Langmuir 1993,9, 1468-1472
1468
X-ray Photoelectron Spectroscopy and Static Time-of-Flight Secondary Ion Mass Spectrometry Study of Dispersion Polymerized Polystyrene Latexes+ Yves Deslandes' Institute for Environmental Chemistry, National Research Council, Ottawa, Ontario K I A OR6, Canada
Don F. Mitchell Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario K I A OR6, Canada
Anthony J. Paine Xerox Research Centre of Canada, Mississauga, Ontario L5K 2L1, Canada Received July 22, 1992. I n Final Form: February 12, 1993 Dispersion polymerized polystyrene particles were prepared using poly(N-vinylpyrrolidone)(PVP) a~ a stericstabilizer. The top surface composition of a series of latex particleswas analyzedusinga combination of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOFSIMS). XPS of 6.5-pm micron particles revealed a top surface composition of 30% steric stabilizer, corresponding to either two monolayers (0.5-0.6 nm) of highly ordered PVP uniformly coating the particles or a patchy surface with islands of PVP on the polystyrene surface. The SIMS experiments showed a mixture of polystyrene and PVP in the very top layer, supporting the patchy model. Similar results were obtained for several 2.5-pm particles synthesized using PVP in combination with Triton and Aerosol AOT "costabilizere". The additional surfactants were not found on the surface. One sample with a much higher polystyrene molecular weight had only 15% PVP on the surface,suggestingthat the majority of the surface polystyrene is simply the anchoring block of the grafted PVP-PS.The results are quantitatively consistent with the grafting mechanism of stabilization in dispersion polymerization.
Introduction Latex particles of 0.1-15 pm can be prepared by dispersion polymerization in the presence of a steric stabilizer.14 During these processes, an initially homogeneous mixture of solvent, monomer, initiator, and steric stabilizer nucleates and then grows the polymer particles.4tM The solvent is selected to be a poor solvent for the polymer being formed, while a good solvent for the stabilizer which is (or becomes through grafting) surface active, protecting the particles from coalescence by steric stabilizati~n.~ Two main classes of dispersion polymerizations which have been investigated in some detail are those in hydrocarbons1*6J0J1and those in polar solvents, such as a l c o h ~ l s . ~ * ~ ~ ~ ~ ' ~ ~ In the case of the nonpolar polymerizations,the stabilizer apparently becomes distributed in microphase regions
* Author to whom correspondence should be addreaeed.
+ Presented, in part, at the Canadian High Polymer Forum, MontGabriel, Quebec, Canada, 1991. (1) Barrett, K. E. J. Dispersion Polymerization in Organic Media; Wiley-Interscience: New York, 1976. (2) h o g , Y.; Reich, S.; Levy,M. B. Polym. J. 1982,14, 131. (3) Taeng, C. M.; Lu, Y. Y.; E l - d e r , M. S.; Vanderhoff, J. W. J. Polym. Sei.: Polym. Chem. Ed. 1986,24, 2995. (4) Lok, K. P.; Ober, C. K. Can. J. Chem. 1985,63, 209. (6) Winnik, M. A.; Lukas, R.; Chen, W. F.; Furlong, P.; Croucher, M. D. Makromol. Chem., Macromol. Symp. 1987, 10111,483. (6) Paine, A. J. J. Collid Interface Sci. 1990,138, 167. (7) Paine, A. J. Macromolecules 1990,23, 3109. (8) Paine, A. J.; Luymes, W.; McNulty, J. Macromolecules 1990,23,
3104. (9) Napper, D. H. J. Colloid Interface Sci. 1997,58, 390. (10) Williamson, B.; Lukas, R.; Winnik, M. A,; Croucher, M. D. J. Colloid Interface Sci. 1987,119, 669. (11) Winnik, M. A.; Williamson, B.; Ruseell, T. P. Macromolecules 1987,20, 899.
0743-746319312409-1468$O4.OO/O
throughout the Darticles.loJ1 On the other hand. the case of poiar solvent polymerizations seems to indicate the stabilizer almost exclusively ends up on the particle surface.12J3 For example, using electron microscopy, we clearly observed a thin surface layer 10-20 nm thick of hydroxypropyl cellulose (HPC)on polystyrene (PS)particles embedded in epoxy resin. A similar study with poly(N-vinylpyrrolidone) (PVP)led to a similar conclusion although the surface layer was less obvious.12 Fluorescence quenching studies also indicated the HPC stabilizer chains were accessible to quenchers in the alcohol phase.13 The objectives of the present study were to examine the dispersion polymerized PS surface in more detail, to determine the uniformity and thickness of the surface layer, and to probe for the presence of certain "costabilizers" used by some a ~ t h o r s . ~ ~ ~ A combination of two surface analysis techniques, XPS and TOF-SIMS, was used. X-ray photoelectron spectroscopy (XPS) has been used extensively to study the surface chemistry of polymeric materials.14J6 The technique uses X-rays to irradiate a surface and generate photoelectrons whose energy is characteristic of each element present in the sample. The electrons that escape the surface are collected and their kinetic energy is measured. On the basis of this energy,their binding energy is calculated. The signals recorded originate from the first few monolayers of the surface since the sampling depth (12) Paine, A. J.; Dealandea, Y.; Gerroir, P. J.; Henrissat, B. J. Colloid Interface Sci. 1990,138, 170. (13) W d k , F. M.;Paine, A. J. Langmuir 1989,5,903. (14) Clark, D. T.; Harrieon, A. J.Polym. Sci.: Polym. Chem. Ed. 1981, 19, 1946. (16) Briggs, D. In Practical Surface Analysis, Briggs, D., Seah, M.P., Eds.; John Wiley and Sone: Chicheeter, United Kingdom, 1989; p 369.
0 1993 American Chemical Society
Dispersion Polymerized Polystyrene Particles
Langmuir, Vol. 9, No. 6, 1993 1469
Table I. Summary of Characterization Data for the Polvrtvrene Particle# (Standard Deviation in Bracket#) ~~
aample ("coeurfadant") M137 (TritonN-57) A (none)
B (TritonN-57) C (AerosolAOT)
M87TP (AerosolAOT)
diameter/pm [GSDI
M,/lOaO
6.35 F1.031 2.49 [LO41 2.51 [1.04] 2.45 [LO41 1.17 [1.161
54 91 91 95 391
of the technique, defined as the depth from which 95% of the signal derives, is approximately 3 times the escape depth of the electron (the electron mean free path). Since the escape depth is about 2.3 nm,1'3J7XPS probes the top 7.5 nm or so of the surface. Secondary ion mass spectrometry (SIMS), in the static mode, is the low primary ion current version of the wellknown "dynamic" SIMS technique.18 The principle of SIMS is the bombardment of the sample surface with primary ions. The momentum of these ions is released within the sample surface causing desorption of neutral particles and a small percentage of positively and negatively charged secondary ions. These ions are collected and their mass measured with a time-of-flight detector. In static SIMS, a primary ion current of less than I O nA/cm2 is used. Under these mild conditions, the lifetime of one monolayer is several hours and the surface integrity of the sample is maintained during the analysis. Fragmentation of the ions is also minimized leading to the generation of both cluster ions having large masses as well as small ions. This allows for the identification of the elemental composition and the chemical structure of materials at the surface. Note that quantitative analysis with SIMS is still problematic. In recent articles, Davies and collaborators have used XPS and SIMS to characterize charged and sterically stabilized polymer colloids.1Q@They were able to provide information on the polymer surface and orientation of the polymer chains responsible for a steric barrier. In this article we report on the utilization of XPS and static TOFSIMS to acquire novel information on the nature and morphology of the surface layer of PVP used as steric stabilizer during the dispersion polymerization of polystyrene latexes. Experimental Section 1. Particle Synthesis. Preparation of the polystyrene
%C 92.8 (0.4) 92.3 (0.5) 92.3 (0.4) 91.7 (1.1) 96.2 (0.3)
ESCA results %O 3.7 (0.4)
% eurface P
%N
W
3.5 (0.4) 3.4 (0.3) 3.6 (0.3) 3.8 (0.5) 2.2 (0.3)
4.2 (0.3) 4.1 (0.4) 4.5 (0.7) 1:5 (0.2)
Cllll
1000
800
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400
BINDING ENERGY (eV)
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I
Figure 1. XPS survey spectrum obtained from the particles having a diameter of 6.5 wm and showingthe preaence of carbon, oxygen, and nitrogen.
allowed to dry at room temperature. This procedure gave a thick dried particle layer (not a fii). Thia procedure waa repeated until complete coverageof the indium foil waa achieved (toavoid recording the signal from the foil) and the sample waa analyzed using X P S and TOF-SIMS. The samples were analped using a PhysicalElectronics X-rayphotoelectron spectrometer,Model 5500 (Perkin-Elmer,Eden Prairie, MN). The size of the analyzed area waa 1X 3 mm. MonochromatizedAl Ka radiation waa used. Survey spectra were collected using pass energies of 188 eV. An electron flood gun waa used to neutralize the charge during the experiment. Binding energies were referenced to the carboncarbon bond which waa assigned a binding energy of 284.6 eV. Atomic compositions were estimated by standard programa provided with the instrument, using the following sensitivity factors: 0.296 for C Is, 0.711 for 0 la, and 0.477 for N 1s. The TOF-SIMS spectra were obtained by using a KRATOS PRISM instrument (Kratos Analytical, Manchester, United Kingdom). A aample area of approximately 1 mm* waa bombarded by 12-keVAr+ ions with a current of less than 1X lW* A. Data acquisition required between 80 and 200 8. Charge neutralization by meam of a beam of electrons synchronously pulsed on every primary gun cycle, in antiphaae with the ion extraction voltage, waa used.
Results and Discussions The XPS survey spectrum obtained from the 6.5-pm particles is shown in Figure 1. An intense carbon peak is seen, along with peaks corresponding to oxygen and nitrogen. The atomic concentrations calculated based on thistype of spectra for particles of different diameters are tabulated in Table I. The spectrum obtained from a pure PVP film cast from ethanol is reproduced in Figure 2. The atomic concentration of the elements of the PVP film correspond, within experimental error, to the theoretical valuesforPVP,i.e.75% carbon, 12.5% oxygen,and 12.5% nitrogen. In order to verify that the PVP was only located at the surface of the particles and not inside the sample, the (16) Szajman, J.; Lieeegan, J.; Jenkin, J.; Leekey, R. G. G. Electron following experiment was undertaken. The particles were Spectrosc. Rekzt. Phenom. 1978,14,247. deposited on a clean glass slide and quickly melted on a (17) Bhatia, Q. S.; Pen, D. H.; Koberstein,J. T. Macromolecules 1988, 21,2166. hot plate at about 280 "C. Another clean glass slide was (18)Gilberg, G. J. Adhes. 1987,21,129. used to shear the molten particles and expose the matter (19)Lynn,R.A.P.;Davis,S.S.;Short,R.D.;Davies,M.C.;Vickerman, located inside the beads. The sample was removed from J. C.; Humphrey, P.; Johnson,D.; H m ,J. Polym. Commun. 1988,29, 365. the heat source and allowed to cool. Figure 3 shows a ... (20) Brindley, A,; Davies, M.C.; Lynn,R. A. P.; Davis, S. S.; Heam, spectrum of this sample. With the exception of a very J.; Watta, J. F.Polymer 1992, 33, 1112. small peak for oxygen (perhaps due to thermal oxidation (21) Paine,A. J.; McNulW, J. J. Polym. Sci.: Polym. Chem. Ed. 1990, during the experiment) only an intense peak for carbon 28,2569. particles described in Table I followed the procedure described elaewhere.221 The samples denoted A, B, and C are the aame aamplesreportedinref 21. Styrene,AIBN,PVP (molwt4O OOO), and optionalcostabiker were mixed in ethanol (M137)or 80/20 (v/v) ethanol/water (M87TP), then polymerized at 70 "C for 24 h. The resulting latex particles were washed several times with ethanol and then water and freeze dried. lH N M R (400 MHz, in CDCg) could not detect PVP in sampleM137 (detectionlimit 50.5%) or M87TP (51%).Particle size and molecular weight data are reported in Table I. A further one to three washings with methanol were performed prior to surface analysis. 2. Surface Analysis. Samples for surface analysis were prepared by dispersing the beads ultrasonically in pure ethanol. A droplet of the suspension waa depositedon an indium foil and
1470 Langmuir, Vol. 9, No. 6, 1993 fn
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E
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Figure 4. Schematicdepicting the basic models describing the I
1000
I 800
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0
BINDING ENERGY (eV)
Figure 2. XPS survey spectrum obtained from a pure PVP thin film, cast from methanol. A
10
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CIlS)
1000
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BINDING ENERGY (eV)
Figure 3. XPS survey spectrum of the interior of the particles
exposed after melting and shearing (see text). The absence of a nitrogen peak indicates that there is no detectable quantity of PVP inside the particles.
at 284.6 eV and its corresponding, less intense, shake-up peak are observed, as expected for polystyrene. It is evident from these data that the particle surface is not pure PVP, since the nitrogen and oxygen contents are not sufficiently high. The consistent 1:l oxygen/ nitrogen ratio, however, is indicative of its presence. It is also noteworthy that in the series of 2.5-pm particles formed with different costabilizers (A-C in Table I), identical surface compositions were observed. There was no indication of Triton N-57 (a nonylphenol ethoxylate with 4.8 EO units having expected atomic percents C = 80.9; 0 = 19.1) in sample B and no evidnece of Aerosol AOT (dioctyl sodium sulfosuccinic acid ester having expected C = 69.0; 0 = 24.1; S = Na = 3.45%) in sample C. This indicates that the costabilizers were not grafted to the surface in significant quantities. From these data, and the observation in all samples of an oxygen/nitrogen ratio within experimental error of 1:1,we conclude that the surfaces are composed primarily of PVP and PS. Assuming this to be the case,we may estimate the apparent fraction of PVP on the surface of each sample using the following equation: fraction PVP = [(% 0 + % N) observed for particles]/ [(% 0 + % N) observed for PVP] (1) These data are included in the final column of Table I. All except the 1.17-pm sample have about 30% PVP in the surface layer, according to XPS. Consequently, different morphological models can be proposed for the surface layer of these polymer beads, with respect to its thickness, composition, and uniformity. The two basic models that can be proposed are schematized in Figure 4. One displays a stretched, continuous uniform layer of PVP, completely hiding the PS, while the second model shows patches of PVP distributed over the polymer beads, and leaving some areas of the bead where polystyreneis directlyexposed to the surroundingenvironment.
two different morphologies of the PVP layer on the particle surface. By use of eq 6, the uniform model has an expected PVP volume fraction of 0.05-0.3% ,while the patchy model detailed in the text predicts more: 0.3-1%. In principle, these patches could have different sizes, morphologies, thicknesses, etc. However, we propose the tentative structure of the patchy model in Figure 4 based on the following discussion. We believe that the first model is not a realistic description of this system. This conclusion is reached based on the calculation of the thickness of a pure PVP layer covering a polystyrene substrate and assuming uniform coverage of the particle. It is possible to predict the spectrum expected for the surface of a material (polystyrene in this case) coated by a thin layer of a different material.22The calculated intensity is taken as the sum of the signals from each atomic layer. The signalfrom one layer of atom being the product of a relative sensitivity factor, an absorption coefficient related to the mean free path of the particular photoelectron, and the elemental concentration in the layer. In analyzing thin films, one can sum the signals from element x in a layer-by-layercalculation accounting for both the concentration of x in each layer and the attenuation of the photoelectron signal due to the depth at which it is generated. The thickness of a monolayer can be estimated by calculating the volume occupied by a carbon atom in polystyrene (excluding the hydrogen atoms since they do not contribute to the XPS spectrum). On the basis of a density of about 1g/cm3,one can calculate that each carbon atom occupies a volume equivalent to a cube having sides equal to 0.27 nm. One monolayer would therefore correspond to a thickness of 0.27 nm. It has been s h o ~ n that ~ ~ the 9 ~ magnitude ~ of the XPS signal, I,, is given by
where a, is the sensitivity coefficient of element x , k,' is the absorption coefficient (the probability of production and escape of an electron from layer i compared with that from the top layer of the sample (i = O)), and N(x,i)is the fraction of element x in layer i. Equation 2 can be rewritten in terms of C,, the atomic concentration of element x W
I, = a,C,
k,'
(3)
i=O
The series summation, Ci",ok,iwhich is equal to (1- kz)-l, can be evaluated based on the equation for k, k, = exp[-(K(E,)'/2 cos 42O)-']
(4) where K = 0.19 is a constant22and E, is the kinetic energy of the electrons. The term cos 42' is a fair representation of the escape angle for electrons from the round surface presented by the spheres. It is therefore possible to (22) Mitchell, D.F. Appl. Surface Sci. 1981, 9, 131. (23) Gallon, T. E. Surf. Sci. 1969, 17, 486. (24) Pons, F.; Le Hericy, J.; Langeron, J. P. Surf. Sci. 1977, 69, 569.
Dispersion Polymerized Polystyrene Particles 2000
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Table 11. Calculated Apparent Atomic Concentration Expected for Different Numbers of Uniform Monolayers of PVP over a PS Substrate no. of PVP layers %C %O %N 0.0
0.0
2.2
2
100.0 95.7 92.2
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;.b
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ATOMIC MASS UNITS
FigureS. StaticSIMSspectrumof the samplehaving adiameter of 6.6 pm.
0 1
0
calculate the intensities generated from each monolayer for each elements and to normalize the sum to 100%. Table I1 shows the calculated apparent compositions for zero, one, two, and three layers of P W on top of pure Polystyrene. Comparison of these values with the experimental results of Table I suggests that there are about two monolayers of atoms over the polystyrene for the large beads and about one monolayer only for the smaller diameter beads. The calculations suggest that, if the uniform coverage model were correct, only two layers of PVP would cover the surface of the polystyrene beads. It is difficult to conceive of such a complete coat, leaving no polysteene exposed, with only two layers of macromolecules. Such a model implies that the chains would have to be ordered in an almost perfect crystalline fashion with the pyrrolidine ring laying nearly flat on the surface. Consequently we might infer that the "patchy" model is a more realistic representation of this system. This inference is supported by the static SIMS results. The SIMS spectrum shown in Figure 5 was obtained from the particles having a diameter of 6.5 pm, while the spectrum of Figure 6 was obtained from the beads that were sheared to expose their interior. The latter shows intense peaks at 39, 51, 91, and 115 Da, clearly corresponding to the SIMS spectrum of polystyrene as reported in the literature.2s The SIMS spectrum of a thin film of PVP showed its more intense peaks a t 41 and 69 Da. The presence, in the spectrum, of peaks at 39,51,91, and 115 (from PS) as well as at 41 and 69 (from PVP) clearly indicates that both PVP and PS are found at the very top surface of the beads. Relevance to the Mechanism of Dispersion Polymerization. In the dispersion polymerization of styrene, an initially homogeneous mixture of monomer, initiator, steric stabilizer (PVP), and solvent is heated to induce polymerization. The solvent (usuallyan alcohol)is selected so that the steric stabilizer chains are soluble but the PS (25) Briggs, D.; Brown, A.; Vickerman, J. C. Handbook of Static Secondary Ion Ma88 Spectrometry (SIMS); John W h y and Sone: Chichestar, England, 1989.
Figure 6. Static SIMS spectrum of the interior of the beads (after melting and shearing). This spectrum corresponds to the spectrum of polystyrene. polymer being formed is insoluble. At very low conversion, the system nucleates and grafted PS-PVP is formed and adsorbed onto the growing particles. During the remainder of the reaction, particles grow by two competing mechanisms: (1)adsorption of monomer and subsequent particle phase polymerization (similar to emulsion polymerization, high molecular weight); (2) accretion of dead polymer formed by termination in the continuous phase (similar to solution polymerization, low molecular weight). The balance between these growth modes depends upon the number of nuclei, the solvency of the medium, and the molecular weight of PVP. Generally, small particles (1-2 pm) are dominated by the former mechanism and large particles (15pm) via the latter. With this picture in mind, we may postulate the further detail of the "patchy" model in Figure 4 from the following considerations. PVP is considered to be anchored by grafting in single chain blobs whose average size is determined by a collapsed single chain molecular weight 40 000 (equivalent to a cubic blob dimension of 4 nm). The separation of such blobs can be estimated from the fraction PVP found on the surface according to eq 5. An internal check on the global volume fraction PVP is available from eq 6 for the two models applied to the five samples in Table I, finding the surface PVP to be at or below the NMR detection limits cited in the Experimental Section. The patchy model suggests the surface is not saturated with PVP in the dry collapsed state, although it may well have been saturated in the extended state in ethanol dispersion during synthesis. blob separation = blob dimension
X
(surface fraction PVP)-'/2 (5)
volume fraction PVP = surface area X thickness X surface fraction PVP (6) particle volume The main effect of particle size on the XPS results is the singularly low surface fraction of PVP in the 1.17-pm sample (about 15% in Table I), which is approximately half that in the other samples. The most likely factor causing this dilution is the much larger molecular weight of polystyrene formed in that reaction (seeTable I). When these molecular weighta are compared to that of PVP (approximately 40 OW), the forecast weight fraction PVP in a 1:l grafted copolymer is 9-43% for these samples, with the lower value for the highest PS molecular weight. Thus the anchoring group is not buried deep inside the particle but rather appears intertwined with, or adjacent
1472 Langmuir, Vol. 9, No.6,1993
to, the surface PVP. The longer the anchor block, the more dilute the surface PVP. Thus, the “patchy” model of dry polystyrene particles is quantitatively consistent with all the characterization information and with the grafting mechanism of stabilization in dispersion po1ymerization.w It is truly amazing that the PS-PVP graft manages to continuouslyreorganize, either by adsorptionldesorption or surface migration during synthesis, to remain on the particle surface despite considerable particle size growth, whether by internal polymerization or by accretion of dead polymer from solution.
Conclusion In this paper we have analyzed XPS and TOF-SIMS spectra obtained from dispersion polymerized polystyrene latexesprepared with PVP as stabilizer. We observed the PVP layer at the surface of the beads with no significant
Deslandes et al.
quantity detected in the interior of the particles. On the basis of calculations of the intensities predicted for a layered material composed of PVP over PS, we have shown that only one or two monolayers would be present on the surface if one assumes a uniform monolayer completely covering the surface and hiding all the polystyrene. Since this would imply a well-ordered arrangementof the chains, the model seems improbable. We believe that a patchy model, where thicker agglomerates of PVP incompletely cover the surface of the particle, is a more realistic model for this sample. The model is also in agreement with the static SIMS results which show the presence of both PS and PVP at the very top surface of the beads.
Acknowledgment. The authors thank Dr. David Surman and Dr. Kevin Robinson from Kratos Analytical for recording the SIMS spectra and Dr. Pierre Plouffe of NRC for the NMR spectra.
