Desorption of Emulsifiers from Polystyrene Latexes Studied by Various

Lu-Tao Weng, Philip C. L. Wong, and Kachun Ho , Suhua Wang, Zhaohua ... Wenjing Xie , Lu-Tao Weng , Chak-Keung Chan , King Lun Yeung , Chi-Ming Chan...
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Langmuir 1997, 13, 2943-2952

2943

Desorption of Emulsifiers from Polystyrene Latexes Studied by Various Surface Techniques: A Comparison between XPS, ISS, and Static SIMS L. T. Weng,†,‡ P. Bertrand,*,† J. H. Stone-Masui,§,| and W. E. E. Stone§,⊥ Universite´ Catholique de Louvain, Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium Received December 6, 1996. In Final Form: March 5, 1997X The results obtained with three surface spectroscopic methods (XPS, ISS, and static SIMS with quadrupole or time-of-flight analyzers) available for the characterization of the particle surface of polystyrene latexes have been compared. The main point was to quantify the desorption of the surfactants used in the emulsion polymerization (Aerosol MA and sodium dodecylbenzenesulfonate) by the study of both unpurified and purified latexes. A comparison was made between these results and those obtained on a latex prepared by emulsion polymerization without any tensioactive stabilizer, therefore bearing only sulfate groups issued from the initiator. Sodium dodecyl sulfate was used as a reference for the presence of sulfate groups. The pure emulsifiers were also investigated separately. The various samples were analyzed as either powders or films, and the results obtained on films are discussed in terms of the migration of the surfactant toward the interface during the preparation of the films. On the other hand, our results showed clearly that static SIMS with a time-of-flight analyzer is the most suitable technique to study the desorption of emulsifier from latex particles. These results demonstrate that, after extensive purification by mixed-bed ion exchange resins, the strong acid emulsifier (Aerosol MA) can be completely desorbed.

Introduction Fundamental colloidal studies on latexes need well characterized surfaces. For example, electrophoretic and conductometric measurements must be done on well defined particles. Adsorption studies of macromolecules (polyelectrolytes, proteins, ...) are also dependent and influenced by the state of the interface. To avoid possible interferences with the emulsifiers introduced in the synthesis of the latexes by emulsion polymerization, extensive cleaning procedures of the dispersions are used with the inherent difficulties involved. It is thus important to know to what extent the surfactant is still present after the latex purification treatments. When particles are synthesized without tensioactive molecules or with the addition of comonomers, the problem of the complete desorption of adsorbed oligomers replaces that of the emulsifier desorption. Even for research undertaken on latexes where the tensioactive stabilizer is considered as a component of the system such as in the practical applications of unpurified latexes (in studies of film formation mechanism or latexes stability in the presence of polymers), it is worthwhile having a means of quantifying the surfactant present at the interface. Characterizations, by X-ray photoelectron spectroscopy (XPS), of the surfaces of purified latexes prepared with and without emulsifiers were performed previously by two of us.1,2 It was possible to detect and quantify the sulfate groups present on these purified latexes. The XPS †

Unite´ de Physico-Chimie et de Physique des Mate´riaux. Present Address: Materials Characterisation and Preparation Centre, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong. § Unite ´ de Chimie des Interfaces. | Research Associate FNRS-UCL (Belgium). ⊥ Service de Physico-Chimie Mine ´ rale (MRAC). X Abstract published in Advance ACS Abstracts, May 1, 1997. ‡

(1) Stone-Masui, J. H.; Stone, W. E. E. In Polymer Colloids II; Fitch, R. M., Ed.; Plenum Press: New York, 1980; p 331. (2) Stone, W. E. E.; Stone-Masui, J. H. In Science and Technology of Polymer Colloids; Poehlein, G. W., Ottewill, R. H., Goodwin, J. W., Eds.; NATO ASI Series E No. 68; Martinus Nijhoff Publishers: Boston, 1983; Vol. 2, p 480.

S0743-7463(96)02078-1 CCC: $14.00

technique was however incapable of detecting the carboxyl (COOH) and hydroxyl (COH) functionalities present in extremely small amounts compared to the C-C(H) contribution to the C 1s peak. Even on the uncleaned latexes made with carboxylated emulsifiers, it was impossible to detect the carboxyl functional group.3 By transmission IR,1 the weak acid emulsifier, still present after extensive cleaning, was detected on latexes prepared without any carboxylated comonomer. These carboxyl groups had already been detected by one of us by potentiometric and conductometric titrations.4 However, with these techniques, carried out on the aqueous dispersions of the latex, it was impossible for us to separate an eventual contribution of sulfate groups from the autodissociation of the carboxyl groups in aqueous media. By XPS, the problem of the presence of sulfate groups was solved, but to detect the presence of the molecular emulsifier, a surface technique providing more molecular information was needed. As static secondary ion mass spectrometry (static SIMS) allows us to study the uppermost layers with a very low detection limit (10-4%) and gives access to surface molecular information and trace analysis, we became interested in this technique in the second part of the eighties. First we detected by FAB-SIMS on extensively purified polystyrene latexes3 the parent ion relative to the carboxylic emulsifier introduced in the synthesis. More recently, we studied, by time-of-flight secondary ion mass spectrometry (ToF-SIMS),5 the desorption of strong acid emulsifiers (sodium dodecyl sulfate and sodium dodecylbenzenesulfonate) from latexes by the cleaning processes. In recent years, these surface analysis techniques have also been used by other groups for the study of various surfaces. Restricting ourselves to the field of polymer colloids analysis, the first to apply ion scattering spectroscopy (ISS) were Fifield and Fitch6 in 1981. These preliminary results were however rather unsuccessful. (3) Stone-Masui, J. H.; Stone, W. E. E. Polym. Int. 1993, 30, 169. (4) Stone-Masui, J. H.; Watillon, A. J. Colloid Interface Sci. 1975, 52, 479. (5) Weng, L. T.; Bertrand, P.; Stone-Masui, J. H.; Stone, W. E. E. Surf. Interface Anal. 1994, 21, 387. (6) Fifield, C. C.; Fitch, R. M. J. Dispersion Sci. Technol. 1981, 2, 267.

© 1997 American Chemical Society

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Concerning XPS analysis, a group at DSM7 was the first, after our previous study of the sulfate groups at the surface of polystyrene latex particles,1,2 to use the technique. By a careful analysis of the C 1s peak, they obtained molecular information concerning butyl acrylate/methyl methacrylate copolymer latexes.7 The surface composition of functionalized polymeric microspheres8,9 and more recently of core-shell latex particles10,11 was also characterized by XPS. Coupled with XPS, X-ray-induced mass spectrometry (XIMS) experiments were also run on a same sample.12 This made it possible to differentiate polymers having almost the same C 1s peak. Used simultaneously with XPS, static SIMS is a powerful technique for the study of the distribution of emulsifiers during film formation from acrylic latexes,13 giving some knowledge of the composition of the layer which influences film adhesion. Both techniques have also been used to examine the polystyrene surface of latex particles prepared by dispersion polymerization in the presence of a steric stabilizer.14 Sterically stabilized (with polyethylene glycol chains) polymer colloids prepared by emulsion polymerization were also characterized by XPS and static SIMS,15 providing insight into the surface structure. More recent studies by XPS and static SIMS on poly(butyl methacrylate) latexes16 and copolymer latexes of poly(butyl methacrylate-methyl methacrylate)17 confirm the possiblity of obtaining qualitative and quantitative data for the components of the copolymer and polymer end groups at the surface of these colloids. Depending on the polymerization procedure, preferential surface enrichment of one monomer was detected. Polymer latex particles with immobilized sugar residues18,19 were also characterized by XPS and ToF-SIMS, providing evidence for the presence of the sugar species at the particle surface. Desorption of sodium dodecyl sulfate adsorbed on biodegradable polymer colloids20 has been followed by static SIMS during the purification process. With the exception of our previous work,1,3,5 this is the only publication concerned with the application of a modern surface analysis method in order to detect the surfactant remaining adsorbed after the cleaning procedures. In this paper, a detailed and systematic comparison between these three surface analysis techniques (XPS, ISS, and static SIMS) is made to investigate the emulsifier presence before and after extensive purification of a (7) Pijpers, A. P.; Donners, W. A. B. J. Polym. Sci., Polym Chem. Ed. 1985, 23, 453. (8) Okubo, M.; Ikegami, K.; Yamamoto, Y. Colloid Polym. Sci. 1989, 267, 193. (9) Okubo, M.; Yamamoto, Y.; Kamei, S. Colloid Polym. Sci. 1989, 267, 861. (10) Dobler, F.; Affrossman, S.; Holl, Y. Colloids Surf., A 1994, 89, 23. (11) Arora, A.; Daniels, E. S.; El-Aasser, M. S.; Simmons, G. W.; Miller, A. J. Appl. Polym. Sci. 1995, 58, 313. (12) Pijpers, A. P. In Scientific Methods for the Study of Polymer Colloids and Their Applications; Candau, F., Ottewill, R. H., Eds.; NATO ASI Series C Vol. 303; Kluwer Academic Publishers: Dordrecht, 1990; p 291. (13) Zhao, C. L.; Dobler, F.; Pith, T.; Holl, Y.; Lambla, M. J. Colloid Interface Sci. 1989, 128, 437. (14) Deslandes, Y.; Mitchell, D. F.; Paine, A. J. Langmuir 1993, 9, 1468. (15) Brindley, A.; Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.Polymer 1992, 33, 1112. (16) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Johnson, D. J. Colloid Interface Sci. 1993, 156, 229. (17) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Johnson, D. Langmuir 1994, 10, 1399. (18) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Vickerman, J. C.; Paul, A. J. J. Colloid Interface Sci. 1993, 161, 83. (19) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Paul, A. J. Langmuir 1993, 9, 1637. (20) Koosha, F.; Muller, R. H.; Davis, S. S.; Davies, M. C. J. Controlled Release 1989, 9, 149.

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polystyrene latex. Our purpose is to compare the advantages and disadvantages of each technique. For the clarity of presentation, in the following, quadrupole SIMS will represent static SIMS using quadrupole analyzer while ToF-SIMS means static SIMS with time-of-flight analyzer. Experimental Section (1) Materials. The polystyrene (PS) latex studied in this work was prepared with potassium persulfate as initiator and Aerosol MA (dihexyl sodium sulfosuccinate, C16H29O4SO3Na, called hereafter AMA, American Cyanamid Co.) as emulsifier. The details of the preparation, similar to those of the latex LS1102-A (The Dow Chemical Co.), were previously given to one of us.21 The latex was extensively purified with mixed-bed ionexchange resins (Amberlite MB1 8% DVB-Analar Grade).4 The unpurified and purified latexes are called hereafter respectively PS1 and PS2. For comparison, two other PS latexes also prepared with potassium persulfate as initiator were examined: one prepared with sodium dodecylbenzenesulfonate (NaDBS), C12H25C6H4SO3Na) as emulsifier (PS3) and the second prepared without emulsifier (PS4).4 The synthesis recipes of these two latter latexes and the purification of PS4 were given previously1,4 (PS4 was called Lg in ref 1). Sodium dodecyl sulfate (NaDS, C12H25SO4Na, Merck p.a., no. 13760) was used as a reference for the presence of sulfate groups. (2) Spectroscopic Methods. (a) XPS. XPS measurements were performed with a SSI X-probe (SSX-100 model 206) photoelectron spectrometer from Surface Science Instruments (Fisons, U.K.), using a monochromatized aluminum anode (10 kV, 20 mA). The spectrometer was operated in the fixed analyzer transmission (FAT) mode. A survey scan (0-1100 eV) was recorded for each sample, followed by the recording of C 1s, O 1s, S 2p, Na 1s, and K 1s peaks where appropriate. The analyzer energy was 150 eV for survey scans and 50 eV for the other peaks. An electron flood-gun, set at 6 eV and 55 µA,22 was used to minimize sample charging. Binding energies were referenced to the C 1s of the C(-C, H) peak, set at 284.8 eV. The pressure in the analyzing chamber was around 10-9 Torr. The experiments were run at room temperature. Data analysis was performed on a HP 9000/310 computer. Peak decomposition was done by an iterative least-squares bestfitting routine, using a Gaussian/Lorentzian ratio of 85/15 and using a Shirley baseline.23 Atomic ratios were calculated from XPS intensities corrected by sensitivity factors calculated from a first principles model,24 which takes into account the cross section (from Scofield25 ), λ ∼ Ek0.7, and the experimentally determined transmission function26 and assumes either no contamination for the latexes or a surface hydrocarbon contamination with x ) 0.3 for the emulsifiers. (x is called the reduced thickness, and 0.3 is an average value found previously for several commercial sulfate samples.)24 (b) ISS and Quadrupole SIMS. ISS and quadrupole SIMS measurements were carried out in the same analysis chamber, which was maintained under ultrahigh-vacuum conditions (residual pressure < 10-9 Torr). The noble gas pressure of the chamber is about 5 × 10-8 Torr during the analysis. Table 1 summarizes some experimental details of the two methods of analysis. In the ISS analyses, the ion beam is normal to the sample surface and the backscattered ions are energy analyzed at a full annular solid angle of 138° with respect to the incident ion beam direction. In the case of quadrupole SIMS analyses, the ion beam is set at 75° from the sample surface normal and the take-off angle of the secondary ions is 15°. The secondary ions are first energy filtered and then mass analyzed in the quadrupole mass (21) El-Aasser, M. S.; Vanderhoff, J. W. private communication to J. H. Stone- Masui, May 1976. (22) Siordia, R. Note Surf. Sci. Instrum. 1987, Nov. 19. (23) Shirley, D. A. Phys. Rev. 1972, B5, 4709. (24) Weng, L. T.; Vereecke, G.; Genet, M. J.; Rouxhet, P. G.; StoneMasui, J. H.; Bertrand, P.; Stone, W. E. E. Surf. Interface Anal. 1993, 20, 193. (25) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. (26) Weng, L. T.; Vereecke,G.; Genet, M. J.; Bertrand, P.; Stone, W. E. E. Surf. Interface Anal. 1993, 20, 179.

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Table 1. Experimental Details for ISS and SSIMS Analyses spectrometer primary ion beam: gas energy sample currenta ion beam rastered area charge neutralization scanned region scan time total ion fluence (ions/cm2) a

ISS

SSIMS

Krotos WG-541 CMA (cylindrical mirror analyzer) 3He+ 2 keV 10 nA 1.5 × 1.5 mm2 heated W filament (2 A) (0.3-0.8)E/E0 400 s 1 × 1015

Riber Q-156 quadrupole Xe+ 4 keV 10 nA 0.33 cm2 electron flood-gun (∼1.8 A and ∼200 eV) and/or W filament (2 A) 200-0 amu for positive mode, 190-100/100-0 amu for negative mode 56 s 1 × 1013

Measured with a Faraday cup.

spectrometer. For the samples analyzed in this study, the charge buildup during analysis was compensated for the positive mode by using electrons (∼10 eV) emitted from a heated W filament and for the negative mode by the same W filament plus a floodgun. In order to increase the intensities of peaks higher than 100 amu in the negative mode, two scanned regions were used: 190-100 and 100-10 amu. In order to generate a semiquantitative comparison between quadrupole SIMS results (see below) of different samples, their spectra should be obtained under similar conditions. In our case, a molecular peak at relatively high mass and common to all samples was selected: 91 amu for the positive mode and 73 amu for the negative mode. Before acquiring a spectrum, the selected peak was first maximized by adjusting the voltages on the quadrupole (principally the extraction lens) and the electron flood beam. This tuning procedure was kept the same for each sample. The time for the optimizing procedure and acquisition of a spectrum corresponded to a total ion fluence of 1013 ions/cm2, which is close to the limiting value of static SIMS conditions.27 (c) ToF-SIMS. ToF-SIMS measurements were performed with a ToF-SIMS spectrometer from Charles Evans & Associates.28,29 In the ToF-SIMS experiments, the sample was bombarded with pulsed Ga+ ions (15 keV). The secondary ions were first accelerated up to (3 keV by applying a bias on the sample. The spreading of the initial energies of the secondary ions is compensated by deflection in three electrostatic analyzers. The analyzed area used corresponds to a square of 100 × 100 µm2. With a data acquisition time of 10 min, the total ion fluence is lower than 1012 ions/cm2, which is lower than the usually accepted dose for static conditions.27 An electron flood-gun (Ek ) 20 eV) was used for charge compensation. Acquisition and data treatment were performed using software from Charles Evans & Associates.28 The best mass resolution which can be obtained with this equipment is M/∆M ≈ 11 000 at mass 28 amu on a Si wafer. For the samples and conditions used in this study, the mass resolution is >3000 at mass 28 amu. This facilitates the identification of the isobars with different compositions, e.g. hydrocarbons, oxygenated or nitrogen-containing fragments, etc. (3) Sample Preparation for Surface Analysis. Except for quadrupole SIMS, the samples can be analyzed either as powders or films. For XPS and ISS analyses, powders were compressed into small stainless steel cups (diameter ) 7 mm). For ToFSIMS analyses, the powders were compressed onto an indium foil substrate. The powder layer formed should be as thin as possible (for the ease of the surface potential control) but sufficiently thick so that the substrate is not detected by ToFSIMS. The films analyzed in this study were prepared by spin-coating. One or two drops of PS or emulsifier solution are placed onto a silicon disk, which is then spun at about 5000 rpm for 30 s. The silicon disks used were previously cleaned by ultrasonication in isopropyl alcohol solution (UCB p.a.). The PS latexes and emulsifiers were respectively dissolved in toluene (Janssen Chimica, p.a.) and Milli-Q-plus water (Millipore), with a concentration of about 4% w/v. By spin-coating, thin and uniform films suitable for surface analysis are obtained. Unless otherwise specified, the results presented below were obtained with powder samples due to the higher surface concentration of functional groups (see below). (27) Briggs, D.; Hearn, M. J. Vacuum 1986, 36, 1005. (28) Schueler, B.; Sander, P.; Reed, D. A. Vacuum 1990, 41, 1661. (29) Schueler, B. Microsc. Microanal. Microstruct. 1992, 3, 119.

Figure 1. XPS S 2p spectrum of the emulsifier Aerosol MA.

Figure 2. XPS S 2p spectrum of the emulsifier sodium dodecyl sulfate.

Results and Discussion (1) XPS. Before presenting the results for the latexes, let us first consider the two emulsifiers. Figures 1 and 2 present respectively the S 2p XPS spectra for the emulsifiers AMA and NaDS. The spectra can be perfectly fitted with the S 2p doublet (S 2p3/2 and S 2p1/2) by imposing the surface area ratio S 2p3/2/S 2p1/2 ) 2 (theoretical degeneracy ratio).25 The S 2p3/2 binding energies of AMA and NaDS are respectively 167.84 and 168.80 eV. These values correspond to those reported in the literature for sulfonate30 and sulfate31 compounds. The difference in binding energy for S 2p3/2 between sulfate (SO4-) or more exactly (O-SO3- ) and sulfonate (SO3- ) is thus about 1 eV. (30) Lindberg, B. J.; Hamrin, K.; Johansson, G.; Gelius, U.; Fahlman, A.; Nording, C.; Siegbahn, K. Phys. Scr. 1970, 1, 286. (31) Briggs, D., Seah, M. P., Eds., Practical Surface Analysis, 2nd ed.; Wiley: Chichester, 1990; Vol. 1.

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The atomic ratios, O 1s/C 1s and O 1s/S 2p, are respectively 0.434 and 6.38 for AMA and 0.325 and 3.54 for NaDS. These values are close to the stoichiometric ones (O/C ) 0.438, O/S ) 7 for AMA; O/C ) 0.333, O/S ) 4 for NaDS). For the latex without purification (PS1), from the solid content (in volume fraction) at the end of the polymerization (φ ) 27.42%) and the particle size measured by turbidimetry (Dt ) 220 nm), the deduced number of particles per milliliter is 5.2 × 1013. The polydispersity ratio P ) DW/DN was 1.005. DW (212 nm) and DN (211 nm) are the weight mean diameter and the number-average diameter determined from transmission electron microscopy. Then, from the known amounts of the ingredients introduced in the recipe and the total surface area developed by the monodisperse particles, we can estimate, for the unpurified sample, the number of atoms per square centimeter for oxygen and sulfur (both from the emulsifier and the initiator). If, as a first approximation, an XPSanalyzed thickness of 5 nm is considered, then the number of carbon atoms can be deduced. Whether the O and S are located on the particle surface or distributed in this volume (in 5 nm surface thickness), the estimated atomic ratios are O/C ) 0.039 and S/C ) 0.007. By XPS, the measured atomic ratios are O 1s/C 1s ) 0.0437 and S 2p/C 1s ) 0.0078. These measured values are in good agreement with our estimations. However, if the oxygen and sulfur atoms were distributed in the entire volume of the particles, the calculated ratios should be for oxygen 0.0047 and for sulphur 0.0008, that is about 10 times lower than those measured by XPS. This means that the O and S atoms are not distributed randomly in the bulk of the particles but are clearly concentrated on their surface. On the other hand, from the known amounts of the emulsifier and the initiator introduced in the recipe, we can estimate that, for the S distributed in the volume analyzed by XPS, there are 53% SO3 and 47% SO4 (assuming that K2S2O8 decomposes totally in 2 SO4). Taking also into account the contribution of the oxygen coming from the ester groups (COO) of AMA, then the O/S ratio should be 5.59. This value is in agreement with that measured by XPS (O 1s/S 2p ) 5.60). As there are two ester groups in a molecule of AMA, we can also estimate that the percentage of carbon atoms found in the carboxyl under the ester form is 0.74%. The binding energy of the carboxyl found in AMA is 288.85 eV (spectrum not given), which is totally in agreement with the mean value cited in the literature.31 Figure 3a shows the C 1s peak for the unpurified sample PS1. As can be seen, no peak appears at the region around 289 eV. This signifies that it is impossible to deduce information concerning the presence of carboxyl groups when they amount to less than 1%. The same conclusion was obtained for a polystyrene latex prepared with potassium stearate as emulsifier.3 The reason for the low sensitivity of carboxyl groups may be that the COO peak is hidden in the background of the large C 1s peak. However, the C-O peak at 286.4 eV with about 2.4% can be detected. It corresponds to C atoms linked to sulfate groups (CO-SO3-) and to (C-O) groups of AMA. As anticipated from the above estimation, both SO3 and SO4 should be present in the unpurified sample. In Figure 3b is shown the fitting of the S 2p peak with two components and assuming a peak width of 1.4 eV, which is found for the other latexes prepared without emulsifier. We have also tried the similar fitting but assuming a peak width of 1.3 eV (the value found for the pure AMA and NaDS), but the fitting (not shown) was very bad. With the fitting presented in Figure 3b, the S 2p3/2 binding energies of these two components are 168.39 and 169.16 eV, respectively. The binding energy of the last component

Weng et al.

Figure 3. XPS (a) C 1s spectrum and (b) S 2p spectrum of the PS latex prepared with Aerosol MA as emulsifier (unpurified).

(169.16 eV) corresponds to the value for SO4. However, the binding energy of the first component (168.39 eV) is 0.5 eV higher than that found for pure AMA. The reason for this shift is not clear. The influence of the environment on the binding energy, which is different in pure AMA and for AMA on PS, must be claimed. Moreover, the relative concentrations of SO3 and SO4 deduced from this fitting (76% for SO3 and 24% for SO4) are not in agreement with those estimated above from the recipe (53% for SO3 and 47% for SO4). For the latex after extensive purification (PS2), the atomic ratios O 1s/C 1s and S 2p/C 1s measured by XPS are 0.0236 and 0.0044, respectively. This means that the O and S contents decrease by 46% and 44%, respectively, after extensive purification. These results can be explained by the elimination of the emulsifier and potassium persulfate residues during purification. Such an explanation seems to be confirmed by the O 1s/S 2p ratio calculated for the oxygen and sulfur eliminated: indeed, the value (5.9) is between the value found for the emulsifier (6.4) and the stoichiometric value for the potassium persulfate (4.0). The fact that it is closer to the value of the emulsifier than that of the persulfate may suggest that the major parts of the oxygen and sulfur eliminated during purification are originated from the emulsifier. However, whether the emulsifier was totally eliminated cannot be answered with the quantitative analysis results. Figure 4a presents a S 2p XPS spectrum fitted similarly as for Figure 3b, namely with two components and assuming a peak width of 1.4 eV. The S 2p3/2 binding energies of the two components are 168.49 and 169.45 eV, respectively. The component at 169.45 eV corresponds to

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Figure 5. ISS spectra of the PS latexes prepared with Aerosol MA as emulsifier: (a) unpurified and (b) extensively purified with mixed-bed ion-exchange resin.

Figure 4. XPS S 2p spectrum of the PS prepared with Aerosol MA as emulsifier (extensively purified with mixed-bed ionexchange resin).

SO4. But the first component at 168.49 eV is 0.65 eV higher than the value found for AMA and 0.10 eV higher than the value of the first component in the unpurified sample (Figure 3b). As the component at 168.39 eV is assigned to SO3 in the unpurified sample, this component may also be assigned to SO3. The conclusion is that there exist both SO3 and SO4 in the purified samples. This means that the emulsifier is not completely desorbed after extensive purification. This is in contradiction with the following static SIMS results, which will show that the emulsifier is not detectable in the purified sample. On the contrary, if the emulsifier is completely desorbed in the purified sample, there exists then only SO4 groups on the particle surface and the S 2p peak should be fitted with only one component. Figure 4b shows such a fitting. As can be seen, the S 2p3/2 binding energy is 169.20 eV, corresponding to SO4. But the peak width (1.84 eV) is higher than the value usually found for the other purified latexes with only sulfate groups (1.40 eV). The reason is not yet clear, but the very low amount of S must be remembered. The conclusion of the above discussion is that it is difficult or almost impossible by XPS to distinguish between sulfonate and sulfate for the latex studied. This is essentially due to two factors: (1) the small difference of binding energy between SO3 and SO4 (about 1 eV) and (2) the small quantity of S present in the latex ( nNa, where ni is the real surface density. However, the effective surface density nS* may be small due to an elastic screening of the other atoms on S.32 As both nS* and PS* are unknown, it is impossible from the present results to deduce which parameter is responsible for the low sensitivity of ISS to sulfur. (3) Quadrupole SIMS. Only the films prepared by spin-coating were analyzed in quadrupole SIMS. This is due to the difficulty in controlling the surface potential and consequently the inability to obtain reproducible spectra with powder samples. (a) Emulsifiers. The positive SIMS spectrum (not shown) of AMA is greatly dominated by a peak at 23 amu (Na+) with the presence of some small peaks corresponding to hydrocarbon fragments, e.g. 27 (C2H3+), 29 (C2H5+), 39 (C3H3+), 41(C3H5+), 43 (C3H7+), etc. amu and several small peaks which can be considered as due to the fragmentation of the AMA molecule:

m/z 13, 16, 17, 25, 32, 41, 48, 64, 80, 81, and 85 while the region above 100 amu is dominated by the peaks at m/z 101, 103, 107, 119, 137, 143, 151, 167, and 183 (the peak at 101 is not present in Figure 6 because of the mass offset of our spectrometer when acquiring the region 190100 amu; it is indeed present when the spectrum is acquired from 190 to 10 amu). These peaks can easily be assigned from the AMA molecule and can be regrouped into three categories: (1) the peaks belonging to the hydrocarbon fragments such as m/z 13 (CH-), 25 (C2H-), etc.; (2) the peaks due to the fragmentation of the carboxylate groups of AMA, such as m/z 143 (C6H13OsC(O)sCH2-), 101 (C6H13O-), 85 (C5H9O-), 71 (C3H3O2-), 41 (C2HO-), etc.; (3) the peaks associated with SO3 groups, such as m/z 183 (Na(SO3)2-), 167 (S2O5Na-), 151 (CH2dC(COOH)sSO3-), 137 (C(COOH)sSO3-), 119 (NaSO4-), 107 (C2H3sSO3-), 103 (NaSO3-), 81 (HSO3-), 80 (SO3-), etc. The peaks at m/z 64 (SO2-), 48 (SO-), and 32 (S-) are due to the fragmentation of SO3-. It should be pointed out that the assignment of some of these peaks is helped by the high mass resolution spectra obtained with ToF-SIMS for the same samples (see below). As a reference, the negative SIMS spectrum of NaDS is presented in Figure 7. The region 10-100 amu of this spectrum looks very similar to that of AMA except for a very intense peak at 97 corresponding to HSO4-. However, in the present case, the peaks at m/z 80 (SO3-), 64 (SO2-), 48 (SO-), and 32 (S-) are due to the fragmentation of the sulfate groups. It is also interesting to note that the intensities of the peaks at 97 (HSO4-) and at 80 (SO3-) are almost the same. The spectrum above 100 amu is very different from that of AMA, with a series of intense peaks which can be assigned from the sodium dodecyl sulfate molecule (C12H25OSO3Na): 183 (C12H23O-), 179

These peaks are m/z 63 (CH3SO+), 85 (C6H13+), 126 (Na2SO3+), and 149 (Na3SO3+). In the negative SIMS spectrum of AMA (Figure 6), the region 10-100 amu is dominated by a series of peaks at

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Figure 8. Positive quadrupole SIMS spectrum for the latex prepared with Aerosol MA as emulsifier (unpurified).

Figure 10. Negative quadrupole SIMS spectrum for the latex prepared with Aerosol MA as emulsifier (unpurified). Figure 9. Positive quadrupole SIMS spectrum for the latex prepared with Aerosol MA as emulsifier (extensively purified). -),

-),

-),

(C6H11SO4 165 (C5H9SO4 137 (C3H5SO4 123 (C2H3SO4-), 119 (NaSO4-), 110 (CH2SO4-). The peak at m/z 103 (NaSO3-) is due to the fragmentation of NaSO4-. The spectrum obtained in this study is similar to that reported in the handbook of Briggs et al.33 but with a higher sensitivity for the peaks at high masses. (b) PS Latexes. The positive SIMS spectra of PS1 and PS2 are presented in Figures 8 and 9, respectively. Except for the presence of two intense peaks at 23 (Na+) and 39 (partially K+) for PS1 (the intensities of these peaks with respect to that of 91 may differ from one spectrum to another, as SIMS is very sensitive to these ions), the two spectra are very similar to those published in the literature for polystyrene.33,34 These spectra are characterized by a series of peaks at m/z 77, 91, 103, 115, 128, 141, 152, 165, 178, etc., which characterize the aromatic structure of polystyrene.34 One difference between the two spectra is that Na+ and K+ coming respectively from the emulsifier and the initiator are easily observed in the unpurified sample while being almost invisible in the purified one (at least for Na+). These results confirm those obtained with ISS and again suggest the presence of the emulsifier and the residue of initiator in the unpurified sample. In this case, we may expect to observe the presence of some important positive fragments of the emulsifier, e.g. 63 (CH3SO+), 85 (C6H13+), 126 (Na2SO3+), and 149 (Na3SO3+), in the unpurified (33) Briggs, D.; Brown, A.; Vickerman, J. C. Handbook of Static Secondary Ion Mass Spectrometry; Wiley: Chichester, 1989. (34) Newman, J. G.; Carlson, B. A.; Michael, R. S.; Moulder, J. F. Static SIMS Handbook of Polymer Analysis; Perkin-Elmer Corporation, Phys. Electr. Divis.: Eden Prairie, 1991.

sample. Unfortunately, at these masses hydrocarbon peaks from pure polystyrene are also present. Due to the low mass resolution of quadrupole SIMS, it is impossible, from these peaks, to deduce any information about the presence or not of the emulsifier. Another difference between the two spectra is the relative intensities of some small oxygenated peaks at m/z 31 (CH3O+), 45 (C2H5O+ or CHO2+), and 59 (C3H7O+ or C2H3O2+). These peaks decrease slightly after extensive purification. These results are also in agreement with those obtained by XPS and ISS. In Figure 10 is shown a negative SIMS spectrum of PS1. The low-mass region of the spectrum is dominated by the peaks CH-, O-, OH-, C2-, and C2H-. In the region m/z 30-100, except the hydrocarbon peaks 36 (C3-), 37 (C3H-), 48 (partially, C4-), 49 (C4H-), 60 (C5-), 61 (C5H-), etc., we observe several peaks due to the presence of SO3 groups: 93 (CHSO3-), 80 (SO3-), 64 (SO2-), 48 (partially, SO-), and 32 (partially, S-). In the region above 100 amu, some important peaks characteristic of the emulsifier are observed: 103 (NaSO3-), 107 (C2H3SO3-), 119 (NaSO4-), etc. These results show that the emulsifier is present on the unpurified latex particles. The important fragments of the emulsifier above 100 amu are, however, absent in the negative SIMS spectrum of PS2 (Figure 11). The S-containing peaks (e.g. SO3-, SO2-, SO-, and S-) observed in the PS1 are only visible when the ordinate is greatly expanded. A series of small S-containing peaks can still be seen: 97 (HSO4-), 96 (SO4-), 81 (HSO3-), 80 (SO3-), 64 (SO2-), 48 (SO-), and 32 (S-) (see the section inserted in the Figure, showing the presence of SO3 and SO4). These results clearly show that the quantity of the emulsifier decreases greatly after

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Table 2. SIMS Intensity Ratios, HSO4/SO3 and (HSO4 + SO4)/(SO3 + HSO3), Obtained for the Pure Emulsifiers and the PS Latexes samples

HSO4/SO3

(SO4 + HSO4)/(SO3 + HSO3)

AMA (SO3) NaDS (SO4) PS1 (AMA as emulsifier, unpurified) PS2 (AMA as emulsifier, purified) PS3 (NaDBS as emulsifier, unpurified) PS4 (without emulsifier)

0.0406 ( 0.0013 0.9180 ( 0.2540 0.0441 ( 0.0130 1.0330 ( 0.2220 0.0852 ( 0.0055 1.1200 ( 0.3660

0.0392 ( 0.0089 1.3010 ( 0.2920 0.0672 ( 0.0192 0.9977 ( 0.1963 0.1303 ( 0.0098 1.3240 ( 0.5190

Figure 11. Negative quadrupole SIMS spectrum for the latex prepared with Aerosol MA as emulsifier (extensively purified).

extensive purification. As indicated previously, the presence of sulfate groups arises from the initiator used in the emulsion polymerization. As SO3 can be due to the fragmentation of SO4, we decided to look at whether the SO3 observed in PS2 is just due to the fragmentation of SO4 or if other SO3 groups coming from the emulsifier are present. For that purpose, we compared the SIMS intensity ratios, HSO4/SO3 and (HSO4 + SO4)/(SO3 + HSO3), obtained with the purified latex with those obtained with the pure sodium dodecyl sulfate. The idea was that if there are only sulfate groups present in the purified latex, these intensity ratios should be the same or at least close to those of sodium dodecyl sulfate. For comparison with PS1, the results obtained with a PS latex (PS3) prepared with sodium dodecylbenzenesulfonate (NaDBS) as emulsifier (unpurified), thus bearing SO3 and SO4 groups, are also included. Furthermore, we analyzed a PS latex (PS4) prepared without emulsifier and purified as PS2. In PS4, only sulfate groups are present. Table 2 summarizes the HSO4/SO3 and (HSO4 + SO4)/(SO3 + HSO3) ratios obtained for the pure emulsifiers and the latexes. It can be observed from Table 2 that, for the unpurified latexes (PS1 and PS3), the SIMS intensity ratios are

slightly higher than those of AMA but much lower than those of NaDS. These results suggest that there is much more SO3 than SO4 in the unpurified samples. This can be easily explained by the presence of the emulsifiers (SO3). For latex PS2, the SIMS intensity ratios are almost the same as those obtained for pure NaDS and the latex prepared without emulsifier (PS4). These results strongly suggest that only sulfate groups are present in PS2. In summary, the results obtained with quadrupole SIMS clearly show that (1) the emulsifier is present in the unpurified sample (PS1), as reflected both by the observation of the characteristic fragments of the emulsifier and by the much higher intensity of SO3 compared to SO4, and (2) the emulsifier is, within the detection limit of quadrupole SIMS, completely desorbed after extensive purification; this comes from the fact that the characteristic fragments of the emulsifier are not detected and only sulfate groups are present on the purified sample (PS2). (4) ToF-SIMS. Both the films prepared by spin-coating and the powders were analyzed by ToF-SIMS. The results presented here refer only to the powder samples and the comparison will be given in the next paragraph. Before presenting ToF-SIMS spectra, it is necessary to point out the fact that the relative intensities in ToF-SIMS spectra are often different from those obtained by quadrupole SIMS for the same samples (but the characteristic peaks appear in both cases). This difference is mainly due to the ion source (4 keV Xe for quadrupole and 15 keV Ga for ToF-SIMS). However, the other effects such as the transmission function, the detection efficiency, the experimental procedure (i.e. in ToF-SIMS, all secondary ions are parallelly detected, while, in quadrupole SIMS, the spectra were optimized with certain characteristic peaks), and the analysis conditions cannot be excluded. In the following, we shall not present the full spectra but only the regions of interest. For the sake of clarity, the spectra for the emulsifier and two latexes are compared directly. Figure 12 shows two regions (60-160 and 400-460 amu) of the positive ToF-SIMS spectra obtained for the emulsifier AMA and the latexes PS1 and PS2, respectively. For the regions < 60 amu (not shown), the difference between PS1 and PS2 is that the peaks of Na+ and K+ dominate the spectrum in the case of PS1 but are very low in the spectra of PS2. This is in total agreement with the results of quadrupole SIMS. From Figure 12, it can be observed that for the emulsifier (top spectrum), except the characteristic fragments as observed in quadrupole SIMS, namely 63 (CH3SO+), 69 (C5H9+), 85 (C6H13+), 126 (Na2SO3+), and 149 (Na3SO3+), a peak at 411 amu, corresponding to the AMA molecule cationized with Na (MNa+), is observed. If we look at the 60-160 amu region of PS1 (center spectrum), we find that besides the peaks characteristic of polystyrene, e.g. 77, 91, 103, 115, etc., peaks at 63, 69, 85, 126, and 149 amu are also observed. As just indicated above, these peaks exist also in the pure polystyrene spectrum. Thanks to the high mass resolution of our ToFSIMS spectrometer, it is possible to deduce if, within these peaks, contributions from fragments of the emulsifier are to be found. For example, the 63 amu peak is formed of

Desorption of Emulsifiers from Polystyrene Latexes

Langmuir, Vol. 13, No. 11, 1997 2951

Table 3. Comparison between Powders and Films: Normalized ToF-SIMS Intensities of SO3- (80 amu) and HSO4- (97 amu) and the Atomic Ratios O1s/C1s and S 2p/C 1s Determined by XPS for the Powders and the Films ToF-SIMS sample PS1 unpurified PS2 purified

film powder film powder

XPS

80/Σ (10-5)

80/73

97/Σ (10-5)

97/73

O 1s/C 1s

S 2p/C 1s

107.1 ( 31.7 154.2 ( 36.0 4.7 ( 0.7 42.2 ( 8.3

1.096 ( 0.098 1.580 ( 0.298 0.036 ( 0.006 0.397 ( 0.007

10.8 ( 2.4 16.1 ( 2.8 5.8 ( 1.7 26.9 ( 6.4

0.112 ( 0.009 0.166 ( 0.024 0.044 ( 0.012 0.252 ( 0.023

0.0375 0.0437 0.0054 0.0236

0.0061 0.0078 undetectable 0.0044

Figure 12. Positive ToF-SIMS spectra for the emulsifier Aerosol MA (up), latex PS1 (center), and latex PS2 (bottom): (a) 60-160 amu region; (b) 400-460 amu region.

two components: CH3SO+ at 62.99 amu and C5H3+ at 63.02 amu (the intensity of the CH3SO+ peak is much lower than that of the C5H3+ peak). Indeed, the fragments of the emulsifiers are observed in these peaks, and the most intense one is C6H13+. In the region above 400 amu, in addition to the molecular ion of the emulsifier (MNa+) at 411 amu, two other peaks at 427 and 443 are also observed. These peaks can be assigned as MK+ and (M - Na)K2+. These results clearly show the presence of the emulsifier in the unpurified sample. On the contrary, for PS2 (bottom spectrum), the characteristic fragments (except C6H13+) and the molecular ions of the emulsifier are absent. The presence of the

very small C6H13+ peak in PS2 seems to suggest that a “trace” of emulsifier may exist in this sample. However, this small peak also exists in the other purified polystyrene samples, e.g. PS4 prepared without any emulsifier and another PS prepared anionically (Pressure Company). The presence of this so saturated peak in polystyrene (a very unsaturated polymer) is strange. It probably comes from the fragmentation of some atmospheric hydrocarbon contamination. In Figure 13 is shown the comparison between the negative ToF-SIMS spectra obtained with AMA, PS1, and PS2. For the emulsifier AMA, the region below 200 amu is similar to the spectrum obtained with the quadrupole machine (Figure 6), i .e. characterized by the presence of a series of peaks at m/z 32 (S-), 33 (SH-), 41/43 (C2HO-/ C2H3O-), 48 (SO-), 64 (SO2-), 80/81 (SO3-/HSO3-), 85 (C5H9O-), 99 (C6H11O-), 101 (C6H13O-), 103 (NaSO3-), 107 (C2H3SO3-), 110 (C4H4O4-), 119 (NaSO4-), and 183 (Na(SO3)2-). The most important fragments above 90 amu are 99 (C6H11O-), 101 (C6H13O-), and 107 (C2H3SO3-). Moreover, the parent ion (M - Na)- is easily observed at mass 365 amu. In the spectrum of PS1 (center spectrum, Figure 13), the peak of the parent ion of AMA ((M - Na)-) and most of its important fragments are present. However, in the spectrum of PS2 (bottom spectrum, Figure 13), the peak due to the parent ion ((M - Na)-) is not visible. The fragments characteristic of AMA in the region > 90 amu are not observed except three small peaks at 99, 101, and 107 amu. The peaks at 99 and 101 amu are not C6H11Oand C6H13O- but the hydrocarbon peaks C8H3- and C8H5-. The peak at 107 amu could be assigned to C2H3SO3-. However, as the more intense fragments of AMA at 99 and 101 amu are totally absent, we think that this small peak is not due to the fragmentation of the emulsifier but presumably to the fragmentation of the chain-end SO4 groups. In conclusion, the ToF-SIMS results support in a much more convincing way the conclusion deduced from the quadrupole SIMS results, namely that the emulsifier is present on the particles in the unpurified sample and desorbed after extensive purification. (5) Comparison between the Results Obtained with Powders and Those Obtained with Films. As shown by the XPS results above, the molecules of the emulsifier and the chain-end groups are concentrated on the surface of the latex particles. When films are prepared from these materials by the spin-coating method, one may ask if this modifies the amount of the functional groups detected. In order to answer this question, we compared the normalized intensities of SO3- (80 amu) and HSO4(97 amu) obtained from the negative ToF-SIMS and the O 1s/C 1s, S 2p/C 1s ratios obtained by XPS for both the film and the powder samples (Table 3). The normalization in ToF-SIMS is done with respect either to the sum of the intensities of all ions (Σ) or to the intensity of a hydrocarbon peak near SO3- and HSO4- (73 amu, C6H-). Both normalizations show the same trends. It can be observed that for the unpurified sample PS1, the intensities of the S-containing peaks (SO3-, SO4-, and S 2p/C 1s) are about 1.5 times higher in the powders than in the films. The difference between the powders and the

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Weng et al.

films is much greater in the case of the purified sample (PS2). In ToF-SIMS the intensities of SO3- and HSO4are 5-10 times higher in the powders. In XPS S 2p is not detected (after 50 scans) in the films. For the purified latex, the lower intensities of the S-containing peaks found for films can be explained by the fact that the chain-end groups SO4 are first homogenized during the dissolution of the latex in the solvent and finally distributed randomly throughout the film. Davies et al.16 arrived at similar conclusions for PBMA latexes prepared without emulsifiers. In the case of the unpurified latex PS1, the smaller difference between the powders and the films may be explained by the fact that during the preparation of the films, except the homogenization of the chain end groups, the molecules of the emulsifier tend to migrate toward the surface.13 Conclusions The results presented in this paper show clearly that XPS and ISS are not suitable for studying the desorption of emulsifier from latexes. For XPS, this is due to the small difference of binding energy between SO3- and SO4and the low content of sulfur; and for ISS this is due to the low sensitivity of S. However, the capability of XPS in quantification is very useful, e.g. for demonstrating the presence of functional groups at the surface of latex particles, at least if the functional group peak is not hidden by a huge peak (as is the case for the C 1s peak and COO functions in amounts less than 1%). Although the presence of emulsifier in the latex before purification can easily be detected by quadrupole SIMS, whether the emulsifier is completely desorbed after purification can only be indirectly deduced from the comparison of the relative intensity of some ions. As only film samples can be used and it is difficult to control the surface potential in quadrupole SIMS, it is not easy to obtain an unambiguous answer to the complete desorption of emulsifier from quadrupole SIMS results. The most suitable technique to study the desorption of emulsifier is ToF-SIMS, as, in this case, the quasi-molecular ions and the characteristic molecular fragments of the emulsifier can be easily monitered in both positive and negative modes. However, as ToF-SIMS is only semiquantitative, it is desirable to combine this technique with XPS in order to get more complete surface information. For the systems studied in this work, our results show that, after extensive purification, the emulsifier (AMA) molecules can be completely desorbed.

Figure 13. Negative ToF-SIMS spectra for the emulsifier Aerosol MA (up), latex PS1 (center), and latex PS2 (bottom): (a) region 30-90 amu; (b) region 90-185 amu; (c) region 350-380 amu.

Acknowledgment. This research was supported by the “Fonds de De´veloppement Scientifique” (FDS) of the Universite´ Catholique de Louvain. The Financial support of the “Fond National de la Recherche Scientifique (FRNS)Loterie Nationale” (Belgium) and the “Re´gion Wallonne” (Belgium) for the purchase of our ToF-SIMS spectrometer is gratefully acknowledged. We would like to thank Mr. C. Poleunis for his technical assistance. For the XPS device, the support of Prof. P. Rouxhet, the FNRS, and the Department of Scientific Policy (Concerted Actions Physical Chemistry of Interfaces and Biotechnology) is gratefully acknowledged. Last but not least, we thank Mr. M. Genet for helpful discussions and technical assistance. LA962078S