Langmuir 1991, 7, 2723-2730
2723
Kinetic and Equilibrium Aspects of Block Copolymer Adsorption Fredrik Tiberg,' Martin Malmsten, Per Linse, and Bjorn Lindman Physical Chemistry 1, Chemical Center, University of Lund, P.O.Box 124, ,9221 00 Lund, Sweden Received February 26, 1991. In Final Form: June 23, 1991
The kinetic and equilibrium adsorption characteristics at hydrophobized silica surfaces of nonionic block copolymers of the Pluronic PE type have been investigated by means of in situ ellipsometry. In parallel, an extended mean-field theory, which takes the reversed temperature phase behavior of the system into account, was employed to model the adsorption. It was observed that both kinetic and equilibrium features of the adsorption process are intimately related to the phase behavior of the bulk copolymer solution. In particular, a very strong increase in the adsorbed amount is observed as the system approaches the phase boundary. The qualitative aspects of this finding were predicted theoretically as being a consequence of a partial phase separation phenomenon due to elevated copolymer concentration in the surface zone.
Introduction Triblock copolymers of the poly(ethy1ene oxide)-poly(propylene oxide)-poly(ethy1ene oxide) type, often referred to by the trade name Pluronic, have found important and versatile applications in a variety of technical fields. The applicability of this type of block copolymers is intimately related to the self-association phenomenon exhibited by these moleculesboth in solution and adsorbed at interfaces, as well as to the efficiency with which these copolymers provide steric stabilization of colloidal dispersions. Block copolymers of this type have been shown to form micelles in dilute aqueous solutions, provided that the ratio between the more hydrophobic poly(propy1eneoxide) (PPO) blocks and the more hydrophilic poly(ethy1ene oxide) (PEO) blocks is suitable and that the temperature and the concentration are sufficiently high.lP2 Furthermore, like other nonionic polymers and low molecular weight surfactants that contain ethylene oxide segments, these copolymers display a reduced aqueous solubility at elevated temperatures manifested by the existence of a lower consolute point. The phase separation temperature a t a given concentration, usually referred to as the cloud point (CP), primarily depends on the segment composition and the molecular weight of the copolymer, but is also affected by the polymer concentration and the presence of cosolutes, e.g., salts, alcohols, and surfactant^.^ Due to the amphiphilic nature of block copolymers, they also tend to adsorb extensively to a large variety of interfaces. Consequently, a great deal of effort has been directed toward the field of block copolymer adsorption. Several comprehensive and wide-ranging theories with applicability to adsorption of copolymers, based on the mean-field theory of Scheutjens and Fleer,4p5have recently been presented. Most of these theories deal with adsorption properties such as surface density, block segment distribution profile, and thickness of adsorbed There have also been numerous parallel developments in (1) Zhou, 2.;Chu, B. J. Colloid Interface Sci. 1988, 126, 171. (2) Wanka, G.;Hoffman, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268.101. ---I
(3) Lindman, B.;Carlsson, A.; KarlstrGm, G.; Malmeten, M. Ado. Colloid Interface Sci. 1990, 32, 183. (4) Scheutjens, J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1979,83,1619. (5) Scheutjens, J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1980,84,178. (6) van Lent, B.; Scheutjens, J. M. H. M. Macromolecules 1989,22, 1931.
the experimental techniques for studying the adsorption phenomena (cf. current reviewslOJ1). Different methods, e.g., surface force measurements,12-14 small-angle neutron scattering (SANS),11J5nuclear magnetic resonance (NMR),leevanescent wave induced fluorescence (EWIF),17 and different hydrodynamic t e c h n i q u e ~ , ~have ~ J ~successfully been applied in order to determine conformational features of adsorbed polymer and surfactant layers. Most of these methods have been used exclusivelyfor equilibrium measurements. Kinetic studies have mainly been performed by means of radio labeling,20 total internal reflection fluorescence (TIRF),21infrared spectroscopy,22 refle~tometry?~ and e l l i p ~ o m e t r y . ~ ~ * ~ ~ Previous adsorption investigations have, in particular, been concerned with the equilibrium aspects of adsorbed layer characteristics. The majority of the experimental work has been dedicated to adsorption Depending on the system, Langmuir or high-affinity (7) Evers, 0.A.; Scheutjens, J. M. H. M.; Fleer, G. J. Macromolecules 1990.23.5221. , ~ , (8) Evers, 0. A.; Scheutjens, J. M. H. M.; Fleer, G.J. J. Chem. SOC., Faraday Trans. 1990,86,1333. (9) Munch, M. R.; Gast, A. P. Macromolecules 1988,21, 1366. (10) Cohen-Stuart, M. A,; Cosgrove, T.; Vincent, B. Adu. Colloid Interface Sci. 1986,24,143. (11) Coegrove, T. J. SOC.Chem., Faraday Trans. 1 1990,86,1323. (12) Taunton, H. J.; Toprakcioglu, C.; Klein, J. Macromolecules 1988, 21. - , 3333. - - - -. (13) Luckham, P. F. Adu. Colloid Interface Sci. 1991, 34,191. (14) Hadziiannou, G.;Patel, S.;Granick, S.;Tirrell, M. J . Am.Chem. SOC.1986,108, 2896. (15) Cosgrove, T.; Heath, T. G.;Ryan, K.; Crowley, T. L. Macromolecules 1987,20,2879. (16) Coegrove, T.; Ryan, K. Langmuir 1990,6, 136. (17) Rondalez, F.; Aweme, D.; Hervet, H. Annu. Reu. Phys. Chem. 1987,38,317. (18) Koopal, L. K.; Hlady, V.; Lyklema, J. J. Colloid Interface Sei. 1988, 121, 49. (19) Cohen-Stuart, M.A.; Waajen, F. H. W. H.; CosgroveT.;Vincent, B.; Crowley, T. L. Macromolecules 1984,17, 1825. (20) Pefferkorn, E.; Carroy, A.; Varoqui, R. J. Polym. Sci., Polym. Phvs. Ed. 1985.23.1997. 121) Lok, B. 'R.;'Cheng, Y.-L.; Robertson, C. R. J. Colloid Interface Sci. 1983, 91, 104. (22) Coegrove, T.; Prestidge, C. A,;Vincent, B. J. Chem. SOC.,Faraday Trans. 1990,86, 1377. (23) Dijt, J. C.; Cohen Stuart, M. A.; Hofman, J. E.; Fleer, G. J. Colloids Surf. 1990, 51, 141. (24) Stromberg, R. R.; Passaglia, E.; Tu-, D. J. J. Res. Natl. Bur. Strand. Sect. A. 1964,68,601. (25) Takahashi, A.; Kawaguchi, M.; Hirota, H.; Kato, T. Macromolecules 1980,13, 884. (26) Gilliland, E. R.; Gutoff, E. B. J . Phys. Chem. 1960,64, 407. ~~
0 1991 American Chemical Society
Tiberg et al.
2724 Langmuir, Vol. 7, No. 11, 1991
isotherms have usually been found. However, a complete picture of the adsorption phenomenon requires an examination of the kinetic and dynamic features of the adsorption process. Not surprisingly, therefore, several papers focusing on these aspects of the adsorption process have been published recently. These concern topics such as conformational rearrangements,31b2adsorption reversibility,S2 preferential ads0rption,2~exchange with bulk solution,20 and effects of micellization on the rate of ad~orption.~~ The present study reports on some aspects of the adsorption of Pluronic PE-type triblock copolymers, in particular Pluronic PE 6200, from aqueous solutions to a hydrophobic surface. The intention of the work was to study the kinetic and equilibrium properties of the adsorption process in relation to the phase behavior of the bulk solution. The results were obtained by means of in situ ellipsometry complemented by surface tension and cloud point measurements. Theoretical model calculations based on an extension of the mean-field theory formulated by Evers et ala7to the case where the reversed temperature dependent phase behavior is taken into account are also presented.
Experimental Section Method and Equipment. All direct adsorption measurements were performed in situ by means of null ellipsometry. A comprehensive review of the methodology of ellipsometry, including the underlying theory, is given by Azzam and Bashara.91 The instrument used was an automated Rudolf thin-film ellipsometer, Type 43603-2003, controlled by a personal computer. A xenon lamp, filtered to 400 nm, was used as the light source. Before the actual adsorption measurements were performed, the complex refractive index of the surface was determined. Thereafter, the polymer sample was added to the thermostated cuvette and the ellipsometric angles \k and A were followedcontinuously. The maximal time resolutioncorresponds to a time interval between the measurementsof 3-4 s. Stirring was performed by a magnetic stirrer at -300 rpm, while rinsing was achieved by a continuous flow (20 mlqmin-') of 250 mL of doubly distilled Millipore water through the cuvette. From the ellipsometric angles,the mean values of the refractive index (nf) and the thickness ( d ) of the adsorbed film were calculated and from these parameters the adsorbed mass in excess of the bulk concentration(ra;vide infra)was calculatedaccordingto Cuypers et a1.u The reason for discussingthe adsorptionprocess in terms of the adsorbed amount rather than the film thickness and refractive index is that the former parameter is subject to much less error than the two latter. The absolute deviations in the adsorbed amount were found to be less than f0.2 mg.m-2. Even though we are not primarily interested in the absolute values of the adsorbed amount, but rather in trends, it is still interesting to note that good agreement is generally found between the adsorbed amounts determined by ellipsometry and those determined by radiolabeling techniques.*37 (27) Kawaguchi, M.; Aoki, M.; Takahashi, A. Macromolecules 1988,
16, 635.
(28) Tadros, Th.F.; Vincent, B. J. Phys. Chem. 1988,84, 1575. (29) van den Boomgaard, Th.; King, T. A.; Tadros, Th.F.; Lyklema, J. J . Colloid Interface Sci. 1987, 116, 1. (30) Dawkins, J. V.; Guest, M. J.; Taylor, G. In The Effect ofPolymers on Dispersion Properties; Academic Press: London, 1982. (31) Leermakere, F.A. M.; Cast, A. P. Macromolecules 1991,24,718. (32) Kawaguchi, M.; Takahashi, A. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 2069. (33) Munch, M. R.;G a t , A. P. J . Chem. SOC.,Faraday Tram. 1 1990, 86, 1341. (34) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, The Netherlands, 1989. (35) Cuypers, P . A.; Corael, J. W.; Jansen, M. P.; Kop, J. M.M.; Hermens, W. Th.; Hemker, H. C. J. Biol. Chem. 1983,268, 2426.
Surface tension measurements were performed at 23 f 1 O C accordingto the drop volume principle. A thorough description of the apparatus used is found in ref 38. Block Copolymers, For this investigationwe used commercially available low-polydispersityblock copolymers, Pluronic PE 6200 and PE 6800, obtained from BASF. These are ABA triblock copolymers, where A is poly(ethy1ene oxide), and B is poly(propy1ene oxide). PE 6200 has an average molecular weight M., = 2660 and a molecular weight distribution Mw/Mn= 1.15, as evidenced by gel permeation chromatography (GPC). For PE 6800 the corresponding values were M., = 11 440 and M.,/M. 1.16. According to the manufacturer, PE 6200 consists of approximately 20 wt 76 ethylene oxide units, while the corresponding value for PE 6800 is approximately 80 wt % . The molecular weights of these Pluronica are relatively low and these molecules are therefore expected to display solution and interfacial properties intermediate in character to that exhibited by surfactants and larger block copolymers. Surface. Polished silicon test slides (thickness -380 Irm, p-type,boron-doped,resistivity 1-20 Dcm) were purchased from OkmeticLtd. The silicon wafers were oxidized thermallyin pure and saturated oxygen at 920 O C for 65 min followed by annealing and cooling in an argon flow to obtain an oxidized layer thickness in the range of 360-370 A. The oxidized wafers were cut into slides with a width of 12.5 mm. The slides were then cleaned in a mixture of 25 % NHdOH (Pro Analysi, Merck),30 % H202 (Pro Analysi, Merck),and HzO (1:1:5, by volume) at 80 O C for 5 min, followed by cleaning in a mixture of 32% HC1 (Pro Analysi, Merck), 30% H202, and H2O (1:1:5, by volume) at 80 OC for 5 min. Then the slides were rinsed twice, with, in order, distilled water, ethanol, and trichloroethylene(ProAnalysi, Merck).The slides were made hydrophobic by treatment with a 0.1 w t % solution of C12(CHs)zSi(Merck) in trichloroethylene(Pro Analysi, Merck) for 90 min. Finally they were rinsed twice in trichloroethylene (Pro Analysi, Merck) and ethanol. This procedure rendered the slides hydrophobic,with a critical interfacetension of 27 mN.m-'." They were then kept in absoluteethanol until use. Before the slides were placed in the ellipsometer cuvette, they were allowed to stabilizein doubly distilled Milliporewater at room temperature (23 "C) for 24 h.
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Theoretical Modeling Background. The Flory-Huggins lattice theory of homogeneous solutionsa can be extended to describe the adsorption of polymers at surfaces. In a heterogeneous system, the solution close to a surface is divided into layers parallel to the surface, where the thickness of the layers corresponds to the size of the solute (or of the polymer segment). Within each layer the densities are constant as a consequence of the mean-field approximation. Density gradients perpendicular to the surface are created by allowing the volume fraction of the components to vary among the layers. Recently, Evers, Scheutjens, and Fleer7 formulated a theory for adsorption of block copolymers from a multicomponent mixture. The theory takes into account the polymer connectivity as well as all polymer conformations. The spacedistribution of the solvent and polymer segments are the primary results, and hence the polymer volume fraction profile as well as the adsorbed amount is easily available. The theory of Evers et al. has been generalized to the case where the block copolymer segments contain internal degrees of freedom." The extension provides a means of ~~
~
(36) de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978,17, 1759. (37) Arnebrant, T.; Nylander, T. J . Colloid Interface Sci. 1986,111, 529. (38) Tornberg, E.J . Colloid Interface Sci. 1977, 60, 50. (39) Wahlgren, M.; Arnebrant, T. J. Colloid Interface Sci. 1990,136, 259. (40) Flory, J. P.Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953.
Kinetic and Equilibrium Aspects of Block Copolymer Adsorption
modeling effective segment-segment interaction parameters which are temperature as well as density dependent. This polymer model was originally devised to describe the existence of a lower consolute point in homogeneous aqueous PEO solutions,42 but has been extended to heterogeneous systems of homopolymer^.^^ The basis of the PEO model is that the distribution of conformations of a segment depends on temperature, and that different conformations interact differently with adjacent polymer segments and solvent molecules. From quantum mechanical calculations,"4the conformations of the OCCO segment were divided into two classes or states, one being polar and having a low energy and a low statistical weight, and one being less polar or nonpolar and having the higher energy and higher statistical weight. At low temperature the former state is dominating, and thus a more favorable polymer-water interaction is obtained, whereas at elevated temperatures the latter state becomes progressively more important, which results in a more unfavorable polymerwater interaction. Model Calculations. On the basis of the molecular weight and the fraction of ethylene oxide, PE 6200 and PE 6800 are described as (EO)e(P0)&0)6 and (E0)lm(P0)3g(E0)104,respectively. Since aqueous solutions of both PEO and PPO display a lower consolute both EO and PO segments are modeled with two internal states, one of which will be assigned polar and the other nonpolar. The model requires 18independent parameters of which4 describe the internal states of EO and PO, whereas the remaining essentially are interaction parameters of Flory-Huggins type. Ten parameters were obtained by fitting calculated P E O - ~ a t e r ~and l * ~PPO-waterq1 ~ phase diagrams to the experimental ones given in ref 45. In order to keep the description as simple as possible, we do not discriminate between EO and PO when considering the four EO-PO cross-interaction parameters as well as the EO and PO interactions with the hydrophobic surface, although these interactions in principal are unequal. This reduces the remaining eight parameters down to only three nonzero parameters. The polarnonpolar cross parameters between EO and PO were chosen as the average of the corresponding cross-interaction parameters of EO and PO, whereas the surface parameters were chosen such that the nonpolar group interacts most favorably and the water molecules most unfavorably with the surface. The values of all parameters are given in Table I. The calculation involves a self-consistent determination of the volume fractions of all species (EO and PO groups and water molecules)and state distributions of the EO and PO groups in each layer of the heterogeneous system. Further aspects of the calculations are given in ref 41. Two different definitions of the adsorbed amount may be distinguished. The one most related to ellipsometry is the adsorbed amount in excess of the bulk concentration defined as rexxi(#i- #b)where #i denotes the volume fraction in layer i and #b the volume fraction in bulk. The sum of layers extends sufficiently far so that the volume fraction attains its bulk value. In the second definition of the adsorbed amount, denoted by r, we only consider primary adsorbed copolymers, i.e., the sum comprises only those polymer molecules which have at least one segment in contact with the surface.
Langmuir, Vol. 7, No. 11,1991 2725
Table I. Internal State Parameters U m (Energy; kJ-mol-') and gm (Statistical Weight), Surface Interaction Parameters X B , . ~ - and State-State Flory-Huggins Interaction Parameters X B ~
\
(41) Linse, P.; Bjtirling, M. Macromolecules, in press. (42) Karlstr6m, G. J . Phys. Chem. 1986,89, 4962. (43) Bjkling, M.; Linse, P.; Karlstrtim, G. J. Phys. Chem. 1990,94, 471. (44) Anderson, M.; Karlstrtim, G. J. Phys. Chem. 1985,89, 4957. (45) Malcolm, G. N.; Rowlingson, J. S. Trans. Faraday SOC.1957,53, 921.
species
state
1
EO
polar nonpolar polar nonpolar
PO
Um
stateno.
water
2 3 4 5
0 Ob 5.0B6b
oC 11.5c
gm 1
Ib Bb 1C 6oC
kTxe,url.,g 10.0 6.7 3.5 6.7 3.5
k TXBB~ state no. state no.
2
3
4
5
1 2 3 4
0.650Bb
5.56Bb 1.266b
1.V
8.5c
od
k3e
1.3e
1.4'
The values correspond to adsorption parameters kTxpoh= 0.825 and ~ Z ' X , , ~ , , = ~ ~1.625 kJ-mol-'. The adsorption parameter x, is related to the interaction parameters by x1 = - A ~ o ( x ~ ~ , , ~ u-r xloivent,lurf.M)where A10 = 1/4 (hexagonal lattice). See also ref 39. From fitto the experimental PEO-water phase diagram.41.42 c From fit to the experimental PPO-water phase diagram." d Taken to be equal. e Taken to be equal.
Although the qualitative effects predicted by the model calculations agree very well with the experimental findings, there are some ambiguities regarding the temperature dependence and the absolute values of the adsorbed amount. In the cases considered, the theoretical modeling predicts the unlimited increase of the adsorbed amount to occur at a higher temperature than that observed experimentally (vide infra). The reason for this is simply that the theoretical cloud point is higher than that observed experimentally due the differences between the theoretically calculated and experimentally measured phase diagrams of aqueous Pluronic P E 6200 solution. Hence, the adsorbed amount should be related to the proximity of the cloud point, and not to the absolute value of the temperature. The discrepancy of the cloud point could be avoided completely by a homogeneous scaling of the interaction parameters or reduced by an improvement of the cross-interaction parameters. Here these parameters were assigned some reasonable values in an ad hoc fashion and have not been optimized. Improvements can also be achieved by taking into account the different size of the solvent molecules and the polymer segments. Since neither of these refinements is important for understanding the adsorption of Pluronic in this study, we have not pursued any further studies along these lines. Additionally, there is an ambiguity of transferring the calculated adsorbed amount rex, expressed as equivalent number of monolayers, to weight per surface area, as obtained from experiment. Therefore, we refrain from such conversions, but notice that in order to obtain a satisfactory agreement, we have to use a lattice size of -4 A,which is reasonable. Rssults As previously discussed, aqueous Pluronic solutions separate into two liquid phases upon heating. To quantify this, we measured the cloud point of Pluronic PE 6200 as a function of concentration up to 10 wt % . The results of these measurements are shown in Figure 1. As can be seen, the cloud point initially decreases strongly with increasing copolymer concentration, whereafter it levels off a t -23 "C. To gain further information on the solution behavior, surface tension measurements were performed. The
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Langmuir, Vol. 7, No. 11, 1991 50
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Figure 3. Adsorption isotherms for PE 6200 at 25 (filled triangles) and 30 "C (open diamonds). The inset shows the isotherms obtained by means of made1calculationsat 30 O C . The cloud points are indicated by dashed lines.
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Figure 2. Surface tension as a function of the concentrationof PE 6200 at 23 O C . cmc is indicated by the arrow. The solid lines are only aids to the eye. surface tension for PE 6200 (against air) as a function of concentration is shown in Figure 2. As for typical surfaceactive compounds, there is an initial steady decrease of the surface tension with increasing concentration, followed by a break in the curve a t a characteristic concentration. Above this concentration the surface tension was observed to be approximately constant. The final value of the surface tension is -45 "em-' while the break point concentration, identified as the apparent cmc, is approximately 1 X 10-3wt %. The Pluronics used in the present study were found to adsorb on hydrophobicallymodified silica surfaces, while pure poly(ethy1ene oxide) did not (results not shown). The experimental adsorption isotherms for Pluronic P E 6200 a t 25 and 30 O C are presented on a logarithmic scale in Figure 3. The inset displays results obtained by theoretical modeling. Initially, the adsorbed amount of PE 6200 increases slowly with copolymer concentration while a very pronounced increase in the adsorbed amount is observed at higher concentrations. This sharp increase in adsorption takes place at a bulk polymer concentration of approximately 5 X wt % at 25 "C. An increase of the temperature to 30 "Cresults in a lowering of this break point concentration to approximately 10-2wt '5%. As can be seen in Figure 4, no such large rise in the adsorbed amount is observed in the isotherm for PE 6800 at 25 "C. Instead, the adsorbed amount levels off and remains quite low a t higher copolymer concentrations. We note that the cloud point is above 100 "C for PE 6800 in water. The temperature dependence of the adsorption was also investigated in more detail for Pluronic PE 6200 (Figure
A
A
1
0 '
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1
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Figure 4. Adsorption isotherm for PE 6800 at 25 "C. The inset showsthe correspondingisotherm obtained by model calculations.
42 tt 0 ' 10
* I
20
* I
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30
40
T/"C Figure 5. Adsorbed amount of PE 6200 ),'I( as a function of temperature at a copolymer concentrationof 1 x 10-1wt %. The inset shows the correspondingtemperature dependenceobtained by means of model calculation at a copolymer concentration of 2 X 10-* vol % The cloud points are indicated by dashed lines.
.
5). This was done a t a constant copolymer concentration of 1 X 10-l wt %. The temperature dependence was observed to be well correlated with the concentration dependence. Hence, well below the cloud point, only minor effects of temperature on the adsorbed amount were observed. As the temperature approachesthe cloud point, however, there is a dramatic increase in the adsorbed amount (Figure 5). To gain more information on the relation between the adsorption characteristics and the phase behavior, some
Langmuir, Vol. 7,No.11, 1991 2727
Kinetic and Equilibrium Aspects of Block Copolymer Adsorption 1.4 j
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tls Figure 7. Adsorbed amount of PE 6200 (rex) as a function of adsorption time ( t )at different bulk concentrations (cb1 cmc): (a) 1 X 10-9, (b) 1 X 10-2, (c) 1wt %.
time-resolved adsorption experiments were performed a t different copolymer concentrations ranging from 3 X lo+ to 1wt % . Figures 6 and 7 illustrate some of the results obtained for PE 6200 in different concentration regions. At concentrations less than 1 X w t % , the initial rate of adsorption is more or less proportional to the bulk polymer concentration and to the square root of the adsorption time, clearly indicating transport-limited adsorption in this region. The adsorpion characteristics change to a large extent as the block copolymer concentration approaches and wt % 1. The initial and overall exceeds the cmc (rates of adsorption are extremely high close to the cmc. An almost total decline in the adsorption rate is observed in a few minutes, and 90% of the saturation coverage is reached in just a few seconds. A t still higher concentrations, the adsorption once more extends over longer times (Figure 7). The overall rate of adsorption is now drastically lowered and 90% of the plateau value is not reached until after more than 30 min at a polymer concentration of 1 wt %, while the initial rate of adsorption remains essentially unaltered. Upon rising after an adsorption time of 8000s, a fraction of the adsorbed block copolymer is desorbed. The fraction desorbed was found to increase with the adsorbed amount, which is illustrated in Figure 8.
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Discussion According to the experimental results presented, the adsorption of PEO-PPO-PEO triblock copolymers onto
Figure 8. Adsorbed amount of PE 6200 (rex) as a function of adsorptiontime (t) at differentbulk concentrations: (a) 1 x 10-6, (c) 1 wt %. Rinsing is indicated by the arrow. (b) 1 x methylated silica from water is quite high, whereas PEO does not adsorb to a detectable extent. Hence, it seems clear that the most hydrophobic moiety of the copolymer, the PPO block, is responsible for the adsorption onto hydrophobic surfaces, while the more hydrophilic PEO chain is extended into the solution. The driving force for adsorption seems to be a combined effect of a hydrophobic attraction between PPO segments of the copolymer and the surface and poor solvency conditions for these segments. From this reasoning it follows that both the total segment density and the adsorbed layer composition will vary strongly within the interfacial region. The preferential adsorption of diblock copolymers has been discussed previously in some detail.6 Adsorption Isotherms. The adsorption isotherm for P E 6200 (Figure 3) onto methylated silica is not of a simple Langmuirian or high-affinity type. Instead, a smooth curved isotherm, involving a very marked rise of the adsorbed amount as the concentration approaches the phase boundary, is observed. A similar isotherm was observed in a previous study on the adsorption of nonionic surfaceactive agents on Graphon by Corkill et a1.& We suggest that the strong increase in adsorption observed at high concentrations of P E 6200 is due to a partial phase separation in the surface region, possibly resulting in formation of multilayers. This notion is supported by the very high adsorbed amounts observed, and by the finding that the break point for the onset of the prominent increase of the adsorbed amount occurs at a lower concentration at a higher temperature (cf.the cloud point curve in Figure 1). Note also that although PE 6200 seems to form micelles in solution (Figure 2), the aggregation is rather limited, as indicated by the small hydrodynamic radius of the micelles (RH= 10-15 A, as evidenced by FT-PGSE NMR diffusion measurements), even at high copolymer concentrations. Hence, no temperature- and concentration-dependent micellization is primarily responsible for the increased adsorption. For P E 6200, the incipient phase separation takes place at a concentration more than 1order of magnitude lower than the phase separation concentration in the bulk solution. Provided that the conditions for phase separation at the interface are similar to those in the bulk, this result indicates that the copolymer concentration in the surface zone is much higher than that in the bulk solution. This is not evident from earlier results, since it is usually found that the segment density profile varies quite gently normal (46) Corkill, J. M.;Goodman, J. F.;Tab,J. R. Trans. Faraday SOC.
1966, 62,979.
Tiberg et al.
2728 Langmuir, Vol. 7, No. 11, 1991 to the surface, and that dilute loops and tails constitute the outer part of the adsorbed layer. (cf., e.g., ref 11). Block copolymers, however, are likely to adsorb in layers that are more compact than those formed by homopolymers, and especially for low molecular weight block copolymers could a high copolymer concentration in the adsorbed layer be expected. However, we cannot exclude the possibility that the finite polydispersity, as well as the possible heterogeneity in composition, contributes to the observed difference. This is particularly likely since the surface-to-volume ratio in the cuvette is very small ( A / V = 0.5 cm-l). The isotherms for PE 6200 were also theoretically calculated with the extended mean-field model described previously. Note, that the excess adsorbed amount (rex) from the model calculations is given in terms of the number of equivalent monolayers, while the corresponding unit for the excess adsorbed amount obtained experimentally is in milligrams per square meter. As can be seen in the inset in Figure 3, the theoretical adsorption isotherm displays the same qualitative features as the experimental isotherm, including the strong increase in the adsorbed amount at elevated copolymer concentrations. Again, this provides strong support for the notion of an incipient phase separation at the surface. Also concluded from Figure 3 is that at low concentration the predicted adsorption is much smaller than the measured one. This discrepancy is partly remedied by employing unequal EO-surface and PO-surface interaction parameters. However, since unequal parameters are not necessary for qualitative understanding and since required experimental data for extracting the interaction parameters unambiguously are lacking, we refrain from doing a more detailed analysis at present. Another contribution to the discrepancy at concentrations below 1X lo-' vol 76 is that the predicted clouding temperature increases faster than the experivol 3' 6 the mentally observed upon dilution; Le., at 2 X predicted clouding temperature is 130 "C (cf. Figure 1). This discrepancy between the experimental and predicted cloud point curves at low concentrations is most likely a consequence of an underestimation of the local segment concentration due to the mean-field approximation. Further testing of the validity of the incipient phase separation hypothesis was attained through the determination of the adsorpion isotherm for PE 6800, a copolymer with a higher EO content and a higher cloud point (>lo0 "C). The result is shown in Figure 4. Notably, no sharp rise of the adsorbed amount is observed in the adsorption isotherm, thus providing further support for the our hypothesis. Again, the model calculations agree very well with the experimental observations (Figure 4, inset). Temperature Effects. The temperature dependence of P E 6200 adsorption is shown in Figure 5. Model calculations, taking the phase behavior into account, are in good agreement with the experimental findings. The general effect is an increasing adsorption with increasing temperature, and as the temperature is raised close to the cloud point, this effect becomes quite dramatic. This trend is expected from our hypothesis, since an increase in temperature results in a decrease of the solvency (Figure 1). A similar temperature dependence has previously been observed for nonionic surfactants by van den Boomgard et al.30 and for cellulose ethers by Malmsten and Lindman.47 Segment Density Profiles. Figure 9 shows the total segment density profiles at two temperatures, 25 and 46.5 "C, of which the latter is 0.1 "C below the cloud point. A (47) Malmsten, M.; Lindman, B. Langmuir 1990,6, 357.
0.2 0.0 5
0
15
10
i
Figure 9. Calculated total volume fraction (&) of Pluronic PE 6200 versus layer number (i). Circles represent the total surface excess, while squares represent contribution from primary ad-
sorbed copolymer molecules. Filled and open symbols represent profiles at 25 and 46.5 "C, respectively. The bulk volume fraction (@) is 2 x 10-3. 3 I
B
' ' ' '
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Figure 10. Calculated EO-PO volume fractionratio ($mj/$w,) of Pluronic PE 6200 versus layer number (i). Circles represent the total surface excess, while squares represent contribution from primary adsorbed copolymer molecules. Filled and open symbols represent profiles at 25 and 46.5 "C, respectively. The bulk volume fraction (&,) is 2 X lo+. prominent feature is that, while the segment profile of primary adsorbed chains (squares) is fairly insensitive to temperature changes,the segment profile of the total excess in the surface region (circles) is extended much further out a t the elevated temperature. The amount of primary adsorbed segments per lattice site and the corresponding excess quantityare r = 2.3 and rex= 2.4 at the lower temperature (T= 25 "C) and 3.5 and 6.7, respectively, at the higher (T = 46.5 "C). The reasonable lattice size (-4 A) obtained from relating the experimental and calculated adsorbed amount and Figure 9 strongly supports the notion of multilayer adsorption close to the cloud point. Moreover, the volume fraction in the first few layers a t 46.5 "C is approximately 0.75, which is comparable to the volume fraction in the polymerrich phase (0.71) at cloud point; the latter obtained from a calculated Pluronic PE 62Wwater phase diagram. This theoretical prediction also confirms our hypothesis that the enhanced adsorption, taking place in the form of multilayers, could be viewed as an incipient phase separation phenomenon that takes place in the surface region. Preferential Adsorption. As indicated above, segregation of EO and PO segments is anticipated. From our model we have calculated the volume fraction of EO and PO segments as a function of the distance from the surface. Figure 10 shows the ratio of the EO and the PO volume fraction profiles,and as for the total volume fraction profile, two temperatures and two definitions of adsorbed amounts are regarded. Considering the ratio based on the volume fractions including segments from the total excess at the surface (circles), we see that there is a enrichment of PO segments
Kinetic and Equilibrium Aspects of Block Copolymer Adsorption
d
0.4
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0
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"
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'
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Figure 11. Calculated fraction of polar states (+b,) of Pluronic PE 6200 versus layer number (i). Dashed and full lines represent EO and PO, respectively, while filled and open symbols represent profiles at 25 and 46.5 O C , respectively. The bulk volume fraction (& is 2 x 10-3.
in the first few layers, whereas the opposite occurs at intermediate distances. When the temperature is increased, the region of enriched EO segments is expelled further away from the surface. Finally, at large distances the ratio levels off toward the bulk value 12/37. The relative excess of PO segments in the first few layers is not large. However, considering the total segment density, the excess in terms of volume fraction amounts to - 4 X 10-l in the first few layers. In those layers where the relative EO excess has its maximum (i = 8 and 14 a t 25 and 46.5 "C,respectively), the volume fraction of EO and PO is approximately equal ( ~ E O / ~ P = O 1). The relative segregation becomes more pronounced if we only consider copolymers which are primary adsorbed at the surface. Although the segregation is not much altered at small separations, the excess of EO segments O is 12 increases rapidly further away. The ~ E O / ~ Pratio at 25 "C and 15 at 46.5 O C in the last layer shown in Figure 10. This is consistent with the picture of the primary adsorbed copolymers as preferentially anchored through the PO block with the EO segments extending away from the surface. As previously mentioned, we have disregarded the additional methyl group of the PO segment when assigning values of the EO-PO as well as the segment-surface interaction parameters. Thus, in a more refined approach where the larger hydrophobic character of PO is fully considered, a more extended EO-PO segregationwill occur. However, the changes are limited, since the polymer segment-water interactions are the main driving force of the segregation. Conformation Equilibrium. The model comprises two states or classes of conformations for EO and PO, respectively (vide supra). Figure 11shows the fraction of the two polar states as a function of the distance from the surface under the same conditions as in Figures 9 and 10. At large separation (i > 151, the bulk values are attained for both temperatures. The reduction of the fraction of polar states in bulk with increasing temperature reflects the larger statistical weight of the nonpolar state. The fact that PO displays a larger fraction of polar conformations, as compared with EO, must not be interpreted as PO being more polar. The reason is that, although, the two more polar states of EO and PO are referred to as polar, their hydrophobic characters are unequal and they interact differently with water (cf. Table I). Also evident from Figure 11 is that fraction of polar states is reduced in the vicinity of the surface. The extension of the more nonpolar (hydrophobic) region is restricted to a few layers at the lower temperature, but as the clouding temperature is approached, the extension of
Langmuir, Vol. 7, No. 11, 1991 2729
the hydrophobic region increases. As for the segment U density, the state distributions in this region ( Z E O , ~ ~= 0.60 and xpowpolar = 0.73) are similar to that for the concentrated phase of the binary solution at cloud point (zEo,~+ = 0.63 and x p ~ , ~ l = a r0.76). Again the theoretical prediction is in accordance with the view of an incipient phase transition at the surface below the cloud point. Kinetics. The adsorption kinetics for block copolymers is often very different from that observed for homopolymers.33 As can be seen in Figure 6, the initial rate of adsorption is approximately proportional to the concentration. Furthermore, at concentrations below w t 9% , the initial part of a r - t1I2plot is linear (results not shown). These findings clearly indicate diffusion-controlled transport of copolymer to the surface. Structural rearrangementa seem to be of minor importance in this concentration regime, since the overall rate of adsorption is mainly determined by the initial rate of adsorption. This is particularly clear at concentrations close to the cmc, where the adsorption is essentially instantaneous. In fact, the adsorption of P E 6200 resembles that of surfactants in this concentration region, rather than that of homopolymers, which is in excellent agreement with previous findings on copolymer system^.^*^^^ Homopolymers, on the other hand, often display slow adsorption kinetics.20~22~24~30 To some extent, this is due to the low molecular weight and low polydispersity of the copolymers studied, although, as clearly shown in Figures 6-9, the mode of adsorption also strongly influences the adsorption kinetics. The slow adsorption kinetics exhibited by most homopolymer systems is often inferred as being due to structural rearrangement and entanglement effects in the adsorbed layer. However, recent measurements by Dijt et al.23 have shown that the adsorption process of monodisperse homopolymers is very fast, at least under some conditions. Hence, the extended adsorption time usually observed for polymer systems could to some extent be attributed to surface exchange of low molecular weight polymer by high molecular weight polymer. At concentrations well above the cmc, we observe that the adsorption extends over relatively long time scales, although the initial rate of adsorption is still very high (Figure 7). We infer that this effect is attributed to the previously discussed incipient phase separation in the surface region resulting in multilayer formation. The concentration at which this extension of the adsorption time scale is first observed corresponds well to the break point of the isotherm earlier identified as the concentration of the incipient phase separation. Note that the much slower overall rate of adsorption at these concentrations seems to indicate that not only multilayer adsorption occurs, but also that there are structural rearrangements of the adsorbed layer. This notion is supported by the model calculations, clearly showing temperature-dependent conformational rearrangement in the adsorbed layer (Figures 9-11). Finally, Figure 8 nicely illustrates the presence of different adsorption-desorption mechanisms. One of these corresponds to the formation of a monolayer ("irreversibly" adsorbed) while the other is probably related to the multilayer formation. The large and rapid desorption observed on rinsing from a high copolymer concentration indicates weak interactions between the apparent multilayers, on one hand, and the surface and the primary adsorbed copolymers, on the other. (48) Munch, M.R.;Gast, A. P.Macromolecules 1990,23,2313. (49) Tasein, J. F.; Siemens, R. L.; Tang, W. T.; Hadziioannou, G.;
Swalen, J. D.; Smith, B. A. J. Phys. Chem. 1989, 93, 2106.
Tiberg et al.
2730 Langmuir, Vol. 7, No. 11, 1991 Conclusions The adsorption isotherm for P E 6200was found to have a smooth curved shape, involving a sharp rise of the adsorbed amount at a critical copolymer concentration. An analogous behavior was observed upon raising the temperature close to the cloud point. The very high adsorbed amounts observed near the phase boundary are thought to be a consequence of an incipient partial phase separation, presumably resulting in the formation of multilayers at the surface. Qualitative aspects of both the isotherm and the temperature dependence were predicted by model calculations with a modified mean-field model that takes the reversed temperature dependent phase behavior of the system into account. Moreover, the model
calculations gave further understanding of the segment density profiles in the adsorbed layers aa well as of the preferential adsorption of PPO segments to the surface. It was also found that the adsorption kinetics was closely related to the equilibrium properties of the adsorption.
Acknowledgment. Prof. Martin Hellsten is gratefully acknowledged for helpful discussions and valuable comments on the manuscript. Hdkan Hagslatt is thanked for help with the FT-PGSE NMR diffusion measurements. This work was financed by grants from the Swedish National Board of Technical Development (STU). Registry No. Pluronic, 106392-12-5.
