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Elemental Distribution within Single Latex Particles: Determination by Electron Spectroscopy Imaging Andre´ Herzog Cardoso,† Carlos Alberto Paula Leite, and Fernando Galembeck* Instituto de Quı´mica, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970, Campinas-SP, Brazil Received October 28, 1997. In Final Form: March 6, 1998 Particles of a poly[styrene-co-(2-hydroxyethyl methacrylate)] latex were prepared and examined by electron spectroscopy imaging (ESI), using an energy-filtered transmission electron microscope (EFTEM). Both whole particles and thin ultramicrotomed sections were examined, in the brightfield and energy-loss modes. The following elemental distributions were determined, for the larger particles in the latex: (i) C is rather uniformly distributed throughout the raspberry-like latex particles; (ii) O is also distributed throughout the particles but with significant fluctuations from point to point, and it is more concentrated in the vicinity of the particle surfaces; (iii) S distribution resembles that of C, but it is absent from the particle-particle contact areas; (iv) K is mostly found in a thin outer particle layer. Elemental distribution patterns in the smaller particles are different: these have a higher O/C ratio, and K is dispersed in the bulk of the particles. The main conclusions are as follows: hydrophobic and hydrophilic domains are distributed throughout the particles; negative charges are trapped in the particle bulk, and countercations are dispersed in the bulk (small particles) or concentrated at the surfaces (large particles); these results are consistent with particle formation by homogeneous nucleation, following previous work by the Fitch and Okubo groups.
Introduction The theoretical and practical interest of the morphology of latex particles led to their intensive study by scattering and electron microscopy techniques.1,2 Observations in the transmission electron microscope are often associated to selective staining methods, to obtain not only the shapes of single particles but also the distribution of different constituent polymers within copolymer and composite particles. Copolymer latexes are obtained with peculiar morphologies, such as core-shell, sandwich-like, and inverted core-shell,3-15 which are relevant to the properties of the dispersed particles, as well as to the properties of the solids and films resulting from their association and aggregation. However, a direct demonstration of the * To whom correspondence should be addressed. E-mail:
[email protected]. † Departamento de Cie ˆ ncias Fı´sicas e Biolo´gicas, Universidade Regional do Cariri (URCA), 60100-100 Crato-Ce, Brazil. (1) Daniels, E. S.; Sudol, E. D.; El-Aasser, M. S. Polymer Latexes: Preparation, Characterization, and Applications, ACS Symposium Series 492; American Chemical Society: Washington, DC, 1992. (2) Poehlein, G. W.; Ottewill, R. H.; Goodwin, J. W. Science and Technology of Polymer Colloids; Martinus Nijhoff: Boston, MA, 1983; Vols. I and II. (3) Cho, I.; Lee, K.-W. J. Appl. Polym. Sci. 1985, 30, 1903. (4) Dimonie, V. L.; El-Aasser, M. S.; Vanderhoff, J. J. Polym. Mater. Sci. Eng. 1988, 58, 821. (5) Lee, D. I.; Ishikawa, T. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 147. (6) Okubo, M. Makromol. Chem., Makromol. Symp. 1990, 35/36, 307. (7) Winzor, C. L.; Sundberg, D. C. Polymer 1992, 33, 3797. (8) Jo¨nsson, J.-E.; Hassander, H.; Jansson, L. H.; To¨rnell, B. Macromolecules 1991, 24, 126. (9) Waters, J. A. Colloids Surf. A: Physochem. Eng. Aspects 1994, 83, 167. (10) Ottewill, R. H.; Schofield, A. b.; Waters, J. A. Colloid Polym. Sci. 1996, 274, 763. (11) Chen, S.-A.; Lee, S.-T. Polymer 1992, 33, 1437. (12) Chen, Y.-C.; Dimonie, V.; El-Aasser, M. S. J. Appl. Polym. Sci. 1991, 42, 1049. (13) Chen, Y.-C.; Dimonie, V.; El-Aasser, M. S. Macromolecules 1991, 24, 3779. (14) Chen, Y.-C.; Dimonie, V.; El-Aasser, M. S. Pure Appl. Chem. 1992, 64, 1691. (15) Rudin, A. Makromol. Chem., Makromol. Symp. 1995, 92, 53.
distribution of the various constituent elements within latex particles has not yet been reported in the literature, to the best of our knowledge. Elemental imaging is currently done using various techniques, for instance, X-ray microprobes, Auger microscopy, and electron spectroscopic imaging (ESI). The principles of ESI and detailed outlines of the possibilities of the method are given by Egerton16 and Reimer.17 ESI is performed using energy-filtered transmission microscopes (EFTEM), and it is adequate for latex study, for the following reasons: (i) it is highly sensitive for the detection of the light elements (carbon, oxygen, nitrogen, sulfur, sodium, and potassium) which are usually found in the latexes, arising from monomers, initiators, and surfactants; (ii) it can reach sub-nanometer resolution; (iii) it is an extremely sensitive technique, which can detect low numbers of atoms of a given element;18-20 (iv) staining is not required, precluding the possibility of image artifacts.21 The ESI technique is rather recent, but it has already been applied to solve many problems of elemental and even chemical distribution within synthetic polymers, by several researchers. Nunes22 et al. have used ESI and electron energy loss spectroscopy EELS (which is also performed using the energy-filtering transmission electron microscope) to study the morphology of a PMMA/silica hybrid material prepared by a sol-gel process. The authors demonstrated that different morphologies in the final material are achieved by using different polymer (16) Egerton, R. F. Electron Energy Loss Spectroscopy in the Electron Microscope; Plenum Press: New York and London, 1986. (17) Reimer, L. Energy-Filtering Transmission Electron Microscopy; Springer Series in Optical Sciences 71; Springer-Verlag: Berlin, 1995. (18) Bauer, R. In Methods in Microbiology; Meyer, F., Ed.; Academic Press: London, 1988; Vol. 20, p 113. (19) Harauz, G.; Ottensmeyer, F. P. Science 1984, 226, 936. (20) Shuman, H.; Somlyo, A. P. Ultramicroscopy 1986, 15, 110. (21) Sawyer, L. C.; Grubb, D. T. Polymer Microscopy; Chapman & Hall: New York, 1987. (22) Silveira, K. F.; Yoshida, I. V. P.; Nunes, S. P. Polymer 1995, 36, 1425.
S0743-7463(97)01167-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/13/1998
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compositions and molecular weights. Wegner23 et al. followed the oxidation of poly(acetylene) by using ESI and EELS. They used the distribution of BF4- and PF6- as an indicator for the penetration of poly(acetylene) by the oxidizing agent and showed that oxidation proceeds homogeneously, on 100 nm to several micrometers depths. Gerharz24 et al. used ESI for the direct visualization of the microphase separated domains in unstained ultrathin sections of the poly(styrene-b-methylphenylsiloxane). These authors found that the best way to visualize microdomain morphology, domain size and the interphase in this system is using inelastic dark-field images at ∆E ) 270 eV (structure-sensitive contrast). Leapman25 et al. accurately identified the coexisting phases by ESI without staining, in a blend of polyethylene and polystyrene. The π*-plasmon peak located at the loss-energy of 7 eV was used, which is specific to the polystyrene polymer. Carboni26 et al. have examined acrylic fibers. They found many irregularly shaped microcavities at the periphery of the fibers. Phosphorus was always detected in these microcavities. Cantow27 et al. studied the interface of polystyrene and poly(1,4-dimethyl-2,6-phenylene oxide). From the elemental distribution of carbon and oxygen across the interface the chemical composition of the interphase was obtained. The group led by Cantow has published many other papers in which ESI was applied to the study of polymers.28-31 However, examples of ESI application to latex morphology and elemental distribution are scarce (indeed, we did not locate any example, in the open literature). The subject of the present paper is the determination of the elemental distribution in the composite latex poly[styrene-co-(2-hydroxyethyl methacrylate)], by ESI. This system is made out of a highly hydrophilic polymer, with polystyrene.32 It has attractive features as a biomedical material, to carry enzymes and antibodies.33 These latex particles display anomalous shapes with uneven surfaces. Okubo34 obtained polymerization kinetics data and transmission electron micrographs, and concluded that the particles consist of copolymer chains, including HEMA-rich and S-rich chains by the end of the polymerization. This is consistent with the homogeneous nucleation model proposed by Fitch and co-workers.35 One evidence found by Okubo was the faster consumption of methacrylic monomer, during the early stages of batch (23) Lieser, G.; Schmid, S. C.; Wegner, G. J. Microsc. 1996, 183, 53. (24) Gerharz, B.; Du Chesne, A.; Lieser, G.; Fischer, E. W.; Cai, W. Z. J. Mater. Sci. 1996, 31, 1053. (25) Hunt, J. A.; Disko, M. M.; Behal, S. K.; Leapman, R. D. Ultramicroscopy 1995, 58, 55. (26) Carboni, V.; Lanzillotta, F.; Carpanese, G. F.; Mariani, P.; Barbatelli, G.; Sbarbatis, A.; Cinti, S. J. Microsc. 1991, 162, 185. (27) Klotz, S.; von Seggern, J.; Kunz, M.; Cantow, H.-J. Polym. Commun. 1990, 31, 332. (28) Cantow, H.-J.; Kunz, M.; Klotz, S.; Mo¨ller, M. Makromol. Chem., Makromol. Symp. 1989, 26, 191. (29) Kunz, M.; Mo¨ller, M.; Heinrich, U. R.; Cantow, H.-J. Makromol. Chem., Makromol. Symp. 1989, 23, 57. (30) Kunz, M.; Mo¨ller, M.; Cantow, H.-H. Makromol. Chem. Rapid Commun. 1987, 8, 401. (31) Kunz, M.; Heinrich, U. R.; Mo¨ller, M.; Cantow, H.-J. Prog. Colloid. Polym. Sci. 1988, 77, 238. (32) Kamei, S.; Okubo, M.; Matsuda, T.; Matsumoto, T. Colloid Polym. Sci. 1989, 267, 861. (33) Okubo, M.; Yamamoto, Y.; Uno, M.; Kamei, S.; Matsumoto, T. Colloid Polym. Sci. 1987, 265, 1061. (34) Kamei, S.; Okubo, M.; Matsumoto, T. J. Polym. Sci. Polym. Chem. Ed. 1986, 24, 3109. (35) (a) Fitch, R. M.; Tsai, C. H. In Polymer Colloids; Fitch, R. M., Ed.; Plenum Publishing Corp.: New York, 1971; p 73. (b) Fitch, R. M. In Encyclopedia of Polymer Science and Engineering; Mark, H. F., Kroschwitz, J. I., Eds.; John Wiley & Sons: New York, 1986; Vol. 3, p 727. (c) Hansen, F. K.; Ugelstad, J. In Emulsion Polymerization; Piirma, I., Ed.; Academic Press: New York, 1982; p 51.
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surfactant free emulsion copolymerization. Moreover, these authors found that the particles with anomalous shapes are formed at a polymerization stage when the bulk viscosity of the particles is still low, and phase separation occurs easily. The authors mentioned that they would further demonstrate phase separation by using OsO4 staining of thin cuts, but this work has not yet been published, to the best of our knowledge. More recently, Martı´n-Rodrı´guez36 and colleagues made a detailed examination of these particles using different techniques. They concluded that particles with different hydrophobicities are obtained, depending on the relative amounts of monomers used. The relative hydrophobicities and the characteristics of the polymer chains were determined using an array of techniques (viscosity, contact angle, adsorption, and NMR and IR spectroscopies). These authors also obtained brightfield TEM pictures and concluded that this copolymer latex is formed by small spheres of poly-HEMA, which have coupled together to coat the PS core, which is different from the previous proposal by Okubo. Experimental Section Materials. Water was doubly distilled from an all-Pyrex apparatus and deionized with a Milli-Q (Millipore) water purification system. Styrene (from Estireno do Brasil) was distilled under reduced pressure in nitrogen atmosphere prior to use. 2-Hydroxyethyl methacrylate (reagent grade, from Aldrich) and the initiator potassium persulfate were used as received. Synthesis of the Latex. The latex was prepared by batch surfactant-free emulsion copolymerization of styrene (S) and 2-hydroxyethyl methacrylate (HEMA) following procedures similar to those developed by Okubo37 and Suzawa.38 The amounts of reagents used are as follows: water, 210.2 g; styrene, 31.2 g; 2-hydroxyethyl methacrylate, 4.5 g; potassium persulfate, 0.1086 g. The polymerization was carried out in a 500 mL glass kettle reactor fitted with condenser, thermometer, glass paddle-type stirrer and a gas inlet providing a constant flow of nitrogen gas. The kettle was kept at the required constant temperature (( 2 °C) using a thermostated water bath. The reaction vessel was loaded with water and the monomers, and heated to 70 °C. After 30 min of N2 purging and stirring of the system, the initiator potassium persulfate dissolved in 4.5 cm3 of water was added to the reaction mixture. The polymerization reaction was carried out at 70 °C for 10 h under constant 300-350 rpm stirring. The product was filtered with a 200 mesh steel sieve, to remove coagulated latex. To remove unreacted monomer, oxidation products, and unwanted electrolyte, the resulting latex was dialyzed against water with daily changes over a 2-month period. The dialysate conductivity reached 2 µS/cm, and remained unchanged for 48 h. The dialysis tubing (a Visking membrane from Sigma) was boiled in several quantities of distilled water, prior to use. After dialysis, part of the latex sample was lyophilized using a benchtop glass apparatus, to recover the solid polymer for spectral characterization. The remainder of the sample was dispersed in water, as required for reaching the desired concentration. Monomer conversion was 94.4%, as determined gravimetrically. The mean particle size diameter was determined by dynamic light scattering using a Coulter N4MD apparatus, from Coultronics. Average diameter is 354 ( 60 nm. The ζ-potential of this latex in KCl 10-3 M is -56.4 mV, as obtained by eletrophoretic measurements with a Brookhaven ZetaPlus instrument. The comonomer content of this latex was determined (36) Martı´n-Rodrı´guez, A.; Cabrerizo-Vı´lchez, M. A.; Hidalgo-A Ä lvarez, R. Colloids Surf. A: Physicochem. Eng. Aspects 1996, 108, 263. (37) Kamei, S.; Okubo, M.; Matsuda, T.; Matsumoto, T. Colloid Polym. Sci. 1986, 264, 743. (38) Tamai, H.; Fujii, A.; Suzawa, T. J. Colloid Interface Sci. 1987, 116, 37.
PS/HEMA Latex Particle Elemental Imaging by 1H NMR at room temperature, using a Varian Gemini 300 spectrometer operating at 300.08 MHz for 1H. The amount of HEMA found is 11% by mol. A more detailed study on the characterization of this latex (MW, monomer distribution, sequence analysis) will be published in another report, following the usual procedures.39 Electron Microscopy. The morphology and the elemental distribution within latex particles, as determined by ESI, was observed using a Carl Zeiss CEM 902 transmission electron microscope, equipped with a Castaing-Henry energy filter spectrometer within the column. The spectrometer uses inelastic electrons to form element specific images. When the electron beam passes through the sample, interaction with electrons of different elements results in characteristic energy losses. A prism-mirror system deflects electrons with different energies to different angles so that only electrons with a well defined energy can be selected. If elastic electrons only are chosen (∆E ) 0 eV), a transmission image with reduced chromatic aberration is obtained. When monochromatic inelastically scattered electrons are selected, electron spectroscopic images (ESI) are formed, in which contrast is dependent on the local concentration fluctuations of a particular element chosen. Clear areas correspond to element-rich domains. Whole individual latex particles as well as microtomed thin sections were examined. Each technique has some advantages. In the first case, the particles are unharmed by foreign chemicals or by stresses developed during the embedding and shearing steps of cut sample preparation. On the other hand, microtomed cuts have uniform thicknesses, which allow us to isolate the effects associated to the variable thickness of the electron beam path, as it crosses the quasi-spherical particles. For individual latex particle examination, one drop of the latex dispersion (1% solids content) was applied to carbon-coated parlodion films supported in 400 mesh copper grids (Ted Pella). To make sure that the whole particles were not excessively thick, they were first observed using ∆E ) 0 eV electrons, then observed again at ∆E ) 20-50 eV. Image contrast inversion was always obtained, showing that a significant number of electrons was transmitted throughout the particles.40 This observation is understood, considering that the 80 keV electrons mean free path within these latex particles is greater than 160 nm for elastic scattering,41 and is estimated as many hundreds of nanometers, for inelastic scattering.42 Thin sections were obtained as follows: latex film was produced by drying the dispersion in a Pyrex glass Petri dish (o.d. 50 mm) at 60 °C in ambient atmosphere, for 24 h. A piece of this latex film measuring roughly 6 × 0.5 mm was embedded in an Epon-Araldite resin (Poly/Bed 812 from Polysciences) using a silicone rubber mold with 5 × 12 × 4 mm cavities, and cured for 48 h at 60 °C. Microtomed thin sections of approximately 80 nm of the latex film were cut in an Ultracut Reichert-Jung microtome using a diamond knife (Drukker) at laboratory temperature (∼27 °C) and mounted on uncoated 400 mesh copper grids (Ted Pella). Elemental images were observed for the relevant elements found in this sample, using monochromatic electrons corresponding to the carbon K-edge, oxygen K-edge, sulfur L-edge and potassium L-edge with an energy-selecting slit of 15 eV in width. The energy-selecting slit was set at 278 ( 6 eV for C, 532 ( 6 eV for O, 165 ( 6 eV for S, and 292 ( 6 eV for K. The images were recorded by a MTI-Dage SIT-66 camera and digitized (512 × 480 pixels, 8 bits) by an IBAS image analyzer software from Kontron running on an IBM PC-AT compatible microcomputer. The three-window technique was used to perform the background subtraction, for each elemental image.43 The actual procedure (39) Herzog Cardoso, A.; Moita Neto, J. M.; Cardoso, A.; Galembeck, F. Colloid Polym. Sci. 1997, 275, 244. (40) We are grateful to Dr. W. Probst (LEO-Zeiss Elektronenmikroskopie Gmbh) for this valuable private communication. (41) Newbury, D. E. In Principles of Analytical Electron Microscopy; Joy, D. C.; Romig, A. D., Jr., Goldstein, J. I., Eds.; Plenum Press: New York, 1986; p 8. (42) Reference 27, p 20. (43) Reimer, L.; Zepke, U.; Moesch, J.; Schulze-Hillert, St.; RossMessemer, M.; Probst, W.; Weimer, E. EELS Spectroscopy: A Reference Handbook of Standard Data for Identification and Interpretation of Electron Energy Loss Spectra and for Generation of Electron Spectroscopic Images; Carl Zeiss: Oberkochen, Germany, 1992.
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Figure 1. Brightfield transmission electron micrographs of PS/HEMA copolymer latex particles. The electron beam was energy filtered, to eliminate inelastically scattered electrons (∆E ) 0). Key: (a) lower magnification; (b) higher magnification. is as follows: the sample is put under focus at ∆E ) 250 eV, so that only electrons which lost this energy to the sample are used in image formation. Then, three pictures of the sample are obtained for each element: one at the higher energy (A) above the absorption threshold, another at the lower energy (B) and a third image at a still lower energy (C). The two low-energy images (Band C) are used to obtain a background image (D) extrapolated to the higher energy level. Finally, a difference image is obtained, subtracting (D) from (A). This is the elemental map.43 Densitometric line scan particle analysis was performed in an IBM PC microcomputer using the public domain Image Tool 1.27 version image analyzer program (developed at the University of Texas Health Science Center in San Antonio and available on the Internet at http://www.ddsdx.uthscsa.edu). The line scans are plots of pixel gray level as a function of position, and the gray level scale used varies from 0 (full black) to 255 (full white). Atomic Force Microscopy. The dry latex film obtained as described above was examined by atomic force microscopy (AFM) in order to observe individual latex particle morphology. The AFM measurements were carried out with a Topometrix Discoverer instrument operating in the true noncontact mode, and using a silicon probe tip.
Results Whole Particle Pictures. Brightfield Images. Brightfield images of whole latex particles obtained by photographic recording are in Figure 1. The images show two characteristics. First, the latex particles are roughly spherical and particle size is highly uniform (average 350 nm diameter), but the particle surfaces are made of convoluted protrusions, as in the “raspberry-like” particle model. The dimensions of the surface protrusions are roughly 20 × 45 nm. Moreover, despite the particle size uniformity there is also a significant population of smaller particles (ca. 60-100 nm diameter).
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Figure 2. Noncontact mode AFM images of PS/HEMA copolymer latex: (a) a self-assembled array of particles (the image size is 10 µm × 10 µm); (b) a raspberry particle morphology.
A second characteristic of the particle surfaces is interesting: some high-electron density areas are seen in the brightfield pictures at some points, often in the apexes of surface protrusions. Given the particle size and the focus depth of the microscope under the present conditions (roughly one-tenth of the diameter of the particles, in this work), it is not possible to have all the protrusions in focus, at the same time. However, we can distinctly observe sharp contrast variations, at the edges of some protrusions. These were verified by observing individual protrusions in distinct under- and overfocus positions (this control is required, to avoid misleading conclusions from having objects at different focus settings, in the same picture). Sharp contrast areas in protrusions are seen, in the enlarged picture in Figure 1b. From these sharp contrast changes, we conclude that there are discrete domains of variable electron density (and thus of variable composition), in the protrusions. Considering the chemical characteristics of this sample, we conclude that these areas are enriched in elements heavier than carbon, and they are enriched in either oxygen, sulfur (from initiator residues) or potassium (which is the counterion to the sulfate initiator residues). Similar observations can be made in the pictures published by Okubo34 and Martı´n-Rodrı´guez,36 but these authors did not comment on them. AFM Images. AFM images of solid films obtained by drying the latex particles show that they pack with an impressive regularity (Figure 2a; a separate paper on the self-assembled arrays of these particles will be published elsewhere). Particle examination under high magnification (Figure 2b) confirms that particles are roughly spherical, but their surfaces are covered with small protrusions, resembling deformed spheroidal bodies with ca. 40-60 nm diameter. In some areas, the pro-
Figure 3. Elemental maps of PS/HEMA copolymer latex particles: (a) brightfield elastic image; (b) carbon map; (c) oxygen map; (d) sulfur map; (e) potassium map. The white lines across the particles indicate the selected region of the particle in which the pixel intensity profile was registered.
truding bodies appear as if they were made of two or more sintered spheroids. Energy-Filtered Images of Whole Particles. Digitally recorded elastic and elemental images of whole particles are in the plate in Figure 3. Line scans of the pixel intensity level across corresponding areas in the different images are in Figure 4. A 0 (gray level zero intensity, black) to 255 (maximum intensity, white) scale is used in these plots. Element Distribution in the Larger Particles. Observation of Figure 3 reveals the following: in the larger particles, carbon signal intensity is a maximum in the particle center and decreases radially. This shows that the amount of carbon is a maximum in the center of each particle image and decreases toward the surface, as expected assuming that carbon distribution is quasiuniform within spherical particles. The brightness observed at the particle central areas shows that the inelastically scattered component of the electron beam emerging from the particle is still very intense, even at the particle centers. A second feature of the carbon images is the existence of a small domain brighter than the surrounding areas, between the two larger particles. This is an evidence for the accumulation of a C-rich component (for instance, polystyrene chains) in these spots. On the other hand, oxygen, which comes from the methacrylic monomer and, to a lesser extent, from initiator residues, is distributed throughout each particle, but it accumulates at the particle surfaces; this is shown by the abrupt jump (ca. 70 units, in the 0-255 gray level scale)
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Figure 4. Line-scan profiles of pixel intensity across elastic and elemental images in whole larger (a-e) and smaller (f-j) PS/ HEMA copolymer latex particles. Key: (a, f) brightfield images; (b, g) carbon maps; (c, h) oxygen maps; (d, i) sulfur maps; (e, j) potassium maps.
in oxygen signal intensity, at the particle surfaces (see Figure 4c). This observation is understood, considering the hydrophilic nature of the oxygenated methacrylic monomer, which leads to its accumulation in the particle surfaces. Oxygen images and the corresponding line scans (Figures 3c and 4c) reveal a very important feature: there is a contrast within the particles, which is not observed in the interparticle region. This is a definite evidence for the existence of domains, richer in the methacrylic monomer than others. Sulfur is also found throughout the particles; its distribution pattern is similar to that of carbon, rather than oxygen. Moreover, the interparticle region has a low content of S, as opposed to C and even
O. This shows that initiator residues are trapped within the particles. Finally, the potassium elemental image is completely different from all others. First, the brightness decreases toward the particle center; this indicates that most potassium ions are strongly concentrated at the outermost part of the large particles. Second, it is the only elemental image in which the perimeter of the particles superimposes well with the perimeter of the brightfield image, which confirms that these ions make a significant contribution to the overall content of the particles outer layers. Element Distribution in the Smaller Particles. Concerning the relationship between the large and the
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Figure 5. Elastic brightfield transmission electron micrograph of unstained ultrathin sectioned PS/HEMA copolymer latex.
small particles, a comparison between the relative image intensities (see Figure 4f-j) allows us to conclude that the small particles have a larger relative content of O and K, as compared to the larger particles. K distribution is rather uniform in the small particles, and K surface accumulation is barely detectable. On the other hand, they have a low sulfur content and a relatively lower C content. This suggests that these particles are largely made out of methacrylic-rich polymer chains. Ultramicrotome Particle Cuts. Brightfield Image. Figure 5 displays a brightfield micrograph of embedded particle thin cut. The observation of these images is required, because the contrast observed in sections of uniform cuts is solely due to differences of domain cross-sections throughout the sample, and thus to changes in the chemical composition rather than topographical changes. The observations made out of this micrograph are as follows. (i) Particle images correspond to ellipsoids, rather than spheres. This is coincident with the shapes observed in pictures of embedded latex particles presented by many authors44,45 and is assigned to the shearing deformation of the embedded particles; ii) there is a discernible contrast within each particle, which is consistent with the idea of chemical heterogeneity within particles. Energy-Filtered Images. Figure 6 presents elastic and elemental images (O and S) from thin microtomed latex film (∼ 80 nm), and the corresponding line scans are in Figure 7. There are two line scans for each picture, to help evaluating the reproducibility of the results. The information obtained from thin section images is more limited, because the contrast of C and K images is too low (images not shown). In the case of C, this is assigned to the low contrast between the particles and the embedding polymer, due to the presence of C in both media. The lack of K contrast, on the other hand, is probably assigned to this element being ion-exchanged out of the particle surfaces, during the embedding process. The O and S elemental images confirm that there is a significant amount of oxygen and sulfur, within the particles. However, there is not any definite increase or decrease in the O concentration from the particles centers to their surfaces.
Figure 6. Elemental maps of unstained ultrathin sectioned PS/HEMA copolymer latex particles: (a) elastic image; (b) carbon map; (c) oxygen map; (d) sulfur map; (e) potassium map. The white lines across the particles indicate the selected region of the particle in which the pixel intensity profile was registered.
A model is now proposed for the morphology of this copolymer latex, which takes into account all of the
reported observations. The general picture is as follows: the larger, prevalent particles are a mosaic of domains of variable chemical composition, which are enriched in one or more of the relevant constituent elements. The negative charge sites associated with the presence of sulfate groups (from initiator residues) are distributed throughout the particles, but their respective counterions accumulate at the particle periphery. The observation of sulfur distributed throughout the particle agrees with the information about the nonexposure of a significant fraction of charged sulfate groups, which arises from latex acid-base titration data, in the literature.46,47
(44) Zhang, L.; Heisenberg, A. Science 1995, 268, 1728. (45) Jo¨nsson, J.-E.; Hassander, H.; To¨rnell, B. Macromolecules 1994, 27, 1932.
(46) Smitham, J. B.; Gibson, D. V.; Napper, D. H. J. Colloid Interface Sci. 1973, 45, 211. (47) van den Hul, H. J.; Vanderhoff, J. W. Br. Polym. J. 1970, 2, 121.
Discussion
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Figure 7. Line-scan profiles of pixel intensity across elastic and elemental images of thin-sectioned PS/HEMA copolymer latex particles; (a) brightfield images; (b) oxygen maps; and (c) sulfur maps.
There are two major contributions for the O distribution in the particles: both sulfate residues and methacrylic polymer contain oxygen, which is rather evenly distributed from the particle center to the periphery, but with a detectable accumulation in the later. Since there is a much larger amount of methacrylic than sulfate, we believe that the O elemental image reflects the methacrylic distribution, rather than the distribution of initiator residues. The conclusion is that the methacrylic copolymer chains are dispersed throughout the particle, with significant spatial variations. However, the polymer chains at the outermost layers are enriched in methacrylic groups. This means that these are not strictly core-and-shell particles, but they have a higher content of hydrophilic components at the outermost layer. The comparison of C/O signal intensities in the smaller and larger particles shows that the small particles are methacrylic-rich. Moreover, the corresponding line scan does not show any abrupt jump at the particle borders, showing that methacrylic monomer is more evenly distributed, in the small particles. The greatest difference between small and large particles is in the case of K distribution, since this is
concentrated at the surface of the later but more evenly distributed in the former. This is understood, considering that the small particles are more polar and consequently they should be more strongly hydrated, allowing for a greater extent of K sorption, while they are dispersed in water. When the particles are dried, the potassium ions remains in their interior. These results show that charge distribution is quite different, in the large and small particles. In the former, electrical charges are separated resembling the pattern observed in clay particles.48,49 In the later, there is much less pronounced charge separation, which could perhaps be perceived (e.g., in electrophoresis experiments) as a low surface charge concentration. This hypothesis can be verified by fractionating the particles and determining the electrical properties of both particle types. Work is now in progress in this laboratory, in this direction. The AFM images presented in this work do not give any information about the elemental distribution in the (48) Sposito, G. The Surface Chemistry of Soils; Oxford University Press: New York, 1984. (49) Grandjean, J. J. Colloid Interface Sci. 1997, 185, 554.
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particles. However, we note a significant contrast in the particle protrusions, represented by clearer and darker points. This observation should also be refined, perhaps using other scanning sensing heads or other observation modes. To conclude, the present results agree with the particle formation hypothesis put forward by Okubo34 and coworkers, and they are not consistent with the other hypothesis in the literature, of poly-HEMA small spheres banded together to coat a polystyrene core. This means, polymer chains are homogeneously nucleated and grow in solution, until they are phase separated, but polymerization continues incorporating both monomers. The resulting chains form growing polymer-swollen particles, in which the chains are distributed, so that the HEMAricher chains are more concentrated at the particle periphery. This distribution is maintained within the dry particles, due to their Tg values being higher than room temperature.
Herzog Cardoso et al.
Conclusion The particles of styrene-2-hydroxyethyl methacrylate copolymer are made out of intermeshed domains of variable chemical composition, consistent with the homogeneous nucleation model adopted by Okubo and colleagues. Moreover, at least two different elemental distribution patterns are observed, associated with particles of different sizes. In the more apolar (higher C/O ratio) and larger particles, ionic groups are mostly separated: sulfate is concentrated in the particle bulk, potassium is in the particle surface. In the methacrylicrich, smaller particles, both types of charged groups are dispersed throughout the particles. Acknowledgment. F.G. acknowledges grants from FAPESP, PRONEX/FINEP/MCT, and CNPq. A.H.C. was awarded a PICDT-CAPES graduate fellowship. LA971167H
