Langmuir 1991, 7, 831-833
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Investigation of Block Copolymer Adsorption onto Aqueous Latex Dispersions by Size Exclusion Chromatography Renliang X U ,Yongzhong ~ Hu,and Mitchell A. Winnik' Department of Chemistry, and Erindale College, University of Toronto, Toronto, Ontario M5S 1A1,Canada
S. Mohanraj2 PolyMicroSpheres, 7017-F Dora1 South Drive, Indianapolis, Indiana 46250
Gerard Riess Ecole Nationale Superieure de Chimie, 3 rue Alfred- Werner, 68093 Mulhouse Cedex, France Received November 23, 1990.I n Final Form: February 14, 1991 Size exclusion chromatographyexperimentsare reported for a polystyrene-poly(ethy1ene oxide) diblock copolymer [PS-PEO]adsorbed onto the surface of a series of PS latex particles of different diameters. In the presence of excess block copolymer,peaks for the latex, the micelle, and unimer were observed. The adsorption process was very rapid (seconds). The latex peak was sensitive to the particle diameter ( d ) and exhibited a linear plot of log ( d ) against retention time. When this plot was extrapolated to the retention time of the micelle,the ordinate value correspondedto the core diameter of the micelle,determined independently from scattering experiments. Size exclusion chromatography (SEC) is normally sensitive to the hydrodynamic radius Rh of an eluting species.3 For small hard sphere species,&and the particle radius are identical, but for polymer coils, the Rh is an effective radius that is smaller than the largest span of the coil dimensions. In many instances, the characteristic radius controlling the elution time of a polymer coil in a SEC experiment is identical with that affecting the intrinsic viscosity of the polymer solution. This identity is the basis of the universal calibration procedure in the analysis of polymer molecular weights by SEC.4 We wish to report an interesting application of SEC to the analysis of polystyrenepoly(ethy1ene oxide) diblock copolymers which form micelles in water.6-6 We examine the retention times (retention volumes) of the micelles themselves and of the block copolymers adsorbed onto a series of colloidal monodisperse latex particles. From this analysis we seem to be able to calculate the core size of the micelles. Consequently, for our core-shell type micelles formed from polystyrene-poly(ethy1ene oxide) block copolymer (PS-PEO) in water where the PS core may safely be assumed to be dense-packed, we can use the core size to calculate the aggregation number of the micelle. Block copolymer micelles are normally characterized by scattering techniques. Rh values are obtained from quasielastic light scattering (QELS), whereas micelle molecular weight is often obtained from small angle scattering of light (SALS), X-rays (SAXS), or neutrons (SANS). Other techniques such as sedimentation, field-flow fractionation (FFF),electron microscopy (EM), etc., provide
* To whom all correspondence should be addressed.
(1)A recipient of a PostdoctoralFellowshipawarded by the National Science and Enmneerinn Council of Canada. (2) This work-was doGe while at Seradyn,Inc., Indianapolis,IN 46225. (3) Penlidis, A.; Hamielec, A. E.; MacGregor, J. F. J. Liq. Cromatogr. 1988. 6 (8-21. 179. (4) Gkbisic, 2.; Rempp, P.; Benoit, H. J. Polym. Sci. 1967,Bb, 753. (5) Xu, R.; Winnik, M. A,; Hallett, F. R.; Riess, G.; Croucher, M. Macromolecules 1991,24, 87. (6) Xu, R.; Winnik, M. A.; Riess, G.; Chu, B.; Croucher,M. Submitted for publication in Macromolecules.
different measures of micelle size.7 It is not an easy task, however, to obtain the core size of a spherical core-shell type micelle. The most reliable information on the radius of gyration of a micelle core is obtained by index-matching techniques in classicallight scattering or by SAXS or SANS measurements.8 Matching techniques are not always suitable, especially for copolymer micelles in aqueous media, and, in addition, SAXS and SANS instruments are not always available. Even if the aggregation number of a micelle is known, one can only estimate the core size provided that the core is free of solvent or the extent of solvent incorporation into the core is known. While this final criterion also applies to the experiments reported here, the SEC method we describe provides a convenient method for the analysis of micelle-forming block copolymers in water.
Experimental Section The polystyrene latex samples were prepared by emulsion polymerization and were fully characterized(cf. Table I). They are monodisperse in size, with diameters ranging from 41 to 223 nm. Certain samples had surface carboxylate groups (denoted by a suffix C for the samples in Table I), a feature irrelevant to our experiments, whereasthe remaining latexsamples contained only sulfate groups. Samples were stored in water at 10% solids, diluted with deionized water (Millipore,Milli-Qgrade) to a final concentration of 1.0 mg/mL. The diblock PS-PEO sample (Jlm5) was synthesized in Mulhouse, France, using standard anionic polymerization techniques.9 The number average molecular weight (M.) of the PS block and the PEO block are 1700 and 6800 g/mol, respectively. M,/M,, of the sample is 1.6. The absence of PS and PEO homopolymers in the copolymer sample has been checked by SEC (THF as the eluent) having dual detectors (UV and refractive index).1° The SEC measurementswere made by using a Micropak TSKGel G5000PW exclusion column (Toyo Soda, purchased from (7) Riess, G.; Hurtrez, G.; Bahadur, P. In Encyclopedia of Polymer Science and Engineering, 2nd ed.;John Wiley & Sons: New York, 1985; Vol. 2, pp 324. (8)T w ,2.; Kratochvil, P. Colloids Surf, in press. (9)Mura, J. Doctoral Thesis, University of Haute Alsace, Mulhouee, France, 1991. (10)Xu, R.; Hu, Y.; Winnik, M. A. J. Chromatogr., in press.
0743-7463/91/2407-0831$02.50/0 0 1991 American Chemical Society
Letters
832 Langmuir, Vol. 7, No. 5, 1991
Table I. Characteristics of Latex Particles. latex sample 41C 69 83 l00C 131C 189C 223
d m , nm 41 69 83 100 131 189 223
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dw,nm 38 63 75 91 119 171 202
d m , mean diameters determined by photon correlation spectroscopy (Coulter N4M); d E M , mean diameter determined by electron a
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Varian)" and a Waters Model 510 pump, with a flow rate of 0.8 mL/min. Doubly distilled water (Milli-Qgrade) was the mobile phase. Two detectors were used: a Varian UV50 UV detector (A = 268 nm) and a Waters R401 differential refractometer (RI). The aqueous copolymer solution of concentration C = 1 mg/mL was prepared by dissolvingthe sample into doubly distilledwater; the solution was then heated at -65 O C for 2 h for complete dissolution. No further dilution was made. The copolymer micelle solution was mixed in a syringe with a series of latex suspensions. The total volume of the mixture waa about 2-5 pL. The ratio of the two liquids in each sample was controlled such that the mixture would produce comparable RI signals of the micelle peak and of the peak for the latex particle peak with copolymer adsorbed. This corresponded to a large excess of block copolymer. Samples were introducedinto the column by a Rheodyne six-way injector.
Results and Discussion The experiments described here derive from a series of observations we made in testing the ability to chromatograph block copolymer micelles and latex dispersions in aqueous media. We found that the block copolymers gave two peaks, due to the elution of micelles at short times, accompanied by a smaller peak a t longer times due to molecularly dissolved species (unimers). Small latex particles did not elute from our TSK G5000PW column, but in the presence of excess surfactant such as sodium dodecyl sulfate (SDS)they eluted cleanly with a retention time related to particle size.lO This observation is similar to the results reviewed by Penlidis et al. on the chromatography of latex particles on columns packed with porous glass beadsa3 In Figure 1 we show a SEC trace of the block copolymer which has been mixed for a few seconds in the barrel of a microliter syringe with an aqueous dispersion of 223 nm diameter PS latex. The top trace is that measured by the RI detector while the lower trace is that detected by UV adsorption at 268 nm. We anticipate the presence of four possible species upon mixing the two solutions: "bare" latex particles, latex particles with copolymer molecule adsorbed, block copolymer micelles, and unimers. We have shown that bare latex particles do not elute from the column, and if bare particles were present in the solution injected on the column, they would not be observed. The first peak in the chromatogram (A, 5.42 min) is assigned to the latex coated with block copolymer. The second peak (B,6.03 min) is due to the block copolymer micelle. This peak and the unimer peak (D, 12.1 min) appear at the same positions in the direct analysis of the block copolymer solution without mixing with latex particles. These assignments were made in an independent study which included collecting samples corresponding to the
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(11)Our SEC column has been exposed repeatedly to dilute aqueous eolutione of SDS. The manufacturer's data sheet indicates that this kind of treatment may change the column characteristics from that of a new column exposed onlytowater. Neverthelesawe alwapobtainreproducible resulta with the column treated in this way.
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t (min) Figure 1. SEC output from the mixture of the PS-PEO micelle solution with a dispersion of PS latex (diameter d = 223 nm). The flow rate was 0.8mL/min. The upper curve is the RI response and the lower one is the UV response at h = 268 nm. 300
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t (min) Figure 2. A plot of the core diameter versus the retention time. The symbols represent the copolymer-stabilized latexes (diamonds) and the copolymer micelles (triangle). The latex diameters were determined from dynamic light scattering measurements and have an uncertainty less than 5%, and the SEC time resolution is 1 a. The error bar on the extrapolated value of d at t = 6.03 min was calculated from d, and d- values based upon the assumption that d = ( a f Aa) exp[-(b h Ab)@ f At)], where a and b are arbitrary fitting parameters. peaks and analyzing their composition by Fourier transform infrared (FTIR) spectroscopy.1° Note that in the UV trace in Figure 1 there is a small shoulder on the main peak corresponding to the much weaker UV absorption of the PS in the micelle. The origin of the minor peak C is still not clear. Several features of this experiment are interesting. The first is the rapidity of the adsorption process which appears to come to equilibrium inside the syringe before the injection is made. There is no change in the relative intensities of the peaks if the mixture was allowed to stand for 30 min before reinjection. A second feature concerns the change in the SEC trace when the block copolymer solution is mixed with a dispersion of latex having a different diameter. In this case, the peak A shifts, depending upon the diameter of the latex, whereas the peaks B and D remain unchanged. This result also confirms the assignment of the peaks B and D. According to the theory of size exclusion chromatography] spherical nonadsorbing solutes should have elution volumes that are linearly related to the logarithm of their diameter^.^ As shown in Figure 2 we obtain this type of behavior for seven latex samples of different
Letters diameters, ranging from 41 to 223 nm, with block copolymer adsorbed. The ordinate in Figure 2 is the particle diameter determined independently prior to treatment with block copolymer. We anticipate an increase in particle dimensions accompanying the adsorption of block copolymer. Studies to examine this point are in process. We also anticipate that adsorption of a monolayer or bilayer of SDS onto the latex will lead to a much smaller increase in particle dimensions. However, latex particles treated with an excess of SDS also elute cleanly from the column giving peaks corresponding to the peak A in Figure 1. In these experiments we noticed that the position of the peak A was the same, within our experimental error limit, independent of whether SDS or the Jlm5 block copolymer sample was added as the surface active agent.1° One possible interpretation of this result is that the SEC experiment senses only the hard-sphere size of the solute but not the flexible shell, whose thickness depends on both the amount of copolymer molecules adsorbed per unit surface and the surface curvature.12 If this is the case, then extrapolation of the linear relation in Figure 2 to a retention time corresponding to peak B should yield (12) Ou-Yang,H.D.;Gao,Z.InScalinginDisorderedMateria&Stokee, J. P., Robbm, M. O., Witten, T. A., Me.; Materiale Research Society: Pittsburgh, PA, 1990, p 163.
Langmuir, Vol. 7,No.5, 1991 833
the diameter of the hard sphere PS core of the micelle. This point is indicated by the open triangle in Figure 2 and corresponds to a core diameter of ca. 7.3 nm. If we assume the core to be dense-packed with a density (1.05 g/mL) equal to that of amorphous PSI we can calculate the mean number N of Jlm5 chains per micelle. We find N 75. From light scattering studies of a series of PS-PEO diblock copolymer micelles in water, we have shown that the characteristic radii and mean aggregation numbers (N)fit very well to the star model for these micelles; e.g., N is proportional to N P S ' / ~where , Nps is the polymerization degree of the PS block cop01ymer.l~From QELS measurements, and in conjunction with the star model, we find a core diameter of 6.8nm and a mean aggregation number of 63 for the Jlm5 diblock copolymer micelles in water.S@
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Acknowledgment. The authors thank NSERC Canada and the Province of Ontario for their support of this research. We also wish to thank Professor H. D. OuYang, Department of Physics, Lehigh University, for informing us of his results prior to their publication. (13) Halperin, A. Macromolecules 1987,20,2943.
