Langmuir l996,11,2179-2186
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Ordering and Adhesion of Latex Particles on Model Inorganic Surfaces V. Granier" and A. Sartre Rhone-Poulenc Recherches, 52, rue de la Haie Coq, 93308 Aubervilliers Cedex, France Received September 6, 1994. In Final Form: March 8, 1995@ The ordering and adhesion of latex particles on model inorganic surfaces are investigated through atomic force microscopy. The particles are made of a soft polymericcore protected by a hydrophilic membrane made of carboxylic acids, and they are dispersed in water. This dispersion is spread on model inorganic surfaces. After water evaporation,there is a monolayer of rather well dispersed particles on the substrate. The measured particle heights are dependent on the scanning rate and on the force of the probe tip acting on the sample. Below the polymer glass temperature, the particles have weak adhesion to the surface and are moved easily by the probe tip. This results in tip-induced organization of the particles. The analysis of particle profiles shows that, above glass temperature, there is a sudden and partial spreading of the particles which is governed by the polymeric chain mobility and not by capillary forces. The particle adhesion and spreading ratio are controlled by acid-base interactions between latex particles and the surface.
Introduction Synthetic latices consist of colloidally-dispersedpolymer particles which can be prepared by a tightly-controlled emulsion polymerization technique. These aqueous dispersions are commonly used as binders for inorganic pigments in a wide range of applications that include paints, adhesives, paper coating, and carpet backing.l Most of the applications require the latex to form a continuous film upon drying and also to bind mineral pigments within a composite layer. The cohesion and adhesion of this composite layer are governed by the adhesion of latex particles on the pigment.2 So it appears that a relevant first step toward understanding properties of the composite is to focus on the ordering and adhesion of latex particles within the coating. The formation and structure of a continuous film from latex have been fully ~tudied.~-'OIt results from coalescence ofthe particles, provided that forces produced during water evaporation are strong enough to overcome both Coulombic repulsion forces of the charged particles and particle rigidity. In contrast, the spreading and the ordering of latex particles on an inorganic surface after water evaporation are poorly investigated fields of research,ll and the measurements are very difficult to perform at the particle scale with conventional techniques such as contact angle analysis and electron microscopy. Thus, only the final stage of latex filmformation can be examined in situ with conventional electron m i c r o ~ c o p y ,and ~ ~ Jcrowding ~ of latex Abstract Dublished in Advance ACS Abstracts. Mav 15. 1995. (1) Daniel, i.C. Science 1988, 125. (2) Lepoutre, P.; Hiraharu, T. J. Appl. Polym. Sci. 1989,37, 2077. (3) Vanderhoff,J. W.; Bradford, E. B.; Carrinpton, - W. K. J. Polym. Sei. Symp. 1973,41, 155. (4) Kast, H. Makromol. Chem. Suppl. 1985, 10111, 447. (5)Joanicot, M.; Wong, K.; Maquet, J.; Chevalier, Y.; Pichot, C.; Graillat, C.; Lindner, P.; Rois, L.;Cabane, B. Prog. Colloid Polym. Sci. 1990, 81,175. (6) Wang, Y.; Kats, A,; Juhue, D.; Winnik, M. A. Langmuir 1992,8, 1435. (7) Winnik, M. A.;Wang, Y.; Haley, F. J. Coatings Technol. 1992,64, @
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particles near the pigment surfaces can be examined only with high-resolution cryogenic scanning electron microsCOPY (cry0-SEM).l4J5 In the present work, we investigate the wetting of model inorganic surfaces by latex particles using a recent surface science technique, atomic force microscopy (AFM).16 This technique allows insulating surfaces to be imaged and surface relief to be described a t the nanometer scale, and in some cases down to the atomic level, without destroying the surface. AFM is a very fast developing field and most of the studies are devoted to imaging biological molecules (cell surface^,^' nucleic acids,18or proteins,lg for example) or to demonstrate the atomic resolution of this technique.20 Recently, AFM was also used to study latex film formation and the evolution of surface structure in latex Its resolution is comparable to, or exceeds, that available by scanning or transmission electron microscopy. Since the samples require no special surface treatment, and the technique is nondestructive, film samples can be examined, annealed, and reexamined many times. In this work AFM is used at a sub-micrometer scale to investigate the wetting of an inorganic surface by latex particles at the particle scale. When a latex dispersion is spread on an inorganic surface, latex particles are brought into contact with this surface during water evaporation. This leads to the following questions: What happens to the latex particles during this process? Is there flocculation or aggregation of latex particles? Are the particles deformed or not?
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51. (8) Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Maquet, J.; Lindner, P.; Cabane, B. Colloid Polym. Sci. 1992,270, 6.
(9) Joanicot,M.;Wong, K.; Cabane,B. Proceedings ofthe 1993 TAPPI Coating Conference; TAPPI Press: Atlanta, GA, 1993. (10)Joanicot, M.; Wong, K.; Richard, J.; Maquet, J.; Cabane, B. Macromolecules 1993, 26, 3168. (11) Granier, V.; Sartre, A,; Joanicot, M. J. Adhes. 1993, 42, 255. (12) Kendall, K.; Padget, J. C . Int. J. Adhes. Adhes. 1982, 2, 149.
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(13) Demejo, L. P.; Rimai, D. S.; Bowen, R. C. J.Adhes. Sci. Technol. 1988,2, 331. (14) Sheehan, J. G.; Whalen-Shaw, M. TAPPI J. 1990, 73, 171. (15)Sheehan, J. G.; Takamura, K.; Davis, H. T.; Scriven, L. E. Adu. Coating Fundam. 1993, 109. (16) Binnig, G.; Quate, C. F.;Gerber, Ch. Phys. Rev. Lett. 1986,56, 930. 117)Gould, S. A. C . ; Drake, B.; Prater, C . B.; Weisenhorn, A. L.; Hansma, H. G.; Hansma, P. K. J. Vac. Sci. Technol. 1990,8, 369. (18)Hansma, P. K.; Elings, V. B.; Marti, 0.;Bracker, C. E. Science 1988,242,209. (19) Marchant, R. E.; Scott Lea, A.; Andrade, J. D.; Bockenstedt, P. J. Colloid Interface Sci. 1992, 148, 261. (20)Albrecht, T. R.; Quate, C. F. J. Appl. Phys. 1987, 62, 2599. (21) Li, Y.; Lindsay, S. M. Rev. Sci. Instrum. 1991, 62, 2630. (22) Wang, Y.; Juhue, D.; Winnik, M. A,; Leung, 0. M.; Goh, M. C. Langmuir 1992,8, 760. (23) Juhue, D.; Lang, J. Langmuir 1993, 9, 792. (24) Goh, M. C.; Juhue, D.; Leung, 0. M.; Wang, Y.; Winnik, M. A. Langmuir 1993, 9, 1319.
0 1995 American Chemical Society
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Figure 1. Schematic view of latex particles on a n inorganic substrate in the case of no flocculation. Particles may remain spherical (top) or they may be more or less deformed (bottom and right) depending on their spreading on the inorganic surface. Table 1. Main Features of the Synthesized Latex Particles
latex
A B
C D
core compositiona (wt 9%) S, 40 BA,60 S,40 BA,60 S,40 BA,60 S,60 BA,40
particle diameted' (nm) 120
("C) 5
surface density of AA groupsc (group/nm2) 3.00
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a Particle core is a statistical copolymer of styrene and butyl acrylate. Particle diameters according to pictures from transmission electronmicroscopy. c Surfacedensity of acrylicacid sequences which are copolymerized with the core polymers to form the hydrophilic membrane.
Latex is a viscoelastic material which can be pictured as something intermediate between a dispersion of a solid and an emulsion (droplets of oil in water). Different possible structures for latex particles after drying of the dispersion can be considered. First, particles may flocculate and form aggregates of dozens of particles. Another possible structure is to consider that there is no flocculation. In this case, particles are rather well dispersed on the inorganic surface and they conserve their own identity. Particles may be more or less deformed depending on their spreading on the inorganic surface (Figure 1). Obviously the ordering and spreading of latex particles may depend on latex characteristics, the chemical nature of the inorganic surface, and drying conditions. In this work we have used atomic force microscopy (AFM) to investigate the ordering and spreading of latex particles (about 100 nm in diameter) on model inorganic surfaces. The system chosen consists of a styrene-butyl acrylate copolymer latex. The main variables of concern in this study are the latex characteristics (glass temperature, Tg, acid level), the chemical nature of the surface, and the drying and annealing conditions. We discuss the mechanisms governing the particle spreading and adhesion. Experimental Section Latex. Four aqueous dispersions of carboxylated latex particles were used in this study. The particles are spherical droplets made of a hydrophobic polymer core surrounded by a hydrophilic layer. The core is made of a statistical copolymer of styrehe (S) and butyl acrylate (BA). The glass transition temperature of these copolymers is between 5 and 41 "C. Hence at room temperature, copolymers A, B, and C are viscoelastic amorphous materials, whereas copolymer D is in the glass state.
Figure 2. A 2000 nm by 2000 nm AFM constant-force image of a cleaved calcium carbonate crystal. The vertical scale is height in nanometers. The core of each latex particle is surrounded by a thin hydrophilic layer made of acrylic acid (AA)sequenceswhich are copolymerized with the core polymers. Full coverage of particles by AA sequences requires a surface density of three AA groups/nm2 of surface. The surface density of AA groups, expressed as the number ofAAgroups/nm2,is in the 0.68-4.30range. The number of AA groups at the surface results in a monolayer coverage of latex A particles. In contrast, latex B particles are not fully covered by AA sequences, and the coverage of latex C and D particles is more than a monolayer. These AA groups have been neutralized at pH = 8.0; hence they are charged and generate long range repulsions between particles, thereby stabilizing the dispersion. After their synthesis, the dispersions were purified in an ultrafiltration cell. The average particle diameter, as measured by electron microscopy, was found to be between 100 and 125 nm, and the corresponding size distribution is very narrow. A complete list of the latex dispersions with the compositions of the particle cores and membranes is given in Table 1. Mineral Substrates. Three chemically different mineral substrates were used in this study: silicon wafers covered by a thin layer of silica (called silica substrate in the following text), mica sheets, and calcium carbonate crystals. AFM Imaging. The atomic force microscope used is a commercial instrument (Digital Instruments Nanoscope I1 system) operated in a constant force mode. All the measurements were performed under ambient conditions,using a 200-pm silicon nitride tip. The x-scan frequency was set a t 5.8 Hz. Quantitativeinformation on particle spreading on a substrate .can be obtained from AFM experiments by measuring particle profiles. Consequently, the roughness of mineral substrates, defined as the maximum displacement, must be smaller than the polymer particle diameter. The silica substrates were used as delivered, their surfaces being very smooth with a roughness less than 1 nm. On the contrary, it was necessary to cleave the calcium carbonate crystals and the mice sheets to obtain very smooth surface (roughness less than 3 nm) (Figure 2). Samples were prepared by placing a 1-pLdroplet of a diluted latex solution onto a substrate. The latex solution concentration was fmed at 0.2 x 10-3 in mass concentration because it corresponds approximately to the same substrate surface coverage as for mineral pigments in typical industrial applications. We have not studied the possible effect of any chemical reaction occurring between the aqueous medium and the substrate, speciallythe calcium carbonate. The diluted latex solutions were prepared with ultrapure water and the dispersions were purified in an ultrafiltration cell after their synthesis. Consequently, we think that there are no chemical impurities in the aqueous medium. The only possiblereaction which might occur is between water molecules and the substrate (dissolution of the substrate for example). The effects of such a reaction are independent of latex characteristics and cannot cloud our results concerning the influence of latex parameters on particle spreading.
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Figure 3. A 5000 nm by 5000 nm AFM constant-force image of latex A deposited on a calcium carbonate crystal after water evaporation. The vertical scale is height in nanometers. The mass concentration of the initial diluted latex solution is 0.2 x 10-3. After drying a t room temperature, the samples were directly imaged in air. For some experiments, the samples were annealed in an oven at different temperatures before AFM imaging.
Results (a)Ordering of Polymer Particles on a Substrate. A typical AFM image obtained with latex A on a calcium carbonate crystal is shown in Figure 3. The “dark mountains” correspond to latex particles, and the bright flat surface between the “mountains”corresponds to the calcium carbonate substrate. This image reveals the ordering of polymer particles after water evaporation. The particles are rather well dispersed on the crystal, and for this chosen concentration they do not cover the whole crystal surface. The substrate surface wetted by a l-pL droplet of the diluted latex solution is about 16mm2,which induces a surface coverage of 12.5 ng of latex/mm2. This calculated surface coverage can be expressed in terms of the percentage of the substrate surface covered by latex particles. A rough calculation,without taking into account latex particle spreading, shows that latex particles cover 20%of the substrate surface when the latex concentration is 12.5 ng/mm2. Obviously when the latex solution concentration is increased, the substrate surface coverage is also increased, and complete coverage can be obtained a t high latex concentrations. A rough calculation shows that the substrate surface is covered by a monolayer of latex particles when the mass concentration of the diluted latex solution is 0.8 x Figure 4 shows an AFM image obtained with a concentrated latex A solution on a calcium carbonate crystal. For this experiment the latex solution concentration was 1.0 x in mass concentration. This concentration corresponds to a substrate surface coverage of 125%,which means that the substrate surface is covered by more than a monolayer of latex particles. This image shows that the whole crystal surface is covered by a latex film as expected and that there are n o aggregates. The most striking feature is the strong periodicity in particle packing, consistent with local face-centered cubic (fcc) structure, which induces the well-known foam structure. Our observations are in agreement with AFM results dealing with the surface structure of poly(buty1 methacrylate) latex film.22924It appears that when latex films are prepared by casting onto a substrate, there is formation of monolayers of latex particles which show long-range periodic order consistent with fcc packing. The AFM image of Figure 3 shows that only a few particles are brought into contact. The same results
Figure 4. A 10000 nm by 10000nm AFM constant-force image of latex A film prepared by casting onto a calcium carbonate crystal. The mass concentration of the initial latex solution is 1.0 x 10-3.
concerning particle ordering were found for the other latices (B, C, and D) and the two other substrates (silica and mica), which indicates that our observations are not dependent on latex or substrate characteristics. This means that upon drying, the latex does not flocculate to form aggregates of dozens of particles and that, as the solution evaporates from the surface, the capillary force pulls only a few spheres together. But a t this point we do not know if particles are deformed or not or if they can form a continuous polymeric film on the substrate. (b)Adhesion and Spreading of Polymer Particles. Polymer particle profiles can be recorded with the image processing system, which gives some information on particle dimensions. Figure 5 shows two typical particle profiles obtained for latex A on a calcium carbonate crystal (AFM image of Figure 3). The first profile is asymmetrical whereas the second one is symmetrical. We think that this difference in profile shape is due to friction effects of the tip on particles. It is known that friction as the tip slides on a surface induces errors in images of polymer films and in force measurements with an atomic force microscope.26*2sThis sliding arises when the cantilever is mounted a t an angle to the surface, like in our microscope. Instruments in which the cantilever is parallel to the surface show little if any of this e f f e ~ t . ~ ’ , ~ ~ Without any friction of the tip (second profile in Figure 5), the particle profiles are symmetrical. When friction effects are present (first .profile in Figure 5), friction between the tip and the polymer particle causes the cantilever to bow forward after the tip reaches the summit of the particle (point B in Figure 5). With the optical lever detection system the forward bow causes the beam to be offset downward, making the tip appear lower down than it is and resulting in asymmetrical profile. The analysis of particle profiles allows their diameter and height to be determined,but the measured dimensions are dependent on some parameters of the AFM ap(25) Fretigny, C.; Jones, S.; Boisset, M. C.; Granier, V.; Sartre, A. Submitted for publication in Langmuir. (26) Hoh, J. H.; Engel, A. Langmuir 1993,9, 3312. (27) Burnham, N. A.; Colton, R. J. J. Vac. Sci. Technol. 1989, A7, 2906. (28) Burnham, N. A.; Dominguez, D. D.; Mowery, R. L.; Colton, R. J. Phys. Rev. Lett. 1990, 64, 1931.
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Horizontal distance A'-A' (nm) : 270 Vertical distance B'-B' (nm) : 32 Figure 5. Two typical particle profiles obtained with the image processing system for latex A on a calcium carbonate crystal. The analysis of particle profiles enables their diameter and height to be determined. the asymmetry of the first profile is due to friction effects of the tip on polymer particles. 45
l a t e x A on calclum carbonate
E
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Figure 6. Average height of latex A particles on a calcium carbonate crystal as a function of the force of the probe tip
acting on the sample. paratus.ll For a latex whose Tgis above room temperature like latex D, height measurements 'are reliable and independent of AFM parameters. In contrast for latices of Tglower than room temperature like latices A, B, and C, height measurements have to be taken with caution. Figure 6 shows the average particle height obtained for latex A as a function of the force of the probe tip acting on the sample. The data show a decrease followed by an increase in height as a function of increasing force. The same variations were obtained with latices B and C. The large variation of the height value is due to the fact that these latex particles are soft and that during the scanning the probe tip deforms the particles. When the force is increased, the latex particle profiles have first the same shape. The decrease of the measured height is due to the penetration of the tip into the polymer particle. This penetration of the tip is possible because of the low glass temperature of the polymer (about 5 "C). As the applied force is increased, the tip penetrates deeper into the particles. Above a certain value of force, the particles appear to be wrinkled by the tip and their surface is not smooth. This phenomenon of particle wrinkling explains
the increase of particle height (Figure 6), which is measured as the position of the highest point ofthe profile. Consequently, for soft materials like polymers of low Tg, height measurements are not reliable. The particle diameter measurements were found to be reproducible and not dependent on parameters of the AFM apparatus. Obviously, the particle diameter value calculated by the image processing system is not the exact value since no processing of experimental profiles taking into account the diameter of the tip and the angle between the cantilever and the analyzed surface is done by the image processing system. The tip convolution effects are very important when imaging a sphere on a flat subIn order to provide more reliable quantitative measurements of sample width, a deconvolution of the probe tip from the image is necessary. Such a deconvolution treatment has been proposed by several authors using a mathematical treatment30 or a simple numerical procedure.29 It results in a more accurate picture of the sample. In our work, AFM images were processed using a deconvolution treatment similar to the procedure developed by Markiewicz et al.29 It was assumed that the tip does not twist during scanning. It was found that tip convolution effects are more important when latex particles remain spherical than when particles are spread on the surface (particles are in this case a portion of a sphere). For latex D whose particles remain spherical at room temperature, the measured particle diameters were found to be respectively 240 and 115 nm before and after image deconvolution. In contrast for latex A whose particles are spread on the surface, tip convolution effects are not very important: the average particle diameters were respectively 280 and 260 nm before and after image deconvolution. Particle dimensions obtained from AFM images with different latices and substrates are presented and compared with each other. The comparison shows that some latex and substrate characteristics are critical for particle spreading and adhesion. Effect of Polymer Glass Transition Temperature (TB). During AFM imaging of a calcium carbonate crystal covered with latex D particles (Tg= 41 "C), we observed an effect of the probe tip, which is to move the particles slowly in the fast ( X I scanning direction until the resistance of the particles is sufficient to cause a deflection of the tip. The 1000 nm by 1000 nm area shown in Figure 7a was scanned 5 times before imaging. The particles, which were initially well dispersed on the crystal surface, were brought into contact by'the probe tip. Latex D particles have weak adhesion to the crystal surface since they are moved by the tip. But we cannot measure their adhesion level because we have no quantitative measure for the forces which are exerted locally by the tip on latex particles and which may depend on tip geometry as well as instrumental parameters. We found that the tip interaction decreases with increasing full-scale scanning range, such that the particles could be imaged without being disturbed by the tip but a t the cost of lower resolution. This phenomenon may be attributed to an effect of the increasedx and y scanning velocities at the higher scan size. Our attempts to image latex particles over a 1000-nm full-scale range resulted in significant particle distortions (see Figure 7a),although the minimum force was used to image the crystal. The analysis of the particle profiles gives 112 f 6 nm in average particle height. This average height of 112 nm is very close to the initial latex particle diameter (29) Markiewicz, P.; Goh, M.C.Langmuir 1994,10,5
(30) Keller, D.Surf. Sci. 1991,253,353.
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latex D (Tg = 41 " C )
a
on CaCO,
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A\\\\\\\\\
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spreading 6 = 250 nm
sudden spreading $ = 250 nm
Figure 8. Profilesof latexD particles(Tg = 41 "C)as a b c t i o n of thermal treatment of the sample.
Figure 7. A 1000 nm by 1000 nm AFM constant-forceimage of latex D particles (Tg = 41 "C)on a calcium carbonate crystal: (a) after water evaporation at room temperature; (b) after annealing 30 min at 80 "C.
measured by electron microscopy (100nm); hence latex D particles remain spherical. During water evaporation, capillary forces do not cause any spreading of latex D particles. It is difficult to determine latex D particle diameter. The particles are moved by the tip during the scanningdue to their weak adhesion to the crystal surface, which induces an error in the measured value. After annealing of the sample a t 80 "C for 30 min, we noticed a sudden spreading of the particles which were no longer moved by the probe tip, as can be seen on Figure 7b. The average particle diameter was found to be 250 nm. This sudden spreading is not due to capillary forces since the sample is dried before annealing. When the droplet of latex D solution is placed on a calcium carbonate crystal and dried for 3 min at 80 "C instead of at room temperature, the particles are not moved by the probe tip and the measured diameter is also 250 nm. Hence, particle spreading is the same after drying at room temperature followed by annealing at 80 "C or after drying at 80 "C. In Figure 8 are depicted the profiles of latex D particles recorded as a function of thermal treatment of the sample. By comparing the profiles for the three types of experiments, we can state that capillary forces do not govern particle spreading although these forces are the "driving force" for the formation of a continuous film from late^.^^^
This spreading phenomenon is attributed to the fact that annealing was performed at a temperature higher than latex Tg Above Tg,. the polymer is in the rubber state and the molecular chain mobility is greatly increased, compared to the glass state (below Tg).The latex particle spreading is not governed by capillary forces but by the latex "mechanical properties". In other words, a particle can spread on a surface only if the polymeric chain mobility is high enough, a condition which was obtained at room temperature for latex A. Above Tg, spreading is observed, and the spreading ratio is certainly dependent on latex and surface characteristics. After water evaporation at a temperature higher than polymer Tg(room temperature for latex A and 80 "C for latex D), the samples were annealed at different temperatures to determine if the spreading observed above polymer Tgcan be modified by further annealing. "he analysis of the particle profiles shows that the latex A particle diameter remains equal to 260 nm even for annealing up to 140 "C.The spreading is not modified by sample annealing. The same result was obtained with latex D: above 80 "C the particle diameter remains equal to 250 nm even after annealing for 12 h at 140 "C. Latex D particle heights were measured as a function of annealing conditions (time, temperature). For this latex, the polymer is in the glass state at room temperature and particle heights are independent ofAFM parameters. When the sample is annealed at 80 "Cbefore AFM imaging, particle height decreases when annealing time is increased and a plateau value of 50 nm is reached after 12h (Figure 9). This plateau value seems to depend on annealing temperature. Indeed for an annealing time of 12 h, the average particle height decreases when annealing temperature is increased. Since particle diameter remains constant and particle height measurements are reproducible for latex D, we are led to conclude that there is a decrease of particle volume during annealing above polymer glass temperature. Upon annealing at elevated temperatures, a diffusion of small polymer chains outside the particles is possible. This diffusion phenomenon has been observed between par(31)Winnik, M.A.; Wang, Y.;Haley, F.J. Coatings Technol. 1992, 64, 51.
2184 Langmuir, Vol. 11, No. 6, 1995 I20
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probe t i p
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d+ A\\\n\\\Y
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Figure 9. Average height of latex D particles on a calcium carbonate crystal as a function of annealing parameters (time and temperature).
ticles in the bulk of a poly(buty1methacrylate) latex film by energy transfer measurement^.^^,^^ AFM study of the surface of the same latex film has shown that surface polymer chains undergo also diffision with a characteristic diffusion coefficient lo4larger than that for the polymers in the interior ofthe film.24 Consequently when annealing is performed above polymer glass temperature, we can assume that small polymer chains diffuse from particles, which induces a decrease of particle height. In order to prove that polymer chains can diffuse outside the particles, latex A film samples were immersed in toluene for 7 days. It was determined33 that latex films have a gel-like behavior and that polymer chains of low molecular weight, which are not covalently linked to one another, were extracted by toluene. Annealing above polymer glass temperature induces certainly the same effect concerning diffision of polymer chains as solvent extraction. Consequently above polymer Tg,there is a sudden spreading which cannot be modified by sample annealing (particle diameter remains constant), even if the temperature is such that the cores are in a liquid state and if polymer chains can diffuse outside the particles. Polymer particles cannot form a continuous polymeric film on pigments. As for wettability of the substrate by latex particles, it is a case ofpartial wetting: the particle contact angle is not zero. ZnfZuence of the Substrate. In this section, all the experiments presented were performed with latex A (Tg = 0 "C). Hence with this latex, the final particle spreading is obtained after water evaporation at room temperature and no further annealing is needed. Figure 10 presents the results obtained with three different substrates (calcium carbonate, mica, and silica). As we have shown in the previous section, 1atexAparticles were found to be fixed and spread on a calcium carbonate crystal. When the substrate is a mica sheet, particles are spread but they are slowly moved by the tip during the scanning, which was not observed on the calcium carbonate crystal. When the substrate is a silicon wafer covered by a silica layer, particles are spread but they are strongly moved by the probe tip. When the full-scale scan range is 1000 nm, the particle shift induced by the tip is about 500 nm per scan. These results show that latex adhesion to the surface is dependent on the chemical nature of the surface. Adhesion is strong with calcium carbonate, weak with (32) Wang, Y.; Zhao, C. L.; Winnik, M. A. J. Chem. Phys. 1991,95, 2143. (33) Cohen-Addad, J. P.; Bogonuk, C.; Granier, V. Macromolecules 1994,27 (18),5032.
mica
+ \\\\\\\\\\\\\\'
silica
Figure 10. Schematic view of latex particles after one scan with the AFM probe tip. Influence of the chemical nature of the substrate on particle adhesion: with a calcium carbonate substrate, the particles are fixed on the surface; with a mica substrate, the particles are slowly moved by the tip during scanning;with a silica substrate, the particles are easily moved by the tip during scanning. silica, and intermediate with mica. The adhesion level cannot be characterized by a number because we have no quantitative measure for the forces exerted by the tip on latex particles. Nevertheless, latex adhesion to different substrates can be quantified by using the particle shift induced by the tip. But it is impossible to make a difference between the adhesion level of two latices on a calcium carbonate substrate since there is no particle shift. Particle diameter is difficult to determine on mica and silica substrates. The polymer particles are moved by the tip during scanning due to their weak adhesion to the substrate, which induces an important error in the measured value. An other way to "see" the influence of the substrate on latex adhesion is to rinse the samples with water after their preparation, or to immerse them into water for a few minutes. After rinsing or immersion, the samples are dried at room temperature and imaged in air. With a substrate of calcium carbonate (strong adhesion), latex particles are fixed on the substrate after rinsing and their diameter is not modified. In contrast with mica or silica (intermediate or weak adhesion), there is no particle on the substrate surface after sample rinsing. In this case, latex adhesion is so weak that particles go back to dispersion although they are spread on the substrate. Effect of Latex Acid Content. The effect of latex acid content on particle spreading was studied with a calcium carbonate substrate and the three latices (A, B, and C) whose Tgis equal to 0 "C. The dispersions were purified in an ultrafiltration cell after their synthesis. Consequently, there is no acid in the water phase and all the acid is associated with the latex itself. In all the experiments, the final spreading was obtained after sample drying at room temperature and the particles were fxed on the substrate (no tip-induced movement during scanning). The average particle diameters measured by AFM are respectively 260,255, and 310 nm for latices A, B, and C. However, as the initial particle diameters in the dispersion are different for these three latices, it is difficult to compare the different diameter values obtained by AFM. So we have defined a spreading ratio R , which is equal to the particle diameter measured by AFM after spreading on the substrate over the initial diameter in the latex dispersion. By use of the R parameter, results obtained with latices of different initial particle diameters can be compared. The R values are respectively 2.17, 2.13, and 2.48 for latices A, B, and C. Figure 11shows the variation of the
Langmuir, Vol. 11,No. 6,1995 2185
Latex Particles on Inorganic Surfaces
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/
v) n
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Figure 11. Variation of the spreading ratio R on a calcium carbonate substrate as a function of the surface density of acrylic acid groups which are copolymerized with the core polymers to form the hydrophilic membranes.
spreading ratio R as a function of the surface density of acrylic acid groups which are copolymerized with the core polymer. When the surface density is in the 0.68-3 range, there is only a little increase of the spreading ratio. But above a surface density of 3 AA groups/nm2, which corresponds to full coverage of latex particles by AA sequences, the increase of the spreading ratio is much greater. This result indicates that the spreading ratio is dependent on the thickness of the hydrophilic membrane when particles are fully covered by AA sequences.
Discussion We have observed the ordering and adhesion of latex particles on model inorganic surfaces. The particle adhesion and spreading depend on the characteristics of the latex and the substrate. The results are summarized in Figure 12. Insially (Figure 12a) the particles are dispersed in water. After water evaporation at room temperature (Figure 12b), there is no aggregate of dozens of particles on pigments. Most of the particles are well dispersed, but some of them are in contact with other particles (see Figure 3) but we do not know if these particles conserve their own identity or if there is coalescence. We knowg that for particles with membranes made of copolymerized acid sequences, which is the case for all the latices used in this work, latex films have foam structures, and polymer interdiffusion is only observed after annealing at very high temperatures. But we do not know if the substrate has an effect on coalescence. When the latex is cast on a mineral substrate, the particle surface energy might be modified and the membranes might break up, allowing coalescence and interdiffusion of the core polymers. In fact AFM images (see Figure 4) have shown that when a substrate is covered by a monolayer of latex particles, the particle packing is the same as for thick latex films and there is no coalescence for a carboxylated latex. Consequently, the transformation of particles in contact with neighboring particles during thermal treatment of the sample may be deduced from studies of latex film structure. The particles isolated on the substrate surface may be more or less deformed, depending on the polymer glass transition (Figure 12c). If the drying or annealing temperature is below the core polymer Tg, the particles remain spherical and their adhesion to the substrate i s weak. In contrast when the sample is dried or annealed at a temperature above polymer Tg,the particles are deformed but they cannot form a continuous polymeric film on an inorganic substrate and their adhesion depends
spherical particles weak adhesion
I
deformed particles adhesion = f(latex, substrate)
I I
Figure 12. Transformations of latex particles upon drying. Initially the latex is dispersed in water (a). After drying, most of the particles are rather well dispersed (b). The particles remain spherical and their adhesion to the substrate is weak if the core polymer Tgis above the compositetemperature during dryingor annealing (c). When polymer Tgis below the composite layer temperature during the coating process, the particles are partially spread and their adhesion and spreading ratio are controlled by acid-base interactions between latex particles and the surface (d).
on the chemical nature of the substrate: it is strong with calcium carbonate and weak with silica. Latex particles do not interact in the same way with calcium carbonate, silica, or mica. This behavior is in agreement with NMR (nuclear magnetic resonance) experiment~,3~9~~ which have shown that the degree of interaction between a styrenebutadiene latex and a pigment is higher for calcium carbonate than for clay. In recent years, mainly as a result of the pioneering work of F o ~ k e s , there ~ ~ ~has ~ ' been increasing awareness of the importance of acid-base interactions in adhesion. Latex particles, whose core polymers are surrounded by a hydrophilic layer formed by the copolymerized acrylic acid sequences, can be considered as acid surfaces. Hence, according to the acid-base theory, latex adhesion to a substrate will be stronger if the substrate is considered as a basic surface. For mineral products, the acido-basicity of the surface can be estimated with the isoelectric point of the surface (IEPS),38which is defined as the pH value of suspensions of the pigments in water at which the (34) Parpaillon, M.; Engstrom, G.; Pettersson, I.; Fineman, I.; Svanson, S. E.;Dellenfalk, B.; Righdahl, M. J.Appl. Polym. Sci. 1985, 30, 581. (35)Lepoutre, P. Prog. Org. Coat. 1989,89: (36)Fowkes, F. M.;Tischler, D. 0.; Wolfe, J. A.; Lannigan, L. A.; Ademu-John, C. M.; Halliwell, M. J. J. Polym. Sci. 1984,22, 547. (37)Fowkes, F. M.J. Adhes. Sci. Technol. 1987,1, 7. (38)Bolger, J. C.In Adhesion Aspects of Organic Coatings; Mittal, K. L., Eds.; Plenum Press: New York, 1983.
2186 Langmuir, Vol. 11, No. 6, 1995
surface charge is zero (the number of positive charges equals the number of negative charges, and the zeta potential is zero). A low IEPS value indicates an acidic surface and a high IEPS value indicates a basic surface. The IEPS values are respectively 2,6, and 9.6 for silica, mica, and calcium carbonate. The silica surface is an acidic surface, the calcium carbonate surface is a basic surface, and the mica surface is intermediate. Hence latex adhesion to the substrate increases with the basicity of the mineral surface, which proves that latex adhesion is controlled by acid-base interactions between polymer particles and the substrate. With an acidic surface like silica, latex particles should be covered by a basic layer to obtain a good adhesion. These acid-base interactions can also explain the observed influence of latex acid content on particle spreading. With a basic surface like calcium carbonate, the spreading ratio increases with the surface density of acrylic acid groups (Figure 11).Hence, when the polymer chain mobility is high enough to allow particle spreading, a condition which is obtained above polymer Tg,the spreading ratio can be controlled by modifying acid-base interactions between latex particles and the surface. The results obtained in this study on the wetting of an inorganic surface by latex particles can be used to understand some properties of latices in industrial applications: (i)The particle spreadingratio increases with latex acid content. This means that the contact surface, and therefore the adhesion, between polymer particles and mineral pigments within the composite layer is increased with a high surface density of acid groups, which should improve the composite cohesion. (ii) We have seen that the adhesion of latex particles is dependent on the chemical nature ofthe substrate. Hence, the specific interactions between polymer particles and pigments might explain to a certain extent the influence of the pigment (clay or calcium carbonate) on composite properties. Clay, an aluminosilicate-like mica, is made of plates. The cristalline structures and the physicochemical properties of faces and edges are different. In particular, the IEPS value is not the same for faces and edges. The IEPS value is about 7.5 for the edge surface whereas it is equal to 6 for the face surface. This means that latex adhesion to clay is higher on edges than on faces. Consequently, for latedclay composites, the inplane strength must be higher than the z-direction strength as observed e ~ p e r i m e n t a l l y . ~But ~ , ~compared ~
Granier and Sartre
to calcium carbonate, the clay surface is less basic. Latex adhesion to the pigment is therefore stronger with calcium carbonate than with clay, which means that calcium carbonate should give a higher strength than clay. Obviously, acid-base interactions cannot explain all the observed differences, and other parameters, like for example pigment shape, also have to be taken into account. (iii)The results on particle spreading might be helpful in understanding the penetration of liquids (water, inks, ...) in the composite layer, since this phenomenon is controlled by the porosity and also by the interfaces within the layer, i.e., pigment surface and polymer particle surface. The most important interfaces are the polymeric ones because mineral surfaces are less reactive with liquids used in industrial applications of latices. By adjusting the specific interactions between the polymer particles and the mineral surface, we could control the particle spreading and consequently the extent of polymerlair interfaces within the coating.
Conclusion This study shows that AFM can be used to investigate the ordering and adhesion of latex particles on an inorganic surface, a t the particle scale. After drying there is a monolayer of rather well dispersed particles on the surface. In particular, there is no flocculation or aggregation. Above polymer Tg,there is a sudden and partial spreading of the particles which is governed by the polymeric chain mobility and not by capillary forces. The particle adhesion and spreading ratio are controlled by acid-base interactions between latex particles and the surface. Increasing the particle surface density of acrylic acid groups can improve particle spreading. Sample annealing above polymer Tginduces the diffusion of low molecular weight polymer chains outside the particles and consequently a decrease of the measured particle height. Since friction effects of the tip on polymer particles were observed for latex of Tglower than room temperature, nanotribology experiments could be performed with an atomic force microscope by measuring friction coefficients on polymeric materials as a function of tip position.
Acknowledgment. The authors thank J. F. d'Allest for the synthesis and characterization of the dispersions and M. Joanicot for many fruitful discussions. LA940717Y
