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PS-PEO Diblock Copolymer at the Cyclohexane/SiO2 Interface: Effect of Micelles on Adsorption Kinetics and Coverage D. E. Gragson,* J. P. Manes, J. E. Smythe, and S. M. Baker Department of Chemistry, Harvey Mudd College, 301 East 12th Street, Claremont, California 91711 Received November 25, 2002 The adsorption behavior and surface excess of an asymmetric poly(styrene)-block-poly(ethylene oxide) copolymer, 4.2% by mass poly(ethylene oxide), from a cyclohexane solution onto a SiO2 surface are studied as a function of polymer concentration. Adsorption experiments were conducted at a temperature below the Θ temperature for polystyrene in cyclohexane (TΘ ) 35 °C). Strong evidence is observed supporting the formation of micelles above a polymer concentration of 0.030 mg/mL at ∼20 °C. This evidence for micelle formation is manifested as a marked jump in the observed surface density (determined by ATRFTIR spectroscopy) and a corresponding change in the rms radius of gyration of the polymer (determined by multiangle light scattering measurements) at high polymer concentrations. The physical association of copolymer onto the surface above and below the critical micelle concentration is explored through rinsing experiments with a good solvent (CCl4) for polystyrene. The results of these experiments suggest limited (monolayer) micelle adsorption and imply that these micelles attach to the surface by association with adsorbed monomolecular copolymer.
I. Introduction Exploring the adsorption of polymers from solution onto solid interfaces is a challenging task due to the very small absolute numbers of molecules comprising a monolayer. Nonetheless, the kinetic mechanism of adsorption of diblock copolymers under a variety of solvent conditions continues to be of interest, particularly as a means to control final polymer interfacial densities.1-8 Most of the diblock/solvent systems studied to date involve solvents which are selective, such that one block preferentially adsorbs onto the surface. The adsorbed block is typically called the anchor block while the block that extends into solution is called the buoy block. Typically, the solvent is good for the buoy block and poor for the anchor block, which leads to preferential adsorption of only the anchor block. Generally, solvents are characterized as selective if there is no adsorption of one block, as is the case for poly(styrene)-block-poly(ethylene oxide) (PS-PEO) adsorption from toluene.6,9 Nonselective solvents differ from selective solvents in that, at least initially, there is no preferential adsorption. Once the initial adsorption occurs, however, displacement of one block for the other can lead to films with molecules anchored preferentially by one * Corresponding author. Present address: Department of Chemistry and Biochemistry, California Polytechnic State University, San Luis Obispo, CA 93407. (1) Pagac, E. S.; Prieve, D. C.; Solomenstev, Y.; Tilton, R. D. Langmuir 1997, 13, 2993-3001. (2) Tripp, C. P.; Hair, M. L. Langmuir 1996, 12, 3952-3956. (3) Pelletier, E.; Stamouli, A.; Belder, G.; Hadziioannou, G. Langmuir 1997, 13, 1884-1886. (4) Zhan, Y.; Mattice, W. L. Macromolecules 1994, 27, 683-688. (5) Munch, M. R.; Gast, A. P. J. Chem. Faraday Trans. 1990, 86, 1341-1348. (6) Field, J. B.; Toprakcioglu, C.; Ball, R. C.; Stanley, H. B.; Dai, L.; Barford, W.; Penfold, J.; Smith, G.; Hamilton, W. Macromolecules 1992, 25, 434-439. (7) Leermakers, F. A. M.; Gast, A. P. Macromolecules 1991, 24, 718730. (8) Munch, M. R.; Gast, A. P. Macromolecules 1990, 23, 2313-2320. (9) Motschmann, H.; Stamm, M.; Toprakcioglu, C. Macromolecules 1991, 24, 3681-3688.
block.2 The equilibrium attachment of a diblock copolymer film adsorbed from a nonselective solvent depends on the segmental adsorption energies of each block.2 Clearly, solvents play an important role in the adsorption of diblock copolymers onto surfaces. If both blocks have some tendency to adsorb, the adsorption will likely be nonselective. An example is PS-PEO in carbon tetrachloride. CCl4 is a good solvent for PS and a poor solvent for PEO, yet both blocks adsorb initially and only in rearrangement does the PEO block displace the PS block.2 The research described here focuses on the adsorption of PS-PEO from cyclohexane at ∼23 ( 3 °C, a poor solvent for both blocks. Under these conditions, either chain can act as the anchor block, at least initially, and thus the solvent is nonselective. Homopolymer adsorption alone is fairly well understood under these conditions,10-16 and our interest here is on how a second block affects adsorption. The mechanism for diblock adsorption from nonselective solvents is generally accepted to proceed by three steps:2 (1) mass diffusion-limited adsorption at the surface of either block, (2) surface diffusion-limited adsorption of the stronger adsorbing block accompanied by an activation barrier to adsorption due to the rearrangement and displacement of the weaker adsorber, and, finally, (3) a slow rearrangement of the remaining blocks with some, but not substantial, additional adsorption. Under conditions where the solvent is unsheared (static), the first step can be rate limiting relative to rearrangement. The kinetics of polymer adsorption are often determined directly by surface forces apparatus,3 null ellipsometry,9 (10) Fu, Z.; Santore, M. M. Macromolecules 1998, 31, 7014-7022. (11) Dijt, J. C.; Cohen-Stuart, M. A.; Hofman, J. E.; Fleer, G. J. Colloids Surf. 1990, 51, 141-158. (12) Mubarekyan, E.; Santore, M. M. Macromolecules 2001, 34, 75047513. (13) Frantz, P.; Grannick, S. Phys. Rev. Lett. 1991, 66, 899-902. (14) Johnson, H. E.; Grannick, S. Science 1992, 255, 966-968. (15) Douglas, J. R.; Johnson, H. E.; Granick, S. Science 1993, 262, 2010-2012. (16) Couzis, A.; Gulari, E. Macromolecules 1994, 27, 3580-3588.
10.1021/la026903i CCC: $25.00 © 2003 American Chemical Society Published on Web 05/14/2003
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Fourier transform IR spectroscopy in the mode of attenuated total reflection (FTIR-ATR),13-19 and total internal reflectance fluorescence (TIRF).10,12 We have used FTIRATR, which allows for the sensitive detection of small amounts of mass and directly gives a measure of Γ, the total surface density (i.e., mass/area) adsorbed. Although monolayer masses are below detection limits for a conventional transmission IR measurement, the multiple reflections and surface selectivity in an ATR geometry provide enough signal to monitor even the early stages of polymer adsorption. The use of IR limits the possible hydrogenated solvents and substrates, but judicious choice of deuterated systems opens a larger set of solvent conditions. While the long-term goal of the project is to explore the effects of block asymmetry, solvent quality, and copolymer concentration on the adsorption kinetics, this study focuses on the observation of a large jump in the adsorbed mass above a critical copolymer concentration for a diblock in a poor solvent. ATR-FTIR is used to explore the adsorption of an asymmetric poly(styrene)-block-poly(ethylene oxide) copolymer, 4.2% by mass poly(ethylene oxide), from a cyclohexane solution onto a SiO2 surface as a function of copolymer concentration. Strong evidence for micelle or aggregate formation of this polymer is correlated with a large jump in adsorbed mass at a critical concentration. Further, these micelles or aggregates are determined to adsorb and be weakly attached to the surface through polystyrene-polystyrene interactions and can be “washed off” by brief rinsing with a good solvent for polystyrene. These observations augment previous models4 and experimental work5,7 where the surface density is actually shown to decrease for concentrations above the critical micelle concentration. This apparent inconsistency is explained by the fact that in our experiments the PS block is in a poor solvent, as the Θ temperature for PS in cyclohexane is TΘ ) 35 °C,20-22 while the previous work only explores systems where the PS block is in a good solvent. II. Experimental Considerations Attenuated total reflection Fourier transform infrared (FTIRATR) spectroscopy is a well characterized technique that has been used in the study of polymer adsorption.13-19 The technique has been used to explore a wide variety of aspects central to understanding polymer adsorption including adsorption kinetics,10,15,16 surface densities,16,17 and displacement kinetics,13,14,17 and it has been used as a structural probe of adsorbed polymer molecules.23 ATR-FTIR is used here to follow the time evolution of the adsorbed mass of polymer by monitoring the integrated area of the spectral peak at 3025 cm-1 arising from the C-H stretches specific to the styrene monomers. Figure 1 shows the spectral region from 2800 to 3140 cm-1 of an adsorbed polymer layer of the PS-PEO copolymer used in the experiments described here. The spectral peak at 3025 cm-1 is used rather than the peaks between 2800 and 3000 cm-1 to minimize the effects of solvent (cyclohexane) background subtraction. The internal reflection element (IRE) is an oxidized silicon rod with an internal incident angle of reflection equal to 34.33°. From the internal incident angle of reflection, the indices of refraction of the substrate and solvent, and the wavelength of the monitored (17) Johnson, H. E.; Granick, S. Macromolecules 1990, 23, 33673374. (18) Frantz, P.; Granick, S. Langmuir 1992, 8, 1176-1182. (19) Johnson, H. E.; Hu, H.; Granick, S. Macromolecules 1991, 24, 1856-1867. (20) Israelachvilli, J. N.; Tirrell, M.; Klein, J.; Almog, Y. Macromolecules 1984, 17, 204-209. (21) Novotny, V. J. J. Chem. Phys. 1983, 78, 183-189. (22) van der Beek, G. P.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1991, 24, 3553-3561. (23) Soga, I.; Granick, S. Langmuir 1998, 14, 4266-4271.
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Figure 1. Sample ATR-FTIR spectrum from a 182.7K PSPEO film adsorbed on an oxidized silicon IRE from CCl4. The integrated area of the peak at 3025 cm-1 is used for the determination of the mass adsorbed and to monitor the adsorption kinetics. spectral peak, the penetration depth of the IR light into the interfacial region is calculated to be 0.40 µm.13-19
dp )
λ 2πn1[sin2 θ - n122]1/2
(1)
where dp is the penetration depth, θ is the angle of incidence, n12 ) n2/n1 (nsilicon ) 3.42 and ncyclohexane ) 1.4266), and λ is the wavelength of light. The measured absorbance, A, is expressed as the sum of contributions from a surface layer and from the bulk as13-15,17-19
A ) Γ + dpc
(2)
where is the absorptivity, Γ is the surface density of adsorbed polymer, dp is the penetration depth (0.40 µm), and c is the concentration of polymer in the bulk solution. The surface density of adsorbed polymer is then obtained via a modified Beer’s Law plot17 (Figure 2) where the integrated area of the spectral peak at 3025 cm-1 is measured as a function of bulk polymer concentration on an IRE that has been previously saturated with either diblock copolymer or homopolymer. The absorptivity, , cannot be determined in cyclohexane, since the PS-PEO aggregates at the surface for moderately low concentrations. Consequently, was determined from the slope of absorbance versus c in solvents other than cyclohexane, and these values were used in determining the adsorbed surface density. Since IR absorbance depends primarily on the species of interest (here the PS), the solvent is assumed to have a relatively small effect on the peak shapes and intensities. In an attempt to validate this assumption, we measured the absorptivity arising from the PS peak at 3025 cm-1 in two different solvents. Figure 2 shows a plot of absorbance versus bulk polymer concentration for several different polymer/solvent systems. The triangles represent measurements for a 200K PS homopolymer in deuterated toluene and show that no polymer adsorbs to the surface, in good agreement with previous experiments.9 The circles represent measurements for a 200K PS homopolymer in CCl4 and show that the surface density is 2.0 mg/m2, again in good agreement with previous experiments.14,16,18 The squares represent the 182.7K PS-PEO copolymer in CCl4 and show that the surface density of the saturated surface is 4.5 mg/mL for a polymer concentration less than 0.01 mg/mL. In each case the
PS-PEO Copolymer at the Cyclohexane/SiO2Interface
Figure 2. Determination of the mass adsorbed and the absorptivity from PS-PEO and PS films adsorbed on an oxidized silicon IRE from CCl4 and toluene. Solid lines are fit to ABS ) Γ + dpc, where is the absorptivity, Γ is the mass adsorbed, dp is the penetration depth (0.40 µm), and c is the concentration of the bulk solution. measured absorptivity shows little dependence on the solvent (values of 0.0087, 0.0090, and 0.0084 m2/mg), and thus we have used a value of 0.0085 m2/mg for the absorptivity of the 182.7K copolymer in cyclohexane. The IRE is prepared for each experiment via a multistep cleaning process.18 First the IRE is soaked in aqua regia (5:3 HNO3/HCl) for several hours followed by thorough rinsing with 18 MΩ Milllipore RQ water. After every 5-8 uses, the oxide layer is removed from the IRE via a 5 min HF (5 vol %) chemical etch followed by thorough rinsing with 18 MΩ water. Otherwise, the IRE is quickly (less than 2 min after acid wash or etch) placed into an ultraviolet ozone cleaner (Jelight) for several hours. The clean IRE is then placed in the ATR cell (Tunnel Cell, Axiom), and the cell is placed into the FTIR spectrometer (Nicolet Magna 560), which is purged of water and carbon dioxide gases. The cleanliness of the IRE is tested by acquiring an empty cell spectrum and verifying that surface contamination is below the detection limit of the instrument. A background spectrum is collected in solvent for each run, and each spectrum collected consists of an average of 100 interferograms and is collected with a resolution of 8 cm-1 using Happ-Genzel apodization. For determination of the absorptivity, an IRE that is saturated with a given polymer is placed in the cell. Spectra are collected for each successive addition of polymer solutions of increasing concentration to the ATR cell. Extrapolation of the absorbance versus concentration data to zero concentration allows determination of the surface coverage, Γ. All spectra were recorded at ambient temperature, 23 ( 3 °C. The PS-PEO copolymer (Mw ) 182 700 g/mol, 4.2% by mass PEO, Mw/Mn ) 1.10) was obtained from Polymer Laboratories and was used as received. Polystyrene homopolymer (212K molecular weight, Mw/Mn ) 1.2) was obtained from Sigma-Aldrich and used as received. HPLC grade carbon tetrachloride (Aldrich), deuterated toluene (>99% d8 from Aldrich), HPLC grade cyclohexane (Aldrich), and HPLC grade toluene (Aldrich) were all used as received. All polymer solutions were prepared and stored in Teflon vials. Light scattering experiments were performed at Wyatt Technologies on a DAWN multiangle light scattering instrument at 23 ( 4 °C. All solutions were filtered through a Teflon membrane (0.45 µm pore diameter) prior to introduction into the instrument (solutions above a concentration of 0.030 mg/mL were filtered at a temperature above 35 °C to prevent removal of the copolymer micelles via filtering).
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Figure 3. Adsorption kinetics for 182.7K PS-PEO adsorbed from cyclohexane solutions of various concentrations at approximately 23 °C. Solid lines are a fit to the stretched exponential function Γ(t) ) Γeq(1 - [exp(t/τon)]β). Table 1. Fit Dataa for the Adsorption of 182.7K PS-PEO Adsorbed from Cyclohexane conc (mg/mL)
Γeq (mg/m2)
τon (min)
β
0.0042 0.0090 0.0468 0.178
4.97 5.96 31.7 33.4
182 89.2 97.6 103
0.584 0.566 0.681 0.600
a
Fit to the stretched exponential Γ(t) ) Γeq(1 - [exp(t/τon)]β).
III. Results Adsorption kinetics data in cyclohexane using the ATRFTIR technique were collected for the 182K diblock for a variety of polymer concentrations. Figure 3 shows representative plots of the time evolution of the adsorption of the 182.7K PS-PEO diblock copolymer onto the native silicon oxide of the IRE surface for several different concentrations. The adsorbed surface density, Γ, was determined as described above by saturating the IRE and using the value determined from the carbon tetrachloride and toluene data. The data were fit to a stretched exponential13-15 with three fitting parameters, the limiting surface density, Γ∞, the effective time constant, τon, and β, which characterizes how much the exponential is stretched in time. The resulting best-fit parameters are shown in Table 1. Further discussion of the fit parameters is left to a future publication, which details the adsorption kinetics by comparing the adsorption process for different block asymmetries, solvent quality, and copolymer concentration. The most striking feature of this figure is the large difference in the limiting surface density for the higher concentration solutions (0.1780 and 0.0468 mg/mL) as compared to the lower concentration solutions (0.0042 and 0.0090 mg/mL). Adsorption from the two higher concentration solutions results in surface densities in excess of 30 mg/mL while adsorption from the lower concentration solutions results in surface densities of approximately 5 mg/mL. Other studies with similar diblock copolymers in good solvents for PS have shown maximum surface
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Figure 4. Root mean square radius of gyration versus polymer concentration for the 182.7K sample in cyclohexane at approximately 23 °C. Data obtained from multiangle light scattering measurements.
densities in the range from 1 to 10 mg/mL,5-9 which is in agreement with our observations at lower concentrations. Since multilayer adsorption is inconsistent with the low observed surface densities in each of these studies, the polymer film is assumed to be a monolayer attached to the surface by the anchor block. In contrast, the much larger surface densities we observe for the higher concentration solutions are not consistent with monolayer adsorption. Rather, our studies indicate that under poor solvent conditions increased polymer-polymer interactions result in increased copolymer adsorption beyond a certain copolymer solution concentration. Further, since the solvent is poor for PS and PEO, both blocks may adsorb and multilayers may form through polymer-polymer association. Increased PS-PS interactions in cyclohexane as the temperature drops below the Θ temperature have been observed in experiments using a surface force apparatus20 and PS homopolymer. With this in mind, in our experiments the attractive forces between PS attached to the surface via PEO anchoring and PS blocks in the bulk solution could reasonably result in multilayer formation. However, elevated surface densities are observed only beyond a certain bulk concentration whereas the above reasoning would suggest continual growth of multilayers even at lower concentrations. An explanation consistent with the abrupt onset of increased Γ for solutions with polymer concentrations in excess of 0.030 mg/mL is aggregate or micelle formation followed by adsorption of these structures. Micellization has been observed in many other block copolymer systems in both aqueous1 and nonaqueous solvents.5,8 We have conducted light scattering experiments using the 182.7K PS-PEO copolymer in cyclohexane as a function of copolymer concentration to explore the possibility of aggregate formation. These results are presented in Figure 4, where the rms radius of gyration (assuming a spherical geometry) is plotted versus copolymer concentration. The transition to a much larger radius of gyration occurring between 0.025 and 0.030 mg/mL is in excellent agreement with the proposed
Gragson et al.
aggregate or micelle formation. It remains unclear yet as to why the aggregates adsorb while the individual monomers do not. The data point at 0.0047 mg/mL was indistinguishable from that for the pure solvent for the rms radius of gyration and is calculated from theoretical models using a 182.7K PS homopolymer and the relationship Rg2 ) aM1/3. Gast et al.5,7,8 have shown that the same PS-PEO copolymer adsorbed in cyclopentane under conditions of very low shear shows the opposite effect in Γ as a function of solution concentration. They observed a lowered surface density above the critical micelle concentration in cyclopentane and propose that as the system relaxes, the diblock rearranges, releasing the weakly adsorbed PS as the more strongly adsorbing PEO replaces PS segments at the surface. Micelles were suggested as the source of the original overshoot at low concentrations, and the critical micelle concentration in cyclopentene was determined by dynamic light scattering to be 0.1 mg/mL. The work by Gast et al. differs from ours because polymer adsorption was monitored at 23 °C, which is above the Θ temperature for PS in cyclopentane (TΘ ) 19.5 °C),5 and thus the PS was under good solvent conditions while our work was performed in cyclohexane below the Θ temperature for PS. Additionally, their experiments were performed under conditions of very low solvent shear, which may keep the micelles from entangling with the PS at the surface, an effect we have seen in neutron reflectivity experiments.24 Here, the PS-PS interactions in poor solvent increase and our results suggest that micelles or aggregates remain associated with the surface, giving a higher Γ. Gast’s experiments, all done with gently flowing solutions and above the Θ temperature, show that micelles in good solvent (for PS) are less likely to adsorb and may, in fact, remove polymer from the interface. Determining whether the adsorption of the aggregates in our system occurs on the bare silica surface or onto a monomolecular layer of copolymer previously anchored to the silica surface via PEO attachment is an important step in elucidating the mechanism of aggregate adsorption. If the attachment of the aggregate is to a layer of copolymer on the surface, then the aggregates should be removed by rinsing with a solvent that is good for the PS block, leaving the PEO attached at the interface. Figure 5 shows a rinsing experiment that suggests that the aggregates are attached via PS-PS interactions between a monomolecular layer and the copolymer aggregates. In this experiment copolymer was allowed to adsorb onto the silica surface until the surface saturated for various concentrations of copolymer in cyclohexane. After saturation the copolymer solution was replaced with CCl4 (a good solvent for PS) and the cell was allowed to sit for several minutes. Upon this brief rinsing with CCl4, the surface density from the solutions in excess of 0.030 mg/mL decreased dramatically (∼30 to ∼10 mg/m2) while the surface density from solutions below 0.030 mg/mL changed very little. This difference observed in the rinsing experiment is in agreement with the proposed adsorption of PS-PEO aggregates onto an adsorbed monomolecular layer via PSPS interactions that are much weaker in the good solvent CCl4. The fact that the surface density from solutions above 0.030 mg/mL after rinsing (∼8-10 mg/m2) is larger than the surface density from solutions below 0.030 mg/mL (∼5 mg/m2) is likely due to the fact that insufficient time was allowed for solvation of all of the entangled PS chains. (24) Baker, S. M.; Callahan, A.; Smith, G. S.; Vradis, A.; Toprakcioglu, C. Physica B: Condensed Matter 1998, 10, 241-243.
PS-PEO Copolymer at the Cyclohexane/SiO2Interface
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The experiments presented here explore the adsorption of a PS-PEO diblock copolymer from a solution of cyclohexane onto a silica surface where both blocks are in poor solvent conditions. Unlike the case of micellar systems in selective solvents under flow, we have found that there is a critical solution concentration (∼0.030 mg/mL) below which the copolymer adsorbs to form an apparently monomolecular film that is stable to rinsing, suggesting adsorption via the PEO block. Above this critical concentration the copolymer adsorbs to form a multilayer film incorporating micellar or aggregated structures which can be mostly removed by rinsing with CCl4, suggesting that the copolymer aggregates are attached through PSPS interactions with a monomolecular polymer film. Light scattering experiments show a dramatic increase in the rms radius of gyration above 0.030 mg/mL, confirming the presence of micelles or less structured aggregates. We are in the process of conducting further studies exploring the adsorption and light scattering of this and other copolymers in a variety of solvents.
Figure 5. Surface density in pure cyclohexane and after a CCl4 rinse for 182.7K PS-PEO adsorbed from cyclohexane at various concentrations and approximately 23 °C.
IV. Conclusions This work represents some initial efforts using ATRFTIR to explore the adsorption of diblock copolymer in our ongoing studies employing several surface techniques.
Acknowledgment. We would like to thank Wyatt Technologies, Santa Barbara, CA, for the light scattering data. This work was supported by the National Science Foundation under an award from the Division of Materials Research (DMR-9623718) and the Division of Undergraduate Education, Research Experience for Undergraduates (DUE-9732111). LA026903I
