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Forces and Hydrodynamic Interactions between Polystyrene Surfaces with Adsorbed PEO-PPO-PEO Michael A. Bevan† and Dennis C. Prieve* Colloids, Polymers, & Surfaces Program and the Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received May 17, 2000 We are presenting the first measurements of purely steric levitation obtained with total internal reflection microscopy (TIRM), using F108 Pluronic triblock adsorbed to a polystyrene particle and a polystyrene film. The steric repulsion measured in these studies appears as a virtual hard wall. We report measurements which simultaneously detect both the “steric” and “hydrodynamic” edges of adsorbed polymer layers. The steric edge occurs at a few nanometers larger separations than the hydrodynamic edge. These measurements are also the first indication of the effects of adsorbed polymer on the net van der Waals attraction in good solvent conditions. The adsorbed polymer strengthens the net attraction for the same separation between the PS substrates but weakens the attraction for the same separation between the outer edges. It is also shown that attraction at small separations is dominated by the properties of the polymer layer, and attraction at large separations is dominated by the properties of the substrates. TIRM experiments are also reported which directly probe the adsorbed polymer layer thickness to establish substrate separation. The estimates from the TIRM experiments were within nanometers of the literature estimate of 15 nm.
Introduction Material properties and processing behavior of polymerically stabilized dispersions arise from interparticle forces and hydrodynamic interactions between particles. Polymeric stabilization is an important mechanism in natural and industrial mixed colloidal and polymeric systems, particularly because of the tendency for polymer to adsorb to particles and surfaces. Despite the considerable theoretical1-3 and experimental4 effort invested in understanding polymeric stabilization, some fundamental aspects are still not well understood. The relative ranges of polymeric (osmotic) and hydrodynamic (lubrication) interactions are important for dispersion stability5 and rheology6 but have proven difficult to measure quantitatively. Despite the fact that repulsion between adsorbed polymer has been studied extensively, the effect of adsorbed polymer on interparticle attraction has been largely ignored, except for several theoretical efforts.7-9 Understanding the effects of the adsorbed polymer on the net attraction is perhaps as critical for explaining polymeric stabilization as is understanding the repulsion. Additionally, there have been many experimental measurements of interactions between surfaces bearing ad* To whom correspondence should be addressed. E-mail:
[email protected]. † Current address: Particulate Fluid Processing Centre, University of Melbourne, Victoria 3010, Australia. (1) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (2) Fleer, G. J.; Stuart, M. A. C.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: New York, 1993. (3) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: New York, 1989. (4) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1992. (5) Stenkamp, V. S.; Berg, J. C. Langmuir 1997, 13, 3827. (6) Elliot, S. L.; Russel, W. B. J. Rheol. 1998, 42, 361. (7) Vold, M. J. J. Colloid Sci. 1961, 16, 1. (8) Vincent, B. J. Colloid Interface Sci. 1973, 42, 270. (9) Parsegian, V. A. J. Colloid Interface Sci. 1975, 51, 543.
sorbed polymer,4 but these have typically been for large interaction forces with a high degree of polymer layer interpenetration. It is possible that measurements performed for these conditions may not be representative of the weak interactions relevant to the behavior and properties of polymerically stabilized colloidal dispersions. In the current study, we exploit the ability of total internal reflection microscopy (TIRM) to measure colloidal interactions on the order of kT10 and its recently developed capability to determine hydrodynamic separation between surfaces.11 We present here the first measurements with TIRM of the interactions between surfaces bearing adsorbed stabilizing polymer layers. While previous TIRM studies have observed “electrosteric” interactions for adsorbed high molecular weight polyelectrolytes,12 the present work measures purely “steric” interactions. This interaction is measured between nonionic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEOPPO-PEO) copolymer adsorbed to a polystyrene particle and a flat polystyrene surface in a high ionic strength aqueous solution, so that repulsion arises only from adsorbed polymer mixing and elastic free energy contributions. The measurements of steric interactions presented here are performed for conditions which are inherently similar to conditions in a stable colloidal dispersion. This is because particle levitation, which is necessary in the TIRM technique, has the same physical requirements as dispersion stabilization; the repulsive and attractive forces must balance each other to within several kT. The onset of steric interaction and zero hydrodynamic separation are measured in each experiment to compare the relative range of osmotic and hydrodynamic interactions between the adsorbed, stabilizing polymer. These measurements are performed for several particle sizes with a fixed adsorbed polymer molecular weight, which allows the range of (10) Prieve, D. C. Adv. Colloid Interface Sci. 1999, 82, 93. (11) Bevan, M. A.; Prieve, D. C. J. Chem. Phys. 2000, 113, 1228. (12) Nagase, T. Ph.D. Dissertation, Carnegie Mellon University, 1995.
10.1021/la0006869 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/12/2000
Polystyrene Surface/PEO-PPO-PEO Interactions
particle attraction to be varied and ultimately the determination of the adsorbed polymer’s contribution to the net attraction. These measurements are also compared with our previous measurements of attraction between bare polystyrene surfaces.13 Finally, we present measurements of the adsorbed polymer layer thickness to aid in determining polystyrene substrate separation. This, in conjunction with steric and hydrodynamic separation, is necessary for distinguishing between polymer and substrate interactions, as well as the continuum van der Waals attraction from favorable polymer mixing. Substrate separation is established as twice the literature hydrodynamic thicknesses for the adsorbed polymer5,14-16 which we confirm with two direct measurements of the adsorbed polymer thickness using TIRM. The first of these measurements changes the solvent quality for the adsorbed polymer layers so that, by monitoring their dimensional collapse, each layer thickness can be determined. The second method uses a bare particle to probe surfaces with and without adsorbed polymer to look at the thickness of one layer. With accurate knowledge of each relevant separation, it is possible to interpret the roles of the adsorbed polymer and the particle in the net interaction.
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Figure 1. Relevant distance for steric interactions between a particle levitated above a wall in TIRM geometry (particle radius and adlayer thickness not to scale).
Experimental Section Potential energy profiles were obtained for the interaction of polystyrene (PS) latex spheres and a PS flat, with both surfaces bearing an adsorbed polymer layer. The surfactant-free PS latex spheres were purchased from the Interfacial Dynamics Corporation with nominal sizes of 1.9, 2.8, 4.2, 6.3, and 9.7 µm. PS spincoated microscope slides were prepared with ∼1 µm thick coatings by the same method described in previous work with bare PS surfaces.13 The 5.1 µm silica particles used in these studies were purchased from Duke Scientific. The adsorbed polymer used in the experiments was F108 Pluronic triblock copolymer [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), or PEO-PPO-PEO, referred to as just “Pluronic” in the remainder of the text] supplied by the BASF Wyandotte Corporation. Although Pluronic is known to be polydisperse, the average molecular weight of the Pluronic is reported as 14 000, with a PEO molecular weight of 10 750 and a PPO molecular weight of 3250.14 All potential energy profiles were obtained using TIRM which has been described in a number of previous publications.10,17-22 Temperature control was implemented on the current apparatus by using heating elements with the current TIRM flow cell in conjunction with a CN7600 adaptive PID controller from the Omega Corporation. A thermocouple was located in the flow cell in the vicinity of the particle of interest. Thermal expansion was handled using Teflon sheet spacers. Finally, the microscope objective was maintained at 25 °C (∼5 mm above a 95 °C surface) by use of an air cooled jacket. The PS particles were sterically levitated (Figure 1) above the flat PS surface by adsorbing layers of Pluronic onto both surfaces. Pluronic was adsorbed onto the particles by shaking them in a 1000 ppm polymer solution for 16 h. The bulk concentration of 1000 ppm is at least three times the concentration necessary to saturate the PS surfaces, according to literature adsorption isotherms.14 The particles were then centrifuged three times at 104 rpm for 10 min to remove unadsorbed polymer. After the first two centrifugations, the supernatant was replaced with double deionized water, and after the third centrifugation, it (13) Bevan, M. A.; Prieve, D. C. Langmuir 1999, 15, 7925. (14) Baker, J. A.; Berg, J. C. Langmuir 1988, 4, 1055. (15) Li, J.-T.; Caldwell, K. D. Langmuir 1991, 7, 2034. (16) Li, J.-T.; Caldwell, K. D.; Rapoport, N. Langmuir 1994, 10, 4475. (17) Prieve, D. C.; Luo, F.; Lanni, F. Faraday Discuss. 1987, 83, 297. (18) Prieve, D. C.; Frej, N. A. Langmuir 1990, 6, 396. (19) Bike, S. G.; Prieve, D. C. Int. J. Multiphase Flow 1990, 16, 727. (20) Walz, J. Y. Ph.D. Dissertation, Carnegie Mellon University, 1992. (21) Walz, J. Y.; Prieve, D. C. Langmuir 1992, 8, 3043. (22) Bevan, M. Ph.D. Dissertation, Carnegie Mellon University, 1999.
Figure 2. Potential energy profiles for 1.9 (O), 4.2 (3), 6.3 (0), and 9.7 µm (]) PS particles sterically levitated with F108 Pluronic. Shown with gravitational contribution subtracted and biexponential fits (-) using eq 1 (only for 1.9, 4.2, and 6.3 µm particles). was replaced with 0.4 M NaCl. The F108 Pluronic was adsorbed to the PS spin-coated surface in the TIRM flow cell by flowing 25 mL of 1000 ppm polymer solution over the surface for 16 h. The bulk solution in the cell was then replaced by flowing 25 mL of 0.4 M NaCl through the cell. The coated PS particles were then introduced into the flow cell and were allowed to settle to the coated PS film where they remained levitated by steric repulsion. The 0.4 M NaCl background solution was used to suppress all electrostatic interactions, although it was shown in prior work that the spin-coated PS surface has no significant native charge.13
Results and Discussion Steric Levitation of F108 Pluronic-Coated Polystyrene Particles. Figure 1 schematically shows (not to scale) the geometry for steric levitation of a particle in a TIRM experiment with relevant separations defined. Figure 2 shows potential energy profiles for the interaction of Pluronic-coated PS particles with Pluronic-coated PS films. The PS particles have nominal sizes of 1.9, 4.2, 6.2, and 9.7 µm. The aqueous medium contains 0.4 M NaCl
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Table 1. Biexponential Fits Using (2) for F108 Pluronic, Sterically Levitated PS Particles PS particle size (µm)
a (kT)
1.9 4.2 6.3
0.26 0.58 0.82
coefficients b (nm-1) c (kT) 0.19 0.21 0.13
-1.1 -2.5 -3.5
d (nm-1) 0.030 0.038 0.034
to suppress electrostatic interactions, so that the particles remain levitated by steric interactions between the adsorbed layers. The gravitational contribution to the net interaction has been subtracted, as described in previous TIRM publications.10,17-22 At large separations, beyond the range of surface forces, there are no interactions shown in Figure 2. With decreasing separation, a negative deviation from the zero interaction at large separations is observed, which is the van der Waals attraction between the particles and the wall. The magnitude of the van der Waals attraction increases with decreasing separation until a sharp repulsion is observed. This is the steric interaction of the two Pluronic layers. The separation in Figure 2 is defined as zero at the minimum in the potential energy profile. To illustrate the importance of the polymer layer thickness for steric levitation when using TIRM, none of the PS particles in Figure 2 could be levitated if the glass slide was used without a PS film. This failure to levitate occurs because the Pluronic layer adsorbed on glass is not as thick as that adsorbed on PS. Owing to the greater hydrophobicity of PS, a greater mass density of Pluronic adsorbs onto PS compared to that of bare glass.22 Lateral crowding of polymer molecules in the adsorbed layer then forces the PEO tails in the Pluronic triblock further into solution, while the PPO block remains anchored.2 Since the van der Waals attraction for glass/water/PS is essentially the same as that for PS/water/PS,13 the shorter range of repulsion for F108 Pluronic on glass prevents levitation. Steric Repulsion. To separate the steric and van der Waals interactions, each potential energy profile in Figure 2 has been fit with the biexponential equation
φ(h) ) ae-bh + ce-dh
Figure 3. Steric repulsion for 1.9 (O), 4.2 (3), and 6.3 µm (0) particles with van der Waals attraction subtracted from the original profiles in Figure 2. Each data set is divided by the particle radius. The exponential fit (-) is shown for each curve. The steric separation is shown for each curve, and a reference for hydrodynamic separation is also shown.
(1)
where the first term represents steric repulsion (a > 0) and the second term represents the van der Waals attraction (c < 0). The functional form was chosen here simply for the reason that it produced the best fit (with the primary alternative being power laws). The coefficients for each particle size are given in Table 1, and the results are shown as the solid curves in Figure 2. The attractive well for the 9.7 µm particle is not significantly deeper than that for the 6.3 µm particle; this is somewhat surprising given the progression of well depths for the three smaller particles. We feel that the well depth for the 9.7 µm particle is uncertain, owing to a significantly greater uncertainty in the gravitational contribution for this particle than for the smaller particles. The remainder of this discussion focuses on the profiles for the 1.9, 4.2, and 6.3 µm particles; the results for the 9.7 µm particle are only meaningful qualitatively as an extreme case of steric levitation of a large particle. Figure 3 shows the remainder of the potential energy profiles after the van der Waals contributions have been subtracted (by subtraction of the second term in eq 1). The curves have also been divided by their respective particle radii to check for scaling with particle size, as suggested by the Derjaguin approximation. Figure 3 displays the curve fits for the steric repulsion, which are
Figure 4. Comparison of the potential energy profile for a levitated 6.3 µm particle (O and -) with the apparent “profile” for a 6.3 µm particle stuck to the bottom surface (- only).
also scaled by particle radii. The coefficients, designated as a and b in Table 1, are the parameters for the steric contributions. The decay lengths, which correspond to the inverse of the b coefficients with increasing particle size, are 5.3, 4.7, and 7.9 nm, with the error due to the fitting procedure being 50 nm between the F108 Pluronic layers, the measurements of the attraction for the bare and the polymer-coated surfaces coincide within experimental error. At separations
