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Film Formation from Concentrated Reactive Silicone Emulsions. 2. Surfactant Distribution D. Guigner,† C. Fischer,‡ and Y. Holl*,†,§ Institut de Chimie des Surfaces et Interfaces (CNRS), BP 2488, 68057 Mulhouse, France, Rhodia Silicones, BP 22, 69191 St Fons Cedex, France, and Institut Charles Sadron (CNRS), 6 rue Boussingault, 67083 Strasbourg, France Received February 6, 2001. In Final Form: July 9, 2001 Drying mechanisms of concentrated, reactive poly(dimethylsiloxane) in water emulsions, stabilized by a nonionic polyethoxylated fatty alcohol, were studied in part 1 of this series. Upon drying, these emulsions form cross-linked polymeric films. The present paper focuses on the fate of the surfactant during and after film formation, studied by attenuated total reflectance (ATR) and infrared microscopy. The effects of various parameters were investigated, namely thickness of the cast layer, surfactant concentration, presence of a CaCO3 filler, and aging of the film after drying. The concentration profile of the surfactant in the film shortly after drying presents a depleted layer on top, a flat vertical line in the bulk, and a strongly enriched layer in contact with the substrate. The vertical line corresponds in fact to an average over surfactant aggregates with a seemingly very high size polydispersity. Drying and coalescence mechanisms have a major influence on the surfactant distribution. In the dry film, the distribution is unstable and, upon aging, both film-air and film-substrate interfaces are progressively enriched with surfactant. More details on the structure of the film after drying are given in part 3 of this series.
Introduction This paper is part 2 of a series devoted to film formation mechanisms from poly(dimethylsiloxane) (PDMS) emulsions in water, stabilized by a nonionic surfactant of the ethoxylated alkyl family (C13E8). In part 1, the drying mechanisms were investigated.1 The main results are recalled in the discussion, as they are very useful in the interpretation of the data presented here, concerning the fate of the surfactant during and after drying. Distribution of surfactants in films prepared from polymer colloids has been a matter of interest for many years. Discussions on the fate of surfactants in latex films began as early as in 1936 when Wagner and Fischer2 suggested that the polymer and the surfactant could form two interpenetrating networks in latex films. Bindschaedler et al.3 have reviewed the main papers published on the subject before 1985. A more recent review on this topic can be found in the general article on latex films written by Keddie in 1997.4 An impressive amount of experimental results using various kinds of infrared and Raman spectroscopic techniques was made available by Urban and co-workers5 and references therein. It was demonstrated that many parameters influence the distribution of the surfactant in a latex film. The main ones are6 the nature of the system (i.e. nature of the polymer, of the surfactant and of the substrate), time (age of the film), total concentration of surfactant in the latex, film formation conditions (temperature and relative humidity), annealing time and temperature, and even mechanical * Corresponding author. † Institut de Chimie des Surfaces et Interfaces (CNRS). ‡ Rhodia Silicones. § Institut Charles Sadron (CNRS). (1) Part 1 of this series: Guigner, D.; Fischer, C.; Holl, Y. Langmuir 2001, 17, 3598. (2) Wagner, H.; Fischer, G. Kolloid Z. 1936, 77, 12. (3) Bindschaedler, C.; Gurny, R.; Doelker, E. J. Appl. Polym. Sci. 1987, 34, 2631. (4) Keddie, J. L. Mater. Sci. Eng. 1997, R21, 101. (5) Zhao, Y.; Urban, M. W. Macromolecules 2000, 33, 2184. (6) Holl, Y. Macromol. Symp. 2000, 151, 473.
Figure 1. Concentration profiles (qualitative shapes) of various surfactants in poly(2-ethylhexyl methacrylate) latex films. Films were aged 3 h and dried at 23 °C, 55% relative humidity. Thicknesses are in the range of a few tens of micrometers. Abbreviations: HTAB ) hexadecyltrimethylammonium bromide; SDS ) sodium dodecyl sulfate; HPCl ) hexadecylpyridinium chloride; NP10 ) polyethoxylated nonylphenol with 10 ethoxy groups. Drawn from data in ref 8.
story of the film7 (whether it was stressed and to what extend). As an example, Figure 1 shows different concentration profiles of surfactants in poly(2-ethylhexyl methacrylate) latex films on glass. The interfaces (studied over thicknesses in the micrometer range) can be either enriched, depleted, or show the same concentration as in the bulk. The shape of these profiles is qualitatively interpreted in terms of desorption of the surfactant from the particle surface when particles come together in the film formation process and mobility of the surfactant in the still wet and then in the dry film.8 The level of understanding of the complex processes leading to distribution of the surfactant in a latex film remains insufficient. For instance, it is not yet possible to give precise explanations on the differences in shape between surfactants. Two similar cationic surfactants, namely HTAB and HPCl (Figure 1), can lead to very different (7) Evanson, K. W.; Urban M. W. J. Appl. Polym. Sci. 1991, 42, 2309. (8) Kientz, E.; Holl, Y. Colloids Surf. 1993, A78, 255.
10.1021/la0101999 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/15/2001
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Table 1. Composition of the Reference Emulsion product
percentages (wt %)
oil (PDMS) cross-linking agent surfactant
73.7 5.2 4.4
product
percentages (wt %)
water catalyst
16.5 0.2
concentration profiles, the reason for this being still unclear. Another point to stress about Figure 1 is that the shape of the concentration profile is well established near the film-air and film-substrate interfaces but little is known about the bulk of the film. Only recently have data appeared on the surfactant in the bulk of the film. The techniques used were photoacoustic infrared spectroscopy9 and Rutherford backscattering.10 For the same purpose, infrared microscopy was used in this work. Besides infrared microscopy, another infrared spectroscopic technique, namely attenuated total reflectance (ATR), gave precise, quantitative information on the micrometer-thick layer in contact with the substrate. Several parameters were investigated: the thickness of the deposited emulsion layer; the concentration of surfactant; the introduction of a CaCO3 filler in the emulsion; the aging of the film after drying. This system being quite complex, the complete description of the structure of the film after drying will be presented in a third paper, to be published.11 Experimental Section Products, Emulsification, and Drying. The system used in this study was described with many details in the previous paper of this series.1 Let us recall the main aspects here. The oil was an R,ω difunctional poly(dimethylsiloxane) (PDMS) polymer (Mw ) 120 000 g/mol, polydispersity index ) 2) from Rhodia, Lyon, France. It contained a cross-linking agent, totally soluble in oil, able to react with the functional groups of PDMS at room temperature with the help of a catalyst. The surfactant was a commercial polyethoxylated fatty alcohol, denoted C13E8, used as received. The hydrophobic part was a linear, saturated alkyl group with 13 carbon atoms. The hydrophilic part contained an average of 8 ethoxy groups. The emulsion was formed by adding surfactant containing water to oil plus cross-linking agent under fast stirring in a metallic reactor. After a few minutes, the w/o emulsion inverted into an o/w emulsion. Stirring was carried on until the mean droplet size reached 0.45 µm. The droplet size distribution was unimodal and Gaussian with a full width at half-maximum of 0.3 µm. The emulsions were then diluted to a water content of 16.5 wt %. The final step was to add the catalyst (the nature of the catalyst cannot be indicated because it is proprietary) in the form of an emulsion and degas the product by stirring under vacuum in order to get rid of all the air bubbles. At this stage, we had an o/w emulsion containing two kinds of droplets: mainly PDMS ones and a few of the catalyst. Crosslinking requiring the contact of the catalyst with PDMS; gelation could only start after coalescence of the droplets, at the end of drying. The composition of the reference (mostly used) emulsion is indicated in Table 1. At 1 h after addition of the catalyst, the emulsion was cast on a glass substrate (15 cm × 6 cm) and allowed to dry at 23 °C and 50% relative humidity (RH). The effects of the following parameters on the surfactant distribution were investigated in this study: thickness of the deposited emulsion layer; surfactant concentration; presence of a filler; aging of the film after drying. Thickness effects were studied using samples of thicknesses 0.19, 2, and 10 mm. (9) Urban, M. W. Prog. Org. Coat. 1997, 32, 215. (10) Tzitzinou, A.; Jenneson, P. M.; Clough, A. S.; Keddie, J. L.; Lu, J. R.; Zhdan, P.; Treacher, K. E.; Satguru, R. Prog. Org. Coat. 1999, 35, 89. (11) Part 3 of this series: Guigner, D.; Fischer, C.; Holl, Y. To be submitted for publication.
The standard surfactant concentration in the emulsion was 4.4 wt %. A lower concentration of 2.6 wt % was used in some cases. In the dry film, these concentrations are 5.3 and 3.1 wt %, respectively. If one assumes that the emulsion is monodisperse, that the surfactant forms a dense monolayer at the oil/water interface, and that the molecular area of C13E8 is 57.9 Å2 12 under these conditions, one can calculate that, when the surfactant concentration is 4.4 wt %, 55% of the surfactant initially introduced in the emulsion remains in water. The concentration of this free surfactant in the water phase is then 12.8 wt %. The cmc of C13E8 is 2.7 × 10-5 M or 149 × 10-4 g/L.12 The phase diagram13 indicates that it is still in the form of globular micelles. In the emulsion containing only 2.6 wt % of surfactant, the amount of nonadsorbed molecules, calculated in the same way, is 22 wt %. The concentration in water is then 4 wt %, and the free surfactant is mainly in the form of spherical micelles. As a filler, precipitated calcium carbonate was used, either untreated (hydrophilic) or pretreated with 2.9 wt % of stearic acid to render its surface hydrophobic. The filler specific area was 20 m2/g. Its concentration was 25 wt %. Surfactant Distribution. Surfactant distribution was followed in 2 ways, namely a local study by infrared microscopy (IRM) and measurements at the interface between the emulsion and the substrate by reflection infrared spectroscopy (ATR-FTIR). Using infrared microscopy (IRM), it was possible to draw concentration profiles of the surfactant along the film thickness at various times during drying. The sample for infrared analysis was prepared in the following way (see also ref 1 for more details). A U-shaped poly(tetrafluoroethylene) (PTFE) spacer of thickness 25 µm was deposited on an IR transparent CaF2 crystal. The area inside the U was 1 × 1 cm2. A small amount of emulsion was put inside the U and covered with a second CaF2 crystal. This cell was sealed with silicone grease and fixed with screws. Only one side was open to allow evaporation of water. This small sample was equivalent to a portion of same sizes in the normal drying emulsion (a figure describing the experimental setup can be found in ref 1). Several pieces of evidence given in ref 1 showed that the small sample for IRM analysis could reasonably be considered as truly representative of the full size normal sample. IR analysis was performed in transmission mode by moving the spot along the vertical direction corresponding to the normal sample thickness. Spot sizes of 50 or 100 µm were used. The complete analysis of all points was repeated several times during the total drying time of 16 days, at 1 h of drying time, 20.5 h, 27.5 h, 44.5 h, etc. Data corresponding to the beginning and to the end of the drying only are shown here. The complete set of data can be found in ref 13. After drying, analyses were also performed by moving the spot along horizontal lines at various depths. The FTIR spectrometer was an “IFS 55” from Brucker, Karlsruhe, Germany, equipped with an IR microscope “IR scope” from the same manufacturer. Resolution was 2 cm-1, and 16 spectra were accumulated to improve the signal-to-noise ratio. The quantitative analysis of IRM data was performed in the following way. The first step was to determine the concentration of water remaining in the sample at a given time. The water band at 1645 cm-1 (OH deformation) was used for quantitative analysis. The area of the corresponding peak was recorded initially (A0) and at times t (At). The fraction of water (compared to the initial amount of water) remaining in the sample at a given time and at a given position along the thickness was calculated by
P ) At/A0 The concentration of water remaining in the emulsion was as follows:
C ) (100P × 16.5)/(16.5P + 83.5) Then, characteristic IR peaks of PDMS and C13E8 (surfactant) had to be chosen. The peaks at 1396 and 1412 cm-1 (Si-CH3, δa) (12) Ueno, M.; Takasawa, Y.; Tabata, Y.; Sawamura, T.; Kawahashi, N.; Meguro, K. Yukagaku 1981, 30, 421. Quoted in the following: Nonionic Surfactants, Physical Chemistry, Surfactant Science Series; Schick, M. J., Ed.; Marcel Dekker: New York, Basel, 1987; Vol 23.
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Figure 2. Infrared spectra of PDMS and C13E8 showing the characteristic peaks chosen for quantitative analysis. were selected for PDMS and at 1460 cm-1 for the surfactant (CH2, δ) (Figure 2). Contributions of PDMS in C13E8 bands and vice versa were subtracted after decomposition of the peaks using the “Grams” software (Galactic Software, Salem, NH). The surfactant over silicone or surfactant over water concentration ratios were determined using the corresponding absorbance ratios and the calibration curves shown in ref 13 established, in transmission, with known mixtures. The concentrations of PDMS and surfactant could be determined from these ratios. It was systematically checked that the sum of the 3 concentrations (PDMS, C13E8, and water) was very close to 100. Reproducibility of IRM results was good, as shown in ref 1. Phenomena taking place at the interface between the drying emulsion and its substrate were studied by attenuated total reflectance (ATR) in Fourier transform infrared spectroscopy (FTIR). The wet emulsion was put in contact with a ZnSe ATR crystal and allowed to dry. Evaporation by the sides was avoided by the use of a small PTFE mold containing the drying sample. Concentration determinations were performed in a way similar to that for IRM analysis except that for PDMS the more intense peak at 1258 cm-1 (Si-CH3, δS) was taken. Calibration curves were established in ATR.13 When the calcium carbonate filler was present, broad and intense peaks appeared at 876, 1440, 1795, 2874, and 2982 cm-1. A lot of overlapping with surfactant and PDMS peaks occurred. The best remaining surfactant peak was at 2850 cm-1, but only qualitative conclusions could be drawn about surfactant concentration at the interface in filled systems. The spectrometer was a “FTS 45A” from Biorad. Resolution was 4 cm-1, and 16 spectra were accumulated for each measurement. The internal reflection unit had sizes of 84 × 9 × 6 mm3, which corresponded to 14 internal reflections. Reproducibility of all these experiments was carefully checked.1
Results The surfactant concentration profiles at various times during drying, determined by IRM for the 10 mm thick emulsion dried in standard conditions (23 °C, 50% RH) and containing 4.4 wt % of surfactant, are shown in Figure 3. There are no points at the top of the layer because the surfactant peak was too small. The surfactant concentration is thus very low in the top region (skin). Under the skin, the concentration profiles are rather flat and close to the nominal value, except in domains were the concentration is well above average and can reach 70 wt % (Figure 3b). These domains correspond to surfactant aggregates. It will be shown in a forthcoming paper11 that these aggregates increase in size as the substrate is approached. In the dry film, the surfactant rich domains (13) Guigner, D. Ph.D. Thesis, University of Mulhouse, Mulhouse, France, 2000.
Figure 3. Infrared microscopy analysis giving surfactant concentration profiles at various times during drying. Emulsion layer thickness: 10 mm. Drying conditions: 23 °C, 50% RH. Surfactant concentration: 4.4 wt %. No filler was used.
were also detected when the spot was moved horizontally at various depths (Figure 4). Figure 4 confirms the conclusion drawn from the data in Figure 3. Near the top of the film (depth 0.36 mm), the surfactant concentration is close to zero. At a depth of 1.15 mm, it is close to the nominal value (around 6 wt %). At 1.75 and 2.34 mm, depleted areas coexist with surfactant rich domains. Deeper (4.32, 7.50, and 9.49 mm), large aggregates were avoided, and between the concentration is slightly lower than the nominal value. Figure 5 shows the evolution of the concentrations at the emulsion/substrate interface during drying. These data correspond to the emulsion containing 4.4 wt % of surfactant and dried in standard conditions (23 °C, 50% RH). The evolution of the water concentration (a decrease followed by a peak) was already mentioned and commented on in ref 1. The surfactant concentration slightly increases at the beginning of the drying process, and when the water concentration starts to increase, it also very strongly increases. It reaches its highest value when the water concentration is at its maximum and then remains
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Figure 6. PDMS, surfactant, and water concentrations versus time at the emulsion/substrate interface (from ATR data) during drying. Surfactant concentration: a ) 4.4 wt %; b ) 2.6 wt %. Emulsion layer thickness: 2 mm. No filler was used. Drying conditions: 23 °C, 50% RH. Table 2. Surfactant Concentration at the End of Drying, at the Dry Film/Substrate Interface, as a Function of Surfactant Concentration Introduced in the Emulsion and Thickness of the Deposited Emulsion Layera initial surfactant concn (wt %) 4.4 film thickness (mm) surfactant concn at the interface (wt %) a
0.19 10
2 36
2.6 10 69
2 27
Drying conditions: 23 °C, 50% RH.
Figure 4. Infrared microscopy analysis showing the variation of the surfactant concentration along horizontal lines at various depths in the dry film (shortly after total drying). These horizontal lines were 2 mm long and located near the center of the sample (see scheme). Spot size: 100 µm. Emulsion layer thickness: 10 mm. Drying conditions: 23 °C, 50% RH. Surfactant concentration: 4.4 wt %. No filler was used.
Figure 5. PDMS, surfactant, and water concentrations versus time at the emulsion/substrate interface (from ATR data) during drying. Emulsion layer thicknesses: 0.19 mm; 2 mm; 10 mm. Drying conditions: 23 °C, 50% RH. Surfactant concentration: 4.4 wt %. No filler was used. The sum of the three concentrations is always very close to 100%.
constant. This plateau value is as high as 70% for a thickness of 10 mm. Correlatively, the PDMS concentration sharply decreases when the surfactant arrives at the interface, increases slightly when water disappears, and remains constant in the dry film. Thickness effects on the surfactant enrichment at the interface are also shown in Figure 5. The enrichment increases when the thickness increases from 0.19 to 2 mm and to 10 mm. Enrichment also increases when the surfactant concentration initially introduced in the emulsion increases from 2.6 to 4.4 wt % (Figure 6).
Figure 7. ATR spectra of emulsions without filler or with filler (25 wt % of stearic acid treated calcium carbonate) after 20 h of drying. Drying conditions: 23 °C, 50% RH. Surfactant concentration: 4.4 wt %. Deposited layer thickness: 2 mm. With the filler, the strong surfactant bands between 2850 and 2950 cm-1 are missing.
Table 2 recalls the dependence of the surfactant concentration at the dry film/substrate interface on the surfactant concentration initially introduced in the emulsion and on the deposited emulsion layer thickness. The study of calcium carbonate filled emulsions was difficult because of the overlap of CaCO3 bands with surfactant bands. However, it can be seen in Figure 7 that the surfactant bands between 2850 and 2950 cm-1 are totally missing after 20 h of drying of a 2 mm thick emulsion containing calcium carbonate, indicating the absence of surfactant migration to the interface in the filled systems. Upon aging of the dry film, only rough, macroscopic observations were made, but they are worth mentioning here. The dry film remained stable, as far as visual
Film Formation from Reactive Silicone Emulsions
Figure 8. Qualitative surfactant concentration profile in the film at the end of the drying of a 10 mm thick layer of emulsion. Drying conditions: 23 °C, 50% RH. Surfactant concentration: 4.4 wt %. No filler was used. This profile does not show the heterogeneities in the center of the film where the concentration can locally reach values as high as at the interface (see text).
observation could assess, during around 8 weeks. Afterward, a greasy layer could be detected at the surface. It was easy to show that it was the surfactant migrating to the surface. The amount of surfactant expelled from the film augmented with time. It seemed that, on the substrate side, the quantity of surfactant also increased progressively with time, after 8 weeks. From the results presented above, the shape of the concentration profile of the surfactant in the film at the end of drying can be deduced (Figure 8). On top of the film, there is a layer, a few hundreds of micrometers thick, strongly depleted in surfactant. In the vicinity of the film/ substrate interface, the concentration is dramatically increased. The exact concentration near the interface depends on several parameters (thickness, nominal surfactant concentration, presence of a filler), as will be discussed later, but it can reach extremely high values, 70 wt % or even more, in the 1 µm thick layer in contact with the substrate. The profile is flat in the central part, at a value close to the introduced amount of surfactant, except in large surfactant rich domains where the concentration can reach values as high as at the interface. The peaks in Figure 3 indicate that the size of the surfactant aggregates can be in the range of several tens of micrometers. Other results11 show that the aggregates increase in size when located deeper in the film. Discussion Drying mechanism and coalescence of polymer droplets have a profound influence on the distribution of the surfactant. Let us first develop some considerations on the initial state of our emulsion and its drying and coalescence mechanisms. In its initial state, this emulsion is already concentrated above the volume fraction of close packed spheres. Thus, droplets are deformed and present 12 flat, circular areas/drop. A geometrical description of such a system14 involves two parameters: the thickness (δ) and diameter (∆) of the flat regions. Upon drying, δ tends to decrease and ∆ to increase. When ∆ increases, the curvature of the Plateau border adjacent to the flat area also increases (see ref 1 for more geometrical considerations about this kind of system). (14) Princen, H. M. J. Colloid Interface Sci. 1979, 71, 55. Princen, H. M.; Aronson, M. P.; Moser, J. C. J. Colloid Interface Sci. 1980, 75, 246. Aronson, M. P.; Princen, H. M. Nature 1980, 286, 370.
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As emulsification usually requires more surfactant than stabilization, the excess surfactant must be free in water. While from the phase diagram of C13E813 globular micelles might be expected, also lamellar phases as detected by Rouvie`re15 using cryomicroscopy may be induced by confinement of the interdroplet medium.16 In this study, the exact structure of the surfactant solution in the interstitial medium between droplets is not known. Another question to raise at this point is whether the surfactant can mix with the polymer, even to a limited extent. The free energy of mixing of the C13E8/PDMS system was calculated using the solubility parameter approach described in Van Krevelen’s book.17 ∆Gm is positive whatever the composition, although only slightly at very high PDMS concentration. Partitioning of the surfactant between the oil and water phases seems thus unlikely. The possibility that a certain amount of surfactant dissolves in the polymer before coalescence will be neglected. The drying mechanism of this system has been described in ref 1. It dries in a pure normal mode (normal to the film surface, vertical in this case). The main motions of water occur vertically and heterogeneities in the water distribution also develop in the vertical direction. Along a vertical axis in the film, the sequence of events occurring during drying is essentially the same whatever the depth, the only difference being that, closer to the air surface, they occur sooner and faster. This statement might have to be corrected near the film-substrate interface as specific phenomena induced by the proximity of the interface can take place. A key point is the local drying rate (the rate of water elimination at a given depth). It decreases from top to bottom. It is now possible to explain the shape of the surfactant concentration profile shown in Figure 8. As water is eliminated by drying, the flat areas between droplets thin, if they are not already at their minimal thickness (the thickness of a hydrated C13E8 bilayer), and increase their extension (increase of ∆). The free surfactant possibly present in the flat areas is transported toward the nearby Plateau borders. Thinning can occur discontinuously, i.e., step by step, each step corresponding to the elimination of one layer of micelles or of one surfactant bilayer, as was shown in a study of disjoining pressure isotherms of thin water films stabilized by sodium dodecyl sulfate above the cmc by Bergeron et al.18 The free surfactant is progressively concentrated in the Plateau borders. The Plateau borders meet at corners which are not all equivalent because all vertexes of the polyhedra resulting from the total deformation of the drops are not equivalent. Even in the relatively simple case where polyhedra are rhombic dodecahedra, there are vertexes of 3- and 4-fold symmetry.19 The local curvature of these sites in the highly deformed drops is different and so is the capillary pressure at the corners. It is thus possible that the surfactant solution is not distributed homogeneously in the channels formed by the connected Plateau borders. This point deserves further studies. When the drying rate is low enough, the system reaches a metastable transparent state where the interstitial medium is too limited in size to significantly scatter visible light. So far, the free surfactant has been transported only over limited distances. (15) Rouvie`re, J. Inf. Chim. 1991, 325, 158. (16) Antelmi, D. A.; Ke´kicheff, P.; Richetti, P. Langmuir 1999, 15, 7774. (17) Properties of Polymers; Van Krevelen, D. W., Ed.; Elsevier: Amsterdam, London, New York, Tokyo, 1990; p 189. (18) Bergeron, V.; Radke, C. J. Langmuir 1992, 8, 3020. (19) Roulstone, B. J.; Wilkinson, M. C.; Hearn, J.; Wilson, A. J. Polym. Int. 1991, 24, 87.
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Figure 9. Scheme of the coalescence of two emulsion droplets possibly trapping some surfactant inside the polymer.
Next step is coalescence. Coalescence is a major event influencing the distribution of the surfactant. It seems likely that coalescence starts in the flat areas and not in the Plateau borders. The adsorbed surfactant is eliminated, at least partially, from the vanishing polymerwater interface. It has to be stressed here that not necessarily all the adsorbed surfactant is displaced. A certain amount can remain trapped at its location before coalescence as sketched in Figure 9. The amount of surfactant which is trapped would be very important to know. It probably depends on the nature of the system (nature of the polymer-surfactant couple) but also on the local drying rate. The desorbed surfactant joins the surfactant solution in the Plateau border, and this solution is also redistributed as coalescence proceeds and takes place in a zone of growing size, involving several drops. As already stressed in this part and in the preceding one,1 coalescence starts on top and progresses toward bottom. When coalescence takes place at a given level in the emulsion, it has already occurred higher and not yet deeper. The permeability of the top layers is much lower than that of the deeper layers where the “coalescence front” is still to come. This explains that the surfactant solution is preferentially expelled toward the bottom. This effect is clearly illustrated by the ATR data near the interface (Figures 5 and 6). Water plus surfactant is expelled toward the interface, water is eliminated, and the surfactant remains at the interface. This is most probably attributable to the coalescence front reaching the proximity of the interface.1 On the air side of the film, the same phenomenon, having taken place much earlier, has expelled the surfactant from the top layers toward the interior of the film. The top region of the film is thus depleted in surfactant. In the bulk of the film, coalescence expels the surfactant solution in separated domains. The main size of these domains strongly depends on the local drying rate. When it is fast (high in the film), the domains tend to remain small and some of them can form a continuous phase. Part 3 in this series11 will show that the upper layers of the film (except the 200 µm thick uppermost region which is totally coalesced) indeed exhibit poor cohesion, probably due to a partly continuous segregated surfactant phase. When the local drying rate is lower, domains tend to become larger and no longer form a continuous phase. Cohesion of the deeper layers of the film increases indeed.11 Coalescence of the polymer droplets expelling the surfactant solution in separated domains is typically a phase inversion phenomenon. This
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phase inversion requires a certain time. It does not have time to take place completely in the upper parts of the film whereas it is more complete in the deeper parts of the film where water elimination and coalescence are slower. Before we close this section of the Discussion, let us make two remarks. First, in a system drying in a pure normal mode like this one, one would have expected a concentration profile in the bulk of the film showing a negative slope (progressively more surfactant as the substrate is approached) and not a vertical line (Figures 3 and 8) (leaving apart the large aggregates in the bulk and the enrichment close to the interface). This means that the surfactant is never transported over very large distances. An examination of the ATR data near the interface gives some hints on the transport phenomenon. In the emulsion containing 4.4 wt % of surfactant, assuming that the surfactant and water concentrations are uniform in the 1 µm thick layer in contact with the substrate (the layer analyzed by ATR) and that the dry content of the emulsion just before coalescence starts is 96%,1 one can calculate that the water necessary for the dilution of the interfacial zone has to come from a layer having a thickness of around 6-8 droplet diameters (3-4 µm). Whereas, the surfactant needed to reach a concentration of 70 wt % (Figure 5) has to come from a layer having a thickness of around 25 droplet diameters. This means that the surfactant solution is progressively concentrated as it moves downward. However, this concentration increase does not propagate far because the surfactant solution is periodically deposited in surfactant rich domains leading to the aggregates. Thus, the concentration profile can remain globally vertical. A similar view was proposed before by Winnik and co-workers following their studies on poly(butyl methacrylate) latex films.20 The second remark concerns the surfactant enrichment at the film-substrate interface. It is often thought that it is due to the thermodynamical trend to minimize the interfacial tension. However, our present results and older ones21 show that there is far more surfactant at the interface than needed to lower the energy. As discussed above, transport phenomena connected with drying and coalescence are much more important to account for the amount of surfactant segregated at the interface. Nevertheless, results from Urban and co-workers7 clearly indicate an effect of the nature of the surface in contact with the film on the surfactant concentration at the interface. Thermodynamics could play an indirect role in driving surfactant molecules at the interface to minimize the energy, and these “pioneers” could attract more molecules afterward. This phenomenon could be a kind of nucleation and growth process. Effects of Parameters. The examined parameters were the thickness of the deposited emulsion layer (10, 2, 0.19 mm), the concentration of surfactant (4.4 and 2.6 wt %), the introduction of a CaCO3 filler in the emulsion, and the aging of the film after drying. The more precise information was gained through ATR measurements at the film-substrate interface, and strictly speaking, the following discussion only pertains to the interfacial region. However, it is likely that the considerations that will be developed below are also valid, to a certain extent, in the bulk of the film. The question is to what extent, exactly, are the results obtained in the interfacial region specific to this area? We do not yet have a clear answer. (20) Wang, Y.; Kats, A.; Juhue´, D.; Winnik M. A. Langmuir 1992, 8, 1435. (21) Zhao, C. L.; Dobler, F.; Pith, T.; Holl, Y.; Lambla, M. J. Colloid Interface Sci. 1989, 128, 437.
Film Formation from Reactive Silicone Emulsions
When the thickness of the deposited emulsion layer decreases, the amount of surfactant segregated to the interface decreases (Figure 5). This is a kinetic effect. The drying and coalescence rates are lower close to the interface of thicker films, the phase inversion and separation have more time to occur, and consequently, more surfactant is rejected toward the interface. The enrichment is very limited for the 190 µm thick film: 10 wt % (Table 2). This confirms that, when drying and coalescence are fast, the surfactant remains trapped inside of the film. It is important to stress that the 190 µm thick film is not equivalent to the skin (of similar thickness) of the thicker films: it dries faster and it retains its surfactant, whereas the skin in thicker films has time to reject it downward. Enrichment at the film-substrate decreases when the total amount of surfactant in the emulsion decreases (Figure 6). The emulsion containing only 2.6 wt % of surfactant dries slightly faster than the one containing 4.4 wt %.1 This normally leads to less segregation. Furthermore, less surfactant is available for the enrichment of the interfacial zone. The result shown in Figure 6 is thus the expected one. However, there is more surfactant at the interface of the 2.6 wt% containing system than simply predicted from the ratio of the nominal concentrations (Table 2). Considering the higher drying rate, there should be less. This shows that accounting quantitatively for the amounts of surfactant at the interface is far from straightforward. Adding a CaCO3 filler to the emulsion has a dramatic effect on the surfactant exudation to the interface: it is totally suppressed (Figure 7). Apparently, the surfactant is attracted by the filler-emulsion interface, very important in area, and is no longer available to migrate to the film-substrate interface. The exact mechanism of this attraction is not yet clear. Again, there is more surfactant at the film-filler interface than needed by thermodynamics. Interestingly, a similar result was obtained by Zhao and Urban5 in a poly(n-butyl acrylate) latex film containing 50 wt % of an organic filler (polystyrene latex particles): the migration of the surfactant (sodium dioctylsulfosuccinate) to the surface was suppressed as compared to a film made of a styrene/n-butyl acrylate copolymer of identical composition. Annealing the film with the polystyrene filler above the Tg of polystyrene (but not at lower temperatures) restored the migration phenomenon. The long-term evolution of the surfactant distribution in the dry film has not yet been studied extensively. We only know that, after several weeks, the dry film rejects surfactant on the air and probably also on the substrate side. The dry film is a cross-linked PDMS/surfactant incompatible mixture. The surfactant is in the form of aggregates. The size distribution of these aggregates is not known, but it is likely that the area of the polymer/ surfactant interface is important. This destabilizes the film. The system tends to increase the size of the aggregates and to expel as much as possible the surfactant toward the air and substrate interfaces. However, this requires that the surfactant crosses pure PDMS domains in which its solubility is nil or extremely low. It could be speculated that the tiny and highly unstable clusters possibly trapped between coalescing droplets (Figure 9) could be the major cause of the evolution of the surfactant distribution in the blend.
Langmuir, Vol. 17, No. 21, 2001 6425
Conclusion The shape of the distribution profile of the surfactant in this system is an inverted S (Figure 8): a depleted zone on top; a vertical line in the bulk; a markedly enriched layer in contact with the substrate. The vertical line in the bulk results in fact from an average over surfactant aggregates scattered in the film and presenting, possibly, a huge size dispersion: from tiny molecular clusters to over micrometer-sized domains. Despite the many questions remaining open (summarized below), this shape of the concentration profile is qualitatively well understood. Drying and coalescence mechanisms play key roles besides thermodynamics (at interfaces and in the polymer/ surfactant blend) and kinetics of phase inversion and separation. The main open questions deserving further studies are as follows: What is the exact structure and distribution of the concentrated surfactant solution in the emulsion just before coalescence? What amount of surfactant desorbes from the dropletwater interface at the moment of coalescence? To what extent are the phenomena occurring in the vicinity of an interface (film-air, film-substrate, polymerfiller interfaces) specific, determined by the presence of the interface? What is, precisely, the size and distribution of the surfactant aggregates in the bulk of the film? This raises the question of the experimental techniques best suited to visualize the aggregates. Besides the techniques cited in this paper (Rutherford backscattering, infrared photoacoustic spectroscopy, infrared microscopy), other possibilities look promising, namely magnetic resonance imaging, already used to study drying22 and cross-linking23 mechanisms of colloids, and confocal Raman spectroscopy.24 There are still sensitivity and resolution limitations that should be overcome. As recalled in the Introduction, the shape of the surfactant distribution in this system is not universal. Several other shapes are possible. Different drying mechanisms, different desorption characteristics during coalescence, possible partial dissolution of the surfactant in the polymer, etc., account for the system specific behaviors. Predictive models allowing one to calculate the distributions knowing the nature of the system and its film formation conditions would be highly desirable. It is difficult to evaluate how far from this goal is the scientific community interested in these questions. Acknowledgment. We thank Rhodia for financial support. It is also a pleasure to thank Drs. P. Branlard, Y. Giraud, M. Feder (Rhodia, St. Fons and Lyon, France), and L. Hamon (CNRS, Mulhouse, France) for helpful discussions. LA0101999 (22) Ciampi, E.; Goerke, U.; Keddie, J. L.; McDonald, P. J. Langmuir 2000, 16, 1057. (23) Wallin, M.; Glover, P. M.; Hellgren, A. C.; Keddie, J. L.; McDonald, P. J. Macromolecules 2000, 33, 8443. (24) Belaroui, F.; Grohens, Y.; Boyer, H.; Holl, Y. Polymer 2000, 41, 7641.
