1884
Langmuir 1997, 13, 1884-1886
Adsorption Kinetics of an Asymmetric Diblock Copolymer: A Surface Forces Apparatus Study Eric Pelletier, Amalia Stamouli, Gerald F. Belder, and Georges Hadziioannou* Department of Polymer Chemistry and Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received September 11, 1996. In Final Form: January 23, 1997X We investigate the formation of ultrathin layers of a diblock copolymerspolystyrene (PS)/poly(2vinylpyridine) (P2VP)sadsorbed from a dilute solution by means of a surface forces apparatus. A simple model, that takes into account the thickness of the P2VP layer and the osmotic pressure within the PS layer, is used to analyze the force-distance curves. An absolute value of surface coverage is determined by comparison with AFM measurements. The volume fraction of P2VP in the anchoring layer is calculated and does not display noticeable change with the incubation time. In addition, the surface coverage variation with the incubation time is compared with theoretical models.
Introduction Much attention has recently been devoted to diblock copolymer layers adsorbed onto solid surfaces from a selective solvent.1,2 One block adsorbs onto the surface and anchors the other block which extends far into the solution. The interest in this system is due to its many possible applications such as steric stabilization of colloidal suspensions or modification of surface friction properties. The structure and the formation of such layers has been theoretically worked out.3-8 In this paper we are interested in studying the structure and the formation of an adsorbed layer of diblock copolymers (P2VP/PS) on mica from a very dilute solution (below the cmc) by means of a surface forces apparatus (SFA). Numerous experimental studies have been performed on this system with various techniques: radiolabeled polymers,9-11 angle-resolved XPS,12 neutron reflectivity,13,14 hydrodynamic studies,15,16 surface plasmons,17 AFM,18,19 and SFA.12,13,20-23 In all these studies, the X
Abstract published in Advance ACS Abstracts, March 1, 1997.
(1) Szleifer, I.; Carignano, M. A. Adv. Chem. Phys. XCIV 1996, 165. (2) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1991, 100. (3) Marques, C.; Joanny, J. F.; Leibler, L. Macromolecules 1988, 21, 1051. (4) Johner, A.; Joanny, J. F. J. Phys. II 1991, 1, 181. (5) Johner, A.; Joanny, J. F. Macromolecules 1990, 23, 5299. (6) Baranowski, R.; Whitmore, M. D. J. Chem. Phys. 1995, 103, 2343. (7) Whitmore, M. D.; Noolandi, J. Macromolecules 1990, 23, 3321. (8) Ligoure, C.; Leibler, L. J. Phys. 1990, 51, 1313. (9) Elaissari, A.; Haouam, A.; Huguenard, C.; Pefferkorn, E. J. Colloid Interface Sci. 1992, 149, 68. (10) Huguenard, C.; Varoqui, R.; Pefferkorn, E. Macromolecules 1991, 24, 2226. (11) Pefferkorn, E.; Elaissari, A.; Huguenard, C. Macromol. Rep. 1992, A29 (suppl 2), 147. (12) Parsonage, E.; Tirrell, M.; Watanabe, H.; Nozzo, R. G. Macromolecules 1991, 24, 1987. (13) Cosgrove, T.; Philipps, J. S.; Richardson, R. M.; Hair, M. L.; Guzonas, D. A. Macromolecules 1993, 26, 4363. (14) Field, J. B.; Toprakcioglu, C.; Dai, L.; Hadziioannou, G.; Smith, G.; Hamilton, W. J. Phys. II 1992, 2221. (15) McKenzie, P. F.; Webber, R. M.; Anderson, J. L. Langmuir 1994, 10, 1539. (16) Webber, R. M.; Anderson, J. L. Langmuir 1994, 10, 3156. (17) Tassin, F. J.; Siemens, R. L.; Tang, W. T.; Hadziioannou, G.; Sxalen, J. D.; Smith, B. A. J. Phys. Chem. 1989, 93, 2106. (18) Stamouli, A.; Pelletier, E.; Koutsos, V.; Van der Vegte, E.; Hadziioannou, G. Langmuir 1996, 12, 3221. (19) Meiners, J. C.; Ritzi, A.; Rafailovich, M. H.; Sokolov, J.; Mlynek, J.; Krausch, G. Appl. Phys. A 1995, 61, 519. (20) Hadziioannou, G.; Patel, S.; Granick, S.; Tirrell, M. J. Am. Chem. Soc. 1986, 108, 2869. (21) Watanabe, H.; Tirrell, M. Macromolecules 1993, 26, 6455. (22) Tirrell, M.; Parsonage, E.; Watanabe, H.; Dhoot, S. Polym. J. 1991, 23, 641. (23) Patel, S.; Tirrell, M.; Hadziioannou, G. Colloids Surf. 1988, 31, 157.
S0743-7463(96)00884-0 CCC: $14.00
thickness of the P2VP layer has been considered as very thin and negligible in comparison with that of the PS one. Our aim is to show that the force-distance profile obtained by means of a SFA can furnish information about the structure of the anchoring (P2VP) layer. In addition, we show that a SFA can be used to follow the kinetics of adsorption despite the fact that other techniques are certainly more suitable for this purpose. Experiments We consider a layer of polystyrene (MWPS ) 75 000 g‚mol-1)/ poly(2-vinylpyridine) (MWP2VP ) 102 000 g‚mol-1) with a polydispersity index of 1.12, adsorbed onto mica surfaces from a dilute toluene solution (0.05 mg‚ml-1). The number of segments in each block of PS and P2VP is NPS ) 721 and NP2VP ) 971. The concentration is below the critical micelle concentration.24 The temperature is kept at T ) 21.0 ( 0.1 °C. The copolymer can 6/5 2/3 be defined by its asymmetry ratio,12 β ) NPS /NP2VP ∼ 27, as moderately asymmetric. The Flory radius of the PS block, which 0.595 is well-solvated, is25 RPS ) 1.86NPS ) 8.9 nm. The P2VP is poorly solvated in toluene. Its radius of gyration is defined12 as RP2VP ) a(3/4πNP2VP/c)1/3 where a is the segment size (a is taken as 0.7 nm for both P2VP and PS) and c is the P2VP volume fraction. The adsorption process occurs inside the liquid cell with the mica surfaces kept well-separated (1 mm). Force-distance measurements are periodically performed to monitor the variation of the force profile with the incubation time (Figure 1). The time of a scan is around 1/4 h.
Discussion When the solution is in contact with the mica surfaces, the P2VP blocks adsorb preferentially onto the mica surfaces. They form a thin layer of thickness e which may be plasticized by the solvent. The PS is soluble in toluene and does not adsorb onto the mica. The PS top layer can be found in different regimes ranging from the mushroom (isolated PS blocks) to the brush (overlapping PS blocks), depending on the respective sizes of the blocks.14 The diblock copolymer used in this study is known to form an intermediate structure, between the above-mentioned regimes, in which the PS blocks just overlap.18 The use of the SFA to follow the adsorption kinetics is challenging, since the measurements are likely to perturb the adsorption processsthe mica surfaces have to be brought into contact and separated.26-28 Thus, this (24) Tang, W. T. Ph.D. Thesis, Stanford University, 1987. (25) Higo, Y.; Ueno, N.; Noda, I. Polym. J. 1983, 15, 367. (26) Luckham, P. F.; Klein, J. J. Chem. Soc., Faraday Trans. 1990, 86, 1363.
© 1997 American Chemical Society
Letters
Langmuir, Vol. 13, No. 7, 1997 1885
Figure 1. Force divided by the mean radius of curvature of the surfaces, F/R, as a function of their separation, D. The measurements have been performed for different incubation times: (9) 2 h; (O) 20 h; (b) 120 h.
Figure 2. Thickness of the P2VP layers, 2e (O), and coefficient proportional to the number of molecules per surface area, AΓ (0), as a function of the incubation time, t.
technique has time limitations which explain the lack of data for short adsorption times. In this study, we assume that we do not perturb the adsorption process too much, since only a few measurements have been performed. In analyzing the force-distance profiles, we have chosen to consider only the strongly repulsive part where the layers are strongly confined. In this part, the osmotic pressure term dominates the measured force.29 The contribution of the P2VP layer to the force profile is considered to be negligible. Thus, we can write the following relationship by considering the thickness, 2e, of the two P2VP layers:3,29
F ) (AΓ)9/4(D - 2e)-5/4 R
(1)
where Γ is the number of molecules per surface area, A is a constant term, R is the average curvature radius of the mica surfaces, and D is the closest distance between the mica surfaces. The choice of eq 1 to model the data is imposed by the aim of analyzing the force-distance curves with as few assumptions as possible concerning the chain conformation. The force-distance curves have been fitted by a rootmean-square method, (AΓ) and 2e being treated as free parameters. Figure 2 shows the variations of AΓ and 2e as a function of incubation time. AΓ increases monotonously whereas 2e varies from 6 nm at short times to a value close to 16 nm at longer times. In order to obtain an absolute value of Γ, we have compared our data obtained for 72 h with the result obtained by AFM for the same incubation time. This procedure allows us to define A and also Γ(t). A4/9/kbT can be compared with the expected theoretical value, ∼8/5πKa15/4, given in ref 29, and applied into eq 1, where kb is the Boltzmann constant, T is the temperature, and K is a universal prefactor estimated to be (1/16)(2π)3/2.30 Numerical estimation gives 1.30 nm15/4, which can be compared to the experimental value of 1.25 nm15/4. The agreement between these data supports the use of eq 1. (27) Klein, J.; Luckham, P. F. Nature 1984, 308, 836. (28) Motschmann, H.; Stamm, M.; Toprakcioglu, C. Macromolecules 1991, 24, 3681. (29) Fredrickson, G. H.; Pincus, P. Langmuir 1991, 7, 789. (30) Broseta, D.; Leibler, L. Europhys. Lett. 1986, 2, 733.
Figure 3. Plot of Γ/Γ0 as a function of time: (b) AFM data from ref 18; (O) SFA measurements.
We now focus on the time dependence of Γ, which shows the increase of the number of molecules involved in the adsorption process with the incubation time (Figure 3). The overlap threshold of the PS blocks can be defined as 2 ) ) 4.02 × 10+15 m-2. Γ(t)/Γ0 varies from 0.9 Γ0 ) 1/(πRPS to 2.3. Tassin et al.17 have shown that, in this range, the PS chains are just weakly stretched. It demonstrates that our system is just at the transition between a brush and a mushroom regime. In order to get more information about the P2VP layer, we can use a relationship3 which connects the number of chains per surface area to the thickness of the P2VP layer:
NP2VPa3Γ ) ce
(2)
where c is the volume fraction of P2VP in the anchoring layer. Figure 4 shows that c (∼0.4) does not change in an appreciable way with the incubation time. The value of c is not too far from the macroscopic value, 0.5, obtained by Parsonage et al.12 with P2VP of comparable molecular weight. The constant value of c is certainly connected to the pseudobrush regime of this copolymer, since variations are expected in the case of a transition from a mushroom to a brush regime. The time dependence of Γ should reveal different kinetic processes.5,8 In the first step of the incubation, the process is governed by the diffusion of the copolymers toward the
1886 Langmuir, Vol. 13, No. 7, 1997
Letters
Figure 4. Plot of c, the volume fraction of P2VP in the anchoring layer, as a function of the incubation time.
surface. However, this process is fast and cannot be followed by a SFA. The second process can be connected to an excluded surface area effect. The anchor blocks have to find area available for adsorption. The third process corresponds to the crossing of the PS layer by the free copolymers to reach the surface. The PS layer can be regarded as a potential barrier which slows down the rate of adsorption. This third process has been theoretically analyzed and a semilogarithmic dependence of Γ with the incubation time is predicted. In this paper, we have chosen to apply the Johner and Joanny model,5 since the theory developed by Ligoure and Leibler8 requires the knowledge of the surface coverage at full equilibrium. The variation of the surface coverage is described by the relation5
kBT(Γa2)1/6 (-NPS(Γa2)5/6) dΓ φe )dt 6πηN a2
(3)
PS
where η is the solvent viscosity and φ is the number of chains per unit volume in the solution, at the edge of the brush. The integration of eq 3 leads to a semilogarithmic dependence of Γ5/6 with the incubation time: -5/3 ln(t) Γ5/6 = B + N-1 PS a
(4)
where B is a constant term. In the integration, we have neglected to account for the first steps of the adsorption process. Figure 5 displays a representation of eq 4 and shows that the predicted linear dependence is obtained. The
Figure 5. Plot of Γ5/6 as a function of ln(t). The full line is the root-mean-square fit.
slope of the curve, determined by a root-mean-square method, is 2.04 × 10-3 nm-5/3 and is in the range of the -1 -5/3 expected theoretical value NPS a ) 2.51 × 10-3 nm-5/3. The good agreement between theory and experiment shows that, in the range of time where the experiments have been conducted, the variation of the surface coverage is certainly dominated by the diffusion-convection process. More precise application of eq 3 requires the knowledge of the surface coverage at short times, which are not accessible with the SFA. Conclusion We have presented a simple way to use force-distance profiles to analyze the conformation of an adsorbed diblock layer. The effects of the adsorbed blocks and those of the blocks extending into the solution have been separated. The kinetic study shows that the P2VP layer is thickening as more chains are adsorbed onto the surface. In addition, the collected data show that the volume fraction of P2VP chains inside the anchoring layer does not change during the adsorption process. The time dependence of the surface coverage has been compared with a theoretical model accounting for the barrier of potential created by the swollen block which slows down the rate of adsorption. These measurements show the possibility to study the anchoring layer through force-distance measurements. It also shows that kinetics studies can be performed with a SFA. Acknowledgment. This research was supported by the Netherlands organization of technology (SON-STW) and the HCM-Network “Functional Materials Organized at Supramolecular Level”. LA960884T
