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Distribution of Surfactants near Acrylic Latex Film Surfaces: A Comparison of Conventional and Reactive Surfactants (Surfmers) Esteban Aramendia,† Jacky Malle´gol,‡,§ Chris Jeynes,| Marı´a J. Barandiaran,† Joseph L. Keddie,*,‡ and Jose´ M. Asua† Institute for Polymer Materials (POLYMAT) and Grupo de Ingenierı´a Quı´mica, Departamento de Quı´mica Aplicada, Facultad de Ciencias Quı´micas, The University of the Basque Country, Apdo. 1072, ES-20080 Donostia-San Sebastia´ n, Spain; Department of Physics, University of Surrey, Guildford GU2 7XH, U.K.; and University of Surrey Ion Beam Centre, Guildford GU2 7XH, U.K. Received November 5, 2002. In Final Form: January 30, 2003 The use of reactive surfactants is a promising way of avoiding the deleterious effects on film properties caused by the segregation of conventional surfactants. In this work, the distributions of conventional and reactive anionic surfactants in acrylic latex films are compared. Atomic force microscopy was used to examine the surface of the films cast from high solids content acrylic latexes, and Rutherford backscattering spectrometry provided depth profiles of the surfactants. It was proven conclusively that the use of surfmers is an effective way of eliminating unwanted surfactant exudation. The amount of conventional surfactant (sodium lauryl sulfate (SLS)) exuded to the surface increased with the temperature at which the films were annealed (60, 90, and 125 °C), but the migration of the surfmer (sodium tetradecyl maleate) was very minimal and only weakly dependent on temperature. An unexpectedly large amount of conventional surfactant was exuded to the film surface when annealed at 125 °C. This result suggests that its transport to the surface might be facilitated by the enhanced mobility of poly(acrylic acid) shells at temperatures above the polymer’s glass transition temperature (ca. 106 °C).
Introduction The use of surfactants is required in emulsion polymers to achieve latex stabilization and particle size control. However, high amounts of surfactants have deleterious effects on the properties of films made from emulsion polymers. As surfactants are only physically attached to the polymer particles, mainly via hydrophobic interaction, they can desorb under certain circumstances. During the film formation process or when the latex is cast at high shear rates, surfactants can migrate toward the film interfaces or concentrate in pockets, creating hydrophilic domains that promote film heterogeneity.1 Both the layer of surfactant at the film’s interfaces and the hydrophilic domains within the film affect final film properties,2 including water sensitivity, adhesion, gloss, and blocking. The migration of physically adsorbed surfactants during film formation is a complex process, which has been thoroughly examined,3-8 but which is still not fully understood. It has been found that the final distribution * To whom correspondence should be addressed. E-mail address:
[email protected]. † The University of the Basque Country. ‡ University of Surrey. § Current address: IRSID-Arcelor group, F-57283 Maizie ` resle`s-Metz, France. | University of Surrey Ion Beam Centre. (1) Asua, J. M.; Schoonbrood, H. A. S. Acta Polym. 1998, 49, 671. (2) Tzitzinou, A.; Keddie, J. L.; Geurts, J. M.; Mulder, M.; Satguru, R.; Treacher, K. E. In Film Formation in Coatings; ACS Symposium Series 790; Provder, T., Urban, M. W., Eds.; American Chemical Society: Washington, DC, 2001; Chapter 4. (3) Keddie, J. L. Mater. Sci. Eng. Rep. 1997, R21, 101. (4) Evanson, K. W.; Torstenson, T. A.; Urban, M. W. J. Appl. Polym. Sci. 1991, 42, 2297. (5) Hearn, J.; Steward, P. A.; Wilkinson, M. C. Adv. Colloid Interface Sci. 2000, 86, 195. (6) Guigner, D.; Fischer, C.; Holl, Y. Langmuir 2001, 17, 3598. (7) Guigner, D.; Fischer, C.; Holl, Y. Langmuir 2001, 17, 6419. (8) Vorobyova, O.; Winnik, M. A. Macromolecules 2001, 34, 2298.
of the surfactant in a latex film depends on the chemical compositions of the surfactant and latex, the conditions of film formation, the molecular weight of the surfactant, and the rheological properties of the polymer particles and the dispersion. The chemical composition of the surfactant influences the compatibility with the polymer. If the surfactant is similar to the polymer, it will dissolve and plasticize the polymer,9,10 but if the surfactant is completely different, it might aggregate and form inverted micelles.11,12 The migration of surfactants can also be influenced by the nature of the polymer. The amount of carboxylic groups in the shell of the particles can modify the chemical compatibility with the surfactant.13,14 In addition, if the surfactant is highly soluble in water, it may be transported toward the upper or the lower interfaces of the film. In this case, the drying kinetics and the distribution of water during film formation are of key importance. It has been proposed that the rate of water evaporation from a latex during film formation influences the distribution of water in the vertical direction in the film (i.e. normal to the substrate).15 If the redistribution of particles by Brownian motion is slow in comparison to the rate at which the water level recedes, then the water distribution in the vertical direction is predicted to be nonuniform. Recent experiments have supported this concept.16 In some cases, the surfactant is carried with the aqueous phase, so that the last region to dry becomes enriched in (9) Edelhauser, H. A. J. Polym. Sci., Part C 1969, 27, 291. (10) Capek, I. Adv. Colloid Interface Sci. 2000, 88, 295. (11) Kientz, E.; Dobler, F.; Holl, Y. Polym. Int. 1997, 34, 125. (12) Du Chesne, A.; Gerharz, B.; Lieser, G. Polym. Int. 1997, 43, 187. (13) Hellgren, A. C. Prog. Org. Coat. 1998, 34, 91. (14) Tang, J.; Dimonie, V. L.; Daniels, E. S.; Klein, A.; El-Aasser, M. S. Macromol. Symp. 2000, 15, 139. (15) Routh, A. F.; Russel, W. B. Langmuir 1999, 15, 7762. (16) Gorce, J.-P.; Bovey, D.; McDonald, P. J.; Palasz, P.; Taylor, D.; Keddie, J. L. Eur. Phys. J. E 2002, 8, 421.
10.1021/la0267950 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/18/2003
Surfactants near Acrylic Latex Film Surfaces
surfactant.7,17 Thus, a slower evaporation rate should favor a more uniform distribution of surfactant. However, other kinetic effects are likely to be relevant in determining the surfactant distribution. With slower evaporation rates, there will be more time for surfactant to desorb from the particles and to migrate to film interfaces. Recent experiments18 have indeed found a greater concentration of surfactant at the surface of latex films when slow drying, in comparison to faster drying, is used. The molecular weight of the surfactant affects surfactant mobility. If the surfactant is small, its diffusion, migration, and transport with the water flux will be faster in comparison to those of larger molecules. On the contrary, high molecular weight surfactants may be trapped inside the film as a result of their slower rate of transport. One promising way to reduce the undesirable effects of surfactant migration is to modify the structure of surfactants so that they can be chemically linked to the polymer. This is the strategy behind polymerizable surfactants or reactive surfactants1,19-23 (also known as surfmers). Owing to the double CdC bond present in their structure, surfmers can copolymerize and therefore remain covalently linked to the polymer, so that desorption is prevented. The absence of surfactant migration imparts substantial benefits to latex coatings, such as better mechanical properties,14 higher electrolyte stability,24,25 increased water resistance,26,27 improved freeze-thaw stability,27,28 and reduced water and water vapor permeability.24 Studies of the surface of latex films by atomic force microscopy (AFM) have been widely reported. However, the study of the heavy elements present near latex surfaces by Rutherford backscattering spectrometry (RBS) is very limited.29,30 RBS is very attractive because it can give precise information about the concentration and distribution of surfactant in dried polymer films. The objective of this work was to compare the distribution of a conventional surfactant (sodium lauryl sulfate) (SLS) to that of a reactive surfactant (sodium tetradecyl maleate) in latex films prepared under identical conditions. The structure of SLS is very similar to that of the reactive surfactant, although it is not precisely the saturated analogue. SLS is, however, commonly used in practice, and so it is sensible to compare its performance to that of the reactive surfactant. AFM was used to probe the surfactant’s topographical features, the surface roughness, and surface defects in the film. RBS was used to determine (17) Juhue´, D.; Wang, Y.; Lang, J.; Leung, O. M.; Goh, M. C.; Winnik, M. A. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1123. (18) Malle´gol, J.; Dupont, O.; Keddie, J. L. Submitted to Prog. Org. Coat. (19) Lam, S.; Hellgren, A. C.; Sjo¨berg, M.; Holmberg, K.; Schoonbrood, H. A. S.; Unzue´, M. J.; Asua, J. M.; Tauer, K.; Sherrington, D. C.; Montoya-Gon˜i, A. J. Appl. Polym. Sci. 1997, 66, 187. (20) Holmberg, K. Prog. Org. Coat. 1992, 20, 235. (21) Guyot, A.; Tauer, K. Adv. Polym. Sci. 1994, 111, 43. (22) Unzue´, M. J.; Asua, J. M. J. Appl. Polym. Sci. 1997, 49, 81. (23) Schoonbrood, H. A. S.; Asua, J. M. Macromolecules 1997, 30, 6034. (24) Greene, B. W.; Sheetz, D. P.; Filer, T. D. J. Colloid Interface Sci. 1970, 32, 90. (25) Greene, B. W.; Saunders, F. L. J. Colloid Interface Sci. 1970, 33, 393. (26) Onodera, S.; Yamamoto, S.; Tamai, T.; Takahashi, H. Jpn. Pat. 06,239,908, 1994. (27) Aramendia, E.; Barandiaran, M. J.; de la Cal, J. C.; Grade, J.; Blease, T.; Asua, J. M. ACS Symp. Ser. 2002, 801, 168. (28) Cochin, D.; Laschewsky, A.; Nallet, F. Macromolecules 1997, 30, 2278. (29) 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. (30) Malle´gol, J.; Gorce, J. P.; Dupont, O.; Jeynes, C.; McDonald, P. J.; Keddie, J. L. Langmuir 2002, 18, 4478.
Langmuir, Vol. 19, No. 8, 2003 3213 Table 1. Formulation of the Seedsa compd MMA BA AA H2O NP30b AsAc TBH
initial charge (g)
592.23 16.33/11.50 1.31 1.90
feed 1 (g) 34.5 34.5 0.88 50.0 1.87
feed 2 (g)
50.0
feed 3 (g)
total (g)
32.85
0.622 0.457
a
34.5 34.5 0.88 725.08 18.20/13.37 1.95 2.38
b
Feeding time, 4 h; temperature, 40 °C. Amount of surfactant used to prepare small/large seeds. Table 2. Formulation of the MMA/BA/AA Seeded Emulsion Polymerizationa initial charge (g)
compd seed water MMA BA AA surfactantb SLS/M14 AsAc TBH
156.51 59.29 11.28 11.28
feed 1 (g)
feed 2 (g) 143.75
feed 3 (g) 100
216.71 216.71 8.60
0.15/0.22 0.05
2.81/4.27 0.652 0.903
a
b
Feeding period, 4 h; digesting time, 1 h; temperature, 50 °C. Total surfactant charge: SLS, 2.96 g; M14, 4.49 g.
the distribution of the heavy elements (namely Na and S) of the surfactants as a function of depth from the film surface. The effects of annealing the films at elevated temperatures were also explored. Experimental Details Chemicals. The following chemicals were used as monomers, as supplied: methyl methacrylate (MMA), butyl acrylate (BA), and acrylic acid (AA) (all technical grade). tert-Butyl hydroperoxide (TBH, from Atofina) and ascorbic acid (AsAc, from Fluka) were used as the initiator system. Sodium lauryl sulfate (SLS) was (from Fluka) used as an ionic surfactant; it consists primarily of sodium dodecyl sulfate. Synperonic NP30, a nonylphenol ethoxylated molecule (Uniqema), was the nonionic surfactant used in the preparation of the latex seed. It was stored at 4 °C prior to use. Sodium tetradecyl maleate (M14) was synthesized following the procedure described elsewhere31 and was stored at 4 °C before use. Doubly deionized water was used throughout the work. Latex Preparation. High solids content MMA/BA/AA copolymer latexes were prepared by seeded semicontinuous emulsion copolymerization. The recipe for the seed is given in Table 1, and the recipe for the seeded reaction is in Table 2. As can be seen in these tables, any compound containing S was avoided (except for the surfactants), so that the RBS analysis was unambiguous. To achieve this objective, a nonionic surfactant was used to prepare the seed, and an organic redox couple (TBH/ AsAc) was used as the initiator. The total molar concentrations of conventional and reactive surfactant were the same in the two types of latex. The SLS comprised 0.61 wt % of the total solids content, and the M14 was 0.92 wt % of the total solids content. The reactions were carried out in a glass reactor equipped with a reflux condenser, a stainless steel stirrer at 200 rpm, a nitrogen inlet (with a 12-15 mL/min flow rate), and a water jacket for temperature control. A heat exchanger fed with tap water was placed between the reactor and the water bath to control any sudden heat production and to keep the reactor at the set temperature (40 °C for the seed and 50 °C for the seeded polymerization). The seed was prepared by means of a semicontinuous emulsion polymerization using three separate feeds: (1) a pre-emulsified mixture of monomers and surfactant with water; (2) the reductant solution; and (3) the oxidant dissolution. The pre-emulsion was (31) Sta¨hler, K. Ph.D. Thesis, Postdam Universita¨t, Germany, 1994.
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prepared by sonication. The sonication of the mixture of water, surfactant, and monomer was done with extreme care to prevent a sudden increase in temperature; the pre-emulsion was sonicated while being introduced to an ice bath. The use of low temperature in the preparation of the seed is of crucial importance. It is well-known that the solubility of nonionic surfactants decreases as the temperature increases. Nonionic surfactants tend to aggregate at high temperatures, and thus they are not effective stabilizers. Two different amounts of nonionic surfactant were used in the initial charge to produce seeds with two different particle sizes. On the other hand, the amount of surfactant in the pre-emulsion was kept constant. The feeding was maintained over 4 h and left overnight to ensure complete conversion. The small seeds had a solids content of 10.4%, a particle diameter of 36 nm, and a pH of 3.7. For the large seeds, the solids content was 10.6%, particle size was 54 nm, and the pH was 3.6. Three separated streams were used in the preparation of the final latex: one with the neat monomers; a second one consisting of the aqueous solution with the surfmer and the oxidant initiator; and a third one containing the reductant aqueous solution. In these reactions the initial charge was purged with nitrogen for 2 h, before starting the feeding. After the feeding period, the system was left to react further for another hour. The acrylic acid was added at the second half of the feed to improve latex stability.32 Latex Characterization. At the end of the reaction, samples were taken to determine the particle size and the conversion of the primary monomers (via gravimetry). The mean particle size was measured by light scattering (Coulter N4 Plus). The amount of coagulum was measured by collecting the coagulum on the reactor wall and stirrer, and by filtering the latex (mesh 63). The result is presented as weight of coagulum per total weight of monomer added. Surfmer conversion was measured by HPLC after extraction from the polymer particles. The analytical method involved the precipitation of the polymer in acetonitrile and ultracentrifugation at 35 000 rpm for 1 h at 20 °C. Differential scanning calorimetry of fully dried latex films identified a glass transition temperature of about 14 °C, regardless of the type of surfactant. Film Formation. Films were cast onto polyester sheets (approximately 10 cm × 10 cm) and allowed to dry at 30 °C under static air in a temperature-controlled chamber for 30 min. After film formation, a representative and uniform piece (less than 1 cm2 in area) was cut from the sheets for use in the AFM and RBS measurements. To study the effect of temperature on surfactant distribution, latex samples were cast at 30 °C and after 20 min (when they were visually dry) they were placed in an oven and annealed in air at 60, 90, or 125 °C for 30 min. The highest temperature was used so that annealing took place above the glass transition temperature for poly(acrylic acid), based on the hypothesis that it could exist as a separate phase in the shells of the particles. Atomic Force Microscopy (AFM). The structure of the top surface of the latex films was characterized using atomic force microscopy (Nanoscope III, Digital Instruments) in the tapping mode with an average set-point amplitude of about 20 nm and a free amplitude of about 25 nm. All measurements used a silicon cantilever (NT-MDT, Moscow, Russia) equipped with an ultrasharp, conical silicon tip having a radius of curvature of about 10 nm. The nominal resonant frequency f0 of the cantilever is 320 kHz, and its spring constant k is 48 N/m. To confirm that an image is representative, the measurements were carried out at least three times, imaging different regions of the same film surface. Rutherford Backscattering Spectrometry (RBS). Rutherford backscattering spectrometry (RBS) measures the energy of particles elastically scattered from the atoms at or near the target sample surface.33,34 The technique thereby determines (32) Aramendia, E.; Barandiaran, M. J.; Grade, J.; Blease, T.; Asua, J. M. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1552. (33) Chu, W. K.; Mayer, J. W.; Nicolet, M. A. In Backscattering Spectrometry; Academic Press: New York, 1978. In Handbook of Modern Ion Beam Analysis; Tesmer, J. R., Nastasi, M., Eds.; Materials Research Society: Pittsburgh, 1995. (34) Earwaker, L. G. Vacuum 1994, 45, 6.
Aramendia et al. the mass of the scattering centers (i.e. the atomic nuclei) through the kinematics (using the conservation of energy momentum), and it determines the depth of the scattering centers from the target surface by considering the electronic (inelastic) energy loss. Particles that scatter from deeper below a surface emerge at a lower energy in comparison to that of particles that scatter at the surface. The RBS cross section was first accurately calculated by Rutherford in 1911 from the Coulomb repulsion of the incident nucleus of the probing beam and the target nuclei; it varies with the square of the atomic number. RBS is therefore ideally suited to obtaining elemental depth profiles of heavy elements in a matrix of lighter elements. As such, it can probe the S and Na contained in the surfactants (and in other species) within the acrylic latex films studied here. Previously, RBS has been similarly employed to determine S profiles in latex films.29,30 RBS experiments were performed on the 2 MV Van de Graaff accelerator, at the University of Surrey, using a 1.0 MeV 4He+ beam at normal incidence. This rather low beam energy E was chosen to achieve a high count rate, which varies as E-2. A high count rate per unit of deposited charge is needed to limit beam damage to the polymer. A 50 mm2 detector with a very large solid angle of 44 msr and a scattering angle of 145° was used in the experiments. To minimize the beam damage, the sample plate was coupled to a coldfinger cooled with liquid nitrogen, and a very low beam current of 5 nA was used. Preliminary experiments showed that nevertheless there was some loss of oxygen in the film (up to 10%) induced by prolonged exposure (10 µC) to the ion beam. In most experiments, a lower charge of 6 µC was therefore collected. The profiles of the elements of greatest interest (K, S, and Na) were not significantly altered by the beam exposure, and so the primary conclusions of the measurements are not affected by any beam damage that occurred. Best-fit elemental depth profiles were extracted from the spectra using the automatic global minimization code DataFurnace,35 which uses the simulated annealing algorithm.36 The spectra obtained in this work were troublesome to fit with the type of minimization used herein (i.e., χ2 difference between the experimental data and the current iteration) because the spectral region of interest has a count rate that is about 2 orders of magnitude below that of the C and O matrix. We therefore used a modified code37 that gives an equal statistical weight to different regions of interest in the spectrum, regardless of whether there is a high or low count rate. The spectra were fitted using prior assumptions based on the known chemistry. Specifically, the elemental concentrations of C, H, O, S, Na, and K were calculated from knowledge of the compounds used in the latex synthesis. A matrix composition (not necessarily the same as the latex composition) was determined from the substrate signal. We assumed no H loss and treated the collected charge as a free parameter. Of course, H is not detected by RBS except through its electronic energy loss, and we cannot infer its concentration accurately because our charge collection is not sufficiently accurate. The accuracy of the analysis is not much disturbed though, since the quantity of interest, the ratio of carbon to the heavy elements, is measured directly. With a good fit to the substrate signal deep in the sample, we then use the program to determine the concentrations of the appropriate surfactant, acrylic acid, and excess surface cations (K and Na) necessary to fit the surface RBS signal for the heavy elements (Na, K, and S).
Results and Discussion Emulsion Polymerization. Table 3 lists the percentage of coagulum, the monomer and surfmer conversions, the particle size, and the solids content of the latexes under investigation. In this table, and throughout the rest of this paper, the latexes made with sodium lauryl sulfate are designated as SLS, and those made with the reactive (35) Barradas, N. P.; Jeynes, C.; Webb, R. P. Appl. Phys. Lett. 1997, 71, 291. (36) Kirkpatrick, S.; Gelatt, C. D.; Vecchi, M. P. Science 1983, 671, 220. (37) NDFv7.7a, which can be found at www.ee.surrey.ac.uk/SCRIBA/ ndf.
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Table 3. Coagulum Percentage and Particle Size of the Latexes Studied latex name
coagulum percent. (%)
monomer convn (%)
surfmer convn (%)
particle size (nm)
solids content (%)
M14-S M14-L SLS-S SLS-L
1.11 0.90 1.71 0.15
100 100 100 99.9
91 74
138.6 151.8 128.0 146.2
53.5 53.6 53.8 53.8
surfactant are designated as M14. The suffixes L and S are used to designate the seed sizes (large and small). For both SLS and M14 latexes obtained from the small seeds, the coagulum content was slightly above 1%. This result is consistent with the fact that a very little amount of surfactant was used with respect to the monomer in the synthesis: 0.61 wt % (with respect to the monomer) for the SLS and 0.92 wt % for the M14, so as to put the same molar amount in every batch. For the latexes obtained from the small seeds (M14-S and SLS-S) a deviation from the theoretical particle size of 115 nm was observed. For the latexes obtained from the large seeds (M14-L and SLSL), better colloidal stability was achieved, and the percentages of coagulum were under 1%. The expected particle size for the latter latexes was 157 nm, but as can be seen in Table 3, slightly lower particle sizes were obtained. The data in Table 3 also show that a high conversion of monomers was achieved, and a high fraction of the surfmer was incorporated into the polymer. Specifically, when the small seed was used, 91% of the surfmer was covalently linked to the polymer particles. With the large seed, 74% of the surfmer was converted. These results are in agreement with both previous experimental38 findings and with theoretical predictions.39 There are two main reasons for this result. One reason is that, for a given solids content, the polymerization rate increases as particle size decreases and the number of particles increases. The second reason is that surfmers react in the outer shell of the particles, and the relative importance of this region increases with decreasing particle size. Surface Morphology. We next consider the surface morphology of the M14 and SLS latexes obtained from small seeds, as displayed in Figure 1. The surface of the M14-S film exhibits some roughness, and individual latex particles can be observed (Figure 1a). A small hill, which is larger than the particle size, is seen in the center of the image (and in other areas outside the field-of-view of this image). This topographical feature is likely to be surfactant used in the preparation of the seed. In contrast, at the surface of the SLS-S film (Figure 1b), individual particles are not seen but only a very irregular layer. It is suspected that the surface is fully covered with SLS, so that the particle identity is obscured. Support of this hypothesis requires the evidence that is provided by RBS analysis. In Figure 1c and d, images are shown of the same films after rinsing with water. Films were immersed in water for 2 min and then shaken dry. The topography of the M14-S film (Figure 1c) did not undergo a significant change. The only difference is that the hill-like features attributed to surfactant (i.e. unreacted surfmer) were no longer observed. The fact that the feature can be removed with water is consistent with the hypothesis that it is composed of surfactant. Similar conclusions have been drawn elsewhere.2,19,30 (Recall that 9% of the surfmer was (38) Schoonbrod, H. A. S.; Unzue´, M. J.; Beck, O. J.; Asua, J. M.; Goni, A. M.; Sherrington, D. C. Macromolecules 1997, 30, 6024. (39) de la Cal, J. C.; Asua, J. M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 585.
unreacted in M14-S.) The surface of the SLS-S film (Figure 1d) is much rougher. Larger hill-like features (up to 1 µm across) are observed. It is suspected that the rough surface is created by the holes left by isolated regions of surfactant that had been rinsed away. Figure 1e and f shows AFM images of the films after annealing (without rinsing them with water) at 125 °C for 30 min. The surfaces of both films are much flatter. Polymer particles of the M14-S film (Figure 1e) have lost their identity. Polymer particle contours cannot be distinguished, but some roughness on that size scale still exists. It appears that particle coalescence has occurred after annealing at 125 °C. Although the SLS-S film surface is planar, regularly spaced and shallow “bumps” are seen, with sizes that are larger than individual particles. It is hypothesized that these bumps might be composed of SLS that has migrated to the surface during annealing. A similar trend is shown in Figure 2 for films cast from latexes made from large seeds. Particle identity is seen in the M14-L film after casting at room temperature (Figure 2a), and it is not much affected by rinsing with water (Figure 2c). After annealing at 125 °C for 30 min, the surface is very flat, and particle coalescence has taken place (Figure 2e). The SLS-L latex displays a different morphology under the same conditions. After casting at room temperature, there is an irregular surface but individual particles cannot be identified (Figure 2b). Presumably, a layer of surfactant is obscuring the surface of the particles. After rinsing (Figure 2d), the particle identities are much more clear, and the surface appears similar to that of M14-L. After annealing at 125 °C (Figure 2f), the surface is planar but not featureless. Small hill-like features are distributed across the surface, and this surface layer might consist of surfactant, but complementary RBS data are required to draw a firm conclusion. The surface topographies observed for the large and small seeds were similar, except after the surfaces had been rinsed with water, in which case the film formed from the latex using smaller seeds was significantly rougher (cf. Figures 1d and 2d). This rough layer is likely to be caused by the redeposition of surfactant after rinsing, although it is not possible to say if it is the nonionic surfactant used to make the seeds or the anionic surfactant used in the subsequent polymerization. To create the small seeds, more nonionic surfactant (NP30) was used in comparison to the case of the large seeds, as shown in Table 1. Some of the surface features in Figure 1d could be the result of migration of this surfactant to the surface, in addition to SLS. Elemental Compositions near the Latex Surfaces. To complement the AFM analysis, the concentration and depth distribution of elements in the latex films were obtained using RBS. From a knowledge of the compounds used in the latex synthesis, the overall elemental composition of the M14 and SLS latexes can be calculated, for both the large and small seeds. The results of the calculation are shown in Table 4. These calculated values are used as a guide in the RBS data analysis. The elements that are heavier than the incoming 4He+ (C, O, Na, S, and K) will appear in the RBS spectra. As H is lighter than 4 He+, it will be forward scattered by incoming 4He+. Typical plots of RBS data from SLS and M14 films cast at room temperature are displayed in Figure 3. On the y-axis, the number of backscattered helium ions is plotted on a logarithmic scale. On the x-axis, the channel number (linearly proportional to the energy of the backscattered ions) is displayed. The channels that correspond to
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Figure 1. AFM topographic images of films formed from latexes produced with small seeds: (a) M14-S after casting at room temperature; (b) SLS-S after casting at room temperature; (c) M14-S after rinsing with water; (d) SLS-S after rinsing with water; (e) M14-S after annealing at 125 °C; (f) SLS-S after annealing at 125 °C. The vertical scale is the same for all images: 50 nm. Scan size: 5 µm × 5 µm.
backscattering from the various elements (C, O, Na, S, and K) at the sample surface are labeled on the axis. The highest number of counts corresponds to the most common elements in the film: C and O. These lighter elements appear at lower channel numbers (lower energies). The elements of greater interest here, S and Na, appear at higher channels. If scattering occurs below the surface, the 4He+ ion emerges with a lower energy, because it loses energy while traveling through the material.
The RBS spectra show no significant difference resulting from the differeing seed sizes, but there are clear differences between the SLS and M14 latexes. Peaks are seen in the SLS-L and SLS-S spectra below channel numbers of 300, namely, corresponding to the presence of S at the surface. A much weaker peak is seen below channel 250, corresponding to Na at the surface. The Na and S are attributed to the presence of SLS. The best fits to the data, obtained using the DataFurnace software,35 are also
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Figure 2. AFM topographic images of films formed from latexes produced with large seeds and dried at room temperature: (a) M14-L after casting at room temperature; (b) SLS-L after casting at room temperature; (c) M14-L after rinsing with water; (d) SLS-L after rinsing with water; (e) M14-L after annealing at 125 °C in air; (f) SLS-L after annealing at 125 °C in air. The vertical scale is the same for all images: 50 nm. Scan size: 5 µm × 5 µm.
included. The data were modeled by describing the sample as consisting of four components (listed in Table 5): (1) latex copolymer containing the bulk concentration of surfactant; (2) surfactant present in a surface layer; (3) acrylic acid; and (4) excess cations (including K present as a trace contaminant in the surfactant). Hence, instead of varying six elements (C, H, O, S, Na, and K) in the fit, with the consequent enormous objective ambiguity in the
spectral interpretation, we considered only three or four chemical components. This approach was discussed at length by Jeynes et al.40 The quality of the fits is excellent. We can therefore say that if only those components used in the fit are present (40) Jeynes, C.; Barradas, N. P.; Rafla-Yuan, H.; Hichwa, B. P.; Close, R. Surf. Interface Anal. 2000, 30, 237.
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Aramendia et al.
Figure 3. RBS plots for films formed at room temperature from latexes having (a) small seeds and (b) large seeds. Symbols indicate the type of surfactant: O, M14; 4, SLS. The solid lines are the best fits to the data. Table 4. Calculated Elemental Composition of the Films Analyzed by RBS latex name
C (at %)
H (at %)
O (at %)
S (at %)
Na (at %)
K (at %)
M14-S M14-L SLS-L SLS-S
33.22 33.15 33.19 33.27
55.27 55.15 55.14 55.25
11.48 11.66 11.64 11.45
0.0158 0.0153 0.0142 0.0141
0.0158 0.0153 0.0142 0.0141
trace trace trace trace
in the sample, then the extracted profiles are true (apart from ambiguity due to the instrumental resolution). The best fits to the data reveal that the M14 data are consistent with the elemental profile expected for the latex copolymer with uniform surfactant distribution. The SLS-L and SLS-S data, on the other hand, are consistent with a thin (20 nm), discontinuous layer of surfactant at the surface. These RBS results support the notion that the layers obscuring the SLS surfaces (Figures 1b and 2b) consist of surfactant. Similar RBS experiments were performed after annealing the SLS and M14 films at various temperatures. In the results shown in Figures 4-6, the top part (a) refers to latexes prepared from small seeds, and the bottom part (b) refers to large seeds, after annealing for 30 min at temperatures of 60, 90, and 125 °C, respectively. The trends found in the films cast at room temperature are also seen after annealing. The composition of M14 films is relatively uniform with depth from the surface, whereas there is a surface excess of Na and S observed in the SLS
Figure 4. RBS plots for films formed from latexes having (a) small seeds and (b) large seeds, after annealing at 60 °C. Symbols indicate the type of surfactant: O, M14; 4, SLS. The solid lines are the best fits to the data.
films. After annealing at 125 °C, the concentration of SLS surfactant at the film surface is particularly high, as indicated by the larger area under the peaks corresponding to Na and S in Figure 6. The size of the seeds does not have any noticeable effect on the trends with increasing annealing temperature. The primary conclusion is that, regardless of annealing temperature, the surfmer is not subject to much exudation to the surface, presumably because most of it is grafted to the polymer. The SLS, in comparison, is in excess at the surface after casting, and it builds up in concentration after annealing at elevated temperatures. Surfactant Depth Profiles in the Latexes. The bestfit analysis of the data shown in Figures 3-6 provides the depth distribution of the components (as defined in Table 5). The depth profiles are shown in Figures 7 and 8 for SLS and M14 surfactants, respectively. These profiles are obtained as discrete layer structures. Of course, the real depth profiles are continuous, and the discrete layering is only an artifact of the finite depth resolution of the technique. The area under these curves is directly proportional to the total excess amount of surfactant at the surface. For the SLS films, Figure 7 shows that the thickness of the surfactant layer increases from about 20 nm to about 100 nm as the annealing temperature is increased. Similarly, the concentration of surfactant at the surface increases. For the SLS-S film (Figure 7a), the
Table 5. Structure Files Used To Fit the RBS Data elemental structure file for M14 films (at %) substance
C
H
O
S
latex excess surfactant acrylic acid cations
33.15 20.7 33.3
55.10 65.5 44.5
11.66 12.1 22.2
0.045 1.7
Na/K 0.045/0 80/20
elemental structure file for SLS films (at %) C
H
O
S
33.15 27.9 33.3
55.10 60.47 44.5
11.66 9.3 22.2
0.045 2.33
Na/K 0.045/0 80/20
Surfactants near Acrylic Latex Film Surfaces
Figure 5. RBS plots for films formed from latexes having (a) small seeds and (b) large seeds, after annealing at 90 °C. Symbols indicate the type of surfactant: O, M14; 4, SLS. The solid lines are the best fits to the data.
Langmuir, Vol. 19, No. 8, 2003 3219
Figure 7. Depth profiles of surfactant in the SLS films at different temperatures: (a) small seeds; (b) large seeds.
Figure 8. Depth profiles of surfactant in M14 films at different temperatures: (a) small seeds; (b) large seeds. Figure 6. RBS plots for films formed from latexes having (a) small seeds and (b) large seeds, after annealing at 125 °C. Symbols indicate the type of surfactant: O, M14; 4, SLS. The solid lines are the best fits to the data.
surface layer consists entirely of surfactant after annealing at 125 °C. The morphology seen in the AFM images obtained from the SLS films is consistent with a continuous surfactant layer.
Only a very slight surface enrichment of surfactant can be seen for the M14 films in Figure 8. The maximum excess surfactant concentration at the surface is only 2% after annealing the film at 125 °C. The excess surfactant is found over depths up to 200 nm. Although experimental uncertainty is higher when analyzing such small concentrations of surfactant, there is a general increase in the total excess amount with increasing annealing temperature. The surfmer reaction with the polymer is very
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Figure 9. Signals under the S (squares) and Na (circles) regions of the RBS plots at different temperatures for latex films containing SLS. The RBS signal is normalized by dividing by the signal at the lowest temperature (30 °C) for ease of comparison. Symbols: 0, S (small seed); 9, S (large seed); O, Na (small seed); b, Na (large seed). The solid lines show the best fit to the S data for the lowest three temperatures only. The dashed lines show the best fit to the Na data for the lowest three temperatures only.
effective at preventing exudation of surfactant, even at elevated temperatures where particle coalescence has taken place (according to the previous AFM analysis). The exudation of SLS or the unreacted surfmer did not depend significantly on the particle size of the seed employed. The greatest difference between the two seed sizes was found for the SLS at 125 °C, where there was a higher surface excess for the small seeds. Likewise, the surface excess of M14 was slightly greater for the latex made from the small seed (despite the fact that the surfmer conversion was higher). This effect of the seed size needs further investigation to explain it fully. Thermal Activation of Surfactant Exudation. The results in Figure 7 show that exudation is particularly pronounced when the SLS-S film is annealed. The effect of temperature on the M14 exudation, in comparison (Figure 8), is very small. Further insight can be gained by considering the exudations of Na and S separately. Whereas S is associated with the anionic organic surfactant molecule, Na can be dissociated from the surfactant as a free cation. The RBS signal areas from Na and S can be used to compare their surface excesses in SLS films at various annealing temperatures (Figure 9). For ease of comparison, the signal is normalized to its value at the lowest temperature (30 °C or 303 K). If the exudation is a thermally activated process, one would expect the rate of exudation, and hence the total amount of surfactant (or ions) exuded, to increase as exp(-E/RT), where E is an activation energy for the diffusion process. The natural logarithm of the signal (which is linearly proportional to the elemental concentration) is therefore plotted against the inverse temperature. Figure 9 reveals that, regardless of seed size, the excess S concentration after annealing the SLS films at 125 °C is much higher than what is predicted by the apparent activation energies of the three lower temperatures. That is, a disproportionate increase in surface excess of S was seen when the temperature was increased from 90 to 125 °C. The Na concentration is closer to what is expected from the trend. A large excess of the anionic surfactant is exuded, but Na cations are not associated with all of the surfactant. In comparison, an unexpectedly strong increase in surfactant or cation exudation is not observed in M14 films at higher temperatures.
Aramendia et al.
These results might be related to the structure of the latex particles. As a result of the method of their synthesis, the particles are expected to have a core-shell structure, with a shell that is enriched in poly(acrylic acid) (PAA). The reported glass transition temperature for pure poly(acrylic acid) (in its acidic form) is 106 °C.41 Thus, the PAA is expected to form a hard, glassy shell at all of the annealing temperatures used, except for 125 °C. Particle coalescence would be impeded by the presence of hard shells, which is consistent with particles retaining their identity, as found with AFM (Figure 2d). Elsewhere, Joanicot et al.42 have already shown that particle coalescence is impeded when the rigidity of a particle shell is increased. At a temperature of 125 °C, a PAA shell would be rubbery and therefore no longer impede coalescence. (Indeed, AFM of the M14 latexes shows that particles are fully coalesced at this temperature (Figures 1e and 2e). Coalescence also certainly occurs in the SLS latexes (Figures 1f and 2f), but it is hidden by the surfactant layer covering the surface.) In the SLS latex, as coalescence proceeds, we suggest that the surfactant is expelled from the interparticle spaces and then segregates to the surface. The resulting excess surface concentration is greater than what is expected for thermally activated (i.e. Arrhenius) diffusion of surfactant. The surface excess of S is always higher than the surface excess for the Na, especially at 125 °C. The Na cations might be associated with acrylic acid groups at the particle surface and therefore be less prone to segregation to the film surface. An additional effect is worthy of consideration. In concentrated emulsions, a few monolayers of water (referred to as a Newton black film) can stabilize a surfactant bilayer.43 When water is present at the particle/ particle interfaces within a latex film, the surfactant serves a “function” in lowering the interfacial energy. At a temperature of 125 °C, any trace amounts of water in the film would be likely to be driven off. Consequently, the surfactant would lose its function, and it might then be expelled from the film as a separate phase. From our data, it is unfortunately not clear whether mobility of the particle surface, loss of trace water, or both trigger the enhanced surfactant exudation. Conclusions We have studied the phenomenon of surfactant exudation in films formed from acrylic latexes prepared by twostep emulsion polymerization stabilized by a conventional surfactant (SLS) and by a surfmer (M14). The conversion of surfmer was high in all the latexes: 74% for a 55 nm seed and 91% for a 36 nm seed. AFM analysis of the latex showed that an irregular layer covered the latex film made using the conventional surfactant. This layer was removed when the films were immersed in water. This layer probably consists of SLS. The surface layer that obscured the particles after annealing the film at 125 °C is likewise probably SLS. By contrast, the latex prepared with the reactive surfactant had a surface in which particle identities were not obscured by a surface layer. After annealing, the surface was very flat, and particles were fully coalesced. These interpretations are supported by RBS analysis. The RBS experiments showed that, in all cases, the surface (41) Hughes, L. J. T.; Fordyce, D. B. J. Polym. Sci. 1956, 22, 509. (42) Joanicot, M.; Wong, K.; Richard, J.; Maquet, J.; Cabane, B. Macromolecules 1993, 26, 3168. (43) Sonneville-Aubrun, O.; Bergeron, V.; Gulik-Grzywicki, T.; Jo¨nsson, B.; Wennerstro¨m, H.; Lindner, P.; Cabane, B. Langmuir 2000, 16, 1566.
Surfactants near Acrylic Latex Film Surfaces
enrichment of Na and S (associated with SLS) was higher for the films containing the conventional surfactant. The very slight excess concentrations of Na and S in the M14 films are consistent with a layer having an excess of up to only 2%. The differences between the M14 and SLS films were even more evident when the films were annealed. There is very little change in the surfactant surface excess in the M14 films, but surfactant excess concentration was found to increase with annealing temperature in the SLS films. RBS analysis reveals that the thicknesses of the surfactant layer increase up to about 100 nm. There was a pronounced increase in the surfactant concentration after annealing the SLS film at 125 °C. This result might be explained by the structure of the latex particles. A hard shell of poly(acrylic acid) might impede particle coalescence below its glass transition temperature of ∼106 °C. At higher temperatures, surfactant exudation might accompany particle coalescence. It might alternatively be triggered by the loss of trace amounts of water.
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The results show that surfactant migration to the surface of acrylic latex films is highly significant, especially after annealing at elevated temperatures. A reactive surfactant (or surfmer), however, can be permanently attached to the polymer. In that case, a minimal surfactant surface excess is found. The use of surfmers is thus undoubtedly an effective means of preventing surfactant migration and segregation. Acknowledgment. The stay of E.A. at the University of Surrey was funded through the Marie Curie Training Centre. We are grateful to Uniqema for funding E.A. and to UCB Chemicals for funding for J.M. Access to the Ion Beam Facility was provided through a grant from the Engineering and Physical Sciences Research Council. We thank IÄ . Lo´pez-Garcı´a for performing the DSC analysis. LA0267950
