shown in Table 1, which shows the plateau adsorption values for the two polymers at 20° and 40°. With the increase of the temperature (from 20 to 40 °C) the plateau value of the adsorption amount increased. Similar results were obtained for Hypermer CG-6 (a) adsorbed on TiO2 (Table 1). In this case the temperature effect is more pronounced. This increase of adsorption with increase of temperature is due to the reduction of solvency for the PEO chains with increase of temperature. Rheological Measurements. Figure 2 shows plots of yield stress as a function of TiO2 volume fraction for the two copolymers. It is clear that when the suspension stabilized by Atlox 4913, the yield stress starts to increase at a low volume fraction (∼0.1), whereas for Hypermer CG-6 (a) such increase occurs at a much higher volume fraction (∼0.4). This indicated that the TiO2 suspensions prepared using Atlox 4913 are weakly flocculated whereas those prepared using Hypermer CG-6 (a) are stable. Evidence for this behavior was obtained using the microscope investigation shown in Figure 3. With Atlox 4913, flocs are formed in the TiO2 dispersions (Figure 3a) whereas for TiO2 stabilized by Hypermer CG-6 (a), the particles are well dispersed (Figure 3b).
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Figure 4. Plot of plastic viscosity as a function of TiO2 (stabilized by copolymers) volume fraction.
acid) (when compared to CG-6 (a)) the backbone of the copolymer does not have sufficient polarity to adsorb on the hydrophilic TiO2 surface. This will cause some preferential adsorption of the PEO chains to the surface, resulting in flocculation of the dispersion as a result of hydrophobic interaction between the backbones which are now pointing toward the solution. When the content of the polymethyacrylic acid in the backbone is increased, i.e., with Hypermer CG-6 (a), the polarity of the backbone becomes sufficient for adsorption on the TiO2 surface, leaving the PEO side chains dangling in solution. This produces effective steric stabilization for the TiO2 dispersions. These results clearly show the importance of the molecular structure of the graft copolymer on dispersion stability and small alteration (i.e., enhancing a higher proportion of methacrylic acid in the backbone) may cause a dramatic effect on dispersion stability. The adsorbed layer thickness of the polymer can be determined from the relative viscosity as described before.1,12 This method can be applied to sterically stabilized dispersions and the analysis was restricted to the dispersion using Hypermer CG-6 (a) which are stable. The basic idea is to fit the relative viscosity data to a hard-sphere equation (Doughty-Krieger13)
[ ( )]
(1)
φeff ) φs [1 + (∆/R)]3
(2)
ηr ) φ 1 -
φeff φm
-[η]φm
where
is the effective volume fraction, [η] is the intrinsic viscosity, and φm is the maximum packing volume fraction. From φeff, one can obtain ∆ using eq 2 provided an estimate of φm is obtained. In the present calculation, we have used the empirical equation of Eilers,14
ηr1/2 - 1 1 ) (η 1/2 -1) + 1.25 φ φm r Figure 3. Optical micrographs of TiO2 suspension stabilized by Atlox 4913 (a) and Hypermer CG-6 (a) (b).
Similar results were obtained when one measures the plastic viscosity as a function of TiO2 volume fraction as shown in Figure 4. The above results clearly indicate the effect of structure in the backbone of the graft copolymer on the stability/ flocculation of the TiO2 dispersions. With Atlox 4913 which contains a relatively small proportion of poly(methacrylic
(3)
Thus plots of (ηr1/2-1)/φ versus (ηr1/2-1) should produce straight lines with a slope of 1/φm from which the maximum volume fraction φm can be obtained. Figure 5 shows the plot of (ηr1/2-1)/φ versus (ηr1/2-1) for TiO2 suspensions stabilized by Hypermer CG-6 (a). From the slope the maximum packing volume fraction of the TiO2 dispersion was obtained and this was found to be 0.668. This value is higher than that for random packing (13) Krieger, I. M. Adv. Colloid Interface Sci. 1972, 3, 111. (14) Eliers, V. H. Kolloid - Z. 1941, 97, 313.
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Figure 5. Plot of (xηr - 1)/φ as a function of (xηr - 1) for TiO2 stabilized by Hypermer CG-6 (a).
Figure 7. Plot of moduli as a function of TiO2 (stabilized by Atlox 4913) volume fraction at 0.1 Hz.
Figure 6. Plot of moduli as a function of TiO2 (stabilized by Hypermer CG-6 (a)) volume fraction at 0.1 Hz.
Figure 8. Plot of storage modulus as a function of temperature for TiO2 suspension stabilized by Hypermer CG-6 (a) at various Na2SO4 concentrations.
Table 2. Adsorbed Layer Thickness as a Function of Volume Fraction for TiO2 Stabilized by Hypermer CG-6 (a) φ
ηr
φeff
∆(nm)
0.459 0.442 0.440 0.438 0.416 0.406 0.375
58.77 46.05 41.34 42.16 28.27 28.73 17.55
0.610 0.601 0.596 0.597 0.572 0.579 0.558
15.6 17.0 16.9 17.2 17.7 19.8 22.4
(0.64) and this can be attributed to the polydispersity of the TiO2 dispersions. The effect of volume fraction of titanium dioxide particles on the adsorbed layer thickness (∆/nm) for the graft copolymer calculated from eq 2 is shown in Table 2. As found previously,1 there is a reduction in adsorbed layer thickness with increase in volume fraction of the dispersions. This is attributed to the interpenetration and/or compression of the chains with increase in φs as discussed before.12 The adsorbed layer thickness of the graft copolymers on TiO2 is relatively higher than that on latex particles. However, due to the polydispersity of the TiO2 particles, an accurate estimation of ∆ is not possible. Figure 6 shows the log-log plots of G*, G′, and G′′ as a function of TiO2 volume fraction. These results were obtained using Hypermer CG-6 (a) as stabilizer adsorbed on TiO2 particle surface at saturation adsorption. The elastic modulus G′ dominates at high volume fraction. The crossover point at which G′ ) G′′ occurs at φs ) 0.39. This crossover point can be taken as an indication of the volume fraction at which the adsorbed layers just overlapped.12 However, the results obtained using Atlox 4913 as a stabilizer (Figure 7) showed a much lower crossover point (∼0.07) suggesting flocculation of the dispersion in this
Figure 9. Plot of storage modulus as a function of temperature for TiO2 suspension stabilized by Atlox 4913 at various Na2SO4 concentrations.
system. Evidence for this may be obtained by applying scaling laws for the change of the moduli with volume fraction of the dispersion. As discussed before,1,12 a scaling relationship between elastic modulus and volume fraction of the particles can be established using the following equation
G′ ) k φm
(4)
where G′ is the elastic modulus, k is a constant, φ is the volume fraction of the particles and m is the exponent. From eq 1 the exponent of m was estimated as 23 for Hypermer CG-6 (a) adsorbed on TiO2 and only 3.4 for Atlox 4913 adsorbed on TiO2. The latter value is an indication of weak flocculation as pointed out by Ball and Brown.15 The effect of addition of electrolyte (Na2SO4) and changing temperature on the viscoelastic properties of (15) Ball, R.; Brown, W. D. Personal communication.
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Figure 10. Plot of shear stress as a function of shear rate for TiO2 suspension stabilized by Hypermer CG-6 (a) at φ ) 0.34.
Figure 11. Plot of shear stress as a function of shear rate for TiO2 suspension stabilized by Atlox 4913 at φ ) 0.27.
the TiO2 suspensions stabilized by both copolymers is shown in Figures 8 and 9 (stabilized by Hypermer CG-6 (a) at φ ) 0.340 and stabilized by Atlox 4913 at φ ) 0.266). It is clear that temperature has little affect on the viscoelastic properties, the elastic modulus is nearly constant in the range of 5-65 °C. However, the moduli values were systematically increased when electrolyte (Na2SO4) was introduced. The higher the electrolyte concentration the higher the elastic modulus. Similar results were obtained using shear stress-shear rate measurements (see Figures 10 and 11). Both shear stress and yield value increased with increase in Na2SO4 concentration. However, for TiO2 suspension stabilized by Hypermer CG-6 (a), at φ ) 0.34 in the absence of salt
the shear stress is quite low and no yield stress was found whereas for TiO2 suspension stabilized by Atlox 4913 even at φ ) 0.27 the shear stress was relatively high and yield stress could be measured. From the above adsorption and rheological investigation, one conclusion can be drawn that Atlox 4913 is not a suitable stabilizer for TiO2 in water medium. However, Hypermer CG-6 (a) may be used as stabilizer for TiO2 suspensions in aqueous solutions, however it is not good enough to prevent flocculation when electrolyte is introduced in the system. A more effective copolymer needs to be chosen which is the future task of our research. LA990811K
