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Self-Aggregation of Triblock Copolymers at the Solid Silica-Water Interface Krister Eskilsson,*,† Lachlan M. Grant,‡ Per Hansson,§ and Fredrik Tiberg‡ Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, Institute for Surface Chemistry, P.O. Box 124, S-114 86 Stockholm, Sweden, and Physical Chemistry, Uppsala University, Box 532, S-75121 Uppsala, Sweden Received January 18, 1999. In Final Form: April 23, 1999
We report on the adsorption and surface aggregation behavior of triblock copoly(ethylene oxidetetrahydrofuran-ethylene oxide) copolymers, EOn/2THFmEOn/2, at the silica-water interface. In this work, we have used both time-resolved fluorescence spectroscopy and atomic force microscopy to study the morphology of the adsorbed layers. We confirm previous suggestions that the copolymers self-assemble at the silica surface to form discrete micelle-like structures. The surface aggregation, however, starts at a much lower copolymer concentration than the corresponding micellization process in solution. The timeresolved fluorescence measurements reveal that the copolymer aggregates are smaller at the surface than those in bulk solution. The AFM measurements that were performed clearly confirm the existence of discrete micelle-like copolymer aggregates at the silica surface. An interesting result seen in the AFM surface topography images is that, for the high molecular weight copolymer, we observe no surface aggregates at some parts of surface. We believe this is related to the polydispersity of the copolymer. Single high molecular weight copolymers may effectively compete for surface area with surface aggregates formed by a copolymer fraction of lower molecular weight.
Introduction Adsorption and surface self-assembly of amphiphilic molecules at various interfaces is of great importance for many industrial processes including surface modification, liquid spreading, and stabilization of emulsions and dispersions. The multiscale organization of the adsorbed amphiphilic molecules is a topic of large scientific and technological interest, since this often determines the function and efficiency of the adsorbed copolymer layer. For quite some time our understanding of adsorption properties was mainly based on the analysis of the adsorption isotherms. During the past decade, however, isotherms have been supplemented by a variety of experimental techniques such as X-ray diffraction, neutron scattering and/or reflection techniques, fluorescence decay spectroscopy, ellipsometry, atomic force microscopy, and calorimetry. These techniques have provided valuable information about the mechanism and thermodynamics of the formation as well as structure of the adsorbed layers.1-22 * To whom all correspondence should be addressed: e-mail,
[email protected]. Fax, 46-46-2224413. † Lund University. ‡ Institute for Surface Chemistry. § Uppsala University. (1) Levitz, P.; El Miri, A.; Keravis, D.; Van Damme, H. J. Colloid Interface Sci. 1984, 99, 485. (2) Levitz, P.; Van Damme, H.; Keravis, D. J. Phys. Chem. 1984, 88, 2228. (3) Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 1302. (4) Rennie, A. R.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Langmuir 1990, 6, 1031-1034. (5) Levitz, P. Langmuir 1991, 7, 1595. (6) McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 1204. (7) Tiberg, F.; Jo¨nsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (8) Tiberg, F.; Jo¨nsson, B.; Lindman, B. Langmuir 1994, 10, 3714. (9) Birch, W. R.; Knewtson, M. A.; Garoff, S.; Suter, R. M.; Satija, S. Colloids Surfaces A 1994, 89, 145-155.
In this work we report on self-assembly of nonionic triblock copolymers at the hydrophilic silica-water interface studied by time-resolved fluorescence decay spectroscopy and atomic force microscopy (AFM). Time-resolved fluorescence decay spectroscopy can give information about aggregation numbers of surface aggregates whereas AFM gives direct observation of the surface aggregate organization, in situ, often with nanometer resolution. During the past few years, these two techniques have been used to characterize adsorbed surfactant layers. However, we are not aware of any studies where these techniques have been employed for copolymer adsorption studies in aqueous media. The copolymers studied in this investigation were a series of poly(ethylene oxide)-polytetrahydrofuran-poly(ethylene oxide) (PEO-PTHF-PEO) triblock copolymers. The interfacial behavior of the copolymers at both hydrophobic and hydrophilic surfaces has earlier been studied extensively by ellipsometry and surface force techniques.22-24 At silica surfaces, adsorption isotherms, (10) Tiberg, F.; Brink, J. In Surfactant Adsorption and Surface Solubilization; Sharma, R., Ed.; American Chemical Society: Washington, DC, 1995; Vol. 615, pp 231-240. (11) Birch, W. R.; Knewtson, M. A.; Garoff, S.; Suter, R. M.; Satija, S. Langmuir 1995, 11, 48-56. (12) Frank, B.; Garoff, S. Langmuir 1995, 11, 4333-4340. (13) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (14) Frank, B.; Garoff, S. Colloids Surfaces A 1996, 116, 31-42. (15) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 32073214. (16) Tiberg, F. J. Chem. Soc., Faraday Trans. 1996, 92, 531. (17) Thomas, R. K. Progr. Colloid Polym. Sci. 1997, 103, 216-225. (18) Kira′ly, Z.; Bo¨rner, R. H. K.; Findenegg, G. H. Langmuir 1997, 13, 3308-3315. (19) Manne, S. Progr. Colloid Polym. Sci. 1997, 103, 226-233. (20) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349-4356. (21) Lachlan, M. G.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294. (22) Eskilsson, K.; Ninham, B. W.; F. Tiberg, F.; Yaminsky, V. V. Langmuir 1998, 14, 7287-7291. (23) Eskilsson, K.; Tiberg, F. Macromolecules 1998, 31, 5075-5083. (24) Eskilsson, K.; Tiberg, F. Macromolecules 1997, 30, 6323-6332.
10.1021/la990051d CCC: $18.00 © 1999 American Chemical Society Published on Web 06/18/1999
Self Aggregation of Triblock Copolymers
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Table 1. Properties of Copolymers polymer
mol wt (g/mol)
na
mb
cmc (wt %)
P224-28 P146-28 P50-14
11900 8400 3200
224 146 50
28 28 14
0.03 0.04 0.02
a Number of ethylene oxide groups. b Number of tetrahydrofuran groups.
layer thicknesses, and surface force data indicate that these copolymers self-assemble and form surface aggregates above a bulk concentration referred to as the critical surface aggregation concentration (csac).23 The interfacial behavior of copolymers at silica do, indeed, in many respects compare well with that observed for short chain poly(ethylene glycol) monoalkyl ethers (CnEm), despite large differences in molecular weight and composition. On silica, short chain poly(ethylene glycol) monoalkyl ether surfactants clearly associate into surface aggregates just below the critical micelle concentration (cmc). Depending on the systems studied, several surface aggregate structures have been proposed. These include discrete ellipsoidal or spherical aggregates and larger bilayer type structures.1-3,16,21 In general, the more hydrophilic nonionic surfactants tend to form small surface micelles, whereas surfactants with, a greater hydrophobic component form larger aggregate structures. The composition of the surface aggregates and the bulk micelles have, for nonionic surfactants, been found to be almost identical.3,10 A difference observed between the adsorption isotherms measured for short chain surfactants and copolymers is the concentration relative to the cmc at which surface aggregates are first detected, i.e., csac. For CnEm surfactants with relatively short ethylene oxide chains (m < 10), surface aggregation starts approximately between 0.5 and 0.9 cmc. The adsorption beyond this concentration is extremely cooperative. For the PEO-PTHF-PEO copolymers, however, this process starts more than 2 orders of magnitude below the cmc.23 The larger surfaceadsorbate interactions observed for copolymers may also result in a larger difference between bulk and surface aggregates both in terms of aggregate size, shape, and composition. Today very little is known about surface aggregates formed by copolymers at the solid-liquid interface. This fact was the main motive for performing the present study. Experimental Section Material. Three different (ethylene oxide-tetrahydrofuraneethylene oxide) triblock copolymers, EOn/2THFmEOn/2, were studied during the course of this work. Depending on the chemical composition, these are referred to as P224-28, P146-28, and P5014, respectively. The first number, n, is the total number of EO groups and the second, m, is the number of THF groups of the copolymer. The triblock copolymers were produced by Akzo Nobel Surface Chemistry, by ethoxylating PTHF polymers of molecular weights of 1000 g/mol and 2000 g/mol, respectively. The latter were purchased from BASF. The copolymer purification procedure, the method used to determine the average n/m ratio (NMR), and cmc of the copolymers (dye solubilization and absorption measurements) have previously been described.24 The polydispersity index Mw/Mn of the copolymer samples is according to the manufacturer between 1.1 and 1.2. A summary of the properties of the copolymers used is given in Table 1. Pyrene (Aldrich) and dimethylbenzophenone (DMBP) (Aldrich 99%) were twice recrystallized from ethanol. The silica particles (Geltech) used as substrate in the fluorescence decay measurements of the surface aggregates had a mean diameter of 0.5
((0.1) µm and density of 2 g/cm3. The size (curvature) of the particles is very important. In an earlier publication, we pointed out that surface aggregates of copolymers do not form on particles, which have a size that is comparable to that of the surface aggregates.23 Macroscopic silica surfaces were used as substrates in the ellipsometric study as well as in the AFM study. These were prepared by the following procedure: Polished silicon wafers purchased from Okmetic Ltd (p-type, boron-doped, resistivity 1-20 Ω‚cm) were oxidized thermally in oxygen atmosphere at 920 °C for ≈1 h, followed by annealing and cooling in an argon flow. This procedure rendered a SiO2 surface layer with a thickness of 300 Å. The oxidized wafers were then cleaned according to the procedure described in ref 24. Before use, the surfaces were dried under vacuum, 0.001 mbar, and then treated in a plasma cleaner (Harrick Scientific Corporation, model PDC3XG) for 5 min prior to the start of the measurement. All aqueous solutions were prepared from Millipore water. Time-Resolved Fluorescence Quenching. Fluorescence decays from pyrene solubilized in the copolymer aggregates were recorded with the single-photon counting technique. The experimental setup has been described in detail elsewhere.25 The width of the laser pulse was short enough not to make deconvolution necessary. The excitation wavelength was 325 nm, and the emission from the probe was selected using a band-pass filter (400 ( 5 nm). The emission from samples containing silica particles where detected front-face. For a detailed description of problems encountered with fluorescence measurements on silica, see ref 3. Sample Preparation. Samples for the fluorescence decay measurements at the silica surface were prepared by adding a certain amount of a stock solution of pyrene in ethanol. After evaporating the ethanol the pyrene was redissolved in the aqueous copolymer solution. The copolymer solution was mixed with a sonicated silica particle suspension to give the desired bulk copolymer concentrations. The concentration of free copolymers was estimated from the adsorption isotherms measured by ellipsometry. The samples that were prepared for bulk aggregate studies (i.e. without silica particles) were prepared in a similar manner. Stock solutions of pyrene and DMPB in ethanol were evaporated and redissolved in the aqueous copolymer solution. Before measurements, all samples were continuously shaken for 24 h to allow for equilibration. Atomic Force Microscopy. Images were obtained with a Nanoscope III atomic force microscope (Digital instruments, CA) using silicon nitride cantilevers (Digital Instruments, CA) with spring constants of 0.21 ( 0.02 nm-1. The spring constant was determined from resonance frequency measurements of loaded and unloaded cantilevers. The cantilevers were treated in a plasma cleaner (Harrick Scientific Corporation, Model PDC-3XG) for 5 min immediately before use. Measurements were performed in contact mode, in situ, in an AFM liquid cell (Digital Instruments, CA). The images presented are deflection images (showing the error in the feedback signal) with low integral and proportional gains and a scan rate of approximately 10 Hz. The only image filtering performed on the images was the linear subtraction from each line inherent in the measurement. All measurements were performed in the temperature range 23 ( 2 °C with single-phase micellar solution of block copolymer. Before images were captured each substrate solution combination was left to equilibrate for at least 30 min and was monitored for at least 2 h. Systematic changes in structure were not observed during this period. Each copolymer system was investigated at least three times to ensure that representative images were obtained. Imaging was performed at a force and separation at which the force on the tip was dominated by the adsorbed copolymers and thus the image provides information about the adsorbed layer topography.19,26 Forces-distance curves were also measured between the tip and the surface. These were also obtained with the use of the Nanoscope III and analyzed as described previously.15,21 It is important to note that the zero of separation is defined to occur (25) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (26) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413.
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when the gradient of force has a high constant (and negative) value, which implies that the tip is in contact with the sample. A very strongly adsorbed film or film that thins very slowly with increasing force may be overlooked and therefore there may be an error in evaluating the zero separation for each complete measurement of force as a function of separation. The zero of force is defined to occur when the gradient in force is very low at a large separation. Data Analysis of Time-Resolved Fluorescence Quenching Measurements. In systems of small micelles with narrow size distributions, fluorescence decays from probes solubilized in the micelles together with quenchers are usually well described by the Infelta-Tachiya equation27,28
F(t) ) A1 exp{-A2t - A3[1 - exp(-Art)]}
(1)
where A1 ) F(0) is the amplitude of the decay curve. In the case of a poissonian distributed quencher, stationary in the micelles during the experimental sampling time (ca. 10 τ0), A2 ) τ0-1, where τ0 is the fluorescence lifetime in the absence of quencher, A3 ) 〈n〉 , the average number of quenchers per micelle, A4 ) xkq, the quenching frequency in micelles containing x quenchers. In eq 1, the deactivation of the probe in a micelle containing x quenchers is described as a pseudo-first-order reaction with a decay rate τ0-1 + xkq. For t . (kq)-1, where the contribution to the intensity from micelles containing quenchers can be neglected, the decays can be described by
Ftf∞(t) ) F(0)e-〈n〉e-t/τ0
(2)
The surfactant aggregation number, N, is obtained from 〈n〉 using the relation
N)
Cs,m 〈n〉 Cq,m
(3)
where Cs,m and Cq,m are the concentration of surfactant and quencher, respectively, present in the aggregates. In the present paper, where the bulk micelles are voluminous, the description of the intramicellar quenching in eq 1 is expected to break down. However, once the “tail” of the decay can be described by eq 2, 〈n〉 can be unambiguously determined. This is because 〈n〉 , which is directly related to the fraction of quencherfree micelles, is independent of the quenching kinetics. With a poissonian distributed quencher the fraction of quencher-free micelles simply equals e-〈v〉. Under such conditions, eq 1 may still give a reasonable description of the curves, but the physical meaning of A4 becomes obscure. If an attempt to fit eq 1 fails, 〈n〉 can be determined from an estimate of F(0) and a fit of eq 2 to the tail of the curve. The pyrene lifetime (τ0) in the polymer solutions was determined in separate experiments by fitting a single-exponential function to the curves recorded in the absence of quencher (DMBP). The quenched decays were analyzed with eq 1. Here A1, A3, and A4 were fitted using the nonlinear LevenbergMarquardt routine, keeping A2 ) τ0-1 fixed. For the copolymer aggregates at the silica surface, τ0 was estimated for samples with approximately 1 pyrene molecule per 50 aggregates. Scattering of the laser pulse perturbed the initial part of the decay. Despite this, τ0 could still be estimated from the data in the tail of the decay curves. To avoid the influence of scattering on the quenched curves it was necessary to use pyrene excimer (self-) quenching instead of a separate quencher. While improving the fluorescence-to-scattering ratio considerably, this is associated with at least two problems. First, in systems of small micelles the “high” concentrations of pyrene necessary to observe quenching may have an influence on the surfactant self-assembly and on the distribution of the pyrene among the micelles. (The latter can give deviations from the Poisson distribution).29,30 In the present systems, these effects can be ruled out. Second, and more relevant here, is the excimer(27) Infelta, P. P.; Gra¨tzel, M.; Thomas J. K. J. Phys. Chem. 1974, 78, 190-195. (28) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (29) Bales, B. L.; Stenland, C. J. Phys. Chem. 1993, 97, 3418-3433.
Figure 1. Simulated monomer fluorescence decay (normalized by e-τ/τ0) from pyrene in micelles. The average number of pyrene per micelle, 〈n〉 , is equal to 0.5. F(0) ) 50000, the natural lifetime of the monomer and the excimer is 270 ns () τ0) and 100 ns, respectively, and the rate constants for excimer formation and dissociation are 106 s-1 and 4 × 106 s-1, respectively. The simulated curve is the total “intensity” from micelles containing 1, 2, 3, etc. pyrene molecules. Also shown is eq 1 (normalized by e-τ/τ0), with F(0) ) 50464, 〈n〉 ) 0.513 (A3), and kq ) 7.78 × 105 s-1 (A4), and the weighted residuals (wres ) F(t)sim - F(t)fit/ {F(t)sim}1/2), as obtained from a weighted fit to the simulated data. to-monomer back reaction which will regenerate excited monomers.31 In systems with a rapid formation of the excimers an analysis using eq 1 is expected to give small errors if there is, on the average, less than one pyrene per micelle.32 However, for the large surface aggregates investigated here, the formation will take place on the same time scale as the back-reaction. To investigate the effect of this on the determination of 〈n〉 (the average number of pyrene molecules per micelle) we simulated the monomer fluorescence decay from the full excimer reaction scheme described by Infelta and Gra¨tzel, see Figure 1.33 The rate constant for the dissociation of the excimer was assumed to be 4 × 106 s-1, in agreement with their experimental results. The other rate constants (see figure legend) were chosen to produce a decay resembling the ones observed in the present study. The result from a fit of eq 1 to the simulated data shows, as expected, that the quenching kinetics is not perfectly captured by the Infelta-Tachyia model. Nevertheless, the estimated value of 〈n〉 deviates only by 2.5% from the true value. To isolate the effect of quenching from the natural decay the curve, in Figure 1, is normalized by the natural decay law, i.e., F(t)/e-τ/τ0 is plotted instead of F(t). The calculation of the surface-aggregation numbers for the different copolymers are based on their adsorption isotherm, it is assumed that all the adsorbed copolymers participate in surface aggregates.
Results Adsorption Isotherms. The adsorption isotherms of the Pn-m copolymers on silica were measured in situ by null-ellipsometry. The isotherms measured for various copolymers on silica are plotted in Figure 2. These have been discussed in detail in previous work, so we will here only give a brief summary.23 The surface excess of the (30) Almgren, M.; Hansson, P.; Wang, K. Langmuir 1996, 12, 38553858. (31) Almgren, M. In Kinetics and Catalysis in Microheterogeneous Systems; Gra¨tzel, M., Kalyanasundaram, K., Eds.; Marcel Dekker: New York, 1991. (32) Zana, R. In New Methods of Investigation; Zana, R., Ed.; Marcel Dekker: New York, 1987; pp 241-294. (33) Infelta, P. P.; Gra¨tzel, M. J. Chem. Phys. 1979, 70, 179.
Self Aggregation of Triblock Copolymers
Figure 2. Adsorption isotherms obtained by ellipsometry for the copolymers P50-14, P146-28, P224-28, respectively. The lines are only drawn to guide the eye.
copolymers was observed to be rather small for all copolymers in the low bulk concentration region. In fact, the adsorption was more or less identical with the adsorption of poly(ethylene oxide) homopolymers in the same low concentration range, provided the molecular weights of copolymers and homopolymers are similar. As the bulk concentration was increased a pronounced increase of the copolymer adsorption was observed. This effect was particularly clear for the Pn-28 copolymers, which exhibited a strong cooperative adsorption increase more than 2 orders of magnitude below the cmc. This increase was inferred to be engendered by the selfassembly of block copolymers in the interfacial region. The adsorption maximum often observed at intermediate bulk concentrations was interpreted as due to polydispersity.23 No maximum was observed for the P50-14 copolymer, which after a steep adsorption increase seems to reach a relatively stable adsorption plateau. The total adsorbed amount was also smaller for P50-14, compared to the Pn-28 copolymers. Fluorescence Decay Spectroscopy on Silica Particles with Adsorbed Copolymers. The fluorescence decay measurements in silica particles suspensions were performed at bulk copolymer concentrations of 5 × 10-4 wt %, 1 × 10-3 wt %, and 8 × 10-3 wt % for P50-14, P14628, and P224-28, respectively. These measurements were performed to characterize the copolymer surface aggregates in the vicinity of the adsorption maximum for the Pn-28 copolymers and at the beginning of the adsorption plateau for the P50-14 copolymer. The bulk concentrations given above are for all copolymers well below the cmc. Note that when the solvent and the silica particle phases were separated by filtration, only traces of the pyrene probe were observed in the filtrate. Hence, the pyrene molecules were solubilized exclusively in the hydrophobic domains of the surface aggregates. Figure 3 gives examples of the time-resolved fluorescence decay curves which were obtained at a bulk concentration of the P146-28 copolymer equal to 1 × 10-3 wt % in the presence of silica particles. The two curves were measured for systems with pyrene concentrations of 1.5 × 10-7 M and 7.8 × 10-6 M. The upper curve represents the single exponential monomer decay at the low pyrene concentration. At the higher pyrene concentration the decay is
Langmuir, Vol. 15, No. 15, 1999 5153
Figure 3. Fluorescence decay curves for surface aggregates of the copolymer P146-28 at a concentration of 8 × 10-3 wt %. The two curves correspond to different concentrations of the pyrene probe 1.5 × 10-7 M (upper curve) and 7.8× 10-6 M (lower curve), respectively.
Figure 4. The reduced fluorescence decay curves for surface aggregates of the copolymers P50-14, P146-28, P224-28, respectively.
initially quenched due to the excimer formation. However, at larger times the two curves become parallel which according to eq 2 is indicative of quenching in discrete aggregates. Another way to illustrate this is to use reduced fluorescence decay plots, where all points in the excimerquenched curve are divided by the best fit value to the single-exponential curve. Figure 4 shows such reduced fluorescence decay curves measured for the different copolymers in the presence of silica particles. For these copolymers, the reduced intensities seem to approach a constant level at long times. This indicates that discrete copolymer aggregates are formed at the silica surface. The calculated aggregation numbers are presented in Table 2 together with data on the lifetimes of the pyrene probes.
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Table 2. Fluorescence Decay Measurement at the Silica Surface polymer P50-14 P146-28 P224-28 aSurface
free polymer concn (wt %) 10-3
1× 8 × 10-3 5 × 10-4
adsorbed amt (mg/m2)
Na
calcd area per aggreg (nm2)
lifetime of probe (ns)
0.7 2.65 1.73
75 23 59
570 121 605
273 271 266
aggregation number.
Fluorescence Decay Spectroscopy on Micellar Solutions of Copolymers. The fluorescence decay measurements on micellar solutions were performed at two different concentrations equal to 0.5 and 2.0 wt %. Concentrations lower than 0.5 wt % could not be studied due to the low concentration of copolymer micelles. The reduced fluorescence decay curves for the different copolymers are displayed in Figure 5. Observe that the curves measured for the 2 wt % samples of the P146-28 and P50-14 copolymers clearly do not reach a plateau. In principle, this means that the lifetime of the pyrene probe is shorter then the time needed to reach the plateau region, despite the fact that dissolved oxygen had been removed from the sample, which results in an increased lifetime of the pyrene probe. The fact that the plateau is not reached indicates that the hydrophobic micelle cores are fairly large. Thus, we cannot distinguish between fragmented or continuous aggregates. Note that this does not imply that we have any other phase than a micellar solution. We have in previous investigations seen that the micellar phase region is very large for these copolymers.34 The number-averaged aggregation numbers calculated by means of the Infelta model are presented in Table 3 together with the lifetime of the pyrene probes. Atomic Force Microscopy. In this work, we also performed atomic force microscopy measurements on macroscopic silica plates to further investigate the adsorption phenomena of these copolymers. The atomic force microscopy technique enables direct observation of surface aggregates and provides some information on the stability and thickness of adsorbed layers. The lateral contrast in an AFM image originates from variations in the tip position normal to the surface. In capturing these images great care was taken to ensure that the surface was scanned at an appropriate force so that the topography of the adsorbed layer is represented in the image. Before discussing this further, we shall look at a typical forcedistance curve measured between an AFM tip and smooth macroscopic silica surface in the presence of a 0.1 wt % P224-28 solution (Figure 6). The reason for using such a high concentration was that the surface-to-volume area in the AFM cell is small with adsorption to air, silicon rubber, and silica surfaces, which makes an estimate of the bulk concentration difficult at low concentrations. Furthermore, with a high copolymer concentration we need not to bother about slow adsorption kinetics. The force-distance curve in Figure 6 clearly shows the presence of an adsorbed layer at the silica surface. As the tip approaches the surface, the force increases from a separation of about 18 nm almost in to hard-wall contact (defined as zero distance). This increase in force is due to steric interactions between the tip and copolymers adsorbed at the surface. Note that there may very well be some copolymers adsorbed on the tip. At separations shorter then 2.5 nm, we observe that the tip is pushed through a high-density region of the copolymer film. From 2.5 nm to hard-wall contact, the tip moves without an increase in the normal force. The general features of the force-distance curve agree well with those previously (34) Holmqvist, P.; Nilsson, S.; Tiberg, F. Colloid Polymer Sci. 1997, 275, 467.
measured between silica spheres immersed in copolymer solution by the interfacial gauge technique.22 Note, however, that the contact area between the tip and the surface is much smaller in an AFM measurement than in the Interfacial Gauge setup. Figure 7 shows adsorbed layers of the P146-28 and P22428 copolymers. Both images are obtained at a concentration of 0.1 wt %, which in both cases is well above the bulk cmc. Light areas in these images represent “high” regions or regions of larger repulsive force, i.e., higher density of copolymers, whereas dark areas indicate low copolymer densities. The two images confirm the presence of copolymer aggregates at the silica surface. The aggregates appear, however, to be very different in size and shape. P146-28 (Figure 7a) form small globular surface aggregates that are rather uniformly distributed over the surface. P224-28, on the other hand, form more extended and distorted structures. These aggregates seem to be larger and more irregularly distributed over the surface. A very large range of interaggregate distances was observed on different parts of the same substrate for the P224-28 copolymer. This may be interpreted as either a massive range in aggregate size or, more likely, large areas of the surface with fewer adsorbed aggregates. This large period range makes it difficult to characterize the size of the P224-28 aggregates, and therefore only qualitative conclusions should be drawn from the AFM image shown. This agrees, however, reasonably well with the picture of the surface aggregates obtained from the fluorescence decay experiments. Discussion Our study shows that both fluorescence spectroscopy and atomic force microscopy can be used to investigate the nature of adsorbed copolymer layers at solid surfaces. Information about the in-plane structure of adsorbed layers and aggregation numbers of surface (and bulk) aggregates are important for understanding adsorption at hydrophilic oxide surfaces. Such data cannot be obtained with techniques such as light and neutron reflectometry, surface force measurements, and ellipsometry, which have become common tools used for adsorption studies. The fluorescence decay data presented in Table 3 illustrates how the bulk aggregation number varies for the different copolymers. For copolymers P146-28 and P5014, the aggregation numbers increase dramatically as the concentration is raised from 0.5 to 2 wt %. Probably reflecting a transformation of the aggregate shapes from spherical too more distorted structures.35 No such increase was observed for the P224-28 copolymer. This finding seems reasonable if one considers the fact that this copolymer has much larger PEO end-blocks. These will, due to mutually repulsive steric interactions, collectively favor spherically shaped copolymer micelles.36 It is worth noting that the aggregation number measured for the P224-28 copolymer agrees relatively well with the value previously reported for this copolymer by Holmqvist and (35) Brown, W.; Schillen, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem 1991, 95, 1850-1858. (36) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145-4159.
Self Aggregation of Triblock Copolymers
Langmuir, Vol. 15, No. 15, 1999 5155 Table 3. Fluorescence Decay Measurement in Bulk Solution polymer
polym concn (wt %)
Na
lifetime of probe (ns)
P50-14 P50-14 P146-28 P146-28 P224-28 P224-28
2.0 0.5 2.0 0.5 2.0 0.5
300 147 128 82 93 89
375 352 375 375 372 369
a
Aggregation number in bulk.
Figure 6. Force vs separation curve for the copolymer P22428 at 0.1 wt % bulk concentration. The steric barrier due to the adsorbed layer can first be detected at approximately 18 nm form the surface. As the separation is decreased, the force steadily increases up to a yield point where the mechanical instability of the film is reached (ca. 2.5 nm from the surface). Such a surfactant curve provides a range of imaging positions. The images shown in Figure 7 were captured near zero applied load. If images are captured at higher force the film is often disrupted by the presence of the AFM tip.
Figure 5. The reduced fluorescence decay curves for the bulk micelles at concentrations 0.5 and 2 wt %. a; copolymer P50-14. (b) Copolymer P146-28. (c) Copolymer P224-28.
co-workers.34 Who from light scattering data estimated the weight averaged aggregation number for P224-28 to be about 105. The main aim of this study was to confirm the presence of surface aggregates on silica surface and characterize some of their properties. The adsorption and surface selfassembly of triblock copolymers at the silica-water interface differ in some aspects from the micellization process in bulk solution even if both processes are driven by the same mechanisms, i.e., hydrophobic attraction induced by the low solubility of THF groups in water. We observed that the aggregation numbers of adsorbed aggregates at the silica surface were significantly smaller than observed for the corresponding bulk micelles. This trend holds for all copolymers studied. This seems not to be in agreement with previous findings for short chain nonionic surfactants. Levitz and co-workers found that the compositions of bulk and surface aggregates of nonionic surfactants were more or less identical, and this was then confirmed by Tiberg et al. also for mixtures of nonionic surfactants.3,10 In contrast to the results obtained for nonionic surfactants, we found rather large discrepancies between the properties of bulk and surface aggregates for the copolymer systems. This difference is attributed to the much larger effective interactions between individual copolymers and the surface than between individual surfactants and the surface. These interactions decrease the size of surface aggregates compared to bulk micelles. Further analysis of our data is complicated by the polydispersity of the copolymer samples. For instance,
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if the strong binding of individual copolymers prevent the necessary collective rearrangements of copolymers at the surface. This rearrangement can be further restrained by obstruction of already formed surface aggregates. We have seen earlier that poly(ethylene oxide) copolymers with a molecular weight of more than 4000 easily can displace, for instance, surface aggregates of pentaethylene glycol monododecyl ethers.37 The adsorption isotherms for the copolymers (Figure 2) show that the initial adsorption of monomers is high for the copolymer P224-28 compared to the P146-28 at low bulk concentrations. At higher concentrations the adsorption of the copolymer P224-28 shows a less-pronounced cooperative increase. Moreover, the maximum adsorbed amount is smaller for P224-28 than for P146-28. At higher concentrations the adsorbed amount begins to decay, probably due to displacement of smaller more hydrophobic copolymer fractions by copolymers with long EO chains. If the relative EO chain length is long enough, the copolymer will behave as a homopolymer and no surface aggregation will occur. For the P22428 copolymer, the argument is supported by the observation that relatively large surface areas are seemingly depleted from aggregates (Figure 7b). We infer that these areas are covered by nonaggregated copolymers, which prevent other copolymer fractions to adsorb and selfassemble at the surface. Furthermore, as can be seen in Table 2, the surface aggregate size decreases when going from P50-14 to P14628, but increases when the copolymer size is increased from P146-28 to P224-28. Today, it is not clear how much of this increase that is due to the fact that an unknown fraction of the P224-28 copolymer was adsorbed as monomers which leads to an overestimation of the calculated aggregation number. We conclude that polydispersity effects complicate the understanding of adsorption of copolymers at hydrophilic surfaces. Most trends observed could nevertheless be qualitatively understood by looking at the adsorption process as a competition between copolymer-surface interactions and cooperative copolymer-copolymer interactions, which promote surface self-assembly. Conclusion
Figure 7. AFM images of the copolymers P146-28 and P22428 at the silica aqueous solution interface. (a) The P146-28 copolymer exhibited the micelle-like aggregates with a regular interaggregate spacing (ca. 17-25 nm) and homogeneous adsorption on all parts of the surface. (b) Adsorbed layers of the P224-28 copolymer produced a wide range of interaggregate spacing. The majority of the surface was covered with aggregates of a similar size to those in the image presented here, while there were large areas of the surface with observable aggregates spaced up to 160 nm apart. One of the probable explanations for this observation is that larger adsorbed copolymers may inhibit aggregate formation.
when the size of the PEO block increases for a constant size of the hydrophobic PTHF block, this leads to stronger interactions between the end-chains and the surface. For copolymers with large PEO blocks, the overall interactions between EO segments and the surface may be more energetically favorable than an aggregate of copolymers. There may also be a kinetic barrier for aggregate formation
Our results show that all copolymers studied in this work tend to form discrete surface aggregates at the silicawater interface. In this study, we have used new tools to study the surface aggregation of copolymers. Previously, our findings were based on measured adsorption isotherms and layer thicknesses measured by ellipsometry, which provided indirect evidence for the existence of surface aggregates.23 We observed that the shape and size of the surface aggregates depends both on the chemical composition of the copolymers, the molecular weight, and the polydispersity. This dependence is clearly nontrivial. Some qualitative conclusions can, however, still be drawn. For instance, the surface aggregates in general have much smaller aggregation numbers than the corresponding bulk micelles. We argue that this is due to the copolymersurface interactions, which oppose the copolymercopolymer attraction leading to self-assembly. We note that large areas of the surface covered by adsorbed P224-28 appear to be depleted from surface aggregates. The reason for this is probably that larger adsorbed copolymers prevent surface aggregate formation, due to their favorable interaction with the surface already as monomers. Polydispersity issues complicate the in(37) Tiberg, F. Unpublished results.
Self Aggregation of Triblock Copolymers
terpretation of our results, and more studies are needed for a good understanding of the change in aggregate size with copolymer composition and polydispersity. Acknowledgment. The Swedish National Board for Industrial and Technical Development (NUTEK) and the
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Swedish Research Council for Engineering Sciences (TFR) are acknowledged for financial support.
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