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Morphology of Core-Shell Polymer Latices during Drying Chaobin He and Athene M. Donald* Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, U.K. Received April 29, 1996. In Final Form: August 23, 1996X The morphology of core shell polymer latices (PMMA-rubber-PMMA) (PMMA, poly(methyl methacrylate)) aggregated upon drying, with and without added salt (MgSO4), was investigated using environmental scanning electron microscopy. Without added salt, the polymer latices form a hexagonal or cubic packing structure upon evaporation of water from the sample, and subsequently the deformation and compaction of the polymer latices occurs. The surface of the aggregated polymer latices appears smooth. With added MgSO4 salt, in general neither a hexagonal nor a cubic packing arrangement is seen as the water evaporates. The packing structure is irregular and porous, and the individual polymer latex particles can always be seen. Moreover, two-dimensional fractal structures were observed in the aggregated state of the highly salted solution.
Introduction The morphology of an aggregated polymer latex is an important factor determining the properties, and hence the application, of such latices. In paint, the aggregation process occurs along with the evaporation of water, so that the volume fraction of the polymer latices gradually increases leading to a dense packed structure and finally film formation. In the field of toughened plastics coreshell latices are frequently used. These are recovered from the reaction solution by an aggregation process, and subsequently redispersed in a polymer matrix to give the final composite structure. In this application film formation would be undesirable, since it would prevent this subsequent redispersal. Indeed it is necessary that the aggregated structure (often referred to as coagulated) is loose, so that this redispersal process is rapid and can lead to a homogeneous distribution in the polymer matrix. The final organization within an aggregated latex is known to depend crucially on the concentration of any added electrolyte, since this will affect the nature of the pairwise interaction potential between the particles. Quantitatively, the way this potential is modified by electrolyte concentration is well described by DLVO theory.1,2 In general there will be a deep primary minimum and a secondary, much shallower, minimum. The depth of this secondary minimum and the height of the potential barrier between this and the primary minimum are strongly affected by changing the electrolyte concentration. Other variables in the theory include the surface charge density on the particles, which is often hard to determine. It is worth noting that during evaporation the concentration of electrolyte will also change. In the bulk, at a low concentration of added electrolyte when the particles are charge stabilized, the well-known colloid crystal structure will form at high volume fractions of latex particles. On the other hand, with a high concentration of added electrolyte the particles become unstable and aggregation occurs, which may be controlled by one of two mechanismssreaction limited or diffusion limited. It is well-known that the structure of an aggregated latex also depends strongly on the rate of aggregation, with a rapid rate leading to a rather loose aggregate in X Abstract published in Advance ACS Abstracts, November 1, 1996.
(1) Derjaguin, B. V.; Landau, L. Acta Physicochim. USSR 1941, 14, 633. (2) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948.
S0743-7463(96)00423-4 CCC: $12.00
which most particles tend to be linked to only one or two other particles, whereas slow coagulation leads to much denser structures.3 However due to the rather small size of most polymer latex particles, investigation of the morphology of aggregated colloidal particles has not always been easy. Only for unusually large particles is optical microscopy a viable technique. Consequently many studies have relied on neutron and light scattering methods,4-6 in which the morphology can only be inferred from an inversion of the scattering curve. Electron microscopy, both scanning (SEM) and transmission (TEM), has also proved helpful.7 However SEM and TEM, although they can resolve a typical small latex particle, are unable to investigate the packing morphology of these particles as they aggregate in solution (usually aqueous), since they must operate under high vacuum. Thus where they are employed in the study of aggregation, there must have been extensive sample preparation prior to examination in the microscope, and the structure must be regarded as a post-mortem analysis, with the possibility of some artifact having been introduced into the packing. The challenge is to see whether we can study the morphologies of aggregated polymer latices in situ, and hence “watch” the development of morphologies in their natural state. Environmental scanning electron microscopy (ESEM) is a modified version of a conventional SEM.8 A differential pumping system and a different type of detector9 not only allows the specimen chamber to contain water vapor during imaging but also allows insulating samples to be examined without coating. These advantages make ESEM well suited for an in situ real time study of the aggregation of polymer latices in solution, which otherwise could not be achieved using conventional SEM. This paper will concentrate on differentiating between the different aggregated morphologies of polymer latices with and (3) Berge, A.; Ellingsen, T.; Skjeltorp, A. T.; Ugelstad, J. Modelling of physical process using monosized polymer particles. In Scientific methods for the study of polymer colloids and their applications; Gandau, F., Ottewill, R. H., Eds.; Kluwer Academic Publishers: Dordrecht, 1989. (4) Ottewill, R. H. Scientific Methods For the Study of Polymer Colloids and Their Applications; Candau, F., Ottewill, R. H., Eds.; Kluwer Academic Publishers: Dordrecht, 1990; p 349. (5) Hahn, K.; Ley, G.; Oberthur, R. Scientific Methods For the Study of Polymer Colloids and Their Applicatins Candau, F., Ottewill, R. H., Eds.; Kluwer Academic Publishers: Dordrecht, 1990; p 463. (6) Bartlett, P.,;Ottewill, R. H.; Pusey, P. N. Scientific Methods For the Study of Polymer Colloids and Their Applications; Candau, F.; Ottewill, R. H., Eds.; Kluwer Academic Publishers: Dordrecht, 1990; p 427. (7) Distler, D.; Kang, G. Colloid Polym. Sci. 1978, 256, 1052. (8) Danilatos, G. D. Microsc. Res. Tech. 1993, 25, 529. (9) Danilatos, G. D. Adv. Electron. Electron Phys. 1990, 78, 1.
© 1996 American Chemical Society
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Figure 1. Core-shell latex (15% w/w) at 1.8 °C, 4.6 τ. Polymer latices are in Brownian motion. Although there are some small regions of aggregation, overall there are no significant areas of aggregation.
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Figure 2. Sample as Figure 1. As evaporation of water proceeds, the polymer latices exhibit partial ordering.
without added salt, and following the changes in this as the water evaporates. Materials and Experimental Protocol The polymer latex solution was supplied by ICI, plc, at a concentration of 30% (w/w) polymer latex in water. A coreshell latex was used with a three-layer structure. It consists of a PMMA (poly(methyl methacrylate)) core, a rubber middle layer, and a thin PMMA outer layer. The total diameter was in the range of 200-250 nm. The samples were placed in the ESEM in a wet state and the pump down sequence described by Cameron at al.10 was followed to ensure there was no premature evaporation of water. The temperature of the stage was maintained at 0-3 °C to minimize any thermal effect of the beam while avoiding the development of ice which could induce artifacts. After suitable imaging conditions were established, the temperature of the sample stage was adjusted either to establish equilibrium conditions for the specimen, so that the rate of water evaporation from the specimen is equal to the condensation rate, or to permit removal of water from the specimen. Care needs to be taken to minimize beam damage by limiting exposure time (or increasing the scanning rate). A low gun voltage can also keep the level of beam damage down, but at the expense of image quality if the gun voltage is reduced too far. This effect may be due to the inability of the electrons to penetrate the thin layer of condensed water which covers the surface of the polymer latices when in a fully hydrated state. Because of this, a high gun voltage is required to penetrate this thin water layer should a good quality of image be desired. Images in this paper were taken with a gun voltage of 18 keV, with a chamber pressure of 3-5 Torr and temperature of 0-3 °C. As well as studying the latex in water, some samples were examined in solutions containing MgSO4 of different molarities, ranging from 0.016 to 0.25 M. For these samples the aggregation was studied by mixing the polymer latex solution and the salt solution (as a 1/1 mix) in a sample holder immediately prior to the examination in the ESEM.
Results Morphologies of Polymer Latices during Drying without Added Salt. Figures 1-5 illustrate the morphology of the latices during the early stages of evaporation of water from the sample. Initially, when the dispersion is still dilute and no evaporation of water has occurred, the latex particles exhibit Brownian motion. The particles tend to be slightly fuzzy at this stage because of their motion. Figure 1 shows an example, in which it can be seen that the majority of the particles are well separated (10) Cameron, R. E.; Donald, A. M. J. Microsc. 1994, 173, 227.
Figure 3. Sample as Figure 1. At late stages, when the particle concentration is high, the polymer latices mainly exhibit a honeycomb packing structure (or hexagonal arrangement).
Figure 4. Sample as Figure 1. Cubic packing is seen in most regions, but with some defects.
with no sign of extensive aggregation yet visible, although there are some small ordered regions. When evaporation of water is allowed to occur by slightly raising the temperature of the stage, the particle volume fraction gradually increases, which forces the latex particles to approach one another more closely. At short distances, the particles repel one another due to the electric double layer, and an equilibrium separation will be set up. As the surface water is removed, some of the particles start to form ordered arrays. This may be due to the formation of a colloidal hard sphere crystal from the charge-stabilized particles, or due to the existence of a
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Figure 5. Higher magnification view of Figure 4. It can be seen that the “black cross” in fact is a packing of five latex particles. This suggests that the packing structure for the latices is fcc.
second minimum in the potential. At this stage, the collision of the particles normally does not lead to sticking. Figure 2 shows an example of the structure at this stage, in which both hexagonal and square packing arrangements can be seen in the ordered region. However at this time there is still a significant fraction of particles that do not exhibit an ordered structure, as a fraction of the particles are still in motion. These less ordered regions appear darker than those parts where there is a high degree of packing, for reasons which will be discussed below. As further evaporation of water occurs, the volume fraction of polymer latex increases still further, and finally reaches its critical concentration in the surface layer, so that all particles are involved in the close-packed arrays. Again both hexagonal (Figure 3) and square (Figure 4) latices can be seen, but the former is much more common than the latter. It should be noted that it is the dark spheres in the micrographs that correspond to the polymer latices, whereas the bright edge is the boundary of the latex which, due to its hydrophilic nature, is covered with a thin layer of water. The origin of this contrast mechanism, in which hydrated latex particles exhibit bright edge contrast, has been described and explained elsewhere.11 Figure 4 shows the square packing arrangement of latex particles. In this figure it can be seen that at some points an apparent “black cross” is visible. Figure 5 is a higher magnification of part of Figure 4, from which it is clear that the apparent black cross is in fact a region in which a fifth particle sits between the other four in a given square. This structure is reminiscent of a face centered cubic packing unit cell (fcc), but recall that we are only able to see the surface of the aggregate by this technique. This packing persists until complete compaction of the latex has occurred, which indicates this structure to be a stable state and not just an intermediate one. However the dominant surface structure is the hexagonal one (Figure 3), corresponding to a close packed layer. A hexagonal structure is the most common packing motif for the polymer latices. Many previous reports on the structure of polymer latices by SEM and TEM have also indicated that the latices mainly exhibited a hexagonal packing arrangement.5,12 (11) Meredith, P.; Donald, A. M. J. Microsc. 1996, 181, 23. (12) Berge, A.; Ellingsen, T.; Skjeltorp, A. T.; Ugelstad, J. Scientific Methods For the Study of Polymer Colloids and Their Applications; Candau, F., Ottewill, R. H., Eds.; Kluwer Academic Publishers: Dordrecht, 1990; p 435.
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If the evaporation of water continues, the small amount of water remaining at the latex boundary is lost and the particles come into intimate contact. At this point deformation and compaction of the polymer latices begin, as evidenced by the fact that the boundaries between particles are no longer visible. Figure 6 is a micrograph of the latex in which partial deformation and compaction have occurred. Only in a small region can particle boundaries still be seen, indicating water is still trapped here, but the majority of the sample has become featureless. Figure 7 shows a sample in which the deformation and compaction process have been completed, with no indication of particle boundaries. The surface of the aggregated latices is now very smooth. This result indicates that diffusion among the outer layer of the latex particles may occur. Morphologies of Polymer Latices during Drying with Added Salt. When electrolytes, such as MgSO4, are added, it is known in general that the morphology of the aggregate will be altered. The addition of salt to a colloidal suspension reduces the electrostatic screening length, so that the electrostatic part of the interparticle potential becomes of short-range and the particle dispersion becomes unstable. In the bulk the particles may aggregate either in the secondary or primary minimasif the former the aggregation may be reversible, but not in the latter case. Either way, this effect may increase the aggregation rate during drying and lead to a looser aggregated structure than that seen in the absence of salt. Figure 8 shows the evolution of the aggregates with time as water evaporates, when there is a low (0.016 M) concentration of MgSO4. Two types of aggregates appear, as shown in Figure 8a. One type of aggregate, shown by an arrow in Figure 8a, is such that the boundary between the aggregated polymer lattices can hardly be seen; this seems likely to correspond to the irreversible aggregate floc. The other type of aggregate, in which the boundary among the polymer latices can still clearly be seen, may correspond to the reversible aggregate floc, and the packing structure, though not as good as those formed from polymer latices without added salt, is still fairly good. With further evaporation of water, the volume fraction of polymer increases until the packing in the surface layer becomes dense. Although there are some regions with a high degree of order in Figure 8b, the regularity of the packing is not as good as those seen when there is no added salt (Figures 3 and 4). As the evaporation of water continues, the water in the interstices between the particles is finally lost. At this point, coalescence occurs and a fairly smooth surface forms with few distinguishable features (Figure 8c). Nevertheless the structure is not as smooth and featureless as the corresponding unsalted situation. With a higher level of salt present, the structure becomes even less regular. Figure 9 shows the development of aggregation in a latex containing MgSO4 at a concentration of 0.072 M at a ratio 1:1 (v/v). Two distinct features of the aggregates at this high concentration of added salt were observed. Firstly, compared with the aggregated structure of lower salt concentration, the size of the irreversibly aggregated floc increases, as shown in Figure 9a. Secondly, the number of this type of aggregated latex flocs also increases compared with those of low salt concentration. With the evaporation of water, the packing of the polymer latices becomes more and more dense but still lacks order. Most of the regions exhibit neither hexagonal nor cubic packing arrangement, as illustrated in Figure 9b. As the evaporation of water continues, the smooth
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Figure 6. Sample as Figure 1. Partial deformation and compaction of polymer latices occurs following further evaporation of water.
Figure 7. Total deformation and compaction occurs after the removal of the last traces of water.
surface of the dense packed latices suddenly becomes very rough, and the packing structure exhibits many voids, as shown in Figure 9c. This structure is distinct from that seen either in the absence of salt or at very low levels of addition. There is no indication of film formation, the particle boundaries are always still visible, and the roughness of the dried aggregate is also in stark contrast to the featureless appearance of figures of Figures 7 and 8c. Discussion To understand the structures observed in these latex systems with and without added salt, we need to consider the form of the interparticle potential. The form of this is shown in Figure 10. In the absence of salt, at small distances there is a deep minimum in the potential energy curve (primary minimum). If particles fall into this deep well, they will irreversibly stick together and the structure is said to be coagulated, but the thermal barrier against this is very high, so that it is to be expected that for most collisions this barrier will not be overcome and aggregation into this state will not occur. Depending on the surface charge density, there may also be a weak second minimum of energy at much larger separations. Whether or not a secondary minimum exists, it is, however, quite possible for the particles to form a colloidal crystal at intermediate spacings due to soft repulsive forces. In this case one may expect an equilibrium to be set up between a colloidal crystal and particles still in solution, with a local phase separation between the two states. As the concentration
Figure 8. (a, top) Aggregation of a core-shell polymer latex (30% w/w) with added salt (0.016 M MgSO4) at a ratio of 1:1 (v/v). It can be seen that there are small regions of irreversible aggregation of polymer latices, while the majority of the particles remain separate. (b, middle) Following Figure 8a, with evaporation of water from the sample, the polymer latices begin to form a dense packed structure. However the regularity of packing for polymer latexes is not as good as those without added salt, as shown in Figures 3 and 4. (c) With further evaporation of water, the polymer latices form a dense packed structure. The surface of this structure is rougher than that shown in Figure 7.
increases (due to loss of water in our experiments), the proportion of particles involved in the crystal will increase. The effect of adding salt is to reduce systematically the height of the potential barrier to the deep minimum and also to increase the depth of the secondary minimum (Figure 10). In this case aggregation may occur with many particles sitting in the primary minimum. Most collisions may be expected to lead to this, and the structure will
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Figure 10. Influence of 1:1 electrolyte, concentration in mol dm-3 at constant surface potential (70 mV) on the interparticle potential.
Figure 9. (a, top) Aggregation of core-shell polymer latex (30% w/w) with added salt (0.072 M MgSO4) at a ratio of 1:1 (v/v). It can be seen that the size and number of irreversible aggregates are higher than those for lower salt content. (b, middle) Following Figure 9a, further evaporation of water leads to aggregation with neither hexagonal nor cubic symmetry. (c, bottom) With the last trace of water, the surface of the sample becomes rough, and the individual particles can still be seen.
thus be more disordered. The aggregation will be irreversible.13 In fact ESEM shows that the number of irreversible aggregate increases with an increase of salt concentration. Any collisions which lead to sticking only in the secondary minimum will on the contrary give rise to a reversible aggregate. Within this well-known theoretical framework, it is now possible to understand the structures that form as the polymer latices aggregate. In the absence of salt, many (13) Jeffrey, G. C.; Ottewill, R. H. Colloid Polym. Sci. 1988, 266, 173.
collisions between particles lead to no significant aggregation, with the particles subsequently moving apart again under their Brownian motion. This is because of the height of the potential barrier to the primary minimum. Some collisions, however, may lead to sticking in a weak secondary minimum or the formation of a colloidal crystal. In this latter case the structure that forms will be well-organized, typically but not invariably leading to the familiar hexagonal close-packed structure. The origin of the appearance of the square lattice is not readily explained. One possible explanation arises from the fact that what we can see in the ESEM is only the surface of a three-dimensional structure. The hexagonal and square packing are of course consistent with different planes in an fcc lattice. Thus the appearance of two different packings in the surface does not imply necessarily the existence of two different types of structure in three dimensions, although this is a possibility that cannot be eliminated. However research on similar systems by scattering methods have indicated evidence for fcc structures in dilute aggregated latices solution.4 Theoretical analysis14-16 and other experimental results17 have also confirmed a dominant fcc packing structure in colloid crystal, with a possible bcc packing structure if the interparticle repulsion is sufficiently soft. The role of surface tension affecting the packing of the surface layer is a factor that may be influencing the predominance of the more closely packed (111) plane of fcc lattice, which is hexagonal. It does appear that once one plane starts to form, it does tend to extend over significant distances, and thus it is possible the plane that forms is determined by local drying conditions, which may be hard to control. It is also clear that the structures that form are far from perfect, in many cases with a given orientation of the lines of particles only extending over a few particle distances before missing particles or “grain boundaries” with a lower packing density intervene. This type of microstructural information is hard to extract from other techniques such as scattering methods, or even postmortem SEM analyses. There are two possible explanations for the appearance of the dark cross in the square lattice structure, corresponding to the visibility of a fifth particle. Assuming that the three-dimensional structure (which ESEM cannot reveal) is a fcc structure (consistent with previous work (14) Robbins, M. O.; Kremer, K.; Grest, G. S. Phys. Rev. Lett. 1986, 57, 2694; J. Chem. Phys. 1986, 88, 3286. (15) Meriier, E. J.; Frenkel, D. J. Chem. Phys. 1991, 94, 2269. (16) Lekkerkerker, H. N. W. Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregation in Solution; Chen, Sow-Hsin, Huang, John S., Tartaglia, Piero, Eds.; p 97. (17) Monovoukas, Y.; Gast, A. P. J. Colloid Interface Sci. 1989, 128, 533.
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Figure 11. A schematic illustration of the three-dimension packing arrangement for an fcc unit cell.
using small angle neutron scattering studying a more dilute concentration of latex solution4), then whether a five or four point pattern of the particles is seen will depend on which layer of the crystal corresponds to the surface layer: the first layer (001) of fcc latices corresponds to {(0,0,0), (1,0,0), (0,1,0), (1,1,0), and (1/2,1/2,0)} whereas the four-point packing represents the second layer (002) or {(0,1/2,1/2), (1/2,0,1/2), (1,0,1/2), and (0,1,1/2)}. Figure 11 shows a schematic three-dimensional packing structure of the fcc unit cell. An alternative explanation is that the same layer is being observed throughout, but in a few places the thickness of the covering layer of water is sufficiently thin that a particle from the layer below is seen. The fact that nowhere do we see an extensive array of the five point pattern suggests this second hypothesis may be correct. Otherwise it implies a high defect density, in which individual unit cells have a translation of half a length of unit cell in z-direction with respect with their neighbors. A final facet of the process of aggregated structure in these unsalted solutions needs to be considered, and that is the origin of the contrast between the particles and their boundaries. It is seen in Figures 3 and 4 that the particles appear dark against a bright background corresponding to the water. As the level of water drops to the point that deformation and compaction are incipient, this effect leads to the situation that the particles whose boundaries are still clear appear to have a “halo”. This effect has been observed previously in acrylic latices viewed in the ESEM. It has been suggested that at least part of the origin for the bright contrast may lie in the high secondary emission of water giving rise to a locally increased secondary electron signal.11 However in the early stages there is an additional contrast effect that needs to be explained. In Figure 2 it is clear that the ordered structure shows the bright contrast attributed to water much more clearly than the disordered regions, which appear comparatively dark. Because the water vapor in the chamber acts as part of the amplification system for the gaseous detector used in these ESEM experiments,9,18 regions will appear bright where the local water vapor pressure is high. This is likely to be higher in the vicinity of ordered regions for the following reason: in the ordered regions there will be an additional capillary pressure acting at the particleparticle boundaries. This local increase in vapor pressure provides a mechanism for local enhancement of the electron signal which is not present in the disordered region which consequently appear comparatively dark. (18) Meredith, P.; Donald, A. M.; Thiel, B. submitted for publication in Scanning.
Figure 12. (a, top) Aggregation of core-shell polymer latex (30% w/w) with added salt (0.25 M MgSO4) at a ratio of 1:1 (v/v). The size and number of irreversible aggreges continues to increase. (b, middle) Following Figure 13a, after further evaporation of water from the sample, there is no sign of hexagonal or cubic packing. (c, bottom) Following Figure 13b, after evaporation of all the water, the surface of the sample is very rough, and the individual particles latices can still be seen.
Some latices cannot be seen in the ordered region because they are covered by too thick a layer of condensed water. The nature of aggregation and the resulting packing structures of polymer latices with a comparatively high concentration of added salt can also be explained according to the curve of interparticle potential plotted as a function of particle separation shown in Figure 10. As the height of the primary maximum decreases, the likelihood of particles passing over this barrier and sticking together at the primary minimum increases. Because of the reduction of the thermal barrier to this minimum, latex particles will tend to stick as soon as they approach one
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another, leading to a rather disorganized aggregate. Furthermore there is a potential energy barrier to the particles separating again, so that the floc tends to be irreversible. This means that a loose, disordered coagulated structure will form with many voids present, in contrast to the close packed structure in the absence of salt, and coagulation is rapid. As long as these internal voids are filled with water, then the surface of aggregated polymer latices still appears even. This can be seen in Figure 9b. However, as the water evaporation continues, these voids lose water and a collapse of the original packing structure follows, since the upper layer latex particles will tend to fall into the emptied voids in the next layer below. This explains why there is a sudden change of the surface morphology of the aggregated latices as the water is removed. By contrast, this result implies that the internal packing of polymer latices without added salt must be very good with no significant amount of internal voids, since otherwise a smooth packing surface could not be seen after the water evaporates. There is thus a systematic change of structure as the salt content is raised. The aggregated structure gets increasingly disordered, and the final dehydrated structure has an increasingly rough surface as the level of salt addition goes up (Figure 12). It is worth noting that the loose, open structure of the colloidal floc observed in the salted latex solutions (shown in Figures 8a, 9a, and 12a) resemble fractal structures. For these, the fractal dimension is an important parameter, and this will be determined by the mechanism of the aggregation. Different types of aggregation mechanisms give different characteristic fractal dimensions ranging from 1.4 to 1.8 for two-dimensional fractals. By studying the fractal dimension, one can deduce the aggregation mechanism. Recently, this subject has attracted increasing attention.19-21 Previous reports on studying fractal dimension of aggregated latices are based on SEM20 or other microscopy19 methods, in which some artifacts are unavoidably (19) Stankiewicz, J.; Vilchez, M. A. C.; Alvarez, R. H. Phys. Rev. E 1993, 47 (no. 4), 2663.
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induced during the sample preparation. In contrast to those obtained by SEM or TEM methods, the twodimensional (because only the surface is seen) fractals obtained by this ESEM method reflects the aggregated latex structure in its native state. As a result, the ESEM method opens a potential new method for the in situ study of the fractal dimension of aggregated particles. This study will form the basis for a subsequent paper. Conclusions The aggregation of polymer latices without added salt results in a close packed structure in which both hexagonal and square lattice planes are observed. The majority of the aggregates display hexagonal packing in the plane of the surface. The morphology of these aggregated latices in the presence of added salt mainly is a randomly packed structure with a comparatively large number of internal voids. With an increase of salt concentration, the final densely packed structure after water evaporation becomes more and more irregular and rough. At intermediate water contents two-dimensional fractal structures are observed. These will be further analyzed to provide additional insight into the aggregation process. ESEM is thus seen to be an effective method for the in situ study of the morphology of aggregated polymer latices. Acknowledgment. The authors thank the EPSRC for the financial support, ICI for supplying the latex samples, Dr John Melrose, Dr Ian Fraser, and Dr Meredith for helpful discussions, and Andy Eddy for technical assistance in using the ESEM equipment. The ESEM instrument was purchased under the Colloid Technology Programme (funded jointly by Unilever, ICI, Zeneca, Schlumberger, and the DTI). LA9604238 (20) Weitz, D. A.; Oliveria, M. Phys. Rev. Lett. 1984, 52 (no. 16), 1433. (21) Ferri, M. C. F.; Giglio, M.; Paganini, E.; Perini, U. Phys. Rev. A 1990, 42 (no. 12), 7347.
