J. Phys. Chem. 1995,99, 3723-3731
3723
Solubilization of Homopolymers by Block Copolymer Micelles in Dilute Solutions: Laser Light Scattering and Viscosity Studies on Micellar Solutions Jose R. Quintana, Ramiro A. Salazar, and Issa Katime* Grupo de Nuevos Materiales, Departamento de Quimica Fisica, Facultad de Ciencias, Campus de Leioa, Universidad del Pais Vasco, Apartado 644, 48080 Bilbao, Spain Received: November 9, 1993; In Final Form: May 13, I994@
Light scattering and viscosity measurements on micellar solutions of block copolymers were carried out in the presence of polyisobutylene. Micelle size and molar mass were determined for micelles of polystyreneblock-poly(ethylene/propylene) in 5-methyl-2-hexanone and 2-pentanone and for micelles of polystyreneblock-poly(ethylenehuty1ene)-block-polystyrene in 4-methyl-2-pentanone in the presence of different percentages of polyisobutylene. As the percentage of homopolymer solubilized in the micellar increases, the size and the aggregation number of the micelles become larger. The temperature effect on micellar solutions was also studied. The presence of homopolymer in the micelle solutions causes the so-called anomalous behavior. This becomes more intense as the molar mass or the percentage of the homopolymer increases.
Introduction When a block copolymer is dissolved in a selective solvent, i.e., a liquid that is a nonsolvent for one of the copolymer blocks but a good solvent for the other, the copolymer may associate reversibly to form micelles in analogy to low molar mass surfactants. The micelles consist of a compact core of the nonsoluble blocks surrounded by a highly swollen shell of the soluble At concentrations above the critical micelle concentration, CMC, all copolymer chains added to the solution aggregate to form micelles. The critical micelle concentration is defined as the concentration at which the experimental method in use can just detect the presence of micelles in the solution when the concentration is increased at constant tem~erature.~.~ Usually, over a wide concentration and temperature range, block copolymer micelles in solution form spherical micelles which have a narrow size distribution and are stable for a long period of However, several studies have also reported the existence of unusually large particles which are unstable. They are usually detected in narrow ranges of temperature and concentration.8-'0 In analogy to low molar mass surfactants, block copolymer micelles can solubilize compounds that otherwise would precipitate in the pure solvent. Solubilization is usually defined as the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic compound or component.'' The phenomenon of solubilization has been extensively studied in aqueous solutions of conventional low molar mass surfactants.'*J3 Solutions of these compounds above the critical micelle concentrationdissolve a certain amount of hydrocarbons which are insoluble in water. The solubilizate molecules are located in the micelle core where the hydrocarbon tails of the surfactant are miscible with the solubilizate molecules. At the same time surfactant micelles increase somewhat their aggregation number, Le., the number of surfactant molecules which constitute the micelle.
* Address for correspondence: Avda, Basagoiti, 8,1 C, 48990 Algorta, Getxo, Vizcaya, Spain. @
Abstract published in Advance ACS Abstracts, February 15, 1995.
Several investigations14-'' have shown that the phenomenon of solubilization can also be observed with block copolymers. Copolymer micelles in a solvent selective for one block can solubilize homopolymers miscibles with the other block, otherwise insoluble. True solubilization of homopolymer occurs only when the molar mass of the homopolymer is lower than the miscible block of the copolymerI6and, in addition, there is a saturation concentration above which homopolymer remains in~oluble.'~ In our laboratory we have undertaken a study of the ability of block copolymer micelles to enhance the solubility of homopolymers in organic liquids. In previous investigations we examined the solubility of different polyisobutylenes in ketone solutions of polystyrene-block-poly(ethylendpropylene)'8 and polystyrene-block-poly(ethylenehutylene)-block-polystyrene copolymer^.'^ In the present paper we report the light scattering and viscosity investigations on block copolymer micelles which have polyisobutylene solubilized in theirs cores. Micellar size and aggregation number were determined for micelles of polystyreneblock-poly(ethylene/propylene) in 5-methyl-2-hexanone and 2-pentanone and for micelles of polystyrene-block-poly(ethy1end butylene)-block-polystyrene in 4-methyl-2-pentanone. The temperature effect on micellar solutions affecting monomer/micelle equilibrium was also studied using light scattering.
Experimental Section Polystyrene-block-poly(ethylene/propylene), SEP, and polystyrene-block-poly(ethylene/butylene)-block-polystyrene,SEBS, are commercial products kindly provided by Shell Espaiia, S.A. Polyisobutylenes, PIB, are also commercial products of BP Chemicals and BASF. The structural characteristics of polyisobutylenes and block copolymers are shown in Table 1. The weight average molar mass, M,, of both copolymers were determined by laser light scattering in tetrahydrofuraneand chloroform at 25 OC, whereas those corresponding to the polyisobutylene samples only in tetrahydrofuran. For each copolymer the difference of molar mass in both solvents was smaller than the experimental error. As both solvents have different refractive index, these copolymers can be considered homogeneous in chemical composition. Polydispersity, MwIMn,was measured by size exclusion chro-
0022-3654/95/2099-3723$09.00/0 0 1995 American Chemical Society
Quintana et al.
3724 J. Phys. Chem., Vol. 99,No. 11, 1995
TABLE 1: Characteristics of Homopolymers and Block CoDolvmers ~~~
2= [q]- k2[T7I2c C
~~
sample
M , (gmol-I)
MJMn
PIB 1 PIB2 PIB3 PIB4 SEP SEBS
10 OOO 14 000 20 700 35 OOO 105 OOO 260 000
1.50 3.00 1.76 1.06 1.08
(4)
wt% PS
0 0 0 0
where c is the polymer concentration, qspthe specific viscosity,
vr the viscosity ratio, [r]the limiting viscosity number, and k, and k2 the Huggins and Kraemer coefficients, respectively.
35 30
Results and Discussion
matography at 25 "C using chloroform as solvent and a standard polystyrene calibration. Polystyrene contents of both block copolymers were determined by W spectroscopy measurements of tetrahydrofuran solutions. Solvents (analytical purity grade) were used without further purification. Solutions were prepared by dissolving the polymers in the ketone at high temperatures (70-100 "C) in sealed flasks. In order to clarify polymer solutions for laser light scattering measurements, they were filtered at room temperature directly into the scattering cells, which were then sealed. Laser light scattering measurements were made using a modified FICA 42000 equipped with a He-Ne laser which emits vertically polarized light at 632.8 nm with a power of 5 mW. To obtain classical Zimm plots, light scattering measurements were taken at ten angles between 37.5 and 150" for each solution at 25 "C. The photogoniodiffusometer was calibrated with pure benzene taking the Rayleigh ratio at 25 "C as 12.55 x cm-'.20 The light scattered by a dilute polymer solution may be expressed as follows*'
where c is the polymer concentration, K an optical constant, A/?(@) the difference between the Rayleigh ratio of the solution and that of the pure solvent, Mu'the weight average molar mass, R i the mean square radius of gyration, no the solvent refractive index, A, the wavelength in vacuum, and A2 the second virial coefficient. The refractive index increments, dnldc, were measured at 632.8 nm using a Brice-Phoenix differential refractometer, previously calibrated with solutions of highly purified NaC1, using a He-Ne laser with a power of 1 mW as light source. The viscosity measurements were made in a Lauda automatic Ubbelohde viscometer model Viscoboy 2, which was placed in a thermostatically controlled bath with a precision of f O . O 1 "C. The viscometer was calibrated using several standard solvents. Kinetic energy corrections were carried out by means of the equation
where e is the density of the liquid, t the efflux time, and A and B are the calibration constants (A = 1.016 x cm2*s-2 and B = 4.3 x cm2). The viscosity measurements were carried out within the polymer concentration range 2 x I c 5 6 x g - ~ m - ~The . basic solution was diluted directly in the viscometer. The data were evaluated according to Huggins and Kraemer equations22
(3)
Structural Parameters. In order to know the influence of the polyisobutylene presence in the micelle core on the structural parameters on the micelles, light scattering measurements of pure copolymers and with different percentages of polyisobutylene were carried out. In all cases the homopolymer concentration was lower than the saturation homopolymer c ~ n c e n t r a t i o n . ' ~The ~ ' ~ light scattering study of copolymer micelles with polyisobutylene was limited to dilute solutions (copolymer concentration 5 5 x to avoid the effect of multiple scattering. The polyisobutylene samples used in this study were chosen in order that their molar masses were large enough to avoid that a significant fraction was taken up into ~ o l u t i o n . ' ~ J ~ Therefore the interpretation of the results assumes that all the solubilized polyisobutylene is placed in the micelle cores. The light scattering technique was employed in the same manner as it was for pure micelles. The refractive index increments were determined for every systems containing micelles with different polyisobutylene percentages in their cores. Photon correlation spectroscopy measurements of polystyreneblock-poly(ethylene/propylene)in 5-methyl-2-he~anone~~ and of polystyrene-block-poly(ethylene/butylene)-bl~k-polys~rene in 4-meth~l-2-pentanone~~ showed that the micelles formed in these systems have narrow size distributions. Taking into account that the copolymer used in this study are homogeneous in chemical composition and in molar mass, we have no reason to believe that the micelles containing polyisobutylene in their cores do not have a narrow size distribution. All measurements carried out at 25 "C produced standard Zimm plots similar to the one shown in Figure 1. The concentration and observation angle dependencies of KclAR are linear as expected for concentrations very much higher than the critical micelle c ~ n c e n t r a t i o n .The ~ ~ weight average molar mass, Mu', determined from the double extrapolation to nil angle and concentration can be considered as the true molar mass of the micelles, since the copolymer samples had a high degree of chemical heterogeneity and the measurements were made under experimental conditions which overwhelmingly favored micelle formation. The values of the second virial coefficient, A2, were determined from the concentration dependence of KclAR at nil angle, and its values range between 2 x and 8 x lov6 m ~ l * c m ~ * gThe - ~ . radii of gyration, RG, of the micelles were determined from the angular dependence of KcIAR at nil concentration. It should be pointed out that the RG values so obtained are only apparent and larger than the true ones due to the core-shell structure of the micelles. The polystyrene shell has a larger refractive index increment in the ketones than the polyolefine core. Weight average molar mass and radius of gyration for pure copolymers and mixtures copolymerhomopolymer at different ratios are shown in Table 2. As expected, an increment in the homopolymer concentration causes an increment in the molar mass of the micelles. However, this increment does not correspond entirely to the amount of polyisobutylene which is placed in the micelle core. On substracting the polyisobutylene amount, it is observed that the molar mass of the micelle itself
Viscosity Studies on Micellar Solutions
J. Phys. Chem., Vol. 99, No. 11, 1995 3725
0
1
1
I
1
1
1
2
4
6
8
10
12
14
(~/g.crn-~+ 100-'.sen2(e/2)). 1o3 Figure 1. Zimm plot for micellar solutions of SEBS120 wt% PIB2 in 4-methyl-2-pentanone at 25 "C.
2000
N 1500
1000
500
0
5
10
15
20
25
% PIB Figure 2. Dependences of aggregation number of SEPPIB in 2-pentanone on the PIB percentage by mass of copolymer (0,PIB1; 0, PIBZ).
also increases, i.e., the aggregation number of the micelle increases with the homopolymer percentage in the micelle. The aggregation number of the micelles as a function of the percentage of polyisobutylene solubilized in the micelle is shown in Figures 2 and 3. As the percentage of the solubilized homopolymer increases, the aggregation number becomes higher and this influence becomes larger with the homopolymer percentage. This behavior does not depend on the block copolymer or on the solvent. The increment in the aggregation number could be due to the fact that in order to solubilize the larger possible amount of homopolymer, the micelle tends to increase its volume. At the same time the micelles decrease their number since the critical micelle concentration is not influenced by the presence of homopolymer.I7 The behavior that the copolymers show is similar to that of conventional surfactants. Thus, an increment in the aggregation
number of their micelles is observed when they solubilize any substance in the micelle cores.26 The molar mass of the polyisobutylene has also a strong influence on the aggregation number of the micelles (Table 2 and Figures 2 and 3). The larger the homopolymer molar mass is, the stronger its influence on the aggregation number is. This fact suggests that as the homopolymer chain become larger, the more the micelle increases its volume in order to arrange the polyisobutylene chains into the micelle core. Although the radius of gyration is apparent, it can give us an idea of the micelle size. In any case the obtained values are remarkably lower than expected for the weight average molar masses. This would suggest that the micelles formed by pure copolymer or containing homopolymer inside have a compact and spherical structure as has been verified by TEMz7928and neutron diffractionz9 for micellar solutions of pure block copolymers. In order to confirm this idea, the relationship radius of gyration-weight average molar mass was analyzed. Logarithm of radius of gyration against logarithm of weight average molar mass for every micellar system studied is plotted in Figure 4. The plot leads to a straight line, and this relationship can be expressed by the equation
R&A
= 1.03 ( ~ + 4 g ~ ~ i - ~ ) ~ . ~( 5~)
The RL vs Mw relationship does not depend on the homopolymer percentage existing in the micelle. This suggests that the introduction of solubilized homopolymer in the micelle does not alter the original shape. Considering a sphere of radius R and mass M,the radius of gyration of this basic particle can be related to those magnitudes by2'
The experimental exponent of RL vs Mw relationship (0.33) agrees with the one corresponding to a sphere (1/3). This suggests that both block copolymers form spherical micelles,
Quintana et al.
J. Phys. Chem., Vol. 99, No. 11, 1995 160 160
N
140
I
120
100
80
60
'0
1
10
30
20
40
% PIB Figure 3. Dependences of aggregation number of SEBSRIB in 4-methyl-2-pentanone on the PIB percentage by mass of copolymer (0,PIB2; 0, PIB3).
TABLE 2: Limiting Viscosity Number, [VI, Radius of Gyration, RL,Weight Average Molar Mass, M,, and Aggregation Number, N,for DiPferent Micellar Systems of Block Copolymers and Their Mixtures with Several Polyisobutylenes at 25 "C
[VI PIB
% PIB
(cm3.g-1)
R: (A)
Mw x 10-6 (gmol-')
0 4 10 20 2,8 4
16.5 16.2 15.9 15.3 16.3 16.2
338 340 357 395 340 350
45 47 57 75 46 55
PIB 1 PIB 1 PIB 1 PIB2 PIB2 PIB2 SEBS/4-methyl-2pentanone
4 10 20 4 10 15
PIB2 PIB2 PIB2 PIB2 PIB 3 PIB3
4 10 20 30 4 10
0
0
2.1
N
2.6
428 430 493 595 426 504
2.5
SEP/S-methyl-2hexanone PIB3 PIB 3 PIB3 PIB4 PIB4 SEP/2-pentanone
2.8
14.5 13.6 13.1 12.1 13.5 13.0 12.4
388 400 420 500 414 456 530
70 80 100 160 89 140 220
666 732 865 1270 815 1212 1822
13.2 12.4 12.1 11.8 11.6 12.5 12.2
239 249 270 285 330 282 311
21 21 26 30 45 26 40
81 78 91 96 133 96 140
and the shape of these micelles does not change when polyisobutylene is solubilized in their cores though the aggregation number increases. As it can be seen in Figure 5, the radius of gyration increases as the homopolymer percentage becomes larger, suggesting an increment in the micellar dimensions. The increment becomes larger with the molar mass of the homopolymer solubilized, following the same behavior as the aggregation number. In order to know better the variations in the micelle dimensions, viscosity measurements of the same polymer systems were carried out. The experimental dependencies of
2.4
'
2.3 1.2
I
1
I
I
I
1
1.4
7.6
7.8
8
8.2
8.4
8.6
log( Mw/g.mol- ') Figure 4. Logarithm of radius of gyration against logarithm of weight average molar mass for the micellar systems of block copolymers and mixtures block copolymer/polyisobutylene: SEP/PIB3/5-methyl-2-hexanone (O),SEP/PIB4/5-methyl-2-hexanone (0),SEPPIB 1/2-pentanone (m), SEPPIB2I2-pentanone(O), SEBSPIBU4-methyl-2-pentanone (A), and SEBSRIB2/4-methyl-2-pentanone (A).
the reduced viscosity, qs&, and of the logarithm viscosity number, In qdc, on the concentration were always linear within the concentration range employed (Figure 6). Extrapolations to nil concentration according to Huggins and Krwmer equations lead to the same values of the limiting viscosity number, [ q ] . They are shown in Table 2. Huggins coefficients were lower than 0.5 in every micellar system. The limiting viscosity number is related to the particle shape and density by the equation30
[q]=
; =
+
(7)
where Y is the shape factor, 5 is the degree of solvation (solvated solvent weight per g of polymer), and e*, e,,, and Qs are the densities of the particle, polymer, and solvent, respectively.For a spherical particle Y is equal to 2.5.
J. Phys. Chem., Vol. 99, No. 11, 1995 3727
Viscosity Studies on Micellar Solutions
400
350
300
250
0
5
10
15
20 Oo /
25
30
35
PIB
Figure 5. Dependences of radius of gyration and radius hydrodynamic of SEBSPIB in 4-methyl-2-pentanone on the PIB percentage by mass of copolymer: RL,PIB2 (0)and PIB3 (0); R,, PIB2 (U)and PIB3 (0).
16.6 16.4 16.2 16.0 15.8 15.6 15.4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
c.1 ~ * / g . c r n - ~ Figure 6. Concentration dependencies of qSdc(A)and In qJc (0)for SEP/10 wt% PIB3 in 5-methyl-2-hexanone at 25 "C.
7.
degree of solvation depends on the block copolymedsolvent system (Figure 7). If the model of the hydrodynamically equivalent sphere is applied to the spherical micelles, the limiting viscosity number can be expressed by Einstein's equation
The degree of solvation becomes smaller as the homopolymer percentage increases. This fact seems logical since the homopolymer molecules are placed in the micelle core, and, therefore, this part of the micelle shows a larger increment in its mass. As the core is less swollen than the shell, a larger increment of the core leads to a lower degree of solvation of the micelle. Contrary to the aggregation number of the micelle, the degree of solvation does not depend on the molar mass of the polyisobutylene. This fact can be explained taking into account that the degree of solvation depends mainly on the interactions polymer segmentlsolvent. The same argument explains that the
where M and R, are the molar mass and the hydrodynamic radius of the micelles and N A is Avogadro's number. The hydrodynamic radius of the micelles was calculated by means of eq 8, and its values are shown in Table 3. As expected, the hydrodynamic radius of the micelles increases as the homopolymer percentage in the micelle becomes larger, in agreement to the behavior that the apparent radius of gyration
The degree of solvation has been calculated for every micellar system applying eq 7. Their values are shown in Table 3. The degree of solvation is plotted as a function of the polyisobutylene percentage for the three copolymer/solvent systems in Figure
Quintana et al.
3728 J. Phys. Chem., Vol. 99, No. 11, 1995 TABLE 3: Degree of Solvation, 6, Hydrodynamic Radius, R,, Radius of the Core, R,,and Thickness of the Shell, Ls, for Different Micellar Systems of Block Copolymers and Their Mixtures with Several Polyisobutylenes at 25 "C ~~
SEP/S-methyl-2hexanone PIB3 PIB3 PIB3 PIB4 PIB4 SEP/2-pentanone PIB 1 PIB 1 PIB 1 PIB2 PIB2 PIB2 SEBS/4-methyl-2pentanone PIB2 PIB 2 PIB2 PIB2 PIB3 PIB3
0 4.0 10.0 20.0 2.8 4.0
4.48 4.31 4.20 4.09 4.43 4.40
486 494 521 566 491 514
233 238 256 285 236 248
250 255 264 281 253 266
0 4.0 10.0 20.0 4.0 10.0 15.0
3.83 3.54 3.38 3.05 3.51 3.35 3.15
543 557 592 682 575 660 156
270 285 309 366 295 346 405
273 273 282 313 280 314 351
0
3.31 3.16 3.06 2.90 2.89 3.19 3.09
348 348 368 382 435 372 426
186 187 202 213 246 200 233
162 162 166 169 189 171 193
4.0 10.0 20.0 30.0 4.0 10.0
shows (Figure 5). Orlanli et al.I5 found a similar behavior of the hydrodynamic radius of micelles formed by polystyrene-bpolybutadiene copolymer when polystyrene or polybutadiene are solubilized by the micelles in heptane or dimethyl formamide, respectively. Micellar shape can also be deduced from the relationship radius of gyration vs hydrodynamic radius. The RHIRGratio is about 0.77 for linear coils3' and 1.29 for hydrodynamic hard spheres. For oblate ellipsoids and rods the ratio R H I R also ~ shows lower values than for hydrodynamic hard spheres.32The RJR; values of the different systems studied are shown in Table 3. The ratio values range between 1.32 and 1.46 indicating that the micelles keep its shape without being affected by the homopolymer solubilized. Even considering the apparent character of radius of gyration, R,fRk values indicate that the assumption of considering the micelles as hydrodynamic spheres is acceptable. To calculate the radius of the core, we assumed that the polyolefin blocks and polyisobutylene chains are located in the core in absence of solvent molecules. Therefore, the density of the bulk polymers was considered. If the core has a spherical shape and its volume can be computed on the basis of the assumption of volume additivity, the core radius is given by
R c = [ 3NMo (--t%PO 4nNA100
@PO
%PIB)]"' @PIB
(9)
where N is the aggregation number, M,,is the weight average molar mass of the copolymer, %PO is the polyolefin percentage in the block copolymer, %PIB is the polyisobutylene percentage by mass of copolymer, and @PO and @PIB (= 0.91 gcmP3) are the density of the polyolefin block and polyisobutylene, respectively. The thickness of the shell, L,,was evaluated by subtracting the core radius from the hydrodynamic radius of the micelles obtained by viscosimetry. Values of both dimensions are shown in Table 3. As expected, the radius of the micelle core increases as the PIB percentage becomes higher, since it is assumed that the
homopolymer is solubilized in the micelle core because it is immiscible with polystyrene. Following the same reasoning, it was expected that the shell thickness would keep approximately constant. However it shows the same behavior as the core radius, i.e., the shell thickness increases with the PIB percentage. There may be two possible explanations for this behavior. Contrary to what was assumed, the core could be solvated showing a lower density and therefore a larger radius. In addition the number of solvent molecules in the core could increase with the PIB percentage, and therefore the core would show a larger increment in its radius. Thus, the shell thickness would keep almost constant. The other possible explanation would be that the increment in the aggregation number, which is caused by the increment in the PIB percentage, leads to an arrangement of the copolymer chains such that the polystyrene blocks become more extended into the solvent medium. In conclusion, the increments in the micellar mass and size are due not only to the addition of homopolymer but also to an increase in the aggregation number of the micelle. In other words, there is a reorganization of the micelles as the homopolymer is solubilized in the micelle core. This behavior was shown by diblock and triblock copolymers. Anomalous Behavior. The presence of a homopolymer which is miscible with the copolymer block that forms the micelle core and immiscible with the block which forms the shell influences also the process of micelle formation. The presence of homopolymer originates a phenomenon which has been called anomalous behavior and has been extensively reported for the case of solutions of pure block copolymer^.^^-^^ This phenomenon is characterized by the existence of a peak in the variation of the scattering light intensity and/or of the dissymmetry, i.e., the ratio of the scattering intensities at 45 and 135", with the temperature. As previous studies" shown, this peak can be small when the temperature increases. However, it becomes larger when the temperature decreases from a high value. This behavior is reproducible and does not depend on the number of temperature cycles. The intensities scattered at different angles for a 5-methyl2-hexanone solution of copolymer SEP at a concentration of 5 x g ~ m at - ~several temperatures are plotted in Figure 8 as log(Kc/AR) against sin2(8/2). On lowering the temperature, the intensity at every angle increases due to the formation of micelles which increase in number as the temperature decreases. The dissymmetry factor, Z, Le., the ratio of the scattering intensities at 45 and 135", shows values very close to unity at any temperature, as it is found in general for a micelle solution of a pure copolymer. Light scattering data for a 5-methyl-2-hexanone solution of SEP in the presence of 4 wt% (on the mass of copolymer) of PIB4 are plotted in Figure 9. At high temperatures (T 1 68 "C) both polymers are dissolved. However as the temperature decreases and the process of micelle formation begins, a high scattering intensity is detected at low angles suggesting the formation of particles much bigger than the micelles themselves. At lower temperatures (T = 25 "C) the number of formed micelles becomes so large that it originates a high level of scattering intensity at any observation angle. This behavior of the scattering intensity when lowering the temperature leads to a dissymmetry factor that goes through a maximum (Figure 10) which coincides with the onset of micellization, as it has already been described for solutions of pure copolymer in several paper^.^^-^* The influence of the homopolymer presence on this anomalous behavior depends on the molar mass of the homopolymer
Viscosity Studies on Micellar Solutions
4.0
J. Phys. Chem., Vol. 99, No. 11, 1995 3729
t-
u
2.5
0
'
0
I
1
1
I
1
1
5
10
15
20
25
30
o /'
35
PIB
Figure 7. Degree of solvation as a function of PIB percentage by mass of copolymer for different micellar systems: SEP/S-methyl-2-hexanone, PIB3 (A) and PIB4 (0);SEP/2-pentanone, PIBl (0)and PIB2 (A);SEBS/4-methyl-2-pentanone, PIB2 (0) and PIB3 (m). -5.0
A a
e U d -5.6 \
s
t
I
A
A
1
1
1
p
y
y
A
1
1
A
A
-
"y
-5.2
-6.0
-
-6.7
n g n n -7.5
I
1
1
1
1
1
0
0.2
0.4
0.6
0.8
1
-7.5
0
I
1
1
I
0.2
0.4
0.6
0.8
1
Figure 8. Log(Kc/AR) as a function of sin2(O/2)for a solution of SEP g ~ m at - ~ in 5-methyl-2-hexanone with a concentration of 5 x different temperatures: 72 (0),68 (m), 65 (A), 63 (O), and 60 (U), and 25 "C (A).
Figure 9. Log(Kc/hR) as a function of sinz(O/2)for a 5-methyl-2g ~ m - in ~ )the hexanone solution of SEP (concentration: 5 x presence of 4 wt% (on the mass of copolymer) of PIB4 and at different 68 (m), 64 (A), 63.5 (O), and 25 OC (0). temperatures: 72 (0).
and on its percentage in the solution. Thus, a micelle solution of copolymer SEP in the presence of 4 wt% of PIB3 does not show any anomalous behavior, and the dissymmmetry factor is kept close to unity at any temperature. However a solution with 10 wt% of PIB3 shows a large increment in the intensity of light scattered to low angles as temperature lowers from 64 to 60 O C . The dissymmetry factor becomes again close to unity at 52.5 "C and keeps these values at lower temperatures. As the PIB3 percentage increases to 20%, the behavior becomes more intense. On decreasing the homopolymer molar mass, micelle solutions of SEP in the presence of PIB2 show only a small anomalous behavior at homopolymer percentage so high as 90 wt% on the mass of copolymer. The influence of the polyisobutylene percentage on the intensity of the anomalous behavior is shown in Figure 11. The dissymmetry factor as a function of the temperature is plotted
for micelle solutions of copolymer SEP with different percentages of polyisobutylene PIB3. On decreasing or increasing the temperature, the dissymmetry factor goes through a maximum at the onset of the micelle formation. The value of this maximum depends on the polyisobutylene percentage, increasing as the percentage becomes higher. The anomalous behavior would be due to the presence of macrostructures larger than the micelles themselves at the onset of the micellization process. The increment in the molar mass of the homopolymer or in its percentage in the solution would cause an increment in the dimensions of these macrostructures and/or its number. This suggests that the homopolymer is the main component of these macrostructures, and they are stabilized by block copolymer molecules. We think that these macrostructures are only stable in a narrow temperature range, and they disappear when lowering the temperature owing to the solubilization of the homopolymer
Quintana et al.
3730 . I . Phys. Chem., Vol. 99, No. 11, 1995
N
12
-5.0
10
-5.5
-6.0
8 6
W
t3)
0
4
-6.5
-7.0
2
-7*5
0 1 20
I
1
1
1
1
1
30
40
50
60
70
80
T
tion: 5 x g - ~ m - ~and ) in the presence of 4 wt% (on the mass of copolymer) of PIB4.
N
I
15
-
10
-
v
I
1
I
I
1
0.2
0.4
0.6
0.8
1
/
0
/OC
Figure 10. Variation of the dissymmetry factor as a function of temperature for a solution of SEP in 5-methyl-2-hexanone (concentra-
20
-8.0
Figure 12. Log(Kc/hR) as a function of sinz(8/2) for a 5-methyl-2hexanone solution of SEP (concentration: 5 x g - ~ m - ~in) the presence of 20 wt% (on the mass of copolymer) of PIB3 and at different temperatures: 75 (0).71 (m), 68 (A), 66 (O),63.5 (O), 60 (A),and 25 'C (0).
1
*B
25
5 -
20
30
40
50
60
70
80
T
/OC Figure 11. Variation of the dissymmetry factor as a function of temperature for solutions of SEP in 5-methyl-2-hexanone (concentration: 5 x g - ~ m - and ~ ) in the presence of different percentages of PIB3: 4 (0),10 (0),and 20 (A) wt% on the mass of copolymer.
chains. This process becomes evident for a 5-methyl-2hexanone solution of SEP in the presence of 20 wt% of PIB3 (Figure 12). Whereas at high temperatures (75 "C) both polymers are dissolved, as the temperature decreases a high scattering intensity is detected at low angles suggesting the formation of macrostructures larger than the micelles themselves. At lower temperatures (60 "C) the micelle formation becomes so extensive that the intensity of light scattered at high angles is also very important. However, in spite of the larger number of micelles existing in the solution, the light intensity at low angles has decreased. This fact suggests that the macrostructures disappear when the micelle number is large enough to solubilize all the homopolymer chains existing in the solution. This behavior is clearly shown in Figure 13 where the intensity of light scattered at 37.5" as a function of temperature is plotted. The intensity goes through a maximum which coincides with the onset of micellization. This feature of the anomalous behavior of micellization appears more rarely than the peak in the dissymmetry factor vs temperature curve. However it has been already described in the literature for some micellar ~ystems.3~~35
20
30
40
50
60
70
80
T 1°C Figure 13. Variation of the scattering intensity at 37.5' as a function of temperature for solutions of SEP in 5-methyl-2-hexanone (concentration: 5 x g - ~ m - and ~ ) in the presence of 20 wt% (on the mass of copolymer) of PIB3.
Acknowledgment. The authors thank to the Vicerrectorado de Investigaci6n de la Universidad del Pais Vasco (VPV 039.310-EC099/92),CYTED and the Comisi6n Interministerial de Ciencia y Tecnologia (CICYT: MAT 464/92-C02) its financial support. R. Salazar wishes to thank the Instituto de Cooperacih Iberoamericana for the award of a research fellowship. References and Notes (1) Tuzar, Z.; Kratochvfl, P. Adv. Colloid Interface Sci. 1976, 6 , 201. (2) Brown, R. A.; Masters, A. J.; Price, C.; Yuan, X. F. In Comprehensive Polymer Science; Booth, C . , Price, C., Eds.; Pergamon Press: Oxford, 1989; Chapter 6, Vol. 2. (3) Quintana, J. R.; Villacampa, M.; Katime, I. Rev. Iberoarner. Polfm. 1992, I , 5. (4)Quintana, J. R.; Villacampa, M.; Muiioz, M.; Andrio, A.; Katime, I. Macromolecules 1992, 25, 3125. (5) Quintana, J. R.;Villacampa, M.; Katime, I. Macromolecules 1993, 26, 601. (6) Quintana, J. R.; Villacampa, M.; Katime, I. Macromolecules 1993, 26, 606.
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