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Atomic Force Microscopy Study of Latex Film Formation Yongcai Wang, Didier Juhu6, Mitchell A. Winnik,* On Man Leung, and M. Cynthia Goh Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 1 A1 Received August 30, 1991. In Final Form: November 21, 1991 Atomic force microscopy (AFM)images are reported for the surfacesof poly(buty1methacrylate)(PBMA) latex films. Films (30 bm thick) prepared by casting onto atomically smooth mica from surfactant-free latex dispersionsshow long-rangeperiodic order consistentwith face-centered cubic packing. Spin-coated films exhibit an open structure composed of particle clusters. Each cluster is highly ordered, but the ordering extends only over 10 or so particles. When the cast films are annealed (6 h, 70 "C), particle-size holes appear in the film surface. We expect that these occur through relaxation of internal defects which allow individual particles to fall from the surface layer into an internal void. Introduction Latex films have become the subject of intense attention' as new techniques provide deeper insights into the processes of ordering, deformation, and fusion that accompany the transformation of an aqueous dispersion of latex particles into a continuous transparent latex film. Scattering studies2indicate that under certain conditions (low salt, no ionic surfactant, monodisperse particles), charge-stabilized latex particles undergo spontaneous ordering in water. This face-centered cubic (fcc)structure is preserved as the water evaporates. The particles touch and stick once the latex volume fraction (4") exceeds 0.74. The forces associated with further evaporation of water drive the deformation of the particles to fill space and form a void-free film. If these forces operate isotropically, the spherical particles should be deformed into rhombic dodecahedra.2 The hexagons often seen by transmission electron microscopy (TEM) in stained thin sections could arise from this kind of process.3 Recent images produced by freeze fracture of nascent films coupled to TEM studies of fracture surface replicas (FFTEM) provide vivid views of an fcc ordering of these rhombic d o d e ~ a h e d r a . ~ ? ~ In the presence of salt and surfactant, ordering in solution should be disrupted.2 Water evaporation would lead to random close packed spheres, which upon deformation would lead to a close packed array of random Wigner-Seitz structures.6 These types of structures have also been seen in a very striking fashion in freeze fracture TEM images of a poly(buty1 methacrylate) (PBMA) latex film formed in the presence of sodium dodecyl sulfate (SDS)~urfactant.~ When these films are subjected to temperatures more than 20 to 30 "C above their glass transition temperature, polymer diffusion occurs across the interparticle interface. On a molecular level, this process can be studied by labeling (1) Karasa, D. R., Ed. Additives for Water-based Coatings; Royal Society for Chemistry: Cambridge, UK, 1990. (2) (a) Joanicot, M.; Wong, K.; Maquet, J.; Chevalier, Y.; Pichot, C.; Graillat, C.; Lindner, P.; Rios, L.; Cabane, B. Prog. Colloid Polym. Sci. 1990, 81, 175. (b) Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Lindner, P.; Cabane, B. Submitted for publication. (3) Kanig, G.; Neff, H. Colloid Polym. Sci. 1975,253, 29. Distler, D.; Kanig, G. Colloid Polym. Sci. 1978, 256, 1052. (4) Wang, Y.; Kats, A.; Juhu6, D.; Winnik, M. A.; Shivers, R. R.; Dinsdale, C. J. Submitted for publication in Langmuir. (5) Roulstone, B. J.; Wilkinson, M. C.; Hern, J.; Wilson, A. J. Polym. Jnt. 1991, 24, 87. (6) Zallen, R. The Physics of Amorphous Solids; Wiley-Interscience: New York, 1983.
some of the latex particles appropriately with deuterium or with donor and acceptor fluorescencedyes and following the interdiffusion with neutron scattering'J' or energy transfer meas~rements.~ FFTEM studies on PBMA latex films demonstrate that particle structure fades and disappears at the point where 40-50% of the latex polymer has interdiffused. Here we report examination of latex film surface by atomic force microscopy (AFM).'O This technique provides high-resolution three-dimensionalimages of the film surface. What is particularly important from the point of view of f i i aging experiments is that AFM does not require pretreatment of the samples and can be operated in an essentially nondestructive mode. Films can be examined, placed in an oven for annealing, removed, cooled, and reexamined many times. Here we show several interesting features of the film formation and annealing process. Experimental Section Materials and Film Preparation. Filmswere prepared from two seta of poly(buty1 methacrylate) latex dispersions whose preparation and characterization have been described.' The larger particles (d = 337 nm) are present as a surfactant-free dispersion (13 wt % solids). The smaller particles (d = 114 nm) are in a dispersion (37 wt % ) containing 2 wt % solids of SDS. Films were prepared by pouring a few drops of latex dispersion onto a freshly cleaved mica surface (10mm X 10mm) and allowing the film to dry slowly at 36 O C for 4 h for the salt-free dispersion of d = 337 nm and at 22 "C for 22 h for the dispersion of d = 114 nm containing 2wt % SDS. The spin-coatedfilms were prepared on mica substrate at lo00 rpm for 40 s and dried further at 36 "C for 4 h. Some of the films were annealed at 70 O C for various amounts of time. The cast films are about 20 pm thick. Spincoated films are about 2 Mm thick. Atomic Force Microscope. The AFM used is a Nanoscope I1from Digital Instruments,Inc., Santa Barbara, CA. It is capable of scanning a region up to 12 pm X 12 pm in size. The tip for imaging is made of microfabricated silicon nitride (SisNd. It is attached to a 100-pm cantilever with a force constant of about 0.58 N/m. The deflection of cantilever is monitored by the deflection of a laser beam reflected off the backside of the cantilever, which is detected by a photodetector. Details of the (7) Hahn, K.; Ley, G.; Schuller, H.; Oberthur, R. Colloid Polym Sci. 1986,64, 1029; 1988,66,631. (8)Linn6, M. A.: Klein, A.; Sperling, . - L. H. J. Macromol. Sci., Phys. 1988, B27, 181, 217. (9) (a) Zhao,C.-L.;Wang, Y.;Hruska,Z.;Winnik,M. A.Mucromolecules 1990,23,4082. (b) Wang, Y.;Zhao, C.-L.;Winnik, M. A. J . Chem. Phys., in press. (c) Winnik, M. A.; Wang, Y.; Haley, F. J. Coatings Technol., in press. (10) (a) Binnig, G.;Quate,C. F.;Gerber,Ch. Phys. Reu.Lett. 1986,56, 930. (b) Bumham, N. A.; Colton, R. J. J. Vac. Sci. Technol. 1989, A7, 2906.
0743-7463/92/2408-0760$03.00/00 1992 American Chemical Society
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Figure 1. Surface image of a latex film prepared by slow evaporation (4h) at 36 "C of water from an aqueous dispersion (ca. 8 wt %) of 337 nm diameter PBMA particles onto a freshly cleaved mica surface. instrumentation are described elsewhere.1° All the AFM measurements presented were made in ambient air. A typical N. The operating force for the sample was on the order of instrument was operated inside a soundproof room covered with Styrofoam for minimizing the sound and air vibration. The data for individual measurements were collected in less than a minute in order to minimize effects due to thermal drift. All the AFM measurements were done in varying height and constant force mode. The z-feedbackcircuit adjusts the z-position of the sample keeping the deflection of the cantilever constant while scanning the sample in x and y directions.
Figure 2. Surface image of a latex film prepared from the dispersion described in Figure 1 by spin coating at 22 "C onto a mica surface, followed by drying in air at 36 "C for 4 h.
Result and Discussion Figure 1shows the surface of a film prepared from the salt-free dispersion of d = 337 nm latex by pouring a few drops onto a freshly cleaved mica surface and allowing the film to dry slowly at 36 "C. The mica surface itself is smooth on the atomic scale. Thus the textures observed in the micrographs are due to features only of the films. The film surface is dominated by a softly undulating smooth surface interrupted in places by rugged dislocations and craters. The most striking features is the strong periodicity in particle packing, consistent with local fcc structure. Near the crevices, packing of particles is more random, and particle deformation is less pronounced. In close-up view, the particle surface are rounded and resemble TEM pictures of the vinyl acrylic latex film surface reported by Padgett.I1 Spin-coated films from the same PBMA latex reveal a completely different type of surface. One sees a random packing of locally ordered aggregates to generate a porous structure (Figure 2). Temperature control during spin coating is not possible for us, and water evaporation is rapid. This cools the surface. We imagine that there is a competition between sticking and deformation, which would occur as long as the temperature remains above the minimum film forming temperature (MFT). Below this temperature, there will be a resistance to deformation as the temperature drops. PBMA has a MFT around 22-25 "C and should be sensitive to temperature changes near room temperature. In addition, the shearing force produced during sample spinning could also lead to the random packed, locally ordered structures one sees in (11) (a) Padget, J. C.; Moreland, P. J. J . Coatings Technol. 1982,55, 39. (b) See ref 1, Chapter 1.
Figure 3. Surface image of a latex film prepared by slow evaporation (10h) at 22 "C of water of 114 nm diameter PBMA dispersion (37 wt %) containing 2 wt ?6 SDS surfactant.
Figure 2. Shearing is known to disrupt the colloidal crystalline phase in aqueous dispersion.12 Films were prepared by slow evaporation of water from a second batch of particles with d = 114 nm. This dispersion contained SDS (2 wt % solids), NaHC03 (0.3 wt %), and latex (37 wt %). Films formed from this latex have been studied in detail in our laboratory, both to assess internal film structure by FFTEM4 and to follow polymer diffusion during film annealing by energy transfer measu r e m e n t ~ .The ~ FFTEM experiments4 always revealed a random close packed structure in the interior of nascent films. Quite to our surprise, these films often showed a highly ordered surface structure. To emphasize this ordering, we show a low-resolution image in Figure 3 to demonstrate that this order persists, with some dislocations over length scales of several micrometers. We conclude this letter with a remarkable picture of the film shown in Figure 1, after annealing this film for 6 h at 70 "C. For this latex, composed of high molecular (12) Ackerson, B. J.; Clark,N.Physica A 1932,118, 221.
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Figure 4. Surface image of the same film shown in Figure 1 -after annealing for 6 h at 70 "C. weight polymer (Mw= 220 000, Mw/Mn= 2.5), we still
expect significant interparticle polymer diffusion. Nevertheless we see, in Figure 4, small holes appearing in the surface. No such holes are seen anywhere in the surface of freshly formed film after drying 4 h at 36 "C. Many such holes appear in the film after 6 h at 70 "C. One of the advantages of AFM over other microscopies is that it permits us to follow the evolution of the surface structure in a single film sample.
To explain the features seen in Figure 4, we assume that the nascent film contained a number of particle-sizevoids in the interior but near the surface. Migration of the holes to the surface must be accompanied by migration of particles into the film interior. To the best of our knowledge, this is an unprecedented observation and is particularly surprising for soft latex, where the tack should lead to adhesion. Films annealed for 2 h at 70 "C do not show these holes but do show depressions which may indicate of the onset of surface rearrangements. This is clearly a phenomena meriting further investigation. In summary, we report the first AFM study of latex film formation and aging. We observe strong ordering in films prepared from salt-free dispersion of PBMA latex and substantial surface ordering in latex films formed in the presence of surfactant where the interior is known to be randomly close packed. Spin coating leads to a more open structure as ordering, adhesion, and rapid evaporative cooling compete during film formation. When films of a high molecular weight latex showing long range surface ordering were heated, holes appeared in the surface consistent with slow diffusion of voids to the surface.
Acknowledgment. We thank the Institute for Chemical Science and Technology, NSERC Canada, and the Province of Ontario for their support of this work. Registry No. PBMA, 9003-63-8.
