Electric Charge Clustering and Migration in Latex Films - American

In Final Form: August 30, 2001. Scanning electric potential microscopy images were acquired from coalesced poly (styrene-butyl acrylate- acrylic acid)...
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Electric Charge Clustering and Migration in Latex Films: A Study by Scanning Electric Potential Microscopy Amauri Jose´ Keslarek, Carlos Alberto Rodrigues Costa, and Fernando Galembeck* Institute of Chemistry, Universidade Estadual de Campinas, Campinas SP, 13083-970, Brazil Received May 29, 2001. In Final Form: August 30, 2001 Scanning electric potential microscopy images were acquired from coalesced poly (styrene-butyl acrylateacrylic acid) latex films having a Tg equal to 15.9 °C. The micrographs show the boundaries between particles even in transparent films (transmittance > 99%), and the boundaries appear as positive domains, relative to the particle cores. Large local electric potential gradients are observed, ca. 1 V/10 nm, but these are smeared when the latex is dialyzed prior to film formation. Electric domain patterns are modified by aging, annealing and exposure to toluene or chloroform vapors. Completely independent patterns are obtained, in each case. The significance of charge distribution to latex cohesion, adhesion and swelling behavior is discussed.

Introduction Latex film formation has been examined in great detail since the early fifties1-4 and the large body of results in the literature has been reviewed,5-8 creating a good understanding of this problem but with some conflicting views. The driving forces for the morphological transformations of particles into a film, during and after water evaporation, are capillary forces as well as the polymer/ water interfacial tension, polymer surface tension and the other forces due to the residual water left among the particles. The relative roles played by the various interfacial tensions in promoting film formation9 have been often discussed, as well as the stresses developed during this process10 and the film formation kinetic regimes.11,12 Many new techniques, e.g. freeze-fracture transmission electron microscopy (FFTEM), environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM), helped revealing morphological details of the intermediate events in the complex latex drying process.8 Greater insight has been achieved thanks to other modern instrumental techniques, such as small angle neutron scattering (SANS) and direct nonradiative energy transfer (NRET), but the actual mechanisms13-15 involved in * Corresponding author: Institute of Chemistry, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas SP, Brazil. (1) Dillon, W. E.; Matheson, D. A.; Bradford, E. B. J. Colloid Sci. 1951 6, 108-117. (2) Brown, G. L. J. Polym. Sci. 1956, 22, 423-434. (3) Sheetz, D. P. J. Appl. Polym. Sci. 1965, 9, 3759-3773. (4) Vanderhoff, J. W.; Bradford, E. B.; Carrington, W. K. J. Polym. Sci., Part C: Polym. Symp. 1973, 41, 155-174. (5) Winnik, M. A. Curr. Opin. Colloid Interface Sci. 1997, 2, 192199. (6) Keddie, J. L. Mater. Sci. Eng. R. 1997, 21, 101-170. (7) Visschers, M.; Laven, J.; German, A. L. Prog. Org. Coat. 1997, 30, 39-49. (8) Steward, P. A.; Hearn, J.; Wilkinson, M. C. Adv Colloid Interface Sci. 2000, 86, 3, 195-267. (9) Lin, F.; Meier, D. J. Langmuir 1996, 12, 2774-2780. (10) Petersen C.; Heldmann C.; Johannsmann, D. Langmuir 1999, 15, 7745-7751. (11) Keddie, J. L.; Meredith, P.; Jones, R. A. L.; Donald, A. M. In Film Formation in Waterborne Coatings; Provder, T., Winnik, M. A., Urban, M. W., Eds.; ACS Symposium 648; American Chemical Society: Washington, DC, 1996; pp 332-348. (12) Routh A. F.; Russel, W. B. Langmuir 1999, 15, 7762-7773. (13) Perez, E.; Lang, J. Macromolecules 1999, 32, 1626-1636. (14) Toussaint, A.; DeWilde, M. Prog. Org. Coat. 1997, 30, 113-126.

deforming spherical particles into void-free films are still the subject of controversy and debate. A “limited parabolic” rheological model was proposed11 and the results showed that the WLF concept and the Adam-Gibbs relationship are applicable, so that master curves for the storage and loss modula can be drawn and the polymer relaxation times determined. From this model, a new definition has been derived for the minimum film formation temperature (MFFT). Latex film aging and annealing have also been examined in a few systems, often using SANS, NRET, and AFM, as well as contact angle, confocal optical microscopy and spectroscopic techniques. The persistence of deformed particle interfaces and their slow disappearance were described by Van Tent et al.,16 who made turbidity measurements and observed the changes in a thin acrylic film interference during the drying and swelling processes. Butt et al.17 observed individual latex particles in a dry latex film with a MFFT of 12 °C and Tg of 18 °C. Particle individuality was strongly decreased after 4 months at room temperature, or after annealing the films at 50 or 60 °C for 4 h, and disappeared following annealing at 80 °C. Film roughness decreased with increasing temperature, and islands assigned to the accumulation of surfactant exudate appeared at the film surface. The role of hydrophilic short chain membranes from surfactant covering soft cores of core-shell latex particles was extensively examined by a French group, showing that the membranes are disrupted when they come into contact during film drying, and they migrate to the film surface. On the other hand, long chain polymeric membranes grafted onto the core remain intact in the dry film.18,19 The extensive work of a Korean group on the effects of the carboxylated random copolymer poly(styrene/alphamethylstyrene/acrylic acid) (SAA) on poly(butyl meth(15) Visschers, M.; Laven, J.; German, A. L. Prog. Org. Coat. 1997, 30, 39-49. (16) Van Tent, A.; Nijenhuis, K. T. Prog. Org. Coat. 1992, 20, 459470. (17) Butt, H. J.; Kuropka, R.; Christebseb, B. Colloid Polym. Sci. 1994, 10, 1218-1223. (18) Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Maquet, J.; Lindner, P.; Cabane, B. Colloid Polym. Sci. 1992, 270, 806821. (19) Joanicot, M.; Wong, K.; Richard, J., Maquet, J.; Cabane, B. Macromolecules 1993, 26, 3168-3175.

10.1021/la010781x CCC: $20.00 © 2001 American Chemical Society Published on Web 11/10/2001

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acrylate) (PBMA) latex film formation yielded some important pieces of information, in the present context: (i) the ethylene glycol-water contact angles and thus the hydrophobic-hydrophilic character of PBMA films containing SAA changed by adjusting the film annealing temperatures and the duration of annealing,20 (ii) the number of holes on the latex film surface increased with aging,21 (iii) the time dependence of the peak-to-valley distance (corrugation height) of latex particles in the film at different annealing temperatures obeys the time/ temperature superposition principle.22 This is interpreted assuming that the SAA layer at particle surfaces retards the interdiffusion of the PBMA molecules across the particle-particle interfaces and slows the gradual coalescence of the particles, resulting in poorer tensile property of these films compared to that of the pure PBMA latex. Film surface flattening was found to be faster than the total chain migration between adjacent particles, by using AFM and NRET measurements,23,24 but films flatten only at annealing temperatures greater than the shell polymer Tg. Particle fusion or surface flattening were not observed25 in a self-arrayed polystyrene film aged at room temperature, but the films annealed above Tg showed a limited particle fusion and a large decrease in the particle heights, while the interparticle distance remained constant. Crystal defects at the film surface grew as a result of annealing, but the number of defects remained constant. The presence of cross-links in the latex particles limits the extent of polymer interdiffusion, but polymer diffusion in latex films with 100% gel content was observed26 and assigned to diffusion of dangling polymer chains. Results from Rutherford backscattering spectrometry (RBS) and AFM show slow changes in the compositional profiles and morphology of surfactants at the air surface of the latex films, regardless of the film-forming temperature and time.27 In some cases, the surfactant features were imaged and they evolve from a thin uniform layer, to a “fingerlike” morphology, then to small flat droplets, and finally to larger, hemispherical “blobs”. AFM has had a fundamental role in the examination of film aging and annealing, and the use of different imaging procedures has provided new results on many polymer systems.28-32 A new possibility for the microanalytical examination of latex film aging is the scanning electric potential microscopy (SEPM) technique. This technique has not yet been widely applied to latex film imaging, but it provides contrast between domains with different electric charge excess, by using a Kelvin-bridge electrode as the scanning probe. Results obtained using (20) Shin, J. S.; Lee, D. Y.; Ho, C. C.; et al. Langmuir 2000, 16, 1882-1888. (21) Park, Y. J.; Lee, D. Y.; Khew, M. C.; Ho, C. C.; Kim, J. H. Colloids Surf., A 1998, 139, 49-54. (22) Lee, D. Y.; Choi, H. Y.; Park, Y. J.; Khew, M. C.; Ho, C. C.; Kim, J. H. Langmuir 1999, 15, 8252-8258. (23) Perez, E.; Lang, J. Langmuir 2000, 16, 1874-1881. (24) Huijs, F.; Lang, J. Colloid Polym. Sci. 2000, 278, 746-756. (25) Goudy, A.; Gee, M. L.; Biggs, S.; et al. Langmuir 1995, 11, 44544459. (26) Tamai, T.; Pinenq, P.; Winnik, M. A. Macromolecules 1999, 32, 6102-6110. (27) Tzitzinou, A.; Jenneson, P. M.; Clough, A. S.; Keddie, J. L.; Lu, J. R.; Zhdan, P.; Treacher, K. E.; Satguru, R. Prog. Org. Coat. 1999, 35, 89-99. (28) Sheiko, S. S. Adv. Polym. Sci. 2000, 151, 61-174. (29) Tsukruk, V. V. Rubber Chem. Technol. 1999, 70, 430-467. (30) Lee, D. Y.; Shin, J. S.; Park, Y. J.; Kim, J. H.; Khew, M. C.; Ho, C. C. Surf. Interface Anal. 1999, 28, 28-35. (31) Raiteri, R.; Butt, H. J.; Beyer, D.; Jonas, S. Phys. Chem. Chem. Phys. 1999, 1, 4881-4887. (32) Miksa, B.; Slomkowski, S.; Marsault, J. P. Colloid Polym. Sci. 1998, 276, 34-39.

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this technique are complementary to other microchemical techniques such as confocal Raman microscopy33 or energyloss imaging,34 but the information on the sample electrical patterning is acquired more directly. Latex films are often loaded with ionic species and this technique is ideal for the observation of ion clustering and dispersion in the film, during aging and annealing. This paper reports on SEPM imaging of films prepared by drying a styrenebutyl acrylate-acrylic acid copolymer latex followed by annealing and aging. Experimental Section The latex was prepared by emulsion polymerization, in a 1 L kettle glass reactor with a cover, fitted with a two-wing turbine type stirrer, a thermometer, a condenser, two burets for adding the redox initiators, and a separation funnel for controlled addition of an emulsion containing monomers, water and surfactants. The reactor was immersed in a thermostatic water bath. Reagents used were: Milli-Q deionized water, ammonium hydroxide, styrene, acrylic acid, butyl acrylate, ascorbic acid, tert-butylhydroperoxide 70% (Trigonox AW-70 from Akzo), Sipomer COPS-I (sodium 1-allyloxy-2-hydroxypropane sulfonate) reactive anionic surfactant in a 40% w/w aqueous solution, and Rhodafac RE-610 (sodium 9EO polyoxyethylene nonylphenyl ether phosphate) anionic surfactant (both from Rhodia). Prior to use, styrene was distilled under a reduced N2 pressure. The monomer emulsion was prepared dissolving 1.61 g of the reactive surfactant (Sipomer COPS-I) and 7.18 g of the phosphate surfactant (Rhodafac RE-610) in 88.8 g of water followed by monomers addition (88.5 g of styrene, 102.0 g of butyl acrylate, and 3.81 g of acrylic acid), and stirring. Water (76.4 g) was initially added to the reactor, and this was purged with N2, stirred at 300-350 rpm and heated to 60 °C, when the surfactants were added (1.79 g of Rhodafac RE-610 and 0.40 g of Sipomer COPSI). The redox initiator reagents ascorbic acid (2.91 g in a 10% w/w aqueous solution) and tert-butylhydroperoxide 70% (1.65 g in a 10% w/w aqueous solution) were continuously added during polymerization, from the respective burets. Thus, the surfactants were added to the starting aqueous solution as well as during the polymerization. The monomer emulsion and the solution of redox initiators were added simultaneously during 5 and 5.5 h, respectively. The temperature was kept at 60-65 °C. After completing the reactant addition, the temperature was kept for an extra 1 h at 60-65 °C, to eliminate residual monomer. The final dispersion was cooled to room temperature and filtered through a 105 µm polypropylene screen, when a small amount of coagulated latex (1.1 g) was collected. The pH was adjusted to 8.0 by using 10% ammonium hydroxide. An aliquot of the latex was extensively dialyzed through a cellulose membrane, until the conductivity of the external water was less than 2 µS. The effective diameters of the latex particles (original and dialyzed) were determined by using photon correlation spectroscopy (PCS), in a ZetaPlus (Brookhaven Inst. Corp.) at 25° C. The dispersion was diluted to 10-4 % of solids content prior to the analysis. The visible light (400-800 nm) transmission spectrum of the film, obtained by drying the latex on a quartz substrate, was determined by using a Hewlett-Packard 8552A Diode Array Spectrophotometer. The dry film was maintained under controlled conditions of temperature (20-22 °C) and humidity (4060%). The thickness of the film was approximately 40 µm. Dry film Tg was determined by DSC and DMA. DSC analysis was made in a TA 2000 (TA Instruments). The temperature range was -80 to 120 °C and the heating rate was 10 °C min-1. Sample weight was approximately 10 mg and the transition temperature was determined from the second run, to eliminate stresses. The DMA (dynamical mechanical analysis) measurements were made using a TA Instruments DMA 983 apparatus. The temperature (33) Belaroui, F.; Grohens, Y.; Boyer, H.; Holl, Y. Polymer 2000, 41, 7641-7645. (34) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1999, 15, 4447-4453.

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Figure 1. Micrographs from the same dry film of the as-prepared latex. A and C: noncontact AFM. B and D: SEPM. Gray-level line scans were acquired, following the straight lines in the upper left side of A and B, see Figure 2. range and heating rate were the same as in DSC, the amplitude was 0.15 mm and the frequency was 1 Hz. The latex was spread over a PTFE sheet substrate and dried in an oven at 40 ( 1 °C. The DMA sample was prepared by cutting an 8.5 × 6.0 mm2 rectangle from a 1.8 mm thick film dried on PTFE. Atomic Force (AFM) and Scanning Electric Potential (SEPM) Microscopy. Dry latex films were prepared by spreading 20 µL of latex (47.2% solids content) on a 0.5 × 0.5 cm2 area of glass slide covers, previously washed with ethanol and dried under air at 22 °C and 40-60% relative humidity. Dry films on glass slides were exposed to solvent vapors within closed glass containers, in the presence of liquid toluene or chloroform contained in open beakers. Another film sample was heated in an air oven for 7 h. Film-coated slide covers were glued to the sample holder and mounted on a Topometrix Discoverer TMX2010 microscope, in which noncontact atomic force microscopy (AFM) and scanning electrical potential microscopy (SEPM) images were obtained simultaneously. Image processing was done using the Topometrix software. The SEPM technique uses the standard noncontact AFM setup, but the sample is scanned with Pt-coated Si tips. An AC signal is fed 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 the fourquadrant photodetector and analyzed by two feedback loops. The first loop is used in the conventional way to control the distance between tip and sample surface, while scanning the sample at constant oscillation amplitude. The second loop is used to minimize the electric field between tip and sample: a second lock-in amplifier measures the tip vibration at 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 that used by Terris,35 who measures the phase displacement of the AC voltage, while in the Topometrix set up we cancel the phase displacement by DC biasing. The image is built using the DC voltage fed 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 study: both use an oscillating electrode separated from the sample by an air gap.36 The major difference between both is the detection technique used, since SEPM uses a phase detection of the mechanic oscillation generated in the frequency of the applied voltage. The system is calibrated and the electric potential measurements are verified using two procedures: (i) measuring electric potentials in the air 10 nm above thin strips of metal deposited on an insulator sheet, to which known voltages were applied; (ii) changing the voltage applied to a metal holder beneath the sample, and acquiring images at different sample holder voltages. The electric potential sensed by the tip at 10 nm from the surface is measured at each pixel, and all the electric potentials for a given sample area are displayed as a gray-level (or eventually color-coded) image. Since the information acquired derives from electrostatic interactions, the effect of buried charges has a quadratic dependence on distance. The sampling depth is dependent on the sample dielectric constant and also on the charge distribution normal to the sample. Consequently, charges buried up to 100 nm beneath the sample-air interface can still interfere in the measurements. However, the effect of a charge at the interface will be respectively four and nine times as large as that of the same charge but 10 or 20 nm beneath the surface. (35) Terris, B. D.; Stern, J. E.; Rugar, D.; Mamin, H. J. J. Vac. Sci. Technol. A 1990, 8, 374-377. (36) Shaw, D. J. Introduction to Colloid and Surface Chemistry, 4th ed.; Book News: Portland, 1992.

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Figure 2. Gray-level intensity scans along the lines drawn in Figure 1A (upper) and B (bottom).

Results The latex used in this work was from a batch with 48.2% weight solids content, as determined by gravimetry following drying at 105 °C, to constant weight. The effective particle diameter of the dialyzed latex, as determined by PCS, was 240 ( 4 nm and the latex polydispersity P ) 0.089 (P ) µ2/Γ2), where µ2 ) (D2* - D*2)q4 and Γ ) Dq2; D* is the average diffusion coefficient (D ) ΣNm2P(q,d)D/ΣNm2P(q,d) and D2 ) ΣNm2P(q,d)D2/ΣNm2P(q,d), where N is the number of particles, m is the particle mass, P is the particle form factor, and q ) [(2πn)2 sin(θ/2)]/λ (the scattering vector, where n is the refractive index of the suspending liquid, λ is the wavelength of the laser light. and θ is the scattering angle). The effective diameter of the original latex was 228 ( 3 nm and the polydispersity was 0.054. Tg of the dry film cast from the dialyzed latex was 16.4 °C and for the original latex film Tg was 15.9 °C, as determined by DSC three months after sample preparation. However, dynamic mechanical analysis (DMA) of the film produced with the original latex after three months showed two separate transitions, at approximately -45 °C and 24 °C. The low-temperature transition is indicative of the existence of domains with a high PBA content, since the Tg of the PBA homopolymer is -50 °C. Residual water content was less than 2% (w/w) in the dry films. Visible light (400-800 nm) transmittance of the dry films was higher than 95% after 1 h drying and higher than 99% after 2 h (100% was adjusted using a silica window). Opalescence developed in the drying dispersion, and the reflected sunlight changed from blue to green to yellow, depending on the angle of visual observation. Noncontact AFM and SEPM images from recently prepared films, acquired ca. 2 h after drying, are in Figure 1. In the higher magnification noncontact picture (Figure 1A), we observe densely packed deformed particles forming a mirrorlike surface, since the maximum height is 20 nm only, or 1/20 the blue light wavelength. Packing density is remarkable, considering that the particles sizes are nonuniform in size. Consequently, there is good space filling by these polydisperse particles. The corresponding SEPM image (Figure 1B) shows that particle cores are negative, relative to the interparticle domains. Contrast is also observed within the negative particle cores, showing

Figure 3. Noncontact AFM (top) and SEPM (bottom) images of a dry film prepared with dialyzed latex.

that they are formed by electrically differentiated domains, even though the particle surfaces appear very smooth in the AFM images. Line scans drawn across Figure 1A,B are in Figure 2, showing in a more quantitative way the difference between the contrast patterns of film surface topography and electric domain distribution. The line scan from Figure 1B was done on an inverted contrast image, to facilitate the comparison with the line scan from the topography image in Figure 1A. A pair of images but at a lower magnification is shown in Figure 1C,D. In the AFM image, we observe film elevations and depressions formed by clusters of many tens of particles. In the SEPM image we observe that these elevations and depressions do not correspond to any electrically differentiated domains. Beyond, there is a strong electric contrast corresponding to two quasihorizontal lines in the SEPM image, which are barely perceptible in the AFM image. This poor correlation between the two types of pictures is a good control of the independence between the two imaging modes. Figure 3 presents the AFM and SEPM micrographs of the film produced by drying the same latex, but after dialysis. The dialyzed latex also provides a very smooth film surface, the densely packed deformed particles are observed in the AFM image and the particle cores appear negative, relative to particle shells. However, particle individuality is less pronounced than in Figure 1B, showing that the removal of dialyzable constituents helps achieving greater particle interpenetration.

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Figure 4. Noncontact AFM (left) and SEPM (right) images of the same dry film shown in Figure 1, but after aging for 1 month (A, B) and 3 months (C, D) at 20-25 °C, 40-60% relative humidity.

The same dry film shown in Figure 1 was stored and examined one month later, and the corresponding images are in Figure 4. Particle contours are now barely seen in the AFM picture, and the film is smoother than that in Figure 1. On the other hand, electric contrast is now highly correlated with topography (as opposed to Figures 1 and 3), showing that the film depressions are the sites for the accumulation of positive charges, while the flatter upper domains are negative, relative to the depressions. Further aging of the same sample for 3 months produces both a new film surface topography pattern and a new distribution of electric domains, which are also shown in Figure 4. In the SEPM image, we observe wavy thick dark (negative) lines within a brighter (positive) continuum. The film surface is very smooth, and there is only a poor correlation between the contrasting domains in the two types of images. Consequently, film roughness and electric charge distribution patterns both change with time, evidencing significant film maturation or aging associated to the mobility of film components, including the ionic species. These evidences for aging effects in the dry film led us to investigate film behavior under exposure to higher temperatures and organic solvent vapors. AFM and SEPM images acquired after heating a recently prepared film at 80 °C for 7 h under air are in Figure 5. The film surface is very flat, the individual particles are hardly discernible and there is a complex, apparently random pattern of electric potential distribution. The range of surface

potentials is now only 0.4 V, less than one-half the voltage range observed in the previous cases. Other film pieces were dried and kept for 3 h under solvent (toluene or chloroform) - saturated atmosphere. The resulting films show peculiar morphologies and domain patterns, in each case (Figure 5). As compared to the other samples, the heated films have a low rugosity and also the lowest electric potential gradients. This points to the effectiveness of constituent mixing at higher temperatures, either due to temperaturedependent mixing properties of the system as represented in its phase diagram, or just to increased diffusion of polymer chains and ionic species, in the film. Discussion The present results confirm the persistence of particle interfaces, even in transparent films. Interfaces are the sites for accumulation of the counterions in the recently cast films, wherefrom they migrate when the film is aged or annealed under heating or even exposed to solvent vapors. The end result of ion migration is not just an entropy-driven homogeneous dispersion in the film, but new patterns of electric potential and thus of ion excess concentration are observed, as seen in Figures 4 and 5. At this point, there is little related information from the literature to help our understanding of these patterns. Perhaps, the most relevant work is that done on counterion clustering in colloidal dispersions, predicting the attraction of colloidal particles with identical charges, in sols.37

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Figure 5. Noncontact AFM (left) and SEPM (right) images of the same dry film shown in Figure 1, but after annealing for 7 h at 80 °C (A, B); after exposure to toluene-saturated atmosphere for 4 h (C, D) or exposure to chloroform-saturated atmosphere (E, F).

The measured electric potentials are very large, as compared to usual potentials for colloidal systems. This is not hard to understand, considering that the dry films are nonconducting, and large voltages are easily found at the interfaces in insulators, e.g., in streaming potential experiments and in aerosols such as water clouds. The preservation of particle individuality in the latex films is facilitated by the slower self-diffusion of ionterminated chains. For instance, sulfonate-ended polystyrene chains migrate up to 1 order of magnitude slower than H-ended chains,38 probably due to the end-to-end

aggregation of the chains, but the concentration dependence of the decreased diffusion lengths has not yet been determined. The ionic groups can then impair the latex film properties, because they decrease the chain diffusion lengths,39 and it is normally expected that polymer interdiffusion leads to maximum film cohesion.40 (37) Wu, J. Z.; Bratko, D.; Blanch, H. W.; Prausnitz, J. M. Phys. Rev. E 2000, 62, 5273-5280. (38) Kim, S. D.; Klein, A.; Sperling, L. H.; Boczar, E. M.; Bauer, B. J. Macromolecules 2000, 33, 8334-8343.

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However, the mosaic of electric charges in the film may also contribute to film cohesion. This will be evaluated in a future work, by modulus and other mechanical determinations on films such as those described in this work. Tg and MFFT are the most often used thermal parameters for predicting the properties of a film, and there have been many discussions on the observed differences between these two temperatures. We can now contribute to this discussion, as follows: the particle interfaces have an ionic composition different from other polymer areas, and diffusion across particle borders depends on other factors beyond the polymer average Tg. The interparticle domains may also help to understand the differences in MFFT reported by some authors for the same latex, but under different film-forming conditions.41 The relative role of film plasticization by water is experimentally well established,9,42 but its mechanism is not completely defined. We can now relate it to the water uptake at the various polymer domains containing either excess cation or anion concentrations, so that different domains would be plasticized to variable extent. Quantitative relationships between hydroplasticization and ion clustering can in principle be established using very recent thermal microscopy techniques. However, this has not yet been done, to the best of our knowledge. Considering the latex film formation process model established by Routh and Russell,12 the films obtained in this work were obtained under the capillary deformation regime, which requires the strain in the film to follow evaporation, and appears at intermediate temperatures. On the other hand, Keddie et al.11 consider that when the temperature is near to the polymer Tg, the rate-limiting step in film formation is the particle deformation, “possibly by viscous flow of the polymer driven by the reduction in surface energy”. In the present work, film formation took place ca. 5 K above the polymer film Tg, and this showed little dependence from serum component removal by dialysis. Films obtained with both the dialyzed and nondialyzed latexes are similar concerning the surface topography, but the electric pattern is more diffuse in the case of the dialyzed latex. Capillary forces depend inversely on the particle sizes, and deforming forces at least an order of magnitude lower than that predicted by the capillary force theory have been reported.43 We now suggest that this discrepancy between predicted and experimental results is related to particle chemical heterogeneity and to the electrostatic attraction between particles and the oppositely charged aqueous film in the later film drying stages, which is not considered in the current models. This is an additional attractive force, but one having different geometrical (39) Kim, K. D.; Sperling, L. H.; Klein, A.; Mammouda, B. Macromolecules 1994, 27, 6841-6850. (40) Van Tent, A.; Nijenhuis, K. T. Prog. Org. Coat. 1992, 20, 459470. (41) Sperry, P. R.; Snyder, B. S.; O’Dowd, M. L.; Lesko, P. M. Langmuir 1994, 10, 2619-2628. (42) Kurita, O.; Fujita, R.; Isikawa, O.; Tsuruoka, K. Colloids Surf., A 1999, 153, 471-476. (43) Kan, C. S. J. Coat. Technol. 1999, 71, 89-97.

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constraints than capillarity and making an important contribution to surface energy. Recent Monte Carlo results are showing that this interaction is sufficiently strong to provoke the attraction of particles with identical charges.37 From Figures 1 and 3, we note the excellent particle packing, irrespective of particle size and of serum content. Both small and large particles underwent extensive deformation, even though the smaller particles should be more strongly deformed by capillary forces. However, from previous results from this laboratory we suggest that the larger particles are chemically different from the smaller ones, and the resulting heterogeneity in particle viscoelastic and interfacial properties is relevant to the particle deformation analysis.44-46 In practical applications, formulation aids such as glycols and other nonvolatile organic substances can accelerate the polymer chains interdiffusion and the consequent particle coalescence.47 On the other hand, the coalescing aids undergo exudation to the dry film interfaces,48 and this can impair the surface film properties. An AFM study of tethered styrene-acrylic copolymer films on flat silicate substrates49 showed a smoother surface after treatment with CH2Cl2, but treatment with cyclohexane led to a relatively rough surface, and a different nanomorphology. Our results in Figure 5 show a strong effect of film surface exposure to the solvent vapors. Not only are the surfaces much smoother, but also the pattern of electric charge distribution is completely changed. Of course, this implies that the films are modified by the sorbed vapors and their constituent ionic species achieve larger diffusivities than in the pristine film. A more complete understanding of the various electric domain patterns observed depends on modeling calculations, what will not be easily done given the many complexities of the systems under study. However, this should not prevent us from imaging the electric patterns of latex films, modifying them and determining their significance for the film properties. Combining this information with microanalytical and other data should lead to more complete models for latex polymer films and perhaps some other solid polymers. Acknowledgment. A.J.K. and C.A.R.C. are Fapesp graduate fellows. F.G. acknowledges support from Fapesp, Pronex/Finep/MCT, and CNPq. LA010781X (44) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1998, 14, 3187-3194. (45) Galembeck, F.; Souza, E. F. In Polymer Interfaces and Emulsions; Esumi, K., Ed.; Marcel Dekker: New York, 1999; Chapter 4. (46) Teixeira-Neto, E.; Leite, C. A. P.; Cardoso, A. H.; Silva, M. C. V. M.; Braga, M.; Galembeck, F. J. Colloid Interface Sci. 2000, 231, 182-189. (47) Winnik, M. A.; Wang, Y.; Haley, F. J. Coat. Technol. 1992, 64, 51-61. (48) Juhue´, D.; Wang, Y.; Lang., J.; Leung, On-M.; Goh, M. C.; Winnik, M. A. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1123-1133. (49) Zhao, B.; Brittain, W. J.; Zhou, W. S.; Cheng, S. Z. D. Macromolecules 2000, 33, 8821-8827.