Langmuir 1992,8, 1435-1442
1435
Freeze-Fracture Studies of Latex Films Formed in the Absence and Presence of Surfactant Yongcai Wang, Aviva Kats, Didier JuhuB, and Mitchell A. Winnik* Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario, Canada M5S 1Al
Richard R. Shivers and Christopher J. Dinsdale Department of Zoology, University of Western Ontario, London, Ontario, Canada N6A 3K7 Received June 26,1991. I n Final Form: December 23, 1991 We report freeze-fracture transmission electron microscopy studies of poly(buty1 methacrylate) latex films for the cases of both large (d = 337 nm) surfactant-free latex and smaller (d = 117 nm) latex in the presence of 2 wt % ! sodium dodecyl sulfate. The large particles pack in face-centered cubic (fcc) ordered domains, and undergo deformation to form perfect rhombic dodecahedra. In the presence of added surfactant, the fcc packing is suppressed, and the particles are deformed irregularly. The smaller latex give random packed structures. In all systems, annealing leads to fading of the image in the fracture surface. Since the diffusion coefficients of the polymer are known from other work (cf. ref 9),we can correlate the evolution of film morphology with the extent of interparticle polymer diffusion. When an aqueous dispersion of soft latex particles is allowed to evaporate, a transparent void-free film is formed.18 This process of latex film formation is the basis of many latex coating technologies. Traditionally the film formation process has been considered in terms of three sequential steps: water evaporation to the point where the particles begin to touch, deformation of the latex spheres to space-filling polyhedra induced by surface and osmotic forces associated with passage of water from the interstitial spaces, and coalescence of the deformed particles to form a mechanically continuous film. As our understanding of latex film increases, it will be convenient to differentiate further stages in the film formation process. For example, at low ionic strength, monodisperse latex particles undergo ~ r d e r i n g . ~This ?~ has consequences on how the latex particles order and pack within the film. It is also important to distinguish coalescence from fusion in the formation and aging of latex films.3 Chevalier et al.3bdefine coalescence as the breakup of the hydrophilic layers surroundingthe particles, allowing the cores to come into contact.3c By fusion we refer to the subsequent interdiffusionof polymer molecules across the particle boundary. This sequence of events is depicted in Figure 1. Interdiffusion is responsible for the growth in mechanical strength of latex films that often occurs when these films age or are annealed. This process has sometimes been referred to as "further coalescence." One of the challenges in the development of a proper picture of the film formation process is to understand how (1) Patton, T. C. Paint Flow and Pigment Dispersion; WileyInterscience: New York, 1979. (2) K-D. R.,E!d.Additiuesfor Water-BasedCoatings;RoyalSociety for Chemistry: Cambridge, U.K., 1990. (3) (a) Joanicot, M.; Wong, K.; Maquet, J.; Chevalier, Y.; Pichot, C.; Graillat, C.; Lindner, P.; Rim, L.; Cabane, B., Prog. Colloid Polym. Sci. 1990, 81, 175. (b) Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Maquet, J.; Lindner, P.; Cabane, B. Polym. Int., submitted for publication. (c) This defmitiondiffers from the more phenomenological definition of coalescence used in the coatings area. In texts on coatings (cf. ref 1, p 253), coalescence during latex film formation refers to the procaee comprisingparticle deformation and interparticle adhesion. The more specificdef~tion of Chevalier et al.,sbwhich we employ here, would considera latex film in which the hydrophilicmembranesremained intact aa not having coalesced. From a physics perspective, the latex droplets in such a film maintain their integrity and thus have not coalesced. (4) Distler, D.; Kanig, G. Colloid Polym. Sci. 1978, 256, 1052.
Latex F i l m Formation Process
Stage I wate I evaDorates
-Close-contact wrticles
Stage II Darticle deforms TZ MFT
_ -
o1
-
-
Stage IU coaleJce"ce* T> Tg
Mechanically rigid film
Figure 1. A representation of the film formation process.
differences in latex structure and composition, as well as in the dispersion formulation, affect the mechanism of film formation. For example, surfactant-freedispersions at low ionic strength form films which dry differently than those containing surfactant. The examples we cite below are typical. Films prepared from our surfactant-free poly(butyl methacrylate) (PBMA) latex dispersions dry uniformly. The thicknesses of these films contract evenly with no patchiness or haziness apparent to the eye. Surfactant-containing films prepared from the same or similar latex dry from the edges inward, accompanied by a propagating front separating a transparent dry region from a turbid moist domain. Here, as the film dries and the particles deform, water and residual surfactant are squeezed out of the interstitial species. In both instances, one ultimately obtains a transparent and void-free film. Chevalier et al.3bhave studied the drying and coalescence
0743-7463/92/2408-1435$03.00/00 1992 American Chemical Society
Wang et al.
1436 Langmuir, Vol. 8,No. 5, 1992 processes by small angle neutron scattering (SANS). Diffraction spots observed in the scattering experiments indicate 3rdering within the liquid phase. When repulsions between the latex particles are strong, the dispersion orders to form a colloidal crystalline phase with face-centered cubic (fcc) packing. This order persists as water evaporates, even as the volume fraction (&) of polymer increases well beyond that (4, = 0.74)at which particle contact occurs. In the range of 4" > 0.74 water evaporation is accompanied by particle deformation. In their view there is a thinning of the hydrophilic regions into membranes whose ultimate fate depends upon the amount and nature of the hydrophilic material present. In the case of films prepared from deuterated latex covered with a surface layer rich in partially neutralized polyacrylic acid (PAA), they could observe by neutron scattering rehydration of the dried films when exposed to H z O . ~The ~ rehydration was not uniform, but it was sufficiently extensive for them to conclude that a largely intact and interconnecting membrane of hydrophilic material (PAA)was present in the film. The membrane separates individual particles and provides channels for diffusion of water. Its presence suggests that little c o a l e s ~ e n c ehas ~ ~occurred. When surfactant is present, drying occurs as a moving front. On the wet side of the front, the latex is present as a concentrated colloidal liquid (& < 0.74).As the front passes, the system undergoes a transition to a "water-inoil"-type emulsion of hydrophobic material in a polymerrich phase. Chevalier et al.3buse the language of phase transitions to describe the thermodynamics of this process. The kinetics of the system are also important. If the matrix is sufficiently fluid, the water-in-oil emulsion will break up into a polymer phase and an aqueous phase. Depending upon the tenacity of surfactant adsorption to the latex, it will be either trapped on the particle surface or exuded into the aqueous phase. Coalescence occurs when the hydrophilic membranes break up, and the polymer cores come into contact. These ideas provide a convenient framework for examining the morphology of latex films. In the nascent film formed at low ionic strength, one expects an ordered array of regular polyhedra reflecting the preexisting order in the colloidal liquid. In films formed in the presence of salt and surfactant, less ordered structures should be present, and one should be able to observe the fate of the hydrophilic material trapped in the film. As these films are subsequently aged or annealed, one expects an evolution of the mophology coupled to polymer diffusion across the interparticle boundary until eventually all memory of the initial structure is lost. Transmission electron microscopy (TEM) of latex films, thinly sectioned and appropriately stained, has been the traditional method for examining film m o r p h ~ l o g yOne .~~~ commonly observes patterns of hexagons, consistent with fcc packing in highly ordered films. When these studies are extended to annealed films, one sometimes finds complete disappearance of structure, consistent with extensive polymer interdiff~sion.~ In other systems, the pattern of hexagons persists for long periods of time.* It is tempting in these cases to infer that the hydrophilic membrane, which one stains and observes in the TEM, has suppressed coalescence and prevented interparticle polymer diffusion. To confirm these ideas, one needs studies of film morphology under conditions where the polymer diffusion can be quantified. One of our objectives, in the work described here, is to make the connection ( 5 ) Vanderhoff, J. W.; Bradford, E. B.; Carrington, W. K. J. Polym. Sci., Polym. Symp. 1973, 41, 155.
between the evolution of film morphology, as determined by freeze-fracture transmission electron microscopy (FFTEM) and polymer interdiffusion as followed by fluorescence techniques. Roulstone et al.6 recently reported the first freezefracture transmission electron microscopy (FFTEM) studies of latex film formation. They examined films formed at room temperature from poly(buty1 methacrylate) latex prepared by surfactant-free emulsion polymerization to give particles of 400-nm diameter. Their exquisite photographs showed ordering of the latex in the initial film accompanied by particle deformation to a structure consistent with a rhombic dodecahedral shape. Roulstone et al. also examined the effects of annealing on film morphology. Here they obtained the very satisfying result that the particle boundaries became more diffuse and eventually disappeared with prolonged aging at elevated temperature. This result is consistent with particle fusion occurring by diffusion of polymer molecules.68 At this time we report our own results using the FFTEM technique to study the coalescence and fusion processes in PBMA latex films. First we examine the case of films prepared from large latex particles (d = 337 nm) prepared by surfactant-free emulsion polymerization. Micrographs from these films provide unambiguous evidence of close packing of rhombic dodecahedral structures in the nascent film. Second we describe the remarkable changes which occur in the film formation process under conditions more typical of commercial coatings, smaller (d = 117 nm) particles and the presence of surfactant. In order to bridge a connection between these experiments, we show that simply adding anionic surfactant to the dispersion of the larger particles disrupts the ordering of the particles in the film. Taken together these experiments confirm the ideas of Chevalier et al.3bdescribed above. Finally, we examine the consequences of annealing the films. These are samples for which we have been able, previously, to measure the interparticle polymer diffusion.*ll Thus, we are able to relate interdiffusion to the evolution of film morphology.
Experimental Section Latex Preparation and Characterization. Latex particles were prepared by seeded semicontinuous emulsion polymerization. The smaller particles (d = 117 nm) are the (sample C.12/ 500 An) anthracene-labeled latex (M,= 6.7 X 104,M. = 3.0 X lo4) described in ref 9. Here, following formation of the seed, BMA and anthryl methacrylate (mole ratio 99:l) were fed into the reactor (80 "C) at a slow rate (0.06 mL/min) in the presence of sodium dodecyl sulfate (SDS), KzSzOa,and sufficient NaHCOs to neutralize the acid formed. Heating was continued for 14 h, and then the reaction was cooled to room temperature. The dispersion was filtered through glass wool to remove any coagulum (none apparent). Films were formed directly from this dispersion. Larger particles (d = 337 nm, M, = 220 OOO, M, = 55 OOO) were prepared from BMA in the absence of surfactant, in a batch process and a reaction temperature of 80 O C . Recipes are given in Table I. (6) Roulstone, B. J.; Wilkinson, M. C.; Hearn, J.; Wilson, A. J. Polym. Int. 1991,24,87. See also Roulstone, B. J.; Wilkinson, M. C.; Heam, J. Polym. Int. 1992, 27, 43, 61. (7) Voyutski, S . S. J. Polym. Sci., Polym. Lett. Ed. 1968,25, 528. (8) Chainey, M.; Wilkinson, M. C.; Hearn, J. J. Polym. Sci., Polym. Chem. Ed. 1985,23, 2947. (9) Zhao, C.-L.; Wang, Y.; Hruska, Z.; Winnik, M. A. Macromolecules 1990,23,4082. (10) Winnik, M. A.; Wang, Y.; Zhao, C.-L. In Photochemical Processes
in Organized Molecular Systems; Honda, K., Ed.; Elsevier: New York, 1991. (11) Wang, Y.; Zhao, C.-L.; Winnik, M. A. J. Chem. Phys. 1991, 95, 2143.
Freeze-Fracture Studies of Latex Films
Langmuir, Vol. 8, No. 5, 1992 1437
Table I. Emulsion Polymerization Recipe and Conditions. Small Particles (Seeded Polymerization) seed second stage water, 20 mL water, 45 mL KPS, 36.0 mg BMA, 3.25 mL SDS, 427.7 mg KPS, 42.8 mg PheMMAIAnMMA,0.44610.393 g NaHCOs, 81.6 mg BMA, 24 mL SDS, 81.0 mg temperature, 80 "C temperature,80 "C time, 22 h time, 1h ~
Large Particles (Batch Polymerization) water, 100 mL BMA, 8 mL KPS, 0.045 g NaHC03,0.087 g temperature, 80 "C time, 24 h a Abbreviations: BMA, butyl methacrylate, KPS, K2S208;SDS, sodium dodecyl sulfa&, PheMMA, 9-phenanthrylmethyl methacrylate; AnMMA, 9-anthryl methacrylate.
Particle diameters and their distributions were determined by dynamiclight scattering using a Brookhaven Model B-90 nanosizer. All samples examined had very narrow size distributions. Molecular weights were determined by size-exclusion chromatography (SEC) in tetrahydrofuran (THF) on samples of dried films which were subsequently dissolved in THF. Molecular weights are nominal in that the SEC columns were calibrated using poly(methy1 methacrylate) standards. Film Preparation. Films were prepared a t 36 "C in disposable aluminum flat-bottomed dishes, 50-mm diameter, by addition of sufficient dispersion (ca. 35 wt %I solids) to form a film ca. 100 p m thick. These films were first annealed for 20 h at 36 "C. Some films were heated further. These were then removed from the substrate, placed in polyethylene pouches, and sent to the University of Western Ontario for FFTEM measurements. Fracture Replication.12J3 Films were prepared for freezefracture by cutting them into narrow (ca. 1 mm) strips which were placed upright in Balzers (Balzers,Liechtenstein) well-type gold specimen holders. A drop of 30 % glycerol containing 0.5 7% Triton-X100 was placed on the pieces of film so that they were completely enveloped in glycerol. The specimens in the well disks were frozen for 10 s in a slurry of liquid nitrogen cooled Freon-22, and then placed in liquid nitrogen until further processing could be done. Freeze-fracture of the frozen film samples was carried out at -115 "C in a Balzers BAF 301 freeze-etch unit (Balzers, Liechtenstein) in a vacuum of 1X lo* Torr according to the protocol of Shivers and Brightman.13 Following fracturing, the broken surface of the pieces of fractured film was replicated with a thin film of platinum (ca. 600 %i) and carbon was added finally to stabilize the replica. Replicas were cleaned by soaking them in absolute chloroform until the replicas separated from the latex. Replicas, on bare 200-mesh copper grids, were examined in a Philips 201 electron microscope operating at an accelerating voltageof 60 kV. Replicas of the films were initially photographed at 10000-45000 diameters and then further enlarged (2.7X) photographically.
Results and Discussion Surfactant-FreeLatex. Once the concentration of an ordered array of soft spheres (droplets), for fcc packing, exceeds = 0.74, further shrinkage must be accompanied by droplet deformation. If the shrinkage is isotropic, each particle and its 12 neighbors will experience a simultaneous flattening of the faces, eventually achieving a void-free film comprised of regular polyhedra which maintain the fcc ordering. (In Figure 2 we show an FFTEM image of (12)Disanayaka, B.; Zhao, C.-L.; Winnik, M. A.; Shivers, R. R.; Croucher, M. D. Langmuir 1990,6, 162. (13) Shivers, R. R.; Brightman, M. W.J. Comp. Neurol. 1976,167,l.
Figure 2. A freeze-fracture TEM image of a floc discovered accidentally while examininga dispersion of latex particles. Note not only the flat faces,which are indicativeof particle deformation, but alsothat the fracture had to pass through a cluster of deformed particles in order to expose a flattened surface. The marker bar represents 100 nm.
Figure 3. Two views of a computer drawing of a solid composed of regular rhombic dodecahedra showing the %fold and 4-fold rotation axes. Reprinted with permission fromref 3a. Copyright 1990 Springer-Verlag New York.
the initial stages of particle deformation.) For the fcc structure, each particle has 12 faces, rhombic in shape. There are six vertices where four edges meet at an angle of 71' and eight vertices where three edges meet at an angle of 109'. Two views of this rhombic dodecahedron, showing the 3-fold and 4-fold rotation axes, are shown in Figure 3. The existence of long-range periodic order and fcc packing in latex films is well documented. In the FFTEM micrographs of Roulstone et one sees at low magnification the finite range of these ordered domains, separated by grain boundaries. We confirm these observations and comment that the extent of orientational order observed bv us and by Roulstone et al. is less than that deduced from neutronscattering experimentsby Chevalier et al.3b The French group suggested two explanations for the strong directional order implied by the nature of the
Wang et al.
1438 Langmuir, Vol. 8, No. 5, 1992
Figure 4. Freeze-fracture micrograph of a film prepared at 36 "C from d = 337 nm PBMA latex and annealed for 20 h at 36 "C. The marker bar represents 370 nm.
Figure 6. Another view of the same film sample as Figure 2 which emphasizes as a structural feature the fcc packing of rhombic dodecahedra in the film. The marker bar represents 370 nm.
Figure 5. Another image from the same film sample in Figure 2. The marker bar represents 370 nm.
diffraction spots observed by SANS, either shear-induced alignment during film preparation or the presence of a single crystallite which dominates the scattering. Our samples have not been sheared. This issue remains unresolved. Our focus is on the local periodic order and the shape of the deformed particles themselves. In Figure 4 one sees in the upper right part of the micrograph a striking view of the close-packed rhombic dodecahedral order drawn on the right-hand side of Figure 3. A grain boundary separates this structure from a slightly twisted orientation which dominates the remainder of the micrograph. Here one sees the alternative view of the structure drawn on the left side of Figure 3. We believe these to be the first unambiguous views of the rhombic dodecahedralstructure in latex films. There are several additional features of interest here and in the accompanying micrographs shown in Figures 5 and 6. The first is the strong tendency for cleavage to occur along the interparticle interface. These films were annealed for 20 h at 36 "C, under conditions in which the films acquire sufficient mechanical strength to be selfsupporting and to survive the trip by courier to the University of Western Ontario. Nevertheless, we have evidence from smaller particles that very little interparticle diffusion of polymer molecules occurs at this tem-
perature, even on a time scale of weeks. Some particles do fracture in these films. One sees chipping of corners in Figure 5 and fracture through the centers of those particles in alternate rows in Figure 4. Fracture reveals a radical interior structure. Thisstructure resembles that seen in the fractured films reported by Roulstone et aL6 One interesting difference between their films and ours is the much stronger tendency of our weakly annealed films to fracture across the particle surfaces rather than through the particles. To understand these differences, which are related to the mechanical strength of the film generated during coalescence, one would need to compare various details of both experiments. To facilitate this comparison, we include in our experimental description the details of film annealing and aging, recipes for particle preparation, and the molecular weights of polymers in the latex. We hope that corresponding information will be provided in future publications by the British group. When these films are heated, extensive polymer interdiffusion occurs. In this system we have been able to observe this diffusion by adding a tiny fraction (0.01%) of an essentially identical latex containing 0.5 mol % of a covalently bound fluorescent dye. The fluorescent particles in the film can be observed by fluorescence microscopy.14 As the films are annealed at 90 "C, the spots grow in size. Diffusion coefficients for the polymers were obtained in a separate set of experiments by energytransfer measurements on labeled smaller diameter particles. At 90 "C, the polymers in the film shown in Figures 4-6 have a diffusion coefficient of 2.3 X cm2s-l,ll and annealing at this temperature for 2 h leads to an effective interpenetration depth of 6 nm. Corresponding to this diffusion is a growth in the mechanical strength of the ~~
(14)Kats, A.; Winnik, M. A. Unpublished observations.
Freeze-Fracture Studies of Latex Films
Langmuir, Vol. 8, No. 5, 1992 1439
Figure 7. A fracture surface of a film sample similar to that in Figure 2 which has been annealed for 2 h at 90 "C. The marker bar represents 370 nm.
Figure 8. A fracture surface of a film prepared at 36 "C from d = 117nm particles in the presence of SDS surfactant, and aged for 20 h at 36 "C. The marker bar represents 370 nm.
film. Freeze-fracturenow propagates through the polymer (Figure 7); there is cohesive rather than adhesive failure. Nevertheless, some vestiges of the fcc structure can be observed in the film. Qualitativelysimilar images, showing thermal-induced fading of the particle boundaries, were also reported by Roulstone et al. Small Latex in the Presence of Surfactant. Commercial latex coatings are often comprised of smaller particles, typically 100 nm in diameter, and contain surfactant, pigment, and a host of other additives. For this reason we were curious to see how the film formation and aging process would be affected for small particles prepared in the presence of 2 w t % SDSas a surfactant. This system is identical to that which we have studied by energy transfer.+ll Here we have determined directly the diffusion coefficients of the polymer in the latex films as a function of temperature. To review briefly, the minimum film-forming temperature for the latex sample C.12/500 An is in the range of 24-30 "C. At 22 "C the films formed are slightly hazy and energy-transfer experiments suggest incomplete particleparticle wetting during coales~ence.~J~ Taken together, these observations indicate that the films formed at 22 "C contain voids ranging in size from tens of angstroms to a few hundred ang~tr0ms.l~ In the temperature range of 3&36 "C, the films formed are transparent and void-free. Energy-transfer experiments indicate that some small amount of mixing of surface chains occurs during coalescence and, more important, that no additional polymer interdiffusion occurs if the films are annealed for up to 90 h at 36 "C. Freeze-fracture images of a weakly annealed film (aged for 20 h, 36 "C) of these smaller particles (d = 117 nm) are
Figure 9. A second image from the film sample shown in Figure 6 indicating some of the variability in texture present in the fracture surface of lightly annealed films. The marker bar represents 370 nm.
(15) The evidence for voids a t the angstrom level comes from energytransfer experiments: Winnik, M. A.; Wang, Y.; Haley, F. J. Coat. Technol., in press.
shown in Figures 8 and 9. This film was formed under conditions identical to those of the surfactant-free larger particles. Cohesive failure predominates during cryo-
1440 Langmuir, Vol. 8, No. 5, 1992
Wang et al.
Figure 10. Drawings of the Wigner-Seitz cell of cubic closepacking (a), compared to two possible cells (b and c) that can occur in random close-packing. Reprintedwith permission from ref 16. Copyright 1970 Royal Society (London).
fracture. This behavior is very different from that in the case of the larger surfactant-free films. We might speculate that the surfactant plays a role in promoting adhesion between the particles in the film. It might do so by promoting a small amount of interdiffusion of surface polymer. Alternatively it might affect only the mode of film drying. These films dry via the propagating front process. It may be that passage of the coalescence front is accompanied by forces of compression strong enough to induce this enhanced adhesion. If one looks closely a t the images of this film, one can see occasional flattened surfaces that can be ascribed to vestiges of the surface of individual deformed particles. These can be seen more clearly in Figure 12 than in Figures 8 and 9. Where apparent, the shapes are not regular, as in Figures 3 and 4, but seem more random in size and shape. Finney16carried out a detailed analysis of the statistical topology of an experimentally constructed random closepacking model of 800 spheres. The particle shape is characterized by irregular Wigner-Seitz cells. In particular, there is a distribution in the number of faces per polyhedron. The most common occurrence is that of 14faceted structures, followed by 15-, 13-, 16-, 12-, and 17- ' faceted cells. Figure 10 shows drawings of two possible polyhedra corresponding to random close-packing.17 When these films are annealed, the texture due to discrete particles is rapidly lost. An example is shown in Figure 11 for a sample annealed for 30 min at 80 "C.The film appears to have undergone complete fusion, and a sample taken for film aged for 3 h a t 80 "C looks very similar. Energy-transfer experiments indicate that the polymers in the sample shown in Figure 11 have diffused a distance of ca. 25 nm across the interparticle boundary. This is sufficient not only to randomizethe conformations of all chains initially adjacent to the particle surface, but also for each particle to have exchanged half its contents with its neighbors. It is thus not surprising that, under these conditions, all vestiges of particle structure have disappeared from the freeze-fracture image. The images obtained from these films annealed for 30 min and 3 h a t 80 "C contain patches of structure like that shown in Figure 12. Similar structures can be seen in Figures 8 and 9 for the fracture surfaces of nascent films. In annealed films, these structures represent 5-10% of the fracture surface, and the size of the structural subunits is characteristic of that of the latex in the nascent film. Such structures are never seen in films prepared from our surfactant-free latex dispersion. We believe that these structures are due to residual surfactant in the film, and return to this point later. We also find it interesting that the smoothfeatures of the fracture surface of these patches (16)Finney, J. L. R o c . R. SOC.London 1970, A319,479. (17)Zallen, R. The Physics of Amorphous Solids;Wiley-Interscience: New York, 1983; p A.54ff.
Figure 11. A fracture surface of a film sample similar to that in Figure 6 which has been annealed for 30 min at 80 "C. The marker bar represents 370 nm.
Figure 12. Another image from the sample shown in Figure 9. The marker bar represents 370 nm.
resemble the surface in freeze-fracture of lipid bilayers, which often fracture through the hydrocarbon core of the bilayer. Large Particles plus Surfactant. While FFTEM is
Langmuir, Vol. 8, No. 5, 1992 1441
Freeze-Fracture Studies of Latex Films
Figure 13. Freeze-fracture micrograph of a film (annealed 20 h at 36 "C) prepared from a dispersion of the d = 337 nm PBMA latex particles to which 2 w t % solids sodium dodecyl sulfate was added. The marker bar represents 1.0 pm.
Figure 14. A 2-fold higher resolution image of a section of the same fracture surface shown in Figure 13. The marker bar represents 100 nm. multibitayer
propagating
a powerful method for observing the texture of a fracture surface, it has the disadvantage that one cannot identify composition. In the preceding section,we ascribed smooth patches in the fracture surface of annealed latex films to the cleavage of surfactant crystals. These structures are absent from all samples prepared from surfactant-free latex. In order to examine this issue more deeply and to asses the effect of postadded surfactant on the film formation process, we added sufficient SDS to the dispersion of 337-nm-diameter PBMA latex so that the films would contain 2 wt % surfactant. Films were then prepared from this mixture and examined by FFTEM. These films dry by propagation of a coalescence front. In Figure 13 we show an image of a fracture surface of this film at low resolution. The film shows a significant degree of order. The system is significantly less ordered than that shown in Figure 4, with no evidence of fcc packing. The fracture plane has a greater tendency here to pass through the particles than for the surfactant-free films, implying somewhat more cohesion in these lightly annealed films. At higher resolution one has a clear view of the polyhedral shape of individual particles particles in the film. The faces of these polyhedra encompass a variety of shapes, with hexagonal faces being among the most common. Figure 14provides the clearest evidence to date of the formation of irregular Wigner-Seitz cells in latex films. Close inspection of Figure 14 reveals a small number of smooth patches with a texture identical to that seen in Figure 12 and attributed to fracture through surfactant multibilayer crystals. These patches are comparable in size to one face of a polyhedron. When the films are annealed (e.g., 8 h at 90 "C), the polyhedral structure of the polymer disappears but these smooth patches do not. Under these circumstances, the FFTEM images resemble those in Figures 11and 12. These results confirm the idea that these structures are due to trapped surfactant in the film. The presence of surfactant crystals in films which dry via a propagating coalescence front is also consistent with the picture of the film formation process developed by Chevalier et al.3b In our view, surfactant becomes concentrated just ahead of this front, and associates or crystallizes to form multibilayer structures, which from time to time become trapped between adjacent layers of
latex film
surfactant
latex f i l m
Figure 15. A cartoon depicting the crystallization of SDS surfactant in the film showing that the crystals assume a shape which captures the particle packing during the drying process.
deformed particles. The kind of structure which we envision is depicted in Figure 15. It would assume the contour of particles in the nascent film, but would undergo little rearrangement during the annealing stage. Upon freeze-fracture, the bilayer core would be the fracture plane of preference. The smoothness of the surfaces seen in Figure 12 is completely consistent with such a process. We know from other experiments with PBMA latex films that, under our annealing conditions, there is very little diffusion of surfactant to the film surface. This diffusion becomes prominent only when significant amounts of a coalescing solvent are added to the dispersion.18 Thus, essentially all the surfactant is trapped in the film for the samples reported here. In the case of the films prepared from the smaller latex, the structures attributed to surfactant represent 5-10 % of the fracture surface, more than one would expect from the small amount of surfactant in the film. In order to account for the prominence of these structures (e.g., Figure 12), we imagine that cleavage during cryofracture propagates selectively through regions of the film containing entrapped surfactant. In annealed films of the larger latex, these structures are less prominent. They are often comparable in size to individual latex particles. It would appear here that these structures are sufficiently distributed so as not to provide a pathway for mechanical failure of the films. It would be very interesting, in future studies, to correlate the presence of surfactant in films to their mechanical strength. Summary We report freeze-fracture TEM studies of poly(buty1 methacrylate) latex films for the cases of both large (d = 337 nm) surfactant-free latex and smaller (d = 117 nm) latex in the presence of 2 w t % sodium dodecyl sulfate. (18) Juhu6, D.; Wang, Y.; Winnik, M. A.; Leung, 0.-M.; Goh, C. Unpublished results obtained from atomic force microscopy measurementa.
1442 Langmuir, Vol. 8,No. 5, 1992
In the former case our results confirm and complement Our major those recently reported by Roulstone et new contribution is unambiguous evidence for rhombic dodecahedron formation during the particle deformation stage of film formation. The smaller particles have a very different behavior. These weakly annealed films indicate a film composed of random packed and deformed particles. Comparison of the TEM and energy-transfer studies of film annealing indicate that all residual traces of particle structure disappear long before molecular interdiffusion of the polymer molecules is complete. Unusual structures appear in the latter set of filmsonce
Wang et al. they are annealed. Similar structures are observed by FFTEM in films prepared from the larger particles to which "factant was added. These structures are attributed to surfactant crystallites entrapped in the film during drying. To understand these resulb, particularly the influence of surfactant on film formation, further experimenta are needed, and these are currentlyin progress in our laboratories.
Acknowledgment. We thank the Institute for Chemical Sciences and Technology, NSERC Canada, and the Province of Ontario for their support of this work.
