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Preparation of uniform spherical titania particles coated with polyurea by the aerosol technique
Preparation of Uniform Spherical Titania Particles Coated with Polyurea by the Aerosol Technique 1 FRANCIS C. MAYVILLE, RICHARD E. PARTCH, AND EGON MATIJEVIC Deparment of Chemistry and Institute of Colloid and Surface Science, Clarkson University, Potsdam, New York 13676 Received October 27, 1986; accepted January 16, 1987 The aerosol technique has been employed in the preparation of uniform colloidal inorganic particles coated with an organic polymer. Specifically, spherical titania cores were produced first by hydrolysis of Ti(IV) ethoxide droplets in contact with water vapor. Hexamethylenediisocyanate liquid was then condensed on these particles and exposed to ethylenediamine vapor. By varying experimental conditions (temperature and flow rate) one attained uniform coatings of polyurea of different thickness on titania. © 1987 Academic Press, Inc.
or complex (hydrous) oxides, sulfides, selenides, phosphates, carbonates, etc., of different metals. Conceptually, the simplest of the developed methods is based on the chemical reactions with aerosols. Droplets of a reactant are exposed to a vapor of a coreactant yielding a new product. The advantages of the technique are several: The final particles are always spherical and their diameter can be predetermined by the size of the initial aerosol droplets. The produced materials are, as a rule, of high purity because in most cases no extraneous ions, surfactants, or any other additives are involved in the process. Finally, the method makes it possible to produce finely dispersed powders by extremely rapid chemical processes which would be difficult to control by other techniques (3). Indeed, uniform titania (4) and alumina (5) powders, as well as polymer colloids, such as polystyrene (6) and polyurea (7), were obtained by the aerosol method. The same procedure can be used for the preparation of solids of mixed chemical composition as demonstrated on examples of titania/alumina (8), ethylvinylbenzene/divinylbenzene (9), and 1This work was supported by the National Science polyurea/TiO2 or polyurea/Al203 (7) systems. Foundation Grant CHE 86-19509. Particles which consist of cores and coatings
In recent years considerable effort has been recorded in the development of procedures for the preparation of colloidal particles uniform in size and shape. This interest was triggered by the need for well-defined systems in fundamental studies of various phenomena such as (hetero)coagulation, adhesion, adsorption, and surface charge characterization, as well as by their usefulness as model systems in the elucidation of corrosion, pollution, flotation, and other processes. Such "monodispersed" powders now find uses in high-tech ceramics, medicine (drug delivery and diagnostics), catalysis, and many other areas of application. In the past, except for polymer latices, uniform particles were generated by individual "recipes" with little physicochemical understanding of the procedures. Furthermore, only occasional success was reported with regard to important inorganic dispersions of different salts or pure metals. At present, several techniques have been developed, yielding a large number of inorganic and organic colloids in a variety of shapes (including spheres) of narrow size distribution (1, 2). Inorganic systems of particular interest so obtained are simple
135 0021-9797/87 $3.00 Journal of Colloid and Interface Science, Vol. 120, No. 1, November 1987
Copyright © 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.
136
MAYVILLE, PARTCH,
of different materials deserve special attention. Changing the particle surface by enveloping cores with layer(s) of other chemical composition can drastically alter optical, electrical, magnetic, adsorptive, and other properties of these solids. This work reports the first such successful application of the aerosol technique by which inorganic (titania) particles were coated with layers of a polymer (polyurea) of different thicknesses,
AND MATIJEVIC
f g h i k I
TITANIA
A schematic diagram of the aerosol assembly used to produce uniform coated colloidal particles is given in Fig. 1. This setup is designed to generate in a continuous manner the original inorganic particles onto which an organic m o n o m e r is condensed and finally polymerized. Sections (a)-(1) were utilized in the preparation of spherical titania cores. Liquid titanium(IV) ethoxide [Ti(OEt)4, Aldrich] was vaporized in the boiler (f), the temperature of which was maintained constant (80-140°C) by circulating ethylene glycol from a constant temperature bath through a surrounding glass jacket. Dried and filtered helium gas, laden with AgC1 nuclei generated at 600 + 10°C in a furnace and bubbled through the liquid in the boiler (f) (at a flow rate of 1.5 d m 3 min-1), carried the Ti(OEt)4 into a condenser (g) where the droplets formed. To improve the uniformity of the droplet size, the aerosol was completely evaporated in a reheater (h) and recondensed in the chamber (i). In order to produce titania particles, droplets of Ti(OEt)4 were brought into contact with water vapor in a reaction chamber (1). Dried and filtered helium gas was bubbled through water in the reservoir (j) at a controlled flow rate (0.42 d m 3 min -1) metered by (d2), and then the water vapor was introduced into (1) through injection ports (k). Finally, the resuiting colloidal titania was either collected in a thermopositor for analysis or carded through the second part o f the assembly in which the particles became coated by the polymer. Journal of Colloid and Interface Science, Vol. 120, No. 1, November 1987
m
Cl
FIG. 1. Aerosolgeneratorassembly.(a) Heliumgas tank, (b) dryingcolumncontainingsilicagel and molecularsieve, (c) Milliporemembrane of 0.1/zm pore size, (d) flowmeters, (e) nuclei generator containing silver chloride, (f) boilercontainingliquid titanium(IV)ethoxide,(g) and (n) chambersfor vaporcondensation,(h) reheater,(i) chamber for recondensingdroplets,(j) containerwith water, (k) water vapor injectionchamber, (1)and (q) reactionchambers, (m) boiler containinghexamethylenediisocyanate(HDI), or toluene 2,4-diisocyanate(TDI),(o) containerwith ethylenediamine(EDA), (19)EDA vapor injection chamber, (r) thermopositor.
An electron micrograph (Fig. 2) illustrates titania particles obtained by the described procedure under conditions given in the legend. The modal diameter and the size distribution of titania particles so obtained could be modified by altering the temperature of the boiler (f) and of the reheater (h), as well as by varying the flow rate of the carder gas. As a rule the average particle size of the resulting titania becomes smaller as the flow rate increases and the boiler temperature decreases. For example, at constant helium flow rate
AEROSOL TECHNIQUE TO PREPARE COATED PARTICLES
!
!
0.5,u,m
FIG. 2. A transmission electron micrograph (TEM) of uniform titania (TiO2)particles obtained from titanium (IV) ethoxideat a temperature of 100°C and flowrate of 1.5 dm3 rain-1, reheater temperature of 150°C, water temperature of 25°C, and water flow rate of 0.42 dm3 min-1.
through the boiler (f) of 1.5 dm 3 min -1 and through the water reservoir (j) of 0.42 dm -3 min- 1the average diameter (with standard deviation) of titania spheres was 0.19 (0.01), 0.46 (0.05), and 1.2 (0.4) ~tm at boiler (f) temperatures of 80, 100, and 115°C, respectively. With increasing temperature the width of the distribution also broadened considerably. In all cases Ti(OEt)4 droplets were first evaporated in the reheater (h) at 150°C and recondensed. It should be noted that colloidal titania was prepared earlier by the aerosol technique, but a different (i.e., falling film) generator was used to produce the alkoxide droplets (4). TITANIA COATED WITH POLYUREA To coat titania cores with the polymer, the solid aerosol generated in (1) was brought into contact with hexamethylenediisocyanate (HDI, Kodak) or toluene 2,4-diisocyanate (TDI, Aldrich) vapor in the boiler (m), the temperature of which was controlled by circulating ethylene glycol (60-100°C). To produce the vapor, dried and filtered helium gas was bubbled through liquid HDI or TDI at a controlled rate (0.19, 0.42, or 1.1 dm 3 min-1). When the mixture of the vaporized m o n o m e r
137
and the aerosol passed through chamber (n), the condensation of HDI (or TDI) on TiO2 particles took place. The so-wetted solids were then exposed to ethylenediamine (EDA) vapor in a tube (q). EDA was supplied by bubbling helium through the reservoir (o) at a controlled flow rate and the vapor was introduced into (q) through the injection chamber (p). The final products were collected on aluminum foil in a thermopositor (r). Experimental conditions must be carefully controlled in order to obtain coated particles. At too high vapor pressures of HDI or TDI condensation of the m o n o m e r droplets takes place, independent of the introduced solids, which on exposure to EDA results in a mixed aerosol consisting of TiO2 and polyurea particles differing in size. This effect is expected and it was theoretically justified (10). However, by proper adjustment of the flow rate and relative concentrations of core particles to the m o n o m e r vapor only coated dispersed powders are produced. The thickness of the polymer layer can be varied by adjusting the vapor pressure of HDI or TDI in the boiler (m) and the length of the reaction tube (n); the higher the temperature or the flow rate and the longer the tube the thicker is the coating. For example, the coating on titania cores of 0.46 ttm in diameter was 0.06, 0.12, and 0.25 ~tm thick, at the flow rates of 0.19, 0.42, and 1.07 dm 3 dm -1, respectively, at a constant temperature of 80°C. The optimal temperature for vaporization of HDI and TDI in the described experimental design is between 60 and 80°C with the flow rate of m o n o m e r vapor of 0.2 dm 3 min -1 and that o f E D A ~ 4 5 cm 3 min -1. The reaction column length was varied between 0.75 and 2 m with most experiments carried out in a 1-m tube. Figure 3 shows three transmission electron micrographs of coated particles prepared under somewhat different conditions to achieve varying thicknesses of the polymer layers. While the polymer coating was clearly discernible in the electron micrographs, the change in surface characteristics of titania Journal of Colloid and Interface Science, Vol. 120, No. 1, November 1987
138
MAYVILLE,
•
PARTCH,
AND
MATIJEVIC
I 0.5/~m
' 0 . 5 ,urn
'0.5/~m FiG. 3. Transmission electron microgs'aphs of three different thicknesses of polyurea coating on titania core particles. In each case the titania was obtained under the same conditions as those in Fig. 2. HDI was kept at 80°C at flow rates (a) 0.19 d m 3 rain-', (b) 0.42 dm 3 rain -~, and (e) 1.07 d m 3 rain-'. The flow rate and temperature of ethylenediamine was held constant at 44.3 cm 3 m i n - ' and 25°C, respectively.
particles due to the polymer was also demonstrated by the electrokinetic measurements. Electrophoretic mobilities were determined as a function of pH with a Doppler electrophoretic light scattering apparatus (DELSA, Langley Ford Instruments) at angles of 15, 22.5, or 30 °. The pH was adjusted with HNO3 or NaOH. Figure 4 shows that isolelectric points (iep) of titania (at pH 4.7) and of polyurea (at pH 7.3) particles differ considerably. The titania particles coated with polyurea show the same dependence of electrophoretic mobilities on pH as the pure polymer, demonstrating that the surface charge characterJournal of Colloid and Interface Science, Vol. 120, No. 1, November 1987
istics of the coated particles are the same as that of polyurea. It is noteworthy that titanium alkoxides react readily and reversibly with isocyanates (11). The fact that no extraneous particles are noted in the final aerosol products suggests that the alkoxide vapor was used up in the first phase of the process, i.e., the formation of titania. However, traces of alkoxide on the particle surface may cause the formation of bonds between titania and polyurea. Particles of different kinds were coated with polymers by other techniques. For example, polymer colloids containing magnetic cores
AEROSOL TECHNIQUE TO PREPARE COATED PARTICLES D 0 A
4
TiO2 Polyureo Ti02 / Potyurea
139
applications can be prepared by proper choice o f core and coating materials. REFERENCES
E >i
i
i
2
3
4
i
i
i
i
i
]
i
-1 -2
FIG. 4. Electrophoretic mobilities of polyurea (HDI) particles (O), titania (TiO2) particles (r~), and polyureacoated titania particles (ZX)dispersed in aqueous solutions of varying pH.
were obtained by the swollen emulsion polymerization o f latex (12) or by protein encapsulation (13, 14). " I m m u n o m i c r o s p h e r e s " were prepared by coupling antibodies to latex particles (15). This w o r k shows that the aerosol m e t h o d is capable o f generating u n i f o r m spherical particles coated with p o l y m e r layers o f desirable structures which m a y not be easily p r o d u c e d by other techniques. Powders for a variety o f
1. Matijevir, E., Annu. Rev. Mater Sci. 15, 483 (1985). 2. Matijevir, E., Langmuir 2, 12 (1986). 3. Ingebrethsen, B. J., and Matijevir, E., J. Colloid Interface Sci. 100, 1 (1984). 4. Vista, M., and Matijevir, E., J. Colloid Interface Sci. 68, 308 (1979). 5. Ingebrethsen, B. J., and Matijevir, E., J. Aerosol Sci. 11, 271 (1980). 6. Partch, R., Matijevir, E., Hodgson, A. W., and Aiken, B. E., J. Polym. Sci. Polym. Chem. Ed. 21, 961 (1983). 7. Partch, R. E., Nakamura, K., Wolfe, K. J., and Matijevir, E., J. Colloidlnterface Sci. 105, 560 (1985). 8. Ingebrethsen, B. J., Matijevir, E., and Partch, R. E., J. Colloid Interface Sci. 95, 228 (1983). 9. Nakamura, K., Partch, R. E., and Matijevir, E., J. Colloid Interface Sci. 99, 118 (1984). 10. Friedlander, S. K., J. Colloid Interface Sci. 67, 387 (1978). 11. Meth-Cohn, O., Thorpe, D., and Twitchett, H. J., J. Chem. Soc. C, 132 (1970). 12. Ugelstad, J., Soderberg, L., Berge, A., and Bergstrom, J., Nature (London) 303, 95 (1983). 13. Widder, K. J., Morris, R. M., Poore, G., Howard, D. P., Jr., and Senyei, A. E., Proc. Natl. Acad. Sci. USA 78, 579 (1981). 14. Widder, K., Flouret, G., and Senyei, A., J. Pharm. Sci. 68, 79 (1979). 15. Rembaum, A., PureAppL Chem. 52, 1275 (1980).
Journal of Colloid and Interface Science, Vol. 120, No. 1, November 1987
or complex (hydrous) oxides, sulfides, selenides, phosphates, carbonates, etc., of different metals. Conceptually, the simplest of the developed methods is based on the chemical reactions with aerosols. Droplets of a reactant are exposed to a vapor of a coreactant yielding a new product. The advantages of the technique are several: The final particles are always spherical and their diameter can be predetermined by the size of the initial aerosol droplets. The produced materials are, as a rule, of high purity because in most cases no extraneous ions, surfactants, or any other additives are involved in the process. Finally, the method makes it possible to produce finely dispersed powders by extremely rapid chemical processes which would be difficult to control by other techniques (3). Indeed, uniform titania (4) and alumina (5) powders, as well as polymer colloids, such as polystyrene (6) and polyurea (7), were obtained by the aerosol method. The same procedure can be used for the preparation of solids of mixed chemical composition as demonstrated on examples of titania/alumina (8), ethylvinylbenzene/divinylbenzene (9), and 1This work was supported by the National Science polyurea/TiO2 or polyurea/Al203 (7) systems. Foundation Grant CHE 86-19509. Particles which consist of cores and coatings
In recent years considerable effort has been recorded in the development of procedures for the preparation of colloidal particles uniform in size and shape. This interest was triggered by the need for well-defined systems in fundamental studies of various phenomena such as (hetero)coagulation, adhesion, adsorption, and surface charge characterization, as well as by their usefulness as model systems in the elucidation of corrosion, pollution, flotation, and other processes. Such "monodispersed" powders now find uses in high-tech ceramics, medicine (drug delivery and diagnostics), catalysis, and many other areas of application. In the past, except for polymer latices, uniform particles were generated by individual "recipes" with little physicochemical understanding of the procedures. Furthermore, only occasional success was reported with regard to important inorganic dispersions of different salts or pure metals. At present, several techniques have been developed, yielding a large number of inorganic and organic colloids in a variety of shapes (including spheres) of narrow size distribution (1, 2). Inorganic systems of particular interest so obtained are simple
135 0021-9797/87 $3.00 Journal of Colloid and Interface Science, Vol. 120, No. 1, November 1987
Copyright © 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.
136
MAYVILLE, PARTCH,
of different materials deserve special attention. Changing the particle surface by enveloping cores with layer(s) of other chemical composition can drastically alter optical, electrical, magnetic, adsorptive, and other properties of these solids. This work reports the first such successful application of the aerosol technique by which inorganic (titania) particles were coated with layers of a polymer (polyurea) of different thicknesses,
AND MATIJEVIC
f g h i k I
TITANIA
A schematic diagram of the aerosol assembly used to produce uniform coated colloidal particles is given in Fig. 1. This setup is designed to generate in a continuous manner the original inorganic particles onto which an organic m o n o m e r is condensed and finally polymerized. Sections (a)-(1) were utilized in the preparation of spherical titania cores. Liquid titanium(IV) ethoxide [Ti(OEt)4, Aldrich] was vaporized in the boiler (f), the temperature of which was maintained constant (80-140°C) by circulating ethylene glycol from a constant temperature bath through a surrounding glass jacket. Dried and filtered helium gas, laden with AgC1 nuclei generated at 600 + 10°C in a furnace and bubbled through the liquid in the boiler (f) (at a flow rate of 1.5 d m 3 min-1), carried the Ti(OEt)4 into a condenser (g) where the droplets formed. To improve the uniformity of the droplet size, the aerosol was completely evaporated in a reheater (h) and recondensed in the chamber (i). In order to produce titania particles, droplets of Ti(OEt)4 were brought into contact with water vapor in a reaction chamber (1). Dried and filtered helium gas was bubbled through water in the reservoir (j) at a controlled flow rate (0.42 d m 3 min -1) metered by (d2), and then the water vapor was introduced into (1) through injection ports (k). Finally, the resuiting colloidal titania was either collected in a thermopositor for analysis or carded through the second part o f the assembly in which the particles became coated by the polymer. Journal of Colloid and Interface Science, Vol. 120, No. 1, November 1987
m
Cl
FIG. 1. Aerosolgeneratorassembly.(a) Heliumgas tank, (b) dryingcolumncontainingsilicagel and molecularsieve, (c) Milliporemembrane of 0.1/zm pore size, (d) flowmeters, (e) nuclei generator containing silver chloride, (f) boilercontainingliquid titanium(IV)ethoxide,(g) and (n) chambersfor vaporcondensation,(h) reheater,(i) chamber for recondensingdroplets,(j) containerwith water, (k) water vapor injectionchamber, (1)and (q) reactionchambers, (m) boiler containinghexamethylenediisocyanate(HDI), or toluene 2,4-diisocyanate(TDI),(o) containerwith ethylenediamine(EDA), (19)EDA vapor injection chamber, (r) thermopositor.
An electron micrograph (Fig. 2) illustrates titania particles obtained by the described procedure under conditions given in the legend. The modal diameter and the size distribution of titania particles so obtained could be modified by altering the temperature of the boiler (f) and of the reheater (h), as well as by varying the flow rate of the carder gas. As a rule the average particle size of the resulting titania becomes smaller as the flow rate increases and the boiler temperature decreases. For example, at constant helium flow rate
AEROSOL TECHNIQUE TO PREPARE COATED PARTICLES
!
!
0.5,u,m
FIG. 2. A transmission electron micrograph (TEM) of uniform titania (TiO2)particles obtained from titanium (IV) ethoxideat a temperature of 100°C and flowrate of 1.5 dm3 rain-1, reheater temperature of 150°C, water temperature of 25°C, and water flow rate of 0.42 dm3 min-1.
through the boiler (f) of 1.5 dm 3 min -1 and through the water reservoir (j) of 0.42 dm -3 min- 1the average diameter (with standard deviation) of titania spheres was 0.19 (0.01), 0.46 (0.05), and 1.2 (0.4) ~tm at boiler (f) temperatures of 80, 100, and 115°C, respectively. With increasing temperature the width of the distribution also broadened considerably. In all cases Ti(OEt)4 droplets were first evaporated in the reheater (h) at 150°C and recondensed. It should be noted that colloidal titania was prepared earlier by the aerosol technique, but a different (i.e., falling film) generator was used to produce the alkoxide droplets (4). TITANIA COATED WITH POLYUREA To coat titania cores with the polymer, the solid aerosol generated in (1) was brought into contact with hexamethylenediisocyanate (HDI, Kodak) or toluene 2,4-diisocyanate (TDI, Aldrich) vapor in the boiler (m), the temperature of which was controlled by circulating ethylene glycol (60-100°C). To produce the vapor, dried and filtered helium gas was bubbled through liquid HDI or TDI at a controlled rate (0.19, 0.42, or 1.1 dm 3 min-1). When the mixture of the vaporized m o n o m e r
137
and the aerosol passed through chamber (n), the condensation of HDI (or TDI) on TiO2 particles took place. The so-wetted solids were then exposed to ethylenediamine (EDA) vapor in a tube (q). EDA was supplied by bubbling helium through the reservoir (o) at a controlled flow rate and the vapor was introduced into (q) through the injection chamber (p). The final products were collected on aluminum foil in a thermopositor (r). Experimental conditions must be carefully controlled in order to obtain coated particles. At too high vapor pressures of HDI or TDI condensation of the m o n o m e r droplets takes place, independent of the introduced solids, which on exposure to EDA results in a mixed aerosol consisting of TiO2 and polyurea particles differing in size. This effect is expected and it was theoretically justified (10). However, by proper adjustment of the flow rate and relative concentrations of core particles to the m o n o m e r vapor only coated dispersed powders are produced. The thickness of the polymer layer can be varied by adjusting the vapor pressure of HDI or TDI in the boiler (m) and the length of the reaction tube (n); the higher the temperature or the flow rate and the longer the tube the thicker is the coating. For example, the coating on titania cores of 0.46 ttm in diameter was 0.06, 0.12, and 0.25 ~tm thick, at the flow rates of 0.19, 0.42, and 1.07 dm 3 dm -1, respectively, at a constant temperature of 80°C. The optimal temperature for vaporization of HDI and TDI in the described experimental design is between 60 and 80°C with the flow rate of m o n o m e r vapor of 0.2 dm 3 min -1 and that o f E D A ~ 4 5 cm 3 min -1. The reaction column length was varied between 0.75 and 2 m with most experiments carried out in a 1-m tube. Figure 3 shows three transmission electron micrographs of coated particles prepared under somewhat different conditions to achieve varying thicknesses of the polymer layers. While the polymer coating was clearly discernible in the electron micrographs, the change in surface characteristics of titania Journal of Colloid and Interface Science, Vol. 120, No. 1, November 1987
138
MAYVILLE,
•
PARTCH,
AND
MATIJEVIC
I 0.5/~m
' 0 . 5 ,urn
'0.5/~m FiG. 3. Transmission electron microgs'aphs of three different thicknesses of polyurea coating on titania core particles. In each case the titania was obtained under the same conditions as those in Fig. 2. HDI was kept at 80°C at flow rates (a) 0.19 d m 3 rain-', (b) 0.42 dm 3 rain -~, and (e) 1.07 d m 3 rain-'. The flow rate and temperature of ethylenediamine was held constant at 44.3 cm 3 m i n - ' and 25°C, respectively.
particles due to the polymer was also demonstrated by the electrokinetic measurements. Electrophoretic mobilities were determined as a function of pH with a Doppler electrophoretic light scattering apparatus (DELSA, Langley Ford Instruments) at angles of 15, 22.5, or 30 °. The pH was adjusted with HNO3 or NaOH. Figure 4 shows that isolelectric points (iep) of titania (at pH 4.7) and of polyurea (at pH 7.3) particles differ considerably. The titania particles coated with polyurea show the same dependence of electrophoretic mobilities on pH as the pure polymer, demonstrating that the surface charge characterJournal of Colloid and Interface Science, Vol. 120, No. 1, November 1987
istics of the coated particles are the same as that of polyurea. It is noteworthy that titanium alkoxides react readily and reversibly with isocyanates (11). The fact that no extraneous particles are noted in the final aerosol products suggests that the alkoxide vapor was used up in the first phase of the process, i.e., the formation of titania. However, traces of alkoxide on the particle surface may cause the formation of bonds between titania and polyurea. Particles of different kinds were coated with polymers by other techniques. For example, polymer colloids containing magnetic cores
AEROSOL TECHNIQUE TO PREPARE COATED PARTICLES D 0 A
4
TiO2 Polyureo Ti02 / Potyurea
139
applications can be prepared by proper choice o f core and coating materials. REFERENCES
E >i
i
i
2
3
4
i
i
i
i
i
]
i
-1 -2
FIG. 4. Electrophoretic mobilities of polyurea (HDI) particles (O), titania (TiO2) particles (r~), and polyureacoated titania particles (ZX)dispersed in aqueous solutions of varying pH.
were obtained by the swollen emulsion polymerization o f latex (12) or by protein encapsulation (13, 14). " I m m u n o m i c r o s p h e r e s " were prepared by coupling antibodies to latex particles (15). This w o r k shows that the aerosol m e t h o d is capable o f generating u n i f o r m spherical particles coated with p o l y m e r layers o f desirable structures which m a y not be easily p r o d u c e d by other techniques. Powders for a variety o f
1. Matijevir, E., Annu. Rev. Mater Sci. 15, 483 (1985). 2. Matijevir, E., Langmuir 2, 12 (1986). 3. Ingebrethsen, B. J., and Matijevir, E., J. Colloid Interface Sci. 100, 1 (1984). 4. Vista, M., and Matijevir, E., J. Colloid Interface Sci. 68, 308 (1979). 5. Ingebrethsen, B. J., and Matijevir, E., J. Aerosol Sci. 11, 271 (1980). 6. Partch, R., Matijevir, E., Hodgson, A. W., and Aiken, B. E., J. Polym. Sci. Polym. Chem. Ed. 21, 961 (1983). 7. Partch, R. E., Nakamura, K., Wolfe, K. J., and Matijevir, E., J. Colloidlnterface Sci. 105, 560 (1985). 8. Ingebrethsen, B. J., Matijevir, E., and Partch, R. E., J. Colloid Interface Sci. 95, 228 (1983). 9. Nakamura, K., Partch, R. E., and Matijevir, E., J. Colloid Interface Sci. 99, 118 (1984). 10. Friedlander, S. K., J. Colloid Interface Sci. 67, 387 (1978). 11. Meth-Cohn, O., Thorpe, D., and Twitchett, H. J., J. Chem. Soc. C, 132 (1970). 12. Ugelstad, J., Soderberg, L., Berge, A., and Bergstrom, J., Nature (London) 303, 95 (1983). 13. Widder, K. J., Morris, R. M., Poore, G., Howard, D. P., Jr., and Senyei, A. E., Proc. Natl. Acad. Sci. USA 78, 579 (1981). 14. Widder, K., Flouret, G., and Senyei, A., J. Pharm. Sci. 68, 79 (1979). 15. Rembaum, A., PureAppL Chem. 52, 1275 (1980).
Journal of Colloid and Interface Science, Vol. 120, No. 1, November 1987