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An Approach to Hierarchically Structured Porous Zirconia Aggregates
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
184, 328–330 (1996)
0626
NOTE An Approach to Hierarchically Structured Porous Zirconia Aggregates Presented here is a simple, inexpensive approach to aggregating colloids into hierarchically structured spherical particles. Successive aggregation steps are used to assemble a particle that is selfsimilar on two size scales and is permeated by an ordered pore network with a bidisperse size distribution. The structure of the micro- and macropore networks as well as the mechanical integrity of the structure can be controlled by varying sintering conditions. q 1996 Academic Press, Inc.
Key Words: liquid chromatography; catalysis; colloid aggregation; zirconia.
INTRODUCTION The rates of fluid flow and mass transfer in beds (packed or fluidized) of porous particulate solids are frequently limited by the characteristic size of the particles, as quantified through dimensionless groups such as Reynolds number, Dahmko¨hler numbers, and Sherwood number (1). For a given pore structure the limiting flow rate may be increased by using a larger particle size, but this decreases the efficiency of mass transfer. It would be advantageous to expand the design to two (or more) size scales—in the simplest case, to vary pore size as well as particle size, and in the extreme case, to vary the mass fractal dimension that would hold over many size scales. In this note we report a method to synthesize spherical ceramic particles which present three distinct size scales. The particles are produced by sequential aggregation of zirconia colloids. In the first step, urea–formaldehyde polymerization induces aggregation of colloids from a stable sol to form ceramic/polymer composite microspheres. In the second step, these microspheres are aggregated by an emulsion method, with the polymer within the microspheres serving as the binder. The structure of a single resulting particle is depicted schematically in Fig. 1. In principle, as the number of sequential aggregations increases, the resulting, increasingly hierarchical structure would come to approximate that of a fractal object such as the Menger sponge (2). The mass-transfer efficiency of fluids within such hierarchical bidisperse pore networks can be superior to that in a random bidisperse pore structure (3), with important implications in the design of catalytic and chromatographic processes. Slow diffusion and mass transfer can lower the overall rate and selectivity of a catalyzed reaction or the resolution and dynamic binding capacity of a chromatographic separation; these problems become more severe with higher-order reaction (or sorption) kinetics (4). These limitations are due to mass transfer resistances within smaller pores. If, however, material is rapidly transported through a macropore network that spans the particle, then the length scale over which slower diffusion occurs is that of the micropore network. Models from reaction engineering (5, 6) and chromatography (7–9) have predicted the improvement in mass transfer to be even greater if significant convective transport occurs within the macropores. Materials with hierarchical structure should also find use in the cracking of large, highly branched hydrocarbons which are less able to travel through smaller pores. This synthesis incorporates two different colloid aggregation techniques,
JCIS 4486
/
6g18$$$361
EXPERIMENTAL PROCEDURE Variations of the PICA procedure are described in detail in Refs. (10, 11). Here, the microspheres are formed by dissolving 7.5 g of urea (Mallinckrodt Specialty Chemical Company, Paris, KY) in 100 ml of a stable zirconia sol with a nominal colloid size of 100 nm (ZrO2 100/20, Nyacol, Ashland, MA), the pH of which has been adjusted to 1.7 by titration with nitric acid. To this is added 12.6 ml of 37 wt% formaldehyde in water and methanol (Fisher Scientific, Pittsburgh, PA), and the mixture is briskly stirred by hand with a glass rod for several seconds to homogenize it. Particles are first observed after 4 min; these are allowed to grow until they are the desired size and are solid enough to collect, another 5–20 min. The reaction is then quenched by diluting the mixture fivefold with deionized water. The micrometer-sized particles, which contain both zirconia colloids and UF resin, are separated from nonaggregated polymer and colloids by repeated sedimentation and decanting of the aqueous supernatant. Finally, the polymer/zirconia aggregates are resuspended in deionized water at a volume fraction of 30–60%. This slurry of micrometer-scale particles is aggregated by the following OE method. A hot oil bath is prepared by placing 800 ml of oleyl alcohol into an 850-ml polypropylene beaker. The oil is heated in a hot water bath (1007C) and agitated by a vibromixer with a 3-cm blade. When the oleyl alcohol reaches 957C, 50 ml of slurry is poured into the hot, agitated oleyl alcohol to form an emulsion. Aggregate particles are collected when they are sufficiently dry and strong (30–90 min) by vacuum filtration, and they are subsequently washed with 2-propanol. The collected particles are dried under ambient conditions for 24 h. Subsequent heating of the particles in
328
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
each of which we have earlier described. Polymerization-induced colloid aggregation (PICA) (10, 11) has been used to synthesize micrometer-scale porous zirconia particles for use in high-performance liquid chromatography (HPLC). In this method, the formation of monodisperse spherical aggregate particles is induced by urea–formaldehyde (UF) polymerization and adsorption. The resulting particles have a high porosity which is exceptionally resistant to high-temperature sintering (12). We have also described an oil emulsion (OE) method to synthesize aggregates tens of micrometers in diameter (13). Here a stable zirconia sol is mechanically dispersed into droplets within an oil phase. Slow removal of the water by extraction into and evaporation from the oil phase leads to a porous aggregate. This process is similar to spray-drying techniques (14), but is easy to implement and produces a high yield of particles of the desired size. Currently we are capable of producing PICA microspheres ranging from 1 to 5 mm in diameter. In the oil emulsion method, particle size and dispersity are easier to control at larger sizes—40 to 150 mm in diameter (15). The PICA and OE methods optimally produce particles of strict and distinct size ranges. Described here are hierarchically porous particles that are synthesized by a sequence of PICA and OE methods. The initial particles, micrometer-scale microspheres, are produced by the PICA procedure. The hierarchically structured 100-mm-scale particles are then formed from the microspheres by an OE method. The novelty of this work is the ease and low cost of synthesizing uniformly sized ceramic spherical particles with highly regulated hierarchical structures using microspheres that contain polymer to serve as a binder in the OE aggregation.
10-24-96 11:26:31
coida
AP: Colloid
329
NOTE
FIG. 1. Schematic representation of a single particle assembled by sequential aggregation. There are multiple length scales within the system, determined by the sizes of the initial colloids, the microspheres, and the particle.
air to high temperature (7507C 6 h, 9007C 3 h, 10507C 1 h) removes the polymer and sinters the the zirconia, increasing the mechanical integrity of the particles. After additional sintering at 9007C for 150 h, aggregates are mechanically strong enough to be packed in a column with a pressure gradient of 100 psi/cm. The pore structure was characterized after heating the sample for 8 h at 3207C at 10 03 Torr to remove surface contaminants. Nitrogen sorption was performed using an ASAP 2000 sorptometer (Micromeritics, Norcross, GA). Mercury porosimetry was performed using a Micromeritics PoreSizer 9320 mercury porosimeter. Micrographs were obtained on a JEOL 8400 scanning electron microscope.
RESULTS Stable, unagglomerated spherical zirconia particles with an ordered, bidisperse pore structure are formed; typical particles are shown in Fig. 2. The particle size distribution can be controlled by varying the mixing conditions and the slurry concentration. The distribution can easily be refined further by elutriation. The BET surface area for this hierarchically structured zirconia is measured at 14 m2 /g by nitrogen sorptometry. This is comparable to earlier reported values of 11 and 13 m2 /g for PICA and OE porous zirconia, respectively (16), and represents the surface area of dense 100-nm colloids. Three peaks are observed in the pore size distribution of this material (Fig. 3). The peaks correspond to (1) the interstices between 100-nm colloids within the microspheres, (2) the interstices between the microspheres within the particles, and (3) the voids between the particles. The porosity and mechanical strength of the aggregates can be adjusted by modifying the sintering protocol (Table 1).
FIG. 2. Scanning electron micrographs of representative aggregates. The material was sintered under the following conditions: 7507C 6 h, 9007C 3 h, 10507C 1 h, with 407C/min ramps between temperatures. The particle size distribution has not been refined after sintering.
The hierarchical structure would also allow the controlled breakup of the particle into smaller particles of regular size and shape. This capability may find uses in processes such as Ziegler–Natta polymerization, in which the mechanical pressure of the growing polymer chains cleaves the aggregate
SUMMARY The process described is a simple and inexpensive method for synthesizing zirconia micrometer-scale aggregates with a well-regulated hierarchical structure. It is easily performed on the laboratory scale and should easily scale up to pilot and commercial scales. Advantage is further taken of the fact that the PICA and OE processes, each described previously, provide aggregates of quite different size scales. When they are performed sequentially, we achieve control over multiple size scales. These aggregates retain specific surface area and pore volume comparable to those of other porous zirconia aggregates, but we expect the transport properties to differ markedly due to the ordered bidisperse pore structure.
AID
JCIS 4486
/
6g18$$$362
10-24-96 11:26:31
FIG. 3. Pore size distributions calculated from data obtained from mercury intrusion. This material was sintered under the following conditions: 7507C 6 h, 9007C 3 h, 10507C 1 h, with 407C/min ramps between temperatures. This is the same material pictured in Fig. 2. The particle size distribution has not been refined after sintering.
coida
AP: Colloid
330
NOTE
TABLE 1 Effect of Sintering Conditions on Nitrogen Adsorption Pore Measurements Sintering conditions
Surface area (m2/g)
Pore volume (cm3/g)
7007C, 4 h 9007C, 220 h
14.3
0.10
9007C, 6 h
16.8
0.15
9007C, 100 h
13.0
0.10
9007C, 6 h 11007C, 230 h
2.9
0.02
10.8
0.17
11007C, 12 h 11007C, 220 h
1.2
0.00
7507C, 6 h 9007C, 3 h 10507C, 1 h
14.1
0.12
7507C, 9007C, 10507C, 9007C,
11.7
0.11
6h 3h 1h 220 h
3. Petropoulos, J. H., Petrou, J. K., and Liapis, A. I., Ind. Eng. Chem. Res. 29, 1281 (1991). 4. Aris, R., ‘‘The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts.’’ Clarendon Press, 1975. 5. Nir, A., and Pismen, L., Chem. Eng. Sci. 32, 35 (1977). 6. Rodrigues, A., Ahn, B., and Zoulalian, A., AIChE J. 28, 541 (1982). 7. Liapis, A. I., Xu, Y., Crosser, O. K., and Tongta, A., J. Chromatogr. 702, 45 (1995). 8. Frey, D. D., Schweinheim, E., and Horvath, C., Biotechnol. Prog. 9, 273 (1994). 9. Afeyan, N. B., Mazsaroff, I., L. Varady, Fulton, S. P., Yang, Y. B., and Regnier, F. E., J. Chromatogr. 519, 1 (1990). 10. Sun, L., Annen, M. J., Lorenzano-Porras, C. F., Carr, P. W., and McCormick, A. V., J. Colloid Interface Sci. 163, 464 (1994). 11. Annen, M. J., Kizhappali, R., Carr, P. W., and McCormick, A. V., J. Mater. Sci. 29, 123 (1994). 12. Lorenzano-Porras, C. F., Reeder, D. H., Annen, M. J., Carr, P. W., and McCormick, A. V., Ind. Eng. Chem. Res. 34, 2719 (1995). 13. Carr, P. W., Funkenbusch, E. F., Rigney, M. P., Coleman, L., Hanggi, D. A., and Schaffer, W. A., U.S. Patent 5,015,373 (1991). 14. Roth, C., and Kobrich, R., J. Aerosol Sci. 19, 939 (1988). 15. Robichaud, M. J., Sathyagal, A. N., Carr, P. W., McCormick, A. V., and Flickinger, M. C., Sep. Sci. Technol., in press. 16. Lorenzano-Porras, C. F., Annen, M. J., Flickinger, M. C., Carr, P. W., and McCormick, A. V., J. Colloid Interface Sci. 170, 299 (1995). 17. Odian, G., ‘‘Principles of Polymerization.’’ Wiley, New York, 1991. DAVID H. REEDER * ,† ANDREW M. CLAUSEN‡ MICHAEL J. ANNEN * ,1 PETER W. CARR† ,‡ MICHAEL C. FLICKINGER† ,§ ALON V. MCCORMICK2, *
initiator particle (17). We envision that such a particle could also be useful in the formulation of toners, electrorheological fluids, and coatings.
ACKNOWLEDGMENTS This work was supported by Grants GM 45988 from the National Institutes of Health and CHE 917029 from the National Science Foundation. D.H.R. gratefully recognizes support through a Biotechnology Training Fellowship through the National Institutes of General Medical Sciences (IT32-GM08347). Thanks also go to the Surface Analysis Center at the University of Minnesota for instrumentation.
REFERENCES
Received February 15, 1996; accepted July 16, 1996
1. Bird, R. B., Stewart, W. E., and Lightfoot, E. N., ‘‘Transport Phenomena.’’ Wiley, New York, 1960. 2. Mandelbrot, B., ‘‘The Fractal Geometry of Nature: Updated and Augmented.’’ W. H. Freeman, 1983.
AID
JCIS 4486
/
6g18$$$362
*Department of Chemical Engineering and Materials Science and ‡Department of Chemistry University of Minnesota Minneapolis, Minnesota 55455 †Institute for Advanced Studies in Bioprocess Technology §Department of Biochemistry University of Minnesota St. Paul, Minnesota 55108
10-24-96 11:26:31
1 Current address: 3M Process Technologies Laboratories, 3M Center, St. Paul, MN 55144. 2 To whom correspondence should be addressed.
coida
AP: Colloid
184, 328–330 (1996)
0626
NOTE An Approach to Hierarchically Structured Porous Zirconia Aggregates Presented here is a simple, inexpensive approach to aggregating colloids into hierarchically structured spherical particles. Successive aggregation steps are used to assemble a particle that is selfsimilar on two size scales and is permeated by an ordered pore network with a bidisperse size distribution. The structure of the micro- and macropore networks as well as the mechanical integrity of the structure can be controlled by varying sintering conditions. q 1996 Academic Press, Inc.
Key Words: liquid chromatography; catalysis; colloid aggregation; zirconia.
INTRODUCTION The rates of fluid flow and mass transfer in beds (packed or fluidized) of porous particulate solids are frequently limited by the characteristic size of the particles, as quantified through dimensionless groups such as Reynolds number, Dahmko¨hler numbers, and Sherwood number (1). For a given pore structure the limiting flow rate may be increased by using a larger particle size, but this decreases the efficiency of mass transfer. It would be advantageous to expand the design to two (or more) size scales—in the simplest case, to vary pore size as well as particle size, and in the extreme case, to vary the mass fractal dimension that would hold over many size scales. In this note we report a method to synthesize spherical ceramic particles which present three distinct size scales. The particles are produced by sequential aggregation of zirconia colloids. In the first step, urea–formaldehyde polymerization induces aggregation of colloids from a stable sol to form ceramic/polymer composite microspheres. In the second step, these microspheres are aggregated by an emulsion method, with the polymer within the microspheres serving as the binder. The structure of a single resulting particle is depicted schematically in Fig. 1. In principle, as the number of sequential aggregations increases, the resulting, increasingly hierarchical structure would come to approximate that of a fractal object such as the Menger sponge (2). The mass-transfer efficiency of fluids within such hierarchical bidisperse pore networks can be superior to that in a random bidisperse pore structure (3), with important implications in the design of catalytic and chromatographic processes. Slow diffusion and mass transfer can lower the overall rate and selectivity of a catalyzed reaction or the resolution and dynamic binding capacity of a chromatographic separation; these problems become more severe with higher-order reaction (or sorption) kinetics (4). These limitations are due to mass transfer resistances within smaller pores. If, however, material is rapidly transported through a macropore network that spans the particle, then the length scale over which slower diffusion occurs is that of the micropore network. Models from reaction engineering (5, 6) and chromatography (7–9) have predicted the improvement in mass transfer to be even greater if significant convective transport occurs within the macropores. Materials with hierarchical structure should also find use in the cracking of large, highly branched hydrocarbons which are less able to travel through smaller pores. This synthesis incorporates two different colloid aggregation techniques,
JCIS 4486
/
6g18$$$361
EXPERIMENTAL PROCEDURE Variations of the PICA procedure are described in detail in Refs. (10, 11). Here, the microspheres are formed by dissolving 7.5 g of urea (Mallinckrodt Specialty Chemical Company, Paris, KY) in 100 ml of a stable zirconia sol with a nominal colloid size of 100 nm (ZrO2 100/20, Nyacol, Ashland, MA), the pH of which has been adjusted to 1.7 by titration with nitric acid. To this is added 12.6 ml of 37 wt% formaldehyde in water and methanol (Fisher Scientific, Pittsburgh, PA), and the mixture is briskly stirred by hand with a glass rod for several seconds to homogenize it. Particles are first observed after 4 min; these are allowed to grow until they are the desired size and are solid enough to collect, another 5–20 min. The reaction is then quenched by diluting the mixture fivefold with deionized water. The micrometer-sized particles, which contain both zirconia colloids and UF resin, are separated from nonaggregated polymer and colloids by repeated sedimentation and decanting of the aqueous supernatant. Finally, the polymer/zirconia aggregates are resuspended in deionized water at a volume fraction of 30–60%. This slurry of micrometer-scale particles is aggregated by the following OE method. A hot oil bath is prepared by placing 800 ml of oleyl alcohol into an 850-ml polypropylene beaker. The oil is heated in a hot water bath (1007C) and agitated by a vibromixer with a 3-cm blade. When the oleyl alcohol reaches 957C, 50 ml of slurry is poured into the hot, agitated oleyl alcohol to form an emulsion. Aggregate particles are collected when they are sufficiently dry and strong (30–90 min) by vacuum filtration, and they are subsequently washed with 2-propanol. The collected particles are dried under ambient conditions for 24 h. Subsequent heating of the particles in
328
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
each of which we have earlier described. Polymerization-induced colloid aggregation (PICA) (10, 11) has been used to synthesize micrometer-scale porous zirconia particles for use in high-performance liquid chromatography (HPLC). In this method, the formation of monodisperse spherical aggregate particles is induced by urea–formaldehyde (UF) polymerization and adsorption. The resulting particles have a high porosity which is exceptionally resistant to high-temperature sintering (12). We have also described an oil emulsion (OE) method to synthesize aggregates tens of micrometers in diameter (13). Here a stable zirconia sol is mechanically dispersed into droplets within an oil phase. Slow removal of the water by extraction into and evaporation from the oil phase leads to a porous aggregate. This process is similar to spray-drying techniques (14), but is easy to implement and produces a high yield of particles of the desired size. Currently we are capable of producing PICA microspheres ranging from 1 to 5 mm in diameter. In the oil emulsion method, particle size and dispersity are easier to control at larger sizes—40 to 150 mm in diameter (15). The PICA and OE methods optimally produce particles of strict and distinct size ranges. Described here are hierarchically porous particles that are synthesized by a sequence of PICA and OE methods. The initial particles, micrometer-scale microspheres, are produced by the PICA procedure. The hierarchically structured 100-mm-scale particles are then formed from the microspheres by an OE method. The novelty of this work is the ease and low cost of synthesizing uniformly sized ceramic spherical particles with highly regulated hierarchical structures using microspheres that contain polymer to serve as a binder in the OE aggregation.
10-24-96 11:26:31
coida
AP: Colloid
329
NOTE
FIG. 1. Schematic representation of a single particle assembled by sequential aggregation. There are multiple length scales within the system, determined by the sizes of the initial colloids, the microspheres, and the particle.
air to high temperature (7507C 6 h, 9007C 3 h, 10507C 1 h) removes the polymer and sinters the the zirconia, increasing the mechanical integrity of the particles. After additional sintering at 9007C for 150 h, aggregates are mechanically strong enough to be packed in a column with a pressure gradient of 100 psi/cm. The pore structure was characterized after heating the sample for 8 h at 3207C at 10 03 Torr to remove surface contaminants. Nitrogen sorption was performed using an ASAP 2000 sorptometer (Micromeritics, Norcross, GA). Mercury porosimetry was performed using a Micromeritics PoreSizer 9320 mercury porosimeter. Micrographs were obtained on a JEOL 8400 scanning electron microscope.
RESULTS Stable, unagglomerated spherical zirconia particles with an ordered, bidisperse pore structure are formed; typical particles are shown in Fig. 2. The particle size distribution can be controlled by varying the mixing conditions and the slurry concentration. The distribution can easily be refined further by elutriation. The BET surface area for this hierarchically structured zirconia is measured at 14 m2 /g by nitrogen sorptometry. This is comparable to earlier reported values of 11 and 13 m2 /g for PICA and OE porous zirconia, respectively (16), and represents the surface area of dense 100-nm colloids. Three peaks are observed in the pore size distribution of this material (Fig. 3). The peaks correspond to (1) the interstices between 100-nm colloids within the microspheres, (2) the interstices between the microspheres within the particles, and (3) the voids between the particles. The porosity and mechanical strength of the aggregates can be adjusted by modifying the sintering protocol (Table 1).
FIG. 2. Scanning electron micrographs of representative aggregates. The material was sintered under the following conditions: 7507C 6 h, 9007C 3 h, 10507C 1 h, with 407C/min ramps between temperatures. The particle size distribution has not been refined after sintering.
The hierarchical structure would also allow the controlled breakup of the particle into smaller particles of regular size and shape. This capability may find uses in processes such as Ziegler–Natta polymerization, in which the mechanical pressure of the growing polymer chains cleaves the aggregate
SUMMARY The process described is a simple and inexpensive method for synthesizing zirconia micrometer-scale aggregates with a well-regulated hierarchical structure. It is easily performed on the laboratory scale and should easily scale up to pilot and commercial scales. Advantage is further taken of the fact that the PICA and OE processes, each described previously, provide aggregates of quite different size scales. When they are performed sequentially, we achieve control over multiple size scales. These aggregates retain specific surface area and pore volume comparable to those of other porous zirconia aggregates, but we expect the transport properties to differ markedly due to the ordered bidisperse pore structure.
AID
JCIS 4486
/
6g18$$$362
10-24-96 11:26:31
FIG. 3. Pore size distributions calculated from data obtained from mercury intrusion. This material was sintered under the following conditions: 7507C 6 h, 9007C 3 h, 10507C 1 h, with 407C/min ramps between temperatures. This is the same material pictured in Fig. 2. The particle size distribution has not been refined after sintering.
coida
AP: Colloid
330
NOTE
TABLE 1 Effect of Sintering Conditions on Nitrogen Adsorption Pore Measurements Sintering conditions
Surface area (m2/g)
Pore volume (cm3/g)
7007C, 4 h 9007C, 220 h
14.3
0.10
9007C, 6 h
16.8
0.15
9007C, 100 h
13.0
0.10
9007C, 6 h 11007C, 230 h
2.9
0.02
10.8
0.17
11007C, 12 h 11007C, 220 h
1.2
0.00
7507C, 6 h 9007C, 3 h 10507C, 1 h
14.1
0.12
7507C, 9007C, 10507C, 9007C,
11.7
0.11
6h 3h 1h 220 h
3. Petropoulos, J. H., Petrou, J. K., and Liapis, A. I., Ind. Eng. Chem. Res. 29, 1281 (1991). 4. Aris, R., ‘‘The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts.’’ Clarendon Press, 1975. 5. Nir, A., and Pismen, L., Chem. Eng. Sci. 32, 35 (1977). 6. Rodrigues, A., Ahn, B., and Zoulalian, A., AIChE J. 28, 541 (1982). 7. Liapis, A. I., Xu, Y., Crosser, O. K., and Tongta, A., J. Chromatogr. 702, 45 (1995). 8. Frey, D. D., Schweinheim, E., and Horvath, C., Biotechnol. Prog. 9, 273 (1994). 9. Afeyan, N. B., Mazsaroff, I., L. Varady, Fulton, S. P., Yang, Y. B., and Regnier, F. E., J. Chromatogr. 519, 1 (1990). 10. Sun, L., Annen, M. J., Lorenzano-Porras, C. F., Carr, P. W., and McCormick, A. V., J. Colloid Interface Sci. 163, 464 (1994). 11. Annen, M. J., Kizhappali, R., Carr, P. W., and McCormick, A. V., J. Mater. Sci. 29, 123 (1994). 12. Lorenzano-Porras, C. F., Reeder, D. H., Annen, M. J., Carr, P. W., and McCormick, A. V., Ind. Eng. Chem. Res. 34, 2719 (1995). 13. Carr, P. W., Funkenbusch, E. F., Rigney, M. P., Coleman, L., Hanggi, D. A., and Schaffer, W. A., U.S. Patent 5,015,373 (1991). 14. Roth, C., and Kobrich, R., J. Aerosol Sci. 19, 939 (1988). 15. Robichaud, M. J., Sathyagal, A. N., Carr, P. W., McCormick, A. V., and Flickinger, M. C., Sep. Sci. Technol., in press. 16. Lorenzano-Porras, C. F., Annen, M. J., Flickinger, M. C., Carr, P. W., and McCormick, A. V., J. Colloid Interface Sci. 170, 299 (1995). 17. Odian, G., ‘‘Principles of Polymerization.’’ Wiley, New York, 1991. DAVID H. REEDER * ,† ANDREW M. CLAUSEN‡ MICHAEL J. ANNEN * ,1 PETER W. CARR† ,‡ MICHAEL C. FLICKINGER† ,§ ALON V. MCCORMICK2, *
initiator particle (17). We envision that such a particle could also be useful in the formulation of toners, electrorheological fluids, and coatings.
ACKNOWLEDGMENTS This work was supported by Grants GM 45988 from the National Institutes of Health and CHE 917029 from the National Science Foundation. D.H.R. gratefully recognizes support through a Biotechnology Training Fellowship through the National Institutes of General Medical Sciences (IT32-GM08347). Thanks also go to the Surface Analysis Center at the University of Minnesota for instrumentation.
REFERENCES
Received February 15, 1996; accepted July 16, 1996
1. Bird, R. B., Stewart, W. E., and Lightfoot, E. N., ‘‘Transport Phenomena.’’ Wiley, New York, 1960. 2. Mandelbrot, B., ‘‘The Fractal Geometry of Nature: Updated and Augmented.’’ W. H. Freeman, 1983.
AID
JCIS 4486
/
6g18$$$362
*Department of Chemical Engineering and Materials Science and ‡Department of Chemistry University of Minnesota Minneapolis, Minnesota 55455 †Institute for Advanced Studies in Bioprocess Technology §Department of Biochemistry University of Minnesota St. Paul, Minnesota 55108
10-24-96 11:26:31
1 Current address: 3M Process Technologies Laboratories, 3M Center, St. Paul, MN 55144. 2 To whom correspondence should be addressed.
coida
AP: Colloid