pubs.acs.org/Langmuir © 2009 American Chemical Society
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Micelles of Polybutadiene-b-poly(ethylene oxide) in Methanol, Cyclohexane, and Methanol þ Cyclohexane Christopher D. Ploetz*,†,§ and Sandra C. Greer*,†,‡,
Department of Chemical and Biomolecular Engineering and ‡Department of Chemistry and Biochemistry, The University of Maryland, College Park, College Park, Maryland 20742. §Current address: Burns & McDonnell Engineering Company, Inc., 9400 Ward Parkway, Kansas City, Missouri 64114. Current address: Office of the Provost, Mills College, Oakland, California 94613 )
†
Received June 3, 2009. Revised Manuscript Received August 8, 2009 We have studied the self-assembly of a polybutadiene-b-poly(ethylene oxide) diblock copolymer in methanol, in cyclohexane, and in the partially miscible binary mixture of methanol þ cyclohexane. Molecular probe experiments indicated that PB89-b-PEO132 (subscripts indicate the number of monomers in each block) forms micelles with PEO cores and PB coronas in pure cyclohexane and micelles with PB cores and PEO coronas in pure methanol. In both pure solvents, dynamic light scattering indicated that the copolymer forms coexisting spherical and cylindrical micelles. In the binary solvent mixture, only spherical micelles are observed. In the methanol-rich phase, spherical micelles form over a wide range of temperatures. In the cyclohexane-rich phase, spherical micelles are present only near the upper critical solution temperature of the mixture. At the critical solvent composition, spherical micelles form in the single-phase region above the critical temperature. Size-exclusion chromatography showed that for the binary solvent mixture the copolymer distributes mostly into the methanol-rich phase and that this preference becomes more pronounced as the temperature decreases.
Introduction Consider a diblock copolymer of roughly equal block lengths, with one block A that is soluble in solvent SA but not in solvent SB and with the other block B that is soluble in SB but not in SA. In each solvent, the copolymers can form micelles, but the micelles will reverse themselves in SB as compared to their behavior in SA: block A will form the corona in SA and the core in SB, and block B will form the corona in SB and the core in SA. Now consider what happens when such block copolymer molecules are in a mixture of SA and SB, where the two solvents can be immiscible, partially miscible, or totally miscible. If the two solvents are partially miscible, then they will mix more and more as the temperature approaches the liquid-liquid critical point, and they will become totally miscible at that critical point. At temperatures well into the two-phase region, where the solvents are not very mixed, we may expect that micelles will still form in SA and reverse micelles will still form in SB. Because the solvents are partially miscible, the bulk solvent in each phase may be less favorable for the corona block than the pure solvent is. Then the interactions between a mixed solvent phase and the micelle corona may lead to less solvation of the micelle corona and to a smaller coronal diameter than in the pure solvent. However, if the micelles in either phase of the mixed solvent are swollen by the solvation of the immiscible solvent phase into the interior of the micelle, then they may be larger than micelles formed in the pure solvents.1 The behavior of any amphiphilic block copolymer in a partially miscible binary solvent mixture that forms two coexisting liquid *To whom correspondence should be addressed. E-mail: christopherploetz@ hotmail.com,
[email protected]. (1) Nagarajan, R.; Ganesh, K. Macromolecules 1989, 22, 4312–4325. (2) Alexandridis, P.; Olsson, U.; Linse, P.; Lindman, B. In Amphiphilic Block Copolymers, Alexandridis, P.; Lindman, B., Eds,; Elsevier Science: Amsterdam, 2000; pp 169-190. (3) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700– 7710. (4) Holmqvist, P.; Alexandridis, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 1149–1158.
13402 DOI: 10.1021/la9019876
phases has received little attention. As discussed above, chemical intuition, corroborated by limited theoretical2 and experimental3,4 work, indicates that dilute solutions of amphiphilic block copolymers in binary solvents should yield micelles in the polar phase and reverse micelles in the nonpolar phase. The only relevant experimental work3,4 developed a three-component (triblock copolymer/ water/“oil”) phase diagram that seems to indicate the presence of spherical micelles in both of the coexisting phases at low copolymer concentrations. However, this work focused on the copolymer-rich corner of the phase diagram. It is not clear that the copolymer-lean side of the phase diagram was investigated thoroughly or that the authors meant to imply the presence of micelles in coexisting phases. Solutions of polybutadiene-b-poly(ethylene oxide) (PB-b-PEO in water have received much attention.5-22 Investigations have determined the configuration of the microphase (e.g., isolated (5) Deng, Y.; Young, R. N.; Ryan, A. J.; Fairclough, J. P. A.; Norman, A. I.; Tack, R. D. Polymer 2002, 43, 7155–7160. (6) Discher, B. M.; Bermudez, H.; Hammer, D. A.; Discher, D. E.; Won, Y. Y.; Bates, F. S. J. Phys. Chem. B 2002, 106, 2848–2854. (7) Forster, S.; Berton, B.; Hentze, H. P.; Kramer, E.; Antonietti, M.; Lindner, P. Macromolecules 2001, 34, 4610–4623. (8) Hentze, H. P.; Kramer, E.; Berton, B.; Forster, S.; Antonietti, M.; Dreja, M. Macromolecules 1999, 32, 5803–5809. (9) Hong, S.; Yang, L. Z.; MacKnight, W. J.; Gido, S. P. Macromolecules 2001, 34, 7009–7016. (10) Jain, S.; Bates, F. S. Science 2003, 300, 460–464. (11) Jain, S.; Bates, F. S. Macromolecules 2004, 37, 1511–1523. (12) Jain, S. M.; Gong, X. B.; Scriven, L. E.; Bates, F. S. Phys. Rev. Lett. 2006, 96, 138304/1–138304/4. (13) Jofre, A.; Hutchison, J. B.; Kishore, R.; Locascio, L. E.; Helmerson, K. J. Phys. Chem. B 2007, 111, 5162–5166. (14) Lang, P.; Willner, L.; Pyckhout-Hintzen, W.; Krastev, R. Langmuir 2003, 19, 7597–7603. (15) Nordskog, A.; Futterer, T.; von Berlepsch, H.; Bottcher, C.; Heinemann, A.; Schlaad, H.; Hellweg, T. Phys. Chem. Chem. Phys. 2004, 6, 3123–3129. (16) Pispas, S.; Hadjichristidis, N. Langmuir 2003, 19, 48–54. (17) Won, Y. Y.; Brannan, A. K.; Davis, H. T.; Bates, F. S. J. Phys. Chem. B 2002, 106, 3354–3364. (18) Won, Y. Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960–963. (19) Won, Y. Y.; Davis, H. T.; Bates, F. S. Macromolecules 2003, 36, 953–955. (20) Won, Y. Y.; Davis, H. T.; Bates, F. S.; Agamalian, M.; Wignall, G. D. J. Phys. Chem. B 2000, 104, 7134–7143.
Published on Web 08/28/2009
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assemblies, networks, etc.) as a function of copolymer concentration7,8 and as functions of molecular mass and copolymer composition (i.e., the relative length of the two blocks).10-12,15-18,22 In “dilute” aqueous solutions (j18 wt %12), PB-b-PEO forms aggregates spanning from spherical micelles (for relatively short PB blocks) to cylindrical (wormlike) micelles to bilayers (for relatively long PB blocks).10-12 We have not found any prior studies of PB-b-PEO in solvents other than water. Thus, we have investigated the behavior of a PB-b-PEO diblock copolymer of narrow polydispersity (1.04) and roughly equivalent block lengths (54.7 wt % PEO) in methanol, in cyclohexane, and in the methanol þ cyclohexane binary solvent mixture. PB-b-PEO was selected for this study because the solubility properties of the two blocks are different,23 which suggests that micellization should occur for a wide variety of solvents, even at low concentrations. The methanol þ cyclohexane mixture is partially miscible and (at 1 atm pressure) exhibits an upper critical solution temperature (UCST) determined experimentally to be between 45 °C24,25 and 48 °C26 and at a critical concentration of 29 wt % methanol (51.7 mol % methanol).24 Our work focuses on the copolymer-lean side of the three-component phase diagram in order to determine whether micelles form in the coexisting liquid phases and/or in the single-phase region above the critical temperature. Before investigating the binary solvent system, the micellization behavior of the copolymer in each of the pure solvents was characterized. In methanol, the copolymer appears to form coexisting spherical and cylindrical regular micelles; changes in temperature and concentration have little effect on the size of the micelles. In cyclohexane, spherical and cylindrical reverse micelles are observed at elevated temperatures, but only cylindrical micelles exist at lower temperatures; there is no effect of concentration on the size of the micelles. In the binary solvent system, micelles do form in both coexisting liquid phases for a small range of temperatures near the upper critical solution temperature. Micelles disappear from the cyclohexane-rich phase as the temperature is lowered but persist in the methanol-rich phase at all temperatures. Micelles are also found above the critical temperature at the critical solvent concentration. The disappearance of micelles from the cyclohexane-rich phase can be explained by the fact that the copolymer preferentially partitions into the methanol-rich phase, which reduces the concentration in the cyclohexane-rich phase until it is below the critical micelle concentration. The temperature effect for this phenomenon is strong: The methanol-cyclohexane partition coefficient increases as the temperature decreases.
Experimental Section Materials. Polybutadiene-b-poly(ethylene oxide) (PBb-PEO) was obtained from Polymer Source, Inc. (catalog number P4603-BdEO). The number-average molecular masses of the polybutadiene (PB) and poly(ethylene oxide) (PEO) blocks are 4800 and 5800, respectively, resulting in approximately 89 repeat units of butadiene and 132 repeat units of ethylene oxide: PB89-b-PEO132. The mass composition of the copolymer is 54.7 wt % PEO. The polydispersity index is ∼1.04, and the PB block is rich in 1,4-microstructure (i.e., each repeat unit is joined at carbons 1 and 4). (23) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley and Sons: New York, 1989. (24) Jacobs, D. T.; Anthony, D. J.; Mockler, R. C.; O’Sullivan, W. J. Chem. Phys. 1977, 20, 219–226. (25) Matsuda, H.; Ochi, K.; Kojima, K. J. Chem. Eng. Data 2003, 48, 184–189. (26) Trejo, A.; Ya~nez, P.; Eustaquio-Rincon J. Chem. Eng. Data 2006, 51, 1070– 1075.
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Methanol (100.0%) was obtained from J. T. Baker. Methanol used for dynamic light scattering (DLS) and size exclusion chromatography (SEC) samples was stored over 3 A˚ molecular sieve beads from Aldrich. Cyclohexane (99.9%) was obtained from Fisher. Cyclohexane used for DLS and SEC samples was stored over 4 A˚ molecular sieve beads from Sigma. Probe molecules (-)R-pinene (99% pure, 97% enantiomeric excess) and rose bengal were also obtained from Aldrich. Silica particles used to determine solvent viscosity by the DLS method were provided by Philip DeShong and Ju-Hee Park. The particles were prepared according to the method of Nozawa et al.27 and were found to be nearly monodisperse with a diameter of approximately 110 nm, as measured by transmission electron microscopy (TEM). We measured the size distribution using DLS and found the hydrodynamic radius to be 68 nm (for a diameter of 136 nm), with a standard deviation of 18 nm. The 136 nm “hydrodynamic diameter” measured by DLS is larger than the TEM diameter because of swelling in the solvent. Sample Preparation. Samples for analysis by DLS and SEC were prepared from as-received PB89-b-PEO132. Solvents were also used as received but were stored over molecular sieve beads to minimize water contamination, which increases the UCST of the polymer-free binary solvent system.24 The UCST of the dehydrated, polymer-free solvent system was measured to ensure that it fell within the range of values reported in the literature (45.5-48.5 °C).24-26 The measured UCST ranged from 45.5 to 46.0 °C, indicating that water contamination of the dehydrated solvents was minimal. Samples of PB89-b-PEO132 in the pure solvents were prepared either from stock solutions (nominally 0.5 wt % copolymer in methanol or cyclohexane) or by adding the copolymer to a glass vial with a Teflon-lined lid and then adding the required mass of the appropriate solvent. Samples of PB89-b-PEO132 in the binary solvent system were prepared by adding the required mass of stock solution (nominally 1 wt % copolymer in cyclohexane) to a glass vial with a Teflon-lined lid; then cyclohexane and methanol were added by mass to achieve the desired concentrations. All masses were measured to (0.1 mg and were corrected for buoyancy. The number of significant figures given for concentrations indicates the precision to which the concentration is known. A small amount of copolymer was lost during sample filtration, but the magnitude of the loss is believed to be negligible. Each sample vial was sonicated in a Fisher Scientific FS6 sonic cleaner to achieve complete mixing. Samples in the binary solvent were sonicated above the critical temperature. Each sample prepared for DLS was filtered four times using the same 0.2 μm Teflon filter element (Pall acrodisc syringe filter or Pall TF membrane disc filter). Finally, a new 0.2 μm Teflon filter element was used as a fifth, polishing filtration step. An exception to this procedure was made for the 0.1 and 0.05 wt % PB89-b-PEO132 samples in pure cyclohexane because newly purchased filters seemed to remove more copolymer than the original filters, thus these samples were not filtered prior to analysis. Samples for polarimetry, dye solvation, and viscometry were prepared from as-received solvent, copolymer, and probe (either (-)-R-pinene or rose bengal) and were not filtered.
Determination of Micelle Configuration Using Molecular Probes. For PB89-b-PEO132, chemical intuition suggests that regular micelles (PB core, PEO corona) should form in methanol whereas reverse micelles (PEO core, PB corona) should form in cyclohexane. To test this hypothesis, two molecular probe experiments were conducted. Rose bengal, a powdery, magenta-colored, hydrophilic dye that is insoluble in nonpolar solvents,28 was used to determine the micelle conformation in cyclohexane. When rose bengal is added (27) Nozawa, K.; Gailhanou, H.; Raison, L.; Panniza, P.; Ushiki, H.; Sellier, E.; Delville, J. P.; Delville, M. H. Langmuir 2005, 21, 1516–1523. (28) Basu, S.; Vutukuri, D. R.; Thayumanavan, S. J. Am. Chem. Soc. 2005, 127, 16794–16795.
DOI: 10.1021/la9019876
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Article to a pure nonpolar solvent, it does not dissolve and the solvent remains clear and colorless (though rose bengal particles do adhere to the air-solvent meniscus and walls of the vial). When an amphiphilic copolymer is added to the mixture of rose bengal and solvent, the solution becomes deeply colored, presumably because of sequestration of the dye in the hydrophilic cores of reverse micelles. Therefore, as shown by Basu et al.,28 rose bengal can be used to indicate the presence of reverse micelles in nonpolar solution. Rose bengal was added to pure cyclohexane and to a 0.2 wt % solution of PB89-b-PEO132 in cyclohexane at ambient temperature. Both samples were sonicated and allowed to equilibrate for several days prior to visual inspection to determine if the addition of PB89-b-PEO132 allowed the rose bengal to dissolve in the unfavorable cyclohexane solvent. Unfortunately, a complementary hydrophobic dye could not be found to test for the existence of regular micelles in methanol. Reichardt’s dye is a solvatochromic, water-insoluble dye that has been used to test for the presence of regular micelles in aqueous solution,28 but we found that it is soluble in methanol and therefore unsuitable for use as a molecular probe in methanol. Thus, a nonpolar chiral probe was used to test for the presence of regular micelles in methanol. When (-)-(R)-pinene is added to methanol, the system separates into methanol-rich and pinene-rich phases.29 We hypothesized that the addition of PB89-b-PEO132 to (-)-(R)-pinene þ methanol will allow additional pinene to be sequestered in the hydrophobic cores of regular micelles, thus increasing the solubility of pinene in the methanolrich phase. The increased pinene solubility will result in an increase in the optical rotation of the methanol-rich phase beyond the baseline rotation. Thus, pinene can be used to indicate the presence of regular micelles in methanol. Pure methanol was mixed with pinene, and the solution was allowed to separate into two phases. A sample of the methanolrich phase was removed, and its optical rotation was measured using a Jasco P-1010 polarimeter (589 nm, (0.2% of measured rotation). Because the two solvents are partially miscible, the methanol-rich phase exhibits significant baseline optical rotation. The methanol-rich phase was returned to the original flask and remixed with the pinene-rich phase. A small amount of PB89-b-PEO132 was added to the solution, which was then allowed to separate into two phases prior to remeasurement of the optical rotation of the methanol-rich phase. These steps were repeated several times in order to determine whether the magnitude of optical rotation increased with the addition of PB89-b-PEO132.
Measurement of Copolymer Distribution by Size Exclusion Chromatography (SEC). SEC analysis results in a plot of signal versus elution time, where signal strength is related to instantaneous polymer concentration and elution time correlates with molecular mass. The total area under the curve is related to the polymer concentration in the original sample.30 Therefore, the relative concentrations of PB89-b-PEO132 in the upper and lower phases of the methanol þ cyclohexane system can be obtained by comparing the signal areas for samples from each phase. Samples of 0.2 wt % PB89-b-PEO132 in a mixture of methanol þ cyclohexane at the critical composition were heated above the critical temperature in the reservoir of a Neslab RTE-111 constant-temperature bath. The samples were shaken and allowed to remain above the critical temperature for at least 15 min prior to rapid cooling to the temperature of interest. The samples were then allowed to equilibrate for several hours until two clear phases formed. After equilibration, a 1.0 mL sample of each phase was extracted and placed in a vacuum oven to evaporate the solvent at ambient temperature. The desolvated copolymer was redissolved in water (NANOpure ultrapure water system, 0.2 μm final filter, resistivity >18 MΩ cm) and filtered once with a 0.2 μm nylon filter (Pall Nylaflo membrane filter). (29) Tamura, K.; Li, H. J. Chem. Eng. Data 2005, 50, 2013–2018. (30) Rodriguez, F.; Cohen, C.; Ober, C. K.; Archer, L. A. Principles of Polymer Systems, 5th ed.; Taylor and Francis: New York, 2003.
13404 DOI: 10.1021/la9019876
Ploetz and Greer The samples were analyzed with a Waters SEC instrument operated at 35 °C using nanopure water as the eluent. The instrument consists of a 717 autosampler, a 1525 binary HPLC pump, three ultrahydrogel columns (120, 250, and 2000), and a 2412 refractive index detector. The relative concentration in each phase was measured by comparing the areas under the signal strength (refractive index) versus elution time curves. Dynamic Light Scattering (DLS). DLS can be used to measure the hydrodynamic radii of small particles in a dilute solution.31,32 When the particles are monodisperse, the normalized scattered-light intensity autocorrelation function, g2(τ), can be fitted to a decaying exponential, as in eq 1, where the decay rate, Γ, the inverse of the characteristic decay time, τc (Γ=1/τc), is the half-width of the scattered-light spectrum and p is the meansquared scattered-light intensity.33 g2 ðτÞ ¼ 1þ p expð-2ΓτÞ
ð1Þ
Polydisperse particles require a more sophisticated analysis.34 The scattered-light field autocorrelation function, g1(τ), can be calculated from the scattered-light intensity autocorrelation function, g2(τ), as in eq 2. Equation 3 is then solved to find the distribution of decay rates, P(Γ), from which the distribution of diffusion coefficients can be calculated using eqs 4 and 5.34 g2 ðτÞ ¼ 1þ jg1 ðτÞj2
ð2Þ
Z g1 ðτÞ ¼
PðΓÞ expð-ΓτÞ dΓ
q ¼
ð3Þ
Γ ¼ Dq2
ð4Þ
4πn θ sin λ 2
ð5Þ
Here, Γ is the decay rate, D is the translational diffusion coefficient, q is the modulus of the scattering vector, n is the refractive index, λ is the wavelength of the incident light, and θ is the scattering angle. Solving eq 1 is straightforward but eq 3 is an “ill-posed” problem, and if the data have finite uncertainties, then multiple solutions can fit the data equally well.34 Therefore, sophisticated algorithms, such as the CONTIN method35 or the method implemented in DynaLS,34 are required to reduce the degrees of freedom and arrive at a meaningful result. The hydrodynamic radius, Rh, can be calculated from D using eq 6, the Stokes-Einstein equation.33,36 D ¼
kB T 6πηRh
ð6Þ
Here, kB is the Boltzmann constant, T is the absolute temperature, and η is the dynamic viscosity of the pure solvent. Rh is a reasonable approximation of the actual size of the particle, but micelles may have a solvation layer that increases this measured radius.37 (31) Caroline, D. In Developments in Polymer Characterization; Dawkins, J. V., Ed.; Elsevier: New York, 1986. (32) Rubinstein, M.; Colby, R. H. Polymer Physics; Oxford University Press: Oxford, U.K., 2003. (33) Yudin, I. K.; Nikolaenko, G. L.; Kosov, V. L.; Agayan, V. A.; Anisimov, M. A.; Sengers, J. V. Int. J. Thermophys. 1997, 18, 1237–1248. (34) Goldin, A. A. Software for particle size distribution analysis in photon correlation spectroscopy. http://www.photocor.com/download/manuals/ dynals_manual.htm (February 17). (35) Provencher, S. W. Makro. Chem 1979, 180, 201–209. (36) Einstein, A. Ann. Phys. 1906, 19, 289–306. (37) Sandoval, C.; Rezende, M. C.; Gonzales-Nilo, F. J. Solution Chem. 2003, 32, 781–790.
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The PhotoCor DLS instrument consists of a JDS uniphase model 1135P (632.8 nm (red)) laser, a photomultiplier tube, a 288channel multitau correlator, and a precision goniometer with a resolution of (0.01°. All data were obtained at a scattering angle of 90°. The instrument was operated in self-beating mode, and a sample time of 2.510-8 s was selected. Temperature control was provided by circulating water through the annulus of the double-walled sample holder using a Neslab RTE-111 constant temperature bath. The uncertainty in the sample temperature is relatively large because of the circulation of air through the laser aperture and because of a temperature offset between the bath and the instrument (Tbath > Tinstrument). Over a range of temperatures, the temperature of the sample, as measured by a thermocouple in a test solution placed in the sample holder, was consistently within (0.5 °C of the value indicated by the DLS instrument thermometer. Copolymer samples in pure solvents and silica particle samples (see below) in both binary and pure solvents that were to be analyzed at elevated temperature were first placed in the reservoir of the Neslab bath. After the temperature of the DLS instrument had stabilized at the desired value, the sample was removed from the bath and placed into the instrument sample holder. Silica particle samples in the binary solvent were shaken prior to being placed in the sample holder to ensure that equilibrium phase separation was achieved because the equilibration time for the mixing of partially miscible phases by diffusion can be very long. Because of the temperature offset between the bath and the instrument, samples were allowed to equilibrate in the sample holder for at least 15 min or until the scattered-light intensity reached a steady state. Measurements indicated that the sample temperatures relaxed to the instrument temperatures in less than the allotted times. Copolymer samples in the binary solvent system to be analyzed at elevated temperatures were heated above the UCST while in the instrument sample holder. The samples were shaken to ensure homogeneity, held above the critical temperature for 3 h, and then cooled to the temperature of interest and held overnight to allow complete phase separation prior to data collection. After the desired temperature had been reached, DLS data were collected for a sufficient period of time to yield an autocorrelation function with little scatter in the data. Generally, a collection time of less than 5 min yielded acceptable results, but samples with low copolymer concentrations sometimes required longer collection times. The autocorrelation functions obtained by DLS were analyzed using DynaLS version 2.8.3 (Alango Ltd.).34 The channels used in the data analysis were selected to exclude anomalous fluctuations at low channel numbers (
