Langmuir 1997, 13, 2993-3001
2993
A Comparison of Polystyrene-Poly(ethylene oxide) Diblock Copolymer and Poly(ethylene oxide) Homopolymer Adsorption from Aqueous Solutions Edward S. Pagac, Dennis C. Prieve, Yuri Solomentsev, and Robert D. Tilton* Department of Chemical Engineering, Colloids, Polymers and Surfaces Program, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received September 9, 1996. In Final Form: January 29, 1997X Although amphiphilic polystyrene-poly(ethylene oxide) diblock copolymers (PS-PEO) adsorb from aqueous solutions to hydrophobic surfaces, they are unable to form an end-anchored brush. Four independent techniquessscanning angle reflectometry, hydrodynamic layer thickness measurements, streaming potential measurements, and total internal reflection microscopysindicate that PS-PEO adsorption is indistinguishable from PEO homopolymer adsorption. The surface concentrations attained by three PS-PEO diblocks are independent of molecular weight (between 67 000 and 479 000) and are indistinguishable from PEO (420 000 molecular weight) surface concentrations. Hydrodynamic layer thicknesses and electrokinetic layer thicknesses are on the order of only a few nanometers for both PEO and PS-PEO diblocks, more characteristic of a homopolymer layer than an extended brush. Furthermore, adsorbed PS-PEO imparts no detectable steric repulsion to the energy of interaction between a Brownian particle and a wall. Apparently, brush formation is prevented because the surface affinity of the large watersoluble PEO block presents a large kinetic barrier to its being completely displaced from the surface by the insoluble PS block.
Introduction When surface active polymers are employed in colloidal systems, the adsorbed layers alter surface forces and interfacial hydrodynamics, in a manner dictated by the adsorbed chain conformation. Accordingly, adsorbed layers can control particle flocculation rates or rates of particle deposition on macroscopic surfaces, as occurs in filtration processes. Amphiphilic diblock copolymers adsorb readily from selective solvents to a wide variety of surfaces. Adsorption in these cases is driven primarily by the poor solvent quality for one of the blocks, the “anchor” block. As that name suggests, block copolymer amphiphiles often adsorb in a surfactant-like fashion, wherein the chains are attached by the strongly adsorbed anchor block, while the soluble block extends from the surface as a single tail, forming an extended polymer brush. This has been supported by both experimental observation and theoretical modeling.1-4 The mutual repulsion and large extension of the tails comprising the brush make them especially attractive as steric stabilizers. In contrast to block copolymer amphiphiles, physisorbed homopolymers have no tendency to form brushes. They generally adsorb to a lesser extent and adopt a conformation consisting of loops, trains, and tails. Brushes can be formed by covalently grafted polymers at high surface densities in good solvents.5 Nevertheless, the possibility of forming brushes by physisorption from solution is extremely attractive, since this method offers processing simplicity and applicability to a greater variety of materials. Other studies have demonstrated the ability of high molecular weight block copolymers to spontane* To whom correspondence should be addressed: e-mail,
[email protected]; fax, (412) 268-7139. X Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Munch, M.; Gast, A. P. Macromolecules 1988, 21, 1366. (2) Evers, O. A.; Scheutjens, J. M. H. M.; Fleer, G. J. J. Chem. Soc., Faraday Trans. 1990, 86, 1333. (3) Patel, S. S.; Tirrell, M. Annu. Rev. Phys. Chem. 1989, 40, 597. (4) Webber, R. M.; Anderson, J. L.; Jhon, M. S. Macromolecules 1990, 23, 1026. (5) Taunton, H. J.; Toprakcioglu, C.; Fetters, L. J.; Klein, J. Macromolecules 1990, 23, 571.
S0743-7463(96)00882-7 CCC: $14.00
ously produce highly extended layers by adsorption from nonaqueous selective solvents.1,3,4,6,7 In this investigation we address the ability of large water-soluble, nonionic block copolymer amphiphiles to form brushes by adsorption from water. We compare adsorption of polystyreneb-poly(ethylene oxide) (PS-PEO) diblock copolymers of three different molecular weights and a poly(ethylene oxide) homopolymer (PEO) having a molecular weight between the larger two PS-PEO samples. At room temperature, water is a good solvent for the EO block and a nonsolvent for the styrene block. Given the widespread application of EO-based nonionic surfactants, some of which are polymeric (e.g., the family of ethylene oxidepropylene oxide triblocks available commercially as “Pluronics”), the adsorption behavior of aqueous EO-based block copolymers is especially interesting. We employ four independent experimental techniques to examine the adsorbed amount, layer thickness, and potential for steric stabilization for both PS-PEO and PEO on three different hydrophobic surfaces. We use scanning angle reflectometry to measure polymer surface concentrations on methylated silica and polystyrene surfaces. We employ hydrodynamic and streaming potential measurements to determine the extension of polymer layers adsorbed to the pore walls of track-etched polycarbonate membranes. Finally, we employ total internal reflection microscopy (TIRM) to observe the effect of polymer adsorption on the interactions between a Brownian polystyrene sphere and a methylated glass slide to gain additional insight into the chain conformation. Each measurement is conducted with a surface best suited to that technique: reflectometry, methylated silica and polystyrene; hydrodynamic/streaming potential, polycarbonate; TIRM, polystyrene and methylated glass. While the comparison of the different surfaces may not be ideal, it is necessary due to the particular requirements of each technique. Furthermore, the adsorption of diblock co(6) Field, J. B.; Toprakcioglu, C.; Dai, L.; Hadziioannou, G.; Smith, G.; Hamilton, W. J. Phys. II 1992, 2221. (7) Stamouli, A.; Pelletier, E.; Koutsos, V.; van der Vegte, E.; Hadziiouannou, G. Langmuir 1996, 12, 3221.
© 1997 American Chemical Society
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Table 1. Summary of Polymer Characteristics and Adsorption to Methylated Silica polymer
Mw
PS-PEO67 PS-PEO375 PS-PEO479 PEO420
6.78 × 3.75 × 105 4.79 × 105 4.20 × 105
a
Mw/Mn 104
PEO (wt %)
1.11 1.20 1.20 1.10
88.8 92.0 90.3 100
N(EO) 1368 7841 9798 9545
N(S)
cmc (µg/mL)
Γb (mg/m2)
Rm (nm)
Rgc (nm)
73 287 458
40a
0.35 ( 0.08 0.34 ( 0.10 0.30 ( 0.11 0.52 ( 0.08
10.1 ( 1.1 24.1 ( 4.6 29.1 ( 6.2 20.4 ( 1.8
13.1 36.3 41.4 40.6
23.5 7.8
Value estimated from turbidity. b Measured on methylated silicon wafer. c Rg of PEO block.35
polymer amphiphiles is primarily a solvent-driven process and should be independent of the surface. All results of the current study point to the similarity of the adsorbed PS-PEO and PEO layers on these hydrophobic surfaces. None of the polymers appears able to form an extended brush spontaneously.
a
Experimental Section Materials. We purchased PS-PEO diblock copolymers from Polymer Laboratories and PEO homopolymers from Pressure Chemical. Table 1 lists some characteristics of these polymers. All water was deionized and further purified by the MilliQ Plus System (Millipore). KCl was obtained from Sigma and used without further purification. We prepared polymer solutions by adding the required mass of polymer to water, sonicating a sealed container for 10 min in an ultrasonic bath, and finally heating to 60 °C for 20 min. The final two steps were repeated as needed until the polymer was completely dissolved. The critical micelle concentrations (cmc’s) of the PS-PEO block copolymers at 25 °C, measured using the procedure of Wilhelm et al.,8 are listed in Table 1. The reader is referred to ref 8 for a complete description of this procedure. The technique uses pyrene as a fluorescent probe to detect the onset of micellization. It is based on the absorbance and emission spectral changes that occur upon pyrene solubilization in micelles. The spectral changes are due to the transfer of the pyrene from a polar to a nonpolar environment (i.e. from water to the micelle core). A Perkin-Elmer Model LF5B fluorescence spectrophotometer was used to monitor these changes. As can be seen in Figure 1a for PS-PEO479, a plot of I338/I332.5 vs log CPS-PEO479 is flat in the low polymer concentration region and sigmoidal in the crossover region. The apparent cmc (cmcapp) shown in the graph is an upper bound to the actual cmc. Figure 1b illustrates the determination of the actual cmc. Following Wilhelm et al.8 the ratio (F - Fmin)/(Fmax - F), where F ) I338/I332.5 and Fmax and Fmin are the maximum and minimum values, respectively, is plotted vs concentration yielding two linear regions. It can be seen that this ratio slightly increases with concentration, even for very low concentrations, suggesting a small contribution to the signal due to pyrene interactions with isolated polymer molecules (“unimers”). We find that the slope in this region is similar to those previously found for lower molecular weight PS-PEO diblock copolymers,8 suggesting that pyrene-unimer interactions do not significantly alter the cmc determination for our larger diblocks. The cmc’s are similar to values found in previous work done with PS-PEO diblock copolymers.8 Since PS-PEO solutions became turbid at concentrations slightly above the cmc, all experiments were conducted with 5.0 µg/mL solutions, i.e., below the lowest cmc of the three block copolymers. The silicon wafers used for reflectometry experiments were first oxidized at 1000 °C for 10-30 min in air, yielding 20-100 nm thick oxide layers. We cleaned the oxidized wafers prior to each experiment following the procedure of Furst et al.9 We then methylated the surfaces by vapor phase silanization using trimethylchlorosilane. Silanization was terminated by rinsing in ethanol. This produced a moderately hydrophobic surface having a 54° advancing contact angle for water. The PS films were made by spin-casting a 1 wt % solution of PS (Mw ) 125000150000, Polysciences) in toluene onto a previously methylated (8) Wilhelm, M.; Zhao, C.; Wang, Y.; Xu, R.; Winnik, M. A.; Mura, J.; Riess G.; Croucher, M. D. Macromolecules 1991, 24, 1033. (9) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566.
b
Figure 1. (a) Determination of the apparent cmc for PSPEO479. I338/I332.5 is the ratio of fluorescence emission peak heights at the indicated wavelengths (nm). (b) Determination of the actual cmc. Plot of (F - Fmin)/(Fmax - F) vs C for PSPEO479. Location of cmc is defined as intersection of high concentration curve with C axis. silicon wafer.10 The wafer was then allowed to dry under a heat lamp for 1 h. Hydrodynamic and electrokinetic experiments were performed with track-etched polycarbonate membranes prepared from polycarbonate sheets, obtained from Bayer. The membranes were prepared by the method of DeSorbo.11 This produces membranes with straight cylindrical pores and a narrow pore size distribution with typical diameters on the order of 100 nm. TIRM experiments were conducted using monodisperse, surfactant-stabilized polystyrene spheres 7.04 ( 0.05 µm in diameter. These were purchased from Duke Scientific and used (10) Cheng, Y. L.; Darst, S. A.; Robertson, C. R. J. Colloid Interface Sci. 1987, 118, 212. (11) DeSorbo, W. Nuclear Tracks 1979, 3, 1.
Absorption of PS-PEO and PEO
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with no further treatment. Glass microscope slides were methylated by vapor phase silanization using trimethylchlorosilane yielding an advancing water contact angle of 64°. Reflectometry. The technique of scanning angle reflectometry9,12-15 is based on the dependence of the reflectivity of a composite interface on the interfacial refractive index profile and on the angle of incidence of parallel (p) polarized light. The technique is especially sensitive to changes in the refractive index profile caused by adsorption when the angle of incidence, θ, is close to the Brewster angle,
θB ) tan-1(nt/ni)
(1)
where nt and ni are the refractive indices of the materials containing the transmitted and incident beams, respectively. The overall intensity reflection coefficient is defined as
RP(θ) ≡ Ip(θ)/I0p
(2)
In eq 2, Ip and I0p are the intensities of the reflected and incident p-polarized beams, respectively. By measurement of the reflectivity Rp(θ) of a p-polarized laser beam over a 4.5° range of angles centered around θB, the surface concentration of the adsorbing species may be determined. This is accomplished by regressing measured reflectivity profiles Rp(θ) according to theoretically generated profiles calculated using the Abele`s matrix method16 and a homogeneous adsorbed layer model. This procedure yields values for the optical average thickness (dp) and refractive index (np) of the adsorbed polymer layer. While dp and np determined in this way are individually dependent on the optical model chosen for the interfacial refractive index profile and may exhibit significant experimental scatter, the surface concentration calculated from these values
Γ)
dp(np - n0) dn/dC
(3)
is both more precise and independent of the model used to describe the adsorbed layer,12,17 i.e., errors in np and dp are mutually compensating. In eq 3, n0 is the refractive index of the bulk solution and dn/dC is the refractive index increment of the adsorbing species. We used the refractive index increment determined previously for PEO homopolymer,18 dn/dC|PEO ) 0.134 cm3/g. The calculated effect of the PS block on the refractive index increment for the ∼10 wt % PS-containing block copolymers examined here is to increase dn/dC|PS-PEO by 17% relative to PEO.19 Thus, we used dn/dC|PS-PEO ) 0.157 cm3/g for the diblocks for the 632.8 nm HeNe laser wavelength. To interpret the reflectivity after polymer adsorption, we employed a two-layer striated interface model (bulk siliconoxide layer-polymer layer-bulk solution) that lumped the thin methylated layer with the oxide layer. When adsorption to PS films is studied, a three-layer model is used (bulk silicon-oxide layer-PS film-polymer layer-bulk solution) where dPS is measured prior to adsorption of the polymer. In order to obtain a surface concentration for the adsorbing species, the thickness of the silicon oxide layer (dox) must be known. We measured dox by recording Rp(θ) prior to the adsorption step and analyzing the data by nonlinear least-squares regression according to a single homogeneous layer optical model. For the oxide layer we used the silicon dioxide refractive index nox ) 1.46. The refractive (12) Schaff, P.; De´jardin, P.; Schmitt, A. Langmuir 1987, 3, 1131. (13) Dijt, J. C.; Cohen Stuart, M. A.; Hofman, J. E.; Fleer, G. J. Colloids Surf. 1990, 51, 141. (14) Leermakers, F. A. M.; Gast, A. P. Macromolecules 1991, 24, 718. (15) Charron, J. R.; Tilton, R. D. J. Phys. Chem. 1996, 100, 3179. (16) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1977. (17) de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759. (18) Polik, W. F.; Burchard, W. Macromolecules 1983, 16, 978. (19) Charron, J. R. Ph.D. Dissertation, Carnegie Mellon University, 1996.
index of bulk silicon is nSi ) 3.882 + 0.019i at the wavelength of 632.8 nm.20 As a consistency check, we measured dox both in air and under water and required agreement between the two before proceeding. The reflectometry measurements were conducted using the apparatus described in ref 9. Polymer adsorbed at 25 °C from solution in parabolic flow at a wall shear rate of 9.0 s-1 in all reflectometry experiments. The polymers studied reached their adsorption plateau in less than 5 min and remained constant for the duration of the longest experiment (24 h). Electrokinetic and Hydrodynamic Measurements. A complete discussion of the experimental protocol for the electrokinetic and hydrodynamic measurements can be found elsewhere.4,21,22 After preparing a membrane, we rinsed it with MilliQ water and placed it into the transport cell4 for a series of hydrodynamic/streaming potential measurements at electrolyte (KCl) concentrations varying from 10-4 to 10-2 M. Polymer was adsorbed in the pores of the membrane by filling each half-cell with 5 µg/mL polymer solution on either side of the membrane and continuously flushing for 12 h. After the polymers were adsorbed, the cell was flushed several times with polymer-free MilliQ water and refilled with electrolyte solution to repeat the series of the hydrodynamic/streaming potential measurements. We determined the pore radius before and after polymer adsorption and calculated the hydrodynamic thickness of the adsorbed polymer layer, Lh, by simply subtracting the two values. For the hydrodynamic measurement, a pressure drop was applied across the membrane by a syringe pump (Harvard Apparatus, Series 11). We measured the flow rate, Q, of polymer-free solvent as a function of the pressure drop across the membrane, ∆P, measured by a Validine D15 pressure transducer. The pressure transducer was calibrated versus a mercury manometer before each series of measurements. All solutions were filtered in-line through two 0.02 µm (Anotec, Whatman) filters. The HagenPoiseuille equation describes the Q(∆P) relationship for laminar flow. Since the pore size distribution was narrow, we analyzed the hydrodynamic measurements in terms of a single pore radius
Q)
πNa4 ∆P 8ηL
(4)
where a is the pore radius, L is the thickness of the membrane, N is the number of pores, and η is the viscosity of the solution. The reader is directed to ref 22 for a thorough discussion of the theory and experimental details of the streaming potential measurements. These were conducted to measure the ζ potential of the pore walls before and after polymer adsorption. Since the ζ potential is measured at the slipping plane, it is very sensitive to the extension of the polymer layer. Thus, the ζ potential can be used to estimate the polymer layer thickness. Rigorous interpretation of the ζ potential in terms of an adsorbed layer thickness is complicated by the possibility that polymer adsorption alters the surface charge density and the double layer structure. Here we assume the polymer layer serves primarily to shift the location of the slipping plane without significantly altering the double layer. We conducted both electrokinetic and hydrodynamic measurements in the same experiment. Electrokinetic data were collected as voltage drop (streaming potential, ∆V), as a function of ∆P. ∆V was measured by Ag/AgCl electrodes immersed in the high- and low-pressure chambers of the cell. The electrodes were connected to a multimeter and readings were taken every 30 s over the course of the experiment. The ζ potential is calculated by numerically solving the following transcendental equation22
L13(ζ,a,1) ∆V )∆P L11(ζ,a,1)
(5)
where L13 and L11 are defined as (20) Palik, E. D., Ed. Handbook of Optical Constants of Solids; Academic Press: London, 1985. (21) McKenzie, P. F.; Webber, R. M.; Anderson, J. L. Langmuir 1994, 10, 1539. (22) Westermann-Clark, G. B.; Anderson, J. L. J. Electrochem. Soc. 1983, 130, 839.
2996 Langmuir, Vol. 13, No. 11, 1997 L11(ζ,a,x) ) -
Pagac et al.
4e2CD x r cosh(ψ(r)) dr 0 kT kTC x r(ψ(x) - ψ(r)) sinh(ψ(r)) dr (6) 0 πη
∫
∫
L13(ζ,a,x) )
kT 2πηe
∫ r(ψ(x) - ψ(r)) dr x
0
(
)
(8)
ψ(1) ) ζ
(
(13)
φgr(h) ) Gh
(14)
where
4 πR3(Fp - Fs) 3
G)
)
L hh kT ψ 1e a
(9)
The “electrokinetic layer thickness,” Le, is defined as the solution of the following equation
(15)
In eq 15 Fp is the density of the particle and Fs is the density of the solution. For a 1:1 electrolyte, the B parameter in eq 13 is given by
( )
where κ is the Debye parameter. In these equations we have assumed that the diffusion coefficients for the positive and negative ions are the same, a satisfactory assumption for KCl. In this paper we discuss three different quantities relating to the ζ potential. ζ0 is defined as the ζ potential of the bare pore wall determined from streaming potential measurements prior to polymer adsorption. ζa is defined as the ζ potential of the pore wall determined from streaming potential measurements after polymer adsorption based on the hydrodynamic layer thickness measured at the same electrolyte concentration, Lh. ζa* is defined as the ζ potential after polymer adsorption based on the average hydrodynamic layer thickness, L h h, measured over all electrolyte concentrations in that experiment. In calculating ζa*, we assume that the slip plane has been moved to the position r ) a - L h h. ζa* is then calculated using
ζa* )
φdl(h) ) B exp(-κh)
B ) 16R
ψ′(0) ) 0
(12)
where φdl, φvdw, and φgr represent the double layer, van der Waals, and gravitational contributions:
(7)
where is the dielectric constant of the solution, C is the molar electrolyte concentration, D is the diffusion coefficient of the ions, k is the Boltzmann constant, T is absolute temperature, and e is the electronic charge. ψ(r) is the solution to the following boundary value problem
1 d dψ(r) r ) (κa)2 sinh(ψ(r)) r dr dr
φ(h) ) φdl(h) + φvdw(h) + φgr(h)
kT e
2
tanh
( ) ( ) eΨ1 eΨ2 tanh 4kT 4kT
(16)
where Ψ1 and Ψ2 are the Stern potentials of the particle and the plate. It should be noted that at the separation distances typically encountered in a TIRM experiment (h > 50 nm), van der Waals forces are negligible compared to the double layer and gravitational contributions. The presence of polymer-induced steric repulsion can be deduced from a repulsive deviation of φ(h) from eq 12. We adsorbed polymer to these spheres by adding one drop of a 10 wt % particle suspension to 20 mL of the polymer solution. The vial was then shaken for 24 h. We adsorbed polymer to the methylated glass slides for 24 h from solutions undergoing parabolic flow at a wall shear rate of 2.0 s-1 in a rectangular flow cell measuring 7.6 cm × 2.54 cm × 0.1 cm, as described by Nagase.27 After adsorption of the polymer, the flow cell was flushed with 20 mL of water and the particle suspension was centrifuged and the supernatant was replaced with water to remove any free polymer. The particles were then injected into the flow cell via a 3.0 mL disposable syringe.
Results and Discussion
where Lh is the hydrodynamic thickness measured at that particular electrolyte concentration (not the average, L h h). Total Internal Reflection Microscopy (TIRM). The reader is directed to references 23-26 for a complete description of the protocol, capabilities, and theory of the TIRM technique. Briefly, TIRM is a technique for determining the instantaneous location of a scattering particle relative to a planar interface, usually the surface of a glass slide. With that location measured over a statistically large number of observations, the potential energy profile of the particle above the interface may be calculated. In the absence of steric repulsion, the potential energy of a sphere of radius R levitated by electrostatic repulsion above a horizontal, planar surface is given by
Reflectometry: Surface Concentrations. Typical reflectivity profiles measured before and after polymer adsorption on the methylated silicon wafers are shown in Figure 2. The final surface concentrations measured for the three diblock copolymers and the homopolymer are reported as Γ in Table 1. They range from 0.3 to 0.5 mg/ m2. Similar surface concentrations were also measured on the PS spin-cast surfaces. The surface concentrations measured were constant for concentrations ranging from 5 to 20 µg/mL for PS-PEO375 (i.e., just up to the cmc), suggesting that we are probing behavior at the adsorption plateau. No experiments were conducted above the cmc for any of the polymers. The surface concentrations, as well as the regressed values of the optical properties, are very similar for all the polymers we investigated. This suggests that the chain conformations are similar for the diblock copolymers and the homopolymer. The PEO homopolymer adsorbed to a surface concentration of 0.52 ( 0.08 mg/m2. For comparison, Dijt et al.13 used reflectometry to measure the adsorption of PEO (Mw ) 400 000) to hydrophilic silica and found surface concentrations of 0.6-0.7 mg/m2. Killmann et al.28 and Van der Beek and Cohen Stuart29 measured adsorption of two PEO homopolymer samples to colloidal silica particles and found maximum surface concentrations of 0.68 mg/m2 (Mw ) 996 000) and 0.61 mg/m2 (Mw ) 570 000), respectively.
(23) Prieve, D. C.; Walz, J. Y. Appl. Opt. 1993, 32, 1629. (24) Prieve, D. C.; Frej, N. A. Langmuir 1990, 6, 396. (25) Liebert, R. B.; Prieve, D. C. Biophys. J. 1995, 69, 66. (26) Pagac, E. S.; Tilton, R. D.; Prieve, D. C. Chem. Eng. Commun. 1996, 148-150, 105.
(27) Nagase, T. Ph.D. Dissertation, Carnegie Mellon University, 1994. (28) Killmann, E.; Maier, H.; Kaniut, P.; Gu¨tling, N. Colloids Surf. 1985, 15, 261. (29) Van der Beek, G. P.; Cohen Stuart, M. A. J. Phys. (Paris) 1988, 49, 1449.
∆V )∆P a
[ ]
( (
L11
) )
Le a Le ζ0, a, 1 a
L13 ζ0, a, 1 -
(10)
where the subscript “a” refers to after polymer adsorption. The potential after adsorption, ζa, is determined from the solution of
L13(ζa, a - Lh, 1)
)[∆V ∆P] L a
11(ζa,
a - Lh, 1)
(11)
Absorption of PS-PEO and PEO
a
b
Figure 2. (a) Reflectivity profile for adsorption of PEO420 (5 µg/mL) to methylated silica, dox ) 53.2 nm, curves correspond to regressions (b) before adsorption and (O) after adsorption. Note that the reflectivity at θB for the uncoated surface is not zero due to the presence of the oxide layer. Γ ) 0.52 mg/m2, np ) 1.354, and dp ) 3.2 nm. (b). Reflectivity profile for adsorption of PS-PEO375 (5 µg/mL) to methylated silica, dox ) 99.8 nm, curves correspond to regressions (b) before adsorption and (O) after adsorption. Γ ) 0.51 mg/m2, np ) 1.353, and dp ) 4.0 nm.
The surface concentrations for the three PS-PEO diblock copolymers in Table 1 are more indicative of homopolymer-type adsorption, rather than the high surface concentrations (1-5 mg/m2) associated with the brush layers that have been shown to form with other diblock copolymer solutions. For example, Hadziioannou et al.30 measured a surface concentration of 1.80 mg/m2 for the adsorption of the diblock copolymer poly(2vinylpyridine)-PS (block molecular weights: 60000 PVP90000 PS) onto mica from the selective solvent toluene (good solvent for PS, nonsolvent for PVP), and Taunton et al.31 found that a PS-PEO diblock (block molecular weights: 3000 PEO-181000 PS) adsorbed to a surface concentration of 2.87 mg/m2 onto mica from the selective solvent toluene (good solvent for PS, nonsolvent for PEO). (30) Hadziioannou, G.; Patel, S.; Granick, S.; Tirrell, M. J. Am. Chem. Soc. 1986, 108, 2869. (31) Taunton, H. J.; Toprakcioglu, C.; Klein, J. Macromolecules 1988, 21, 3333.
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It should be noted that these examples are somewhat different from the situation encountered in this study due to the fact that only one block could adsorb (PS is nonadsorbing onto mica in the above studies) whereas both the PS and the PEO blocks are able to adsorb from water to the methylated silica. This suggests that there are two important issues to consider: the solvent selectivity for one of the blocks and the affinity of the surface toward both of the blocks. Another indication that the adsorbed layers for our three diblocks are similar to that of the homopolymer is the fact that the surface concentration is not an increasing function of the diblock molecular weight. Surface concentration has been shown both experimentally13,29 and theoretically32 to be independent of molecular weight for high molecular weight homopolymers adsorbing from good solvents. This is far from the expectation for brushes. Self-consistent field and scaling theories predict the surface concentration (Γ) to scale with the soluble block length to some power between approximately 0.6 and 1.0 1,2,33,34 depending on the relative sizes of the two blocks. It should be noted that an extensive set of data on block copolymers that do form highly extended adsorbed layers do not seem to support this dependence of surface concentration on the soluble block size.3 It is interesting to note that the surface concentrations for the PS-PEO diblocks are consistently lower than that for the PEO homopolymer. The difference in these values cannot be explained by the difference in the refractive index increment used to calculate each value. There is only a 17% difference between the refractive index increments for the diblocks and the homopolymer, whereas the surface concentration difference is over 50%. Some insight into the adsorbed polymer conformation can be obtained by comparing the size of a polymer molecule in solution to the area it occupies at an interface. This can be accomplished by first calculating the mean radius per adsorbed polymer chain Rm from the surface concentration. These are shown in Table 1 and may be compared with the radius of gyration Rg for PEO having the same degree of polymerization as the PEO block in the respective copolymer. Rg is calculated using the result Devanand and Sesler35 found for PEO in water at 30 °C
Rg ) 0.0215Mw0.583 (nm)
(17)
The ratio of Rg/Rm is an approximate measure of how the chain dimensions change upon adsorption. The ratio of Rg/Rm for the PS-PEO and PEO chains all range between 1.3 and 2.0, suggesting that the diblocks and the homopolymer share similar conformations on the surface and that both adsorb beyond the point where unperturbed chains would begin to overlap. It is interesting to note that in previous studies with diblock copolymers that adopted extended conformations, as shown by more direct measures of chain extension, similar values of Rg/Rm have been found. For example, the results of Hadziioannou et al.30 yield Rg/Rm ) 2.0 for adsorption of PVP-PS onto mica from toluene. Since we found that of the polymers we investigated PEO homopolymer had the largest value of Rg/Rm (2.0), it is clear that merely comparing the apparent size of the polymer in solution and its area occupied at the interface is not sufficient to determine (32) Scheutjens, J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1979, 83, 1619. (33) de Gennes, P.-G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (34) Marques, C.; Joanny, J. F.; Liebler, L. Macromolecules 1988, 21, 1051. (35) Devanand, S.; Sesler, J. C. Macromolecules 1991, 24, 5943.
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Figure 3. Hydraulic resistance measurements on polycarbonate membrane prior to and after polymer (5 µg/mL) adsorption from 0.005 M KCl: (b) before PEO420 adsorption, pore radius a ) 162.2 nm; (O) after PEO420 adsorption, a ) 158.9 nm; (9) before PS-PEO479 adsorption, a ) 138.2 nm; (0) after PS-PEO479 adsorption, a ) 136.4 nm.
Figure 4. Streaming potential measurements on polycarbonate membrane prior to and after polymer (5 µg/mL) adsorption from 0.005 M KCl: (b) before PEO420 adsorption, ζ0 ) -31.9 mV; (O) after PEO420 adsorption, ζa ) -13.2 mV; (9) before PSPEO479 adsorption, ζ0 ) -31.9 mV; (0) after PS-PEO479 adsorption, ζa ) -15.8 mV.
whether the molecule is in an extended or thin conformation. A direct measurement of the layer thickness is required, such as Hadziioannou et al.30 did indeed perform using the surface forces apparatus. While the next section addresses layer thickness in a more direct manner, we note that the optical layer thicknesses for both PS-PEO and PEO are on the order of 3-4 nm. While the optical thickness is weighted toward the center of mass and hence does not necessarily reflect the maximum chain extension, consistency between this value for PS-PEO and PEO argues for similarity rather than dissimilarity of chain conformation. In summary, the similar final surface concentrations for PS-PEO of three molecular weights and for PEO homopolymer indicate that PS-PEO adopts a homopolymer-like conformation on methylated silica surfaces. Hydrodynamic/Electrokinetic Measurements. Whereas optical thicknesses are weighted toward the center of mass of the adsorbed layer, hydrodynamic thicknesses are controlled by the extension of the tails. As such they can readily distinguish between brushlike and thin layers. Webber et al.4 previously showed that diblock copolymer adsorption from nonaqueous solvents can have a dramatic effect on the hydrodynamic pore size if they form brushes. They measured hydrodynamic layer thicknesses as large as 30% of the bare pore radius (i.e., Lh ) 13.5 nm in ∼45 nm radius pore) for a poly(methacrylic acid)/poly(butyl methacrylate) polymer with block molecular weights of 602 and 43 736, respectively, in methyl ethyl ketone. Hydrodynamic and streaming potential measurements are highly stable and reproducible.4,21 These measurements are typically reproducible to within 1% for the hydrodynamics and 2-5% for the electrokinetics. Typical results are presented in Figures 3 and 4. It should be noted that after adsorption and subsequent replacement of the polymer solution by polymer-free electrolyte solution, the ζ potential and hydrodynamic layer thickness did not change over a 1 month period. This suggests that the adsorption is effectively irreversible and the adsorbed chain conformation is unchanging. The average layer thickness (L h h) for each set of hydrodynamic measurements is calculated as the average
of Lh measured at various electrolyte concentrations. While polymer mushrooms and brushes are characterized by layer extensions in excess of Rg, the measured Lh values, presented in Tables 2-4, were only a small fraction of Rg (see Table 1 for Rg) for each of the polymers investigated. To within experimental error, Lh was independent of ionic strength. The extremely small values of L h h and the similarity of PEO and PS-PEO layers clearly show that these PS-PEO diblock copolymers do not form any sort of extended layer at the polycarbonate/water interface. The L h h values obtained for the PS-PEO diblock copolymers may exhibit a slight increase with increasing molecular weight. It is interesting that PEO420 yields the largest L h h. This may be due to the fact that Lh is controlled mainly by tails and PEO will have two tails per chain, while PS-PEO most likely has only a single tail due to the insolubility of the PS end. Given that PS-PEO and PEO were found to attain similar surface concentrations on hydrophobic surfaces, the tail density would be higher for PEO than for PS-PEO. However, the trends in L h h should not be overinterpreted since they are close to the experimental resolution limit. They do clearly show that no brush exists for these diblock copolymers. Consistent with the reflectometry study on methylated silica, hydrodynamic data indicate that the PS-PEO chains on polycarbonate surfaces are no more extended, and possibly less extended, than are PEO chains. The effect of polymer adsorption on the ζ potentials can be seen by comparing the measured potentials before and after adsorption, ζ0 and ζa, presented in Tables 2-4. We attribute the decrease in the magnitude of the ζ potentials before and after adsorption to a shift in the position of the slip plane. An interesting point is that polymer adsorption slightly increases the ζ potential compared to that of the bare pore at the lowest electrolyte concentration examined for each sample. This small increase at low bulk polymer concentrations and low ionic strengths has also been seen previously for PEO homopolymer adsorbing to AgI particles.36 The reasons for this increase are still not well (36) Eremenko, B. V.; Platanov, B. E. Proc. Int. Conf. Colloid Surf. Sci. 1975, 363.
Absorption of PS-PEO and PEO
Langmuir, Vol. 13, No. 11, 1997 2999
Table 2. Results of Hydrodynamic/Streaming Potential Measurements for PS-PEO Polymer MW ) 67 800; L h h )1.0 ( 0.6 nm
a
[KCl] (M)
pore radius before (nm)
ζ0 (mV)
ζa (mV)
Lh (nm)
Lea (nm)
ζa* b (mV)
0.0005 0.001 0.005 0.01
69.6 ( 0.7 69.1 ( 0.4 68.6 ( 0.4 69.0 ( 0.5
-23.4 ( 1.9 -29.6 ( 1.1 -32.5 ( 1.3 -27.7 ( 1.5
-28.4 ( 0.5 -28.0 ( 0.5 -17.0 ( 1.0 -11.9 ( 1.5
2.1 ( 0.6 0.6 ( 0.3 0.6 ( 0.3 0.5 ( 0.3
-10.9 1.0 3.4 2.7
-21.9 -24.9 -26.1 -20.4
Calculated from eq 10. b Calculated from eq 9. Table 3. Results of Hydrodynamic/Streaming Potential Measurements for PS-PEO Polymer MW ) 479 000; L h h )1.9 ( 0.5 nm
a
[KCl] (M)
pore radius before (nm)
ζ0 (mV)
ζa (mV)
Lh (nm)
Lea (nm)
ζa* b (mV)
0.00025 0.0005 0.001 0.005
138.5 ( 0.4 138.2 ( 0.8 137.8 ( 0.2 137.9 ( 0.7
-23.5 ( 1.0 -30.4 ( 0.9 -36.7 ( 0.3 -31.9 ( 0.9
-32.5 ( 0.2 -25.9 ( 0.7 -23.4 ( 0.2 -15.8 ( 0.7
2.4 ( 1.1 1.8 ( 0.7 1.7 ( 0.2 1.8 ( 0.7
-10.0 2.6 5.1 3.3
-21.2 -26.1 -29.9 -20.2
Calculated from eq 10. b Calculated from eq 9. Table 4. Results of Hydrodynamic/Streaming Potential Measurements for PEO Polymer MW ) 42 0000; L h h ) 3.2 ( 1.1 nm
a
[KCl] M
pore radius before (nm)
ζ0 (mV)
ζa (mV)
Lh (nm)
Lea (nm)
ζa* b (mV)
0.0001 0.00025 0.0005 0.001 0.005
161.3 ( 1.2 161.8 ( 0.4 161.9 ( 0.4 160.4 ( 2.0 162.2 ( 0.6
-18.5 ( 0.5 -26.7 ( 0.1 -34.9 ( 0.2 -33.3 ( 0.2 -31.9 ( 1.1
-24.3 ( 0.2 -27.6 ( 0.1 -29.1 ( 0.1 -21.5 ( 1.7 -13.2 ( 1.7
4.5 ( 0.4 3.2 ( 0.3 3.1 ( 0.5 1.8 ( 0.5 3.2 ( 0.6
-13.2 -0.5 2.1 3.7 2.6
-16.6 -22.6 -27.5 -23.9 -15.7
Calculated from eq 10. b Calculated from eq 9.
understood but may be related to a disturbance of the ion distribution near the surface or to the orientation of the ethylene oxide units when the layer is very thin. This slight increase is followed by a steadily decreasing magnitude of the ζ potential for the polymer-coated surfaces as the ionic strength increases. The trend is evident for all the polymers studied. Each experimental series finishes at the highest ionic strengths with ζa values that are 40-50% of ζ0. To compare electrokinetic and hydrodynamic probes of adsorbed layer structure we use the ζ potential calculated from experimental data (eq 11) to calculate an “electrokinetic layer thickness” (Le) via eq 10. Conversely, we hh can predict ζa, which we call ζa*, from the measured L and eq 9. While the electrokinetic and hydrodynamic thickness of an adsorbed layer are not equal, it has been shown both theoretically37,38 and experimentally39 that they converge for low values of κLh. In our experiments this condition is always met (i.e., κLh < 1.0). It can be seen from comparison of ζa* and ζa in Tables 2-4 that L h h gives a reasonable estimate of the effect of polymer adsorption on the value of the electrostatic potential at the slip-plane. On the other hand, Le can be a very poor estimate for Lh. Figure 5 helps explain why this is so. At lower electrolyte concentrations the layer thickness becomes very sensitive to small changes in the measured potential. Experimental error (typically 2-5%) in the measurement of ζa can thus yield large errors in the electrokinetic layer thickness, Le. For example, a 5% error in the ζ potential at a 0.1 mM electrolyte concentration can result in a 58% change in calculated layer thickness, whereas a 5% error in Lh would produce only a 0.5% change in ζa/ζ0. While the experimental error can greatly affect the calculated layer thickness, it still does not fully explain (37) Koopal, L. K.; Hlady, V.; Lyklema, J. J. Colloid Interface Sci. 1988, 121, 49. (38) Cohen-Stuart, M. A.; Waajen, F. H. W. H.; Dukhin, S. S. Colloid Polym. Sci. 1984, 262, 483. (39) Cohen-Stuart, M. A.; Mulder, J. W. Colloids Surf. 1985, 15, 49.
Figure 5. Relationship between ζ potential and the layer thickness calculated from that potential for various electrolyte concentrations: ‚‚‚, 0.1 mM; - - -, 1.0 mM; ;, 10 mM.
the deviation found between the measured hydrodynamic thickness, Lh, and the calculated electrokinetic thickness, Le, based on the measured potential, ζa. This disagreement between Le and Lh has been seen previously40,41 and is usually explained by a decrease in the surface charge of the substrate due to the polymer adsorption. The decrease in the surface charge may be due to the lowering of the dielectric constant of the medium near the interface and/or to the tendency of PEO to bind protons and thereby carry them to the negatively charged surface. (40) Garvey, M. J.; Tadros, Th. F.; Vincent, B. J. Colloid Interface Sci. 1975, 55, 440. (41) Churaev, N. V.; Nikologorskaja, E. A. Colloids Surf. 1991, 59, 71.
3000 Langmuir, Vol. 13, No. 11, 1997
Another comparison between the diblock copolymers and the homopolymer can be made by crudely estimating the thickness of the polymer layer as the Debye length where ζa begins to differ significantly from ζ0. At this value of κ-1, a significant fraction of the double layer decay occurs within the polymer layer as the Debye length approaches the location of the slip plane. Only then will the polymer layer have much effect on the magnitude of the ζ potential. Comparing the values of ζa to ζ0 for the different ionic strengths in Tables 2-4, one finds that the potentials begin to deviate at Debye lengths of 4.3, 9.6, and 13.6 nm for the PS-PEO67, PS-PEO479, and PEO420 samples, respectively. It is interesting to note that all these correspond to approximately 5Lh. Consistent with the Lh measurements, this thickness estimate is largest for PEO homopolymer. The similarity of the results for the largest PS-PEO and the PEO and the fact that all measures of the layer thickness are small (relative to the radius of gyration), once again suggest that these polymers are adsorbing in a manner expected of homopolymers. A final observation concerns the ionic strength dependence of the bare pore ζ potentials. The small increase in magnitude of ζ0 with increasing ionic strength indicates that the surface charge in the polycarbonate pores varies with KCl concentration. TIRM. We used TIRM to determine the effect of adsorbed PS-PEO layers on the interactions between a polystyrene sphere and a methylated glass microscope slide and in turn to infer the adsorbed layer extension. Under high ionic strength conditions (>5.0 mM), an uncoated sphere does not remain levitated above an uncoated glass slide due to the screening of the repulsive electrostatic force. On the basis of experiments3,4,30 conducted with nonaqueous amphiphilic diblock copolymers of similar molecular weight, the PS-PEO diblock copolymers would reasonably be expected to form layers sufficiently thick to provide steric stabilization if they formed brushes. However, we observed no enhanced stabilization upon PS-PEO479 adsorption to the sphere and slide at high electrolyte concentrations. In fact, the particles strongly adhere to the slide, as they would in the absence of polymer. Experiments were also conducted at lower ionic strengths where electrostatic repulsions keep the particles levitated. Figure 6 shows the potential energy profiles for two different spheres under identical conditions of low ionic strength (0.45 mM NaCl) interacting with the methylated glass surface in the presence or absence of the adsorbed polymer PS-PEO479. The solid curves are the theoretical fits based on eq 12. The right side of the curves corresponds to the gravity-dominated region where the slope is directly related to the gravitational force. The left side is the region where electrostatic and steric forces dominate. Not only does the adsorbed polymer not contribute an additional repulsive energy of interaction, but it allows the particle to approach the wall more closely, i.e., decreasing the magnitude of the repulsion. The form of the wall-sphere repulsion does not change. The closer separation portion of the potential profile is dominated by the electrostatic double layer repulsion in both cases, as demonstrated by the agreement between the regressed Debye length and that measured from the conductivity of the solution. Instead of adding a repulsive energy component due to steric forces, the polymer layer causes a drastic decrease in the magnitude of the electrostatic repulsion. This is a result of the decrease in the apparent surface potential |Ψ s| calculated via eq 16 from 92.3 mV without polymer to 19.1 mV with adsorbed polymer. This is far outside the usual 5-10% variation in potential
Pagac et al.
Figure 6. Potential energy profiles for polystyrene spheres levitated above a glass slide in 0.45 mM KCl (κ-1 ) 14.3 nm): (b) in the absence of polymer, G ) 0.19 pN, hm ) 148.2 nm, Ψs ) 92.3 mV, κ-1 ) 14.8 nm; (O) after adsorption of PS-PEO479 (5 µg/mL), G ) 0.20 pN, hm ) 108.8 nm, Ψs ) 19.1 mV, κ-1 ) 14.5 nm. Solid curves correspond to theoretical fits using the above parameters.
measured for large numbers of uncoated particles in a suspension. It should be noted that in this experiment it is not possible to separate the surface potentials of the sphere and the slide; therefore they are assumed to be equal. We conclude from these TIRM observations that PSPEO does not impart any significant steric repulsion to the interaction between a polystyrene particle and a methylated glass slide, and by inference we conclude that it does not form a brush. Furthermore, at low ionic strengths, TIRM provides evidence that PS-PEO adsorption decreases the average surface potential. In addition to shifting the slip plane, this type of potential reduction may play a role in the decreasing ζ potential caused by PS-PEO and PEO adsorption in streaming potential measurements. Brush Formation? The results discussed up to this point strongly suggest that the adsorbed PS-PEO diblock copolymers examined in this study do not extend far into solution. Rather they assume an adsorbed conformation similar to that of homopolymer PEO. The question then arises: why do these polymers not form brushlike layers? The work cited previously in this paper1,3,4,30 is strong evidence that diblock copolymers can indeed form highly extended layers. A point to note is that the majority of this work has focused on organic systems where the solvent is strongly selective for one block and the soluble block is not surface active. In these cases, the soluble block is entirely nonadsorbing whereas the insoluble block strongly adsorbs to the interface. As noted earlier in this paper, this is not the case for aqueous PS-PEO solutions. Although water is strongly selective (good solvent) for the PEO block, PEO does adsorb to all surfaces examined in this study. This is preventing brush formation from occurring. It should be noted that block copolymers with smaller PEO blocks (i.e., “Pluronics”) have been shown to adsorb in an extended brushlike fashion.42 We find that the adsorption of the large PEO blocks encountered in this study imposes too large of a kinetic barrier to allow brush formation. (42) Berg, J. C.; Baker, J. A. Langmuir 1988, 4, 1055.
Absorption of PS-PEO and PEO
Ou-Yang and Gao43 studied adsorption of PEO-based associative polymers (approximate molecular weights of 100 000) on PS latex, focusing on the effect of molecular architecture on the type of layer formed. They found layer thicknesses on the order of 2Rg. This suggests brush formation, although the possibility of intermolecular associations resulting in a thicker adsorbed layer could not be dismissed. Rodgers and Santore44 recently reexamined this same set of data to determine if molecular architecture could be the cause of increased layer thickness as surface concentration increases. They showed that a sharp change in hydrodynamic thickness occurs as the surface concentration approaches 1.0 mg/m2 regardless of molecular architecture. They concluded that the growth of the hydrodynamic layer thickness occurs universally when the surface becomes “saturated”. This suggests a possible explanation for the results found in this study. In the Rodgers and Santore44 study, a surface concentration of 0.5 mg/m2 yielded a hydrodynamic layer thickness of only 2-4 nm. This is indeed very similar to the values found in this study, suggesting that the PS-PEO polymers are simply not reaching sufficiently large coverages to experience chain extension. It has been shown previously45,46 that a “saturated” layer of PEO at the air-water interface yields approximately 9-11 Å2 per EO unit. The PS-PEO surface concentrations in this study correspond to areas per EO unit of between 21 and 25 Å2, whereas the PEO homopolymer yields a value of 14 Å2 per EO unit. Clearly, one characteristic of the aqueous PS-PEO system that distinguishes it from the nonaqueous diblock systems that have been examined is the surface affinity of the soluble PEO block. In that regard, a system studied by Amiel et al.47 is conceptually similar to the current system. They adsorbed water-soluble poly(tert-butylstyrene)-sodium poly(styrenesulfonate) diblocks (PtBSNaPSS) onto silica surfaces from water. In this case, the PtBS block was insoluble, and the NaPSS block was soluble but surface active. Under high ionic strength conditions, the diblock attained large surface concentrations (∼2.0 mg/m2), suggesting it was able to adopt an extended conformation at the surface, but the NaPSS homopolymer adsorbed to a much lesser extent (0.45 mg/ m2). At 0.45 mg/m2, the average distance between NaPSS chains is much larger than its radius of gyration. If the NaPSS block were to behave similarly during PtBSNaPSS adsorption, it would therefore leave a significant amount of area on the surface exposed. This low coverage would offer little resistance to adsorption of the PtBS “anchor” block, leading ultimately to brush formation and larger surface concentrations. In contrast, the distance between adsorbed PEO homopolymer chains in our study was approximately equal to its radius of gyration, resulting in a nearly “saturated” layer. Since an amphiphilic diblock copolymer may be expected to adopt a conformation reminiscent of a unimolecular micelle in solution, the first contacts that form (in a kinetic sense) between adsorbing PS-PEO diblocks and the surface will be PEO segments. Since these rapidly adsorb to achieve a large surface coverage, access of the PS block to the surface will be hindered and evolution to a brush will be retarded. Theory2 has predicted that the segment density profile for an adsorbed diblock copolymer in a selective solvent goes through a minimum near the junction of the two blocks. This is due to the repulsion between the two (43) Ou-Yang, H. D.; Gao, Z. J. Phys. II 1991, 1, 1375. (44) Rodgers, S. D.; Santore, M. M. Macromolecules 1996, 29, 3579. (45) Kuzmenka, D. J.; Granick, S. Macromolecules 1988, 21, 779. (46) Sauer, B. B.; Yu, H. Macromolecules 1989, 22, 786. (47) Amiel, C.; Sikka, M.; Schneider, J. W., Jr.; Yi-Hua, T.; Tirrell, M.; Mays, J. W. Macromolecules 1995, 28, 3125.
Langmuir, Vol. 13, No. 11, 1997 3001
monomer types and results in a depletion of monomers. This idea may explain the lower surface concentrations found for the diblocks as compared to the homopolymer. There may exist a region of depleted EO density that surrounds the PS core when the molecule is adsorbed at the interface. This would result in less surface area available for adsorption of the long PEO block and an overall lower surface concentration. In addition, Xu et al.48 found that the adsorbed layer thicknesses formed by adsorption of PS-PEO diblock copolymer micelles to latex particles in water was strongly dependent on the curvature of the surface. For flat surfaces, as used for reflectometry and TIRM in this study, they suggested that as the PS core forms contacts with the surface, more EO surface attachments are formed resulting in a thinner, “patchier” layer. This effect should be amplified inside the pores used in the hydrodynamic/ electrokinetic measurements. Following similar reasoning, the low surface concentrations and layer thicknesses of the diblock copolymers adsorbed from nonmicellar solutions likely result from surface affinity of each block and the low curvature of the surfaces. Conclusions PS-PEO diblock copolymers adsorb from nonmicellar aqueous solutions to hydrophobic surfaces, but they do not form brushes. They attain low surface concentrations that are similar to PEO surface concentrations, and they show no apparent dependence on molecular weight. At the concentrations studied the diblocks do not form the highly extended adsorbed layer that has been seen previously with other block copolymers in nonaqueous selective solvents. By all means employed here, the PSPEO diblock adsorption is essentially indistinguishable from PEO homopolymer adsorption. Adsorbed layers of both the diblocks and the homopolymer on polycarbonate are characterized by hydrodynamic layer thicknesses much smaller than the unperturbed radius of gyration. Their effect on the electrokinetic potential of the surface measured at various ionic strengths also indicates that the PS-PEO and PEO layers are of similar thickness. PS-PEO also decreases the surface potential and thereby weakens the electrostatic forces acting between a Brownian polystyrene sphere and a methylated glass slide at low electrolyte concentrations. No evidence of enhanced stability due to steric repulsion between surfaces coated with PS-PEO layers is observed. Although brush formation is often driven by the insolubility of the anchor block, it is likely that the surface affinity of the large, soluble EO block presents a large kinetic barrier that prevents the PS-PEO chains from forming an end-anchored brush. Acknowledgment. This material is based on work supported in part by the National Science Foundation under Grants CTS-9308569, BES-9501145, and CTS9420780 and by E. I. du Pont de Nemours & Co., Inc., through a Young Faculty Award. We also thank the Keck Foundation for fellowship support for E.S.P. We thank Dr. John Anderson for valuable discussions during the course of this work and Dr. Michael Domach for the use of the fluorescence spectrophotometer. LA9608829 (48) Xu, R.; D’Unger, G.; Winnik, M. A.; Martinho, J. M. G., d’Oliveira, J. M. G. Langmuir 1994, 10, 2977.
