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Preparation of polymer colloids by chemical reactions in aerosols
Preparation of Polymer Colloids by Chemical Reactions in Aerosols II. Large Particles 1
KANEHIRO NAKAMURA, 2 RICHARD E. PARTCH, AND E G O N MATIJEVIC Department of Chemistry and Institute of Colloid and Surface Science, Clarkson University, Potsdam, New York 13676
Received July 22, 1983; acceptedOctober 11, 1983 Polymer and copolymercolloidshave been prepared in modal diameters up to 30 um by reacting aerosol droplets of styreneor "divinylbenzene"(whichwas a mixture of ortho, meta, and para isomers with ethylvinylbenzene)with the vapor of trifluoromethanesulfonicacid in a speciallydesignedgenerator. The properties of the resulting particles (size distribution and surface characteristics) depend on the temperatures of the boiler and of the reaction chamber as well as on the monomer-to-initiator mass ratio. INTRODUCTION
This work represents an entirely different experimental approach to achieving large A large number of polymer colloids has polymer colloids, i.e., by chemical reactions been prepared by the conventional techniques in aerosols. It has been shown that droplets of emulsion or dispersion polymerization. of monomers (e.g,, p-tertiarybutylstyrene) disThese finely dispersed systems are characterpersed in a gas (helium) can be polymerized ized by exceedingly narrow size distribution of spherical particles. As a rule, their modal to solid particles if a vapor initiator (trifluodiameter is in the micrometer or submicro- romethanesulfonic acid, TSFA) is added to meter range. Recently, considerable effort the aerosol (3). Under proper conditions these powders were of narrow size distribution and has been made to produce such lattices of much larger sizes (10-20 urn). Ugelstad (1) could be dispersed in aqueous solutions of achieved this goal by swelling monodispersed appropriate surfactants. The advantages of the aerosol procedure polymer colloids of less than 1 tzm in diameter are several. No surfactant is necessary to prowith the solvent, surfactant, and monomer, duce the polymer colloid particles and their Following second polymerization, uniform size can be predetermined by the diameter of spheres in excess of 10 # m were obtained. the monomer droplets. Furthermore, each Growing large latex particles by emulsion poaerosol droplet acts as a separate "container." lymerization under gravity-free conditions was The technique can also be employed for the also attempted in the Columbia space shuttle preparation of polymers in colloidal state when laboratory during two different flights (2). Apno initiator is required. For example, polyurea parently, no significant change in modal diparticles were obtained by reacting droplets ameter was observed as compared to normal of toluene-diisocyanate with ethylenediamine conditions. vapor (4). This work describes the synthesis of styrene polymer and divinylbenzene/ethylvinylbenSupported by the NSF Grant CHE-80 13684. zene copolymer particles in the size range 102On leavefromthe ScienceUniversityof Tokyo,Tokyo, Japan. 20 t~m by the aerosol teehnique using the c o r 118 0021-9797/84 $3.00 Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and InterfaceScience, Vol. 99, No. 1, May 1984
119
POLYMER COLLOID PREPARATION IN AEROSOLS, II
responding liquid monomers as starting materials and TFSA as the vapor initiator. EXPERIMENTAL
Choice of materials. The monomers tested were styrene, 4-vinyl-pyridine, 4-vinyl-l-cyclohexene, 5-vinyl-2-norbornene, acrylonitrile, and butyl acrylate, all commercial products. "Divinylbenzene" (Fluka Chemical Corp.) was actually a mixture of ortho, meta, and para isomers and it contained ---50% of ethylvinylbenzene. Since all aerosols were prepared by the evaporation/condensation technique no purification of the starting materials was undertaken. In principle the initiation of polymerization can be accomplished either by radiation (UV or 3' rays) or by appropriate chemicals. In order to use the aerosol technique for the preparation of polymer colloids the chemical initiator must be chosen which will react fast since the contact times with the monomer droplets may be rather short. For this reason test experiments with various monomers and initiators were first carried out in bulk. For acrylonitrile and butyl acrylate, azo-tbutane activated by UV light was used as initiator. With medium pressure mercury or common sun lamps the polymerization required at least 5 min which was considered inadequate in the aerosol technique employed. Trifiuoromethanesulfonic acid (TFSA), boron trifluoride, and trifluoroacetic acid polymerized styrene, 4-vinyl-pyridine, 5-vinyl-2-norbornene, and divinylbenzene in bulk. However, only TFSA acted fast enough to be effective in the aerosol systems and was chosen for these studies. The two monomers used in this work are styrene and "divinylbenzene." Techniques. The monomer droplets can be prepared either by nebulizing the corresponding liquids or by the evaporation/condensation sequence. The latter technique was applied in studies of chemical reactions in aerosols (5) using a falling film generator (6, 7). It was also
shown that finely dispersed inorganic powders of narrow size distribution (8-10), as well as polymer colloids (3) can be obtained by an analogous procedure. For the purpose of this project the equipment was redesigned in order to produce larger, yet uniform droplets. A schematic diagram of the entire set-up is given in Fig. I. Helium (a) used as cartier gas was filtered through a Millipore membrane of 0.1 gm in pore size (b) and bubbled at a controlled flow rate (1.4 dm 3 min -z) through the boiler (d). The vapor was condensed in glass tubing (e), the resulting aerosol evaporated in reheater (f), and recondensed in tube (g) at ~ 2 0 ° C . This procedure assured better uniformity of aerosol droplets. Helium was introduced at flow rates between 34 and 51 cm 3 rain -1 into the reservoir (h) containing the initiator TFSA which was then transferred into the chamber (i) where it was mixed at 20°C with the monomer droplets. The polymerization is completed in the unheated reaction tube (j) and the resuiting polymer colloid particles are collected
et) f
g
c
i b
k PIG. 1. Schematic diagram of the apparatus used to produce polymer colloids: (a) helium gas tank; (b) Millipore membranes of 0.1 #m pore size; (c) flow meters; (d) boiler; (e) chamber for forming droplets; (f) reheater; (g) chamber for reforming droplets; (h) initiator reservoir; (i) initiator injection chamber; (j) reaction chamber; (k) therrnopositor. Journal of Colloid and Interface Science,
Vol.99, No. 1, May 1984
NAKAMURA, PARTCH, AND MATIJEVI(~
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in the thermopositor (k). The temperatures of The resulting polymer particles were exthe boiler and of the reheater were kept con- amined either in a transmission or in a scanstant by circulating oil from a bath. ning electron microscope. For this purpose Two different variations of the boiler (Fig. the solids were dispersed in an aqueous so1d) were used; improved reproducibility was lution of sodium dodecylsulfate and redeposachieved with the apparatus shown in Fig. 2A. ited by sedimentation on appropriate sample A detailed sketch of the reactor vessel (Fig. holders. For scanning electron microscopy the l i) is shown in Fig. 2B, the design of which specimens were coated with gold. prevented turbulence on contact of the aerosol Since "divinylbenzene" was actually a mixwith the TFSA vapor. ture of monomers, the liquid aerosol obtained In work with styrene the boiler temperature by the evaporation/condensation technique ranged between 40 and 60°C, while "divi- was collected and analyzed for composition nylbenzene" aerosols were generated at 50- by gas chromatography and nuclear magnetic 90°C. The reheater temperature was kept be- resonance spectroscopy. Both techniques tween 35 and 55°C for the former and between showed that the droplets contained divinyl45 and 75°C for the latter system. benzene and ethylvinylbenzene in the apIn earlier aerosol studies using the falling proximate ratio of 2:3. The polymerized parfilm generator heterogeneous nuclei (e.g., NaC1 ticles would, therefore, have the same comor AgC1) were initially introduced in order to position. produce more uniform droplets. The generator The polystyrene powder obtained by the described above was proven to be more suited aerosol process was dissolved in chloroform for liquids of higher vapor pressure and no and analyzed by gel permeation chromatogadvantage was detected when foreign nuclei raphy. The molecular weight was found to be were present. Thus, in this work all aerosol low (ranging between Mw = 1000 and 10,000) droplets were selfnucleated. and of broad distribution. The molecular
\
b
J° b
!
f
b
b
/ d
g A
"
120ram '150mrn--
FIG. 2. (A) Schematicdiagram of the boiler: (a) helium gas inlets; (b) stopcocks;(c) bubblers; (d) liquid monomer; (e) inlet from the constant-temperaturebath; (f) outlet to the constant-temperaturebath; (g) monomer vapor laden helium outlet. (B) Schematicdiagram of the initiator injection chamber: (a) liquid monomer droplets; (b) initiator vapor. Journal of Colloid and Interface Science, Vo[. 99, No. 1, May 1984
POLYMER COLLOID PREPARATION IN AEROSOLS, II
weight of the copolymer latex could not be determined because no solvent was found to dissolve the particles.
121
process was checked by the absence of a Tyndall beam at the exit of the reheater.
Polystyrene RESULTS
A large number of experiments was carded out to establish the best conditions for the preparation of colloid polymers of larger modal diameters and of narrow size distributions. The most critical parameters that control the properties of liquid aerosols of monomers of high vapor pressure are the temperatures of the boiler (Fig. l d) and of the reheater (Fig. 1f), which were altered in a systematic way. To achieve reasonable uniformity of the droplets it is essential to reevaporate the formed aerosol. The minimum temperature necessary to completely volatilize the droplets in (f) depends on the conditions in the aerosol generator (flow rate, boiler temperature, etc.) as well as on the design of the reheater. The completeness of the evaporation
Table I lists several selected sets of conditions studied in the preparation of polystyrene particles along with a brief description of the product appearance. It is apparent that a relatively small change in the listed parameters greatly affects the size, uniformity, and even the shape of the resulting polymer colloids. Spheres of narrow size distribution ( ~ 10 t~m in modal diameter) with smooth surfaces can be obtained as illustrated in Fig. 3a (Table I, sample 4; sample 3 is of similar quality). If the boiler temperature is higher, smooth spherical particles ofbimodal distribution are generated (Fig. 3b; Table I, sample 9); particles of --~20 ~zm in diameter, are in mixture with spheres I0 times smaller. As a rule an increase in the boiler temperature yields colloid polymers of larger modal sizes (up to 20 tzm), al-
TABLE I Conditions for the Generation of Polystyrene Particles by the Aerosol Procedure Flow rate Boiler temperature (°C)
Reheater temperature (°C)
Boiler section (drn 3 rain -j)
1
40
35
1.8
39
2
40
35
1.4
106
3 4 (Fig. 3a) 5 (Fig. 4) 6
50 50 50 50
45 45 45 55
1.8 1.8 1.4 1.4
34 51 51 106
7
60
30
1.4
20
8
60
30
1.4
39
60 60 60
40 50 55
1.4 0.9 1.4
39 20 106
Styrene samples
9 (Fig. 3b) 10 11
Initiator section (ern 3 min -~)
Particle description
Polydispersed spheres (up to 15 um), a smooth surface Bimodal spheres with a small amount of large particles (10 t~m) Uniform spheres (10 ~m), smooth Uniform spheres (10 urn), smooth Polydispersed spheres, rough surface Large spheres (10-20 t~m) and irregular particles Polydispersed spheres (10 ~m) and irregular particles Bimodal spheres (~20 #m and small particles) Bimodal spheres (~20 and 2 #m) No particles Large spheres (10-20 #m) and irregular particles
a Numbers in parentheses indicate modal diameters. Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
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NAKAMURA, PARTCH, AND MATIJEVIt~
a
I
I I
10 ~m
b
t
I
!
I
20M.m FIG. 3. Scanning-electron micrographs of polystyrene particles described in Table I: (a) sample 4; (b) sample 9.
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POLYMER COLLOID PREPARATION IN AEROSOLS, II
though of broader distribution. In many cases an increase in the initiator flow rate yields either solids of rough surface or of irregular shape (Fig. 4; Table I, sample 5). It is also interesting to note that these particles seem to be brittle; the cracked spheres show a viscous polymer core.
Divinylbenzene/Ethylvinylbenzene The analysis of aerosol droplets of commercial "divinylbenzene" showed these to consist of a mixture of isomers of two monomers. Consequently, the resulting colloidal particles must be copolymers of the same species. The solids obtained by initiation with TFSA differed from those of pure polystyrene in their surface characteristics. In most cases the generated solids were a mixture of particles having smooth and wrinkled (roughened) appearance.
123
Table II gives a number of conditions used in the preparation ofcopolymer colloids with the description of the major characteristics of the produced particles. Figure 5a (Table II, sample 1) illustrates such a system having a rather narrow size distribution; the enlarged section (Fig. 5b) reveals the differences in the surface structure. As the boiler temperature was raised (to 90°C) mostly smooth particles were generated (Fig. 6a; Table II, sample 10). The modal particle diameter increases with the boiler temperature but it depends also on the temperature of the reheater. Figure 6b (Table II, sample 5) is a scanning electron micrograph of a copolymer of distinctly bimodal distribution, which was accomplished by cooling the liquid aerosol with ice-water after exiting the reheater (Fig. I f). In all other experiments the tube (Fig. lg) was kept at room temperature. Figure 7 shows the results of an experiment
l
t
lOFm FIG. 4. Scanning-electron micro~raph of polystyrene panicles (Table 1, sample 5). Journal of Colloid and Interface Science, Vol. 99, No,
1, May 1984
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NAKAMURA, PARTCH, AND MATIJEVIC TABLE II Conditions for the Generation of Divinylbenzene/Ethylvinylbenzene Copolymer Particles by the Aerosol Procedure ~
Sample
Boiler temperature (°C)
Reheator temperature (°C)
1 (Figs. 5a, b) 2 3
50 50 50
45 55 65
50 70 70 70 70 90 90 90
85 45 55 65 75 35 55 75
4 5 (Fig. 6b) 6 7 8 9 10 (Fig. 6a) 11
Particledescription Spheres ( ~ 5 urn), b smooth and rough surface Spheres ( ~ 7 um), some small spheres, rough surface Spheres ( ~ 8 urn), with small irregular particles, rough surface Spheres ( ~ 8-10/~m), with very rough surface Bimodal spheres (larger, ~ 12 t~m, smaller, ~ 2 #m) Spheres ( ~ 8 - 1 0 #m), mostly smooth surface Polydispersed spheres, smooth surface Polydispersed spheres, smooth surface Spheres ( ~ I0 t~m) glued together, smooth surface Spheres ( ~ 8-12 um), smooth surface Spheres ( ~ 15/~m) with some small particles
In all experiments the flow rate in the boiler section was 1.4 dm 3 min -1 and in the initiator section 39 em ~ rain -l. b Numbers in parentheses indicate modal diameters.
in which the falling film generator was used for droplet formation rather than the boiler in the apparatus shown in Fig. 2A. Setting the temperature of the generator and reheater at 80 and 100°C, respectively, rather large, smooth particles were obtained. DISCUSSION
This work has demonstrated that the aerosol procedure can be used to generate polymer or copolymer colloids. This technique makes it possible to obtain, under certain conditions, spherical particles of rather large modal sizes (10-30 #m) by a direct polymerization process. The particle properties depend in a sensitive way on the experimental conditions by which the monomer droplets are formed and reacted with the vapor initiator. In addition to smooth spheres, in certain cases rather rough surfaces result. This surface morphology, which is more common to copolymer colloids prepared at lower boiler temperatures, may be due to the shrinkage of particles in the course of the polymerization process. Alternately, it is possible that unreacted monomer diffuses to the particle surface where it polymerizes on further contact with gaseous TFSA, producing surface Journal of Colloid and Interface Science, Vol. 99, No. l, May 1984
irregularities. The finding that this effect is more pronounced with the divinylbenzene! ethylvinylbenzene particles could be ascribed to the ability of this copolymer to crosslink (which is not the case with polystyrene). Such crosslinking would affect the density difference between the liquid droplets and the resulting solids, causing an increased shrinkage in course of polymerization. The low molecular weight of the polymer colloids obtained by the aerosol technique makes them distinctly different from the analogous materials prepared by the emulsion (or dispersion) polymerization process. The early chain termination process in the aerosol droplet may be caused by the interaction of trapped initiator anions with the cationic end of the growing polymer. The reason that the strongly acidic initiator (TFSA) causes polymerization of monomer aerosol droplets, while some other initiators do not, may be due to the fact that no water is present in the studied system. In contrast, emulsion polymerization carried out in aqueous media facilitates the release of protons from more weakly acidic initiators which are responsible for the polymerization.
125
POLYMER COLLOID PREPARATION IN AEROSOLS, II
I
!
20 p.m
b
r
I
10~m FIG. 5. (a) Scanning-electron micrograph of divinylbenzene/ethyvinylbenzene copolymer particles (Table II, sample 1). (b) Enlarged micrograph of the same sample.
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NAKAMURA, PARTCH, AND MATIJEVIC
a
I
" '1
10 p.m
I_
b
,
lOFm
I
FIG. 6. Scanning-electron micrograph of copolymer particles described in Table II: (a) sample 10; (b) sample 5.
Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
POLYMER COLLOID PREPARATION IN AEROSOLS, II
127
1
20 p.m F/G. 7. Scanning-electron micrograph of copolymer particles obtained in the failing film generator. Temperatures: generator 80°C; reheater 100°C; flow rates: monomer 1.8 dm 3 min -z, initiator 106 cm3 min-I
T h e p r o p a g a t i o n steps o f c a t i o n i c p o l y m e r i z a t i o n are k n o w n to b e faster .than those i n d u c e d b y free radicals. F o r this reason, t h e t y p e o f i n i t i a t o r u s e d in this w o r k is a d v a n tageous for the g e n e r a t i o n o f p o l y m e r colloids b y the aerosol technique. REFERENCES 1. Ugelstad, J., Mork, P. C., Kaggerud, K. H., Ellingsen, T., and Berge, A., Advan. Colloid Interface Sci. 13, 101-140 (1980). 2. Vanderhoff, J. W., El-Aasser, M. S., Micale, F. J., Sudol, E. D., Tseng, C. M., Silwanowicz, A., Kornfeld, D. M., and Vicente, F. A., private communication.
3. Partch, R., Matijevi~, E., Hodgson, A. W., and Aiken, B. E., J. Polym. Sci., Polym. Chem. Ed. 21, 961 (1983). 4. Nakamura, K., Partch, R. E., and Matijevi6, E., to be published. 5. McRae, D., Matijevi6, E., and Davis, E. J., J. Colloid Interface Sci. 53, 411 (1975). 6. Nicolaon, G., Cooke, D. D., Kerker, M., and Matijevi~, E., J. Colloid Interface Sci. 34, 534 (1970). 7. Nicolaon, G., Cooke, D. D., Davis, E. J., Kerker, M., and Matijevi6, E., J. Colloidlnterface Sci. 35, 490 (1971). 8. Visca, M., and Matijevi6, E., J. Colloid Interface Sci. 68, 308 (1979). 9. Ingebrethsen, B. J,, and Matijevi6, E., J. Aerosol Sci. 11, 271 (1980). 10. Ingebrethsen, B. J., Matijevi6, E., and Partch, R. E., J. Colloid Interface Sci. 95, 228 (1983).
Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
KANEHIRO NAKAMURA, 2 RICHARD E. PARTCH, AND E G O N MATIJEVIC Department of Chemistry and Institute of Colloid and Surface Science, Clarkson University, Potsdam, New York 13676
Received July 22, 1983; acceptedOctober 11, 1983 Polymer and copolymercolloidshave been prepared in modal diameters up to 30 um by reacting aerosol droplets of styreneor "divinylbenzene"(whichwas a mixture of ortho, meta, and para isomers with ethylvinylbenzene)with the vapor of trifluoromethanesulfonicacid in a speciallydesignedgenerator. The properties of the resulting particles (size distribution and surface characteristics) depend on the temperatures of the boiler and of the reaction chamber as well as on the monomer-to-initiator mass ratio. INTRODUCTION
This work represents an entirely different experimental approach to achieving large A large number of polymer colloids has polymer colloids, i.e., by chemical reactions been prepared by the conventional techniques in aerosols. It has been shown that droplets of emulsion or dispersion polymerization. of monomers (e.g,, p-tertiarybutylstyrene) disThese finely dispersed systems are characterpersed in a gas (helium) can be polymerized ized by exceedingly narrow size distribution of spherical particles. As a rule, their modal to solid particles if a vapor initiator (trifluodiameter is in the micrometer or submicro- romethanesulfonic acid, TSFA) is added to meter range. Recently, considerable effort the aerosol (3). Under proper conditions these powders were of narrow size distribution and has been made to produce such lattices of much larger sizes (10-20 urn). Ugelstad (1) could be dispersed in aqueous solutions of achieved this goal by swelling monodispersed appropriate surfactants. The advantages of the aerosol procedure polymer colloids of less than 1 tzm in diameter are several. No surfactant is necessary to prowith the solvent, surfactant, and monomer, duce the polymer colloid particles and their Following second polymerization, uniform size can be predetermined by the diameter of spheres in excess of 10 # m were obtained. the monomer droplets. Furthermore, each Growing large latex particles by emulsion poaerosol droplet acts as a separate "container." lymerization under gravity-free conditions was The technique can also be employed for the also attempted in the Columbia space shuttle preparation of polymers in colloidal state when laboratory during two different flights (2). Apno initiator is required. For example, polyurea parently, no significant change in modal diparticles were obtained by reacting droplets ameter was observed as compared to normal of toluene-diisocyanate with ethylenediamine conditions. vapor (4). This work describes the synthesis of styrene polymer and divinylbenzene/ethylvinylbenSupported by the NSF Grant CHE-80 13684. zene copolymer particles in the size range 102On leavefromthe ScienceUniversityof Tokyo,Tokyo, Japan. 20 t~m by the aerosol teehnique using the c o r 118 0021-9797/84 $3.00 Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and InterfaceScience, Vol. 99, No. 1, May 1984
119
POLYMER COLLOID PREPARATION IN AEROSOLS, II
responding liquid monomers as starting materials and TFSA as the vapor initiator. EXPERIMENTAL
Choice of materials. The monomers tested were styrene, 4-vinyl-pyridine, 4-vinyl-l-cyclohexene, 5-vinyl-2-norbornene, acrylonitrile, and butyl acrylate, all commercial products. "Divinylbenzene" (Fluka Chemical Corp.) was actually a mixture of ortho, meta, and para isomers and it contained ---50% of ethylvinylbenzene. Since all aerosols were prepared by the evaporation/condensation technique no purification of the starting materials was undertaken. In principle the initiation of polymerization can be accomplished either by radiation (UV or 3' rays) or by appropriate chemicals. In order to use the aerosol technique for the preparation of polymer colloids the chemical initiator must be chosen which will react fast since the contact times with the monomer droplets may be rather short. For this reason test experiments with various monomers and initiators were first carried out in bulk. For acrylonitrile and butyl acrylate, azo-tbutane activated by UV light was used as initiator. With medium pressure mercury or common sun lamps the polymerization required at least 5 min which was considered inadequate in the aerosol technique employed. Trifiuoromethanesulfonic acid (TFSA), boron trifluoride, and trifluoroacetic acid polymerized styrene, 4-vinyl-pyridine, 5-vinyl-2-norbornene, and divinylbenzene in bulk. However, only TFSA acted fast enough to be effective in the aerosol systems and was chosen for these studies. The two monomers used in this work are styrene and "divinylbenzene." Techniques. The monomer droplets can be prepared either by nebulizing the corresponding liquids or by the evaporation/condensation sequence. The latter technique was applied in studies of chemical reactions in aerosols (5) using a falling film generator (6, 7). It was also
shown that finely dispersed inorganic powders of narrow size distribution (8-10), as well as polymer colloids (3) can be obtained by an analogous procedure. For the purpose of this project the equipment was redesigned in order to produce larger, yet uniform droplets. A schematic diagram of the entire set-up is given in Fig. I. Helium (a) used as cartier gas was filtered through a Millipore membrane of 0.1 gm in pore size (b) and bubbled at a controlled flow rate (1.4 dm 3 min -z) through the boiler (d). The vapor was condensed in glass tubing (e), the resulting aerosol evaporated in reheater (f), and recondensed in tube (g) at ~ 2 0 ° C . This procedure assured better uniformity of aerosol droplets. Helium was introduced at flow rates between 34 and 51 cm 3 rain -1 into the reservoir (h) containing the initiator TFSA which was then transferred into the chamber (i) where it was mixed at 20°C with the monomer droplets. The polymerization is completed in the unheated reaction tube (j) and the resuiting polymer colloid particles are collected
et) f
g
c
i b
k PIG. 1. Schematic diagram of the apparatus used to produce polymer colloids: (a) helium gas tank; (b) Millipore membranes of 0.1 #m pore size; (c) flow meters; (d) boiler; (e) chamber for forming droplets; (f) reheater; (g) chamber for reforming droplets; (h) initiator reservoir; (i) initiator injection chamber; (j) reaction chamber; (k) therrnopositor. Journal of Colloid and Interface Science,
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NAKAMURA, PARTCH, AND MATIJEVI(~
120
in the thermopositor (k). The temperatures of The resulting polymer particles were exthe boiler and of the reheater were kept con- amined either in a transmission or in a scanstant by circulating oil from a bath. ning electron microscope. For this purpose Two different variations of the boiler (Fig. the solids were dispersed in an aqueous so1d) were used; improved reproducibility was lution of sodium dodecylsulfate and redeposachieved with the apparatus shown in Fig. 2A. ited by sedimentation on appropriate sample A detailed sketch of the reactor vessel (Fig. holders. For scanning electron microscopy the l i) is shown in Fig. 2B, the design of which specimens were coated with gold. prevented turbulence on contact of the aerosol Since "divinylbenzene" was actually a mixwith the TFSA vapor. ture of monomers, the liquid aerosol obtained In work with styrene the boiler temperature by the evaporation/condensation technique ranged between 40 and 60°C, while "divi- was collected and analyzed for composition nylbenzene" aerosols were generated at 50- by gas chromatography and nuclear magnetic 90°C. The reheater temperature was kept be- resonance spectroscopy. Both techniques tween 35 and 55°C for the former and between showed that the droplets contained divinyl45 and 75°C for the latter system. benzene and ethylvinylbenzene in the apIn earlier aerosol studies using the falling proximate ratio of 2:3. The polymerized parfilm generator heterogeneous nuclei (e.g., NaC1 ticles would, therefore, have the same comor AgC1) were initially introduced in order to position. produce more uniform droplets. The generator The polystyrene powder obtained by the described above was proven to be more suited aerosol process was dissolved in chloroform for liquids of higher vapor pressure and no and analyzed by gel permeation chromatogadvantage was detected when foreign nuclei raphy. The molecular weight was found to be were present. Thus, in this work all aerosol low (ranging between Mw = 1000 and 10,000) droplets were selfnucleated. and of broad distribution. The molecular
\
b
J° b
!
f
b
b
/ d
g A
"
120ram '150mrn--
FIG. 2. (A) Schematicdiagram of the boiler: (a) helium gas inlets; (b) stopcocks;(c) bubblers; (d) liquid monomer; (e) inlet from the constant-temperaturebath; (f) outlet to the constant-temperaturebath; (g) monomer vapor laden helium outlet. (B) Schematicdiagram of the initiator injection chamber: (a) liquid monomer droplets; (b) initiator vapor. Journal of Colloid and Interface Science, Vo[. 99, No. 1, May 1984
POLYMER COLLOID PREPARATION IN AEROSOLS, II
weight of the copolymer latex could not be determined because no solvent was found to dissolve the particles.
121
process was checked by the absence of a Tyndall beam at the exit of the reheater.
Polystyrene RESULTS
A large number of experiments was carded out to establish the best conditions for the preparation of colloid polymers of larger modal diameters and of narrow size distributions. The most critical parameters that control the properties of liquid aerosols of monomers of high vapor pressure are the temperatures of the boiler (Fig. l d) and of the reheater (Fig. 1f), which were altered in a systematic way. To achieve reasonable uniformity of the droplets it is essential to reevaporate the formed aerosol. The minimum temperature necessary to completely volatilize the droplets in (f) depends on the conditions in the aerosol generator (flow rate, boiler temperature, etc.) as well as on the design of the reheater. The completeness of the evaporation
Table I lists several selected sets of conditions studied in the preparation of polystyrene particles along with a brief description of the product appearance. It is apparent that a relatively small change in the listed parameters greatly affects the size, uniformity, and even the shape of the resulting polymer colloids. Spheres of narrow size distribution ( ~ 10 t~m in modal diameter) with smooth surfaces can be obtained as illustrated in Fig. 3a (Table I, sample 4; sample 3 is of similar quality). If the boiler temperature is higher, smooth spherical particles ofbimodal distribution are generated (Fig. 3b; Table I, sample 9); particles of --~20 ~zm in diameter, are in mixture with spheres I0 times smaller. As a rule an increase in the boiler temperature yields colloid polymers of larger modal sizes (up to 20 tzm), al-
TABLE I Conditions for the Generation of Polystyrene Particles by the Aerosol Procedure Flow rate Boiler temperature (°C)
Reheater temperature (°C)
Boiler section (drn 3 rain -j)
1
40
35
1.8
39
2
40
35
1.4
106
3 4 (Fig. 3a) 5 (Fig. 4) 6
50 50 50 50
45 45 45 55
1.8 1.8 1.4 1.4
34 51 51 106
7
60
30
1.4
20
8
60
30
1.4
39
60 60 60
40 50 55
1.4 0.9 1.4
39 20 106
Styrene samples
9 (Fig. 3b) 10 11
Initiator section (ern 3 min -~)
Particle description
Polydispersed spheres (up to 15 um), a smooth surface Bimodal spheres with a small amount of large particles (10 t~m) Uniform spheres (10 ~m), smooth Uniform spheres (10 urn), smooth Polydispersed spheres, rough surface Large spheres (10-20 t~m) and irregular particles Polydispersed spheres (10 ~m) and irregular particles Bimodal spheres (~20 #m and small particles) Bimodal spheres (~20 and 2 #m) No particles Large spheres (10-20 #m) and irregular particles
a Numbers in parentheses indicate modal diameters. Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
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NAKAMURA, PARTCH, AND MATIJEVIt~
a
I
I I
10 ~m
b
t
I
!
I
20M.m FIG. 3. Scanning-electron micrographs of polystyrene particles described in Table I: (a) sample 4; (b) sample 9.
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POLYMER COLLOID PREPARATION IN AEROSOLS, II
though of broader distribution. In many cases an increase in the initiator flow rate yields either solids of rough surface or of irregular shape (Fig. 4; Table I, sample 5). It is also interesting to note that these particles seem to be brittle; the cracked spheres show a viscous polymer core.
Divinylbenzene/Ethylvinylbenzene The analysis of aerosol droplets of commercial "divinylbenzene" showed these to consist of a mixture of isomers of two monomers. Consequently, the resulting colloidal particles must be copolymers of the same species. The solids obtained by initiation with TFSA differed from those of pure polystyrene in their surface characteristics. In most cases the generated solids were a mixture of particles having smooth and wrinkled (roughened) appearance.
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Table II gives a number of conditions used in the preparation ofcopolymer colloids with the description of the major characteristics of the produced particles. Figure 5a (Table II, sample 1) illustrates such a system having a rather narrow size distribution; the enlarged section (Fig. 5b) reveals the differences in the surface structure. As the boiler temperature was raised (to 90°C) mostly smooth particles were generated (Fig. 6a; Table II, sample 10). The modal particle diameter increases with the boiler temperature but it depends also on the temperature of the reheater. Figure 6b (Table II, sample 5) is a scanning electron micrograph of a copolymer of distinctly bimodal distribution, which was accomplished by cooling the liquid aerosol with ice-water after exiting the reheater (Fig. I f). In all other experiments the tube (Fig. lg) was kept at room temperature. Figure 7 shows the results of an experiment
l
t
lOFm FIG. 4. Scanning-electron micro~raph of polystyrene panicles (Table 1, sample 5). Journal of Colloid and Interface Science, Vol. 99, No,
1, May 1984
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NAKAMURA, PARTCH, AND MATIJEVIC TABLE II Conditions for the Generation of Divinylbenzene/Ethylvinylbenzene Copolymer Particles by the Aerosol Procedure ~
Sample
Boiler temperature (°C)
Reheator temperature (°C)
1 (Figs. 5a, b) 2 3
50 50 50
45 55 65
50 70 70 70 70 90 90 90
85 45 55 65 75 35 55 75
4 5 (Fig. 6b) 6 7 8 9 10 (Fig. 6a) 11
Particledescription Spheres ( ~ 5 urn), b smooth and rough surface Spheres ( ~ 7 um), some small spheres, rough surface Spheres ( ~ 8 urn), with small irregular particles, rough surface Spheres ( ~ 8-10/~m), with very rough surface Bimodal spheres (larger, ~ 12 t~m, smaller, ~ 2 #m) Spheres ( ~ 8 - 1 0 #m), mostly smooth surface Polydispersed spheres, smooth surface Polydispersed spheres, smooth surface Spheres ( ~ I0 t~m) glued together, smooth surface Spheres ( ~ 8-12 um), smooth surface Spheres ( ~ 15/~m) with some small particles
In all experiments the flow rate in the boiler section was 1.4 dm 3 min -1 and in the initiator section 39 em ~ rain -l. b Numbers in parentheses indicate modal diameters.
in which the falling film generator was used for droplet formation rather than the boiler in the apparatus shown in Fig. 2A. Setting the temperature of the generator and reheater at 80 and 100°C, respectively, rather large, smooth particles were obtained. DISCUSSION
This work has demonstrated that the aerosol procedure can be used to generate polymer or copolymer colloids. This technique makes it possible to obtain, under certain conditions, spherical particles of rather large modal sizes (10-30 #m) by a direct polymerization process. The particle properties depend in a sensitive way on the experimental conditions by which the monomer droplets are formed and reacted with the vapor initiator. In addition to smooth spheres, in certain cases rather rough surfaces result. This surface morphology, which is more common to copolymer colloids prepared at lower boiler temperatures, may be due to the shrinkage of particles in the course of the polymerization process. Alternately, it is possible that unreacted monomer diffuses to the particle surface where it polymerizes on further contact with gaseous TFSA, producing surface Journal of Colloid and Interface Science, Vol. 99, No. l, May 1984
irregularities. The finding that this effect is more pronounced with the divinylbenzene! ethylvinylbenzene particles could be ascribed to the ability of this copolymer to crosslink (which is not the case with polystyrene). Such crosslinking would affect the density difference between the liquid droplets and the resulting solids, causing an increased shrinkage in course of polymerization. The low molecular weight of the polymer colloids obtained by the aerosol technique makes them distinctly different from the analogous materials prepared by the emulsion (or dispersion) polymerization process. The early chain termination process in the aerosol droplet may be caused by the interaction of trapped initiator anions with the cationic end of the growing polymer. The reason that the strongly acidic initiator (TFSA) causes polymerization of monomer aerosol droplets, while some other initiators do not, may be due to the fact that no water is present in the studied system. In contrast, emulsion polymerization carried out in aqueous media facilitates the release of protons from more weakly acidic initiators which are responsible for the polymerization.
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POLYMER COLLOID PREPARATION IN AEROSOLS, II
I
!
20 p.m
b
r
I
10~m FIG. 5. (a) Scanning-electron micrograph of divinylbenzene/ethyvinylbenzene copolymer particles (Table II, sample 1). (b) Enlarged micrograph of the same sample.
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NAKAMURA, PARTCH, AND MATIJEVIC
a
I
" '1
10 p.m
I_
b
,
lOFm
I
FIG. 6. Scanning-electron micrograph of copolymer particles described in Table II: (a) sample 10; (b) sample 5.
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POLYMER COLLOID PREPARATION IN AEROSOLS, II
127
1
20 p.m F/G. 7. Scanning-electron micrograph of copolymer particles obtained in the failing film generator. Temperatures: generator 80°C; reheater 100°C; flow rates: monomer 1.8 dm 3 min -z, initiator 106 cm3 min-I
T h e p r o p a g a t i o n steps o f c a t i o n i c p o l y m e r i z a t i o n are k n o w n to b e faster .than those i n d u c e d b y free radicals. F o r this reason, t h e t y p e o f i n i t i a t o r u s e d in this w o r k is a d v a n tageous for the g e n e r a t i o n o f p o l y m e r colloids b y the aerosol technique. REFERENCES 1. Ugelstad, J., Mork, P. C., Kaggerud, K. H., Ellingsen, T., and Berge, A., Advan. Colloid Interface Sci. 13, 101-140 (1980). 2. Vanderhoff, J. W., El-Aasser, M. S., Micale, F. J., Sudol, E. D., Tseng, C. M., Silwanowicz, A., Kornfeld, D. M., and Vicente, F. A., private communication.
3. Partch, R., Matijevi~, E., Hodgson, A. W., and Aiken, B. E., J. Polym. Sci., Polym. Chem. Ed. 21, 961 (1983). 4. Nakamura, K., Partch, R. E., and Matijevi6, E., to be published. 5. McRae, D., Matijevi6, E., and Davis, E. J., J. Colloid Interface Sci. 53, 411 (1975). 6. Nicolaon, G., Cooke, D. D., Kerker, M., and Matijevi~, E., J. Colloid Interface Sci. 34, 534 (1970). 7. Nicolaon, G., Cooke, D. D., Davis, E. J., Kerker, M., and Matijevi6, E., J. Colloidlnterface Sci. 35, 490 (1971). 8. Visca, M., and Matijevi6, E., J. Colloid Interface Sci. 68, 308 (1979). 9. Ingebrethsen, B. J,, and Matijevi6, E., J. Aerosol Sci. 11, 271 (1980). 10. Ingebrethsen, B. J., Matijevi6, E., and Partch, R. E., J. Colloid Interface Sci. 95, 228 (1983).
Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984