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Surface composition of coalesced acrylic latex films studied by XPS and SIMS
Surface Composition of Coalesced Acrylic Latex Films Studied by XPS and SIMS C. L. ZHAO, F. DOBLER, T. PITH, Y. HOLL, AND M. LAMBLA 1 Institut Charles Sadron ( C R M - E A H P ) , CNRS, ULP, 4 rue Boussingaull, 67000 Strasbourg, France
Received January 5, 1988; accepted May 2, 1988 Surface concentrations of two anionic surfactants (sodium dodecyl sulfate (SDS) and sodium dodecyl diphenyl ether disulfonate (SDED)) in coalesced acrylic latex films (methyl methacrylate and butyl acrylate copolymer, 45 55 wt%) were studied by XPS and SIMS. For SDS, surface enrichment is very pronounced and can reach saturation in the layer analyzed by XPS ( ~ 5 0 / k ) , Film maturation for 3 months gives rise to SDS expulsion toward interfaces. For SDED, enrichment is much less pronounced and during maturation the surfactant surface concentration does not change. It has been shown that the interfaces of 3-day-old latex films were almost saturated in the layer studied by SIMS (10-15 A), whatever the nature of surfactant, the side of the film (air or substrate), and the average concentration. These XPS-SIMS results are useful in the interpretation of adhesion properties of films. © 1989 Academic Press,lnc. INTRODUCTION
Synthetic latices, like natural latices from which natural rubber is produced, are constituted by a colloidal dispersion of small polymer particles stabilized in an aqueous continuous phase by amphiphilic molecules. Synthetic latices are used in two forms: dry powders (e.g., SBR and ABS) obtained after water elimination and latices used directly to form films by coalescence. The latter have become of growing importance in comparison with polymer solutions because they present the major economic and ecologic interest of avoiding the use of an organic solvent. Applications of those kinds of latices are found in the fields of paints, adhesives, paper and coating industries, and reinforcing fiber pretreatment. The latex is cast on a support, water evaporates, and the particles lose their shape and coalesce. The result is a continuous polymer film. Thus the essential properties of latices are the ability to form films and adhesion of the films onto various substrates. To whom correspondence should be sent.
Although there is a relatively large body of literature (1, 2) devoted to the study of the film formation mechanism, more work is needed for a fully satisfactory state of knowledge of the whole process to be reached. For instance, very little is known about what happens to the stabilizing agents during film formation. Adhesion of polymer films on different supports is a difficult problem too. Large differences exist between adhesion of films issuing from a latex and adhesion of films cast from a solution (3). These two kinds of films are also different in other properties like water diffusion (2), gas permeability (4), water uptake (5), mechanical properties (6), and glass transition temperature (7). It happens that the mechanism of latex film formation and nonpolymeric components like initiator residues and emulsifiers largely influence the properties of latex films. We were interested in the influence of emulsifiers on adhesion properties of latex films. We studied the emulsifier distribution in film thickness with a special interest in the film-support and film-air interfaces. As far as
437 Journal of Colloidand InterfaceScience,Vol. 128,No. 2, March15, 1989
0021-9797/89 $3.00 Copyright© 1989by AcademicPress,Inc. All rightsof reproductionin any formreserved.
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adhesion is concerned, the film-support in- ing 10 days and progressively disappeared. The terface is the most important. But the film- disappearance was interpreted by a degradaair interface is important too: it indirectly in- tion by atmospheric oxygen. These results, fluences adhesion by its hydrophilicity and based on one polymer-surfactant couple and permeability. And it also has some effect on on one experimental technique, seem not to other film properties like surface appearance be transposable to other systems. and physical and chemical aging. We studied latices constituted by a methyl In general, emulsifiers are incompatible with methacrylate (45 wt%)-butyl acrylate (55 the polymeric matrix. That incompatibility wt%) copolymer stabilized by one of two anlikely gives rise to an enrichment of the inter- ionic surfactants: sodium dodecyl sulfate faces with emulsifiers. What is the extent of (SDS) and sodium dodecyl diphenyl ether dithis enrichment? How does it depend on time sulfonate (SDED). These latices form films at and surfactant nature? To what extent can room temperature. Glass plates were used as polymer-surfactant interactions account for support. The surfaces of the films were anathe phenomenon? And finally, how are adhe- lyzed at various depths by different techniques. sion properties influenced by the emulsifier By ATR-FTIR (Attenuated Total Reflectionenrichment of the interfaces? These were the Fourier Transformed Infrared) the thickness basic questions at the start of our work. Cor- of the analyzed layer was around 2 #m. The relation between surfactant distribution and results of this study were published elsewhere adhesion of films is important because it in- (9). Over that thickness, the emulsifier controduces a new criterion for the choice of centration at the interfaces is higher than the emulsifier besides the more classical one based average concentration over the entire film (the nominal concentration) by a factor between on latex-stabilizing ability. Little work has been done on the behavior 1.2 and 4. The enrichment factor depends on of the surfactant during coalescence and on --the surfactant nature: enrichment is more its influence on film properties. Voyutskii (7) first discussed the problem of emulsifier be- pronounced with SDS than with SDED; --the interface considered: enrichment is havior during coalescence. According to this higher at the air side; author, when the solubility of the surfactant in the polymer is very low or the concentration --the nominal concentration: the higher the very high, it accumulates between particles nominal concentration, the higher the surface during water evaporation and forms a bicon- concentration; tinuous phase with the polymer matrix. This --the coalescing time: surface concentrahydrophilic network can make the film more tion increases with coalescing time (only for sensitive to water but can also play a reinforc- SDS). ing role and improve the mechanical properties. On the other hand, when the solubility of Interpretation of the enrichment was prothe suffactant is high or the concentration low, posed. Three processes are to be considered: it can dissolve in the polymeric matrix. It then 1. initial enrichment at both sides when the acts as a plasticizer, it facilitates the coaleslatex is cast on the substrate in order to mincence, and the film will be homogeneous. All imize the interfacial free energy; intermediates can exist between these two sit2. extra enrichment at the air side due to uations. Bradford and Vanderhoff (8) obthe nonadsorbed surfactant transported by served an exudation phenomenon by scanning water flux; electron microscopy during the coalescence of a poly (styrene-butadiene) latex poststabilized 3. long time migration toward both sides by a nonionic surfactant, the nonyl phenol in the solid state when the surfactant is inethylene oxide. The exudates grew in size dur- compatible with the polymer. Journal of Colloid and InterfaceScience, Vol. 128, No. 2, March 15, 1989
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By XPS (X-Ray Photoelectron Spectroscopy) and SIMS (Secondary Ion Mass Spectrometry) the probed depths are much thinner: around 50 and 10 A, respectively (this work). These methods permit more precise analysis of the composition of the layer which influences adhesion, and they allow one to correlate surface composition and adhesion work (3). Furthermore, comparison between XPSSIMS and ATR-FTIR results is interesting. MATERIALS AND METHODS 1. F I L M P R E P A R A T I O N
The latices studied were based on classical acrylic systems leading to copolymers suitable for coating applications. Methyl methacrylate (MMA) and butyl acrylate (BuA) are commercially available monomers stabilized by 20 ppm of hydroquinone. To purify them before polymerization they were passed through a column filled with basic and neutral alumina powders. The initiator was analytical grade K2S208 (purity > 99%) used at a concentration of 0.15 wt%. The two anionic surfactants used to stabilize the latex particles were SDS (C~2H25SO4Na supplied by Fluka, purity > 99.8%) and SDED,
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0.133 and 0.170 t~m. The Tg of such a system is around 275°K. The films were formed by coalescing the latices at 22°C and 50% relative humidity on a carefully pretreated polished glass plate. The plates were first immersed in a sulfochromic bath for several hours, then rinsed with water, wiped with fine paper which had been soaked in THF, and finally dried in an oven. This technique generated reproducible substrate surfaces, as verified by contact angle measurements (20 ° _+2 with water). The contact angle was 20 ° because of the THF which remained on the plate, as shown by a SIMS analysis of the substrate before use. The films became transparent after a drying time of about 3 h and their thickness was about 70/~m. Samples for analysis were always taken from the center of the substrate to eliminate edge effects. The films were studied after a coalescing time of 3 days or 3 months. The film-air interface was studied directly, and the film-substrate and the substrate itself were studied after the film was peeled off. The samples are given the notation L if they contain SDS and D if they contain SDED. The emulsifier concentration is specified in the notation. For instance, IA designates a film containing 4 wt% SDS and D2 a film containing 2 wt% SDED. 2. XPS ANALYSIS
SO3Na
SO3Na
The latter was purified from Dowfax 2AI (Dow Chemical Co.) by repeated dissolutions in methanol and precipitations in acetone. Bidistilled water was used throughout. The MMA and BuA (45:55, wt%) copolymer latices (43 wt% solid content) were synthesized by a semicontinuous emulsion polymerization process in a glass reactor under nitrogen atmosphere. The polymerization temperature was 70°C, and the feeding time 3 h. The seed polymerization was performed in such a way that the particle size remained almost constant with different surfactant concentrations, between 0.5 and 4 wt% (percentage of solid content). Particle sizes for all latices are between
XPS spectra were recorded on a VG ESCALAB MKII spectrometer using an A1Ka exciting radiation from an X-ray source operated at 13.5 kV and 10 mA. The constant analyzer energy (CAE) mode was chosen with a pass energy of 50 eV. The pressure in the analysis chamber was around 10 -8 Torr (1 Torr = 133.3 N. m-2). An Apple computer system was used for spectrometer control and data treatment. Due to the small content of sulfur and sodium in the films, 20 scans were accumulated for each of them to increase the signal-to-noise ratio. Typically, about 60 rain were needed for recording the spectra for Cls, Ois, Na~s, and S2p core levels. The radiation damage of the Journal of Colloid and Interface Science, Vol. 128,No. 2, March 15, 1989
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sample upon exposure to X rays was checked by other investigators (I0, 1 1); it was shown that damage was not significant up to an exposure time of 1 h. A general XPS spectrum of an L4 sample is given in Fig. 1 as an example. The main elements in the system C, O, S, Na contribute to the spectrum through XPS a n d / o r Auger peaks. Even silicon, probably coming from the glass support, can be detected. The emulsifier was detected in XPS experiments by the presence of sulfur. But sulfur coming from the initiator introduces an error in the estimation of the amount of emulsifier at the interfaces. The contribution from the initiator (0.15 wt% of the solid content) to the sulfur signal was subtracted, assuming a homogeneous distribution of the initiator fragments through the film thickness. This assumption is the only one possible. Interfaces might be enriched in initiator fragments but that enrichment cannot be measured. At any rate, the higher the surface concentration of the emulsifier, the lower the error due to the initiator contribution.
AL.
The ratio of atom i to atom j (Ni/Nj) has been derived from the XPS data using
N, = I, × kj.jX
Nj
1j
kioti• i
with I the intensity of the XPS signal (peak area), k the instrumental transmission function, a the ionization cross section, and ~ the mean-free path of emitted electrons. The term kjaj~j/kiai~i is called the relative sensitivity factor of atoms i and j . It was experimentally determined by measuring Ii/lj in model polymers with known NdNj ratios. The model polymers were classical ones like PA6, PET, PEO, and PDMS or less classical ones like PSiPh or PPS. The Ni/N; ratios in the bulk were controlled by microanalysis and it was checked that there was no XPS angular dependence. Of course, one does not know ifNi/ Nj in the bulk is equal to Ni/Nj at the surface. But for each ratio of interest, between three and five different products were used and an average value was taken. The values found for Szp/Cls, Najs/C~s, and Si2p/Cls were 1.85, 8.6, and 0.92, respectively. These values are close
0 is
Na is
ClS
Na KLL
0
Binding Energy
FlG. 1. GeneralXPS spectrum of an L4 film. Film/glass interface. Source:A1Ka; 13.5 kV, 10 mA, pass energy: 50 eV. Journal of Colloid and Interface Science, Vol: 128, No. 2, March 15, 1989
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to the theoretical ones which can be calculated taking into account the photoelectron cross sections of Scofield and Rielman et al. (12) and the inelastic mean-free paths calculated by Ashley et al. (13) or measured by Clark et al. (14) or Andrade et aL (15) and assuming a classical kinetic energy dependence of the transmission function (16). The S / C atomic ratio derived from the XPS data is directly related to the emulsifier surface concentration. Angular dependence experiments were performed on some samples. The validity of the results depends on the surface roughness. The only way to assess rugosity in our case is to estimate it on electron microscopy pictures. Rugosity is in the range 0. I zm, which is considerable on an atomic scale. We will only discuss the angular dependence results qualitatively. 3. SIMS ANALYSIS Static SIMS involves bombarding a surface with a rastered ion beam of low current density and mass analyzing the ions emitted by the sample. Under static conditions, the secondary ions are related directly to the molecular structures at the surface. The advantage of static SIMS lies not only in its surface sensitivity (one to two atomic layers) but also in its molecular specificity and trace-analytical capability. The work done by Briggs and coworkers (17-21) has established the performances and potentials of this technique in the field of polymer surface studies. The secondary ion mass spectrometer used was from VG Ltd. equipped with a MIG 300 ionic nanoprobe. The ion beam was generated by field ionizing the liquid gallium atoms from a very fine source. The conditions used to record static secondary ion mass spectra were: Ga ion energy, 7 keV; area covered by ion beam, 4 ram2; ion beam density, 5 × 10-9 A/ cm2; tension for accelerating flooding electrons, 500 V; target bias, 12 V; basic pressure, 2 × 10-9 Tort. Under these conditions, the total ion dose was 1.8 × 10 ~3ions/cm 2, more than the limit (10 J3ions/cm 2) recommended by Briggs. This
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will be taken into account in interpreting the SIMS results. Figure 2 shows two positive SIMS spectra taken at two different points on a 3-day-old L2 film at the substrate side. The two spectra are highly similar, which indicates the reproducibility of the method and the homogeneity of the surface on a macroscopic scale. These spectra are shown as given by the apparatus; the next spectra will be presented in the more classical form of line segments. In order to limit recording time and thus surface damage, only the range 50-75 amu has been recorded carefully. This range is assumed to be typical enough for our purposes. A spectrum of the pure copolymer is presented later on. The pure copolymer film is obtained by precipitating a latex in methanol, then extensively rinsing the precipitate in bidistilled water and methanol and dissolving it in ethyl acetate. The solution is cast on a glass plate and dried in a carefully decontaminated vacuum oven at 60°C for several days. RESULTS
1. XPS RESULTS
Qualitative Analysis: Cls Peak Shape Figure 3 shows that the C~s peak shape in films containing SDS (air side) evolves de2 ~ SDS F/G interface
i ~'o
s5
60
15
i
,
70
7s
rn/e ( amu ) FIG, 2. Positive SIMS spectra recorded at two different points on an L2 film. Film/glass interface. Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
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ZHAO ET AL. Cls
- - t h e air side is always more concentrated in surfactant than the glass side;
Cls
i
i
r
I
280
285
290
280
i
285
.
i . 290
B i n d i n g E n e r g y ( eV )
I~G. 3. Carbon ls XPS peaks of films (air side) containing various concentrationsof SDS. The main peak is always assumed to have a binding energyof 285 eV. (A) Films aged 3 days. (B) Films aged 3 months.
pending on the surfactant concentration in the film and on the age. In the L0.5 film aged 3 days the main peak is rather broad and there is a second peak relatively well resolved at 3.8 eV from the main one. As concentration and age increase the main peak becomes thinner and the second peak tends to disappear; it has vanished in the L2 (not given) and L4 films aged 3 months. This is also true on the substrate side. As an illustration, in films containing SDS and aged 3 days, the ratios of the second peak height to the main peak height are 0.193, 0.164, 0.153, and 0.087 when concentrations increase from 0.5 to 1,2, and 4%, respectively. For films stabilized by SDED the evolution of the Cls peak shape with concentration is the same: the higher the concentration, the thinner the main peak and the lower the second peak. But age seems not to influence the shape.
maging gives rise to further enrichment: after 3 months' coalescing time the S / C ratio is higher than after 3 days. For the L2 and L4 films on the air side, after 3 months' maturation time the S/C ratios are 0.072 and 0.079, respectively, very close to 0.083, the value in the pure emulsifier. Figure 5 derives directly from Fig. 4 and shows the weight percentage of surfactant at the interfaces (assuming no orientation effect; see Discussion) as a function of the percentage in the film. The surface concentration is between 20 and 95 wt%. Angular dependence studies have been performed on L4 and L2 samples, air and glass sides (see Fig. 6 for L4 results). The S / C and N a / C ratios decrease as the exit angle (angle between the entrance direction of the analyzer and the normal to the surface) increases. This corresponds to an increase in carbon atomic content in comparison with sulfur and sodium
F/A, 3 months
u~6.{I
2,0
Quantitative x P s Results 0
Figure 4 presents the S / C atomic ratio at the interfaces as a function of the same ratio in the film for both sides of SDS-stabilized samples aged 3 days or 3 months. One can observe that - - t h e interfaces are much more concentrated in surfactant than the bulk; Journal of Colloid and Interface Science, V o l .
128, N o . 2, M a r c h 15, 1989
1.0
2.0
3,0 s/c xlo 3 ( volume )
FIG. 4. SIC atomic ratio determinedby XPS as a function of the average S/C atomic ratio in SDS-containing films(L0.5, LI, L2, and L4 samples). El, film-airinterface: 3 m0nthsi II, film-g/ass interface: 3 months; O, film-air interface: 3 days; e, film-g/ass interface: 3 days; equality of the surfaceratio with the averageratio.
443
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f
% 4% $DS
=-,'~
~8.0 ~ - - ~ _ ~
~.A, 3 months
60 F/G, 3 days 4a
2.0 o~
2*oO
410°
6'00 Exit Angle
FIG. 6. Angular dependence of the S / C atomic ratio of L4 films. [], film-air interface: 3 months; II, film-glass interface: 3 months; ©, film-air interface: 3 days; e , filmglass interface: 3 days. 0
1io
2i6
3.0 '
4.0 ' WSDS (%) Wp
2. SIMS RESULTS
(volume)
FIG. 5. SDS surface concentration (wt%) as a function of average concentration, WsDs/Wp(%). [3, film-air interface: 3 months; It, film-glass interface: 3 months; (3, film-air interface: 3 days; O, film-glass interface: 3 days.
when the uppermost layers are approached. The L2 sample gives the same kind of results. Figure 7 is equivalent to Fig. 4 for SDED. It shows the S/C ratio at the interfaces as a function of the S/C ratio in the film. SDED behaves differently from SDS:
Figure 9 presents the SIMS spectra of pure SDED, pure copolymer, D0.7 and D3.5 films aged 3 days, at the air side. The spectra at the glass side are very similar and will not be presented. The D3.5 spectrum is almost equivalent to the pure SDED spectrum. But the D0.7
°~3.~
F/A
-- S DED enrichment at the air side is less pronounced than for SDS; - - a t the glass side there is no SDED enrichment: surface and bulk concentrations are equivalent; --unlike SDS, SDED does not migrate toward interfaces between 3 days and 3 months.
2.0
1,0
F/g
The glass substrates were analyzed after the 3-day-old films were peeled off. Figure 8 shows the S/Si atomic ratio on the glass as a function of the S/C ratio in the film. No sulfur is detected by XPS on the glass plates before use. It is clear that a certain amount of emulsifier remains on the substrate after the film is peeled off and this amount increases with the surfactant content in the film.
|
= .... i
o
1.0
2,0
3.0 SIC xlO 3
(vo[ume)
FiG. 7. S / C atomic ratio determined by XPS as a function of the average S/C atomic ratio in SDED-containing films. [], film-air interface: 3 months; II, film-glass interface: 3 months; ©, film-air interface: 3 days; e , film-glass interface: 3 days; - - -, equality of the surface ratio with the average ratio. Journal of Colloid and Interface Science, VoL 128, No. 2, M a r c h 15, 1989
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ZHAO ET AL.
Carbons 1 give the main peak, which is broadened by the peaks associated with carbons 2 SDS and 3. The carboxylic carbon gives a second peak separated by 3.8 eV from the main one. ~ 0.3 On our films, the carbon peak shape depends on the amount of emulsifier at the interface. The higher the emulsifier surface concentration, the lower the XPS peaks associated with 0.2 (n carbons from the polymer (the lower the carboxylic peak and, to a lesser extent, the thinner the main peak). The L0.5 film aged 3 days 0.1 (air side) clearly exhibits the carboxylic peak from the polymer. On the other hand, in the L2 and L4 films aged 3 months (air side) this i peak has disappeared (Fig. 3). In the latter LO 'LO 3.0 S/Cxl0 3 case, the surfaces seem to be covered by a layer ( latex film ) of pure emulsifier whose thickness is more PqG.8. S/Si atomicratio on the glasssubstrateafterthe than the XPS sampling depth ( ~ 6 0 ~,). Thus film was peeled off, as a function of the volumeaverage the shape of the XPS carbon peak gives an S/C ratio in the film. e, SDS-containingfilms;II, SDEDcontainingfilms. 0.4
spectrum exhibits slight differences. In the latter case the polymer probably contributes to the spectrum. Figure 10 is the same for pure SDS, L0.5, L2, and IA films (air side, aged 3 days). Again the glass side spectra look very much like the air side ones. All film spectra are highly similar to the pure emulsifier spectrum. DISCUSSION
C
l
+l
l. QUALITATIVEXPS The XPS carbon peak of acrylic and methacrylic polymers exhibits a relatively complex shape resulting from the superposition of four single peaks corresponding to four different kinds of carbon atoms (22). These are: (1) the carbons from the main chain and from the methacrylic methyl, (2) the carbon in the a position from the carbonyl, (3) the carbon singly bonded to oxygen, (4) the carboxylic carbon. Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
*
i I ,, I If
J ,,I
+i
5o ~'5 +'o .'5 ~o ~'s m/e
(amu)
FIG.9. PositiveSIMSspectraofpureSDED.Latexfilms: F/A interface,aged3 days.(A) Pure copolymer.(B) I)0.7. (C) D3.5. (D) Pure SDED.
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of 0.072 for L2 and 0.079 for L4, values close to 0.083 (1/12), the theoretical ratio in pure emulsifier. The difference with 0.083 can be interpreted in several ways.
JJ~ c= m
L', Itl
It /,,
.[[ A
,,Ill I 5'0
5'5
445
6'0 ITI/e
,i
II
6'5 70 (amu)
I
7'5
FIG. 10. PositiveSIMSspectraof pure SDS.Latexfilms: F/A interface,aged 3 days. (A) L0.5. (B)L2. (C) L4. (D) Pure SDS. indication of the surfactant surface concentration. The qualitative shape analysis shows that for SDS-stabilized films the interfacial concentration increases with concentration in the film and maturation time at both sides. For SDED-stabilized films the interfacial concentration increases with bulk concentration but aging seems not to affect the emulsifier amount at the interfaces.
(1) There is a certain error in atomic ratios determined by XPS. The extent of this error is difficult to evaluate but it is unlikely that it can account for the difference, especially for L2 films. (2) The layer studied by XPS is not really saturated with surfactant. Some polymer is still there but in too small an amount to be detected by the carbon peak shape. This hypothesis cannot be ruled out. (3) The surface of the sample is contaminated by hydrocarbons coming from the atmosphere of the laboratory or from the XPS chamber. The carbon in excess would decrease the S/C ratio. But polymers have a low surface free energy and the sticking coefficient of hydrocarbons on polymer surfaces is low (23). Furthermore our films have always been carefully protected from contamination in the laboratory and the hydrocarbon content of the XPS chamber is low as verified by mass spectrometry. The contamination hypothesis is unlikely. (4) Another possibility is to postulate an orientation effect of the surfactant molecules which could present their hydrophobic part toward air in order to minimize the interfacial tension. A scheme is shown in Fig. 11, the
air
2. QUANTXTATIVEXPS The quantitative analysis ( S / C ratios, Figs. 4-7) confirms the results of the qualitative analysis. The SDS enrichment is markedly more pronounced than the SDED one. The high level of SDS surface concentration should be emphasized. For the L2 and LA films aged 3 months (air side) the carbon peak shape indicates a top layer saturated with surfactant. The quantitative analysis reveals a S / C ratio
TITTTITI lll11111; IIIIIIIIIIIIII
polymer
FIG. 11. Modelof SDS moleculeorientationat the filmair interface.L, length of the hydrophobicpart; M, length of the hydrophilicpart. Journal of Colloid and Interface Science, Vol. 128, No. 2, M a r c h 15~ 1989
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ZHAO
lines represent the lauryl groups (-C12H25) (length L) and the circles the sulfate groups (-SO4Na) (length M). This model, thermodynamically likely, explains the angular dependence results which show a decrease of the S / C ratio when analysis becomes more superficial. Nevertheless complete agreement between the model and the angular dependence data is not observed for several reasons. First, the error on the photoelectron meanfree paths is considerable (24) as well as that on the lengths L and M. Second, the surface is relatively rough and the emulsifier layers are probably rather disordered. To sum up this part of the discussion, it should be noted that some samples can be saturated with emulsifier over the XPS sampling depth and the molecules in this layer probably orient in such a manner as to present their hydrophobic part toward air. The S / C atomic ratio measured by XPS is influenced by two factors: concentration and orientation of the emulsifier. This introduces a certain ambiguity in the XPS resuits. A careful angular dependence study, when possible, allows us to solve the problem to a certain extent. In our case, the concentration effect seems predominant over the orientation effect.
Comparison of Air Side and Substrate Side The air side of the films is always more concentrated than the substrate side (see Figs. 4 and 7). Figure 8 shows that there is a certain amount of emulsifier left on the glass substrate when the film is peeled off. This amount is enough to account for the difference detected by XPS between the two sides. The film-support interface is the most important for adhesion but its analysis is not straightforward. It would be interesting to know the total amount ofsurfactant at the interface, i.e., the emulsifier on the film after peeling plus the one left on the substrate. This is impossible to determine by XPS because a reference is always needed and the reference is carbon on the film and silicon on the glass. Only comparisons between Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
ET AL.
samples are possible. A surfactant concentration increase on the film side is always associated with an increase on the glass side; thus, the emulsifier amount on the film is representative of the total quantity at the interface.
Aging Aging has different effects on the SDS- and SDED-stabilized films. During maturation SDS surface concentration increases. SDS migrates toward the interfaces because of its incompatibility with the polymer matrix. SDED is different: surface concentration is fixed after 3 days. SDED is more compatible with the polymer or diffusion from the bulk to the surfaces is too slow to be detected after 3 months. It is impossible to give a clear answer. The diffusion coefficient in water is lower for SDED than for SDS (to be published). It is likely that SDED diffusion in films is very slow but it is not possible to assert that it is the key point in explaining the SDED-SDS difference.
Comparison between XPS and A T R - F T I R Comparison between XPS and ATR-FTIR results is interesting. In spite of the difference in sampling depths, the general conclusions which can be drawn in both cases are essentially similar. The only difference is in the enrichment factor (the ratio of the surface concentration to the average concentration): the maximum factor is 4 in the thickness sampled by A T R - F T I R ( ---2 ~m) but it can reach 50 in the much thinner layer ( ~ 5 0 A) studied by XPS. 3. SIMS Figure 9 shows the SIMS spectrum of the pure methyl methacrylate-n-butyl acrylate copolymer. Neither this copolymer nor pure polybutyl acrylate has been studied before. Our spectrum can only be compared to PMMA and PnBMA, studied by Briggs et al. (18, 20), Gardella and Hercules (25), and Campana et
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al. (26). Comparison with spectra published by Briggs et aL shows some clear differences. The most important ones are the intensities of the peaks at 51, 53, and 65 ainu, which are much more intense in our spectrum than in theirs. This is probably due to the difference in the products. The n-butyl acrylate content (55 wt%) strongly influences the spectrum. Pure PnBA will be studied in our laboratory in order to confirm this point. The major contribution from the methyl methacrylate content of the copolymer is to the intensities of the peaks at 59 and 69 amu. According to Briggs (18) the former would be due to cleavage ofthe carboxylate radical +O ~ C - - O - - CH3 and the latter to the C~H~ cyclopropyl derivative generated from the methacrylate backbone. The differences with Briggs spectra can also be due, to a certain extent, to differences in experimental conditions. SIMS spectra are still highly dependent on experimental conditions and it is not easy to find the right conditions for rigorously static SIMS and good-quality spectra. In our case, as we are mainly concerned with comparisons between latex films, pure emulsifiers, and pure copolymer, the most important point is to keep the recording conditions as constant as possible from one sample to another. Finally, contribution of an impurity to a SIMS spectrum is always possible. Contamination could come from the oven in which the film was dried but probably not from solvent residues because it is very unlikely that ethyl acetate can produce peaks at 51, 53, and 65 amu. The important differences between the copolymer spectrum and the latex film spectra and the similarities of the latter with the pure emulsifier spectra indicate that the contribution of the copolymer to the top surface composition is weak or nil. In the uppermost layers one finds the emulsifier which decreased the latex-air and latex-glass interfacial tensions when the latex was cast on its substrate (9). This is particularly clear with the SDED-stabilized films (Fig. 9). For D3.5 films the layer
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analyzed by SIMS is almost saturated with emulsifier. For D0.7 films a weak contribution from the copolymer can be detected (peak at 65 amu). In the layer analyzed by XPS the SDED surface concentration is only 40 and 7%, respectively. The thinner the analyzed layer, the higher the emulsifier surface concentration. This result was already established on a much larger scale by ATR-FTIR (9). On the substrate side of the films one observes the same type of results in spite of the emulsifier left on the glass plate. This indicates that there is probably very little polymer in direct contact with glass. Polymer-glass interactions cannot exert an important influence on adhesion properties. With SDS-stabilized films (Fig. 10) the same kind of conclusions can be drawn. There are weak differences between the latex film spectra which cannot be attributed to the contribution of the copolymer. They could be due to some differences in the surfactant molecule orientation at the surface. A contamination effect is also possible. More work is needed to make some points of this SIMS analysis clearer. In a further study full-range (0-200 amu) spectra will be recorded. Even if SIMS remains a difficult method for polymer surface studies it produces extremely valuable and interesting results and will probably develop widely in the future. CONCLUSION
This XPS-SIMS study of the surface concentrations of two anionic surfactants (SDS and SDED) in coalesced acrylic latex films (methyl methacrylate and butyl acrylate copolymer, 45:55 wt%) has shown the following results. The study by XPS of the S/C and N a / C atomic ratios and of the carbon peak shape leads to relatively precise knowledge of the structure and composition of the interfaces. SDS and SDED behave quite differently. For SDS, surface enrichment is very pronounced and can reach saturation in the layer analyzed by XPS (50 A). Molecules orient at Journal of Colloid and lnte(face Science, Vol. 128, No. 2, March 15, 1989
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ZHAO
the air-film interface in order to adopt the thermodynamically most favorable configuration, i.e., the hydrophobic part presented to air. The difficulty in interpreting the S / C atomic ratio has been emphasized. Complementary information sources like carbon peak shape and angular dependence results are necessary to draw precise conclusions. Film maturation during 3 months gives rise to SDS expulsion toward interfaces due to the incompatibility of the emulsifier with the polymer matrix. For SDED, enrichment is much less pronounced. During maturation the surfactant surface concentration does not change. An exact interpretation o f the latter difference is difficult. The slowness of SDED diffusion in films can be invoked but it is not possible to assert that this point is determining. For both systems, the air side of the films is more concentrated than the substrate side. The emulsifier left on the glass after the film is peeled off can account for the difference. Qualitatively the conclusions of the A T R F T I R (9) and XPS studies are identical. The difference in the sampling depths of the two methods did not allow us to postulate such a result. But surface concentration in the layer sampled by XPS can reach 50 times the average concentration whereas this factor is only contained between 1.2 and 4 in the layer sampled by IR. With SIMS, the sampling depth becomes comparable to the size of the emulsifier molecules. By comparison between surface mass spectra of latex films, pure emulsifiers, and pure copolymer, it has been shown that the interfaces of 3-day-old latex films were almost saturated whatever the side (air or substrate) and the average concentration. This layer is probably formed as soon as the latex is east on its support in order to minimize interfacial tensions and it remains there after water evaporation. Polymer-support interactions cannot influence adhesion of the films. This does not mean that adhesion is only determined by surfactant-substrate interactions. It will be Journal of ColloM and Interface Science, Vol. 128, No. 2, March 15, 1989
ET AL.
shown (3) that the thickness of the emulsifier layer at the film-glass interface and the polymer-surfactant interactions strongly influence adhesion. The difficulties in studying polymer surfaces by SIMS have been emphasized. A proper quantitative use of the technique would require a high level of experimental precaution. The usefulness of these X P S - S I M S results in interpreting the adhesion data is shown in Ref. (3). ACKNOWLEDGMENTS We thank Dr. S. Affrossman for helpful discussions. One of us (C.L.Z.)is indebted to the Ministryof Education of the People's Republic of China for financial support of his Ph.D. work in France. REFERENCES 1. Dillon, R. E., Matheson, L. A., and Bradford, E. B., J. Colloid ScL 6, 108 (1951); Brown, G. L., J. Polym. Sci. 22, 423 (1956); Sheetz, D. P., J. Appl. Polym. Sci. 9, 3759 (1965); Vanderhoff,J. W., Brit. Polym. J. 2, 161 (1970). 2. Vanderhoff,J. W., Bradford, E. B., and Carrington, W. K., J. Potym. Sci. Symp. 41, 155 (1973). 3. Zhao, C. L., Pith, T., Holl, Y., and Lambla, M., Brit. Polym. J. (in press). 4. Chainey, M., Wilkinson, M. C., and Hearn, J., J. Polym. Sei. Polym. Chem. Ed. 23, 2947 (1985). 5. Snuparek, J., Bidman, A., Hanus, J., and Hajkova, B., J. Appl. Polym. Sci. 28, 1421 (I983). 6. Basset,D. R., in "Scienceand Technologyof Polymer Colloids" (G. W. Poehlein, R. H. Ottewill, and J. W. Goodwin, Eds.), Vol. 1, p. 220. NATO AS1 Series, 1981. 7. Voyutskii,S. S., J. Polym. Sci. 32, 528 (1958). 8. Bradford,E. B., and Vanderhoff,J. W., J. Macromol. Sci. Chem. 1, 335 (1966); Bradford, E. B., and Vanderhoff, J. W., Y. Macromol. Sci. Phys. B 6, 671 (1972). 9. Zhao, C. L., Holl,Y., Pith, T., and Lambla,M., Colloid Polym. Sci. 265, 823 (1987). 10. Copperwaite,R. G., Surf Interface Anal. 2, t7 (1980). 11. Stone, W. E. E., and Stone Masui, J. H., in "Science and Technology of Polymer Colloids" (G. W. Poehlein, R. H. Ottewill,and J. W. Goodwin,Eds.), Vol. II, p. 480. NATO ASI Series, 1983. 12. Scofield,J. H., J. Electron Spectrosc. Relat. Phenom. 8, 129 (1976); Rielman, R. F., Msezane, A., and Manson, S. T., J. Electron Spectrosc. Relat. Phenom. 8, 389 (1976).
COALESCED ACRYLIC LATEX FILMS 13. Ashley, J. C., J. Electron Spectrosc. Relat. Phenom. 28, 177 (1982); Ashley, J. C., and Tung, C. J., Surf Interface Anal 4, 52 (1982). 14. Clark, D. T., Thomas, H. R., and Shuttleworth, D., J. Polym. Sci. Polym. Lett. 16, 465 (1978); Clark, D. T., and Thomas, H. R., J. Polym. Sci. Chem. 15, 2843 (1977). 15. Hall, S. M., Andrade, J. D., Ma, S. M., and King, R. N., J. Electron Spectrosc. Relat. Phenom. 17, 181 (1979). 16. Fadley, C. S., Baird, R. J., Siekhaus, W., Novakov, T., and Bergstrom, S. A. L., Y. Electron Spectrosc. Relat. Phenom. 4, 93 (1974); Proctor, A., and Hercules, D. M., Appl. Spectrosc. 38, 505 (1984). 17. Briggs, D., and Wootton, A. B., Surf. Interface Anal. 4, 109 (1982).
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18. Briggs, D., Surf Interface AnaL 4, 151 (1982). 19. Briggs, D., Surf Interface Anal. 5, 113 (1983). 20. Briggs, D., Hearn, M. J., and Ratner, D. B., Surf. Interface Anal 6, 184 (1984). 21. Briggs, D., Surf Interface Anal. 8, 133 (1986). 22. Pijpers, A. P., and Donners, W. A. B., J. Polym. Sci. 23, 453 (t985). 23. Holm, R., and Storp, S., Surf. Interface Anal 2, 96 (1980). 24. Andrade, J. D., "Surface and Interfacial Aspects of Biomedical Polymers," p. 141. Plenum, New York/London, 1985. 25. Gardetla, J. A., and Hercules, D. M., Anal. Chem. 52, 226 (1980). 26. Campana, J. E., DeCorpo, J. J., and Colton, R. J., Appl. Surf Sci. 8, 337 (1981).
Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
Received January 5, 1988; accepted May 2, 1988 Surface concentrations of two anionic surfactants (sodium dodecyl sulfate (SDS) and sodium dodecyl diphenyl ether disulfonate (SDED)) in coalesced acrylic latex films (methyl methacrylate and butyl acrylate copolymer, 45 55 wt%) were studied by XPS and SIMS. For SDS, surface enrichment is very pronounced and can reach saturation in the layer analyzed by XPS ( ~ 5 0 / k ) , Film maturation for 3 months gives rise to SDS expulsion toward interfaces. For SDED, enrichment is much less pronounced and during maturation the surfactant surface concentration does not change. It has been shown that the interfaces of 3-day-old latex films were almost saturated in the layer studied by SIMS (10-15 A), whatever the nature of surfactant, the side of the film (air or substrate), and the average concentration. These XPS-SIMS results are useful in the interpretation of adhesion properties of films. © 1989 Academic Press,lnc. INTRODUCTION
Synthetic latices, like natural latices from which natural rubber is produced, are constituted by a colloidal dispersion of small polymer particles stabilized in an aqueous continuous phase by amphiphilic molecules. Synthetic latices are used in two forms: dry powders (e.g., SBR and ABS) obtained after water elimination and latices used directly to form films by coalescence. The latter have become of growing importance in comparison with polymer solutions because they present the major economic and ecologic interest of avoiding the use of an organic solvent. Applications of those kinds of latices are found in the fields of paints, adhesives, paper and coating industries, and reinforcing fiber pretreatment. The latex is cast on a support, water evaporates, and the particles lose their shape and coalesce. The result is a continuous polymer film. Thus the essential properties of latices are the ability to form films and adhesion of the films onto various substrates. To whom correspondence should be sent.
Although there is a relatively large body of literature (1, 2) devoted to the study of the film formation mechanism, more work is needed for a fully satisfactory state of knowledge of the whole process to be reached. For instance, very little is known about what happens to the stabilizing agents during film formation. Adhesion of polymer films on different supports is a difficult problem too. Large differences exist between adhesion of films issuing from a latex and adhesion of films cast from a solution (3). These two kinds of films are also different in other properties like water diffusion (2), gas permeability (4), water uptake (5), mechanical properties (6), and glass transition temperature (7). It happens that the mechanism of latex film formation and nonpolymeric components like initiator residues and emulsifiers largely influence the properties of latex films. We were interested in the influence of emulsifiers on adhesion properties of latex films. We studied the emulsifier distribution in film thickness with a special interest in the film-support and film-air interfaces. As far as
437 Journal of Colloidand InterfaceScience,Vol. 128,No. 2, March15, 1989
0021-9797/89 $3.00 Copyright© 1989by AcademicPress,Inc. All rightsof reproductionin any formreserved.
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ZHAO ET AL.
adhesion is concerned, the film-support in- ing 10 days and progressively disappeared. The terface is the most important. But the film- disappearance was interpreted by a degradaair interface is important too: it indirectly in- tion by atmospheric oxygen. These results, fluences adhesion by its hydrophilicity and based on one polymer-surfactant couple and permeability. And it also has some effect on on one experimental technique, seem not to other film properties like surface appearance be transposable to other systems. and physical and chemical aging. We studied latices constituted by a methyl In general, emulsifiers are incompatible with methacrylate (45 wt%)-butyl acrylate (55 the polymeric matrix. That incompatibility wt%) copolymer stabilized by one of two anlikely gives rise to an enrichment of the inter- ionic surfactants: sodium dodecyl sulfate faces with emulsifiers. What is the extent of (SDS) and sodium dodecyl diphenyl ether dithis enrichment? How does it depend on time sulfonate (SDED). These latices form films at and surfactant nature? To what extent can room temperature. Glass plates were used as polymer-surfactant interactions account for support. The surfaces of the films were anathe phenomenon? And finally, how are adhe- lyzed at various depths by different techniques. sion properties influenced by the emulsifier By ATR-FTIR (Attenuated Total Reflectionenrichment of the interfaces? These were the Fourier Transformed Infrared) the thickness basic questions at the start of our work. Cor- of the analyzed layer was around 2 #m. The relation between surfactant distribution and results of this study were published elsewhere adhesion of films is important because it in- (9). Over that thickness, the emulsifier controduces a new criterion for the choice of centration at the interfaces is higher than the emulsifier besides the more classical one based average concentration over the entire film (the nominal concentration) by a factor between on latex-stabilizing ability. Little work has been done on the behavior 1.2 and 4. The enrichment factor depends on of the surfactant during coalescence and on --the surfactant nature: enrichment is more its influence on film properties. Voyutskii (7) first discussed the problem of emulsifier be- pronounced with SDS than with SDED; --the interface considered: enrichment is havior during coalescence. According to this higher at the air side; author, when the solubility of the surfactant in the polymer is very low or the concentration --the nominal concentration: the higher the very high, it accumulates between particles nominal concentration, the higher the surface during water evaporation and forms a bicon- concentration; tinuous phase with the polymer matrix. This --the coalescing time: surface concentrahydrophilic network can make the film more tion increases with coalescing time (only for sensitive to water but can also play a reinforc- SDS). ing role and improve the mechanical properties. On the other hand, when the solubility of Interpretation of the enrichment was prothe suffactant is high or the concentration low, posed. Three processes are to be considered: it can dissolve in the polymeric matrix. It then 1. initial enrichment at both sides when the acts as a plasticizer, it facilitates the coaleslatex is cast on the substrate in order to mincence, and the film will be homogeneous. All imize the interfacial free energy; intermediates can exist between these two sit2. extra enrichment at the air side due to uations. Bradford and Vanderhoff (8) obthe nonadsorbed surfactant transported by served an exudation phenomenon by scanning water flux; electron microscopy during the coalescence of a poly (styrene-butadiene) latex poststabilized 3. long time migration toward both sides by a nonionic surfactant, the nonyl phenol in the solid state when the surfactant is inethylene oxide. The exudates grew in size dur- compatible with the polymer. Journal of Colloid and InterfaceScience, Vol. 128, No. 2, March 15, 1989
COALESCED ACRYLIC LATEX FILMS
By XPS (X-Ray Photoelectron Spectroscopy) and SIMS (Secondary Ion Mass Spectrometry) the probed depths are much thinner: around 50 and 10 A, respectively (this work). These methods permit more precise analysis of the composition of the layer which influences adhesion, and they allow one to correlate surface composition and adhesion work (3). Furthermore, comparison between XPSSIMS and ATR-FTIR results is interesting. MATERIALS AND METHODS 1. F I L M P R E P A R A T I O N
The latices studied were based on classical acrylic systems leading to copolymers suitable for coating applications. Methyl methacrylate (MMA) and butyl acrylate (BuA) are commercially available monomers stabilized by 20 ppm of hydroquinone. To purify them before polymerization they were passed through a column filled with basic and neutral alumina powders. The initiator was analytical grade K2S208 (purity > 99%) used at a concentration of 0.15 wt%. The two anionic surfactants used to stabilize the latex particles were SDS (C~2H25SO4Na supplied by Fluka, purity > 99.8%) and SDED,
439
0.133 and 0.170 t~m. The Tg of such a system is around 275°K. The films were formed by coalescing the latices at 22°C and 50% relative humidity on a carefully pretreated polished glass plate. The plates were first immersed in a sulfochromic bath for several hours, then rinsed with water, wiped with fine paper which had been soaked in THF, and finally dried in an oven. This technique generated reproducible substrate surfaces, as verified by contact angle measurements (20 ° _+2 with water). The contact angle was 20 ° because of the THF which remained on the plate, as shown by a SIMS analysis of the substrate before use. The films became transparent after a drying time of about 3 h and their thickness was about 70/~m. Samples for analysis were always taken from the center of the substrate to eliminate edge effects. The films were studied after a coalescing time of 3 days or 3 months. The film-air interface was studied directly, and the film-substrate and the substrate itself were studied after the film was peeled off. The samples are given the notation L if they contain SDS and D if they contain SDED. The emulsifier concentration is specified in the notation. For instance, IA designates a film containing 4 wt% SDS and D2 a film containing 2 wt% SDED. 2. XPS ANALYSIS
SO3Na
SO3Na
The latter was purified from Dowfax 2AI (Dow Chemical Co.) by repeated dissolutions in methanol and precipitations in acetone. Bidistilled water was used throughout. The MMA and BuA (45:55, wt%) copolymer latices (43 wt% solid content) were synthesized by a semicontinuous emulsion polymerization process in a glass reactor under nitrogen atmosphere. The polymerization temperature was 70°C, and the feeding time 3 h. The seed polymerization was performed in such a way that the particle size remained almost constant with different surfactant concentrations, between 0.5 and 4 wt% (percentage of solid content). Particle sizes for all latices are between
XPS spectra were recorded on a VG ESCALAB MKII spectrometer using an A1Ka exciting radiation from an X-ray source operated at 13.5 kV and 10 mA. The constant analyzer energy (CAE) mode was chosen with a pass energy of 50 eV. The pressure in the analysis chamber was around 10 -8 Torr (1 Torr = 133.3 N. m-2). An Apple computer system was used for spectrometer control and data treatment. Due to the small content of sulfur and sodium in the films, 20 scans were accumulated for each of them to increase the signal-to-noise ratio. Typically, about 60 rain were needed for recording the spectra for Cls, Ois, Na~s, and S2p core levels. The radiation damage of the Journal of Colloid and Interface Science, Vol. 128,No. 2, March 15, 1989
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ZHAO
ET
sample upon exposure to X rays was checked by other investigators (I0, 1 1); it was shown that damage was not significant up to an exposure time of 1 h. A general XPS spectrum of an L4 sample is given in Fig. 1 as an example. The main elements in the system C, O, S, Na contribute to the spectrum through XPS a n d / o r Auger peaks. Even silicon, probably coming from the glass support, can be detected. The emulsifier was detected in XPS experiments by the presence of sulfur. But sulfur coming from the initiator introduces an error in the estimation of the amount of emulsifier at the interfaces. The contribution from the initiator (0.15 wt% of the solid content) to the sulfur signal was subtracted, assuming a homogeneous distribution of the initiator fragments through the film thickness. This assumption is the only one possible. Interfaces might be enriched in initiator fragments but that enrichment cannot be measured. At any rate, the higher the surface concentration of the emulsifier, the lower the error due to the initiator contribution.
AL.
The ratio of atom i to atom j (Ni/Nj) has been derived from the XPS data using
N, = I, × kj.jX
Nj
1j
kioti• i
with I the intensity of the XPS signal (peak area), k the instrumental transmission function, a the ionization cross section, and ~ the mean-free path of emitted electrons. The term kjaj~j/kiai~i is called the relative sensitivity factor of atoms i and j . It was experimentally determined by measuring Ii/lj in model polymers with known NdNj ratios. The model polymers were classical ones like PA6, PET, PEO, and PDMS or less classical ones like PSiPh or PPS. The Ni/N; ratios in the bulk were controlled by microanalysis and it was checked that there was no XPS angular dependence. Of course, one does not know ifNi/ Nj in the bulk is equal to Ni/Nj at the surface. But for each ratio of interest, between three and five different products were used and an average value was taken. The values found for Szp/Cls, Najs/C~s, and Si2p/Cls were 1.85, 8.6, and 0.92, respectively. These values are close
0 is
Na is
ClS
Na KLL
0
Binding Energy
FlG. 1. GeneralXPS spectrum of an L4 film. Film/glass interface. Source:A1Ka; 13.5 kV, 10 mA, pass energy: 50 eV. Journal of Colloid and Interface Science, Vol: 128, No. 2, March 15, 1989
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ACRYLIC
to the theoretical ones which can be calculated taking into account the photoelectron cross sections of Scofield and Rielman et al. (12) and the inelastic mean-free paths calculated by Ashley et al. (13) or measured by Clark et al. (14) or Andrade et aL (15) and assuming a classical kinetic energy dependence of the transmission function (16). The S / C atomic ratio derived from the XPS data is directly related to the emulsifier surface concentration. Angular dependence experiments were performed on some samples. The validity of the results depends on the surface roughness. The only way to assess rugosity in our case is to estimate it on electron microscopy pictures. Rugosity is in the range 0. I zm, which is considerable on an atomic scale. We will only discuss the angular dependence results qualitatively. 3. SIMS ANALYSIS Static SIMS involves bombarding a surface with a rastered ion beam of low current density and mass analyzing the ions emitted by the sample. Under static conditions, the secondary ions are related directly to the molecular structures at the surface. The advantage of static SIMS lies not only in its surface sensitivity (one to two atomic layers) but also in its molecular specificity and trace-analytical capability. The work done by Briggs and coworkers (17-21) has established the performances and potentials of this technique in the field of polymer surface studies. The secondary ion mass spectrometer used was from VG Ltd. equipped with a MIG 300 ionic nanoprobe. The ion beam was generated by field ionizing the liquid gallium atoms from a very fine source. The conditions used to record static secondary ion mass spectra were: Ga ion energy, 7 keV; area covered by ion beam, 4 ram2; ion beam density, 5 × 10-9 A/ cm2; tension for accelerating flooding electrons, 500 V; target bias, 12 V; basic pressure, 2 × 10-9 Tort. Under these conditions, the total ion dose was 1.8 × 10 ~3ions/cm 2, more than the limit (10 J3ions/cm 2) recommended by Briggs. This
LATEX
441
FILMS
will be taken into account in interpreting the SIMS results. Figure 2 shows two positive SIMS spectra taken at two different points on a 3-day-old L2 film at the substrate side. The two spectra are highly similar, which indicates the reproducibility of the method and the homogeneity of the surface on a macroscopic scale. These spectra are shown as given by the apparatus; the next spectra will be presented in the more classical form of line segments. In order to limit recording time and thus surface damage, only the range 50-75 amu has been recorded carefully. This range is assumed to be typical enough for our purposes. A spectrum of the pure copolymer is presented later on. The pure copolymer film is obtained by precipitating a latex in methanol, then extensively rinsing the precipitate in bidistilled water and methanol and dissolving it in ethyl acetate. The solution is cast on a glass plate and dried in a carefully decontaminated vacuum oven at 60°C for several days. RESULTS
1. XPS RESULTS
Qualitative Analysis: Cls Peak Shape Figure 3 shows that the C~s peak shape in films containing SDS (air side) evolves de2 ~ SDS F/G interface
i ~'o
s5
60
15
i
,
70
7s
rn/e ( amu ) FIG, 2. Positive SIMS spectra recorded at two different points on an L2 film. Film/glass interface. Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
442
ZHAO ET AL. Cls
- - t h e air side is always more concentrated in surfactant than the glass side;
Cls
i
i
r
I
280
285
290
280
i
285
.
i . 290
B i n d i n g E n e r g y ( eV )
I~G. 3. Carbon ls XPS peaks of films (air side) containing various concentrationsof SDS. The main peak is always assumed to have a binding energyof 285 eV. (A) Films aged 3 days. (B) Films aged 3 months.
pending on the surfactant concentration in the film and on the age. In the L0.5 film aged 3 days the main peak is rather broad and there is a second peak relatively well resolved at 3.8 eV from the main one. As concentration and age increase the main peak becomes thinner and the second peak tends to disappear; it has vanished in the L2 (not given) and L4 films aged 3 months. This is also true on the substrate side. As an illustration, in films containing SDS and aged 3 days, the ratios of the second peak height to the main peak height are 0.193, 0.164, 0.153, and 0.087 when concentrations increase from 0.5 to 1,2, and 4%, respectively. For films stabilized by SDED the evolution of the Cls peak shape with concentration is the same: the higher the concentration, the thinner the main peak and the lower the second peak. But age seems not to influence the shape.
maging gives rise to further enrichment: after 3 months' coalescing time the S / C ratio is higher than after 3 days. For the L2 and L4 films on the air side, after 3 months' maturation time the S/C ratios are 0.072 and 0.079, respectively, very close to 0.083, the value in the pure emulsifier. Figure 5 derives directly from Fig. 4 and shows the weight percentage of surfactant at the interfaces (assuming no orientation effect; see Discussion) as a function of the percentage in the film. The surface concentration is between 20 and 95 wt%. Angular dependence studies have been performed on L4 and L2 samples, air and glass sides (see Fig. 6 for L4 results). The S / C and N a / C ratios decrease as the exit angle (angle between the entrance direction of the analyzer and the normal to the surface) increases. This corresponds to an increase in carbon atomic content in comparison with sulfur and sodium
F/A, 3 months
u~6.{I
2,0
Quantitative x P s Results 0
Figure 4 presents the S / C atomic ratio at the interfaces as a function of the same ratio in the film for both sides of SDS-stabilized samples aged 3 days or 3 months. One can observe that - - t h e interfaces are much more concentrated in surfactant than the bulk; Journal of Colloid and Interface Science, V o l .
128, N o . 2, M a r c h 15, 1989
1.0
2.0
3,0 s/c xlo 3 ( volume )
FIG. 4. SIC atomic ratio determinedby XPS as a function of the average S/C atomic ratio in SDS-containing films(L0.5, LI, L2, and L4 samples). El, film-airinterface: 3 m0nthsi II, film-g/ass interface: 3 months; O, film-air interface: 3 days; e, film-g/ass interface: 3 days; equality of the surfaceratio with the averageratio.
443
COALESCED ACRYLIC LATEX FILMS i
f
% 4% $DS
=-,'~
~8.0 ~ - - ~ _ ~
~.A, 3 months
60 F/G, 3 days 4a
2.0 o~
2*oO
410°
6'00 Exit Angle
FIG. 6. Angular dependence of the S / C atomic ratio of L4 films. [], film-air interface: 3 months; II, film-glass interface: 3 months; ©, film-air interface: 3 days; e , filmglass interface: 3 days. 0
1io
2i6
3.0 '
4.0 ' WSDS (%) Wp
2. SIMS RESULTS
(volume)
FIG. 5. SDS surface concentration (wt%) as a function of average concentration, WsDs/Wp(%). [3, film-air interface: 3 months; It, film-glass interface: 3 months; (3, film-air interface: 3 days; O, film-glass interface: 3 days.
when the uppermost layers are approached. The L2 sample gives the same kind of results. Figure 7 is equivalent to Fig. 4 for SDED. It shows the S/C ratio at the interfaces as a function of the S/C ratio in the film. SDED behaves differently from SDS:
Figure 9 presents the SIMS spectra of pure SDED, pure copolymer, D0.7 and D3.5 films aged 3 days, at the air side. The spectra at the glass side are very similar and will not be presented. The D3.5 spectrum is almost equivalent to the pure SDED spectrum. But the D0.7
°~3.~
F/A
-- S DED enrichment at the air side is less pronounced than for SDS; - - a t the glass side there is no SDED enrichment: surface and bulk concentrations are equivalent; --unlike SDS, SDED does not migrate toward interfaces between 3 days and 3 months.
2.0
1,0
F/g
The glass substrates were analyzed after the 3-day-old films were peeled off. Figure 8 shows the S/Si atomic ratio on the glass as a function of the S/C ratio in the film. No sulfur is detected by XPS on the glass plates before use. It is clear that a certain amount of emulsifier remains on the substrate after the film is peeled off and this amount increases with the surfactant content in the film.
|
= .... i
o
1.0
2,0
3.0 SIC xlO 3
(vo[ume)
FiG. 7. S / C atomic ratio determined by XPS as a function of the average S/C atomic ratio in SDED-containing films. [], film-air interface: 3 months; II, film-glass interface: 3 months; ©, film-air interface: 3 days; e , film-glass interface: 3 days; - - -, equality of the surface ratio with the average ratio. Journal of Colloid and Interface Science, VoL 128, No. 2, M a r c h 15, 1989
444
ZHAO ET AL.
Carbons 1 give the main peak, which is broadened by the peaks associated with carbons 2 SDS and 3. The carboxylic carbon gives a second peak separated by 3.8 eV from the main one. ~ 0.3 On our films, the carbon peak shape depends on the amount of emulsifier at the interface. The higher the emulsifier surface concentration, the lower the XPS peaks associated with 0.2 (n carbons from the polymer (the lower the carboxylic peak and, to a lesser extent, the thinner the main peak). The L0.5 film aged 3 days 0.1 (air side) clearly exhibits the carboxylic peak from the polymer. On the other hand, in the L2 and L4 films aged 3 months (air side) this i peak has disappeared (Fig. 3). In the latter LO 'LO 3.0 S/Cxl0 3 case, the surfaces seem to be covered by a layer ( latex film ) of pure emulsifier whose thickness is more PqG.8. S/Si atomicratio on the glasssubstrateafterthe than the XPS sampling depth ( ~ 6 0 ~,). Thus film was peeled off, as a function of the volumeaverage the shape of the XPS carbon peak gives an S/C ratio in the film. e, SDS-containingfilms;II, SDEDcontainingfilms. 0.4
spectrum exhibits slight differences. In the latter case the polymer probably contributes to the spectrum. Figure 10 is the same for pure SDS, L0.5, L2, and IA films (air side, aged 3 days). Again the glass side spectra look very much like the air side ones. All film spectra are highly similar to the pure emulsifier spectrum. DISCUSSION
C
l
+l
l. QUALITATIVEXPS The XPS carbon peak of acrylic and methacrylic polymers exhibits a relatively complex shape resulting from the superposition of four single peaks corresponding to four different kinds of carbon atoms (22). These are: (1) the carbons from the main chain and from the methacrylic methyl, (2) the carbon in the a position from the carbonyl, (3) the carbon singly bonded to oxygen, (4) the carboxylic carbon. Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
*
i I ,, I If
J ,,I
+i
5o ~'5 +'o .'5 ~o ~'s m/e
(amu)
FIG.9. PositiveSIMSspectraofpureSDED.Latexfilms: F/A interface,aged3 days.(A) Pure copolymer.(B) I)0.7. (C) D3.5. (D) Pure SDED.
COALESCED ACRYLIC LATEX FILMS
of 0.072 for L2 and 0.079 for L4, values close to 0.083 (1/12), the theoretical ratio in pure emulsifier. The difference with 0.083 can be interpreted in several ways.
JJ~ c= m
L', Itl
It /,,
.[[ A
,,Ill I 5'0
5'5
445
6'0 ITI/e
,i
II
6'5 70 (amu)
I
7'5
FIG. 10. PositiveSIMSspectraof pure SDS.Latexfilms: F/A interface,aged 3 days. (A) L0.5. (B)L2. (C) L4. (D) Pure SDS. indication of the surfactant surface concentration. The qualitative shape analysis shows that for SDS-stabilized films the interfacial concentration increases with concentration in the film and maturation time at both sides. For SDED-stabilized films the interfacial concentration increases with bulk concentration but aging seems not to affect the emulsifier amount at the interfaces.
(1) There is a certain error in atomic ratios determined by XPS. The extent of this error is difficult to evaluate but it is unlikely that it can account for the difference, especially for L2 films. (2) The layer studied by XPS is not really saturated with surfactant. Some polymer is still there but in too small an amount to be detected by the carbon peak shape. This hypothesis cannot be ruled out. (3) The surface of the sample is contaminated by hydrocarbons coming from the atmosphere of the laboratory or from the XPS chamber. The carbon in excess would decrease the S/C ratio. But polymers have a low surface free energy and the sticking coefficient of hydrocarbons on polymer surfaces is low (23). Furthermore our films have always been carefully protected from contamination in the laboratory and the hydrocarbon content of the XPS chamber is low as verified by mass spectrometry. The contamination hypothesis is unlikely. (4) Another possibility is to postulate an orientation effect of the surfactant molecules which could present their hydrophobic part toward air in order to minimize the interfacial tension. A scheme is shown in Fig. 11, the
air
2. QUANTXTATIVEXPS The quantitative analysis ( S / C ratios, Figs. 4-7) confirms the results of the qualitative analysis. The SDS enrichment is markedly more pronounced than the SDED one. The high level of SDS surface concentration should be emphasized. For the L2 and LA films aged 3 months (air side) the carbon peak shape indicates a top layer saturated with surfactant. The quantitative analysis reveals a S / C ratio
TITTTITI lll11111; IIIIIIIIIIIIII
polymer
FIG. 11. Modelof SDS moleculeorientationat the filmair interface.L, length of the hydrophobicpart; M, length of the hydrophilicpart. Journal of Colloid and Interface Science, Vol. 128, No. 2, M a r c h 15~ 1989
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lines represent the lauryl groups (-C12H25) (length L) and the circles the sulfate groups (-SO4Na) (length M). This model, thermodynamically likely, explains the angular dependence results which show a decrease of the S / C ratio when analysis becomes more superficial. Nevertheless complete agreement between the model and the angular dependence data is not observed for several reasons. First, the error on the photoelectron meanfree paths is considerable (24) as well as that on the lengths L and M. Second, the surface is relatively rough and the emulsifier layers are probably rather disordered. To sum up this part of the discussion, it should be noted that some samples can be saturated with emulsifier over the XPS sampling depth and the molecules in this layer probably orient in such a manner as to present their hydrophobic part toward air. The S / C atomic ratio measured by XPS is influenced by two factors: concentration and orientation of the emulsifier. This introduces a certain ambiguity in the XPS resuits. A careful angular dependence study, when possible, allows us to solve the problem to a certain extent. In our case, the concentration effect seems predominant over the orientation effect.
Comparison of Air Side and Substrate Side The air side of the films is always more concentrated than the substrate side (see Figs. 4 and 7). Figure 8 shows that there is a certain amount of emulsifier left on the glass substrate when the film is peeled off. This amount is enough to account for the difference detected by XPS between the two sides. The film-support interface is the most important for adhesion but its analysis is not straightforward. It would be interesting to know the total amount ofsurfactant at the interface, i.e., the emulsifier on the film after peeling plus the one left on the substrate. This is impossible to determine by XPS because a reference is always needed and the reference is carbon on the film and silicon on the glass. Only comparisons between Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989
ET AL.
samples are possible. A surfactant concentration increase on the film side is always associated with an increase on the glass side; thus, the emulsifier amount on the film is representative of the total quantity at the interface.
Aging Aging has different effects on the SDS- and SDED-stabilized films. During maturation SDS surface concentration increases. SDS migrates toward the interfaces because of its incompatibility with the polymer matrix. SDED is different: surface concentration is fixed after 3 days. SDED is more compatible with the polymer or diffusion from the bulk to the surfaces is too slow to be detected after 3 months. It is impossible to give a clear answer. The diffusion coefficient in water is lower for SDED than for SDS (to be published). It is likely that SDED diffusion in films is very slow but it is not possible to assert that it is the key point in explaining the SDED-SDS difference.
Comparison between XPS and A T R - F T I R Comparison between XPS and ATR-FTIR results is interesting. In spite of the difference in sampling depths, the general conclusions which can be drawn in both cases are essentially similar. The only difference is in the enrichment factor (the ratio of the surface concentration to the average concentration): the maximum factor is 4 in the thickness sampled by A T R - F T I R ( ---2 ~m) but it can reach 50 in the much thinner layer ( ~ 5 0 A) studied by XPS. 3. SIMS Figure 9 shows the SIMS spectrum of the pure methyl methacrylate-n-butyl acrylate copolymer. Neither this copolymer nor pure polybutyl acrylate has been studied before. Our spectrum can only be compared to PMMA and PnBMA, studied by Briggs et al. (18, 20), Gardella and Hercules (25), and Campana et
COALESCED
ACRYLIC
al. (26). Comparison with spectra published by Briggs et aL shows some clear differences. The most important ones are the intensities of the peaks at 51, 53, and 65 ainu, which are much more intense in our spectrum than in theirs. This is probably due to the difference in the products. The n-butyl acrylate content (55 wt%) strongly influences the spectrum. Pure PnBA will be studied in our laboratory in order to confirm this point. The major contribution from the methyl methacrylate content of the copolymer is to the intensities of the peaks at 59 and 69 amu. According to Briggs (18) the former would be due to cleavage ofthe carboxylate radical +O ~ C - - O - - CH3 and the latter to the C~H~ cyclopropyl derivative generated from the methacrylate backbone. The differences with Briggs spectra can also be due, to a certain extent, to differences in experimental conditions. SIMS spectra are still highly dependent on experimental conditions and it is not easy to find the right conditions for rigorously static SIMS and good-quality spectra. In our case, as we are mainly concerned with comparisons between latex films, pure emulsifiers, and pure copolymer, the most important point is to keep the recording conditions as constant as possible from one sample to another. Finally, contribution of an impurity to a SIMS spectrum is always possible. Contamination could come from the oven in which the film was dried but probably not from solvent residues because it is very unlikely that ethyl acetate can produce peaks at 51, 53, and 65 amu. The important differences between the copolymer spectrum and the latex film spectra and the similarities of the latter with the pure emulsifier spectra indicate that the contribution of the copolymer to the top surface composition is weak or nil. In the uppermost layers one finds the emulsifier which decreased the latex-air and latex-glass interfacial tensions when the latex was cast on its substrate (9). This is particularly clear with the SDED-stabilized films (Fig. 9). For D3.5 films the layer
LATEX
447
FILMS
analyzed by SIMS is almost saturated with emulsifier. For D0.7 films a weak contribution from the copolymer can be detected (peak at 65 amu). In the layer analyzed by XPS the SDED surface concentration is only 40 and 7%, respectively. The thinner the analyzed layer, the higher the emulsifier surface concentration. This result was already established on a much larger scale by ATR-FTIR (9). On the substrate side of the films one observes the same type of results in spite of the emulsifier left on the glass plate. This indicates that there is probably very little polymer in direct contact with glass. Polymer-glass interactions cannot exert an important influence on adhesion properties. With SDS-stabilized films (Fig. 10) the same kind of conclusions can be drawn. There are weak differences between the latex film spectra which cannot be attributed to the contribution of the copolymer. They could be due to some differences in the surfactant molecule orientation at the surface. A contamination effect is also possible. More work is needed to make some points of this SIMS analysis clearer. In a further study full-range (0-200 amu) spectra will be recorded. Even if SIMS remains a difficult method for polymer surface studies it produces extremely valuable and interesting results and will probably develop widely in the future. CONCLUSION
This XPS-SIMS study of the surface concentrations of two anionic surfactants (SDS and SDED) in coalesced acrylic latex films (methyl methacrylate and butyl acrylate copolymer, 45:55 wt%) has shown the following results. The study by XPS of the S/C and N a / C atomic ratios and of the carbon peak shape leads to relatively precise knowledge of the structure and composition of the interfaces. SDS and SDED behave quite differently. For SDS, surface enrichment is very pronounced and can reach saturation in the layer analyzed by XPS (50 A). Molecules orient at Journal of Colloid and lnte(face Science, Vol. 128, No. 2, March 15, 1989
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the air-film interface in order to adopt the thermodynamically most favorable configuration, i.e., the hydrophobic part presented to air. The difficulty in interpreting the S / C atomic ratio has been emphasized. Complementary information sources like carbon peak shape and angular dependence results are necessary to draw precise conclusions. Film maturation during 3 months gives rise to SDS expulsion toward interfaces due to the incompatibility of the emulsifier with the polymer matrix. For SDED, enrichment is much less pronounced. During maturation the surfactant surface concentration does not change. An exact interpretation o f the latter difference is difficult. The slowness of SDED diffusion in films can be invoked but it is not possible to assert that this point is determining. For both systems, the air side of the films is more concentrated than the substrate side. The emulsifier left on the glass after the film is peeled off can account for the difference. Qualitatively the conclusions of the A T R F T I R (9) and XPS studies are identical. The difference in the sampling depths of the two methods did not allow us to postulate such a result. But surface concentration in the layer sampled by XPS can reach 50 times the average concentration whereas this factor is only contained between 1.2 and 4 in the layer sampled by IR. With SIMS, the sampling depth becomes comparable to the size of the emulsifier molecules. By comparison between surface mass spectra of latex films, pure emulsifiers, and pure copolymer, it has been shown that the interfaces of 3-day-old latex films were almost saturated whatever the side (air or substrate) and the average concentration. This layer is probably formed as soon as the latex is east on its support in order to minimize interfacial tensions and it remains there after water evaporation. Polymer-support interactions cannot influence adhesion of the films. This does not mean that adhesion is only determined by surfactant-substrate interactions. It will be Journal of ColloM and Interface Science, Vol. 128, No. 2, March 15, 1989
ET AL.
shown (3) that the thickness of the emulsifier layer at the film-glass interface and the polymer-surfactant interactions strongly influence adhesion. The difficulties in studying polymer surfaces by SIMS have been emphasized. A proper quantitative use of the technique would require a high level of experimental precaution. The usefulness of these X P S - S I M S results in interpreting the adhesion data is shown in Ref. (3). ACKNOWLEDGMENTS We thank Dr. S. Affrossman for helpful discussions. One of us (C.L.Z.)is indebted to the Ministryof Education of the People's Republic of China for financial support of his Ph.D. work in France. REFERENCES 1. Dillon, R. E., Matheson, L. A., and Bradford, E. B., J. Colloid ScL 6, 108 (1951); Brown, G. L., J. Polym. Sci. 22, 423 (1956); Sheetz, D. P., J. Appl. Polym. Sci. 9, 3759 (1965); Vanderhoff,J. W., Brit. Polym. J. 2, 161 (1970). 2. Vanderhoff,J. W., Bradford, E. B., and Carrington, W. K., J. Potym. Sci. Symp. 41, 155 (1973). 3. Zhao, C. L., Pith, T., Holl, Y., and Lambla, M., Brit. Polym. J. (in press). 4. Chainey, M., Wilkinson, M. C., and Hearn, J., J. Polym. Sei. Polym. Chem. Ed. 23, 2947 (1985). 5. Snuparek, J., Bidman, A., Hanus, J., and Hajkova, B., J. Appl. Polym. Sci. 28, 1421 (I983). 6. Basset,D. R., in "Scienceand Technologyof Polymer Colloids" (G. W. Poehlein, R. H. Ottewill, and J. W. Goodwin, Eds.), Vol. 1, p. 220. NATO AS1 Series, 1981. 7. Voyutskii,S. S., J. Polym. Sci. 32, 528 (1958). 8. Bradford,E. B., and Vanderhoff,J. W., J. Macromol. Sci. Chem. 1, 335 (1966); Bradford, E. B., and Vanderhoff, J. W., Y. Macromol. Sci. Phys. B 6, 671 (1972). 9. Zhao, C. L., Holl,Y., Pith, T., and Lambla,M., Colloid Polym. Sci. 265, 823 (1987). 10. Copperwaite,R. G., Surf Interface Anal. 2, t7 (1980). 11. Stone, W. E. E., and Stone Masui, J. H., in "Science and Technology of Polymer Colloids" (G. W. Poehlein, R. H. Ottewill,and J. W. Goodwin,Eds.), Vol. II, p. 480. NATO ASI Series, 1983. 12. Scofield,J. H., J. Electron Spectrosc. Relat. Phenom. 8, 129 (1976); Rielman, R. F., Msezane, A., and Manson, S. T., J. Electron Spectrosc. Relat. Phenom. 8, 389 (1976).
COALESCED ACRYLIC LATEX FILMS 13. Ashley, J. C., J. Electron Spectrosc. Relat. Phenom. 28, 177 (1982); Ashley, J. C., and Tung, C. J., Surf Interface Anal 4, 52 (1982). 14. Clark, D. T., Thomas, H. R., and Shuttleworth, D., J. Polym. Sci. Polym. Lett. 16, 465 (1978); Clark, D. T., and Thomas, H. R., J. Polym. Sci. Chem. 15, 2843 (1977). 15. Hall, S. M., Andrade, J. D., Ma, S. M., and King, R. N., J. Electron Spectrosc. Relat. Phenom. 17, 181 (1979). 16. Fadley, C. S., Baird, R. J., Siekhaus, W., Novakov, T., and Bergstrom, S. A. L., Y. Electron Spectrosc. Relat. Phenom. 4, 93 (1974); Proctor, A., and Hercules, D. M., Appl. Spectrosc. 38, 505 (1984). 17. Briggs, D., and Wootton, A. B., Surf. Interface Anal. 4, 109 (1982).
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18. Briggs, D., Surf Interface AnaL 4, 151 (1982). 19. Briggs, D., Surf Interface Anal. 5, 113 (1983). 20. Briggs, D., Hearn, M. J., and Ratner, D. B., Surf. Interface Anal 6, 184 (1984). 21. Briggs, D., Surf Interface Anal. 8, 133 (1986). 22. Pijpers, A. P., and Donners, W. A. B., J. Polym. Sci. 23, 453 (t985). 23. Holm, R., and Storp, S., Surf. Interface Anal 2, 96 (1980). 24. Andrade, J. D., "Surface and Interfacial Aspects of Biomedical Polymers," p. 141. Plenum, New York/London, 1985. 25. Gardetla, J. A., and Hercules, D. M., Anal. Chem. 52, 226 (1980). 26. Campana, J. E., DeCorpo, J. J., and Colton, R. J., Appl. Surf Sci. 8, 337 (1981).
Journal of Colloid and Interface Science, Vol. 128, No. 2, March 15, 1989