3866
Langmuir 1996, 12, 3866-3875
Surface Chemical Characterization Using XPS and ToF-SIMS of Latex Particles Prepared by the Emulsion Copolymerization of Methacrylic Acid and Styrene M. C. Davies,*,† R. A. P. Lynn,†,‡ J. Hearn,§ A. J. Paul,| J. C. Vickerman,⊥ and J. F. Watts∇ Department of Pharmaceutical Sciences, University of Nottingham, Nottingham NG7 2RD, U.K., Department of Chemistry and Physics, Trent University, Nottingham NG11 8NS, U.K., Centre for Surface and Materials Analysis, Armstrong House, Oxford Road, Manchester M1 7ED, U.K., Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, U.K., and Department of Materials Science and Engineering, University of Surrey, Guildford GU2 5XH, U.K. Received February 23, 1996X A series of colloids based on poly(styrene) were prepared by emulsion copolymerization with various proportions of methacrylic acid. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to monitor changes in particle surface composition and indicated a substantial enrichment of the methacrylic acid component. Further evidence for the presence of carboxyl groups at the particle surfaces was provided by electrophoretic mobility measurements, which showed a marked increase over the pH range studied.
1. Introduction Many of the industrial and biological applications of polymer colloids require systems with specifically-tailored surface properties.1-7 A common means of preparing latex systems with the desired surface characteristics involves the incorporation of one or more comonomers (usually bearing appropriate functional groups) into the polymerization recipe.6 Whilst the ability to modify latex surface properties in this way is extremely useful, the incorporation of comonomers inevitably complicates the polymerization kinetics and particle nucleation and formation mechanisms. This, in turn, means that the copolymer compositions of the particle bulk and surface may vary considerably, and both may also differ from the initial monomer composition. An understanding of the copolymer composition of particle surfaces in relation to the bulk copolymer and/or initial monomer composition is important for many colloid applications. For example, the use of latex particles in biomedical and immunological applications may involve covalent linkage of surface functional groups to a biomolecule of interest (enzyme, antibody, peptide, oligosaccharide, etc.). In theory, the surface coverage of post* To whom correspondence should be addressed. † University of Nottingham. ‡ Present address: SmithKline Beecham Pharmaceuticals, New Frontiers Science Park, Third Avenue, Harlow, Essex CM19 5AW, U.K. § Trent University. | Centre for Surface and Materials Analysis. ⊥ UMIST. ∇ University of Surrey. X Abstract published in Advance ACS Abstracts, June 1, 1996. (1) Rembaum, A.; Yen, S. P. S.; Molday, R. S. J. Macromol. Sci., Chem. 1979, A13 (5), 603. (2) Rembaum, A.; Yen, S. P. S.; Cheong, E.; Wallace, S.; Molday, R. S.; Gordon, I. L.; Dreyer, W. J. Macromolecules 1976, 9 (2), 328. (3) Orsini, A. J.; Ingenito, A. C.; Needle, M. A.; Debari, V. A. Cell Biophys. 1987, 10, 33. (4) Delair, T.; Pichot, C.; Mandrand, B. Colloid Polym. Sci. 1994, 272 (1), 72. (5) Pichot, C. Makromol. Chem., Macromol. Symp. 1990, 35/36, 327. (6) Upson, D. A. J. Polym. Sci., Polym. Symp. 1985, 72, 45. (7) Blackley, D. C. In Science and Technology of Polymer Colloids; Poehlein, G. W., Ottewill, R. H., Goodwin, J. W., Eds.; NATO-ASI Series E (No. 67); Martinus Nijhoff: Dordrecht, The Netherlands, 1983; Vol. 1 (Preparation and Reaction Engineering), p 203.
S0743-7463(96)00169-2 CCC: $12.00
grafted molecules could be varied by manipulating the latex recipe to alter the surface density of reactive functionalities. Similarly, ‘overcoating’ of polymer latices in emulsion polymerization is widely used industrially to control particle surface properties. In these examples, the ability to define changes in surface composition would be valuable in optimizing the design of the colloids. A variety of analytical methods have been used to determine the distribution of monomers or functional groups in polymer colloids, including titration8-10 and NMR spectroscopy.11 There have also been a number of recent reports on the use of surface analysis techniques such as X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) to evaluate the surface compositions of both homo- and copolymer latices.12-20 In the present study, XPS and time-of-flight SIMS (ToF-SIMS)21,22 have been used, in conjunction with conventional methodologies, for the determination of the surface compositions of carboxylated latex particles based on styrene (ST). These were prepared by the incorporation of various quantities of methacrylic acid (MAA) in an (8) Okubo, M.; Kanaida, K.; Tsunetaka, M. J. Appl. Polym. Sci. 1987, 33, 1511. (9) Greene, B. W. J. Colloid Interface Sci. 1973, 43 (2), 449. (10) Emelie, B.; Pichot, C.; Guillot, J. J. Dispersion Sci. Technol. 1983, 5 (3-4), 393. (11) McDonald, C. J. J. Dispersion Sci. Technol. 1984, 5 (3-4), 365. (12) Pijpers, A. P.; Donners, W. A. B. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 453. (13) Okubo, M.; Okegami, K.; Yamamoto, Y. Colloid Polym. Sci. 1989, 267, 193. (14) Okubo, M.; Yamamoto, Y.; Kamei, S. Colloid Polym. Sci. 1989, 267, 861. (15) Koosha, F.; Muller, R. H.; Davis, S. S.; Davies, M. C. J. Controlled Release, 1989, 9, 149. (16) Brindley, A.; Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F. Polymer 1992, 33 (5), 1112. (17) Deslandes, Y.; Mitchell, A. F.; Paine, A. J. Langmuir 1993, 9 (6), 1468. (18) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Paul, A. J. Langmuir 1993, 9 (7), 1637. (19) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Johnson, D. Langmuir 1994, 10 (5), 1399. (20) Davies, M. C.; Lynn, R. A. P.; Hearn, J.; Paul, A. J.; Vickerman, J. C.; Watts, J. F. Langmuir 1995, 11, 4313-4322. (21) Standing, K. G.; Chait, B. T.; Ens, W.; McIntosh, G.; Beavis, R. Nucl. Instrum. Methods 1982, 33, 198. (22) Bletsos, I. V.; Hercules, D. M.; Greifendorf, D.; Benninghoven, A. Anal. Chem. 1985, 57, 2384.
© 1996 American Chemical Society
Characterization of Latex Particles
Langmuir, Vol. 12, No. 16, 1996 3867
Table 1. Polymerization Recipes for the Colloids (Total Monomer Concentration 1% v/v) volume addeda (mL) colloid 1 styrene methacrylic acid potassium persulfate soln (0.1% w/v) water (double dist) total volume (mL) a
colloid 2
1.0
colloid 3
colloid 4
4.0
0.9 0.1 4.0
0.8 0.2 4.0
0.7 0.3 4.0
95.0 100.0
95.0 100.0
95.0 100.0
95.0 100.0
Equivalent monomer compositions (% v/v): latex no.
ST
MAA
1 2 3 4
100% 90% 80% 70%
10% 20% 30%
attempt to introduce different surface coverages of the carboxylate functionality. The monomers comprise the following structures:
Apart from the significance of carboxylate groups for biomolecule immobilization, carboxylated polymer latices have many industrial applications (e.g. latex foam rubber, paper coating) which benefit from the increased chemical and mechanical stability conferred by the carboxylates.7 2. Materials and Methods 2.1. Monomer and Initiator Purification. Styrene monomer (ST, 99%, Aldrich, Gillingham, U.K.), containing 10-15 ppm p-tert-butylcatechol as a polymerization inhibitor, was purified by distillation at 40-50 °C at 5 mmHg pressure under nitrogen. Methacrylic acid monomer (MAA, 98.5%, Aldrich), containing 1000 ppm hydroquinone and 250 ppm hydroquinone monomethyl ether as polymerization inhibitors, was purified by distillation under a nitrogen atmosphere at 60 °C and 10 mmHg pressure using an air condenser. Both purified monomers were stored under nitrogen at 4 °C and protected from light until required. Potassium persulfate initiator (Analar grade, BDH, Poole, U.K.) was recrystallized twice from double-distilled water and dried in a dessicator. This process was repeated after two weeks’ storage. 2.2. Preparation of Latex Particles. In order to minimize contamination of the 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 four colloids based on ST were prepared with increasing proportions of MAA comonomer. The monomer compositions and polymerization recipes are shown in Table 1. The procedure for emulsifier-free polymerization described elsewhere23,24 was followed. The concentrations of monomer and potassium persulfate initiator were 1% v/v and 1.48 × 10-4 mol dm-3, respectively. The polymerizations were continued for 24 h at 70 C and allowed to cool. The colloids were then filtered through Whatman No. 1 filter paper (Whatman, U.K.) to remove polymer aggregates. The percentage yield of polymer in latex form was estimated by taking a known volume of the cooled filtered latex and evaporating to dryness. The weight of polymer was determined, and in each case, the yield was greater than 80%, indicating satisfactory monomer-to-polymer conversion. (23) Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Br. Polym. J. 1973, 5, 347. (24) Lynn, R. A. P. Ph.D. Thesis, University of Nottingham, U.K., 1991.
2.3. Cleaning and Storage of Latex Particles. The colloids were cleaned in two stages. Ten milliliter samples were initially dialyzed against double-distilled water for 14 days with regular changes of the dialyzate. The process was monitored by performing a UV scan on each dialyzate. The colloid was judged to be clean when the UV absorbance of the supernatant was negligible over a wide range of wavelengths at the maximum sensitivity of the spectrometer (Kontron Uvikon 860, Kontron, St. Albans, U.K.). However, UV scans on the sample supernatants indicated the presence of water-soluble material after this period (probably oligomers rich in poly(methacrylic acid)). This material was removed by passing 1 mL samples through a Sephadex G75 column (Pharmacia, Milton Keynes, U.K.). Purified latex samples were stored in chromic acid-cleaned glass tubes at 4 °C until required. 2.4. Characterization of Colloids. 2.4.1. Size and Electrophoretic Mobility Determination. Particle size measurements were carried out by photon correlation spectroscopy.25,26 Briefly, 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) to yield particle size and polydispersity data. The polydispersity, Q, is derived from Koppel’s method of cummulants25 and is used to express the polydispersity numerically. Using this approach, monodisperse latices have a value of Q ) 0.03, but correlative data from electron microscopy suggest that values of Q less than 0.1 reflect particle sizes that are narrowly distributed. A total of 10 measurements were recorded for each sample, and mean size and polydispersity values were calculated. The determination of electrophoretic mobility (EPM) and ζ potential (ZP) was performed using laser Doppler anemometry.25-27 Measurements were carried out using a Malvern Zetasizer II (Malvern Instruments Ltd). EPM measurements were made over the pH range 3.0-7.9 using phosphate-citrate buffers of constant ionic strength (0.01 M). Samples were prepared by the addition of 200-300 µL 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. 2.4.2. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were obtained using a VG ESCALAB Mk II electron spectrometer (VG Scientific Ltd, East Grinstead, Sussex, U.K.) employing Mg KR X-rays (hν ) 1253.6 eV) at an electron take-off angle of 45°. 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 wide scan (0-1000 eV) was recorded for each sample (single scan) followed by the C 1s, O 1s, and S 2p regions, where appropriate (5 scans). The analyzer was operated in fixed analyzer transmission (FAT) mode with a pass energy of 50 eV (wide scan) and 20 eV (C 1s, O 1s, and S 2p regions). Data analysis was performed on a VGS 5000 data system based on a DEC PDP 11/73 computer. The methodology employed for peak fitting of the C 1s and O 1s envelopes has been described in detail elsewhere.28 Typically, 1.5-1.6 eV line widths and Gaussian/Lorentzian ratios of 30% were employed for the components of the C 1s envelope. Atomic percentage values and elemental ratios were calculated from the peak areas using sensitivity factors and background subtraction. Spectra were (25) Cummins, H. Z., Pike, E. R., Eds. Photon Correlation and Light Beating Spectroscopy; NATO-ASI Series B (No. 3); Plenum: New York, 1974. (26) Cummins, H. Z., Pike, E. R., Eds. Photon Correlation Spectroscopy and Velocimetry; NATO-ASI Series B (No. 23); Plenum: New York, 1977. (27) Earnshaw, J. C., Steer, M. W., Eds. The Applications of Laser Light Scattering to the Study of Biological Motion; Plenum: New York, 1983. (28) Sherwood, P. M. A. In Practical Surface Analysis (by Auger and X-ray Photoelectron Spectroscopy); Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, 1985; p 445.
3868 Langmuir, Vol. 12, No. 16, 1996
Davies et al.
Table 2. Particle Size (dz), Polydispersity Index (Q), Electrophoretic Mobility (EPM), and ζ Potential (ZP) monomer composition (% v/v) ST 100 90 80 70
MAA 10 20 30
EPMb (µms-1 cm V-1) dza
(nm)
192 257 243 242
Q 0.042 0.053 0.034 0.036
ZP (mV)
pH 3.0 pH 7.9 pH 3.0 pH 7.9 -5.05 -2.40 -1.18 -0.64
-5.66 -4.48 -3.80 -3.62
-65.9 -31.3 -15.4 -8.3
-73.9 -58.4 -49.6 -47.2
a Mean of 10 measurements, coefficient of variation
