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Solution Properties of Diblock Copolymers of Polystyrene-block-polybutadiene M. A. Awan,†,‡ V. L. Dimonie,† D. Ou-Yang,§ and M. S. El-Aasser*,†,| Emulsion Polymers Institute and Departments of Chemistry, Physics, and Chemical Engineering, 111-Research Drive, Lehigh University, Bethlehem, Pennsylvania 18015 Received October 13, 1995. In Final Form: October 31, 1996X A combination of transmission electron microscopy (TEM), dynamic light scattering (DLS), static light scattering (SLS), and flow field-flow fractionation (flow-FFF) techniques were used to study the solution behavior of diblock copolymers of polystyrene-block-polybutadiene in hexane. Results obtained from all these techniques showed that the diblock copolymer forms micelles in selective solvents. Analysis via DLS indicates that, in hexane, the micelles have a hydrodynamic diameter (Dh) of ∼140 nm. It was also found that each micelle is composed of a few hundred polymer molecules. Flow FFF was used to analyze the diblock copolymer in hexane and in a solution of hexane and ethylbenzene. The data obtained via flow FFF show the presence of both single polymer molecules and micelles. It was also observed that, upon titration of a diblock copolymer solution in hexane by ethylbenzene, the micelles started to dissolve, and at 23% ethylbenzene the micelles completely dissolved. Moreover, the diblock copolymer solution in hexane was titrated with styrene, and the effect of the micellar structure was studied by both dynamic light scattering (DLS) and static light scattering (SLS).
Introduction Previous studies have shown that diblock and triblock copolymers form micelles of close association upon dissolution into a selective solvent.1,2 These block copolymers, amphipathic in nonaqueous and amphiphilic in aqueous media, proved to have potential applications for steric stabilization,3,4 for dye or pigment vehicles in printing technology,5 for drag-reducing agents,6 and as additives for lubricants and coating materials.7,8 Recently, micelles have been used as a tool for transport in areas as diverse as drug delivery.9,10 Upon dissolving into a selective solvent (thermodynamically good solvent for one block), at very low concentration, the diblock copolymer’s chains remain free in order to maximize their transitional entropy. As the concentration is increased, one reaches the critical micelle concentration (cmc), where it becomes favorable for the insoluble block to associate and trade its transitional freedom in order to reduce its enthalpy. The net result is the formation of micelles consisting of a “core” of insoluble segments and a “shell” of soluble segments. Previously, a wide range of experimental techniques were adopted to investigate the solution properties of the diblock copolymers such as the critical micelle concentration (cmc), hydrodynamic size of the micelles, apparent molar mass * To whom all correspondence should be addressed. † Polymers Institute. ‡ Department of Chemistry. § Department of Physics. | Department of Chemical Engineering. X Abstract published in Advance ACS Abstracts, January 1, 1997. (1) Tuzar, Z.; Kratochvil, P. Adv. Colloid Interface Sci. 1976, 6, 201. (2) Price, C. In Development of Block Copolymers; Goodman, I., Ed.; Applied Science: New York, 1982; Vol. 1, p 39. (3) Piirma, I Polymeric Surfactants; Marcel Dekker: New York, 1992. (4) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (5) Napper, D. H. J. Colloid Interface Sci. 1977, 58, 390. (6) Piirma, I. Polymeric Surfactants; Science Series 42; Marcel Dekker: New York, 1992. (7) Becher, J. E.; Marker, L.; Bradford, R. D.; Aggrwal, S. J. Polym. Sci. 1963, C4, 473. (8) Fowkes, F. M.; Pugh, P. J. ACS Symposium 240; American Chemical Society: Washington, DC, 1984. (9) Emmelius, M.; Hoerpel, G.; Ringsdorf H.; Schmidt, B. In Polymer Science and Technology; Chiellini, E., Giusti, P., Migliaresi, C., Nicolais, L., Eds.; Plenum: New York, 1986; Vol. 34, p 313. (10) Kramer, O. Ed. Biological and Synthetic Polymer Network; Elsevier Applied Science: New York, 1988.
(Mw,app) of the micelles, internal structure, and transport properties and steric stabilization capability in selective and mixed solvents.11-15 The investigation of their solution behavior was complicated due to their composition, polydispersity, and optical and geometric heterogeneties. Further, the cmc’s of the amphiphilic and amphipathic diblock and triblock copolymers are orders of magnitude lower compared to those of the low molecular weight surfactant. Due to the low cmc it is difficult to determine via DLS and SLS the concentration at which the polymer chains begin to associate. In recent years a significant effort was devoted to study the solution behavior of these diblock copolymers, and various state-of-the-art techniques were applied, e.g., nuclear magnetic resonance spectroscopy (NMR),16 dynamic light scattering (DLS),17 static light scattering (SLS),18 size exclusion chromatography (SEC),19 and small-angle X-ray scattering (SAXS).20 In very dilute solutions diblock and triblock copolymers exist as a single collapsed polymer molecule (see Figure 1A). The formation of micelles (see Figure 1B) is favored by the reduction of the enthalpic contribution,21 and further, it depends on the diblock copolymer composition, the chemical nature of the solvent, the molar mass, the temperature, and the manner in which the solution was prepared. The most common emerging application of these diblock copolymers is steric stabilization of the dispersion processessfor example, emulsion and dispersion polym(11) Tuzar, Z.; Sikora, A.; Petrus, V.; Kratochvil, P. Makromol. Chem. 1977, 178, 2743. (12) Ibrahim Unal, H.; Price, C.; Budd, P. M.; Mobbs, R. H. Eur. Polym. J. 1994, 30 (9), 1037. (13) Xu, R.; Winnik, M. A.; Hallett, F. R.; Riess, G.; Croucher, M. D.; Macromolecules 1991, 24, 87. (14) Corner, T.; Gerrard, T. J.; Price, A. Colloids Surf. 1984, 11, 219. (15) Berman, N. S. Annu. Rev. Fluid Mech. 1978, 10, 47. (16) Spevacek, J. Makromol. Chem., Rapid Commun. 1982, 3, 697. (17) Tang, W.; Hadziioannou, G.; Frank, C. W.; Smith, B.; Cotts, P. Am. Chem. Soc., Polym. Prepr. 1986, 27 (2), 107. (18) Elias, H. G. In Light Scattering of Polymer Solution; Huglin, M. G., Ed.; Academic Press: London, 1972. (19) Prochazka, K.; Bednar, B.; Tuzar, Z.; Kocirik, M. J. Liq. Chromatogr. 1989, 12, 1023. (20) Rigby, D.; Roe, R.-J. Macromolecules 1984, 17, 1778. (21) Leibler, L.; Orland, H.; Wheeler, J. C. J. Chem. Phys. 1983, 79, 3550.
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of styrene monomer in hexane medium. Therefore, we investigated the solution behavior of Stereon 730A in hexane and in solutions of hexane/styrene or hexane/ ethylbenzene. The current investigation utilized electron microscopy, dynamic light scattering (DLS), static light scattering (SLS), and flow field-flow fractionation (flow FFF). These results are discussed in terms which can help to elucidate the mechanism of particle nucleation in anionic dispersion polymerization and the mechanism of stabilizer adsorption.26 DLS was used to investigate the micellar size, the size distribution, and the change in the micelle size with concentration of diblock copolymer in hexane. SLS was applied to determine the net number of polymer chains per micelle, the apparent molar mass of the micelle (Mw,app), and the apparent radius of gyration (Rg). Since flow FFF is commonly used to determine particle size and distribution,27,28 in collaboration with Giddings we employed this technique to determine the presence of single polymer molecules and micelles and their relative amounts at different concentrations of the diblock copolymers. The most significant results received from this analysis via SLS and flow FFF showed that micelles prefer to dissolve rather than to be swollen by the styrene or ethylbenzene in hexane. Theory Dynamic Light Scattering. To determine particle size and size distribution, the technique of dynamic light scattering (DLS) has been utilized extensively in the past few decades. In DLS the fluctuation of the intensity of scattered light can be related to the self-diffusion of micelles in solution. The fluctuation of scattered light intensity can be expressed in terms of the normalized second-order autocorrelation function, g2(tc):29
g2(tc) )
〈I(tc) I(0)〉 (I(0))2
(1)
where tc is the delayed time and I(tc) is the intensity of the scattered light at the delayed time tc. This correlation function of the scattered light intensity can be written, in a normalized form, as
Figure 1. Possible structures of diblock copolymer in hexane (poor solvent for polystyrene): (A) single diblock copolymer molecule; (B) micelle of diblock copolymer.
erization.22,23 The number of uses for such applications is growing rapidly because of their excellent resistance to freeze-thaw cycles and resistance to added electrolytes. Additionally they have proven to be an excellent steric barrier against the process of agglomeration.24 The most efficient steric stabilizer for nonaqueous applications was found to be the amphipathic, block, and graft copolymers which consist of a block which is insoluble in the dispersing medium and another block soluble in the medium. The insoluble block is adsorbed on the surface, while the soluble block acts as a stabilizing component and provides a barrier against coalescence. In a recently published paper of this series25 we reported that the diblock copolymer polystyrene-block-polybutadiene with 23% polystyrene terminal block, Stereon 730A, is the most appropriate stabilizer for preparing uniform micron size particles via anionic dispersion polymerization (22) Jialanella, G. L.; Firer, E. M.; Piirma, I. J. Polym. Sci., Polym. Chem. Ed. 1992, 30, 1925. (23) Leemans, L.; Fayt, R.; Teyssie, Ph. Polymer 1990, 31, 106. (24) Higgins, J. S.; Dawkins, J. W.; Shakir, S. A. Polymer 1986, 27, 931. (25) Awan, M. A.; Dimonie, V. L.; El-Aasser, M. S. J. Polym. Sci., Polym. Chem. Ed. 1996, 34, 2633.
g2(tc) ) 1 + C[g(1)(tc)]2
(2)
where C is an empirical constant which depends on the sampling interval and scattering geometry and g(1)(tc) is the normalized first-order autocorrelation function. For a monodisperse system, the normalized first-order autocorrelation function can simply be written as
g(1)(tc) ) exp(-Γtc)
(3)
where Γ is the decay rate constant (line width of the broadening of the frequency distribution of scattered light), which can be written in the form of a transitional diffusion constant:30
Γ ) DTq2
(4)
DT is related to the particle radius by the Stokes-Einstein relationship and can be expressed as (26) Awan, M. A.; Dimonie, V. L.; Filippov, L. K.; El-Aasser, M. S. Langmuir, submitted. (27) Giddings, J. C. Science 1993, 260, 1456. (28) Giddings, J. C.; Caldwelland, K. D.; Jones, H. K. Particle Size Distribution II: Assessment and Characterization; Provder, T., Ed.; ACS Symposium Series 332; American Chemical Society: Washington, DC, 1991; p 192. (29) Morrison, I. D.; Grabowski, E. F.; Herb, C. A. Langmuir 1985, 1, 496. (30) Chu, B.; Wang, J.; Shuely, W. J. Polymer 1990, 31, 805.
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k BT 6πηRh
(5)
where kB is the Boltzmann constant, T is the temperature (K), η is the viscosity of the solvent, and Rh is the hydrodynamic radius. Thus the particle size of the monodisperse suspension can be obtained from the measured autocorrelation function via eqs 1-5. In eq 4 q is the magnitude of the scattering vector and is given by
q)
θ sin( ) (4πn λ ) 2
(6)
where n is the refractive index of the solvent, q is the angle of detection for the scattered light, and λ is the wavelength of light. Static Light Scattering (SLS). The mass-average molar mass of the solute (Mw) and the second virial coefficient (A2), which reflects the solvent-solute thermodynamic interaction, can be evaluated by static light scattering.31,32 From the Zimm plot the apparent molar mass (Mw,app), apparent mean-square radius of gyration 〈Rg2〉, and second virial coefficient interaction parameter A2 of the micelles can be calculated using eq 7:33
Kc 1 + 2A2c ) ∆R(θ) P(θ)Mw
(7)
where K is an optical constant at concentration c, ∆R(θ) is the excess scattering intensity (Rayleigh ratio) of the polymer solution at concentration c, P(θ) is the scattering form factor, and A2 is the second virial coefficient. The optical constant is given in eq 8:
λ (dn dc )
K ) 2π2n2
2
NA-1
-4
(8)
where n is the refractive index of the solvent, (dn/dc) is the refractive index increments of polymer solution, and NA is Avogadro’s number. Moreover, the scattering factor P(θ) can be written as
1 θ 16π2 2 Rg sin2 )1+ 2 2 P(θ) 3λ
()
(9)
where λ is the wavelength of the incident light (in vacuum), θ is the angle of detection, and Rg is the radius of gyration. Producing a Zimm plot, plotting the left hand side of eq 7 against a function of angle and concentration, provided a graphical picture of solving eq 7 for Mw, Rg, and A2. Mechanism of Flow Field-Flow Fractionation (Flow FFF). The complete theory, experimental details, and detailed mechanism of flow FFF were described by Giddings.27,28 Flow FFF is an elution technique based on the principle of flow displacement. In this technique, the components of a solution are separated on the basis of the flow in a thin ribbon-like channel. This channel divides particles into narrow fractions, where each fraction is uniquely characterized by size or by mass. The driving force is imposed by an independent stream of fluid penetrating through permeable channel walls and forcing entrained species toward the accumulation wall, which is covered by a permeable membrane. The displacement velocity of the respective components (bands) depends on the mean position of the component (bands) in the parabolic flow profile. Those components which are driven into equilibrium distributions to the walls are in slow flow and move in the flow stream at a low rate. In flow FFF, a sample with a continuous distribution of sizes can be separated. When solution is injected in the ribbon channel, it travels with the flowing liquid in the ribbon channel having a parabolic flow. The channel flow propels the components of the solution toward the outlet. However, the displacement velocities of component A and component B depend on the mean position of the bands in the parabolic profile. The positions are (31) Yeng, A. S.; Frank, C. W. Polymer 1990, 31, 2089. (32) Tuzar, Z.; Kratochvil, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum: New York, 1993; Vol. 15, p 1. (33) Krause, S. J. Phys. Chem. 1964, 68, 1948.
controlled by the right-angle cross-flow. The cross-flow used in flow FFF must be strong enough to drive the components of interest into localized laminae within the parabolic profile. In flow FFF the retention time tr has been formulated as34
1 t° ) 6λ′ coth - 2λ′ tr 2λ′
[ ( )
]
(10)
where t° is the void time (the emergence time of a nonretained tracer) of nonretained component and λ′ is the retention parameter, which depends on one or more of the sample properties. In flow FFF, λ′ is given by34
λ′ )
DV° w2V°e
(11)
where D is the ordinary diffusion coefficient of the retained component, V° is the channel void volume, w is the channel thickness, and V°e is the cross-flow. The diffusion coefficient can be calculated from the Stokes-Einstein equation, and λ′ can be written as34
λ′ )
kBTV° 3πηdw2V°e
(12)
where kB is the Boltzmann constant, T is the absolute temperature, V° is the channel void volume, η is the viscosity of the solution, and d is the particle diameter.
Experimental Section Materials and Sample Preparation. The diblock copolymer polystyrene-block-polybutadiene Stereon 730A, a commercial product used in this study, was kindly provided by the Firestone Latex and Rubber Division, Akron, OH. The dilute solution of these diblock copolymers in tetrahydrofuran was used to determine the molar mass via GPC, 14.7 × 104 g/mol with a Mw/Mn of 1.05. The chemical analysis of this stabilizer, provided by Firestone, showed 23% pure polystyrene end block. Repeated precipitation was conducted to remove any trace of antioxidant used to avoid cross-linking because of double bonds in polybutadiene. The purification process consisted of repeated precipitation from toluene (HPLC grade, Aldrich) into methanol (HPLC grade, Aldrich). After precipitation, the diblock copolymer was vacuum-dried at 25 °C until no noticeable change in the polymer weight was observed. The diblock copolymer was kept in hexane for three to four days until all of the diblock completely dissolved. All the solutions studied had a bluish tint (characteristic of micelles) and were stable on prolonged storage for months. Initially, a solution having a concentration of 2.37 × 10-5 mol/L was prepared in predistilled hexane, and further dilution was made as required. Hexane, styrene, and ethylbenzene of spectroscopic grade (Aldrich) were distilled via vacuum line. Then the purity of these solvents was checked before use. Filtration was carried out using a 25 mm filter assembly with 0.2 µm pore size Millipore filters.
Results and Discussion Electron Microscopy. A solution of the diblock copolymer of Stereon 730A was prepared in hexane at a concentration of 2.43 × 10-6 mol/L. Samples for transmission electron microscopy (TEM) were prepared by placing a drop of the diblock copolymer solution on a copper grid having a carbon-coated formvar film. In one case the grid was freeze-dried with liquid nitrogen, and in another, the grid was dried at room temperature and diblock copolymer was stained with osmium tetroxide (OsO4) vapors before examination with the electron microscope. OsO4 stains only polybutadiene chains because it reacts selectively with the olefinic bonds of the polybutadiene component and appears black in the electron micrographs. Micrographs taken by TEM Philips 400T (Figure 2A) were selected to measure the particle size and size distribution (34) Brimhall, S. L.; Mayers, M. N.; Caldwell, K. D.; Giddings, J. C. Science 1972, 176, 296.
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Figure 2. Transmission electron micrographs of polystyrene-block-polybutadiene diblock copolymer (Stereon 730A) from hexane solution: (A) sample dried at room temperature and stained with OsO4 (larger particles of polystyrene 200 nm were used as internal standard); (B) sample freeze-dried. Table 1. Characterization of Stereon 730A Diblock Copolymer Polystyrene-block-polybutadiene via Dynamic Light Scattering (DLS), Static Light Scattering (SLS) in Hexane, and Transmission Electron Microscopy (TEM)a light scattering results Dh (at 90°) (nm) 141.2 ( 10
Mw,appb (× 10-7 g/mol) 4.89
no. of chains/ micelle, Naggb 349
TEM results Rg (nm) 58
micelle diameter (nm) 45 ( 5
Mw,app (× 10-7 g/mol) 2.9
no. of chains/ micelle, Nagg 202 ( 10
a
Dh, hydrodynamic diameter; Mw,app, apparent molecular weight of the micelle; Nagg, aggregation number; Rg, apparent radius of gyration. b Measured by SLS.
using the Zeiss Mop-3 analyzer. The results are presented in Table 1. These micrographs show that the diameter of the micelles is 45 ( 5 nm and that the distribution is fairly narrow. TEM micrographs of the unstained freezedried sample show that the micelles form regular patterns and that the distance between each particle is 70 ( 5 nm. This pattern may be due to repulsive interactions between the micelles. Moreover, the narrow micelle size distribution allows us to estimate the aggregation number of the diblock copolymer chains using eq 13:
4 Nagg ) πr3FNA/Mw 3
(13)
where r is the radius of the micelle (cm), F is the density of the diblock copolymer (g/cm3), NA is Avogadro’s number, and Mw is the molar mass of Stereon 730A (g/mol). The number of polymer chains per micelle was found to be 202 ( 10, and thus, the apparent molar mass of the micelle is 2.9 × 107 g/mol (Mw,app ) NaggMw). Light Scattering. A Brookhaven Instruments multiangle light-scattering goniometer with an Ar ion laser operating at 0.1 W and λ ) 488 nm was used to measure the intensity fluctuation as a function of concentration, temperature, and styrene contents. The apparatus included a specimen cell assembly with a temperature control and index-matching liquid systems. The temperature was controlled within 0.2 °C fluctuation. Filtration and circulation of the index-matching fluid were
maintained via peristaltic pump and membrane filter. The optical detector assembly used a 200 mm focal length lens as part of its optical transfer system. The autocorrelation function of the scattered photon was determined using a Brookhaven BI-2030 AT-72 channel four-bit digital correlator. Static Light Scattering. The polymer solution was diluted with hexane, and the change in the scattering intensity was recorded. The results given in Figure 3 show that, at low concentrations, the change in the scattering intensity has a weak dependence on the concentration of diblock copolymer. As the concentration was increased to above 9 × 10-11 mol/L, a progressive increase in intensity was observed. This is taken as the cmc of the diblock copolymer in hexane. However, above a concentration of 3 × 10-7 mol/L, a decrease in the scattering intensity was observed. This decrease in the scattering intensity is due to multiple scattering taking place at higher micelle concentration. To determine the apparent molar mass Mw,app and apparent radius of gyration Rg, a solution of the diblock copolymer in hexane was prepared. Hexane with a solubility parameter δ ) 7.3 (cal/cm3)1/2 is a better solvent for polybutadiene [δ ) 8.1 (cal/cm3)1/2] and a poor solvent for polystyrene [δ ) 9.1 (cal/cm3)1/2].35 The scattered intensity from semidilute (2.25 × 10-7 mol/L) to very dilute (35) Brandrup, J.; Immergut, E. H. Polymer Handbook; John Wiley: Toronto, 1975; p PIV-354.
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B
Figure 3. Effect of dilution of the diblock copolymer with hexane on the scattering intensity measured via static light scattering at 30 °C.
A solutions (2.25 × 10-11 mol/L) at 20-140 °C was observed. By fitting the actual data to the Zimm equation (eq 7), the apparent molar mass (Mw,app), apparent radius of gyration (Rg), and second virial coefficient were extracted, and the results are given in Table 1. The results given in Table 1 show that, using the diameter of dried micelles (45 ( 5 nm), the corresponding calculated apparent molar mass (Mw,app ) 2.9 × 107 g/mol) and the aggregation number (Nagg ) 202 ( 10) for each micelle observed via TEM are lower than the corresponding values obtained via SLS. The difference in the diameter observed via both techniques (TEM and DLS) results from the different physical states of the micelles. Samples studied via TEM were dried, and the polybutadiene shell of the diblock copolymer was collapsed, as opposed to DLS conditions where the polybutadiene block is swollen. Further, the polybutadiene block may degrade under the high energy of the electron beam during TEM analysis. This degradation of the polybutadiene may lead to further shrinkage of the micelle. The radii of gyration obtained from the angle dependencies of Kc/∆R at zero concentration are also apparent due to the core/shell structure of the micelles and the different refractive indices of the blocks. Therefore, the apparent radius of gyration (Rg) value obtained via SLS is only qualitative. Dynamic Light Scattering. Micelle Formation. Hexane is a good solvent for polybutadiene and a precipitant for polystyrene chains. Therefore, it was expected that polystyrene-block-polybutadiene diblock copolymer would form micelles in a dilute solution in this solvent. The intensity fluctuation of scattered light from the solution of diblock copolymer in hexane was determined by dynamic light scattering (DLS) and provided single-exponential autocorrelation curves for all diblock copolymer concentrations, i.e., up to 4.2 × 10-6 mol/L. The experimental data were treated with a cummulant method to estimate the diffusion coefficient of the micelle. The hydrodynamic diameter Dh, calculated from the diffusion coefficient using eq 5, was 141.2 ( 10 nm, and the distribution was found to be unimodal. However, the micelle diameter observed via TEM in the freeze-dried specimen as measured from the center to center particle interdistance was very close to 70 ( 5 nm (Figure 2B). Effect of Dilution and Solvency. The effect of the amount of added styrene monomer upon the state of the diblock copolymer micelle formation was investigated by titrating the diblock copolymer solution in hexane with styrene. A 6.41 × 10-5 mol/L solution of diblock copolymer was prepared in hexane, and styrene was selected as titrant. Styrene was used because it is a good solvent [δ ) 9.3 (cal/cm3)1/2] for polystyrene blocks [δ ) 9.1 (cal/cm3)1/2].
Figure 4. (A) Effect of pure styrene addition on the micelle diameter of diblock copolymer (Stereon 730A) in hexane measured via dynamic light scattering. Initial concentration of the diblock copolymer in hexane ) 6.41 × 10-5 mol/L. (B) Effect of pure styrene addition on the scattering intensity of the diblock copolymer solution 6.41 × 10-5 mol/L (Stereon 730A) in hexane, measured via static light scattering at q ) 20° and T ) 30 °C.
The change in the micelle size was recorded by DLS, and the scattering intensity was measured with SLS; the results are shown in parts A and B of Figure 4, respectively. Figure 4A shows that the size of the micelle decreases slightly with the addition of styrene (up to 22% styrene), followed by a sharp increase in diameter. The initial slight decrease in micelle diameter (from 141 to 115 nm) could be due to changing the solvency of the hexane medium due to addition of styrene, making it a slightly “poor” solvent for the polybutadiene block [the solubility parameter of polybutadiene is 8.1 (cal/cm3)1/2]. Another possible reason for the initial decrease is that, with the addition of styrene, the diblock copolymer chains prefer to exist as single polymer molecules over micellar structures. Therefore, the observed decrease in the micelle’s diameter could be the net result of smaller aggregation number per micelle and slightly poor solvency of the medium for polybutadiene. The increase in micelle diameter above 22% styrene could be due to the swelling of the core or to deformation of the micelle followed by reorganization of the diblock copolymer of the micelle in a network in which the polystyrene block of one micelle may associate with the neighboring polystyrene chains. At a styrene content above 50% this association could not be maintained and the micelle network disappeared. The disappearance of the micelles in the presence of styrene is the outcome of the solvency of the medium, which becomes more favorable for the polystyrene core with
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Figure 5. Effect of addition of a styrene solution of diblock copolymer (1.78 × 10-5 mol/L Stereon 730A) on the scattering intensity of a diblock copolymer (Stereon 730A) solution (1.78 × 10-5 mol/L) in hexane, measured via static light scattering at q ) 20° and T ) 30 °C.
increased concentration of styrene in the hexane/styrene mixture. However, the SLS results in Figure 4B show that the observed scattering intensity was decreasing with the addition of styrene. The drop in the scattering intensity is the net outcome of a decrease in both the aggregation number of each micelle and the number of micelles per unit volume. On the basis of the results by SLS presented in Figure 4B, an optimum composition of the solution required for the complete dissolution of the micelle is in the range 20-25% styrene in the hexane/ styrene mixture. Effect of Solvency. The effect of solvency of the medium was further elucidated by preparing two solutions with the same concentrations (1.78 × 10-5 mol/L) of diblock copolymer, one in hexane and the other in styrene. The diblock copolymer solution in hexane (1.78 × 10-5 mol/L) was titrated with the diblock copolymer solution in styrene (1.78 × 10-5 mol/L). The observed change in the scattering intensity with the addition of styrene solution was plotted and is shown in Figure 5. In this case, the scattered intensity drop was slower than that found when pure styrene was added (see Figure 4B), and the total amount of the styrene solution required for the complete dissolution of the micelles was around 65-70%. These results show that solvency can affect the dissociation of the micelles. The combined results of Figures 4 and 5 indicate that both solvency and dilution are effective for the dissociation of the micelles. It is reported that diblock copolymers in a selective solvent can exist as single polymer molecules and as micelles. Both single polymer molecule (free polymer molecules) and micellar species exist in a dynamic equilibrium.21 However, by light scattering, it is difficult to identify the coexistence of both single polymer molecules and micelles.30 Therefore, the flow FFF technique was selected both to confirm the presence of single polymer molecules along with the micelles and to determine the cmc of diblock copolymer in a mixture of hexane and ethylbenzene. Flow Field-Flow Fractionation (Flow FFF). In order to quantify the ratio of micelles to single polymer molecules and to separate both species of the diblock copolymers in hexane and in a mixture of hexane and ethylbenzene, flow FFF was used. Solutions of diblock copolymer Stereon 730A in hexane at concentrations of 7.5 × 10-6 mol/L and 4.2 × 10-6 mol/L were used to study
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Figure 6. Flow field flow fractionation (flow FFF) fractograms of polystyrene-block-polybutadiene (Stereon 730A) showing the effect of dilution (in hexane) on the retention time of the micelles at 25 °C. Conditions: cross-flow ) 0.35 ml/L; void volume ) 0.64 mL.
Figure 7. Flow FFF fractograms showing the effect of polystyrene-block-polybutadiene (Stereon 730A) concentration in hexane on the micelle diameter at 25 °C. Conditions: crossflow ) 0.35 ml/L; void volume ) 0.64 mL.
the micelle hydrodynamic size and the relative amount of both single polymer molecules and micelles. The results, given in Figure 6, show that the diblock copolymer in hexane exists as a mixture of both single polymer molecules and micelles. The single polymer molecules in a solution of hexane eluted prior to the micelles, and the base line resolution was achieved within a short time. The hydrodynamic size of the corresponding species can be determined by applying eq 10. The results given in Figure 7 show that the hydrodynamic size of the first eluted peak is in the range 7-10 nm. This size corresponds to single polymer molecules or some kind of association between a few diblock copolymer molecules. The hydrodynamic size for a peak eluted within 5-6 min is in the range of 130 nm. A hydrodynamic size of 130 nm can be attributed to a large cluster of diblock copolymer chains. The areas belonging to the respective peaks of single polymer molecules and micelles at 4.2 × 10-6 mol/L and 7.5 × 10-6 mol/L were cut and weighted. The relative mass ratio between single polymer molecules and micelles was calculated, and results are given in Table 2. The results obtained via flow FFF given in Table 2 also show that, by increasing the diblock copolymer concentration in hexane from 4.2 × 10-6 mol/L to 7.5 × 10-6 mol/L, an
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in the solvency of the medium shifts the equilibrium toward single polymer molecules with a decrease in micelle size. Summary and Conclusions
Figure 8. Influence of the solvency of the medium on the size and relative distribution of micelles and free polymer molecules of polystyrene-block-polybutadiene (Stereon 730A) in hexane/ ethylbenzene. The initial concentration of Stereon 730A in hexane is 7.5 × 10-6 mol/L. The solvency of the medium is adjusted by addition of ethylbenzene. Flow FFF conditions: cross-flow ) 0.35 ml/L; void volume ) 0.64 mL. Table 2. Effect of Concentration of Stereon 730A in Hexane on the Free Polymer/Micelle Ratio and Micelle Size Measured with Flow FFF at Room Temperature conc × 106 mol/L)
free polymer/ micelle
micelle diameter (nm)
4.2 7.5
5.2 12.5
115 130
increase in the micelle size and the relative weight ratio of single polymer molecules to micelles was observed. The possible reason(s) for the observed increase in the micelle size and the corresponding increase in the weight ratio of free polymer chains relative to micelles cannot be rationalized on the basis of the typical low molecular weight surfactant micelles. However, it could be possible that micelles of the diblock copolymers are not compact structures like the micelles of small molecules. Moreover, with the further addition of solvents, e.g., hexane, the associated structure becomes loose. The polymer chains may start leaving the micelles. Another reason could be due to an inherent limitation of the flow FFF technique. Therefore, a more systematic study may be required. In another set of experiments the 7.5 × 10-6 mol/L solution of diblock copolymer in hexane was diluted with pure ethylbenzene. The amount of the ethylbenzene was increased from 10% to 23%, as shown in Figure 8. The purpose for the addition of ethylbenzene in small steps was to study the effect of the medium’s solvency and dilution on the association behavior of Stereon 730A. With the addition of ethylbenzene [δ ) 8.8 (cal/cm3)1/2] (a good solvent for polystyrene and polybutadiene), the time required for the micelles to elute started decreasing.35 At 23% ethylbenzene a single peak was observed near the void volume. This suggests either that all of the clusters dissolved or that a major portion of these clusters dissolved and the amount of the remaining clusters was below the detection limit of the instrument. A decrease in the retention time of the eluting peak (see Figure 8) as the ethylbenzene content increases is an indication of the decrease in the micelle’s hydrodynamic size. The increase
Results obtained via TEM and DLS showed that diblock copolymers of polystyrene-block-polybutadiene form monodisperse micelles in hexane. The hydrodynamic diameter of the micelles observed by both DLS and flow FFF is 140 ( 5 and 130 ( 5 nm, respectively. However, the diameter of the dried micelles observed with TEM is almost three times smaller than the size observed via DLS or flow FFF. This differences in size is probably a net result of the following factors: (a) the collapse of the extended conformation of the polybutadiene block (shell) with the evaporation of hexane during TEM sample preparation and (b) a shrinkage of the micelle size under the high energy electron beam due to the soft polybutadiene block. Further, the deswelling of the micelles during sample preparation for TEM may lead to a small distortion of the shape. Therefore, the estimation of the micelle molar mass from the electron micrograph may be less reliable. Results obtained via DLS and flow FFF confirmed that the diblock copolymers form stable micelles in hexane. Moreover, it was observed via DLS and shown in Figure 4A that, for styrene concentrations below 20% in hexane, the micelle size decreased slightly with the addition of styrene. This trend indicates that styrene does not swell the micelles. On the basis of the solubility parameters of hexane, styrene, polybutadiene, and polystyrene, hexane can be considered a better solvent for the polybutadiene blocks which are situated in the corona of the micelle. Therefore, the addition of styrene up to 2023% may not be enough to overcome the interaction between hexane and polybutadiene. Therefore, at low styrene concentrations the polystyrene core remains unswollen by styrene. The flow FFF results can be utilized as evidence that, under the prevailing conditions used in our anionic polymerization, the hexane medium contains both single polymer molecules and micelles.25 The weight ratio of single polymer molecules in solution relative to micelles increases with the increase of the diblock copolymer concentration (Table 2). Upon the addition of a good solvent (styrene) for the polystyrene core of the diblock copolymer, the scattered intensity decreases, as observed via static light scattering, and the size of the micelle decreases from 140 to 112 nm, as observed via dynamic light scattering. A further addition of styrene or ethylbenzene using either SLS or flow FFF methods showed complete dissolution of the micelles above 23% styrene or ethylbenzene. However, results obtained via DLS showed that micelles still exist, and an abrupt increase in their size was observed via DLS above 23% styrene in hexane, suggesting a type of network formation. A further addition of styrene leads to complete dissolution of the micelles. Dissolution of the micelles using both flow FFF and SLS confirms that the micelles are not being swollen either by the styrene monomer or by the ethylbenzene. Acknowledgment. The authors would like to extend gratitude to Dr. Giddings, Mr. Mike Miller for flow FFF studies (University of Utah, Salt Lake City, UT), and the Firestone and Latex and Rubber Division, Akron, OH, for providing samples of Stereon 730A. LA9508699
