Langmuir 1995,11, 4454-4459
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Atomic Force Microscopy Study of Polystyrene Latex Film Morphology: Effects of Aging and Annealing Andrea Goudy, Michelle L. Gee,* Simon Biggs, and Sylvia Underwood’ School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia Received June 15, 1994. I n Final Form: July 21, 1995@ The surface topography of polystyrene latex thin films (-1-pm thick) deposited from surfactant-free latex dispersions onto mica was investigated by atomic force microscopy (AFM) as a function of particle size, annealing time and temperature, and aging at ambient temperatures. The films were observed t o be polycrystalline, consisting of hexagonal domains of long-range,close-packed particles. Greater particle monodispersity resulted in fewer crystal defects. Films aged at ambient temperatures did not exhibit interparticle fusion, within the resolution limits of the AFM. Limited particle fusion is observed when the films are annealed: the particle heights decreased dramatically but only when heated at temperatures above the glass transition of polystyrene, whereas the interparticle distance between adjacent, closepacked particles was observed to remain constant under all annealing and aging conditions. A smaller particle sized latex resulted in a faster rate of interparticle fusion, thought t o be due t o a greater capillary force between the smaller particles. Crystal defects at the film surface became larger in size as a result of annealing, but there was no measurable increase in the number of defects.
Introduction Polymer latices are colloidaldispersions of discrete, soft polymer particles commonly suspended in an aqueous continuous phase. The ease with which spherical latex particles of a narrow size distribution can be prepared leads to their widespread use as model systems for fundamental studies in colloid ~cience.l-~ Spreading of these opaque dispersions onto a substrate followed by the subsequent evaporation of water can, in certain cases, yield transparent, coherent, polymeric film^.^-^ Synthetic latex films have widespread industrial applications, such as paints, paper, and textile coatings. Physical properties of latex films, such as transparency, surface roughness, porosity, and tensile strength, can be varied depending on the composition ofthe polymer latex and film formation conditions. Previous ~ t u d i e s have ~ - ~ described the film-formation process as being divided into three distinct stages: Stage 1. The particles exhibit increasingly restricted Brownian motion during water evaporation until they come into contact (latex volume fraction > 0.74). The uniformity of the packing at this stage depends on both the polydispersity and the ionic strength of the original latex dispersion.8 Stage 2. Further, slower evaporation ofwater leads to deformation and coalescence of the soft deformable particles, primarily due to capillary forces. Coalescence in this sense means that the particles come into contact. Coulombic repulsion between charged particles, viscous
* To w h o m correspondence should b e addressed. E-mail:
[email protected]. Current address: IC1Valchem, Newsom St., Ascot Vale. Victoria 3032, Australia. Abstract published inAdvanceACSAbstracts, October 1,1995. (1) Ottewill, R. H.; Shaw, J. N. Discuss. Faraday SOC.1966,42,154. (2) Watillon, A,;Joseph-Petit, A. M. Discuss. Faraday SOC.1966,42, 143. (3) Ottewill, R.H.; Shaw, J . N. J . Electroanal. Chem. 1972,37,133. (4)Dillon, R. E.; Matheson, L. A,; Bradford, E. B. J . Colloid Sei. 1951,6,101. (5) Brown, G. L. J . Polym. Sci. 1956,22,423. ( 6 ) Voyutskii, S. J . Polym. Sci. 1958,32,528. (7) Bradford, E. B.; Vanderhoff, J. W. J . Macromol. Chem. 1966,1 (2), 335. (8) 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.
and elastic deformation of the polymeric particles, as well as steric repulsion due to external stabilizers oppose this coalescence.8 Stage 3. Some degree of fusion (often termed “autoadhesion”6),i.e., interdiffusion of polymer chains across the particle-particle interface, results in, ideally, a mechanically continuous film. Further dehydration ofthe film occurs by diffusion of water, either through capillary channels between the deformed spheres or through the polymer itself. Films formed from high-volume-fractiondispersions of monodisperse, charge-stabilizedparticles will tend to order during the first stage of film formation. Such films have been shown to be crystalline in nature, having face centered cubic (fcc) structures.8 In the subsequent deformation process (stage 21, the particles acquire flat faces and transform into polyhedra. Small-angle neutron scattering (SANSY and freeze-fracture transmission electron microscopy (FFTEMP9 studies support the postulationlothat the individual particles in close-packed films become rhombic dodecahedra. Thus, each closepacked particle in such a film has 12 faces (contact regions with adjacent particles). During stage 3, where fusion occurs, particle contours either remain visiblell or totally fade.7 Direct evidence ofparticle fusion has been obtained by fluorescence technique^,^ SANS,12-14and direct nonradiative energy-transfer e x p e r i m e n t ~ . ’ ~The J ~ ’degree ~~ of fading of particle contours with aging or annealing of the film is an indirect measure of the extent of fusion. The advent of the atomic force microscope (AFM) has led to a revived interest into the formation of latex films since it is able to provide high-resolution images of film surfaces and also offers the advantage that samples do not require pretreatment. This is highlighted by recent
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(9) Shivers, R. R.; Dinsdale, C. J.; Wang, Y.; Kats, A,; Juhu’e, D.; Winnik, M. A.Langmuir 1992,8, 1435. (10) Lissant, K. J. J . Colloid Interface Sci. 1966,22,462. (11) Kanig, G.; Neff, H. Colloid Polym. Sci. 1975,253,29. (12) Anderson, J. E.; Jou, J. H. Macromolecules 1987,20,1544. (13) Hahn, K.; Ley, G.; Oberthur, R. Colloid Polym. Sei. 1988,266, 631. (14) Hahn, K.; Ley, G.; Schuller, H.; Oberthur, R. Colloid Polym. Sei. 1986,264,1092. (15) Zhao, C. L.;Wang,Y.; Hruska, Z.; Winnik,M. A. Macromolecules 1990,23(181, 4082. (16) Wang, Y . ;Zhao, C. L.; Winnik, M. A. J . Chem. Phys. 1991,95 (31, 2143. ~~~
0743-7463/95/2411-4454$09.00/0 0 1995 American Chemical Society
AFM Study of PS Latex Film Morphology AFM studies on the surface structure of poly(buty1 methacrylate) (PBMA) latex films.17-19 However, there are some recently postulated limitations to the accuracy of atomic force microscopy. These include the effects of finite tip as well as possible interference effects due to interactions between the probe tip and the surface,22 all of which must be considered in such studies. The main objectives of the present investigation are, by means of atomic force microscopy, to establish a correlation between thevariation in surface topography oftransparent latex films with particle size, aging, and annealing conditions. To date, emphasis in this field has been on films composed of latex with glass transition temperatures (T,) at or below room temperature. It is for this reason that, in the present study, we have turned our attention to polystyrene latex films, since polystyrene has a distinctly higher Tg(100 "C). We intend to compare what is already known about the effects of aging and annealing on the structure of low-T, latex films with the high-T, polystyrene latex films. Previous studiesg have found that the presence of surfactant disrupts the ordered packing commonly observed in monodisperse latex films. In addition, the introduction of surface-active agents as part of the polymerization process has been associated with the appearance of surface exudates, upon film aging.12 This phenomenon was attributed to the incompatibility of the surfactant with the latex polymer, giving rise to the eventual expulsion of the surfactant from the film in the advanced stages of film coalescence. Hence, to prevent this disruption to ordered packing of the latex particles, a surfactant-free polymerization technique was employed for the polystyrene latex synthesis.
Experimental Section Materials. All water used was obtained from a three-stage W 1cm-I, Millipore Milli-Q system (conductivity
