Langmuir 1988,4, 1055-1061
1055
Investigation of the Adsorption Configuration of Poly(ethy1ene oxide) and Its Copolymers with Poly(propylene oxide) on Model Polystyrene Latex Dispersions James A. Baker1 and John C. Berg*p2 Department of Chemical Engineering BF-IO, University of Washington, Seattle, Washington 98195 Received February 23, 1988. I n Final Form: April 7, 1988 Theoretical prediction of the performance of adsorbed polymeric dispersion stabilizersrequires information on the configuration of polymers at the solidjliquid interface. Parameters describing polymer adsorption configuration on aqueous polystyrene latices are evaluated for representative linear homopolymers, block copolymers, and random copolymers of poly(ethy1ene oxide) and poly(propy1ene oxide). Effective hydrodynamic adsorption layer thicknesses, determined with photon correlation spectroscopy, are reported for the different polymer systems as a function of molar mass, concentration, and time from initial mixing. Specific adsorption isotherms are also determined. The data suggest that the polymers adsorb in the extended configuration, as loops and/or tails.
Introduction The use of adsorbed macromolecules to modify the aggregation state, sedimentation behavior, and rheological properties of colloidal dispersions represents an industrially significant, albeit largely empirical, technology. Steric stabilization has been the focus of much recent experimental and theoretical research directed at developing a fundamental understanding of the effects of polymers on dispersion p r ~ p e r t i e s . ~ -Much l~ of the theoretical work has been directed at the development of statistical mechanical models of polymer a d ~ o r p t i o nwhile , ~ ~ experimental efforts have focused largely on the measurement of stability domains for representative model polymerparticle system^.^-'^ The quantitative comparison of theoretical and experimental stability behavior requires specific information on the properties of the adsorbed polymeric stabilizer, much of which must also be obtained experimentally. The effectiveness of an adsorbed stabilizer is largely determined (1) Present address: 3M Company, 3M Center 208-1-01, St Paul, MN 55144. (2) To whom correspondence should be addressed. (3) Sato, T.; Ruch, R. Stabilization of Colloidal Dispersions by Polymer Adsorption; Marcel Dekker: New York, 1980. (4) Napper, D. H.Polymeric Stabilization of Colloidal Dispersions; Academic: New York, 1983. (5) Hesselink, F. Th.; Vrij, A.; Overbeek, J. Th. G. J. Phys. Chem. 1971, 75, 2094. (6) Scheutjens,J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1979,83,1619; 1980, 84, 178. (7) Cowell, C.; Vincent, B. In The Effect of Polymers on Dispersion Properties; Tadros, Th. F., Ed.; Academic: New York, 1982; p 263. (8) Vincent, B. In Science and Technology of Polmyer Colloids; Poehlein, E. W., Ottewill, R. H., Goodwin, J. W., EMS.; NATO AS1 Series; Nijhoff: Boston, 1983; Vol. 2, p 335. (9) Tadros, Th. F. In The Effect of Polymers on Dispersion Properties; Tadros, Th. F., Ed.; Academic: New York, 1982; p 1. (10) Ash, S. G.; Clayfkld, E. J. J. Colloid Interface Sci. 1976,55,645. (11) Killmann, E.; Wild, Th.; Gutling, N.; Maier, H. Colloids Surf. 1986, 18, 241. (12) Napper, D. H.; Netschey, A. J . Colloid Interface Sci. 1971, 37, 528. (13) March, G. C.; Napper, D. H. J . Colloid Interface Sci. 1977, 61, 383. (14) Napper, D. H. J. Colloid Interface Sci. 1970, 33, 384. (15) Tadros, Th. F.; Vincent, B. J. Phys. Chem. 1980,84, 1575. (16) Kato, T.; Nakamura, K.; Kawaguchi, M.; Takahashi, A. Polym. J . 1981, 13, 1037. (17) Cosgrove, T.; Vincent, B.; Crowley, T. L.; Cohen-Stuart,M. A. In Polymer Adsorption and Dispersion Stability; Goddard, E. D., Vincent, B., Eds.; ACS Symposium Series 240; American Chemical Society: Washington, DC, 1984; p 147. (18) Kayes, J. B.; Rawlins, D. A. Colloid Polym. Sci. 1979, 257, 622.
0743-7463/88/2404-lO55$01.50/0
by the amount of polymer adsorbed (specific adsorption, r) and the apparent thickness of the adsorbed macromolecular layer (adsorption layer thickness, 6). The apparent adlayer thickness is of paramount importance in that it determines not only the range of the steric repulsion but also its distance dependence, as embodied by the segment density distribution. In the absence of a measured segment density distribution (as by small angle neutron scattering), the adsorption layer thickness can be scaled to an assumed expression for the segmental concentration profile, which is then used to evaluate theoretical stability behavior! In order to evaluate theoretical expressions for dispersion stability, it is thus necessary to obtain experimental values for r and 6, both of which are strongly influenced by polymer structure, molar mass, and adsorption conf i g ~ r a t i o n . ~The , ~ determination of these parameters for representative poly(ethy1ene oxide) (PEO) linear homopolymers and poly(ethy1ene oxide)-poly(propy1ene oxide) (PEO-PPO) copolymers, adsorbed on model polystyrene latex dispersions, is the focus of the present work. Homopolymers and copolymers of poly(ethy1ene oxide) were selected for study because they are widely used in practice as dispersion stabilizers and f l o c c ~ l a n t s . ~In ~~J~~~~ addition, they are readily obtainable in a range of molar masses and in relatively pure form. The aggregation behavior of dispersions stabilized by poly(ethy1ene oxide) and its copolymers has also been extensively investigated. Napper14carried out one of the first systematic experimental investigations of PEO-stabilized colloids, determining the effect of ion valence on critical flocculation temperatures (CFTs) for two poly(viny1 acetate) dispersions stabilized with different molecular weight fractions of PEO homopolymers. In a subsequent publication, Napper and Netschey12reported the results of incipient flocculation rate measurements for commercial PEO-PPO block copolymers (Pluronics F108, F68, F38, and P104). In both cases, no parameters describing polymer adsorption configurations (i.e., adsorbed amount, adsorbed layer thickness) were determined. Ash and Clayfieldlosubsequently measured the critical flocculation concentrations (CFCs) for a polystyrene latex dispersion as a function of electrolyte concentration, (19) Technical Data on PLURONIC Polyols; BASF Wyandotte Corp.; Parsippany, NJ. (20) UCON Fluids and Lubricants, Union Carbide Corp.; Ethylene Oxide Derivatives Div.; Danbury, CT.
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polymer concentration, polymer molecular weight, and Table I. Properties of PEO Homopolymers and PEO-PPO polymer composition. In addition, they measured adCoDolsmers sorption isotherms for the two types of polymers that were total molar mass (M,) studied: PEO homopolymers and PEO-PPO random homopolvmer GPC LS MJM, (statistical) copolymers. N o attempt was made to deterPEO mine the thickness of the adsorbed stabilizing layer. SE-2 17000 18000 1.10 Tadros and Vincent15 recently reported results of sta35 000 39 000 1.07 SE-5 bility measurements for a model PEO-PPO block co86 000 1.02 SE-8 80 000 polymer (Pluronic P75) adsorbed upon a polystyrene latex SE-15 135000 145000 1.03 259 000 252 000 1.04 SE-30 dispersion. In addition, the specific adsorption of Pluronic SE-70 668 000 594 000 1.04 P75 on the model latex was determined as a function of SE-150 1190000 996 000 1.05 electrolyte concentration and temperature. However, the molar mass adsorption layer thickness was not determined, making copolymer total PEO PPO comparison to theoretical stability predictions difficult. A recent series of papers by Vincent et a1.7-9J7,21-24 Pluronic provides the best examples found in the literature of L64 2 900 1150 1750 F68 8 350 6 600 1750 comprehensive studies of a nature similar to the present 2050 4 150 2 100 P75 research. The dispersions used in these investigationswere 2250 PS4 1950 4 200 model polystyrene latices which had been characterized 2250 4 600 2 350 P85 with respect to particle size distribution and potential. 2750 L92 3 650 900 The polymeric stabilizers evaluated included poly(viny1 P94 2750 1850 4 600 2750 F98 acetate-co-vinyl alcohol) (PVAA) copolymers, PEO hom10 250 3 000 P104 3250 2 600 5 850 opolymers, and a family of PEO homopolymers chemically 3250 P105 6 500 3 250 grafted to the latex particles. F108 3250 4 000 10 750 Polymer adsorption isotherms were measured, as were polymer segment density distributions (using small-angle Meroxapol 108 4 550 4 004 546 scattering). In addition, the apparent thicknesses of the 2484 3 100 252 616 adsorbed polymer layers were determined both as a 7 172 1378 8 550 258 function of electrolyte concentration (CFCs) and temUcon perature (CFTs), and the experimental stability data were 50-HB-260 1000 500 500 compared to theoretical predictions of dispersion stability. 1700 850 50-HB-660 850 Although this series of papers represents a definitive 1450 1450 50-HB-2000 2 900 attempt to correlate theoretical and experimental stability 50-HB-5100 2000 4 000 2 000 behavior for sterically stabilized dispersions, the range of polymers studied was limited to linear homopolymers and Garvey et al.25have demonstrated that the adsorbed amount is independent of particle size for monodisperse PVAA adsorbed statistical copolymers. In addition, it is difficult to draw on model polystyrene latices. The adsorption layer thickness, general conclusions regarding the relative effectiveness of however, depended strongly on particle size. Throughout this random copolymers versus homopolymers in promoting work, it will be assumed that the adsorbed amounts determined dispersion stability. This ambiguity arises because the for Latex G may be used in conjunction with the adlayer thickchemical nature of the polymer segments (i.e., PVAA nesses determined for Latex B to describe polymer adsorption versus PEO) was varied between each class of polymer. configuration on the smaller latex particles. The objective of the present work is to determine exAll latices were supplied as aqueous dispersions at 2.5 wt % perimental values of r and 6 for a series of chemically solids. The dispersions were prepared by persulfate-initiated similar, but structurally different, PEO linear homoemulsion polymerization of styrene and thus were inherently electrostatically stabilized by surface sulfate groups incorporated polymers and PEO-PPO block copolymers. These results during their preparation. The latex dispersions as supplied by will then be used to evaluate the effect of polymer structhe manufacturer were stated to be “free of other electrolytes, ture and molar mass on the adsorption configuration of residual styrene monomer and surfactants”. Water used in the polymeric stabilizers. The determination of theoretical and preparation of colloidal dispersions, polymer solutions, and experimental stability behavior of dispersions stabilized electrolyte solutions was prepared by distillation of deionized water by these model polymers will be the subject of a future in a two-stage quartz still. manuscript. The model polymer systems chosen for investigation are listed ~~~
Materials and Methods The model dispersions selected for study were two commercially available polystyrene latices obtained from Polysciences Inc. (Warrington, PA). Measurements of the amount of adsorbed polymer (specific adsorption) were determined by using a 900adsorption layer thicknesses were nm-diameter latex (Latex G); determined by using a 56-nm-diameter latex (Latex B). The use of two model dispersions, though undesirable, was necessitated by the conflicting requirements (rapid sedimentation time versus small particle size) of the measuring techniques used to determine I’ and 6. (21) Cowell, C.; Vincent, B. J. Colloid Interface Sci. 1982, 87, 518. (22) Cowell, C.; Vincent, B. J. Colloid Interface Sci. 1983, 95, 573. (23) Long,J.; Osmond, D. W. J.; Vincent, B. J. Colloid Interface Sci. 1973, 42, 545. (24) Vincent, B.; Luckham, P. F.; Waite, F.A. J. Colloid Interface Sci. 1980, 73, 508.
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in Table I. The polymers selected for use as a model stabilizers were primarily commercially available ABA block copolymers and random copolymers of PEO and PPO; however, a series of well-characterized,highly monodisperse PEO linear homopolymers was also studied. Homopolymers spanning a 3-decade range of molar mass were obtained from Polymer Laboratories Ltd. (Shrewsbury, U.K.). These polymers, selected to serve as a reference system, were extremely well-characterized PEO fractions manufactured by Toya Soda for use as gel permeation chromatography (GPC) standards. The molar masses and polydispersity indices (Mw/Mn)were independently obtained by using gel permeation chromatography, light scattering, and viscosity rneasurement~.~~~’~ The molar masses referred to in the present study are those determined by using GPC. Note that all of the homopolymers exhibited a very narrow molar mass distribution ( M w / M n< L l ) , with the highest degree (25) Garvey, M. J.; Tadros, Th. F.; Vincent, B. J. Colloid Interface Sei. 1976, 55, 405.
Langmuir, Vol. 4,No. 4,1988 1057
Polymer Adsorption Configurations of polydispersity exhibited by the very low molar mass fractions (SE-2 and SE-5). The Pluronics were commerical PEO-PPO-PEO (ABA) block polymers manufactured by BASF Wyandotte Corp. (Wyandotte, MI) and marketed as nonionic, water-soluble surfactant^.'^ Pluronics were anticipated to adsorb on hydrophobic polystyrene with the PPO B-block attached as a train to the surface and the PEO A-blocks extending out into bulk solution as tails, at least under good solvency conditions. The primary disadvantage of the Pluronics as a model system is related to their source; i.e., they are commercial products of unknown purity. Accordingly, the molar masses presented in Table I are only approximate, calculated on the basis of the stoichiometry of the polymerization (the two PEO blocks in the Pluronic molecular formula were assumed to be statistically equal in length). In addition, the polydispersities of these samples were unknown. Two other families of water-soluble PEO-PPO copolymers are listed in Table I. The Meroxapols were also obtained from BASF Wyandotte Corp. (Wyandotte, MI). The Meroxapols, PPOPEO-PPO (BAB) copolymen, were essentially inverted Pluronics. Because of the presence of two hydrophobic PPO blocks, the Meroxapols were expected to adsorb strongly on hydrophobic surfaces like polystyrene, leaving the central PEO block extending into bulk solution as a single loop or a series of short loops and trains. Tails were not expected for adsorbed Meroxapols. The Ucons were linear (50-50) PEO-PPO copolymers manufactured by Union Carbide Corp. (Tarrytown, NY). Although the literatureBg refers to Ucons as random (statistical)copolymen of PEO and PPO, the Union Carbide product literaturez0lists a structural formula more indicative of an ABC block copolymer, where A and C are PEO blocks of different molar mass. This structure is similar to Pluronic, although Ucons have only one hydroxylated PEO block of sufficient length to extend into bulk solution as a tail. The adsorption profile for a Ucon can be either a trainjtail of loop/train configuration, depending upon the solvent quality and PEO block length. Specific adsorptions and apparent adsorption layer thicknesses were determined for the PEO-PPO block copolymers listed in Table I as a function of bulk polymer concentration. The results, while primarily of pragmatic interest for theoretical computations of dispersion stability, are discussed in the next section. Because of the limited quantities of PEO homopolymers available, it was not possible to determine specific adsorption isotherms for these polymers. A static adsorption method was used to determine specific adsorption isotherms on Latex G (900-nm diameter); details of the method are provided elsewhere.28 Latex G was used in the adsorption measurements because these particles have similar surface chemistry to the latex used in adlayer thickness measurements (56-nm diameter, Latex B) but are of sufficient size to permit separation from the supernatant liquid by centrifugation in a reasonable time. Initial polymer concentrations were varied between 0 and 600 ppm. The polymer concentration in the supernatant was determined after 24 h from the turbidity of tannic acid-PEO complexes by using the method of Attia and Rubio.29 Several methods are available for determining apparent polymer adlayer thicknesses on particles, including viscometric measurements, { potential measurements, and photon correlation spectroscopy (PCS).4 These techniques all involve the measurement of an apparent hydrodynamic radius for both a bare and polymer-coated particle, the difference between the two radii being the desired adlayer thickness. PCS was selected for use in the present work because it is fast and produces more accurate measurements of layer thicknesses than either viscometry or { potential determinations. In addition, PCS is highly sensitive to the presence of adsorbed "tails", at least for the case where (26) Kastens, A. s. In Polyethers. Part I; Gaylord, N. G.; Ed.; Wiley-Interscience: New York, 1963; p 231. (27) Schmolka, I. R. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1967; p 309. (28) Baker, J. A. Electrosteric Stabilization: The Dynamics of Polymer Adsorption at Charged Interfaces and Implications for Dispersion Stability; Ph.D. Dissertation; University of Washington, 1987. (29) Attia, Y. A.;Rubio, J. Br. Polym. J. 1975,7,135.
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equilibrium polymer concentration (g/mlx IO4) 1. Adsorption isotherms for Pluronics: (a) sDecific adsorption on Latex G; (b) adlayer thickness on Latex B. 0,P75; X, P85; 0 , F68; A, F98; 0, F108.
the adsorbed polymer chains are free draining. Recent theoreticalZ4 and e ~ p e r i r n e n t a l ' ~ ~evidence ' ~ ~ ~ ~ ~ points ~ ~ ~ ~ ~to the importance of tails of determining the effectiveness of a steric stabilizer. Photon correlation spectroscopy was used to determine the mean diffusion coefficient for all modeldispersions in the presence and absence of adsorbed stabilizer. The diffusion coefficient was evaluated by using a cumulants fit to the normalized autocorrelation function obtained for each sample. The apparent hydrodynamic radius (RH)of the particles (or polymer-coated particles) was calculated from the mean diffusion coefficient (D) by application of the Stokes-Einstein equation?2 Measurements carried out on both bare and polymer-coated particles yielded the apparent adsorption layer thickness as the difference in apparent hydrodynamic radii between polymer-covered and bare particles. The PCS measurements were carried out on a Brookhaven Model BI-2030 72-channel digital correlator. The correlator was used in conjunction with a Spectra-Physics Model 124B 15-mW He-Ne laser and a Model BI-POOSM motorized goniometer. Reference PCS conditions (scattering angle, 90"; latex concentration, 1.5 X g/mL; pH 3.0; T = 25 "C) were used for all measurements. Measurements were made 1 h after polymer addition and 24 h after polymer addition. Measurements were also carried out as a function of polymer molar mass at a fixed bulk polymer concentration corresponding to a position on the adsorption plateau for each class of polymer. These concentrations were determined from measurements of the adsorption thickness isotherms for each polymer on the smallest g/mL for the Pludiameter latex and corresponded to 5 X ronics, 2.5 X lo4 g/mL for the Meroxapols and Ucons, and 5 X g/mL for the PEO homopolymers.28
Results and Discussion Specific adsorption and adsorption layer thickness isotherms are plotted in Figures 1-3 for representative Plu(30) Cohen-Stuart, M.A.; Waajen, F. H. W. H.; Cosgrove, T.; Vincent, B.; Crowley, T. L. Macromolecules 1984, 17, 1825. (31) Cohen-Stuart, M.A,; Scheutjens, J. M. H. M.; Fleer, G. J. In Polymer Adsorption and Dispersion Stability; Goddard, E. D.; Vincent, B., Eds.; ACS Symposium Series No. 240; American Chemical Society: Washington, DC, 1984; p 53. (32) Weiner, B. B. In Modern Methods of Particle Size Analysis; Barth, H. G., Ed.; Wiley: New York, 1984; p 1.
1058 Langmuir, Vol. 4,No. 4, 1988 I6 I
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Figure 2. Adsorption isotherms for Meroxapols: (a) specific adsorption on Latex G; (b) adlayer thickness on Latex B. 0,252; 0 , 108; A, 258. I
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equilibrium polymer concentration (g/mlx IO4) Figure 3. Adsorption isotherms for Ucons: (a)specific adsorption on Latex G; (b) adlayer thickness on Latex B. 0,50-HB-260; 0 , 50-HB-2000; A, 50-HB-5100.
ronics, Meroxapols, and Ucons. The adsorption isotherms exhibit the expected behavior, with both adlayer thickness and specific adsorption increasing with PEO block size (MJPEO)) and bulk polymer concentration (CJ. The thickness isotherms have essentially the same shape as conventional high-affinity specific adsorption isotherms, rising rapidly at low concentrations and levelling off to pseudoplateaus a t bulk polymer concentrations corresponding to the plateau adsorption ~ o v e r a g e . ~ As expected from the work of Kayes and Rawlins,ls the specific adsorption values for Pluronics reach their plateau values at bulk polymer concentrations slightly higher than the critical micelle concentrations (cmc) for these surface-active polymers (ea. 8 X lo-" g/mL). In addition, the
concentration corresponding to the onset of the adsorption plateau is well-above the concentration required to create a monolayer of polymer segments at the particle surface (assuming all segments are in contact with the surface). This provides strong evidence in support of a polymer adsorption configuration involving thin PPO loops and trains attached to the surface, with longer PEO tails extending away from the surface. Trains refer to portions of the polymer in which all segments are in direct contact with the surface. Both the thickness and specific adsorption isotherms are somewhat rounded in the subplateau transition region, indicative of a certain degree of polydispersity, which is expected for these commercial polymers. At equilibrium polymer concentrations well above the plateau values (>0.004g/mL for the high molar mass Meroxapols and Ucons) a slight increase in the adsorbed layer thickness is observed. This may represent the onset of multilayer adsorption or alternatively may arise as an artifact of the minute increase in viscosity of the dispersion medium at these polymer concentrations. None of the data in Figures 1-3 has been corrected for viscosity changes, since Kayes and RawlinP demonstrated that such corrections are negligible for Pluronics at similar bulk polymer concentrations. The experimental adlayer thickness data have relatively large scatter, particularly at very low values of C,. This is unavoidable with the thin layers produced by PEO-PPO copolymer adsorption. It should be noted, however, that the experimental error in these data is significantly lower than the error for the corresponding PCS measurements conducted by Kayes and Rawlins.ls It is instructive to compare the plateau surface coverages determined in the present work to those reported in the literature. Tadros and Vincent16 obtained a plateau adsorption value of approximately 8.5 X and 1.2 X g/m2 for Pluronics F68 and F108, respectively. The latter measurements apply to adsorption on a 312-nm-diameter, carboxylated latex; the measurements of Tadros and Vincent were carried out with a 236-nm-diameter, sulfate-stabilized latex. The specific adsorption isotherms shown in Figure 1 are in generally good agreement with these literature values for the plateau adsorption (within experimental error, typically fO.OOO1 g/m2). The largest deviations are observed for the lowest molar mass polymer (P75). For this polymer we obtained a plateau adsorption value approximately 0.0003 g/m2 less than the value determined by Tadros and Vincent. This is not a serious deviation, however, since the largest experimental errors always arise with the lower molar mass polymers, for which the adsorbed amounts, and hence changes in bulk polymer concentration, are smallest. The experimental and literature plateau adsorption values for Pluronic F68 agree to within a few percent; however, the data of Kayes and Rawlins for F108 adsorption are approximately 0.0002 g/m2 less than our experimental values, although still within the estimated experimental error. These results suggest that the plateau adsorption values for Pluronics are approximately independent of particle size, at least within the range of experimental error. This result is not surprising for block copolymer adsorption, where the driving force for adsorption is the specific interaction between the hydrophobic polystyrene surface and the hydrophobic PPO block of the Pluronics. The dependence of the apparent adlayer thickness on molar mass was determined for each of the three model
Langmuir, Vol. 4, No. 4, 1988 1059
Polymer Adsorption Configurations IO0
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Figure 5. PEO molar mass dependence of plateau adlayer thickness for PEO-PPO copolymers on Latex B. 0 , Pluronics; A, Meroxapols; 0 , Ucons. (-) Pluronics least-squaresh e ; (- - -) calculated PEO 2R,; (--) calculated total 2R,; (---) PEO contour length. copolymer systems, as well as the PEO homopolymers. These measurements were carried out at bulk polymer concentrations corresponding to the adsorption plateau. The results are plotted in Figure 4 and 5. Error bars are not shown, due to the use of logarithmic axes; however, errors are typically fl-2 nm. Figure 4 presents data for the molar mass dependence of adsorption layer thickness for the model PEO homopolymers measured at two concentrations corresponding to the adsorption plateau.16 Measurements of apparent adlayer thickness were made at two times, corresponding to 1 and 24 h after initial mixing. Least-squares fitting of the 1-h data in Figure 4 yields a dependence of thickness on molar mass corresponding to MWo2'. Two data points, corresponding to SE-15 and SE-70, exhibit anomalous scatter from the least-squares line. If these points depenare omitted from the regression analysis, a Mw0.35 dence is obtained. Both of these molar mass dependencies are lower than the Mw0.5 or Mwo.6 proportionalities predicted by theory for an isolated linear homopolymer in a O-solvent or a good solvent, respectively.33p34 This suggests that the adsorbed polymers adopt a configuration flatter than the random coil configuration obtained in a O-solvent. Comparison of (33) Flory, P. J.; Krigbaum, W. R. J. Chem. Phys. 1960, 18, 1086. (34)Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (35) Cabane, B.; Duplessix, R. J . Phys. (Les Ulis, Fr.) 1982,43,1529, 1579.
the measured adlayer thicknesses with light-scattering data for PEO homopolymers in free solution16i36reveals that the adsorption configuration is approximately equal to or slightly greater than twice the free solution radius of gyration (RJ. The measured PEO thicknesses decrease significantly after 24 h, reaching a constant value (approximately 3.5 nm) independent of molar mass. This result, along with dependence of the adlayer thickness, the inital Mw0,35 suggests that the homopolymers initially adsorb in a loop/train/tail configuration, with tails providing the main contribution to the hydrodynamic thickness. Over time, however, the adsorption configuration changes to a flatter arrangement as the free tails gyrate through space, eventually finding bare patches of particle surface on which to adsorb additional segments. The result is a shift in adsorption configuration from one in which the tails determine the thickness to one in which the loops dominate the thickness profile. Such a change in adsorption configuration could arise from subtle differences in adsorption energetics for polymer segments and solvent molecules on the solid surface. The time-dependent layer thickneses observed in the present study support this hypothesis and also point to the possible importance of kinetic (diffusive) effects on the measured adlayer thicknesses. Thus, the time required for a free tail to undergo diffusion, find a surface site that is energetically favorable for adsorption, and then adsorb segments on the surface is reflected in the time dependence and magnitude of the measured adlayer thickness. Adlayer thicknesses were also determined at a bulk polymer concentration of 5 X 10" g/mL, in order to ensure that measurements were in fact made at the adsorption plateau. After 1 h, the PEO adlayer thicknesses were found to increase with Mw0.43.This value is significantly lower than the Mark-Houwink coefficient (Mw0.78-M Wo.82) obtained for the molar mass dependence of intrinsic viscosity of high molar mass PEO in free solution,36again suggesting an adsorption configuration perturbed significantly from a random coil configuration. After 24 h the adlayer thicknesses again reach an essentially constant value (8 nm) which is independent of molar mass. These results are again consistent with an initial loop/ train/tail adsorption configuration, with subsequent tail adsorption acting to decrease the adlayer thickness with time. The mean thickness after 24 h (8 nm) is significantly greater than the 24-h value obtained at 5 X g/mL (3.5 nm). This suggests that the mean loop size (or remaining tail length) is greater at the higher polymer concentration, which is expected, since fewer vacant surface sites exist for tail adsorption at the higher surface coverage. It must be emphasized that the observed decrease in layer thickness with time shown in Figure 4 is a real effect and does not arise from flocculation/sedimentation phenomena or polymer polydispersity. The former can be ruled out by the observation that the sample polydispersities are relatively low (
