J. Phys. Chem. B 2000, 104, 1439-1446
1439
Thermal Behavior and Association Properties of Polystyrene-b-Poly(ethylene/ butylene)-b-Polystyrene Triblock Copolymer in N-Octane/4-Methyl-2-pentanone Solutions Jose´ R.Quintana, Estı´baliz Herna´ ez, Irune Inchausti, and Issa Katime* Grupo de NueVos Materiales, Departamento de Quı´mica Fı´sica, Facultad de Ciencias, Campus de Leioa, UniVersidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain ReceiVed: July 28, 1999; In Final Form: December 2, 1999
Static and dynamic light scattering measurements have provided good information concerning the temperaturedependent associative behavior of a triblock copolymer polystyrene-b-poly(ethylene/butylene)-b-polystyrene dissolved in solvent mixtures of n-octane and 4-methyl-2-pentanone. Both are selective solvents, and when the solvent composition is varied, the solvent mixture shifts from a selective solvent of the copolymer end block to a selective solvent of the middle block. For solvent mixtures with a high 4-methyl-2-pentanone percentage (selective solvent for polystyrene blocks), two kinds of particles have been detected in solution. The smallest ones had a hydrodynamic radius close to 14 nm and were considered as free copolymer chains because they were only detected at temperatures higher than the critical micelle temperature (CMT). Below this temperature, a second kind of particle was detected with a hydrodynamic radius close to 37 nm, and those particles were considered as ordinary micelles with a core formed by poly(ethylene/butylene) blocks. For solvent mixtures with a high n-octane percentage (selective solvent for poly(ethylene/butylene) blocks) three kind of particles were detected. The smallest ones had a hydrodynamic radius close to 14 nm and corresponded to free copolymer chains. These particles were only detected at temperatures higher than CMTs. Below the critical micelle temperatures a second kind of particle of Rh ≈ 47 nm was detected, and those particles were considered flowerlike micelles with a core formed by polystyrene blocks. A third kind was detected at any temperature. The hydrodynamic radii of these particles were around 200 nm. Those were considered to be aggregates with a loose structure different from that of micelles. n-Octane/4-methyl-2pentanone mixtures with similar solvent content behaved as a nonselective solvent, and no micelles were detected at 25 °C. Finally, the self-association process is enthalpy-driven, yielding closed association structures in the form of flowerlike micelles in solvent mixtures with a high n-octane content and ordinary micelles in solvent mixtures with a high 4-methyl-2-pentanone content.
Introduction The ability of diblock and triblock copolymers to selfassemble into micelles has been established for over 2 decades. Most studies have been concerned with AB and ABA block copolymers in selective solvents of A blocks, i.e., a thermodynamically good solvent for A blocks and precipitant for B blocks. However, the study of the self-association of triblock copolymers in selective solvents of the middle blocks has attracted great interest in recent years.1-8 The reason is that, whereas the former copolymer systems have been well characterized, much less understanding has been achieved about the solution properties of ABA block copolymers in solvents that are relatively good for the B blocks. It is now well recognized9-11 that for symmetrical triblock copolymers in selective solvents for the outer blocks the micelles consist of two main regions, an inner core containing the insoluble middle blocks and an outer corona containing both the soluble outer blocks and the solvent molecules. In most cases, these micelles have spherical shape and a narrow size distribution. In other words, the micellization process obeys the closed association mechanism, which assumes a dynamic equilibrium between the copolymer chains and micelles with an association number N. * To whom correspondence should be addressed. Address: Avda. Basagoiti, 8, 1°C, 48990 Getxo, Vizcaya, Spain
The association phenomenon of triblock copolymers in selective solvents for the middle block would lead to a variety of possible aggregate structures. Flowerlike micelles would be formed where the middle block forms a loop so that both outer blocks stay in the micelle core. However, the existence of branched structures at low concentrations or gellike networks at high enough concentrations would also be possible because of a bridging function of the soluble middle blocks extended between the small clusters formed by the outer blocks. The additional entropic penalty to the flowerlike micelle formation that arises from the loop formation of the corona blocks could cause some outer blocks to extend into solution, favoring the branched structure formation. However, the removal of poorly solvated blocks from the core into solution also produces an interfacial free energy penalty favoring, therefore, the flowerlike micelle formation. Thus, intermediate structures between both extremes mentioned above are also possible. The association phenomenon will depend on the balance between these two competing forces, which are, in turn, influenced by the interactions between the outer block and the solvent and by the relative lengths of the two different chemical blocks. This would explain the apparently contradictory results published in recent years. In dilute solutions of a few such systems, some authors12,13 failed to detect any kind of multimolecular association while others1-3,5,14 have found well-
10.1021/jp9926544 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/27/2000
1440 J. Phys. Chem. B, Vol. 104, No. 7, 2000 defined micelles. On the other hand, some authors4,15 have reported the existence of loose and polydisperse aggregates rather than standard micelles. Several authors6,16-18 have found thermoreversible gels in solutions of ABA block copolymers in selective solvents of B blocks at remarkable low concentrations. To improve our understanding of these systems, we have investigated the aggregation process of ABA copolymers in selective solvents of block B in both dilute14,19,20 and semidilute solutions.18,21-23 In a recent paper20 we have studied the selfassociation of a polystyrene-b-poly(ethylene/butylene)-b-polystyrene copolymer in n-octane/4-methyl-2-pentanone mixtures of different compositions. n-Octane is a selective solvent for poly(ethylene/butylene), whereas 4-methyl-2-pentanone is selective for polystyrene, and by varying the solvent composition, we were able to control the kind of aggregates. These solvents are practically isorefractive and therefore the light scattering analysis is simplified because effects of preferential solvation can be neglected. Viscosity and light scattering measurements were carried out as a function of solvent mixture composition at 25 °C. For solvent mixtures with a high 4-methyl-2-pentanone content only one kind of particle was detected. These particles had a high molar mass and a small size. However, for solvent mixtures with a high n-octane content two particles of different size were detected: the smaller, more numerous ones were considered as free copolymer chains, whereas larger ones were assumed to be aggregates with a loose structure different from that of ordinary micelles. The n-octane/4-methyl-2-pentanone mixture with 50% ketone behaved as a nonselective solvent, and no aggregates were detected at 25 °C and under the experimental concentrations. Given that several authors have reported the existence of micelles in selective solvents for the middle block, we have extended our study of SEBS solutions in n-octane/4-methyl-2-pentanone mixtures to examine the temperature-dependent association behavior of the above triblock/solvent mixture system in dilute solutions. Experimental Section Materials and Solution Preparation. The polystyrene-bpoly(ethylene/butylene)-b-polystyrene sample, SEBS3, is a commercial product kindly provided by Shell Espan˜a, S.A. The sample has been previously characterized in detail.24 Its average molar mass, polydispersity index, and polystyrene weight percentage are 260 000 g mol-1, 1.18, and 30 wt %, respectively. Solvents purchased from Panreac and Fluka were analytical purity grade and were used as received. Solvent samples used for light scattering measurements were filtered using a 0.02 µm aluminum oxide membrane filter. Solvent mixtures were made up by volume. Solutions were obtained by dissolving a known amount of copolymer in the solvent mixtures in sealed flasks under gentle agitation and then heating them at 70 °C for several hours to ensure complete solution. Copolymer solutions were filtered at 60 °C through PTFE Acrodisc CR filters (0.2 µm pore size) into light scattering cells in order to clarify them for light scattering measurements. Static Light Scattering. Thermodynamic parameters of the micellization process were determined by a light scattering technique. Measurements were performed on a modified FICA 42000 light scattering photogoniodiffusometer. Both the light source and the optical block of the incident beam were replaced by a He-Ne laser that emits vertically polarized light at 632.8 nm. Investigations of the thermodynamics of micellization of block copolymers in organic solvents have shown that it is far
Quintana et al. better experimentally to carry out measurements in which the concentration is kept constant and the scattered light intensity is monitored over a temperature interval to find the critical micelle temperature, CMT, than to keep the temperature constant and to vary the concentration to find the critical micelle concentration, cmc.9 The critical micelle temperature of a solution at a given concentration is the temperature at which the formation of micelles can just be detected experimentally. For block copolymers in organic solvents it has been shown25 that, within experimental error,
d ln c d ln cmc ) dT-1 d CMT-1
(1)
To establish critical micelle temperatures, measurements of light scattered intensity were made at series of temperatures within the range 25-90 °C at scattering angles of 45° and 135°. Dynamic Light Scattering. A light scattering spectrometer Antec was used. Intensity correlation function measurements were carried out in the self-beating mode by using a Brookhaven BI-9000 AT 522-channel digital correlator. A He-Ne laser operating at 632.8 nm was employed as the light source. We accepted only those photoelectron count time correlation functions where the measured baseline, i.e., the average value of the correlation function at very long delay times, agreed with the computed baseline within ∼0.1%. A nonnegative constrained least-squares method26 was used for the data analysis of the dynamic light scattering results. Results and Discussion A conventional way to study the association process is to determine the temperature dependence of the scattered light intensity. From this dependence the critical micelle temperatures can be determined for several concentrations and the thermodynamic functions of micellization calculated. On assuming a closed association mechanism, the standard free energy and standard enthalpy of micelle formation, ∆G° and ∆H°, per mole of the copolymer in the micelle, are given by the relations
∆G° ≈ RT ln cmc
(2)
d ln cmc d T-1
(3)
∆H° ≈ R
[
]
where cmc is the critical micelle concentration. The two standard states are the copolymer molecules and micelles in ideal dilute solution at unit molarity. Taking into account eq 1, the standard enthalpy of micellization can be expressed by
∆H° ≈ R
[
d ln c d CMT-1
]
(4)
After integration we have
ln c ≈
∆H° + constant R CMT
(5)
provided that ∆H° is approximately constant in the temperature interval of interest. The temperature dependence of the light intensity scattered at two angles, 45° and 135°, has been studied for three n-octane/ 4-methyl-2-pentanone mixtures: 20/80, 30/70, and 80/20. Parts a and b of Figure 1 show the scattered intensity at 45° and the dissymmetry factor, i.e., the ratio of the scattering intensity at 45° to the intensity at 135°, for two SEBS3 copolymer solutions
Polystyrene-b-Poly(ethylene/butylene)-b-Polystyrene
J. Phys. Chem. B, Vol. 104, No. 7, 2000 1441
Figure 2. Plots of the logarithm of the concentration as a function of the reciprocal of the absolute critical micelle temperature for solutions of SEBS3 in different n-octane/4-methyl-2-pentanone mixtures: 20/ 80 (0), 30/70 (O), 80/20 (4).
Figure 1. Plots of scattering light intensity at 45° (b) and dissymmetry factor (O) against temperature for two solutions of SEBS3 in two different n-octane/4-methyl-2-pentanone mixtures: 20/80 (a) and 80/ 20 (b). Concentrations of solutions were c ) 3.00 × 10-4 g mL-1 (a) and c ) 3.07 × 10-3 g mL-1 (b). The intersection of the dotted lines defines the critical micelle temperature.
in n-octane/4-methyl-2-pentanone (20/80) and (80/20). The curve shape of the scattered intensities is due to the influence of temperature on the equilibrium between micelles and free copolymer chains. The SEBS3 solutions in n-octane/4-methyl2-pentanone (20/80) behave practically as ordinary micelle solutions. At the upper end of the temperature range studied only free chains exist, whereas at low temperatures the equilibrium is overwhelmingly in favor of micelle formation. When the temperature is lowered from a high value, a sharp increase in the scattered intensity is observed because of the appearance of micelles. Once the micelles are predominant in the copolymer solution at lower temperatures, the intensity increase becomes slight. On the other hand, the dissymmetry variation with the temperature shows a slight anomalous behavior. In general, for solutions of copolymers with small molar mass the dissymmetry factor remains practically constant and close to unity. However, in our case we have found a small peak on lowering the temperature that appeared just at the onset of the intensity increase. Similar curves, I ) f(T) and Z ) f(T), have been found when increasing and decreasing the temperature for all systems studied. Plots similar in shape to the one shown in Figure 1a were also obtained for all solutions examined corresponding to solvent mixtures with 20 and 30% n-octane. The temperature dependence of the light intensity scattered at 45° for the solvent mixture with 80% n-octane shows quite a different behavior. A sharp scattering peak accompanied by a strong angular dissymmetry in scattering is observed between the micelle and unimer regions in Figure 1b. The presence of both peaks in the scattered intensity and in the dissymmetry
factor has been detected in every concentration studied for this solvent mixture. This remarkable phenomenon has been extensively reported on various block copolymer solutions and is known as anomalous micellization.27-29 Recently,30,31 it has been proposed that the anomalous micellization could have its cause in the possible composition heterogeneity of the block copolymer. If there exists a small proportion of copolymer chains with a higher content of the insoluble blocks, when the solvent quality decreases as a consequence of, for example, lowering the temperature, these copolymer chains could precipitate before the onset of micellization of the major chains is reached. Later, as the micelle formation of the major component progresses, the insoluble chains can become part of the micelles. This explanation also would be supported by the investigation of the solubilization of poly(isobutene) by micelles of polystyrene-b-poly(ethylene/ propylene) block copolymer in 5-methyl-2-hexanone.32 The addition of poly(isobutene) to the micellar solution favors the anomalous behavior found in micellar solutions of the block copolymer. The critical micelle temperatures defined as the temperatures at which the presence of micelles in the solution can just be detected have been estimated from the intersection of the two straight line portions corresponding to the unimer and micelle regimes despite the presence of the anomalous scattering. In Figure 1 the CMTs correspond to the crossing point of the dotted lines. Critical micelle temperatures were determined for solutions of SEBS3 in three n-octane/4-methyl-2-pentanone solvent mixtures covering a range of concentration between 1.2 × 10-5 and 5.0 × 10-3 g mL-1 for 20/80, 2.3 × 10-5 and 5.3 × 10-3 g mL-1 for 30/70, and 1.0 × 10-3 and 6.0 × 10-3 g mL-1 for 80/20. Plots of ln c as a function of (CMT)-1, obtained on the basis of eq 5, for solutions of SEBS3 in the three solvent mixtures are shown in Figure 2. All the plots were linear within experimental error over the temperature ranges studied. The standard Gibbs energy, ∆G°, enthalpy, ∆H°, and entropy of micellization, ∆S°, were calculated from the experimental data by means of eqs 2 and 5. These values, per mole of copolymer chain, are shown in Table 1. The standard Gibbs energy of micellization shows negative values for every system studied, as expected. The standard entropy of micellization is unfavorable to the micellization process as predicted by simple statistical arguments. The
1442 J. Phys. Chem. B, Vol. 104, No. 7, 2000
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TABLE 1: Thermodynamic Data of the Micellization Process of Copolymer SEBS3 in n-Octane/ 4-Methyl-2-pentanone Binary Mixtures at 50 °Ca % n-octane mol-1
∆H°/kJ ∆S°/kJ mol-1 K-1 ∆G°/kJ mol-1 cmc/g mL-1
20 (SLS)
30 (SLS)
80 (SLS)
80 (DLS)
-215 -0.57 -29.2 1.90 × 10-5
-112 -0.30 -16.1 2.46 × 10-3
-37.8 -0.07 -15.0 3.70 × 10-3
-41.7 -0.08 -14.5 4.49 × 10-3
a Data were calculated from static light scattering (SLS) and dynamic light scattering (DLS) measurements.
standard enthalpy of micellization also shows negative values, and therefore, the micelle formation of the triblock copolymer in the three solvent mixtures is an enthalpy-driven process. The ∆H° negative values stem largely from the exothermic interchange energy that accompanies the replacement of copolymer segment/solvent interactions by copolymer segment/copolymer segment and solvent/solvent interactions on micelle formation. When the thermodynamic magnitudes for the three micelle systems are compared, important differences can be observed and attributed to several factors. In the solvent mixture with 80% n-octane, the standard enthalpy of micellization shows a much lower value than in mixtures with 20 and 30% of n-octane probably because of the fact that the looping geometry that the middle poly(ethylene/butylene) block needs to form the micelle makes some polystyrene blocks come out of the core and extend into the solution. Therefore, the number of polystyrene/n-octane interactions replaced by polystyrene/polystyrene and n-octane/ n-octane interactions will be smaller. In the solvent mixtures with a lower percentage of n-octane, the polyolefin blocks will form the micelle core surrounded by a shell formed by the outer polystyrene blocks, and thus, a very low probability that some polyolefin blocks stay out of the micelle core exists. Therefore, the replacement of poly(ethylene/butylene)/4-methyl-2-pentanone interactions will be high. The existence of the polystyrene blocks out of the micelle together with the looping geometry of the middle block will lead to less order in the micelles formed in solvent mixtures with a high n-octane percentage, and therefore, the standard entropy of micellization for these micelle systems is also markedly smaller. The loss of exothermic energy interchange that accompanies the core formation reduces largely the driving force for micellization. So the standard Gibbs energy of micellization becomes less negative for solutions with 80% n-octane; i.e., SEBS3 in this solvent mixture has a lower critical micelle temperature and a higher critical micelle concentration. A very similar behavior has been reported for poly(oxypropylene)-bpoly(oxyethylene)-b-poly(oxypropylene) in water,3 polyterbutylstyrene-b-polystyrene-b-polyterbutylstyrene in N,N-dimethylacetamide,5 and polystyrene-b-poly(ethylene/butylene)b-polystyrene in n-octane.14 It is not less interesting to compare the micellization thermodynamics of SEBS3 in both solvent mixtures with a low n-octane percentage. An increase in the n-octane percentage leads to a decrease in the absolute value of every standard magnitude of micellization. The decrease in the value of ∆H° suggests that the number of interactions PEB/4-methyl-2pentanone-substituted for interactions PEB/PEB and 4-methyl2-pentanone/4-methyl-2-pentanone along the micellization is reduced. This decrease should be caused by an increase of favorable PEB/n-octane interactions in the unassociated chains and in the micelle core. The decrease observed in the standard entropy of micellization will be related to the decrease in the
association number and to the increase in the micelle solvation that take place as n-octane is added to 4-methyl-2-pentanone.20 So the stability of the micelles decreases, showing lower critical micelle temperatures and higher critical micelle concentrations as the n-octane percentage increases. This behavior agrees with the fact that no micelles of SEBS3 were detected in solutions of the solvent mixture n-octane/4-methyl-2-pentanone (50/50).20 This behavior is similar to that found for the micellization of a polystyrene-b-poly(ethylene/propylene) copolymer in n-dodecane/1,4-dioxane mixtures.33 Both solvents were also selective solvents for each block. Since the addition of one selective solvent does not improve the solvent quality with respect to the micelle shell, it suggests that core formation is mainly responsible for the micelle stability. This result agrees with those reported for the micellization of a polystyrene-b-poly(ethylene/ propylene) block copolymer in 5-methyl-2-hexanone/2-chlorobutane34 and 5-methyl-2-hexanone/2-pentanol mixtures.35 The addition of a precipitant, 2-pentanol, did not change significantly the solvent quality of the mixture with respect to the core block, and therefore, ∆H° and ∆S° hardly changed with the solvent mixture composition. On the other hand, the addition of a good solvent, 2-chlorobutane, changed markedly the solvent quality of the solvent mixture with respect to the core block, and this could explain the important changes observed in the micellization thermodynamics when the solvent mixture composition was varied. To get more extensive information on the aggregate systems studied, especially for the copolymer solutions with a high n-octane content where an anomalous behavior of micellization has been observed, dynamic light scattering (DLS) measurements were carried out. The size distribution functions of the different copolymer solutions have been analyzed as a function of copolymer concentration, solvent composition, and temperature. The equivalent hydrodynamic radius, Rh, can be calculated by knowing the translational diffusion coefficient, D, which is experimentally determined by DLS experiments, and by using the Stokes-Einstein relation
Rh )
kBT 6πηD
(6)
where kB is Boltzmann’s constant, T is the absolute temperature, and η is the viscosity of the solvent. The nonnegative constrained least-squares method was used to analyzed the intensity autocorrelation functions measured at a scattering angle of 45° where qRh < 1 with q being the magnitude of the momentum transfer vector. Figure 3 gives the size distributions in terms of Rh as a function of solvent mixture composition for triblock copolymer solutions with c ) 5.0 × 10-3 g mL-1 and at 25 °C. The plots are given in the semilogarithmic form, and the intensity contribution function is expressed in arbitrary units but normalized to the highest value at each solvent composition; i.e., the area under each peak is a measure of its scattering intensity contribution but on a relative scale. The dynamic light scattering results, shown in Figure 3, confirm the different association behaviors found for n-octane/ 4-methyl-2-pentanone mixtures with a high content of n-octane or 4-methyl-2-pentanone. The solvent mixtures with n-octane percentages of 0, 20, and 30% show a single narrow peak at Rh ≈ 30 nm. This peak is assigned to ordinary micelles. The mixture with an n-octane percentage of 40% also shows a single narrow peak but at a hydrodynamic radius of 14 nm. In this case the peak is assigned to free copolymer chains. The addition of n-octane to copolymer solutions of 4-methyl-2-pentanone
Polystyrene-b-Poly(ethylene/butylene)-b-Polystyrene
J. Phys. Chem. B, Vol. 104, No. 7, 2000 1443
Figure 3. Intensity contribution function against equivalent hydrodynamic radius, Rh, for solutions of SEBS3 in different n-octane/4-methyl-2pentanone mixtures at 25 °C. Copolymer concentration ) 5.0 × 10-3 g mL-1.
increases the cmc to hinder micelle formation under the experimental conditions used. At 25 °C no micelles were detected for solvent mixtures with n-octane percentages of 40, 50, and 60%. For the mixture 50/50 two peaks were detected at hydrodynamic radii of 12 and 85 nm. The largest peak would correspond to free copolymer chains and the other to some kind of aggregate. According to the relative heights of both peaks, the number of aggregates in the solution would be very small. In the mixture with 60% n-octane the peak corresponding to these aggregates appears at a higher hydrodynamic radius and shows a greater importance. Two peaks are also observed for a mixture with 70% n-octane, but the first peak appears at Rh ≈ 48 nm and the aggregate peak continues increasing. In this case the first peak is assigned to flowerlike micelles. For the 80/20 mixture, the same two peaks appear, but now the micelles are the predominant particles in the copolymer solution. From Figure 3 we conclude that when the composition of the solvent mixtureis varied, it is possible to go from ordinary to flowerlike micelle solutions through solutions where no micelles could be detected. For solvent mixtures with a high n-octane content, aggregates different from micelles exist in solution independent of whether the other particles in the solution are micelles or free copolymer chains. The temperature influence in the association process of SEBS3 has also been studied for the different n-octane/4-methyl2-pentanone mixtures. As an example, the intensity contribution function versus equivalent hydrodynamic radius is plotted in Figure 4 for two solutions of 5.0 × 10-3 g mL-1 SEBS3 in two n-octane/4-methyl-2-pentanone mixtures, 20/80 and 80/20, and at several temperatures. For the 20/80 solvent mixture a single narrow peak is observed at each temperature. At 75 °C a peak appears at Rh ≈ 14 nm and is attributed to free copolymer chains. At temperatures equal to or lower than 65 °C a peak appears at Rh ≈ 32 nm and is assigned to ordinary micelles
with a core formed by the poly(ethylene/butylene) blocks. These results agree with the static light scattering measurements from which a solution with c ) 5.0 × 10-3 g mL-1 should have a CMT of 73 °C. The DLS results for the 80/20 mixture are quite different. Two peaks appear at any temperature. At temperatures equal to or higher than 55 °C the first peak appears at Rh ≈ 16 nm and is attributed to unassociated copolymer chains. At temperatures equal to or lower than 50 °C the first peak appears at Rh ≈ 41 nm and is attributed to flowerlike micelles whose cores are formed by polystyrene blocks. The second peak is assigned to large aggregates. The importance of these aggregates increases as the temperature decreases until the CMT is reached. Once the micelles are detected in the copolymer solution, as the temperature decreases, the micelle peak increases and the large aggregate peak decreases. Table 2 shows the DLS results for solutions of SEBS3 (c ) 5.0 × 10-3 g mL-1) in n-octane/4-methyl-2-pentanone mixtures of different composition and at several temperatures. Behaviors similar to both mentioned above are shown by solvent mixtures with a high content of either 4-methyl-2-pentanone or n-octane. As the percentages of both solvents become similar, the temperature at which the micelle peak is detected decreases. The temperature dependence of the micellization process of SEBS3 solutions in a n-octane/4-methyl-2-pentanone (80/20) mixture has also been studied as a function of the copolymer concentration. As an example, the intensity contribution functions versus equivalent hydrodynamic radius are plotted in Figure 5 for a concentration of 1.0 × 10-3 g mL-1. The results show two separated peaks at temperatures equal to or higher than 25 °C. The first peak appears at Rh ≈ 14 nm and is attributed to unassociated copolymer chains. The second appears at Rh ≈ 110 nm and is assigned to aggregates. As was mentioned above, this peak gains importance as the temperature decreases. At 20 °C flowerlike micelles are detected in the copolymer
1444 J. Phys. Chem. B, Vol. 104, No. 7, 2000
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Figure 4. Intensity contribution function versus equivalent hydrodynamic radius, Rh, for two solutions of SEBS3 in n-octane/4-methyl-2-pentanone (20/80) and (80/20) at different temperatures. Copolymer concentration ) 5.0 × 10-3 g mL-1 .
TABLE 2: Equivalent Hydrodynamic Radii at Which the Intensity Contribution Function Shows a Peak for Solutions of SEBS3 (c ) 5.0 × 10-3 g mL-1) in Different n-Octane/4-Methyl-2-pentanone Mixtures and at Several Temperaturesa T/°C
20/80
75 70 65 60 55 50 45 40 35 30 25 20
14 (100)
a
37 (100) 36 (100) 36 (100)
28 (100)
30/70 14 (100) 16 (100) 14 (100) 16 (100) 9 (61), 39 (100) 40 (100) 39 (100) 36 (100) 36 (100) 35 (100)
40/60
50/50
60/40
70/30
80/20
14 (100), 150 (12) 14 (100), 190 (10)
10 (100), 142 (7)
13 (100), 135 (18)
10 (100), 125 (16)
18 (100), 128 (66) 18 (100), 173 (65) 20 (100), 188 (63) 42 (44), 178 (100)
14 (100)
18 (100) 16 (100) 13 (100) 10 (100)
14 (100)
12 (100)
14 (100), 228 (14) 13 (100), 180 (21) 15 (100), 210 (18)
14 (100) 14 (100) 13 (100) 14 (100) 12 (100), 44 (94)
12 (100) 14 (100) 13 (100), 65 (12) 12 (100), 85 (10)
15 (100), 222 (23) 20 (100), 220 (38) 21 (100), 244 (48) 23 (100), 246 (67)
12 (100)
10 (100), 92 (26) 12 (100), 172 (18) 14 (100), 182 (30) 22 (82), 213 (100) 48 (67), 245 (100) 53 (47), 238 (100)
53 (33), 226 (100) 44 (100), 186 (89) 39 (100), 227 (63)
The relative height of the peaks is shown in parentheses.
solution and three separated peaks are observed, suggesting that the three kinds of particle coexist in the copolymer solution. The DLS results for solutions of SEBS3 in the mixture n-octane/ 4-methyl-2-pentanone (80/20) at different copolymer concentrations and temperatures are shown in Table 3. According to the DLS results, we have plotted the lowest temperature at which no micelles were detected (unfilled circles) and the highest temperatures at which a micelle peak was observed (filled circles) in Figure 6. A straight line was plotted in a way that separates both kinds of symbol. The thermodynamic data shown in Table 1 were calculated from the straight line. The square symbols correspond to CMTs determined by SLS measurements. There is a small difference between the critical micelle temperatures determined by both experimental methods. We think that, owing to the existence of the anomalous scattering phenomenon observed in this solvent mixture, the determination of CMTs by SLS measurements shows a large experimental error.
Conclusions The triblock copolymer polystyrene-b-poly(ethylene/butylene)-b-polystyrene chains associate in two very different ways when dissolved in selective solvent mixtures of n-octane and 4-methyl-2-pentanone. The solvent mixtures with a high content of 4-methyl-2-pentanone behave as selective solvents of the end copolymer blocks (polystyrene). In these mixtures only a kind of particles has been detected with a hydrodynamic radius close to 37 nm, and they have been considered as ordinary micelles. In a previous paper20 we concluded that these micelles had a high association number and a small size. On the other hand, the solvent mixtures with a high n-octane content behave as selective solvents of the middle copolymer block, poly(ethylene/ butylene). In these mixtures, two kinds of particles have been detected. The smallest particles appeared only at temperatures lower than CMTs and had a hydrodynamic radius close to 47 nm. They were considered as flowerlike micelles with a core formed by polystyrene blocks. The largest aggregates were
Polystyrene-b-Poly(ethylene/butylene)-b-Polystyrene
J. Phys. Chem. B, Vol. 104, No. 7, 2000 1445
TABLE 3: Equivalent Hydrodynamic Radii at Which the Intensity Contribution Function Shows a Peak for Solutions of SEBS3 in n-Octane/4-Methyl-2-pentanone (80/20) at Different Copolymer Concentrations and Temperaturesa T/°C 75 70 65 60 55 50 45 40 35 30 25 20 a
1.0 × 10-3 g mL-1
2.0 × 10-3 g mL-1
3.0 × 10-3 g mL-1
13 (100), 108 (6)
12 (100), 77 (12) 13 (100), 93 (10)
12 (100), 94 (10) 13 (100), 114 (10)
14 (100), 108 (16)
14 (100), 157 (3)
14 (100), 106 (33) 13 (100), 122 (44) 12 (100), 124 (74) 14 (100), 120 (87) 16 (80), 116 (100) 17 (73), 114 (100) 23 (57), 46 (100), 130 (89)
14 (100), 127 (34) 14 (100), 120 (65) 16 (100), 148 (76) 18 (85), 154 (100) 38 (37), 162 (100) 28 (72), 146 (100)
9 (100), 103 (25) 13 (100), 149 (51) 14 (100), 153 (65) 16 (100), 158 (92) 41 (23), 164 (100) 42 (36), 187 (100) 36 (97), 124 (100) 33 (100), 174 (76)
3.7 × 10-3 g mL-1
5.0 × 10-3 g mL-1 13 (100), 135 (18)
16 (100), 138 (60) 20 (83), 177 (100) 46 (43), 194 (100)
18 (100), 128 (66) 18 (100), 173 (65) 20 (100), 188 (63) 42 (40), 178 (100) 53 (33), 226 (100)
47 (46), 188 (100) 37 (100), 176 (73) 36 (100), 156 (98)
44 (100), 186 (89) 39 (100), 227 (63)
The relative height of the peaks is shown in brackets.
Figure 6. Plot of the logarithm of the concentration as a function of the reciprocal of the absolute temperature for solutions of SEBS3 in the 80/20 n-octane/4-methyl-2-pentanone mixture: (b) the lowest temperatures at which no micelles were detected by DLS; (O) the highest temperatures at which micelles were detected by DLS; (0) CMTs determined by SLS.
Figure 5. Intensity contribution function versus equivalent hydrodynamic radius, Rh, for a solution of SEBS3 in n-octane/4-methyl-2pentanone (80/20) at 25 and 20 °C. Copolymer concentration ) 1.0 × 10-3 g mL-1 .
detected at every studied temperature, and their contribution to the scattered intensity increased as temperature decreased until the critical micelle temperature was reached. Once flowerlike micelles appeared in the solution, the larger aggregate contribution decreased, increasing that of the micelle. The larger aggregates exist in a smaller proportion, and their size, Rh ≈ 200 nm, is much larger than that of ordinary micelles. In a previous paper20 we concluded, according to what Raspaud et al.4,15 found, that these aggregates have a loose structure. In the mixtures with similar content for both solvents we failed to detect any kind of micelles under the experimental conditions used. We found the same phenomenon for a copolymer polystyrene-b-poly(ethylene/propylene) in n-dodecane/ 1,4-dioxane mixtures.36 The static light scattering measurements suggest clearly that the micellization process is enthalpy-driven, yielding closed
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