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Rheological Properties of Silica Suspensions in Aqueous Solutions of Block Copolymers and Their Water-Soluble Components
Journal of Colloid and Interface Science 241, 293–295 (2001) doi:10.1006/jcis.2001.7708, available online at http://www.idealibrary.com on
NOTE Rheological Properties of Silica Suspensions in Aqueous Solutions of Block Copolymers and Their Water-Soluble Components
Dynamic moduli of fumed silica suspensions in aqueous solutions of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers and PEO homopolymers were measured as a function of surface coverage. Since the block copolymers and PEO are adsorbed on the silica surface through hydrogen bonding between the ether oxygen and the silanol group on the silica surface, the interaction between the silanol groups, which is dominant for the aggregation of silica particles, should be prohibited. Dynamic moduli in the silica suspensions were strongly related to the stability of the silica suspensions and the block copolymer, and the longest PEO portion was useful for stabilizing the silica particles. However, the PEO homopolymer did not support stability of the silica particles, suggesting that chain conformation of the PEO portion in the block copolymer is different from that for the PEO homopolymer. °C 2001 Academic Press Key Words: rheology; silica suspension; PEO–PPO–PEO block copolymer; PEO; adsorption; stability; silanol group; hydrogen bonding.
INTRODUCTION A great deal of effort has been expended in attempting to understand adsorption of block copolymer. There have been numerous parallel developments in theoretical treatments and experimental techniques, and the importance of block copolymer adsorption has been addressed in some textbooks (1–3) and reviews (4–6). Several comprehensive theories based on both mean field theory and scaling theory have been presented as ways of predicting surface densities, segment distribution profiles, and thickness of adsorbed layers. Methods such as surface force measurements, optical reflection and scattering techniques, reflection and scattering methods with neutrons, and atomic force microscopy have successfully been used to determine conformational features of the adsorbed block copolymers. Since poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers can exhibit amphiphilic properties in their aqueous solutions, they are often used not only in basic researches but also in many applications (7, 8). They are commercially available as Poloxamers and Pluronics, and due to variation of copolymer composition and molecular weight a wide range of amphiphilic properties can be controlled. Thus, the block copolymers associate to form micelles as well as liquid crystalline phases and they also tend to adsorb extensively onto a variety of interfaces. There have been many experimental reports on the adsorption of PEO–PPO– PEO block copolymers at water–solid interfaces (2, 7, 8–16). Kinetics and equilibrium aspects of the adsorption of PEO–PPO–PEO onto silicon wafers have been studied as functions of hydrophobicity gradient (9) and temperature
(9–11). The layer thicknesses of the block copolymers adsorbed at the hydrophobic and hydrophilic surfaces have been measured by the optical reflection and scattering techniques (11–15) and potential measurements (16): the layer thickness adsorbed on the former surfaces was proportional to the PEO mass with a given power (12–16), whereas on the latter surface the thickness was thin as 2–6 nm (11, 16). However, little is known about the effect of PEO–PPO–PEO block copolymers on the stability of the solid particles, taking into account the adsorption behavior of the block copolymers and the rheological properties of the dispersions. For the dispersed particles, polymer lattices and colloidal silica particles were used for the adsorption of PEO–PPO–PEO block copolymers. Although fumed silica particles are often employed for the adsorption of various polymers from aqueous and nonaqueous solutions (1, 2, 4), they are not used as adsorbent for the adsorption of PEO–PPO–PEO block copolymers. In this paper, by using hydrophilic fumed silica we report some aspects of the effects of the block copolymer adsorption on both the stability of the silica particles and the rheological properties of the silica suspensions as functions of polymer concentration and composition of the block copolymer.
EXPERIMENTAL Samples. Pluronic PEO–PPO–PEO samples, F-108, F-68, and L-31, were kindly supplied by Asahidenka Co. and used without further purification. According to the manufacturer of PEO–PPO–PEO, the PEO composition is 80 wt% for F-108 and F-68 and 10 wt% for L-31. Two PEO samples with molecular weights of 11 × 103 (PEO-11) and 5 × 103 (PEO-5) were purchased from Polyscience Ins. Co. and used as received. The molecular weight distributions of the PEO-11 and PEO-5 are 1.05 and 1.10, respectively. Moreover, the molecular weights of the PEO-11 and PEO-5 have almost the same magnitude as the total mass of the PEO portion in F-108 and F-68, respectively. Aerosil 130 silica powder supplied from Nippon Aerosil Co. was treated as described previously before use (17). From the manufacturer of the Aerosil 130 silica, the primary silica particles have an average diameter of 16 nm, a surface area of 130 m2 /g, and a silanol group density of 2.5/nm2 , but in air the silica particles tend to form aggregates due to hydrogen bonding between the silanol groups. Water purified using a Millipore-Q system was used for a dispersion medium and a solvent for the polymer samples. For preparation of the silica suspensions in water and in aqueous polymer solutions, a weighed amount of Aerosil 130 was mixed with the solvent or preprepared aqueous polymer solutions with dose concentrations of 0.05, 0.1, 0.15, and 0.2 g/100 mL in a centrifuge glass tube. The resulting suspensions were made homogeneous by mechanical shaking in an air incubator at 25 ± 0.1◦ C, and the silica concentration was fixed at 0.8 wt%. Adsorption of polymers. The amounts of the polymers adsorbed on the silica surface were determined as follows. The silica suspensions were centrifuged using a Kubota 6700 centrifuge in order to sediment the silica. The polymer concentration Cp in the removed supernatant was determined by an Ohtsuka Denshi DMR-1030 refractometer.
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0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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Rheological measurements. Dynamic moduli measurements of the suspensions were performed using a Paar Physica DCR capillary. The frequency ranges were changed from 2 to 200 rad/s. The measuring temperature was fixed at 25 ± 0.1◦ C.
RESULTS AND DISCUSSION Adsorption behavior. Although the PEO–PPO–PEO block copolymers are well known to form micelles, their reported critical micelle concentration (cmc) values depend strongly on the method of investigation and differ by several orders of magnitude (7, 8, 18). However, a comprehensive study by Lopes and Loh (19) gave consistent data of the cmc value for a wide range of the PEO–PPO–PEO block copolymers in water using different techniques. According to their data, the dose concentrations in this study are below the cmc values of the corresponding PEO–PPO–PEO block copolymers. Hereafter, we will discuss the resulting data without taking into account the micellization of the PEO–PPO–PEO block copolymers in bulk solutions. Figure 1 shows adsorption isotherms of the respective polymers. The adsorbed amount almost attains a plateau at the highest dose concentration of 0.2 g/100 mL, irrespective of the polymer. The plateau adsorbed amount increases with increase in the molecular weight for the respective polymer series. The magnitude of the plateau adsorbed amounts for the block copolymers is similar to that at the hydrophilic silicon wafer surface, where the plateau adsorbed amount is independent of the PPO content in the range 0–30% (11). Moreover, the adsorbed amounts of PEO portions calculated from the plateau adsorbed amounts of F-108 and F-68 were almost the same as the adsorbed amounts of the PEO-11 and PEO-5 in the plateau region, respectively. This means that the PPO portion in the block copolymer cannot easily adsorb on silica surfaces as compared to the PEO portion since PPO is insoluble in water at intermediate and high molecular masses (11). Rheological properties. Hydrogen bonding between the ether oxygen and the silanol groups at the silica surface is responsible for the adsorption of the block copolymers and PEO homopolymers, leading to suppression of an aggregated structure of the silica particles in water maintained through interaction between the silanol groups on the silica surface. Thus, the stability of dilute silica suspensions in the presence of the polymer should strongly depend on the initial added polymer concentration as well as the polymer character. Moreover, the stability of the silica suspensions could be related to rheological properties of the silica suspensions (20). The silica suspensions in the presence of the polymer solutions with the dose concentrations of 0.05 and 0.2 g/100 mL as well as in the absence of any polymer were used for the dynamic moduli measurements. From
FIG. 1. Plots of adsorbed amount (A) per unit area of silica surface against polymer concentration (Cp ) in supernatant of PEO–PPO–PEO block copolymers and PEO homopolymers on the silica surfaces: F-108 (s), F-68 (h), L-31 (n), PEO-11 (d), PEO-5 (j).
FIG. 2. Frequency dependence of the storage G 0 (a) and loss G 00 (b) moduli for 0.8 wt% silica suspensions in water (j) and 0.2 g/100 mL PEO–PPO–PEO block copolymers and PEO homopolymers: F-108 (s), F-68 (n), L-31 (h), PEO-11 (d), PEO-5 (m).
the adsorption isotherms as shown in Fig. 1, at the highest dose concentration of 0.2 g/100 mL the silica surface should be fully covered by the polymers, whereas at the lowest dose concentration of 0.05 g/100 mL no full surface coverage occurs due to less adsorbtion than in the plateau region. The differences in both the adsorbed amount and characteristics of the polymer should induce changes in dynamic moduli as shown in Figs. 2 and 3. The variation of the storage G 0 and loss G 00 moduli with the frequency ω for silica suspensions in the 0.2 g/100 mL F-108 solution is similar to that observed for polymer solutions, i.e., G 0 ∝ ω2 and G 00 ∝ ω. Such a frequency dependence indicates that the silica particles in the silica suspension are sterically stabilized by the adsorbed F-108 chains, although the respective moduli are slightly larger than those for the silica suspensions in water. As mentioned above, the amounts of PEO portions in the adsorbed block copolymers were equivalent with the adsorbed amounts of the PEO homopolymer with the similar molecular weight to the PEO portion in the corresponding block copolymer. However, the silica suspension at the full surface coverage of PEO-11 shows the largest dynamic moduli as shown in Fig. 2, namely PEO-11 is not suitable to stabilize the silica suspension as compared to F-108. This may be due to the difference in the conformation between the adsorbed PEO homopolymer chain and the PEO portions in the block copolymer: the block copolymer layer adsorbed on the silica particles should be thicker than that with the PEO homopolymer due to surface aggregation of the block copolymer (21). In the other wards, the PEO portions in the block copolymer form longer stabilizing corona-like chains on the silica surfaces. On the other hand, the dynamic moduli of the silica suspension at the lower surface coverage of F-108 are much larger than those at the full surface coverage and their frequency dependencies become weaker, suggesting that flocculation
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adsorbed chain between the PEO portion in the block copolymer and the PEO homopolymer. Except for the block copolymer with the smallest composition of PEO, at the partial surface coverage the dynamic moduli were in the same order of the magnitude, but the magnitude was increased by one order of magnitude as compared to that without polymers.
REFERENCES
FIG. 3. Frequency dependence of the storage G 0 (a) and loss G 00 (b) moduli for 0.8 wt% silica suspensions in water, 0.05 g/100 mL PEO–PPO–PEO block copolymers, and PEO homopolymers. Symbols are the same as in Fig. 2.
of the silica particles causes formation of stronger aggregated structures in the silica suspensions. Similar surface coverage dependencies of the dynamic moduli are observed for silica suspensions in the presence of F-68 and PEO, but the magnitude of the storage module is one order of magnitude higher than that at the full surface coverage of F-108. An opposite surface coverage dependence of the dynamic moduli was observed for silica suspensions in the presence of L-31. The dynamic moduli at the full surface coverage of L-31 are the same order of magnitude as those in the presence of F-68 and the PEO homopolymers, whereas the dynamic moduli at the low surface coverage of L-31 are almost comparable to those for the silica suspension dispersed in water. The adsorbed L-31 chains at the low surface coverage benefit neither stabilization nor flocculation of the silica particles due to the shorter chain length.
CONCLUSIONS From the measurements of dynamic moduli, the effect of adsorption of the polymers on fumed silica particles by changes in surface coverage was more clearly observed in dilute silica suspensions. The longest PEO portion in the block copolymer was the most useful for stabilizing the silica particles at full surface coverage. However, the PEO homopolymer with the same molecular weight as the PEO portion in the corresponding block copolymer showed good flocculation power. This may be due to the difference in the conformation of the
1. Fleer, G. J., Cohen Stuart, M. A., Scheutjens, J. M. H. M., Cosgrove, T., and Vincent, B., “Polymers at Interfaces.” Chapman & Hall, London, 1993. 2. Kawaguchi, M., in “Structure and Properties of Multiphase Polymeric Materials” (T. Araki, Q. Tran-Cong, and M. Shibayama, Eds.), p. 361. Dekker, New York, 1998. 3. Jones, R. A. L., and Richards, R. W., “Polymers at Surfaces and Interfaces.” Cambridge University Press, Cambridge, 1999. 4. Kawaguchi, M., and Takahashi, A., Adv. Colloid Interface Sci. 37, 219 (1992). 5. Helperin, A., Tirrel, M., and Lodge, T. P., Adv. Polymer Sci. 100, 31 (1992). 6. Leger, L., Rapha¨el, E., and Hervet, H., Adv. Polymer Sci. 186, 138 (1999). 7. Alexandridis, P., and Hatton, T. A., Colloids Surfaces 1, 96 (1995). 8. Alexandridis, P., Current Opin. Colloid Interface Sci. 2, 478 (1997). 9. Van de Steeg, L. M. A., and Golander, C.-G., Colloids Surfaces 105, 55 (1991). 10. Tiberg, F., Malmsten, M., Linse, P., and Lindman, B., Langmuir 7, 2723 (1991). 11. Malmsten, M., Linse, P., and Cosgrove, T., Macromolecules 25, 2474 (1992). 12. Killmann, E., Mair, H., and Baker, J. A., Colloids Surfaces 31, 51 (1988). 13. Baker, J. A., and Berg, J. C., Langmuir 4, 1055 (1988). 14. Baker, J. A., Pearson, R. A., and Berg, J. C., Langmuir 5, 339 (1989). 15. Lee, J., Martic, P. A., and Tan, J. S., J. Colloid Interface Sci. 131, 252 (1989). 16. Barnes, T. J., and Prestidge, C. A., Langmuir 16, 4116 (2000). 17. Kawaguchi, M., Hayakawa, K., and Takahashi, A., Polymer J. 12, 265 (1980). 18. Holland, R. J., Parker, E. J., Guiney, K., and Zeld, F. R., J. Phys. Chem. 99, 11981 (1995). 19. Lopes, J. R., and Loh, W., Langmuir 14, 750 (1998). 20. De Silva, G. P. H. L., Luckham, P. F., and Tadros, Th. F., Colloids Surfaces 50, 263 (1990). 21. Eskilsson, K., and Tiberg, F., Macromolecules 31, 5075 (1998). Masami Kawaguchi1 Tadahiro Yamamoto Tadaya Kato Chemistry Department for Materials Faculty of Engineering Mie University 1515 Kamihama, Tsu Mie 514-8507, Japan Received February 13, 2001; accepted May 11, 2001; published online July 20, 2001
1 To whom correspondence should be addressed. Fax: 81-59-231-9471. E-mail: [email protected].
NOTE Rheological Properties of Silica Suspensions in Aqueous Solutions of Block Copolymers and Their Water-Soluble Components
Dynamic moduli of fumed silica suspensions in aqueous solutions of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers and PEO homopolymers were measured as a function of surface coverage. Since the block copolymers and PEO are adsorbed on the silica surface through hydrogen bonding between the ether oxygen and the silanol group on the silica surface, the interaction between the silanol groups, which is dominant for the aggregation of silica particles, should be prohibited. Dynamic moduli in the silica suspensions were strongly related to the stability of the silica suspensions and the block copolymer, and the longest PEO portion was useful for stabilizing the silica particles. However, the PEO homopolymer did not support stability of the silica particles, suggesting that chain conformation of the PEO portion in the block copolymer is different from that for the PEO homopolymer. °C 2001 Academic Press Key Words: rheology; silica suspension; PEO–PPO–PEO block copolymer; PEO; adsorption; stability; silanol group; hydrogen bonding.
INTRODUCTION A great deal of effort has been expended in attempting to understand adsorption of block copolymer. There have been numerous parallel developments in theoretical treatments and experimental techniques, and the importance of block copolymer adsorption has been addressed in some textbooks (1–3) and reviews (4–6). Several comprehensive theories based on both mean field theory and scaling theory have been presented as ways of predicting surface densities, segment distribution profiles, and thickness of adsorbed layers. Methods such as surface force measurements, optical reflection and scattering techniques, reflection and scattering methods with neutrons, and atomic force microscopy have successfully been used to determine conformational features of the adsorbed block copolymers. Since poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers can exhibit amphiphilic properties in their aqueous solutions, they are often used not only in basic researches but also in many applications (7, 8). They are commercially available as Poloxamers and Pluronics, and due to variation of copolymer composition and molecular weight a wide range of amphiphilic properties can be controlled. Thus, the block copolymers associate to form micelles as well as liquid crystalline phases and they also tend to adsorb extensively onto a variety of interfaces. There have been many experimental reports on the adsorption of PEO–PPO– PEO block copolymers at water–solid interfaces (2, 7, 8–16). Kinetics and equilibrium aspects of the adsorption of PEO–PPO–PEO onto silicon wafers have been studied as functions of hydrophobicity gradient (9) and temperature
(9–11). The layer thicknesses of the block copolymers adsorbed at the hydrophobic and hydrophilic surfaces have been measured by the optical reflection and scattering techniques (11–15) and potential measurements (16): the layer thickness adsorbed on the former surfaces was proportional to the PEO mass with a given power (12–16), whereas on the latter surface the thickness was thin as 2–6 nm (11, 16). However, little is known about the effect of PEO–PPO–PEO block copolymers on the stability of the solid particles, taking into account the adsorption behavior of the block copolymers and the rheological properties of the dispersions. For the dispersed particles, polymer lattices and colloidal silica particles were used for the adsorption of PEO–PPO–PEO block copolymers. Although fumed silica particles are often employed for the adsorption of various polymers from aqueous and nonaqueous solutions (1, 2, 4), they are not used as adsorbent for the adsorption of PEO–PPO–PEO block copolymers. In this paper, by using hydrophilic fumed silica we report some aspects of the effects of the block copolymer adsorption on both the stability of the silica particles and the rheological properties of the silica suspensions as functions of polymer concentration and composition of the block copolymer.
EXPERIMENTAL Samples. Pluronic PEO–PPO–PEO samples, F-108, F-68, and L-31, were kindly supplied by Asahidenka Co. and used without further purification. According to the manufacturer of PEO–PPO–PEO, the PEO composition is 80 wt% for F-108 and F-68 and 10 wt% for L-31. Two PEO samples with molecular weights of 11 × 103 (PEO-11) and 5 × 103 (PEO-5) were purchased from Polyscience Ins. Co. and used as received. The molecular weight distributions of the PEO-11 and PEO-5 are 1.05 and 1.10, respectively. Moreover, the molecular weights of the PEO-11 and PEO-5 have almost the same magnitude as the total mass of the PEO portion in F-108 and F-68, respectively. Aerosil 130 silica powder supplied from Nippon Aerosil Co. was treated as described previously before use (17). From the manufacturer of the Aerosil 130 silica, the primary silica particles have an average diameter of 16 nm, a surface area of 130 m2 /g, and a silanol group density of 2.5/nm2 , but in air the silica particles tend to form aggregates due to hydrogen bonding between the silanol groups. Water purified using a Millipore-Q system was used for a dispersion medium and a solvent for the polymer samples. For preparation of the silica suspensions in water and in aqueous polymer solutions, a weighed amount of Aerosil 130 was mixed with the solvent or preprepared aqueous polymer solutions with dose concentrations of 0.05, 0.1, 0.15, and 0.2 g/100 mL in a centrifuge glass tube. The resulting suspensions were made homogeneous by mechanical shaking in an air incubator at 25 ± 0.1◦ C, and the silica concentration was fixed at 0.8 wt%. Adsorption of polymers. The amounts of the polymers adsorbed on the silica surface were determined as follows. The silica suspensions were centrifuged using a Kubota 6700 centrifuge in order to sediment the silica. The polymer concentration Cp in the removed supernatant was determined by an Ohtsuka Denshi DMR-1030 refractometer.
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0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
294
NOTE
Rheological measurements. Dynamic moduli measurements of the suspensions were performed using a Paar Physica DCR capillary. The frequency ranges were changed from 2 to 200 rad/s. The measuring temperature was fixed at 25 ± 0.1◦ C.
RESULTS AND DISCUSSION Adsorption behavior. Although the PEO–PPO–PEO block copolymers are well known to form micelles, their reported critical micelle concentration (cmc) values depend strongly on the method of investigation and differ by several orders of magnitude (7, 8, 18). However, a comprehensive study by Lopes and Loh (19) gave consistent data of the cmc value for a wide range of the PEO–PPO–PEO block copolymers in water using different techniques. According to their data, the dose concentrations in this study are below the cmc values of the corresponding PEO–PPO–PEO block copolymers. Hereafter, we will discuss the resulting data without taking into account the micellization of the PEO–PPO–PEO block copolymers in bulk solutions. Figure 1 shows adsorption isotherms of the respective polymers. The adsorbed amount almost attains a plateau at the highest dose concentration of 0.2 g/100 mL, irrespective of the polymer. The plateau adsorbed amount increases with increase in the molecular weight for the respective polymer series. The magnitude of the plateau adsorbed amounts for the block copolymers is similar to that at the hydrophilic silicon wafer surface, where the plateau adsorbed amount is independent of the PPO content in the range 0–30% (11). Moreover, the adsorbed amounts of PEO portions calculated from the plateau adsorbed amounts of F-108 and F-68 were almost the same as the adsorbed amounts of the PEO-11 and PEO-5 in the plateau region, respectively. This means that the PPO portion in the block copolymer cannot easily adsorb on silica surfaces as compared to the PEO portion since PPO is insoluble in water at intermediate and high molecular masses (11). Rheological properties. Hydrogen bonding between the ether oxygen and the silanol groups at the silica surface is responsible for the adsorption of the block copolymers and PEO homopolymers, leading to suppression of an aggregated structure of the silica particles in water maintained through interaction between the silanol groups on the silica surface. Thus, the stability of dilute silica suspensions in the presence of the polymer should strongly depend on the initial added polymer concentration as well as the polymer character. Moreover, the stability of the silica suspensions could be related to rheological properties of the silica suspensions (20). The silica suspensions in the presence of the polymer solutions with the dose concentrations of 0.05 and 0.2 g/100 mL as well as in the absence of any polymer were used for the dynamic moduli measurements. From
FIG. 1. Plots of adsorbed amount (A) per unit area of silica surface against polymer concentration (Cp ) in supernatant of PEO–PPO–PEO block copolymers and PEO homopolymers on the silica surfaces: F-108 (s), F-68 (h), L-31 (n), PEO-11 (d), PEO-5 (j).
FIG. 2. Frequency dependence of the storage G 0 (a) and loss G 00 (b) moduli for 0.8 wt% silica suspensions in water (j) and 0.2 g/100 mL PEO–PPO–PEO block copolymers and PEO homopolymers: F-108 (s), F-68 (n), L-31 (h), PEO-11 (d), PEO-5 (m).
the adsorption isotherms as shown in Fig. 1, at the highest dose concentration of 0.2 g/100 mL the silica surface should be fully covered by the polymers, whereas at the lowest dose concentration of 0.05 g/100 mL no full surface coverage occurs due to less adsorbtion than in the plateau region. The differences in both the adsorbed amount and characteristics of the polymer should induce changes in dynamic moduli as shown in Figs. 2 and 3. The variation of the storage G 0 and loss G 00 moduli with the frequency ω for silica suspensions in the 0.2 g/100 mL F-108 solution is similar to that observed for polymer solutions, i.e., G 0 ∝ ω2 and G 00 ∝ ω. Such a frequency dependence indicates that the silica particles in the silica suspension are sterically stabilized by the adsorbed F-108 chains, although the respective moduli are slightly larger than those for the silica suspensions in water. As mentioned above, the amounts of PEO portions in the adsorbed block copolymers were equivalent with the adsorbed amounts of the PEO homopolymer with the similar molecular weight to the PEO portion in the corresponding block copolymer. However, the silica suspension at the full surface coverage of PEO-11 shows the largest dynamic moduli as shown in Fig. 2, namely PEO-11 is not suitable to stabilize the silica suspension as compared to F-108. This may be due to the difference in the conformation between the adsorbed PEO homopolymer chain and the PEO portions in the block copolymer: the block copolymer layer adsorbed on the silica particles should be thicker than that with the PEO homopolymer due to surface aggregation of the block copolymer (21). In the other wards, the PEO portions in the block copolymer form longer stabilizing corona-like chains on the silica surfaces. On the other hand, the dynamic moduli of the silica suspension at the lower surface coverage of F-108 are much larger than those at the full surface coverage and their frequency dependencies become weaker, suggesting that flocculation
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adsorbed chain between the PEO portion in the block copolymer and the PEO homopolymer. Except for the block copolymer with the smallest composition of PEO, at the partial surface coverage the dynamic moduli were in the same order of the magnitude, but the magnitude was increased by one order of magnitude as compared to that without polymers.
REFERENCES
FIG. 3. Frequency dependence of the storage G 0 (a) and loss G 00 (b) moduli for 0.8 wt% silica suspensions in water, 0.05 g/100 mL PEO–PPO–PEO block copolymers, and PEO homopolymers. Symbols are the same as in Fig. 2.
of the silica particles causes formation of stronger aggregated structures in the silica suspensions. Similar surface coverage dependencies of the dynamic moduli are observed for silica suspensions in the presence of F-68 and PEO, but the magnitude of the storage module is one order of magnitude higher than that at the full surface coverage of F-108. An opposite surface coverage dependence of the dynamic moduli was observed for silica suspensions in the presence of L-31. The dynamic moduli at the full surface coverage of L-31 are the same order of magnitude as those in the presence of F-68 and the PEO homopolymers, whereas the dynamic moduli at the low surface coverage of L-31 are almost comparable to those for the silica suspension dispersed in water. The adsorbed L-31 chains at the low surface coverage benefit neither stabilization nor flocculation of the silica particles due to the shorter chain length.
CONCLUSIONS From the measurements of dynamic moduli, the effect of adsorption of the polymers on fumed silica particles by changes in surface coverage was more clearly observed in dilute silica suspensions. The longest PEO portion in the block copolymer was the most useful for stabilizing the silica particles at full surface coverage. However, the PEO homopolymer with the same molecular weight as the PEO portion in the corresponding block copolymer showed good flocculation power. This may be due to the difference in the conformation of the
1. Fleer, G. J., Cohen Stuart, M. A., Scheutjens, J. M. H. M., Cosgrove, T., and Vincent, B., “Polymers at Interfaces.” Chapman & Hall, London, 1993. 2. Kawaguchi, M., in “Structure and Properties of Multiphase Polymeric Materials” (T. Araki, Q. Tran-Cong, and M. Shibayama, Eds.), p. 361. Dekker, New York, 1998. 3. Jones, R. A. L., and Richards, R. W., “Polymers at Surfaces and Interfaces.” Cambridge University Press, Cambridge, 1999. 4. Kawaguchi, M., and Takahashi, A., Adv. Colloid Interface Sci. 37, 219 (1992). 5. Helperin, A., Tirrel, M., and Lodge, T. P., Adv. Polymer Sci. 100, 31 (1992). 6. Leger, L., Rapha¨el, E., and Hervet, H., Adv. Polymer Sci. 186, 138 (1999). 7. Alexandridis, P., and Hatton, T. A., Colloids Surfaces 1, 96 (1995). 8. Alexandridis, P., Current Opin. Colloid Interface Sci. 2, 478 (1997). 9. Van de Steeg, L. M. A., and Golander, C.-G., Colloids Surfaces 105, 55 (1991). 10. Tiberg, F., Malmsten, M., Linse, P., and Lindman, B., Langmuir 7, 2723 (1991). 11. Malmsten, M., Linse, P., and Cosgrove, T., Macromolecules 25, 2474 (1992). 12. Killmann, E., Mair, H., and Baker, J. A., Colloids Surfaces 31, 51 (1988). 13. Baker, J. A., and Berg, J. C., Langmuir 4, 1055 (1988). 14. Baker, J. A., Pearson, R. A., and Berg, J. C., Langmuir 5, 339 (1989). 15. Lee, J., Martic, P. A., and Tan, J. S., J. Colloid Interface Sci. 131, 252 (1989). 16. Barnes, T. J., and Prestidge, C. A., Langmuir 16, 4116 (2000). 17. Kawaguchi, M., Hayakawa, K., and Takahashi, A., Polymer J. 12, 265 (1980). 18. Holland, R. J., Parker, E. J., Guiney, K., and Zeld, F. R., J. Phys. Chem. 99, 11981 (1995). 19. Lopes, J. R., and Loh, W., Langmuir 14, 750 (1998). 20. De Silva, G. P. H. L., Luckham, P. F., and Tadros, Th. F., Colloids Surfaces 50, 263 (1990). 21. Eskilsson, K., and Tiberg, F., Macromolecules 31, 5075 (1998). Masami Kawaguchi1 Tadahiro Yamamoto Tadaya Kato Chemistry Department for Materials Faculty of Engineering Mie University 1515 Kamihama, Tsu Mie 514-8507, Japan Received February 13, 2001; accepted May 11, 2001; published online July 20, 2001
1 To whom correspondence should be addressed. Fax: 81-59-231-9471. E-mail: [email protected].