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Novel Bowl-Shaped Morphology of Crew-Cut Aggregates from Amphiphilic Block Copolymers of Styrene and 5-(N,N-Diethylamino)isoprene Izabel Cristina Riegel† and Adi Eisenberg* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada, H3A 2K6
Cesar L. Petzhold and Dimitrios Samios Instituto de Quı´mica, Universidade Federal do Rio Grande do Sul, Av. Bento Gonc¸ alves, 9500, CP 15003, Porto Alegre, RS, Brazil 91501-970 Received September 24, 2001. In Final Form: February 5, 2002 A new morphology of crew-cut aggregates prepared from highly asymmetric triblock copolymers of 5-(N,N-diethylamino)isoprene and styrene in dilute solution is reported. After quaternization of the polar block, using dimethyl sulfate, the copolymers consist of a long block of polystyrene (PS) with short poly[5-(N,N,N-diethylmethylammonium)isoprene] (PAI) blocks at both chain ends. The aggregates were prepared by first dissolving the copolymers in a common solvent for both blocks and then adding water to induce the segregation of the PS chains. 1,4-Dioxane, THF, or a DMF/THF mixture was used as the common solvent in the preparation of these structures. The bowl-shaped aggregates are essentially highly polydisperse spheres, containing an asymmetrically placed single void space, which has broken through the surface. The continuous phase is composed of an assembly of reverse micelles (PAI core and PS corona) with hydrophilic PAI chains surrounding the structure at the polymer/aqueous solution interface. It is believed that the formation of the bowl-shaped morphology is under kinetic control and does not represent an equilibrium state. A possible mechanism for the formation of this aggregate is proposed, based on two other previously reported crew-cut morphologies from diblock copolymers. This study illustrates the importance of the preparative conditions on the self-assembly of nonequilibrium aggregates from amphiphilic block copolymers.
Introduction A new class of polar monomers containing aliphatic tertiary amino groups, known as 5-(N,N-dialkylamino)isoprenes, was developed and described some time ago.1 Since they are dienes, they can be polymerized anionically. The polymerization behavior of these amino-functionalized monomers and the structure and properties of the resulting homopolymers have been investigated.2 Amphiphilic block copolymers with specific architectures and well-defined molecular weight distributions can be obtained through anionic copolymerization of 5-(N,N-dialkylamino)isoprene with a nonpolar monomer such as styrene.3 Moreover, positively charged ionic blocks are obtained by quaternization of the tertiary amino group.4 These quaternized species show interesting solution behavior, suggesting possible applications in catalysis and as drug delivery systems. Amphiphilic block copolymers can self-assemble into a large variety of microstructures in selective solvents that are good for one block and poor for the other. Crew-cut * To whom correspondence should be addressed. E-mail:
[email protected]. † Permanent address: Instituto de Quı´mica, Universidade Federal do Rio Grande do Sul, Av. Bento Gonc¸ alves, 9500, CP 15003, Porto Alegre, RS, Brazil 91501-970. (1) Petzhold, C. L.; Stadler, R.; Frauenrath, H. Makromol. Chem., Rapid Commun. 1993, 14, 33. (2) Petzhold, C. L.; Morschha¨user, R.; Kolshorn, H.; Stadler, R. Macromolecules 1994, 27, 3707. (3) Petzhold, C. L.; Stadler, R. Macromol. Chem. Phys. 1995, 196, 2625. (4) Petzhold, C. L.; Monteavaro, L. L.; Stefens, J. Polym. Bull. 2000, 44, 447.
micellelike aggregates5 represent a new type of aggregate formed in solution by the self-assembly of highly asymmetric amphiphilic block copolymers. The aggregates are termed crew-cut because the dimensions of the core are much larger than those of the corona, as opposed to the star micelles, in which the core is small and the corona is relatively large. Because of the large size of the core block, crew-cut aggregates are prepared by first dissolving the copolymer in a common solvent for both blocks and then adding water (precipitant) to the solution to induce aggregation of the long hydrophobic segments. As the addition of water progresses, the quality of the solvent for the long block (e.g., polystyrene) decreases. The micellization of the hydrophobic segments starts when the water content reaches a critical point, that is, a critical water content or cwc, usually given in wt % of water. Frequently, water is added to the colloidal solution beyond this point, followed by dialysis to remove the common solvent from the aggregates. When it is desirable to know the morphology at some intermediate water content, a large amount of water is usually added at that point, which quenches the aggregates. By this method, the corresponding morphology at a particular water content can be isolated into pure water, after dialysis. Previous studies have explored extensively the preparation of crew-cut aggregates of various morphologies from amphiphilic diblock copolymers of polystyrene-b-poly(acrylic acid) (PS-b-PAA). It has been found that the morphologies are influenced by many factors, that is, the (5) (a) Gao, Z.; Varshey, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923. (b) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (c) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168.
10.1021/la015592t CCC: $22.00 © 2002 American Chemical Society Published on Web 03/16/2002
Novel Bowl-Shaped Morphology
composition5b,6 of the block copolymer, the copolymer concentration,7 the type of the common solvent,8 the type and concentration of added ions,9 and other factors. As a consequence of thermodynamic versus kinetic control during the formation of the aggregates, equilibrium, nearequilibrium, and metastable morphologies can be prepared.10 Spheres, rods, and vesicles can be equilibrium morphologies in a range of mixed solvents; also, regions where two structures coexist in equilibrium were found between the different morphological regions.7 On the other hand, under some other preparative conditions, nearequilibrium and nonequilibrium morphologies such as branched short rods, tubules, branched tubules, large compound vesicles, and porous spheres, among others, were also isolated.11 The growing interest in potential applications of amphiphilic block copolymers has motivated many studies on the self-assembly of such polymers.12 Recently, many reports have dealt with the various morphologies of nanoaggregates formed from block copolymers. In particular, the investigation of the solvent effect on the morphological control of such aggregates has received much attention.13 This report describes the preparation and observation of an additional nonequilibrium morphology of crew-cut aggregates, consisting of a bowl-like structure, prepared from 5-(N,N,N-diethylmethylammonium)isoprene and styrene triblock copolymers (PAI-b-PS-b-PAI). A mechanism of formation is proposed based on two previously encountered crew-cut morphologies from diblock copolymers. It is suggested that the bowl-shaped structure is kinetically trapped; to our knowledge, it has not been encountered in block copolymer aggregates in dilute solution in the past. Experimental Section Polymer Synthesis and Characterization. Triblock copolymers were obtained via sequential anionic polymerization of 5-(N,N-diethylamino)isoprene followed by styrene and again by 5-(N,N-diethylamino)isoprene. The polymerization reaction was carried out in benzene, using sec-butyllithium as the initiator. The 5-(N,N-diethylamino)isoprene monomer was synthesized and purified as described elsewhere.14 Styrene and benzene were distilled over calcium hydride under nitrogen and dried over di-n-butylmagnesium (1.0 M solution in heptane). The polymerization reaction was carried out in a 1000 mL stirred glass reactor (Buechi) with a thermostated cooling jacket under a N2 atmosphere. Initially, benzene and 5-(N,N-diethylamino)isoprene were added through a glass ampule to the reaction vessel and (6) Shen, H.; Eisenberg, A. Macromolecules 2000, 33, 2561. (7) Shen, H.; Eisenberg, A. J. Phys. Chem. B 1999, 103, 9473. (8) (a)Yu, Y.; Eisenberg, A. J. Am. Chem. Soc. 1997, 119, 5980. (b) Yu, Y.; Zhang L.; Eisenberg, A. Macromolecules 1998, 31, 1144. (9) (a) Yu, K.; Zhang, L.; Eisenberg, A. Science 1996, 272, 1777. (b) Zhang, L.; Eisenberg, A. Macromolecules 1996, 29, 8805. (10) (a) Zhang, L.; Eisenberg, A. Macromolecules 1999, 32, 2239. (b) Zhang, L.; Shen, H.; Eisenberg, A. Macromolecules 1997, 30, 1001. (11) (a) Yu, K.; Zhang, L.; Eisenberg, A. Langmuir 1996, 12, 5980. (b) Cameron, N. S.; Corbierre, M. K.; Eisenberg, A. Can. J. Chem. 1999, 77, 1311. (12) (a) Discher, B. M.; Won, Y. Y.; Ege, D. S.; Lee, J. C. M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143. (b) Armes, S. Mater. World 2000, 8, 15. (c) Liu, G. J. Chin. J. Polym. Sci. 2000, 18, 255. (d) Ahmed, F.; Alexandridis, P.; Neelamegham, S. Langmuir 2001, 17, 537. (e) Resendes, R.; Massey, J. A.; Temple, K.; Cao, L.; PowerBillard, K. N.; Winnik, M. A.; Manners, I. Chem.sEur. J. 2001, 7, 2414. (f) Foster, S.; Berton, B.; Hentze, H. P.; Kramer, E.; Antonietti, M.; Lindner, P. Macromolecules 2001, 34, 4610. (13) (a) Li, Z. C.; Shen, Y.; Liang, Y. Z.; Li, F. M. Chin. J. Polym. Sci. 2001, 19, 297. (b) Butun, V.; Armes, S. P.; Billigham, N. C.; Tuzar, Z.; Rankin, A.; Eastoe, J.; Heenan, R. K. Macromolecules 2001, 34, 1503. (c) Svesson, B.; Olsson, U.; Alexandridis, P. Langmuir 2000, 16, 6839. (14) Mannebach, G.; Morschha¨user, R.; Petzhold, C. L.; Stadler, R. Macromol. Chem. Phys. 1998, 199, 909.
Langmuir, Vol. 18, No. 8, 2002 3359 Table 1. Molecular Characteristics of Triblock Copolymers of Styrene and 5-(N,N-Diethylamino)isoprene copolymera
PAI contentb (mol %)
Mn (g/mol)c
Mw/Mnc
PAI11-b-PS228-b-PAI11 PAI6-b-PS120-b-PAI5
8.8 8.3
26 800 13 900
1.19 1.18
a The numbers indicate the number average degrees of polymerization of each block. b The PAI content was determined by NMR relative to that of the PS block. c Parameters determined by SEC.
cooled to 10 °C. sec-Butyllithium was then added into the reactor; the reaction mixture became slightly yellow as a consequence of the initiation step. At a polymerization temperature of 10 °C, the 5-(N,N-diethylamino)isoprene polymerizes quickly, that is, full conversion can be obtained within a few minutes.15 Styrene was added to the reactor 30 min after the initiation step. Its immediate incorporation into the living chain end can be detected by the appearance of an orange color. The styrene was allowed to polymerize for 5 h to ensure a complete monomer conversion. Finally, a second aliquot of 5-(N,N-diethylamino)isoprene was added to the reactor and the polymerization was allowed to proceed for an additional 30 min. The reaction was terminated with degassed methanol. The copolymers were precipitated in methanol and dried under reduced pressure. The degrees of polymerization and the polydispersity of the copolymers were determined by size exclusion chromatography (SEC), performed in THF as the eluent on a Waters GPC instrument equipped with a differential refractometer detector and PS/DVB columns (Waters Styragel). A calibration curve with a polystyrene standard was used. The copolymer composition was determined by 1H NMR. The spectra were recorded on a Varian VXR 200 Hz spectrometer in deuterated chloroform at room temperature. The calculation of the degree of polymerization of the polystyrene relative to that of the poly[5-(N,N-diethylamino)isoprene] was performed as previously described.3 Two sets of copolymers with different molecular weights but with approximately the same composition were investigated. The relevant data for the materials are summarized in Table 1. After synthesis, the copolymers were quaternized using dimethyl sulfate as the alkylating agent, according to recommended procedures,4 in order to obtain the positively charged blocks. Preparation of the Aqueous Solutions. The colloidal solutions were prepared by first dissolving the triblock copolymers in the organic solvent, or in a mixture of organic solvents, and stirring overnight. In the present study, 1,4-dioxane, THF, and DMF/THF mixtures (5/95, 10/90, 30/70, 50/50, 70/30, and 95/5 wt %) were used as common solvents. The copolymer concentrations in the initial solution were 0.01, 0.1, and 1 wt %. In the case of pure solvents, after the copolymer dissolution and stirring, deionized water was added at a rate of 0.5 wt % per minute, with vigorous stirring, until the water content reached ca. 30 wt %. When mixtures of solvents were used, the addition of water was performed at the same rate, until predetermined water contents were reached, that is, 5, 15, 25, and 35 wt %. After that, a large amount of water was added to the solutions in order to quench the resulting morphologies. The solutions were then dialyzed against water for 3 days to remove the organic solvent. The dialyzed colloidal solutions are stable. In this specific case, transmission electron micrographs obtained from freshly prepared solutions were compared to micrographs obtained from solutions aged for up to 6 months. The same morphology and virtually the same sizes and size distribution were found. The PS-PAA aggregates studied earlier (micelles and large compound micelles (LCMs)) showed no changes over periods of 6 years. Transmission Electron Microscopy (TEM). Transmission electron microscopy experiments were carried out on a JEM2000FX microscope operating at 80 kV. The dialyzed colloidal solutions were diluted by a factor of 10-20 in order to prepare the TEM samples. A drop of the very dilute solution was placed onto copper grids precoated with Formvar and carbon and allowed to dry in air. Since the solutions are dialyzed after quenching, the PS is far below its glass transition, so the morphology is unaffected by (15) Bieringer, R.; Abetz, V. Polymer 2000, 268, 1728.
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Table 2. Materials, Preparative Conditions, and Morphological Characteristics of Bowl-Shaped Structures copolymer composition
initial concn (wt %)
organic solvent
avg diameter (nm)
std dev (nm)
coexisting morphologies
11-b-228-b-11
0.1 1 0.01 0.1 1 1 1
dioxane dioxane THF THF THF DMF/THFa dioxane
380 340 144 90 320 560 170
130 120 40 120 250 120 30
spheres none spheres and rods rods none spheres none
6-b-120-b-5 a
10/90 (wt %) DMF/THF mixture; the morphology was quenched from 25 wt % of water.
the detailed procedure used to prepare the grids. This was confirmed for other morphologies when dioxane-water mixtures were used, which could be freeze-dried. The morphologies obtained after dialysis were identical to those obtained by freezedrying. The TEM images were recorded with a Gatan digital CCD camera, Bioscan model 792. The sizes of the aggregates were measured directly from the digital files.
Results and Discussion Typical examples of several bowl-shaped structures prepared from a 1 wt % copolymer solution in THF can be seen in Figure 1a. Such structures were encountered only under some preparative conditions, the variables being the nature of the common solvent, initial copolymer concentration, and copolymer molecular weight. As can be seen in Table 2, which deals with the preparative conditions and corresponding morphological characteristics, the aggregates were formed at all three concentrations (1, 0.1, and 0.01 wt %) when THF was used as the common solvent. Exclusively bowl-shaped structures were found at 1 wt % initial copolymer concentration when THF or 1,4-dioxane was used as the common solvent; however, coexistence of bowl-shaped structures with other morphologies was found at lower initial copolymer concentrations (0.1 and 0.01 wt %). Also, in the DMF/THF mixtures, bowls were found only in a 10/90 (wt %) DMF/THF mixture, quenched from 25% of water. One notable characteristic of such aggregates is that, irrespective of the preparative conditions, they always showed a large size distribution. A typical distribution, obtained from TEM images, is shown in Figure 1b. Besides 1,4-dioxane and THF, DMF was also used as a possible common solvent to prepare aggregates from the higher Mw triblock copolymer; however, in pure DMF, the bowl-shaped structures were not encountered over the range of polymer concentration used (0.01, 0.1, and 1 wt %). Instead, primary micelles were found with a diameter of 23 ( 3 nm. A detailed study on the morphologies found from the two sets of the triblock copolymers and their corresponding diblocks (same PS block and with one PAI block of the same length) in different solvents will be reported soon.16 The diblocks were obtained by withdrawing an aliquot from the reactor before the second addition of the aminoisoprene monomer took place. In regard to the nature of the common solvent, homopolymers of PAI are completely soluble in THF, whereas they are insoluble in dioxane or DMF. Therefore, even though DMF is able to solubilize the PAI-b-PS-b-PAI copolymer, it is not a common solvent for both blocks. Also, on formation of crew-cut aggregates from PS-b-PAA, the solvent content in the PS core, at a same water content, is highest in THF, followed by dioxane and DMF.8 Therefore, since the bowl-shaped morphology was found in THF and dioxane but not in DMF, one can conclude that the affinity of the common solvent with both the corona and the core chains is an important factor governing (16) Riegel, I. C.; Petzhold, C. L.; Samios, D.; Eisenberg, A. Manuscript in preparation.
Figure 1. (a) Bowl-shaped structures prepared from a 1 wt % solution of PAI11-b-PS228-b-PAI11 in THF; (b) size distribution obtained from several micrographs of the same morphology; the average diameter and the standard deviation are 320 and 250 nm, respectively.
the formation of such aggregates. The polymer-solvent interactions were discussed in a previous paper for the PS-b-PAA copolymer, where the χ parameters were compared with the solubility parameters (δ) and dieletric constants ().8 These numbers are not available for the aminoisoprene. We believe that the morphology encountered in these triblock copolymer systems is related to two other structures previously encountered in diblock copolymers of PS-b-PAA and PS-b-PEO (polystyrene-b-poly(ethylene oxide)), that is, the compound micelles (LCMs)6 and the porous spheres.11a Given the sizes, the size distributions,
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Figure 2. Schematic drawing of a cross section of a bowlshaped structure.
and wall thicknesses of these aggregates, their internal structure should be identical to that of the compound micelles. In addition, the process of formation of the void spaces most likely resembles that encountered in the porous sphere structures. A schematic picture of the internal structure of a cross section of a bowl-shaped aggregate is given in Figure 2, based on what had been proposed previously as the internal structure of a typical LCM.6 LCMs are solid, highly polydisperse spheres which are composed of an assembly of reverse micelles; they consist of islands of the polar chains (the cores of the reverse micelles) in a continuous phase of PS. The outer shell is hydrophilic because of the presence of the polar chains at the hydrophobic polymer/aqueous solution interface, which surrounds the whole structure. The formation of LCMs involves first the formation of a highly swollen large sphere in the early stages of water addition. On continued water addition and simultaneous solvent extraction from the PS core, due to the low viscosity of the system and high chain mobility at this stage, a homogeneous shrinkage of the whole structure is favored and no voids are generated. In this way, classical LCMs are formed in the absence of complications. Because of the very large size of some of these assemblies, they are subject to settling due to gravity, but they can be resuspended upon stirring. Such a morphology was frequently encountered for PS-b-PAA copolymers containing very short hydrophilic segments, for example, PS200-b-PAA4,5 as well as in a range of PSb-PEO copolymers.9 In the porous spheres, the continuous phase is hydrophobic, similar to that in the LCMs, but there are large cavities within each aggregate. The cavities are generated on addition of water to the block copolymer solutions in organic solvents under conditions resembling those which give rise to LCMs. However, the formation of porous spheres is a result of complications that arise at moderately high viscosities, when the solvent is being extracted from the PS-rich phase. One possible mechanism for the formation of the porous sphere structure is that it is a result of a relatively rapid water addition process. Under those circumstances, since the chain mobility will be considerably decreased, the extraction of the solvent from the surface region of the precursor structure may result in the formation of a “skin” of high viscosity surrounding the structure. The diffusion of water or solvent through the skin may still be relatively rapid, but the hardened skin will prevent the homogeneous shrinkage of the whole precursor sphere. Upon continued water addition to the solution, more and more solvent will be extracted from the sphere, and since the skin of the structure is relatively rigid, spaces are formed which are devoid of polymer; that is, they consist essentially of organic solvent and some water. Another possible mechanism suggests that, on water addition, a thermodynamic phase separation occurs
Figure 3. Aggregates prepared from a 1 wt % solution of PAI11b-PS228-b-PAI11 in dioxane. Note the presence of a structure containing two inclusions, resembling the porous sphere morphology.
within the highly swollen precursor spheres, to yield polymer-rich and polymer-poor regions. The spaces thus formed will be filled with solvent/water mixtures, possibly with some dissolved polymer. Such a phase separation phenomenon is analogous to that encountered in many two- or three-component partially miscible liquid systems, where a breakup into two liquid phases can occur on changing the temperature or composition. Porous spheres were encountered previously not only in PS-b-PEO11a but also in PS-b-PAA with additional homopolystyrene.17 Based on the above considerations of previously found morphologies in similar copolymer systems, a possible mechanism can be proposed for the formation of the newly found bowl-shaped structure. At the beginning of water addition to the copolymer/organic solvent solution, the first structure formed as a consequence of the selfassembly, that is, segregation of the PS chains and subsequent secondary aggregation, is an apparently homogeneous large sphere. At this point, the viscosity of the system is low, the diffusion of solvent molecules is rapid, and the chain mobility is high. Upon further addition of water, more of the organic solvent is extracted through the skin (or surface) of the sphere. When the viscosity of the core becomes high enough due to the decreased solvent content or some other factor (e.g., phase separation of the hydrophilic short blocks), the low mobility of the chains can cause the external shell to harden or, alternatively, a liquid-liquid phase separation may occur. From this point on, the formation of the bowl-shaped structures will closely resemble the formation of porous spheres. The internal viscosity and the rate of water addition are the main factors that will determine which one of these will be the resulting morphology. If the solvent/water-filled bubbles (inclusions) within the structure have the chance to coalesce, a single large bubble will result and bowlshaped structures can be formed; if not, porous spheres are produced. The driving force for the coalescence is the interfacial energy (which leads to the formation of a single bubble if the core viscosity is low enough). As evidence that the bowl-shaped structures and the porous spheres are related, a structure showing two inclusions in the same entity is shown in Figure 3, among many bowl-shaped structures. This indicates that, in this aggregate, the coalescence process did not proceed completely toward the formation of a true bowl shape. If the single bubble were to be located in the center of the structure, then a morphology resembling that of a (17) Zhang, L.; Eisenberg, A. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 1469.
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Figure 4. Aggregates prepared from a 1 wt % solution of PAI11b-PS228-b-PAI11 in THF. Note aggregates showing a centrally located void.
Figure 5. Aggregates prepared from a 1 wt % solution of PAI11b-PS228-b-PAI11 in THF. The cracked membrane of some structures can be seen distinctly.
Riegel et al.
Figure 6. (a) Proposed steps leading to the formation of a bowl-shaped structure; (b) micrographs illustrating the various steps; the images were chosen from many micrographs and do not represent the process in the same aggregate.
solvent and initial copolymer concentration, as well as the copolymer molecular weight (since for the lower molecular weight copolymer this morphology was found only when dioxane was used as common solvent, Table 1). These results confirm the important role of the three abovementioned morphogenic factors, which have been discussed in a previous paper for the PS-PAA system.19 Additionally, the architecture of the copolymer was also found to be an important morphogenic factor. In regard to this, these structures were not encountered in similar diblock copolymers of the same PS and one PAI block lengths, that is, PAI11-b-PS228 and PAI6-b-PS120, under the same preparative conditions. Instead, LCMs, vesicles, and coexisting vesicles and rods were found.16 This behavior may be attributed to the lower core viscosity of the diblocks relative to the triblocks. In the diblocks, the flow will depend solely on the sliding of the polystyrene corona chains past each other, whereas in the triblock system, one has to pull the amino isoprene chains out of the phase-separated aggregates to achieve flow. The extra aminoisoprene block in the triblock copolymer (when comparing to the analogous diblock) probably is responsible for physical cross-linking, which increases the viscosity.
vesicle would be seen, with the notable difference that the wall thickness would be much greater than that in a typical vesicle prepared from the same PS block length. From the range of the wall thicknesses in the bowls, typically greater than 150 nm, it is clear that they are not vesicles, since vesicles prepared from block copolymers of 200-300 PS units have wall thickness around 20-30 nm.18 Although some structures in which the bubble seems to be located at the center were seen (Figure 4), such images could also result from either a top or bottom view of a true bowlshaped aggregate. In the absence of tilt micrographs, it is not possible to conclude unequivocally which of these possibilities corresponds to what has been seen. If the location of the single bubble is not exactly in the center, the asymmetry of forces acting on it will move the bubble in the direction of the thinner wall and thus create asymmetric structures. Once the bubble is close to the edge, the high surface-to-volume ratio in the thin regions will drive the bubble to break through the surface. A complete breakthrough is seen in Figure 5, in which the cracked membrane can be seen distinctly. Once the breakthrough has occurred and healing of the periphery of the breakthrough has taken place, we can refer to the aggregate as a true bowl. A possible mechanism, showing the detailed steps involved in the process of formation of the bowl-shaped structures, is represented in Figure 6. The formation of the bowl-shaped structures will only be possible within a narrow range of values of the modulus of the core and the internal viscosity of the precursor spheres. The former needs to be high enough to prevent the homogeneous shrinkage of the core (which otherwise would yield LCMs), and the latter has to be relatively low to permit the coalescence of the multiple small bubbles into a single larger one (which otherwise would yield porous spheres). This critical combination of properties will be achieved depending on the nature of the common
A new bowl-like morphology was encountered under some preparative conditions during the investigation of dilute solution morphologies of triblock copolymers consisting of poly[5-(N,N,N-diethylmethylammonium)isoprene and polystyrene, PAI-b-PS-b-PAI. To prepare the structures, the copolymers were first dissolved in a common solvent for both blocks and subsequently water was added to the solution. The bowl-shaped aggregates are essentially spheres, containing an asymmetrically placed single void space, which has broken through the surface. The continuous phase is composed of an assembly of reverse micelles (PAI core and PS corona) with hydrophilic PAI chains surrounding the structure at the polymer/aqueous solution interface. At a 1 wt % initial copolymer concentration, pure bowl morphologies were found in dioxane and THF as common solvents. However, coexistence with other morphologies (primary micelles and, less frequently, rods) was seen at lower initial copolymer concentrations (0.1 and 0.01 wt %) and also when a mixture of solvents (DMF/THF) was used. This is the first time that the bowl-shaped morphology has been reported; it constitutes an additional morphology seen in the crew-cut aggregates. A possible mechanism of formation of such a structure is proposed, based on the formation of two previously encountered crew-cut mor-
(18) Luo, L.; Eisenberg, A. J. Am. Chem. Soc. 2001, 123, 1848.
(19) Zhang, L.; Eisenberg, A. Polym. Adv. Technol. 1998, 9, 677.
Conclusion
Novel Bowl-Shaped Morphology
phologies from diblock copolymers, that is, LCMs and porous spheres. It is concluded that the bowl shape is a kinetically trapped morphology; therefore, its occurrence illustrates the importance of kinetic versus thermodynamic factors in the self-assembly of highly asymmetric block copolymers.
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Acknowledgment. The authors thank the Brazilian foundation CAPES for the scholarship granted to I.C.R. We also thank NSERC (Canada) and PADCT/CNPq (Brazil) for financial support. LA015592T
