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Langmuir 1997, 13, 2578-2581
Multiple Morphologies of Crew-Cut Aggregates of Polybutadiene-b-poly(acrylic acid) Diblocks with Low Tg Cores Yisong Yu, Lifeng Zhang, and Adi Eisenberg* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3W 2K6 Received October 31, 1996. In Final Form: February 17, 1997
Introduction When diblock copolymers are dissolved in a solvent that is selective for one of the blocks, colloidal aggregates or micelles are formed as a result of the self-assembly of the insoluble block.1-3 The aggregates have compact cores of the insoluble blocks, which are surrounded by a corona composed of the soluble blocks. If one of the blocks is hydrophilic and the other block is hydrophobic, then depending on solvent and composition, the block copolymer can form either regular micelle-like aggregates (in aqueous media), which have a hydrophobic core and a hydrophilic corona,4-9 or reverse micelle-like aggregates (in nonaquous media), which have a hydrophilic core and a hydrophobic corona.10-12 To date, most of the effort has been devoted to block copolymer aggregates in which the dimensions of the corona are large relative to those of the cores. Aggregates of this type are generally referred to as star micelles. Micelles of another kind, namely, “crew-cut” micelles, have received attention only recently.13-20 Crewcut micelles consist of relatively large cores and relatively small corona chains. The word crew-cut was proposed by Halperin et al.,21 based on an earlier theoretical work of de Gennes.22 Recently, Zhang and Eisenberg16,17 reported that block copolymer aggregates of six different stable morphologies, i.e., spheres, rods, lamellae, and vesicles, as well as simple * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Price, C. In Developments in block copolymers; Goodman, I., Ed.; Applied Science Publishers: London, 1982; Vol. 1, p 39. (2) Selb, J.; Gallot, Y. In Developments in block copolymers; Goodman, I., Ed.; Applied Science Publishers: London, 1985; Vol. 2, p 27. (3) Tuzar, Z.; Kratochvil, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15, p 1. (4) Xu, R.; Winnik, M. A. Macromolecules 1991, 24, 87. (5) Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu, R.; Winnik, M. A. Macromolecules 1991, 24,1033. (6) Prochazka, K.; Kiserow, D.; Ramireddy, C.; Tuzar, Z.; Munk, P.; Webber, S. E. Macromolecules 1992, 25, 454. (7) Tuzar, Z.; Kratochvil, P.; Prochazka, K.; Munk, P. Collect. Czech. Chem. Commun. 1993, 58, 2362. (8) Astafieva, I.; Zhong, Z. F.; Eisenberg, A. Macromolecules 1993, 26, 7339. (9) Qin, A.; Tian, M.; Ramireddy, C.; Webber, S. E.; Munk, P. Macromolecules 1994, 27, 120. (10) Desjardins, A.; Eisenberg, A. Macromolecules 1991, 24, 5779. (11) Desjardins, A.; Van de Ven, T. G. M.; Eisenberg, A. Macromolecules 1992, 25, 2412. (12) Gao, Z.; Desjardins, A.; Eisenberg, A. Macromolecules 1992, 25, 1300. (13) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923. (14) Honda, C.; Sakaki, K.; Nose, T. Polymer 1994, 35, 5309. (15) Zhang, L.; Barlow, R. J.; Eisenberg, A. Macromolecules. 1995, 28, 6055. (16) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (17) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (18) Zhang, L.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777. (19) Yu, K.; Eisenberg, A. Macromolecules 1996, 29, 6359. (20) Yu, K.; Zhang, L.; Eisenberg, A. Langmuir 1996, 12, 5980. (21) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31. (22) de Gennes, P. G. In Solid State Physics; Liebert, L., Ed.; Academic Press: New York, 1978; Supplement 14, p 1.
S0743-7463(96)01058-X CCC: $14.00
reverse micelle-like aggregates, and a new morphology consisting of large spheres with hydrophilic surfaces filled with reverse micelle-like assemblies can be prepared in dilute solution. The aggregates were made from polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblocks of an identical PS block length and relatively short PAA blocks of various lengths. The aggregates of the various morphologies were prepared by dissolving the copolymers in N,N-dimethylformamide (DMF) and then adding water to induce microphase separation. Finally, aqueous solutions of aggregates were obtained by dialyzing the resulting solution against water to remove DMF. Aggregates of various morphologies also were observed from polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymers. In addition to those described above, morphologies such as tubular structures, porous spheres, “pincushions”, and a range of other geometries were also obtained.19,20 This shows that completely nonionic copolymers can also give aggregates of multiple morphologies. In the aggregates of multiple morphologies mentioned above, the core-forming block consists of polystyrene, a material with a glass transition temperature (Tg) of ca. 100 °C. Thus, when the aggregates are isolated in water, the glass transition is very much higher than room temperature. One question that arises, therefore, is whether preservation of the morphology under various conditions, e.g., while electron microscopy is being performed on the materials, is due to the fact that the glass transition temperature is so high or whether other forces preserve the morphology, with the glass transition temperature being only one of the contributing factors. To answer this question, we synthesized a range of block copolymers consisting of butadiene (BD) and acrylic acid (AA). Butadiene was chosen because it can be prepared by anionic polymerization with a wide range of controlled 1,4- and 1,2-microstructures. These various structures have glass transition temperatures that range from -90 to -8 °C. This paper reports that aggregates of various morphologies can be prepared from these diblock copolymers also and that these aggregates can be investigated by the usual techniques, including electron microscopy. Therefore, a high Tg is not a necessary condition for the preservation of the various morphologies either in dilute solution or in dry state. Experimental Section Anionic polymerization of butadiene has been described in detail in the scientific literature. It is well-known that solvent polarity has a significant effect on the microstructure of polybutadiene (PBD)23 and therefore also on the glass transition temperature of the polymer. Polybutadiene prepared in apolar solvents such as cyclohexane and toluene has very high 1,4content (ca. 90%). Conversely, polybutadiene prepared in a more polar solvent, such as tetrahydrofuran (THF), has a very low 1,4-content (ca. 12%). When butadiene polymerization is carried out in a mixture of a polar and an apolar solvent, the 1,4-content of the resulting PBD varies depending on the ratio of the two solvents. The higher the 1,4-content, the lower the glass transition temperature. In the present study, the syntheses of the polybutadienes, the core-forming blocks, were carried out either in cyclohexane or in a mixture of cyclohexane and diethyl ether, or THF, in order to obtain different 1,4-contents. When cyclohexane or a mixture of cyclohexane and diethyl ether were used as the polymerization solvents, an equal volume of THF containing diphenylethylene in 5-fold molar excess relative to the initiator was added before the tert-butyl acrylate (t-BuA) polymerization because t-BuA cannot polymerize in an apolar solvent because of the inherent side reactions of the propagating (23) Yu, Y. S.; Dubois, Ph.; Jerome, R.; Teyssie, Ph. Macromolecules 1996, 29, 2738.
© 1997 American Chemical Society
Notes
Langmuir, Vol. 13, No. 9, 1997 2579 Table 1. Characteristics of PBD-b-PAA Block Copolymers
carbons sample 1,4-microstructure Tg compositiona in BD no. (%) (°C) (PBD-b-PAA) backbone PIb 1 2 3 4 5
90 55 55 55 12
-90 -65 -65 -65 -8
360-b-16 430-b-17 430-b-80 1345-b-360 340-b-25
1370 1330 1330 4170 760
1.08 1.10 1.10 1.10 1.09
a The numbers indicate block length, e.g., 360-b-16 indicates a butadiene block length of 360 units attached to an acrylic acid block of 16 units. b PI ) Mw/Mn.
enolate with the ester carbonyl during anionic polymerization. The addition of diphenylethylene to polybutadiene lithium solutions resulted in the highly delocalized, sterically hindered, weak diphenylhexyl lithium anion, which has been routinely utilized as an efficient initiator for a variety of acrylate polymerization in order to avoid carbonyl attack. The latter would lead to deactivation of a portion of the initiator and subsequent loss of molecular weight control of the acrylate monomer. The poly(tert-butyl acrylate) (P-t-BuA) block in the block copolymer was hydrolyzed to its acid form, poly(acrylic acid) (PAA), by using methane sulfonic acid as the catalyst, as described elsewhere.24 Size-exclusion chromatography (SEC) measurements were carried out with THF as a solvent on a Varian 5010 liquid chromatography apparatus equipped with a refractive index detector. Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer 16 PC apparatus. Nuclear magnetic resonance (1H-NMR) spectra were recorded on a Varian XL-300 spectrometer at room temperature. The 1,2- and 1,4-contents of the polybutadiene block were determined by 1H-NMR using the integration of the signals at 4.9 and at 5.4 ppm, respectively, of the methylene protons of the 1,2 and 1,4 units. SEC was used to measure the degree of polymerization and the polydispersity of the polymers.23 The degree of polymerization of the poly(tert-butyl acrylate) block was measured by FTIR relative to the degree of polymerization of PBD block.25 To prepare solutions of the crew-cut aggregates, the diblock copolymers were first dissolved in THF, which is a common solvent for both polybutadiene and poly(acrylic acid) blocks. Subsequently, deionized water was added to the polymer-THF solution dropwise at a very slow rate with vigorous stirring. As the addition of water progressed, the quality of the solvent for the PBD block decreased gradually. The aggregations of the PBD block of the copolymer, as indicated by the appearance of turbidity in the solution, typically occurred when the water content reached 11 wt %. The addition of water was continued until 33 wt % of water had been added. Then the solutions were poured into a 2-fold excess of water. The resulting colloidal solution was placed in a dialysis bag and dialyzed against distilled water to remove the THF. In the present study, the initial polymer concentration in THF (before the addition of water) was 0.25 wt %. Transmission electron microscopy (TEM) was carried out on a Philips EM410 microscope operating at an acceleration voltage of 80 kV. For the preparation of TEM specimens, copper EM grids were first precoated with a thin film of Formvar (J. B. EM Services Inc) and then coated with carbon. The micelle solutions were diluted with water by a factor of 5, and then a drop of the diluted solution was placed on the grid. The grids, after drying in air for a few hours, were shadowed with palladium-platinum alloy at an angle of ca. 30°. A detailed description of the experimental conditions can be found elsewhere.16,17
PBD chains were approximately 90%, 55%, and 12%. As reported,26 the various polybutadienes have glass transitions at ca. -90, -65, and -8 °C, respectively. The sample designation includes both block length and 1,4-content. For example, PBD360 (90% 1,4)-b-PAA16 indicates a block length of 360 units for the polybutadiene of 90% 1,4-content attached to a PAA chain of 16 units. It is worth noting that although samples 1 and 5 have a similar number of BD units in the blocks, the number of backbone carbon atoms, and therewith the lengths of the PBD main chains, is quite different because of differences in the microstructure. 1,4-PBD has four carbons in the main chain, while 1,2-PBD has only two carbons, with the other two forming a short branch. The total number of backbone carbons in the BD chain in PBD360 (90% 1,4)b-PAA16 is 1370 and is thus almost 2 times larger than that in PBD340 (12% 1,4)-b-PAA25 where it is 760. The number of backbone carbon atoms in the BD blocks is given in Table 1. The aggregate morphologies of the five samples listed in Table 1 were investigated. Aggregates were formed when the water content in the polymer solution in THF reached ca. 11 wt %. In the range 11-43 wt % water content, the aggregates were stable. As soon as the water content exceeded 43 wt %, the aggregates precipitated, even though the water addition was slow. However, the aggregates could be stabilized by pouring the 33 wt % solution into a 2-fold excess of water. This phenomenon implies that there is an unstable region in the phase diagram. The micrographs showing aggregates prepared from sample PBD360 (90% 1,4)-b-PAA16 are characterized by the presence of various morphologies, such as rods, vesicles, ribbons, and large spheres on the same micrograph. Parts A-D of Figure 1 show, respectively, typical examples of the rods, vesicles, large ribbons, and large spheres obtained from this sample. One reason for the coexistence of the various morphologies is that boundaries of various morphologies in the phase diagram are being crossed as water is added to the polymer-organic solvent mixture. As the solvent becomes progressively worse for the core-forming block, the aggregates may change from one morphology to another. If the kinetics of chain exchange and rearrangement of the aggregates is considerably slower than the change of solvent quality, some of the aggregates of the previous morphologies might survive while the aggregates with the new morphologies are formed. Thus, the observation of multiple morphologies is due to a rapid crossing of regions of stability of the different morphologies while water is added. Figure 1A shows rods of some polydispersity in that the radii are in the range 15-30 nm. Most of the rods are extremely long compared with their diameters. It is interesting to compare the rod diameters with the endto-end distance of polybutadiene chains in the unperturbed state (unperturbed dimension r0), which can be calculated from the following equation:27
r0 ) 0.085M1/2
Table 1 reports the characteristic features of the PBDb-PAA diblock copolymers used in the present study. All the block copolymers have a narrow and monomodal molecular weight distribution. The 1,4-contents of the
The value 0.085 applies to 90% 1,4-PBD. Thus for PBD360 (90% 1,4) -b-PAA16, r0 is ca. 12 nm. The radii of the rods (15-30 nm) are thus similar in size to the unperturbed dimensions of the butadiene chains. A vesicular morphology is shown in Figure 1B. The vesicles in this figure have coalesced into a rodlike
(24) Deporter, C. D.; Ferrence, G. M.; McGrath, J. E. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1993, 34, 574. (25) Zhong, X. F.; Varshney, S. K.; Eisenberg, A. Macromolecules 1992, 25, 7160.
(26) Yu, Y. S.; Dubois, Ph.; Jerome, R.; Teyssie, Ph. Macromolecules, submitted. (27) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley & Sons: New York, 1989; Vol. VII/33.
Results and Discussion
2580 Langmuir, Vol. 13, No. 9, 1997
Notes
Figure 1. TEM pictures of multiple morphologies of crew-cut aggregates formed from PBD360-b-PAA16 block copolymer with a 90% 1,4-content: (A) rodlike aggregates; (B) vesicles; (C) compound ribbon-like aggregates; (D) LCMs.
aggregate, but the vesicular structure of the individual units is clear, especially near the edges of the rod. The polydispersity in the size of the vesicles is relatively low. The vesicles have a wall thickness of ca. 25 nm and diameters of ca. 70 nm. It is worth noting that because of the very low contrast in the electron density between the copolymer blocks, it is difficult to distinguish the micelle corona layer from the core region. However, it is obvious that the corona layer on the core surface must be very thin because of the low content of the corona-forming block. It was calculated to be on the order of 0.4 nm in this case. Therefore, the thickness of the corona layer is not considered in the estimation of the number of molecules per vesicle (Nagg). From the volume of a typical sized vesicle and that of a single PBD360-b-PAA16 chain, Nagg is estimated to be 5000. The area per corona chain at the core surface (Ac) is ca. 3.5 nm2. Figure 1C shows a ribbon morphology. The widths and thicknesses of the ribbons are ca. 250 and 160 nm, respectively. The dimensions of these ribbons are much more uniform than those shown in Figure 1A. The ribbons are also very long, and very straight in this case. Since the dimensions are considerably larger than those of shown in Figure 1A, a compound morphology is probably involved.16,17 Large spheres are seen in Figure 1D. The diameters of the spheres vary from 120 to 400 nm. The large spheres are probably large compound micelles (LCMs) that are solid, highly polydisperse spheres consisting of an assembly of reverse micelles, with a hydrophilic surface.16,17 Some of the spheres appear to be vesicles. Figures 2 and 3 show that the morphology changes from spherical to nonspherical as the hydrophilic PAA block length decreases. The same trend has been observed in aggregates of PS-b-PAA block copolymers16,17 and PS-bPEO block copolymers.19 The driving forces involved in the change from spherical to nonspherical aggregates in those systems have been discussed.16,17 The morphological
Figure 2. TEM picture of compound ribbon-like aggregates formed from PBD430-b-PAA17 block copolymer with a 55% 1,4content.
changes are believed to result, in part, from the changes of the degree of stretching of the hydrophobic blocks in the core regions as the length of the hydrophilic corona block decreases and, in part, to corona chain interactions.17 PBD430(55% 1,4)-b-PAA17 forms compound ribbon-like aggregates. Figure 2 shows a typical example. The sizes of the ribbons are quite polydisperse. The widths are in the range 100-300 nm, and the heights, calculated from the length of the shadowed region, range from 60 to 200 nm. It appears that some of the ribbons stand vertically (indicated by arrows in the Figure 2). This vertical positioning can be deduced from the directly measurable width and height as estimated from the shadow length. PBD430 (55% 1,4)-b-PAA80 forms irregular aggregates, some of which are almost spherical. These small spheres are most likely normal “crew-cut” micelles consisting of a polybutadiene core and a poly(acrylic acid) corona. The diameter of the spheres in the dry state is ca. 50 nm. Figure 3 shows a typical example. The highly irregular
Notes
Figure 3. TEM picture of aggregates formed from PBD430-bPAA80 block copolymer with a 55% 1,4-content. Some primary near-spherical shapes are seen.
Figure 4. TEM picture of primary near-spherical aggregates formed from PBD1345-b-PAA360 block copolymer with a 55% 1,4content.
larger aggregates are most likely a result of the coalescence of the primary spherical aggregates. PBD1345 (55% 1,4)-b-PAA360 also forms an irregular sphere morphology (Figure 4). The diameters of the spheres are ca. 40 nm. Again, primary crew-cut micelles are involved here, since r0 ) 22 nm. Nagg is calculated to be 256 and Ac ) 20 nm2/chain. PBD340 (12% 1,4)-b-PAA25 forms mainly a compound ribbon morphology. However, some spheres are also observed. Figure 5 shows an example. The width of the ribbons varies from 120 to 180 nm. The thickness of the ribbons aggregates is ca. 45 nm, but the surfaces are quite irregular. The diameter of the spheres is ca. 30 nm. From the above results, it can be seen that a high glass transition temperature is not a necessary condition for the preservation of the various morphologies. This conclusion is based on the fact that the various morphologies such as rods and vesicles can be produced not only from PS, which has a high Tg core (ca. 100 °C),16,17 but also from PBD, which has a low Tg core (ca. -90 to -8 °C). Finally, it is worth inquiring to what extent the glass transition of the bulk polymer influences the morphology, or whether it is the chain dimensions that are responsible.
Langmuir, Vol. 13, No. 9, 1997 2581
Figure 5. TEM picture of compound ribbon-like aggregates formed from PBD340-b-PAA25 block copolymer with a 12% 1,4content.
We believe that the morphologies of aggregates are mainly controlled by the dimensions of the chains that form the core and the corona under preparative conditions. The Tg is largely irrelevant because the morphologies are fixed in dilute solution, where the Tg of the core-forming block does not exert a measurable influence, while the coil dimensions do. In the case of PS-b-PAA diblock copolymers, for example, two polymers, PS200-b-PAA15 and PS410b-PAA16, both have the same Tg for the core as well as similar PAA lengths, but the former gives aggregates with a rodlike morphology and the latter gives vesicles.16 In the present study, it appears that the morphologies of aggregates are also determined by the dimensions of the chains that form the core and the corona, which, in turn, are functions of the structures of the polymers. For instance, a ribbon-like morphology is obtained from PBD360 (90% 1,4)-b-PAA16, from PBD430(55% 1,4)-b-PAA17 and from PBD340 (12% 1,4)-b-PAA25, with Tg’s ranging from -90 to -8 °C. Conclusion It is shown that aggregates of multiple morphologies can be obtained from PBD-b-PAA diblock copolymers, which have a soft core-forming block (PBD). The glass transition temperature (Tg) of the core-forming block in bulk ranges from -90 to -8 °C, depending on the microstructure of PBD. It is clear that the aggregates are able to retain their morphologies even though the Tg of the core-forming block is much lower than the temperature at which the aggregates are formed or studied. Since morphologies form in dilute solution, the mobility of the polymer chain under preparative conditions is not affected by the Tg of the bulk polymer. It appears that it is the coil dimensions that determine largely the morphology rather than the glass transition temperature of the core-forming block. Acknowledgment. The authors thank The Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support of this research. LA961058N
