Langmuir 1994,10, 1399-1409
1399
Evidence for Preferential Surface Enrichment in the Surface Chemical Analysis of a Series of Poly(buty1 methacrylate-co-methylmethacrylate) Latices Using X-ray Photoelectron Spectroscopy and Static Secondary Ion Mass Spectrometry M. C. Davies,*l+R. A. P. Lynn,+?$S. S. Davis,+J. Hearn,s J. F. Watts) J. C. Vickerman,l and D. Johnsonl Department of Pharmaceutical Sciences, University of Nottingham, Nottingham NG7 2RD, U.K., Department of Chemistry and Physics, Trent University, Clifton Lane, Nottingham NG11 8NS, U.K., Department of Materials Science and Engineering, University of Surrey, Guildford GU2 5 X H , U.K., and Centre for Surface and Materials Analysis, Armstrong House, Oxford Road, Manchester M1 7ED, U.K. Received June 30, 1993. I n Final Form: February 17, 1994’ The surface chemical analysis of a series of poly(buty1methacrylate-co-methylmethacrylate) (MMABMA) latices, prepared by persulfate-initiated emulsion polymerization and characterizedusing standard colloid techniques, has been undertaken using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The surface compositions of these latices were compared with the bulk polymer compositions and the surface compositions of solvent-cast (latex) polymer films. While the average particle size of the near-monodispersed colloidswas found to be independent of composition,there was a marked increase in electrophoreticmobility and zeta potential values as the proportion of the BMA in the monomer mixture was increased. The XPS and SIMS analyses both revealed a higher level of MMA component of the copolymer in the latex films than would be anticipated from the bulk composition. In contrast, a significant enrichment of the BMA component of the copolymer was observed in the surface of solvent-cast films. These findings are briefly discussed where appropriate in terms of the particle formation mechanism, but in the main, this paper highlightstheir relevance to the study of surface chemistry of complex polymer colloids.
Introduction Many of the industrial and biological applications of polymer colloids require systems with specifically-tailored particle properties. In many cases, this involves the incorporation of one or more comonomers (often bearing appropriate functional groups) into the polymerization recipe. The objective with these compositions is to introduce functional groups into the particle surface which are then available for reaction with biological molecules such as monoclonal antibodies.lv2 The synthesis of these types of polymer particles was pioneered by Rembaum and c o - ~ o r k e rfor s ~various ~~ biological and immunological applications. The presence of surface functionalities may alsoconfer favorable properties on latices used in industrial applications. While the ability to modify latex properties by the use of comonomers is extremely useful, the use of additional monomers inevitably complicates the polymerization kinetics and particle nucleation and formation mechanisms. This, in turn, means that the copolymer composition of the particle bulk and surface may vary
* To whom correspondence should be addressed. + University of Nottingham. t Present address: SmithKline Beecham Pharmaceuticals, Great Burgh, Yew Tree Bottom Road, Epsom, Surrey KT18 5XQ, U.K. f Trent University. 11 University of Surrey. 1 Centre for Surface and Materials Analysis. Abstract published in Advance ACS Abstracts, April 15, 1994. (1) Rolland, A,: Bourel, D.: Genetet, B.; Le Verge, R. Int. J. Pharm. @
1987, 39, 173.
(2) Illum, L.; Jones, P. D. E. Methods Enzymol. 1985, 112,67. (3) Rembaum,A.; Yen, S. P. S.;Molday,R. S. J . Macromol. Sci. Chem. 1979, A13 (5), 603. (4) Rembaum, A.; Yen, S. P. S.;Cheong, E.; Wallace, S.; Molday, R. S.; Gordon, I. L.; Dreyer, W. J. Macromolecules 1976, 9 (2) 328.
considerably, and may also differ from the initial monomer composition. The most important factors governing the copolymer composition in such systems are the free-radical copolymerization reactivity ratios (in emulsion) of the monomers concerned, their aqueous and mutual solubilities, their solubilities in the polymer phase (partition coefficient), the loci of polymerization of the monomers and the preparation process. The partitioning of the monomers between the various phases is dependent on experimental conditions such as the presence of emulsifier, the temperature, the pH (particularly for ionogenic monomers), the ionic strength of the medium, and the relative concentrations of the various monomers. The locus of monomer polymerization, which may be in the aqueous phase, inside the particles or at the particle-water interface, also depends on experimental variables (pH, ionic strength, physicochemical properties of comonomers). The reactivity ratios for a pair of monomers can provide an indication of the composition of the copolymer and a number of bulk reactivity ratios are available.6 In emulsion systems, factors such as the aqueous solubilities of the monomers will influence both the loci and kinetics of polymerization with consequent changes in the reactivity ratios. It should also be noted that the factors above refer to “batch” polymerizations, that is, those in which the monomers are added as a single shot at the start of the reaction. An alternative method involves the addition of the monomer mixture at a rate equal to, or less than, the rate at which it can polymerize (“semicontinuous” poly(5) Greenley,R. 2.Free radical copolymerization reactivity ratios. In Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; John Wiley & Sons: New York, 1989; Vol. 11, pp 153-226.
0743-7463/94/2410-1399$04.50/00 1994 American Chemical Society
Davies et al.
1400 Langmuir, Vol. 10, No. 5, 1994
merization). Under these conditions, a single copolymer composition corresponding to the monomer ratio is obtainede6 In general, the nature of the particle nucleation and formation mechanisms in emulsion copolymerizations will be dependent upon the properties of the monomers chosen and the reaction conditions. It is likely, however, that a homogeneous nucleation mechanism will dominate in systems involvingwater-soluble monomers while a micellar mechanism will obtain for monomers with low aqueous solubility. In many systems, a combination of different nucleation and growth mechanisms may be responsible for the observed kinetics or particle characteristics. Chen and Chang have investigated the kinetics and mechanisms of emulsifier-free emulsion copolymerizations involving styrene with ionic7 and nonionic comonomer.8 These authors have suggested that in the polymerization of nonionic comonomers with styrene, a surface-phase polymerization mechanism occurs in the growth phase, as previously predicted for styrene homopolymeri~ation.~ However, in the case of ionic comonomers, the kinetic data indicate that polymerization may occur either in the surface phase or throughout the particle, depending on the reactivity ratio of the comonomer with styrene.7J0 From the above discussion it is evident that the difficulties in predicting polymer compositions in emulsion polymerizations involving two or more monomers are very significant. However, a knowledge of the surface polymer composition vis-a-vis the bulk and/or monomer composition is clearly important for many applications. To date, the majority of attempts to determine the distribution of monomers or functional groups have involved conventional techniques such NMR spectroscopyll or conductometric titration12-l4 A few reports have been published over the last decade in which X-ray photoelectron spectroscopy (XPS) has been used to elucidate the surface composition of copolymer 1ati~es.l"'~ Recently, we have exploited the quantitative elemental and chemical state information of XPS combined with the detailed molecular structural data from secondary ion mass spectrometry (SIMS) for the characterization of a number of polymer colloid^.^^^^ In this paper, we extend these studies to examine the use of (6)Vanderhoff, J. W. J. Polym. Sci.: Polym. Symp. 1985, 72, 161. (7)Chen, S.-A,; Chen, H.-S. J. Polym. Sci., Part A: Polym. Chem. 1988,26 (4),1207. (8)Chang, H.-S.; Chen, S.-A. J. Polym. Sci., Part A: Polym. Chem. 1990.28. 2547. (9)Hearn,J.; Wilkinson, M. C.; Goodall, A. R.; Chainey, M. J.Polym. Sci., Polym. Chem. Ed. 1985,23, 1869. (IO) Chen, S.-A,;Lee, S.-T.; Lee, S.-J. Makromol. Chem. Macromol. Symp. 1990, 35/36, 349. (11)McDonald, C. J. J. Dispersion Sci. Techol. 1984,5 (3-4),365. (12)Okubo, M.; Kanaida,K.; Tsunetaka, M. J.Appl. Polym. Sci. 1987, 33. 1511. (13)Greene, B. W. J. Colloid Interface Sci. 1973, 43 (2), 449. (14)Emelie, B.;Pichot, C.; Guillot,J. J.Dispersion Sci. Techol. 1983, ~
-5 (3-4) ~ _,,__-. - 393
(15) Pijpers, A. P.; Donners, W. A. B. J. Polym. Sei., Polym. Chem. Ed. 1985, 23, 453. (16)Okubo, M.; Okegami, K.; Yamamoto, Y. Colloid Polym. Sci. 1989,
267. 193.
- - . I
(17)Okubo, M.; Yamamoto, Y.; Kamei, S. Colloid Polym. Sci. 1989,
267, 861.
(18) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Johnson, D. J. Colloid Interface Sci. 1993, 156, 229. (19)Koosha, F.;Muller, R. H.;Davis,S. S.; Davies, M. C. J.Controlled Release 1989, 9, 149. (20)Davies, M. C.;Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Paul, A. J. Langmuir 1993,9, 1637. (21)Davies, M.C.;Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Paul, A. J. J. Colloid Interface Sci. 1993, 161, 83. (22)Brindley, A.;Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F. Polym. Commun. 1992, 33 (5),1113. (23)Brindley, A.; Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F. Submitted for publication in J. Colloid Interface Sci.
Table 1. Polymerization Recipes for PMMA/PBMA Copolymer Latex Particles (Total Monomer Concentration 1% (v/v)) volume added (mLP material 1 2 3 4 5 6 7 methylmethacrylate 1.0 0.8 0.6 0.5 0.4 0.2 butyl methacrylate 0.2 0.4 0.5 0.6 0.8 1.0 potassiumpersulfate 4.0 4.0 4.0 4.0 4.0 4.0 4.0 soln (1.25% (w/v)) water (doubledist.) 95 95 95 95 95 95 95 totalvolume(mL) 100 100 100 100 100 100 100 Equivalent monomer compositions(% v/v): latex 1,MMA 100%; latex2,MMA80%,BMA20%;latex3,MMA60%,BMA40%;latex 4,MMA 50%, BMA 50%; latex 5, MMA 40%, BMA 60%; latex 6, MMA 20%, BMA 80%; latex 7,BMA 100%.
SIMS and XPS, in combination with conventional colloid characterization methodologies, for the determination of the surface polymer compositions of a series of poly(buty1 methacrylate-co-methyl methacrylate), PMMA/PBMA, copolymer colloids prepared by persulfate-initiated emulsion polymerization. Since the surface characteristics of the respective homopolymerlatices were discussed in detail previously,la this series provides a means of establishing the validity of this approach to the surface characterization of copolymer colloids.
Materials N-Butyl methacrylate (BMA) and methyl methacrylate (MMA) were obtained from Polysciences, Warrington, PA. The potassium persulfate and chloroform (Analar grades) were obtained from BDH, Poole, Dorset, U.K. Deuterated chloroform (CDC13, Gold Label) and tetramethylsilane (NMR grade) were obtained from Aldrich, Gillingham, Dorset, U.K. Spectropor membrane tubing (MW cut-off 12-14000) was supplied by Spectrum Medical Industries, Los Angeles, CA.
Methods Purification of Monomers a n d Initiator. MMA and BMA were purified to remove small quantities of inhibitors such as hydroquinone or hydroquinone monomethyl ether. Purification was carried out according to the method of Riddle.% A 100-mL portion of MMA monomer was washed 5 times with 20-mL aliquots of a 20% (w/v) NaC1/5% (w/v) NaOH solution, then distilled a t 63 OC in a nitrogen atmosphere a t a pressure of about 200 mmHg. For BMA, the procedure involved washing 100 mL of monomer 10 times with 20-mL aliquota of a 1%NaOH/25% (w/v) NazC03 solution, followed by distillation a t 80 "C in a nitrogen atmosphere a t a pressure of about 15 mmHg. Purified monomers were stored protected from light, at 4 OC under nitrogen until required. Potassium persulfate initiator was recrystallized twice from double-distilled water and dried in a desiccator. This process was repeated after 2 weeks of storage. Preparation of Polymer Latex Particles. In order to prevent adventitious contamination of the polymer latex samples during preparation and subsequent manipulations, all glassware was cleaned before use with chromic acid and rinsed repeatedly with double-distilled water. A series of PMMA and PBMA homopolymers and PMMA/ PBMA copolymer latices were prepared according to the recipes shown in Table 1,by emulsifier-free emulsion polymerization of various proportions of MMA and BMA. The colloids were prepared using procedures and apparatus described in detail elsewhere.26 Briefly, 95 mL of double-distilled water was heated to 70 OC (* 1 "C) under a nitrogen atmosphere. The polymerizations were started by the addition of the required quantity of monomer@)(total monomer concentration of 1% (v/v)),followed immediately by 4 mL of potassium persulfate solution (initiator) (24)Riddle, E.H. In Monomeric Acrylic Esters; Reinhold Publishing Corporation: New York, 1954;pp 7-25. (25)Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Br. Polym. J. 1973,5, 347.
Surface Enrichment of Copolymer Colloids for a final concentration of 1.85 X 103 mol dm4. Each polymerization was stirred by means of a magnetic follower and the stirring rate was maintained at a constant level for all experiments. The polymerizations were continuedfor 24 h at 70 "C.When the colloids had cooled, any polymer aggregates were removed by filtration through Whatman No. 1 filter papers (Whatman,UK). The percentage yields of polymer in latex form were determinedby taking a known volume of the cooled, filtered latex and evaporatingto dryness on a rotary evaporator. After oven drying, the weight of polymer was determined. In each case the value was greater than 80% indicating satisfactory monomer-to-polymer conversion. It is likely that the conversion is considerably higher than this as some aggregated latex was present on the nitrogen inlet and also the magnetic follower. Cleaning and Storage of Latex Systems. All latices were cleaned by exhaustive dialysis of 10-mLsamples against doubledistilled water. The dialysatewas changed every 24 h for a period of 14 days by which time its conductivity was equal to that of water. Purified latex sampleswere stored in chromicacid-cleaned glass tubes at 4 "C until required.
Characterization of Polymer Colloids NMR Spectroscopy. The bulk compositions of the polymers obtained from the PMMA/PBMA latex series were determined by proton NMR. About 10 mL of each latex was freeze-dried and approximately 40 mg was dissolved in deuterated chloroform (CDC13). Tetramethylsilane (TMS) was added as a reference and spectra were recorded using a 90-MHz Varian EM390 spectrometer (Varian Associates). Size and Electrophoretic Mobility Determination. Size measurements were carried out by photon correlation spectroscopy (PCS).26v27Briefly, the system comprised a helium neon laser (Siemens, Germany), a Malvern K7025, 64-channel,multibit correlator (Malvern Instruments Ltd., U.K.), and a Commodore PET 2001-32N microcomputer (Commodore Business Machines, U.S.A.). Correlation data were analyzed by a Malvern Applications Program (7025 Spect. I. VI; Malvern Instruments, Ltd., U.K.) to yield particle size and polydispersity data. The polydispersity, Q, is derived from Koppel's method of cumulants26 and is used to express the polydispersity numerically. By use of this approach, monodisperse latices have a value of Q = 0.03, but correlative data from electron microscopy suggest that values less than 0.1 reflect particle sizes that are narrowly distributed. A total of 20 measurements were recorded for each sample and mean size and polydispersity values calculated. The determination of electrophoretic mobility (EPM) and zeta potential (ZP)was performed using the technique of laser Doppler anemometry.2628 Measurements were carried out using a Malvern Zetasizer I1 (Malvern Instruments, Malvern, U.K.). EPM measurements were made over the pH range of 3.0-8.3 using phosphate-citrate buffers of constant ionic strength (0.01 M). Samples were prepared by the addition of 200-300 pL of latex to 5 mL of the appropriate buffer solution. This level of dilution provided a suitable scattered light intensity without compromising buffering capacity. Five measurements were made for each latex sample at each pH. X-ray Photoelectron Spectroscopy. XPS spectra were obtained using a VG Scientific ESCALAB Mk I1 (26) Cummins, H.Z., Pike, E. R., Eds. Photon Correlation and Light Beatrng Spectroscopy; NATO-AS1Series B (No. 3); Plenum Press: New York, 1974. (27) Cummins,H.Z., Pike,E. R. Eds.Photon Correlation Spectroscopy and Velocimetry; NATO-AS1 Series B (No. 23); Plenum Press: New York, 1977. (28) Earnshaw, J. C., Steer, M. W. Eds. The Application of Laser Light Scattering to the Study of Biological Motion; Plenum Press; New York, 1983.
Langmuir, Vol. 10, No. 5, 1994 1401 electron spectrometer (VG Scientific, Ltd., East Grinstead, Sussex, U.K.) employing Mg K a X-rays (photon energy, hv = 1253.6eV), and an electron take-off angle of 45O. The base pressure of the spectrometer was typically 10-9 mbar. The X-ray gun was operated at 10 kV and 20 mA, corresponding to a power of 200 W. A survey spectrum (0-1000 eV) was recorded for each sample (single scan) followed by the Cls, Ols, and S2p regions where appropriate (five scans). The analyzer was operated in fixed analyzer transmission (FAT) mode with a pass energy of 50 eV (survey spectrum) and 20 eV (Cls, Ols, and S2p regions). Data analysis was performed on a VGS 5000 data system based on the DEC PDP 11/73computer. Themethodology employed for peak fitting of the Cls and 01s envelopes has been described in detail elsewhere.29 Typically, 1.51.6 eV line widths and a GaussiadLorentzian mix with a Lorentzian contribution of 30% were employed for the components of the Cls envelope. Atomic percentage values were calculated from the peak areas using the sensitivity factors provided with the data system and background subtraction. Spectra were corrected for sample charging by referencing photoelectron peaks to C-C/C-H at 285 eV. Secondary Ion Mass Spectrometry. SIMS spectra were acquired using a modified VG Scientific SIMSLAB quadrupole instrument (VG Scientific, Ltd., East Grinstead, Sussex, U.K.) which has been described in detail elsewhere.30 In this work the surface was bombarded with 2-keV Arostriking the surface a t an incident angle of 30' and focused into an area of 0.3 cm2. An electron flood gun (VG LEG31,500 eV energy, 0.1 nA to 10 A cm-2 current density) was utilized to mitigate sample charging for both positive and negative ion spectra, in a manner described p r e v i o ~ s l y .The ~ ~ argon atom flux density was 3 X 109 cm-2 s-1 corresponding to a total dose for setting up and spectral acquisition of 2 X 10l2cm-2 per sample (600 s), well within the established regime for static SIMS spectra of -undamaged" ~ u r f a c e s . 3 A ~ ~DEC ~ ~ PDP 11 based computer system was used for data acquisition and analysis. Preparation of Samples for Surface Analysis. Samples for SIMS and XPS analysis were prepared by casting the cleaned latices onto an acetone-washed aluminum foil substrate mounted on a sample stub and drying at room temperature. The aim here was to obtain a thin film and thereby obviate severe sample charging for the SIMS analysis, which can hinder spectral acquisition. Preliminary experiments indicated that optimal negative ion signals from the polymer were obtained from latex films sufficiently thick to occlude the aluminum substrate (monitored by the intensity of the m / z 59 (AlOz-) ion). Charging of the films during acquisition of negative ion spectra was mitigated with a low-energy electron flood. For XPS analysis, relatively thick films were prepared by the addition of multiple drops of sample to the foil. This technique was sufficient to completely cover the aluminum substrate. For both SIMS and XPS studies, films were prepared just prior to analysis. The surface chemistry of PMMNPBMA latex films was compared with that of solvent-cast films prepared in the followingmanner. Approximately 10-mLportions of each of the cleaned colloids were freeze-dried, dissolved in (29) Sherwood, P. M.A. In Practical Surface Analysis (by Auger and X-ray Photoelectron Spectroscopy); Briggs, D., Senh, M. P., Eds.; John W h y : Chichester, 1985, p 446. (30) Brown, A.;Vickerman, J. C. Surf. Interface Anal. 1986,8, 75. (31) Briggs, D.;H e m , M. J. Vacuum 1 9 8 6 , s (11-12), 1006. (32) Hearn, M.J.; Briggs, D. Surf. Interface Anal. 1986, 9, 411.
Davies et al.
1402 Langmuir, Vol. 10, No. 5, 1994 Table 2. Bulk Compositions of PMMA/PBMA Latex Copolymers from Proton NMR Data bulk copolymer monomer
comDosition ( % vlv) MMAIBMA -1 100 20180 40160 50150 60140 80120 1001-
composiaon (mol % )"
integru'
-OC& (3.6 ppn
. I .
I
-0CH.-
Table 4. Elemental Composition in Atomic Percent (Excluding Hydrogen) of the Surfaces of Solvent Cast and Latex Films of PMMA/PBMA Copolymers monomer composition film (% vlv) MMAIBMA typeo % C b -1100
8 15 11 11 10 4
4 14 22 29.5 37 27
0.0 (0.0) 28.6 (27.1) 45.9 (49.8) 57.1 (59.8) 66.3 (69.1) 86.0 (85.6) 100 (100)
100 (100) 71.4 (72.9) 54.1 (50.2) 42.9 (40.2) 33.7 (30.9) 14.0 (14.4) 0.0 (0.0)
60140
Table 3. Electrophoretic Mobility (EPM), Zeta Potential (ZP), Particle Size (dz), and Polydispersity Index (Q)of the Series of PMMA/PBMA CoDolymer Latex Particles -
0.9
(20.0) NDc
73*5 79.1 (80.0)
S
71.9 78.8
(77.5) 27*4 21.2 (22.5) 0.7 ND
S
65.1 74.8
(74.3)
S
67*7 72.8 (72.6)
L
80120 1001-
%S
25.6 20.9
S 20180
Initial monomer compositions (mol %) in parentheses.
% Ob
34 3 25:2
(25.7) 0.6 ND
32.1 27.2 34 1 (71.4) 30:5
0.2 (27.4) ND 0.5 (28.6) ND
~~
monomer composition ( % v/v) MMA BMA 100 80 60 50 40 20
20 40 50 60 80 100
dzo (nm)
Q
156 149 149 152 148 155 171
0.036 0.041 0.055 0.044 0.055 0.047 0.038
EPMb (Mums-1cmV-1) 3.40 3.91 4.48 4.94 5.24 5.84 6.35
ZP (mV) -44.4 -51.0 -58.3 -64.3 -68.3 -76.1 -82.8
.
0 Mean of 20 measurements,coefficient of variation
