162
Langmuir 1990, 6, 162-168
where
where 1 is the arc length, L the total length, and b/K the cross sectional length of the surface in the y direction. Evaluation of this expression gives
Thus u is determined to order cy2. The electrostatic free energy per unit area is from eq A2 and A9
p1sech E& ~ 0 t Eo h + pl) 1 ( u ) - uo(o)= 1 ( T ) ( c o t h 2 Eo + p1coth to+ p : - 1)'
(A331
so that the mean electrostatic free energy of bending per unit length is
Polymer Colloid Morphology Studied by Freeze-Fracture Electron Microscopy Bimsara Disanayaka, Cheng-Le Zhao, and Mitchell A. Winnik* Department of Chemistry and Erindale College, University of Toronto, 80 S t . George St., Toronto, Ontario, Canada M5S 1Al
Richard Shivers Department of Zoology, University of Western Ontario, London, Ontario, Canada N 6 A 3K7
Melvin D. Croucher Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1 Received January 24,1989. I n Final Form: June 12, 1989
The technique of freeze-fracture electron microscopy has been applied to study the morphology of poly(methy1 methacrylate) and polystyrene polymer colloid particles. Both aqueous emulsion-polymerized and nonaqueous dispersion-polymerized particles have been examined. We distinguish essentially two types of fracture patterns. One is composed of irregularly organized globules; the other has a welldeveloped structure with radiant lines directed from the center to the surface of the particles. The fracture pattern does not seem to be simply a material property of the polymer, and the polymerization medium is not the determining factor. We believe that the fracture pattern is characteristic of polymer particle growth mechanism. Introduction Polymer colloids are commonly referred to as latexes when they are water-based and as nonaqueous dispersions (NADs) when an organic solvent constitutes the continuous phase. The technique of emulsion polymerization leading to stable latex was developed in the 1940s. Since then, there has been tremendous progress in both industrial applications and in fundamental understanding of these systems.'-3 Considerable effort has been made in order to understand the mechanism of polymer parti(1) Smith, W. V.; Ewart, R. H. J . Phys. Chem. 1948, 18, 592. (2) Emulsion Polymers and Emulsion Polymerization; ACS Symposium Series 165, American Chemical Society: Washington, DC, 1981. ( 3 ) Science and Technology of Polymer Colloids; Poehlein, G. W., Ottewill, R. H., Goodwin, J. W., Eds; NATO AS1 Series; Martinus Nijhoff Publishers, 1983, Vol. 1.
0743-7463I90 12406-0162$02.50I O
cle formation, with the three principal schools of thought being micellar nucleation,' homogeneous n ~ c l e a t i o nand ,~ nucleation inside monomer droplets (miniemulsion).' While the first two mechanisms are most commonly encountered in emulsion systems, the third nucleation mechanism is only effective when monomer droplets are very small. NAD systems were invented a t ICI. They can be thought of as the hydrocarbon equivalent of emulsionpolymerized latexes, but they have not been investigated as thoroughly. The processes of aqueous emulsion and nonaqueous dispersion polymerizations are fun(4) Fitch, R. M.; Tsai, C. H. In Polymer Colloids; Fitch, R. M., Ed.; Plenum Press: New York, 1980; p 209. (5) Ugelstad, J.; El-Aasser, M. S.; Vanderhoff, J. W. J . Polym. Sci., Polym. Lett. Ed. 1973, 11, 503.
0 1990 American Chemical Society
Freeze-Fracture Microscopy of Polymer Colloids
damentally similar.6 The early stages of particle formation may differ (NAD polymerizations, in fact, begin as precipitation polymerizations), but once colloidally stable particle nuclei are formed, subsequent growth by a combination of polymerization within the particles and in solution followed by particle capture is believed to be essentially the same. Following the pioneering works of Harkins7 and Smith and Ewart,’ it was established that certain types of latex particles are nucleated from monomers solubilized inside micelles by free radical capture from the aqueous phase. These then grow by polymerization of monomers adsorbed via diffusion from the monomer droplet reservoir. Because of the important role of micelles in such a mechanism, this type of emulsion polymerization is known as the micellar nucleation mechanism. While this mechanism explains fairly well the kinetic results of emulsion polymerization of less water-soluble monomers such as styrene (limiting solubility 3.5 mM at 25 “C), it does not hold for more water-soluble monomers like methyl methacrylate (MMA, 150 mM a t 25 “ C ) . Fitch and Tsai4*’developed another mechanism, known as the homogeneous nucleation mechanism, to account for the behavior of more water-soluble monomers. In this mechanism, an initiator radical formed in the aqueous phase continues to add monomer molecules dissolved in the water phase until the growing oligomeric radical exceeds its solubility. The oligomeric chain then collapses (precipitates upon itself) to form a tiny colloidal particle. Primary particles thus appear. There are two possible fates for the newly formed primary particles. They can either grow by polymerization of adsorbed monomer or they can undergo limited flocculation with existing larger particles or other primary particles. According to Y eliseyeva? these processes compete. The former will dominate only if the extent of equilibrium adsorption of the emulsifier onto the surface of primary particles is sufficient to provide colloidal stability and occurs fast enough to prevent coagulation. The polarity of monomers is not directly important, but it is related to the dynamic adsorption of emulsifier. A more hydrophobic surface, for example, corresponds t o a faster emulsifier adsorption, which will reduce the flocculation tendency of particles. In view of these considerations, the formation of primary particles is a key factor for understanding the nucleation and growth mechanisms in emulsion polymerization. Little is known about this process, largely because only a limited number of experimental techniques are available that can provide information about it. An oxygen plasma etching technique was used by Yeliseyevag to reveal smaller globules within acrylate latex particles, and these were interpreted to be the primary particles formed a t the initial stages of the polymerization. With this hypothesis, the numbers of primary particles were measured for a series of acrylic copolymer latexes. There are, however, problems with this technique. We note, first, that because the method of oxygen plasma etching is destructive the size of these small globules decreased with etching time. Therefore, the results have a certain subjectiveness depending on the etching time empirically chosen. In addition, the possibility exists that these small globules were only artificial creations of the oxygen etching. 6.
(6) Fitch, R. M.; Kamath, Y. K. J . Colloid Interface Sci. 1976, 54, (7) Harkins, W. D. J. Chem. Phys. 1945, 13, 381. (8) Fitch, R. M.; Prenosil, M. B.; Sprick, K. J. J . Polym. Sci. C
1969, 27, 95.
(9) Yeliseyeva, V. I. In Emulsion Polymerization; Piirma, I., Ed.; Academic Press: New York, 1982; Chapter 7.
Langmuir, Vol. 6, No. 1, 1990 163
Fitch and Tsai’O isolated aqueous polymerization oligomer and latex polymerization oligomer of methyl methacrylate. They found a critical polymerization degree of 65-66 for PMMA to self-nucleate. This corresponds to primary particles of about 26 A in diameter. Goodall et al.” determined the molecular weight of polystyrene oligomeric radicals, which corresponded to a polymerization degree of 10. As direct evidence of primary particles, Bassett12 presented scanning electron microscopy result of hard latex particles. A “raspberry” particle structure was clearly observable. With this technique, however, one is only able to look at the particle surface. Once the initial polymer particles are formed, monomer can partition between the aqueous and particulate phases. For many years, it was thought that growth of the particles proceeds through polymerization of monomer within the monomer-swollen particles and that the monomer is uniformly distributed throughout the particles. More recently, this view has changed in light of fairly convincing evidence that the growing styrenepolystyrene particles consist of an expanding polymerrich core surrounded by a monomer-rich shell. For example, systematic work by Williams et al.13-15 using kinetic and molecular weight development data and electron microscopy in conjunction with butadiene tagging and osmium tetroxide staining, as well as autoradiography experiments, demonstrated that polystyrene formed a t later stages of the reaction is found preferentially near the outside of the particle. He concluded that the outer shell is the main locus of polymerization. Similar conclusions have been reached recently by Hearn et a1.I6 in the case of surfactant-free emulsion polymerization of styrene and by Chang et a1.l’ for the same system but incorporating a surfactant comonomer. Although the results seem conclusive, they are nevertheless based solely on aqueous styrene emulsion polymerization studies. The aim of our present research is to try to obtain some new information on the subjects of primary particle formation and monomer-polymer particle growth mechanisms. We applied the freeze-fracture transmission electron microscopy (FFTEM) technique to the study of poly(methy1methacrylate) particles prepared in both aqueous and nonaqueous dispersion media, as well as to aqueous polystyrene particles for comparison. The freeze-fracture transmission electron microscopy (FFTEM) technique was introduced by Steerla and developed by Moor et al.” for studying biological systems. While FFTEM has become a very important tool for biological studies, it has received relatively little attention in the polymer colloid domain.20 It has been used for particle (10) Fitch, R. M.; Tsai, C. H. In Polymer Colloids; Fitch, R. M., Ed.; Plenum Press: New York, 1971; p 103. (11) Goodall, A. R.; Wilkinson, M. C.; Hearn, J. In Polymer Colloids; Fitch, R. M., Ed.; Plenum Press: New York, 1980; p 629. (12) Bassett, D. R. In Science and Technology of Polymer Colloids; Poehlein, G. W., Ottewill,R. H., Goodwin, J. W., Eds.;NATO AS1 Series; Martinus Nijhoff Publishers, 1983; Vol. 1, p 220. (13) Grancio, M. R.; Williams, D. J. J. Polym. Sci., Polym. Chem. Ed. 1970,8, 2617. (14) Grancio, M. R.; Williams, D. J. J . Polym. Sci., Polym. Chem. Ed. 1970, 8, 2733. (15) Keusch, P.; Williams, D. J. J . Polym. Sci., Polym. Chem. Ed. 1973, 11, 143. (16) Hearn, J.; Wilkinson, M. C.; Goodall, A. R.; Chainey, M. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 1869. (17) Chang, H. S.; Chen, S . A. Makromol. Chem. Rapid Commun. 1987, 8, 297. (18) Steer, R. L. J . Biophys. Biochem. Cytol. 1957,3, 45. (19) Moor, H.; Muhlethaler, K.; Waldner, H.; Frey-Wyssling, A. J. J . Biophys. Biochem. Cytol. 1961, 10, 1. (20) Menold, R.; Luttge, B.; Kaisser, W. Adu. Colloid Interface Sci. 1976, 5, 281.
Disanayaka et al.
164 Langmuir, Vol. 6, No. I , 1990 size analysisz1and i n a micellization study" of graft copoly m e r s in a microemulsion system. Briefly, t h i s technique consists of rapidly freezing a sample, a dispersion of particles in a continuous medium for example, b y using liquid nitrogen a n d then fracturing the frozen sample with a cooled knife. R a p i d freezing is used i n order to prevent structural changes during sample preparation. T w o types of fracture can be produced, depending on the propagation pathway of the fracture plane: (a) the fracture goes through the particles and (b) the fracture goes through t h e particle/matrix interface leading to either protrusions or depressions, as schematized below:
P, P r o t r u s i o n ,
E, Extrusion,
FS, F r o c r u r e d Surface
As p o i n t e d out b y M e n o l d et al.,*' case a is o f t e n observed for polymer colloids. Nevertheless, case b m a y occur depending o n experimental conditions. It is known that at cryogenic temperatures (-196 "C) major polymer relaxation mechanisms (a,0)a r e frozen and most polymers experience brittle fracturez3 (for example, the impact strength is a b o u t 12-20 k J / M 2 for PMMA and PS at t e m p e r a t u r e s well below T,, compared to 5090 k J / M 2 for the styrene-butadiene copolymer). The small values of t h e i m p a c t s t r e n g t h indicate a weak energy dissipation upon loading, t h u s forming the physical basis of polymer freeze fracture. If the fractured surface is shadow cast appropriately with platinum/carbon and followed b y a removal of the remaining polymer material from the mask, a replica film is isolated. Electron micrographs of these films reveal t h e morphology of fractured particles. We believe that t h i s technique is a powerful and useful tool which gives direct insight i n t o particle morphology.
Experimental Section The surfactant-free emulsion polymerization technique was used for both methyl methacrylate and styrene monomers in order to produce large monodisperse particles. The stability of these latex particles is ensured by negatively charged sulfate end groups which come from the decomposition of the initiator, potassium persulfate (KPS). We will refer to surfactantfree emulsion-polymerized poly(methy1 methacrylate) (PMMA) and polystyrene (PS) latexes as E-PMMA and E-PS, respectively, in the following discussion. The recipe for preparing E-PS particles is as follows:24water, 150 g; styrene, 15 g; KPS, 0.08 g. The recipe for preparing EPMMA latex is exactly the same as above, except that MMA replaces styrene. Monomer and water were introduced into a 250-mL three-necked flask, equipped with a condenser, mechanical agitator, and a nitrogen inlet and outlet. The whole system was purged with nitrogen for a t least 20 min while stirring a t the reaction temperature, 70 "C. The initiator was introduced and polymerization started. The reaction time was 24 h. Scanning electron micrographs showed that both E-PMMA and E-PS were monodisperse in size. Particle diameters are 5000 A for E-PMMA and 4300 A for E-PS. For comparison, a monodisperse aqueous PMMA latex with a mean diameter of 0.586 km was purchased from Seragen Diag(21) Reed, R.; Barlow, J. R. Polymer 1972, 13, 226. (22) Candau, F.; Boutiller, J.; Tripier, F.; Wittman, J. C. Polymer
1979,20, 1221.
(23) Kausch, H. H. Polymer Fracture; Springer-Verlag, 1987; p 240. (24) Shirahama, H.; Suzawa, T. Poiym. J . (Tokyo) 1984,16(11), 795.
nostic Inc., Indianapolis. We refer to this product as SDPMMA latex. Nonaqueous dispersions of PMMA particles were synthesized in our laboratory by using the same procedure as described p r e v i o u ~ l ywith , ~ ~ isooctane as the solvent. Degraded butyl rubber (PIB, Hardman, Inc.), a copolymer, poly(isobuty1ene-coisoprene), containing 1.5 mol % unsaturation and with a nominal molecular weight of 10 000 was used as the stabilizer. The monomer mole percent of stabilizer incorporated into the particles was determined by 'H 400-MHz NMR and in certain instances by UV spectroscopy. Some particle samples were labeled in the stabilizer phase with pyrene (Py) groups (ca. one pyrene per ten polymers of MW = 10 000). The labeled stabilizer was prepared by treating a solution of butyl rubber in CH,Cl, with singlet oxygen. After reduction with LiAlH,, the polymer contained allylic alcohol groups. These were treated with 4-(1-pyreny1)butyryl chloride to introduce a small fraction of pyrene groups attached to the PIB via an ester linkage. The details of this synthesis will be reported elsewhere.26 NAD particles were prepared with this labeled stabilizer in a manner identical with that described for unlabeled particles stabilized with butyl rubber. No differences in particle size and size distributions were observed. The Py groups are useful for fluorescence studies of particle morphology and serve as a convenient label for determining the PIB content of the particles. The PIB in the particles is present as a graft copolymer with PMMA. Analysis of the various sample preparations yields the following PIB contents (monomer mol %): sample PIB-PMMA1 (unlabeled) 1.5%;PIB-Py-PMMA-1 (Py labeled) 1.25%; PIBPy-PMMA-2 (Py labeled) 1.62%. All particles were washed by repeated centrifugation, decanting of the supernatant, and redispersion in fresh solvent. The latex particles were then freezedried from water, whereas the NAD particles were freeze-dried from cyclohexane. The samples were stored in the dry powder form. Freeze-Fracture Electron Microscopy. Particles were prepared for freeze-fracture by preparing a suspension of the particles (ca. 30% by weight) with 30% aqueous glycerol in the presence of a small amount (ca. 0.5%) of Triton-X100. Single droplets of this suspension were placed on gold specimen disks (Balzers, Liechtenstein) and frozen in a slurry of liquid-nitrogen-cooled Freon-22. Each droplet was 1.4-1.8 mm in diameter. Frozen disks were stored in liquid nitrogen until further processing could be carried out. Alternatively, suspensions of similar concentrations of the particles were prepared by using a hydrocarbon oil (immersion oil for optical microscopy) as the continuous medium. Frozen and treated in the same manner as the aqueous glycerol suspensions described above, these samples gave essentially identical freeze-fracture electron micrographs as those obtained from the samples prepared by using the more polar medium. Freeze-fracture of the frozen particle suspensions was carried out a t -110 "C in a Balzers BAF 301 Freeze-Etch Unit (Balzers, Liechtenstein) in a vacuum of 1 X lo4 Torr, following the procedure of Shivers and BrightmamZ7 The exposed fractured surface of the particle suspension was replicated with a thin film (ca. 60 nm) of platinum and carbon and then cleaned in chloroform by soaking the replica in several changes of fresh chloroform over a period of 48 h. Cleaned replicas were picked up on bare 200-mesh copper grids and examined in a Philips 201 electron microscope operating at an accelerating voltage of 60 kV. Particles were initially photographed a t 30 000 diameters and then further enlarged photographically.
Results and Discussion Figures 1 and 2 show typical electron micrographs of freeze-fractured E-PMMA and S D - P M M A particle surfaces, respectively. T h e magnification is approximately 30 000 ( t h e marker bar represents 100 nm). Several fea(25) Williamson, B.; Lukas, R.; Winnik, M. A.; Croucher, M. D. J . Colloid Interface Sci. 1987, 119(2),559. (26) Disanayaka, B.; Winnik, M. A.; Croucher, M. D. J . Colloid Interface Sci., in press. (27) Shivers, R. R.; Brightman, M. W. J . Comp. Neurol. 1976, 167, 1.
Freeze-Fracture Microscopy of Polymer Colloids
Figure I . Freeze-fracture electron micrograph of surfactantfree emulsion-polymerizedPMMA particles, E-PMMA. In each of the figures, the marker bar represents 100 nm.
hngmuir, Vol. 6, No. 1, 1990 165 domly organized structure seemingly composed of tiny beads less than 100 A in diameter. The first point is in fact a measure of reproducibility of FFTEM technique. Although we do not know the exact procedure of the preparation of SD-PMMA particles, their structure resembles that of our surfactant-free emulsionpolymerized PMMA particles. Point b might mean that the adhesion strength between particles and glycerin glass is not much higher than the cohesion strength of PMMA. Because freeze-dried EPMMA and SD-PMMA particles do not disperse readily in aqueous glycerol, a detergent (Triton X-100)was used to help in dispersing the polymer particles prior to freezing. Consequently, the surface of the PMMA particles was covered by a stabilizer surfactant layer, which was suddenly frozen for fracturing. This frozen stabilizer layer could constitute the so-called weak boundary layer (WBL) at the particle/medium interface?* The deformed particles observed in the micrographs (point c) most likely result from plastic deformation during fracturing. This process can be significant even a t cryogenic temperatures" and may be promoted by dissolved nitrogen in the particles picked up during the freezing step. Points d and e contain information about particle structure. The raspberry-like surface structures seen in replica in Figures 1 and 2 are reminiscent of those reported by Bassett" from SEM studies of latex particles. He attributed the origin of the structures to primary particle coagulation contributing to latex growth, a point of view which would have been easier to substantiate if he could have seen similar structures in the particle interiors. With SEM, one can see only particle surfaces. Here (Figures 1and 2 ) the FFTEM technique provides us with an opportunity to examine the internal, cross sectional structure of our PMMA latex particles. We observe a rough texture a t the fractured surface which may have been created as a result of primary particle coagulation. From this point of view, the graininess that characterizes this surface may be due to vestiges of small particles. The spatial scale of the raininess in Figures 1 and 2 is on the order of 80-100 , and objects of that size should be resolvable in these experiments. We emphasize, therefore, that no well-distinguished primary particles can be identified in the micrograph. This means that the coagulated primary particles a t the surface have undergone a further process leading to homogenizationof internal structure. The factors leading to primary particle fusion need to be investigated. Coalescence, for example, can play a very important role in this process. Fitch et al! studied the kinetics of MMA emulsion polymerization in the absence of surfactant. They found that when the initiator concentration was sufficiently high ([I] > 0.925 M) the system showed classical heterogeneous polymerization kinetics behavior. We were careful to choose similar reaction conditions (here [I] = 1.97 M). We therefore assume that in our reaction polymerization of MMA took place simultaneously in a large number of isolated loci during the synthesis of the E-PMMA particles. The most difficult question about the particle formation mechanism is how to relate the idea of isolated loci of polymerization and the grainy texture seen in the fractured particle surfaces in Figures 1 and 2. The key aspect of this texture is that it is so uniform, suggesting that it
1
Figure 2. Freeze-fracture electron micrograph of commercial PMMA particles, SD-PMMA. tures can be readily observed (a) Two electron micrographs have almost the same aspects. (b) The fracture plane goes through some of the particles in an irregular pathway and also through the particle/medium interface, producing depressions and protrusions. (c) The fractured particle surfaces are often deformed. (d) The particle surfaces seen from depressions and protrusions are not smooth; they have a raspberry-like texture. (e) The internal surface of the fractured particles reveals a ran-
(28) (a) Bikerman, J. J . Adheaive Age 1959.2.23. (b) Zhao, C. L.; Dobler, F.; Holl, Y.;Lambla. M.J . Colloid Interface Sei. 1989,128.437. (29) Reference 23,p 244.
Disannyaka et al.
166 Langmuir, Vol. 6, No. I , I 9 9 0
0
I
i I
m
Froeluriig
1
shadowing
PI SOYIC.
\
I
6
Figure 4. Schematic representation of protrusion formation during fracturing of EPS samples.
Figure 3. Freeze-fracture electron micrograph of surfactantfree emulsion-polymerizedpolystyrene particles, E-PS.
is formed from the partial fusion of similarly sized particles. Polymerization is unlikely to take place homogeneously inside large monomer-swollen polymer particles, because we would not expect such a process to form the kind of surface structures seen in Figures 1 and 2. One possible explanation is that primary particles continue to grow by polymerization of adsorbed monomer, until they become unstable and undergo flocculation among themselves and with larger particles. This process would have to occur during the entire polymerization period in order to explain why we see this particular type of texture across the whole internal structure of the final particles. Figure 3 shows a freeze-fracture electron micrograph of E-PS particles. Three observationscan be made. First, polystyrene particles do not break as easily as PMMA particles; second, no globules are observable; third, we can distinguish well-defined radiant lines directed from the center to the surface of the particles. As discussed in the Introduction, both PS and PMMA have comparable impact strength. The relatively easy fracture of EPMMA particles, compared with E-PS particles. may be due to an imperfect coalescence of primary particles after coagulation. The fact that no grainy structure can be recognized implies that primary particle coagulation is not a dominant factor in styrene polymerization. An alternative mechanism must be operating here. Emulsion polymerization of styrene in the presence of surfactant has been thoroughly studied by Williams et a1,13-15 and in the absence of added surfactant by Hearn et al." and by Chang et al." These various experiments all seem to support a core-shell growth model. The hypothesis that we make is that the surface fracture pattern reflects the particle growth mechanism. The radiant lines may signify that polystyrene particles grow by preferential polymerization a t the particle surface. That these "landmarks" survive until the end of polymerization suggests that polymerization does not occur uniformly inside the particles.
There is at this time no rigorous explanation for the mechanism of radial structure formation. We believe that this structure indicates the existence of radial forces in the surface layer of the particle during the particle growth process. This could arise from the fact that simultaneous polymerization and absorption of monomer within the surface layer exert radial stresses on the swollen polymer network. Also contributing would be the tendency of sulfate end groups within this network to remain a t the polymer-water interface, inducing an extensional deformation of the chains to which they are attached. One other feature of the micrograph in Figure 3 requires comment, the tongue-like shadows (called "horns" by R~barts~'.~') which emanate from many of the E-PS particles. These shadows are cast by protrusions formed during fracture of the E-PS particles. Their presence empbasizes that fracture of these particles is not simply a radial process and that fracture sometimes occurs with the kind of plastic deformation depicted in Figure 4. This type of deformation has been seen previously, particularly with polystyrene latex particles. It is an example of one of the classic artifacts associated with plastic deformation during freeze-fracture. There is a vivid description of this and other freeze-fracture artifaets in papers by Sleytr and Robartsm and by Dunlop and Robarts.3' Figures 5 and 6 are the freeze-fracture electron micrographs of PMMA particles, PIB-PMMA-1 and PIB-PyPMMA-2. They are prepared by nonaqueous dispersion polymerization of MMA in isooctane in the presence of butyl rubber. During the polymerization,grafting occurs, leading to formation of a steric stabilizer layer of PIB a t the particle surface and entrapment of grafted PIB in the particle interior. The particle diameters are 1.35 pm for PIB-PMMA-1 and 1.25 Fm for PIB-Py-PMMA-2. With the PMMA-PIB NAD particles, we observed that fractured particle surfaces were much easier to produce than with the emulsion-polymerized PMMA or PS particles. Furthermore, the fracture plane passed almost exclusively through the center of the particles. We believe that the presence of an interconnected PIB phase inside the NAD prepared particle^^'.^^ could be one reason for the easy fracturing. Another possibility is that the PMMA particles produced in organic medium have a looser packing than those produced in water. More experiments need to be done to clarify this point. (30) Slew,U. B.; Robarts, A. W. d. Microscopy 1977,IIO. 1. (31) Dunlop. W. F.; Rob&, A. W. J . Ultmstructure Res. 1972.40,
391.
(32) Winnik. M. A. In Photophysics of Polymers; Hoyle. C. E., Torkeelson, J. M..E%.; ACS Sympasium Series 358; American Chemical Society: Washington. DC, 1987. (33) Winnik. M.A.; Pekcnn, 0.;Chen, L. S.; Cmucher, M.D. Macmmolecules 19sB,21,55.
Langmuir, Vol. 6, No. I, 1990 167
Freeze-Fracture Microscopy of Polymer Colloids
I '1
Figure 5. Freezefracture electron micrograph of NAD-polymerized PMMA particles, PIB-PMMA-1.
Figure 6. Freezefracture electron micrograph of NAD-polymerized PMMA particles, PIB-Py-PMMA-2. These particles are prepared from PIB labeled to the extent of 0.170with pyrene groups. In Figure 6, we can see a halo-like outer zone at the periphery of the particle. The nature of the corona is not known. With a thickness of about 500 A, it represents about 23% of the total volume of the particle. From the point of view of composition, it cannot be the collapsed pure stabilizer layer. Another hypothesis is that the corona is a stabilizer-rich layer. We are tempted to
infer from comparison of the micrographs in Figures 5 and 6 that the presence of the corona depends in some way on the presence of the pyrene groups attached to the stabilizer in the labeled particles. This is a difficult point to establish rigorously. We do see the same type of structure in the electron micrographsof another pyrenelabeled PMMA particle system, PIB-Py-PMMA-1. Further experiments are in progress in our laboratory to help us understand this phenomenon. A common feature in Figures 5 and 6 is the presence of radiant lines, directed from the center region to the surface of the particle, the same pattern as is seen for PS particles prepared by the aqueous emulsion polymerization technique. According to Barrett and Thomas,% nonaqueous dispersion polymerization of MMA proceeds by two steps: nucleation by precipitation of PMMA as a result of the decrease of solvency as MMA monomers are consumed, followed by growth of particles by polymerization inside nuclei accompanied by diffusion of residual MMA monomer into the particles. The growing particles have to be, of course, stabilized by the incorporation of stabilizer. This mechanism has been found to explain satisfactorily our previous results on the same system.= The similarities in fracture surface 'structure for the PS particles prepared by surfactant-free emulsion polymerization and the PMMA particles prepared by dispersion polymerization in isooctane suggest that both systems share common features of the particle growth mechanisms. E-PS polymerization is thought to be largely heterogeneous and dominated by a monomer-rich shell a t the particle exterior. It may well be that in isooctane growing PMMA NAD particles have a similar monomer concentration gradient across the particle diameter with polymerization occurring preferentially in the peripheral region. This is, to our knowledge, the first suggestion of heterogenous growth of NAD particles. By itself, this idea does not provide an explanation of all aspects of the PMMA-PIB particle formation and growth aspects. We still need to explain, for example, how grafted PIB comes to be trapped within the particle interior in the form of an interconnected network. More experiments are needed. It is interesting to speculate on why E-PS and PMMAPIB particles share a common particle growth mechanism different from that of E-PMMA. The major factor is most likely the solubility of the propagating oligomeric radical in the continuous medium. This solubility is high for PMMA in water and low for PS in water and for PMMA in isooctane. As a consequence, the latter two systems have little tendency to form independent primary particles which grow, eventually coagulate, and partially coalesce. In summary, by using the freeze-fracture electron microscopy technique, we have been able to distinguish particles formed by different nucleation and growth mechanisms. With surfactant-free emulsion-polymerized PS particles, the fracture pattern corresponds to the surfacephase polymerization model. With PMMA particles prepared by the surfactant-free emulsion polymerization technique, our results indicate that coagulation of primary particles is an important process during the entire polymerization period. When similar types of PMMA particles are prepared by nonaqueous dispersion polymerization, we find that a different mechanism is operative: a (34)
Barrett. K. E.J.: Thomas, H.R.J. Polym. Sei., Polym. Chem.
Ed 1969,17,2621.
168
Langmuir 1990,6, 168-172
homogenous nucleation process followed by a preferential growth at the periphery of the particles. This, in turn, resembles the surface-phase polymerization mechanism of styrene in aqueous emulsion polymerization.
Acknowledgment. We thank NSERC Canada for their support of this research. Registry No. PMMA, 9011-14-7; PS, 9003-53-6.
Effect of Dispersed Manganese Oxides on the Decomposition of Permanganate Solutions Liang-zhong Zhao,+ Jack G. Davis, Jr., and Vaneica Y. Young* Department of Chemistry, University of Florida, Gainesville, Florida 32611 Received July 21, 1988
The effect of manganese oxide particles dispersed on carbon substrates upon the decomposition of dilute, neutral permanganate solutions is investigated. Dispersed Mn,O, and MnO were prepared by vapor deposition, and dispersed MnO, was prepared by solution deposition. In all cases, dispersed oxides were more active than thick or bulk oxides. Introduction The effect of dispersion of a metal upon its catalytic activity has been intensely investigated in recent years. The dispersed metal is usually prepared as a distribution of small particles supported on inert substrates, such as silica, alumina, or carbon. In most cases, it has been shown that the catalytic activity changes as the average particle size changes.'+ A fundamental goal of studies such as these is to characterize those properties of the particles, e.g., electronic structure, morphology, chargetransfer behavior, etc., which are responsible for the enhanced catalytic activity. Metal oxides also exhibit greater catalytic activity when discontinuously dispersed on a substrate. These materials are commonly prepared by chemical impregnation of powdered supports7 followed by activation, e.g., calcining, to give crystallites of the metal oxide on the surfaces of individual support particles. However, the activation step occurs a t such high temperatures that lateral diffusion on the surfaces of the support particles prevents precise control of the size distribution of the particles. As one example of many which could be cited, the results of Cimino et a1.' will be mentioned. These authors used X-ray photoelectron spectroscopy (XPS),X-ray diffraction, and optical measurements to show that CrOJSiO, catalysts contain molecularly dispersed Cr(V1) and a bimodal distribution of Cr,O, particles with sizes 120 and 2400 A. We are conducting research aimed a t ascertaining the electronic structure of small particles of metal oxides and its correlation with their catalytic activity. The former
'
Present address: Institute of Chemistry, Academia Sinica, Beijing, China. (1) Yates, D. J. C.; Sinfelt, J. H. J . Catal. 1967,8, 348. (2) Boudart, M.; Aldag, A. W.; Ptak, L. D.; Benson, J. E. J . Catal. 1968, 11, 35. (3) Lam, Y. L.; Sinfelt, J. H. J. Catal. 1976, 42, 319. (4) Dominguez, J. M.; Yacaman, M. J. J. Catal. 1980, 64, 223. ( 5 ) Fuentes, S.; Figueras, F. J. Catal. 1980, 61, 443. (6) Graydon, W. F.;Langan, M. D. J. Catal. 1981.69. 180. (7) Sinfelt, J. H. Science 1977, 195, 641. (8) Cimino, A.; DeAngelis, B. A,; Luchetti, A.; Minelli, G. J. Catal. 1976, 45, 316.
0743-7463/90/2406-0168$02.50/0
studies are facilitated by supporting the metal oxide particles on carbon substrates. Experimentally, the electronic structure of these systems is studied by using core and valence level XPS. The catalytic activity of these systems may be studied by choosing an appropriate reaction. In this paper, we report the effect of dispersed MnO, Mn203,and MnO, on the decomposition of dilute, neutral potassium permanganate solutions. Experimentally, this is a very simple reaction to study because the kinetics is slow enough that the course of the reaction can be followed spectroph~tometrically.~~~~ Although only a few will be cited, many researchers have investigated the kinetics of the oxidation of organic compounds by permanganate in acidic, neutral, and alkaline solutions by using spectrophotometry a t a wavelength corresponding to an absorption maximum in the first excitation band of permanganate.l'-l4 Here, neutral solutions have been used because the rate of reaction is thermally increased in strongly acidic15or strongly alkaline15*16 solutions. The rate of reaction has also been reported to increase as a result of exposure to visible The decomposition product MnO has been shown to catalyze the decomposition reaction.'6 Mn(II1) in the solid has been proposed as an intermediate in the decomposition r e a ~ t i o n . ' ~ ~Manganous ~' ion has been shown to react with permanganate in 3 M HClO, s o l ~ t i o n . ' ~It is wellknown that it autocatalyzes oxidations by permanga(9) Kachan, A. A.; Sherstoboeva, M. A. Zh. Neorg. Khim. 1958, 3, 1089. (10) Iogansen, A. V.; Grushina, N. M. Khim. Fiz. 1982,1, 121. (11) Wiberg, K. B.; Fox, A. S. J. Am. Chem. SOC.1963,85, 3487. (12) Freeman, F.; Fuselier, C. 0.; Armstead, C. R.; Dalton, C. E.; Davidson, P. A.; Karchesfski, E. M.; Krochman, D. E.; Johnson, M. N.; Jones, N. K. J . Am. Chem. SOC.1981,103, 1154. (13) Toyoshima, K.; Okuyama, T.; Fueno, F.J . Org. Chem. 1980, 45, 1600. (14) Ogino, T. Tetrahedron Lett. 1980,21, 177. (15) Zimmerman, G. J. Chem. Phys. 1955,23, 825. (16) Narita, E.; Hashimoto, T.; Yoshida, S.; Okabe, T. Bull. Chem. SOC.Jpn. 1982,55, 963. (17) Duke, F. R. J. Phys. Chern. 1952, 56, 882. (18) Shafirovich, V. Ya.; Shilov, A. E. Kinet. Katal. 1979,20, 1156. (19) Adamson, A. W. J . Phys. Colloid Chem. 1951,55, 293.
0 1990 American Chemical Societv
