10576
Langmuir 2004, 20, 10576-10582
Heterogeneity in Styrene-Butadiene Latex Films Juliane P. Santos,†,‡ Pascale Corpart,§ Kenneth Wong,‡ and Fernando Galembeck*,† Institute of Chemistry, Universidade Estadual de Campinas, P.O. Box 6154, 13083-970 Campinas SP, Brazil, Rhodia Recherche CRA, 93308 Aubervilliers, France, and Physical-Chemistry Laboratory, Rhodia Brasil Ltda, P.O. Box 921, 13140-000 Paulı´nia SP, Brazil Received July 6, 2004. In Final Form: September 14, 2004 Low-Tg styrene-butadiene (SB) latex films were investigated by noncontact atomic force microscopy and scanning electric potential microscopy, revealing a number of different morphologies and electric potential patterns across films cast from the same SB latex dispersions under the same conditions. Surface leveling and charge dispersion throughout the films are, thus, restrained even at temperatures above Tg and the minimum film-formation temperature. An unprecedented electric pattern is observed, in which the particle cores are more positive than the contacting particle outer layers. Different packing patterns, including cubic and hexagonal arrays, coexist in neighboring areas. Zonal centrifugation of the SB latex in sucrose density gradient shows that particles cover a broad range of densities. Thus, film surface heterogeneity is at least partly due to particle heterogeneity. Fractal dimensions of topographic profiles are lower than those of the electric potential profiles, showing that charge mobility is much more restrained than polymer chain motion at the film surface and that it imposes a limit to the charged chain-ends motion.
1. Introduction The physicochemical characteristics of latex films and dispersions have been extensively investigated as a result of their importance for academic and industrial applications. Latexes have been considered as model colloids1 because of their impressive narrow particle size distribution. However, other characteristics of the latex dispersions such as distribution of chemical groups throughout the particle surface and core, as well as comonomer, molecular weight, and charge distribution within the particle, and also the reorganization of the chemical constituents within the particles when they move from one environment to another (i.e., the transformation from an aqueous to a dry environment) have been scarcely explored. Many researchers have assumed that these properties do not change from one particle to another, but others have observed unexpected latex behavior due to particle heterogeneity. For instance, Buscall and Ottewill2 suggested that the anomalous coagulation behavior of some latexes under NaCl concentrations lower than the critical coagulation concentration could be due to particle heterogeneity. This unexpected dispersion instability can be predicted by a model3 that takes into account the nonuniform distribution of charges on particle surfaces instead of the uniform charge distribution generally used in the classical models. Latex heterogeneity can be related to the following aspects: (i) particle heterogeneity within the dispersions due to the emulsion polymerization reaction1,4 and (ii) film heterogeneity due to latex particle heterogeneity inherent to the dispersion;1 latex particle heterogeneity * To whom correspondence should be addressed. E-mail:
[email protected]. † Universidade Estadual de Campinas. ‡ Rhodia Brasil Ltda. § Rhodia Recherche CRA. (1) Galembeck, F.; Souza, E. F. In Polymer Interfaces and Emulsions; Esumi, K., Ed.; Marcel Dekker: New York, 1999; p 119. (2) Buscall R.; Ottewill, R. H. Polymer Colloids; Elsevier: London, 1985; chapter 5. (3) Velegol, D.; Thwar, P. K. Langmuir 2001, 17, 7687. (4) Vanderhoff, J. W. Chem. Eng. Sci. 1993, 48, 203.
inherent to the reorganization of the chemical constituents within the particles and serum when they move from the aqueous to the dry environment;5-7 surfactant migration;8-16 and complex drying patterns.17-21 The presence of any kind of heterogeneity influences the properties of the latex dispersions and respective films. For example, it is common in the industry to have problems with the unexpected coagulation and different amounts of coagulum from one to another batch of the same product. Moreover, latex film heterogeneity can influence film properties such as adhesion, mechanical strength, water absorption, and film brightness. There is a lack of techniques that allow accessing latex particle heterogeneity, and, thus, information in the literature concerning latex heterogeneity is limited. A preparative technique suitable for the detection of heterogeneity in latex particle dispersions is the zonal (5) Teixeira-Neto, E.; Galembeck, F. Colloids Surf., A 2002, 207, 147. (6) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1999, 15, 4447. (7) Teixeira-Neto, E.; Kaupp, G.; Galembeck, F. J. Phys. Chem. B 2003, 107, 14255. (8) Bradford, E. B.; Vanderhoff, J. W. J. Macromol. Chem. 1966, 1, 335. (9) Joanicot, M.; Wong, K.; Richard, J.; Maquet, J.; Cabane, B. Macromolecules 1993, 26, 3168. (10) Distler, D.; Kanig, G. Colloid Polym. Sci. 1978, 256, 1052. (11) Juhue´, D.; Wang, Y.; Lang, J.; Leung, O.-M.; Goh, M. C.; Winnik, M. A. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1123. (12) Malle´gol, J.; Gorce, J.-P.; Dupont, O.; Jeynes, C.; McDonald, P. J.; Keddie, J. L. Langmuir 2002, 18, 4478. (13) Aramendia, E.; Malle´gol, J.; Jeynes, C.; Barandiaran, M. J.; Keddie, J. L.; Asua, J. M. Langmuir 2003, 19, 3212. (14) Belaroui, F.; Cabane, B.; Dorget, M.; Grohens, Y.; Marie, P.; Holl, Y. J. Colloid Interface Sci. 2003, 262, 409. (15) Belaroui, F.; Hirn, M. P.; Grohens, Y.; Marie, P.; Holl, Y. J. Colloid Interface Sci. 2003, 261, 336. (16) Belaroui, F.; Grohens, Y.; Boyer, H.; Holl, Y. Polymer 2000, 41, 7641. (17) Hwa, J. C. H. J. Polym. Sci., Part A-2 1964, 785. (18) Winnik, M. A.; Feng, J. J. Coat. Technol. 1996, 68, 39. (19) Routh, A. F.; Russel, W. B. AIChE J. 1998, 44, 2088. (20) Routh, A. F.; Russel, W. B. Langmuir 1999, 15, 7762. (21) Salamanca, J. M.; Ciampi, E.; Faux, D. A.; Glover, P. M.; McDonald, P. J.; Routh, A. F.; Peters, A. C. I. A.; Satguru, R.; Keddie, J. L. Langmuir 2001, 17, 3202.
10.1021/la048319a CCC: $27.50 © 2004 American Chemical Society Published on Web 10/27/2004
Heterogeneity in SB Latex Films
centrifugation in the density gradient.1 Using this technique, it was possible to separate a polystyrene homopolymer, an industrial polyvinyl alcohol, and other latexes into many particle fractions1 that were analyzed by suitable spectroscopic techniques (infrared and nuclear magnetic resonance). For copolymerized latexes, the comonomer distribution among particles can be obtained using the zonal centrifugation in the density gradients only when there is a correlation between monomer content and particle density. A technique that has been successfully employed on the characterization of irregular charge distribution within latex particles is rotational electrophoresis.22-24 Some interesting results have shown that bare polystyrene particles present significant charge nonuniformity22 that is minimized by the adsorption of ionic surfactant within the particles23 or by annealing the particles above their Tg.24 Furthermore, particle heterogeneity within films can be accessed by ESI-TEM (energy-loss spectroscopy imaging in transmission electron microscopy),6 AFM (atomic force microscopy), SEPM (scanning electric potential microscopy),5 and BEI (backscattered electron imaging).5 Heterogeneity in latex particles within submonolayers was evidenced by this group.5-7 The irregular distribution of sulfate and potassium ions in dry PS-HEMA {poly[styrene-co-(2-hydroxyethyl methacrylate)]} latex particles was found by using ESI-TEM.6 In another case, the asymmetric distribution of ionic and other constituents in PS-AAM [poly(styrene-co-acrylamide)] latex films was detected by SEPM.5 SEM (scanning electron microscopy), AFM, and nearfield optical microscopy images show the clustering of particle sub-populations with similar chemical compositions within a PS-AAM submonolayer.7 Heterogeneity in latex films due to surfactant migration8-16 has been demonstrated in several cases. Bradford and Vanderhoff8 were the pioneers in showing the nonhomogeneity in latex films. They observed, by monitoring latex film formation by TEM, that the elimination of particle contours is concurrent to the exudation of some materials from the interior of the latex film, which were identified as the stabilizers from the latex dispersions. Heterogeneity in coalesced SBA latex films was detected by TEM.9 The TEM micrographs showed the presence of lumps assigned to hydrophilic materials discontinuously distributed throughout the coalesced film. Distler and Kanig10 have suggested that the presence of nonhomogeneous morphologies within latex films can be related to the incompatibility between hydrophilic and hydrophobic polymer. As an evidence of this inhomogeneity, they showed that a normally transparent film might turn opaque, or even show iridescence, when swollen with water. An interesting contribution showing the effect of irregular lateral drying on surfactant distribution along the film surface was reported by Juhue´ et al.11 For the films in which a drying front moves from the edges inward, they observed at the central area of the film (“the last drop”) that the rate of surfactant migration to the surface was much higher than at the other sites of the film. They suggested that the surfactants were accumulated at the central area of the film during the drying process. Keddie et al.12 have also observed an excess of watersoluble species (surfactant, counterions) at the polymer(22) Feick, J. D.; Velegol, D. Langmuir 2002, 18, 3454. (23) Feick, J. D.; Chukwumah, N.; Noel, A. E.; Velegol, D. Langmuir 2004, 20, 3090. (24) Feick, J. D.; Velegol, D. Ind. Eng. Chem. Res. 2004, 43, 3478.
Langmuir, Vol. 20, No. 24, 2004 10577
air interface. The water-soluble species are driven to the surface by the water flow during the drying stage, and when water evaporates they are deposited between the low-Tg particles inhibiting their coalescence. In another work,13 the same authors compared the distribution of conventional and reactive surfactants at the polymer surface. Within coalesced films, conventional surfactant exudates to the surface, but the exudation of reactive surfactant is very low. Recently, Holl et al.14 followed the desorption of labeled sodium dodecyl sulfate from a monodisperse core-shell latex by small-angle neutron scattering. They observed that most surfactant desorbs from the particles when the dispersion contains 20 wt % water. In another paper from the same group,15 surfactant distribution along the latex film thickness and surface was evaluated by using confocal Raman and attenuated total reflection infrared. There was an excess of surfactant at the film-air and filmsubstrate interfaces. The migration of surfactant to the interfaces is mainly driven by its tendency to lower the interfacial energy of the polymer-air and polymersubstrate interfaces.16 In this work, we report a number of different morphologies and electric potential patterns in styrene-butadiene (SB) latex films prepared from a single latex batch, as evidenced by AFM and SEPM. We also present some results on zonal centrifugation in the density gradient of the same SB latex. 2. Experimental Section 2.1. Preparation of the Latex. The particles were made through a semi-continuous procedure; monomers were continuously added to an initial start-up load. For the SB latex, the composition of the reaction mixture was as follows: styrene (62%), butadiene (30%), ammonium persulfate (4%), and acrylic acid (4%). A small amount of surfactant was added to control the number and the size of the growing particles. The important feature of this polymerization reaction is that the pH must be low; indeed, if the acrylic acid is neutralized, it tends to remain dissolved in the serum instead of binding to the particle core.25,26 2.2. Sample Preparation. The SB latex used in this work is the original latex dispersion obtained from the emulsion polymerization reaction without further purification. The films of SB latex were prepared by spreading one drop of the dispersion (50% w/w) on freshly cleaved mica and drying at 25 °C in 4448% relative humidity during 24 h. After drying the dispersion for approximately 10 min, there is the formation of a skin at the dispersion surface. The dispersion dries laterally from the edges to the center, and the microscopy examination was done in spots halfway between the edge and the center of the dry film. 2.3. AFM and SEPM. AFM and SEPM images were acquired from the same sample area, at the same time, in a Topometrix Discoverer scanning probe microscope. The noncontact AFM mode was used to obtain topographic information on the latex films. Topographic changes were sensed by monitoring the detector signal amplitude at a 300 × 300 pixel resolution and 1-µm/s scan rate. The instrument was calibrated with a standard grid formed by a regular array of 0.9-µm-spaced SiO2 cylinders with diameters of 1.1 µm and heights of 240 nm, on a Si surface. The probes used were made of silicon coated with platinum with a nominal radius of 20 nm, which was verified by imaging the tip in a field-emission gun scanning electron microscope. SEPM in the Topometrix Discoverer microscope uses the standard noncontact AFM setup but with the following modifications: the Pt-coated Si tip is fed with an alternating current (AC) signal, 10 kHz below the frequency of the normal AFM oscillator, which matches the natural frequency of mechanical oscillation of the cantilever-tip system (40-70 kHz). During a measurement, the mechanical oscillation of the tip is tracked by (25) Richard, J. Polymer 1992, 33, 562. (26) Greene, B. W. J. Colloid Interface Sci. 1973, 43, 462.
10578
Langmuir, Vol. 20, No. 24, 2004
Santos et al.
Figure 1. Noncontact AFM and SEPM images of the SB latex film on mica. Areas displaying hexagonal and cubic packing are seen within the white rectangles located at the center and bottom right side of the image, respectively. Table 1. Characteristics of the SB Latex
SB latex
solid content (wt %) 50
pH
zeta potential (mV)
effective diameter (nm)
conductitvity (mS/cm)
viscosity (cP)
MFFT (°C)
Tg (°C)
1.9
-61
153 ( 4
8.4
34.2
23
25
the four-quadrant photodetector and analyzed by two feedback loops. The first loop is used in the conventional way to control the distance between the tip and the sample surface, while scanning the sample at a constant oscillation amplitude. The second loop is used to minimize the electric field between the tip and the sample: a second lock-in amplifier measures the AC frequency oscillation while scanning and adds a DC bias to the tip, to cancel the phase displacement in the AC oscillation. This technique differs from the one used by Terris et al.,27 who measures the phase displacement of the AC voltage; in the Topometrix setup, the phase displacement is canceled by direct current (DC) biasing. The images are built using the DC voltage applied to the tip, at every pixel, thus, detecting electric potential gradients throughout the scanned area. This technique is reminiscent of the oscillating electrode technique for monolayer surface potential measurements:28 both use an oscillating electrode separated from the sample by an air gap. The major difference between both techniques is the detection technique used, because SEPM uses a phase detection of the mechanical oscillation generated in the frequency of the applied voltage. The SEPM technique has a good lateral resolution as compared to other electric mapping techniques, because the sample-to-tip distance is kept at 10 nm only, while the methods based on electric force measurements depend on scanning at 40 nm or more. A detailed study of the errors introduced by scanning at different heights as well as the tip shape was published by Hong et al.29 Image processing as well as fractal dimension calculations were performed in a personal microcomputer using the Topometrix analysis program. 2.4. Zonal Centrifugation in the Density Gradient. The SB latex was fractionated by zonal centrifugation in a density gradient. The centrifugation experiments were performed using a linear density gradient made by mixing water and an aqueous sucrose solution (20 wt %) using a two-chamber mixing cell connected to a peristaltic pump.30 A total of 100 µL of SB latex (2 wt %) was layered on the top of the preformed sucrose density gradient, followed by centrifugation at 15 krpm for 2 h in a Sorvall RC26 Plus centrifuge. The densities of the bands obtained were determined by imaging the centrifuge tubes with a digital camera and line-scanning the tube pictures, along the tube axis, for the acquisition of scattered line profiles with the Image-Pro Plus 4.0 (Media Cybernetics) software. (27) Terris, B. D.; Stern, J. E.; Rugar, D.; Mamin, H. J. Phys. Rev. Lett. 1989, 63, 2669. (28) Shaw, D. J. Introduction to Colloid and Surface Chemistry; Butterworth: London, 1996. (29) Hong, J. W.; Park, S.; Khim, Z. G.; Hou, A. S. Appl. Phys. Lett. 1996, 69, 2831. (30) Neto, J. M. M.; Monteiro, V. A. R.; Galembeck, F. Colloids Surf., A 1996, 108, 83.
3. Results Table 1 presents the main physicochemical characteristics of the SB latex studied in this work. Latexes are often stable at basic pH because of electrostatic repulsion among ionized groups present in the particles, and an interesting feature of this SB latex is its stability even at low pH. This suggests that steric effects can also stabilize the SB latex. The zeta potential and viscosity of the latex are constant in the pH range of 2-10. The films are transparent, because the MFFT (minimum film-formation temperature) is 23 °C, and they were cast at 25 °C. SEPM and AFM images of the most often observed topographic and electric potential patterns within four SB latex films prepared under the same conditions are shown in the Figures 1-4. The AFM and SEPM images are from domains located between the edge and the center of the films. The images were examined, and some relevant topographic and electric potential parameters were calculated. Some image features are compared in Table 2. In Figure 1, the AFM image shows deformed particles packed in hexagonal (image center) or cubic arrays (right end of the image). The SEPM image shows that the ionic constituents responsible for the observed electric potentials are unevenly distributed across the map: the negative charge components tend to accumulate in the interparticle regions, thus, creating a core-shell structure. However, both the core and the shell show small contrasting spots, revealing that they are also electrically nonuniform at a shorter distance scale. Figure 2 presents SEPM and AFM images from another area of the SB latex film. The AFM image shows that the particles are also hexagonally distorted. The degree of particle distortion is highly variable, and a few particles depart largely from the predominating behavior: in a few cases, the deformation is so intense that the particles actually appear as lozenges, while in others they appear more isometric. These observations are very important to exclude the hypothesis of apparent particle distortion due to scanning artifacts. In this field, the particle shell is more negative than the core as well as in Figure 1, but there is an asymmetric distribution of the negative charge domains within the shells: the left downside of the particle shells is often more negative than the others. Figure 3 shows AFM and SEPM images from a third film area. The surface is very smooth, and the height differences across
Heterogeneity in SB Latex Films
Langmuir, Vol. 20, No. 24, 2004 10579
Figure 2. Noncontact AFM and SEPM images of a second area of the SB latex film on mica.
Figure 3. Noncontact AFM and SEPM images of a third area of the SB latex film on mica.
Figure 4. Noncontact AFM and SEPM images of a fourth area of the SB latex film on mica. Table 2. Comparison between Topographic and Electric Potential Features Observed in the Images Shown in Figures 1-4 feature
Figure 1
Figure 2
Figure 3
Figure 4
maximum height (nm) maximum electric potential difference (mV) discernible particle boundaries (AFM) discernible particle boundaries (SEPM) particle shape
64 -0.35 yes yes deformed spheres, forming elongated hexagons in the x-y plane 1.51 1.64
42 -0.33 yes yes deformed spheres, forming elongated hexagons in the x-y plane 1.53 1.72
24 -0.65 no yes flat
29 -0.17 no yes flat
fractal dimension from the AFM line scan fractal dimension from the SEPM line scan
the scanned line are less than 6 nm. Particle borders are not completely discernible in the AFM image, but they are easily perceived in the SEPM micrograph, where the interparticle spaces are more positive than the particle
core areas. Finally, the images in Figure 4 have some points in common with Figure 3, but they also have some marked differences: the film surface is very smooth and particle limits are more visible in the SEPM image, but
10580
Langmuir, Vol. 20, No. 24, 2004
Santos et al.
Figure 5. Fractal dimension determination of line scans from Figures 1 and 2 (see the line position in the inset).
Figure 6. Pictures of the density gradient tubes and light scattering profiles after a fractionation experiment of SB latex (a) and PS reference latex (b).
they are more negative than the particle cores (while in Figure 3 particle interstices were more positive than the cores). The respective line scans together with the box count versus feature size plot used for calculation of the line fractal dimension from the AFM and SEPM images in Figures 1 and 2 are presented in Figure 5. These results are summarized in Table 2. The fractal dimension shows how a rugged line fills the two-dimensional space.31 It is a convenient alternative to the conventional roughness parameters [average roughness (Ra), maximum profile peak height (Rp), maximum height of the profile (Rt)] for describing the surface texture of nonsmooth objects, because previous work from Chesters et al.32,33 has shown that the fractal dimension has a better correlation with other surface properties (i.e., gas adsorption and surface cleanability) than the conventional roughness parameters. The fractal dimension was calculated using the box(31) Kaye, B. H. Chaos and Complexity; VCH: New York, 1993. (32) Chesters, S.; Wen, H. Y.; Lundin, M.; Kasper, G. App. Surf. Sci. 1989, 40, 185. (33) Chesters, S.; Wen, H. Y.; Kasper, G. Solid State Technol. 1991, 34, 73.
counting method. To do this, the line profile is filled with boxes with different sizes. For each given box size, the number of boxes that intersect the profile is counted. The number of box profile intersections (box count) versus their respective box size (feature size) on a log-log scale produces a straight line (Richardson plot).32,33 The slope of the Richardson plot is the fractal dimension of the profile. For the images shown in Figures 1 and 2, the fractal dimensions of the electric potential line scans are higher than those from the topographic line scans. To have some information on whether the origin of the heterogeneity observed in the AFM and SEPM images can be due to the latex dispersion, irregular drying, or both, the SB latex studied in this work was subjected to zonal centrifugation in a sucrose density gradient. Figure 6a shows a picture of the density gradient tube after the SB fractionation experiment together with its scattered light profile. Two fractions were obtained under isopycnic conditions. One fraction is in a narrow band with an average density of about 1.03 g‚cm-3, and the other is broad, with a density spanning over the 1.00-
Heterogeneity in SB Latex Films
1.03-g‚cm-3 range. Figure 6b shows one example of a density gradient for a PS reference latex that presents only one fraction with a density of about 1.04 g‚cm-3.34 4. Discussion The morphological and electric potential patterns of the SB latex films examined in this work are highly variable. This may be due, at least partly, to latex particle heterogeneity.1 Density gradient results have shown that the SB latex particles cover a broad density range, showing that they have significant monomer composition heterogeneity. Particle populations with different chemical compositions can be generated as a result of the influence of many factors of the emulsion polymerization process. Particles formed from monomers with different reactivities and polarities, as in the present case (styrene, butadiene, and acrylic acid) can be formed at different sites through different mechanisms,4 thus, generating particle populations with particular comonomer composition.7 During the film formation, water evaporates from the dispersion leading to an increase in the latex solid content, as well as in the ionic strength. The increase in the electrolyte concentration within the dispersion provokes a decrease of the Coulomb repulsion between the charged particles. Depending on the ionic strength, these charged particles can undergo reversible flocculation or irreversible coagulation.35 Different particle populations within the dispersion can undergo flocculation or coagulation at different salt concentrations, producing segregated domains within the dispersion that settle side by side or interspersed within the films.7 An important factor that can influence film morphology is the irregular film drying.17-21 On the basis of the qualitative model previously developed by Winnik and Feng18 and quantitatively described by Routh and Russel,19 during the drying of a latex dispersion droplet there are drying and particle packing fronts that propagate from the edges of the film toward the center of the dispersion. The propagation of the drying front is controlled by capillary pressure and, thus, by surface tension. Continued water evaporation from the close-packed regions draws water from the central fluid region, transporting particles that settle in the limits of the close-packed region and the remaining dispersion. This flow of solvent through the close-packed region sets a pressure gradient which is balanced by the capillary pressure. The capillary pressure is the result of the curved water surfaces joining adjacent particles. At the point when the pressure at the closepacked region exceeds the capillary pressure, drying from the edges occurs.19,21 The time required for the pressure at the close-packed region to overcome the opposing capillary pressure is called open time. It is a function of the capillary pressure, surface tension, and radial position. The presence of particles with different chemical compositions can affect the interfacial tensions and, thus, the capillary pressure. This can generate points within the film that present different open times. Therefore, although the different areas within the film were all analyzed under the same annealing conditions, the resulting morphologies are also dependent on the open time characteristic of each particle cluster within the film. Furthermore, the different levels of particle deformation, packing, and flattening observed in the micrographs can be explained considering the variations in the surface-tension-driven forces and (34) Teixeira-Neto, E.; Leite, C. A. P.; Cardoso, A. H.; da Silva, M. D. V. M.; Braga, M.; Galembeck, F. J. Colloid Interface Sci. 2000, 231, 182. (35) Lai, S. K.; Wu, K. L. Phys. Rev. E 2002, 66, 041403.
Langmuir, Vol. 20, No. 24, 2004 10581
the resistive viscoelastic forces inherent to each particle subpopulation. Another quite interesting feature is the coexistence of hexagonal and cubic packing within the film area shown in Figure 1, even though the majority of the particles are packed in a hexagonal array. The main difference between these two sets of particles is their shape. The particles within the cubic packing are more elongated than the ones within the hexagonal packing. Previous work in the literature36 showed the coexistence of hexagonal and cubic arrays within the same film of poly(methyl methacrylate)rubber-poly(methyl methacrylate) latex using environmental scanning electron microscopy. The authors pointed out that the appearance of two different packings in the surface does not necessarily imply the occurrence of two structure types in three dimensions. Winnik et al.37 have also demonstrated the occurrence of different arrays, bodycentered cubic and face-centered cubic, within meltpressed PS latex films by freeze-fracture transmission electron microscopy. They have pointed out that the differentiation of these arrays within the films depends on the level of the distortion of the particles within the films that is directly dependent on the experimental conditions in which the films have been prepared. The variable level of particle deformation observed within the AFM image (Figure 2) such as distorted hexagons or lozenge-shaped particles can be related to nonhomogeneous distribution of copolymer domains along the particle itself. Therefore, under a particular stress the particles can be deformed in different ways in the different directions. The presence of particles with nonuniform deformation was also observed previously in this laboratory.7 A very interesting feature of the present SEPM images concerns their patterns of electric potential distribution within the SB latex films. Some SEPM images of the SB latex films show that negative charges are clustered within particle interstices. It is the first time that this pattern is observed in synthetic latex. This pattern was previously observed in natural rubber latex.38 Furthermore, the SEPM images also show the opposite pattern, with shells more positive than the cores within the films. Previous work39-41 with model latexes revealed a majority of cases in which the particle shells are more positive than their cores, and this was assigned to the distribution of sulfate polymer chain end groups throughout the particles while the counterions where accumulated in the interparticle spaces formed by the drying serum. These results from SEPM were confirmed by microchemical mapping by ESI-TEM. The asymmetric distribution of charges within the latex particles as observed in the case of Figure 2 has been previously observed for latexes of PS-AAM5 and PSHEMA.6 The dipolar nature of PS-AAM latex particles5 was demonstrated by using SEPM and BEI, and the interaction of particle dipoles was used to explain the alignment of the particles within films cast on mica. In the case of PS-HEMA,6 the dipolar or multipolar nature of the particles was shown by ESI-TEM through the asymmetric distribution of sulfur and potassium within (36) Chaobin, H.; Donald, A. M. Langmuir 1996, 12, 6250. (37) Sosnowski, S.; Li, L.; Winnik, M. A.; Clubb, B.; Shivers, R. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 1123. (38) Rippel, M. M.; Costa, C. A. R.; Galembeck, F. To be published. (39) Braga, M.; Costa, C. A. R.; Leite, C. A. P.; Galembeck, F. J. Phys. Chem. B 2001, 105, 3005. (40) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Colloids Surf., A 2001, 181, 49. (41) Keslarek, A. J.; Costa, C. A. R.; Galembeck, F. Langmuir 2001, 17, 7886.
10582
Langmuir, Vol. 20, No. 24, 2004
the particle core and shell, respectively. This was assigned to the seggregation of polymer chains with different hydrophilicities within the particle core, which induces a partition of the countercations in the particle shell. The dipolar nature of the PS-HEMA particles was used to explain the high level of particle alignment in the colloidal crystal and in the macrocrystal. The dipolar nature of the latex particles has also been demonstrated by rotational electrophoresis experiments.22,23 The authors observed that bare PS particles present a heterogeneous distribution of charges.22 They suggest that particles with an irregular distribution of charges can be formed by the coagulation of primary particles containing different numbers of charged groups, during the emulsion polymerization reaction or by the conversion of sulfate end groups (from the initiator) to hydroxyl or carboxyl groups, with age. The different morphologies and patterns of the electric potential distribution observed in the present work can be understood considering that the local chemical composition within the latex films controls the partition of polymeric chains and charges throughout these domains. The dissimilarity between AFM and SEPM images shown in Figures 3 and 4 may be understood considering the following arguments: (i) The ionic species are not evenly distributed throughout the film, thus, creating the large electric potential contrast observed even when polymer chain interdiffusion has occurred to a great extension. This feature is consistent with a previous finding in the literature, following which the diffusion of polymer chains containing specific ionic constituents is slower than that of the neutral chains.42 (ii) SEPM has a higher sampling depth, as compared to AFM. The former depends on Coulomb’s law, while the latter depends on short-range van der Waals forces. Consequently, charge clusters beneath the sample are also observed. To confirm which one is the dominant factor, we plan to do experiments evaluating simultaneously the kinetics of charge dispersion and neutral chain migration within latex films by SEPM and AFM, respectively. To evaluate the effect of the SEPM sampling depth, we will investigate particle submonolayers and thick films. These experiments will be performed with an instrument that allows very good environmental control. (42) Kim, S. D.; Klein, A.; Sperling, L. H. Polym. Adv. Technol. 2002, 13, 403.
Santos et al.
The fractal dimension of the lines scanned from the images in Figures 1 and 2 is higher for the electric potential map than for the height distribution, thus, showing that the ionic constituents within the film have a more complex distribution pattern than the neutral chain components. Recalling that mass transfer driven by concentration gradients and by surface tension normally smooth out the surface and, thus, they decrease the fractal dimension, it is possible to suggest that either the ionic constituents diffuse slower than the neutral chains or else their equilibrium distribution is not uniform because of some kind of microphase separation. However, to properly understand the electric potential patterns and their time dependence, further experimental and theoretical developments on the subject of ion distribution throughout polymer phases are necessary. Up to now it is not possible to strictly predict which are the driving forces involved in this complex process nor how the complex electric patterns affect the particle viscoelastic response. 5. Conclusions Films from a low-Tg and low-MFFT latex show a number of topographic and electric potential patterns, even in contiguous areas, and this heterogeneity is at least partly due to particle heterogeneity. Fractal dimensions of topographic and electric potential profiles show that the electric charges in the film are much less mobile than those in the polymer chains and they restrict the mobility of chains ended with charges, thus, preventing full film leveling driven by surface tension. The film heterogeneity revealed in this work may be relevant to some practical problems related to latex coating performance: uneven swelling and the concurrent development of mechanical tensions, nonuniform dyeing, wetting, and adhesion properties. Acknowledgment. J.P.S. acknowledges a fellowship from FAPESP. This work was supported by Pronex/Finep/ MCT and CST/Rhodia. This is a contribution from the Millenium Institute for Complex Materials (PADCT/CNPq). LA048319A
