862
Langmuir 2000, 16, 862-865
Adsorption of Cationic Polymer Micelles on Polyelectrolyte-Modified Surfaces Maria Ruela Talingting, Yanhui Ma, Chris Simmons, and S. E. Webber* Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712 Received February 2, 1999. In Final Form: September 1, 1999
Introduction Modification of surfaces by sequential adsorption of a preformed structure to form larger scale arrays has been actively pursued since the demonstration of this concept by Iler for the particular case of charged polystyrene latexes.1,2 It is usually observed that the latex particles form a hexagonal close-packed array with a particleparticle spacing of twice the particle radius (i.e., they are in contact). In the “random sequential adsorption” model it is assumed that adsorption of a particle is irreversible and random, except for self-avoidance.3 If the particles are strongly charged, then electrostatic repulsion will tend to separate them and reduce the surface density below the so-called “jamming limit” of 0.55.3 The particle positions are expected to be correlated and the pair distribution function (〈g(r)〉) is predicted to be liquidlike with the first maximum at a separation larger than the particle diameter (the hard sphere limit).4 If the particles were free to move after attachment, then one would expect a two-dimensional (2D) crystal lattice to develop, to minimize particle-particle electrostatic repulsions. The present work grew out of our interest in modifying aminated surfaces with polymer micelles by carbodiimide coupling in order to characterize the micelles via scanning electron microscopy (SEM) or atomic force microscopy (AFM).5 More recently we reported the effect of pH and ionic strength on the adsorption of a polymer micelle with a polyacid corona to aminated surfaces.6 In this latter case we believe that acid-base interactions drive the adsorption process. In the work presented herein a cationic polymer micelle is adsorbed onto an anionic polystyrene sulfonate layer and electrostatic attraction should be the primary driving force. Our polymer micelles have a protonated poly(2-vinylpyridine) corona and a polystyrene core, and all results are for 0.1 M HCl solutions. At this ionic strength the Debye screening length is ca. 0.96 nm, so the repulsive forces between the individual adsorbed micelles is most likely steric in origin rather than electrostatic. A fairly regular array of the micelles forms on the surface with the minimum center-to-center on the order of 125 nm (ca. 2.6 times the micelle hydrodynamic radius). The polymer micelles appear to be firmly bound to the surface, presumably as a result of the many points of electrostatic attraction. The present system is a new (1) Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569. (2) For a review see: Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (3) Feder, J. J. Theor. Biol. 1980, 87, 237. (4) Adamczyk, A.; Zembala, M.; Siwek, B.; Warszynski, P. J. Colloid Interface Sci. 1990, 140, 123. (5) Karymov, M.; Procha´zka, K.; Mendenhal, J.; Martin, T. J.; Munk, P.; Webber, S. E. Langmuir 1996, 12, 4748. (6) (a) Ma, Y.; Webber, S. E. Polym. Prepr. 1998, 39, 425. (b) Ma, Y. Ph.D. Dissertation, May 1999.
example of random sequential deposition of “soft spheres” and layer-by-layer polyelectrolyte modification of a surface. So far as we can ascertain from the literature, the deposition of preformed particles to create regular and sparse arrays is not a common observation. Other than the examples presented herein and in an earlier report from our laboratory,6a there is the work of Johnson and Lenhoff 7 and Adamczyk et al.4 on latex particles at very low ionic strength, the “surface micelles” of Eisenberg and Lennox,8 and the deposition of Au-loaded PS-b-PVP or PS-b-PEO micelles from toluene solution onto various substrates reported by Spatz et al.9 For the latex case it seems clear that it is the electrostatic repulsion that maintains a sparse distribution of particles. For the PSb-PVP or PS-b-PEO micelles in toluene the ionic species are expected to reside in the micelle core with no net charge on the particles such that the regular and sparse distribution of micelle cores must be a result of steric stabilization by the corona. In many respects the results reported here are most similar to the “surface micelles”, which are prepared by Langmuir-Blodgett (LB) techniques with pure water in the subphase. The corona was charged but it is not obvious how important electrostatics will be under these conditions. The “starfish” and “jellyfish” motifs are analogous to our suggestion below that our cationic micelles are “flattened” upon adsorption.8c We note a similar suggestion by Tsukruk et al. concerning the collapse of dendrimeric polyelectrolytes upon electrostatic layer-by-layer deposition.10 Experimental Section Polymer Micelles. The details of these materials have been described elsewhere.11 The diblock polymer was prepared by anionic polymerization and is composed of a polystyrene block (DP ) 350, weight fraction 0.392) and poly(2-vinylpyridine) (DP ) 550), with a total molecular weight of ca. 93 000, Mw/Mn ) 1.14. The polymer micelle is prepared by dissolving the diblock polymer in a series of mixed solvents, eventually ending with 0.1 M HCl. In this solution the poly(2-vinylpyridine) block is protonated (PVPH+) and makes up the micelle corona while the micelle core is polystyrene (PS). The radius of the PS core is 12 nm according to small-angle neutron scattering (SANS).12 The micelle hydrodynamic radius and radius of gyration are 49 and 33.8 nm, respectively. The Rg/Rh ratio (0.69) is consistent with a core-shell model. The micelle molecular weight was determined from static light scattering to be 13.4 × 106, which corresponds to an aggregation number of ca. 140. Therefore in principle ca. 8 × 104 protonated pyridine groups are available to interact with the anionic surface. It is expected that only the outer periphery of the corona will be protonated because of the high charge density that would exist near the core-corona interface.13 We have not measured the ζ potential of our polymer micelles and hence do not have an experimental estimate of the surface charge density. (7) Johnson, C. A.; Lenhoff, A. M. J. Colloid Interface Sci. 1996, 179, 587. (8) (a) Meszarnos, M.; Eisenberg, A.; Lennox, B. R. Faraday Discuss. 1994, 98, 283. (b) Li, S.; Hanley, S.; Khan, I. Varshney, S. K.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9, 2243. (c) Zhu, J.; Hanley, S.; Eisenberg, A.; Lennox, R. B. Makdromol. Chem., Macromol. Symp. 1992, 53, 211. (9) Spatz, J. P.; Herzog, T.; Mo¨ssmer, S.; Ziemann, P.; Mo¨ller, M. In Micro- and Nanopatterning Polymers; Ito, H., Reichmanis, E., Nalamasu, O., Ueno, T., Eds.; ACS Symposium Series 706; American Chemical Society: Washington, 1998; pp 12-25. (10) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2179. (11) Talingting, M. R.; Munk, P.; Webber, S. E.; Tuzar, Z. Macromolecules 1999, 32, 1593. (12) Plestil, J. Private communication.
10.1021/la9901027 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/07/1999
Notes
Langmuir, Vol. 16, No. 2, 2000 863
A back-of-the-envelope calculation suggests that the surface charge density may not be so different than that of typical polystyrene latexes.14 We believe that the new feature encountered for polymer micelles of the type described herein is the combination of charge and flexibility of the corona. Surface Modification and Attachment of Micelles. The quartz disks are first cleaned by soaking for 15 min in “piranha solution” (250 mL of concentrated H2SO4 added slowly to H2O2 chilled in an ice bath. Caution: heat liberated and final solution very reactive). The disks are rinsed in distilled water for 5 min, and dried at 110 °C unless they are used immediately The disks are placed in pH 2 HNO3, just before attachment of the silane. A freshly made solution of (3-aminopropyl)dimethylethoxylsilane (United Chemical Technologies) is used as follows: 2 vol % of the silane is added to 95% ethanol/5% water and allowed to react for 5 min in order to form the silanol. Then the HNO3treated disks are placed into solution and allowed to react for 5 min with slight agitation to encourage homogeneous and complete reaction. After removal they are rinsed with pure ethanol and stored in a desiccator until ready for use. The disk has a contact angle (static) with water of 50° ( 5° over the entire surface. The polyelectrolyte modification procedure is as follows: the aminated quartz disks are placed in 0.1 M HCl for 5 min, and then placed in a 0.1 wt % sodium polystyrene sulfonate (PSS-Na+) in 0.1 M HCl solution for 5 min. The disk is then rinsed with 0.1 M HCl for 5 min and placed in a 0.1 wt % (1 mg/mL) of the PS-b-PVPH+ micelles for 5 min. We have found essentially no dependence on the concentration of the micelles or the reaction time unless the micelle concentration is reduced by several orders of magnitude, in which case a longer reaction time (days) yields the same arrays as discussed later. After micelle adsorption the disk is rinsed in 0.1 M HCl for 10 min with mild agitation. Vigorous washing does not appear to remove the micelles from the surface. Multiple AFM scans of the same region in contact or shear force mode do not appear to dislodge the particles from the surface. Pair Distribution Function. The x, y coordinates of the centers of all particles in the SEM image were obtained by using the public domain program NIH Image (Ver. 1.61). The 2D pair distribution function was obtained by explicitly computing the number of micelles at distance r (in 25 nm steps) for each of the micelles in the middle 25% of the SEM image. The normalization of g(r) is defined from the relation
∫
2π σg(r) r dr )
∫ dN(r) ) N
(1)
where σ is the surface density of micelles and N is the total number of micelles. Therefore in the explicit counting ∆N(r)i is the number of micelles located between r and r + ∆r from micelle i, and
g(r)i ) ∆N(r)i/2πσr∆r
Scientific Instruments, Sunnyvale, CA). Images were obtained in either tapping (noncontact) mode or contact mode with PSI Ultralevers. Each probe was made up of a sharp silicon conical tip with a nominal tip radius of 100 Å mounted on a silicon cantilever. The cantilever had a force constant of 3.2 N/m and oscillated near its resonant frequency of 90 kHz. As part of some preliminary near field scanning optical microscope (NSOM) experiments lower resolution shear force images in the noncontact mode were obtained using 50-75 nm uncoated pulled glass fibers on an Aurora NSOM (Topometrix).
Results +
The PS-b-PVPH micelles adsorb very quickly onto the surface, reaching an equilibrium coverage within minutes at the usual concentration of 1 mg/mL. A typical SEM image is presented in Figure 1a, and similar images are obtained from contact, tapping, and shear mode AFM. The AFM image has a rather uniform valley-to-peak height of ca. 20 nm (the peaks correspond to the dark region of the SEM), which is slightly less than the diameter of the core (24 nm). While the array of attached micelles does not form an evident lattice structure, it is clear that the average closest approach is not random as is demonstrated by the 2D pair-distribution function (Figure 1b). 〈g(r)〉 is the average of the ca. 130 micelles in the middle of Figure 1a. The general appearance of 〈g(r)〉 was identical for a number of independently prepared samples (ca. 10) using either AFM or SEM images. The shape of 〈g(r)〉 is typical of the liquid state and the first maximum occurs at ca. 125 nm, which is approximately 2.6 times the hydrodynamic radius (49 nm). Of course, in dealing with such a soft particle as a polymer micelle it is not obvious what is meant by the “radius”, so one could also define the “effective hard sphere radius” of the micelle as 62.5 nm. We compute the approximate fractional surface coverage (0.12) on the basis of the dark regions in SEM or the regions with a significant height above the baseline in AFM. We presume that this adsorption pattern is the result of the micelle-micelle repulsion between previously adsorbed and newly adsorbed micelles (see below). Since the points of initial attachment are random, formation of a 2D lattice would be imperfect unless the adsorbed micelles are mobile after attachment. Our results seem to be qualitatively in accord with the model proposed by Adamczyk et al. for the adsorption of electrostatically charged disks onto a surface and our experimental 〈g(r)〉 curves strongly resemble their Monte Carlo simulations.4
(2)
We denote the position of the first maximum in 〈g(r)〉 as rmax#1 and note that for a hard sphere of radius a that rmax#1/a ) 2. Imaging Instrumentation. SEM images were obtained using a Philips 515 scanning electron microscope which was equipped with a Polaroid camera. All SEM samples were sputter-coated with ca. 10 nm of Au/Pd on a Ladd benchtop sputter coater using a 60/40 Au/Pd target at 2.5 kV/20 mA for 45 s. AFM imaging of the dried micelle adsorbed on the surface was performed under ambient conditions on a PSI AFM (Park (13) (a) Misra, S.; Mattice, W. L.; Napper, D. H. Macromolecules 1994, 27, 7090. (b) Zhulina, E. B.; Birshtein, T. M. Macromolecules 1995, 28, 1491. (c) Lyatskaya, Y. V.; Leermakers, F. A. M.; Fleer, G. J.; Zhulina, E. B.; Birshtein, T. M. Macromolecules 1995, 28, 3562 and references to earlier work therein. (14) The hydrodynamic radius of the micelle is 49 nm. If we assume that only the terminus of the protonated PVP chains contribute to the surface charge density, then there are ca. 140 charges per micelle (the aggregation number), or σ ) 0.08 µC/cm2 (clearly an underestimate). The number of pyridine moieties per micelle is 140 × 550 ) 7.7 × 104. Therefore σ is on the order of f42.6 µC/cm2, where f is the fraction of charged PVP units that contribute to the surface charge density. If f is on the order of 0.05-0.1, which seems physically reasonable, the value of σ is similar to typical polystyrene latexes.
Discussion 4
Adamczyk et al. and Johnson and Lenhoff 7 have observed similar liquidlike pair distributions with large interparticle separations for latex particles adsorbed on mica at a very low ionic strength. These latter authors compared several different ionic strengths. At 0.003 mM, which corresponds to a Debye length of 175 nm, they observed a 〈g(r)〉 distribution with rmax#1/a ) 4.5 and a coverage of 0.14. At higher ionic strength (20 mM) the surface coverage (0.48) approached the “jamming limit” of 0.55 and the peak in 〈g(r)〉 was approximately 2.6 times the particle radius. Layer-by-layer adsorption of oppositely charged latexes onto charged surfaces often yields hexagonal, closed-packed regions or clusters of closely spaced particles15,16 and is quite different than the liquidlike pattern reported herein or by Johnson and Lenhoff.7 (15) Bliznyuk, V. N.; Campell, A.; Tsukruk, V. V. In Organic Thin Films (Structure and Applications); Frank, C. W., Ed.; ACS Symposium Series 695, American Chemical Society: Washington, DC, 1998; Chapter 16, p 220. (16) Fulda, K.-U.; Kampes, A.; Krasemann, L.; Tieke, B. Thin Solid Films 1998, 327-329, 752.
864
Langmuir, Vol. 16, No. 2, 2000
Notes
compare the micelle corona spacing and the solution phase micelle dimensions. Eisenberg and Lennox et al. observed some images that are very similar to ours for “surface micelles” formed by polystyrene-polyelectrolyte diblock polymers at the water-air interface.8 Their surface micelles look more ordered than those presented herein, which is reasonable because they should be free to move along the liquid interface. These images were not analyzed with respect to the pair distribution or surface coverage, as the emphasis of the work was on different morphologies that could be obtained with surface micelles. The theoretical calculations of Adamczyk et al.4 focused on electrostatic interactions, but the MC results were obtained using a potential
(
φ ) φ0
Figure 1. (a) SEM of PS-b-PVPH+ micelles deposited on quartz/-NH3+PSS- surface. The white bar is 1 µm (sample coated with ca. 12 nm Au). The dark regions correspond to the micelle core position (peak-to-valley height approximately 20 nm according to AFM of uncoated samples). (b) Pair distribution function 〈g(r)〉 (see eqs 1 and 2 and the discussion of calculation in the text).
Spatz et al. have recently reported depositing polystyrene-block-poly(2-vinylpyridine) (PS-b-PVP) or polystyrene-block-poly(ethylene oxide) (PS-b-PEO) polymer micelles from toluene, a good solvent for the polystyrene component, onto glass, mica, or carbon-coated TEM grids.9 The micelles had been treated with HAuCl4 or LiAuCl4 for PS-b-PVP and PS-b-PEO, respectively. This latter step not only improves the contrast for TEM but also seems to improve the quality of the ordered array of micelle cores that results. The spacing between the micelle cores is larger as the degree of polymerization of the diblock polymer increases, which presumably is the result of the steric repulsion by the PS corona. These authors do not
)
2 exp(-κaHm) Hm + 2
(3)
where Hm is the dimensionless edge-to-edge separation of the disk (or the projection of a sphere)17 and φ0 describes the potential of the disks in contact. For Hm < 0 (overlapping disks) φ ) ∞ is assumed. The damping term, κa, is ascribed to a screened electrostatic interaction (κ is the inverse Debye length and a is the radius of the disk) but in reality it could be an approximation for the steric interaction of the outer portions of the polymer micelle. As pointed out above, at 100 mM ionic strength κ is on the order of unity and if a for these micelles is taken to be on the order of 34 or 49 nm (Rg and Rh, respectively), then κa . 1 is expected, which corresponds to the “hard disk” limit. However the properties of our micelle surface distribution (coverage ) 0.12 based on the high-density core region, rmax#1/Rh ) 2.55) corresponds to the MC simulation results of Adamczyk et al. for relatively small κa (a quantitative comparison is impossible). However it is possible that with respect to steric stabilization the “effective size” of the micelles is greater than 2Rh. By way of comparison, Johnson and Lenhoff obtain a center-tocenter separation of 2.52a and a surface coverage of 0.48 at 20 mM ionic strength.18 The reason rmax#1/a is so much larger than 2 is not discussed by these authors. It is possible that the density of polymer micelle adsorption is essentially equivalent to a latex particle of equal Rh, but the distribution of mass for the micelle is much more sparse because so much of the mass is concentrated in the core and the corona is so expanded. It also seems plausible to us that as the cationic corona is attracted to the anionic surface that the corona chains may be pulled onto the surface, effectively “flattening” the polymer micelle, thereby extending the range of the corona-corona steric repulsion. We suggest this in part because the separation between centers for our system is considerably larger than for the corresponding PS-b-PVP(HAuCl4) surface-adsorbed polymers of Spatz et al.9 The relative sharpness of the AFM images (the width of the 20 nm high feature is on the order of 65-70 nm with very distinct valleys between peaks) implies than the adsorbed polymer micelles are rather compact. However since all AFM images are taken in the dry state, information about spatial relations during the adsorption itself is lacking. (17) Hm is related to the smallest center-to-center separation (rm) obtained upon random placement of a disk onto a surface that already contains disks and is given by Hm ) (rm - 2A)/2A. Strictly speaking a and A both describe the disk diameter but κa is treated as an independent parameter in the MC simulations. (18) These authors quote an edge-to-edge distance of 30 nm at 20 mM in their paper, which when combined with the nominal particle diameter of 116 nm yields rmax#1/a ) 2.52.
Notes
Langmuir, Vol. 16, No. 2, 2000 865 Scheme 1
A representation of these ideas is presented in Scheme 1. The top panel illustrates the polymer micelle in solution, the middle panel illustrates the possible stretching of the adsorbed cationic chains upon adsorption, and the bottom panel illustrates the spatial relationships described in the preceding. In summary, the present work and the work of others suggest that adsorption of polymer micelles can yield a 2D array with a regular and sparse distribution of the micelle cores that is controllable by corona-corona steric interactions, even though the surface attachment is driven by electrostatics. This situation is quite distinct from the (19) For an unusual example of the formation of “clustered” mesostructures by latexes at the air-water interface see: Ghezzi, F.; Earnshaw, J. C. J. Phys. Condens. Matter 1997, 9, L517. (20) Ruela Talingting, work in progress.
case of latex deposition on surfaces.19 It is also worth noting that the use of micelles with polyelectrolyte coronas opens up the possibility of a layer-by-layer deposition approach,20 which has been a very active field in recent years. Acknowledgment. This research was supported by the NSF (CTS-9870881) and the Robert A. Welch Foundation (Grant F-356). We thank Professor K. Shih and Professor D. vanden Bout for allowing us to use their AFM instruments and J. Mendenhall for his assistance with SEM measurements. We acknowledge very helpful discussions with Professor Petr Munk concerning these results. LA9901027
