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On the Origins of Morphological Complexity in Block Copolymer Surfactants

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Science  18 Apr 2003:
Vol. 300, Issue 5618, pp. 460-464
DOI: 10.1126/science.1082193

Abstract

Amphiphilic compounds such as lipids and surfactants are fundamental building blocks of soft matter. We describe experiments with poly(1,2-butadiene-b-ethylene oxide) (PB-PEO) diblock copolymers, which form Y-junctions and three-dimensional networks in water at weight fractions of PEOintermediate to those associated with vesicle and wormlike micelle morphologies. Fragmentation of the network produces a nonergodic array of complex reticulated particles that have been imaged by cryogenic transmission electron microscopy. Data obtained with two sets of PB-PEOcompounds indicate that this type of self-assembly appears above a critical molecular weight. These block copolymers represent versatile amphiphiles, mimicking certain low molecular weight three-component (surfactant/water/oil) microemulsions, without addition of a separate hydrophobe.

The manipulation of interfacial curvature and topology plays a central role in the creation of soft materials (1). Living cells, mayonnaise, and fracturing fluids used in oil recovery are representative examples of heterogeneous systems containing coexisting hydrophilic and hydrophobic domains assembled in precise ways through the action of ubiquitous amphiphilic compounds such as lipids, soaps, and other surfactants (2). Macromolecular surfactants, such as poly(butadiene-b-ethylene oxide) (PB-PEO) diblock copolymers, offer important materials advantages not associated with conventional low molecular weight amphiphiles (3). For example, vesicles and wormlike micelles can be chemically cross-linked without structural disruption, leading to dramatic modifications of static and dynamic mechanical properties (4, 5). Over the past decade, Eisenberg and co-workers (68) have demonstrated that block copolymers also afford access to a cornucopia of complex morphological assemblies through clever processing strategies.

For several years we have explored various similarities and differences in the thermodynamic and processing properties of conventional and polymeric amphiphiles (4, 911). Here, we document a surprising result: network formation in two-component dilute aqueous solutions of PB-PEO. Previous experiments with relatively low molecular weight macromolecular surfactants produced three basic structural elements: spheres (S), cylinders (C), and bilayers (B). These zero-, one-, and two-dimensional structures are readily dispersed in water at low concentration as spherical micelles, wormlike micelles, and vesicles, respectively, mimicking the well-established states of aggregation created when low molecular weight amphiphiles are mixed with water (12). Here we demonstrate the formation of “Y-junctions,” which assemble into a dense, three-dimensional network (N) accompanied by macroscopic phase separation. Isolated fragments of the network have been evaluated with cryogenic transmission electron microscopy (cryo-TEM), from which we deduce that the Y-junction is preferred thermodynamically above a critical surfactant molecular weight at compositions between those associated with the B and C morphologies. These findings are consistent with theoretical predictions recently developed (13, 14) and demonstrated (15, 16) with traditional three-component (surfactant/oil/water) microemulsions, signaling a fundamental transition in self-assembly behavior as the amphiphile molecular weight is increased beyond a critical value.

Two sets of PB-PEO diblock copolymers (Scheme 1), each containing constant molecular weight poly(1,2-butadiene) blocks (Mn,PB = 2500 and 9200 g/mol) and varying weight fractions of PEO (wPEO), were synthesized with anionic polymerization techniques described elsewhere (17). Fifteen compounds with degree of polymerization NPB = 46 and 0.30 ≤ wPEO ≤ 0.64, and 13 with NPB = 170 and 0.24 ≤ wPEO ≤ 0.62, were prepared; in all cases, the compounds had relatively narrow molecular weight distribution, Mw/Mn < 1.1. These diblock copolymers were mixed with measured amounts of distilled water, generally at a concentration of 1 weight percent (wt%), and stirred at room temperature.

Scheme 1.

Solution morphologies were characterized with cryo-TEM, a powerful microscopy technique capable of imaging molecular-scale structures in thin (∼100 to 300 nm) films of vitrified aqueous solutions. The results for the 1% mixtures are summarized in Fig. 1. With NPB = 46, we find the classic sequence of dispersed structures (B, C, and S) separated by mixed morphology regimes (B + C and C + S) with increasing PEO content, in agreement with an earlier report (11). Increasing the size of the hydrophobic block by nearly four times (NPB = 170) dilates the dimensions of the hydrophobic cores by about three times (not illustrated here) and shifts the composition window for wormlike micelles (and perhaps vesicles and spherical micelles) to lower values of wPEO.

Fig. 1.

Morphology diagram for PB-PEO in water (1 wt%) as a function of molecular size and composition, where NPB and wPEO are the degree of polymerization and weight fraction of the PB and PEO blocks, respectively. Four basic structural motifs—bilayers (B), Y-junctions (Y), cylinders (C), and spheres (S)—have been identified by cryo-TEM, as illustrated in the micrographs, (A to C). At NPB = 170, decreasing wPEO from the cylindrical condition (wPEO = 0.42) toward that associated with vesicles (wPEO = 0.24) results in an increasing population of Y-junctions. A single-phase dispersion of branched, wormlike micelles at wPEO = 0.39, denoted CY (Fig. 2A), is followed by network (N) formation and macroscopic phase separation at wPEO = 0.34 (Fig. 2, B and C). Only the classic structures (B, C, and S) were found at NPB = 46, indicating a fundamental structural transition from classic two-component (surfactant/water) to three-component (surfactant/oil/water) phase behavior above a critical diblockcopolymer molecular mass. Speculated (dashed lines) and established (solid lines) morphological boundaries do not reflect precise transition conditions. Open and filled symbols refer to previous (11) and current experimental results, respectively. Bars (A to C), 100 nm.

However, the most dramatic structural changes we found were at compositions between the B and C regions. Decreasing the length of the PEO block leads to the formation of Y-junctions in the nominally cylindrical micelles. Even for wPEO = 0.42, which we associate with the C region in Fig. 1, the wormlike micelles contain occasional branches; two have been captured in the cryo-TEM image shown in Fig. 1B. Decreasing wPEO to 0.39 induces striking morphological changes (Fig. 2A). Branched cylindrical loops, branched linear wormlike micelles, and Y-junctions terminated by enlarged spherical caps characterize this representative image. Macroscopically, this solution is opaque (milky) but showed no tendency to phase separate even after sitting quiescently for several months.

Fig. 2.

Cryo-TEM images obtained from 1 wt% aqueous solutions of (A) wPEO = 0.39 and (B and C) wPEO = 0.34 PB-PEO diblockcopolymers. The single-phase solution denoted CY in Fig. 1 (wPEO = 0.39) contains a dispersion of micelles constructed from linear and looped cylindrical segments, Y-junctions, and spherical caps. Macroscopic phase separation at wPEO = 0.34 is manifested in large, dense networkparticles (B) containing a preponderance of loops and Y-junctions. A smaller networkfragment (C) provides a more detailed glimpse of the morphology associated with the networkphase. Bars (A to C), 200 nm.

Unlike any of the other specimens examined in this study, dilute mixtures of the wPEO = 0.34 diblock copolymer phase separate macroscopically at room temperature. This process is slow but unmistakable. Several hours to several days after cessation of stirring, an opaque layer develops above a relatively clear majority (water) phase. Increasing the polymer concentration greatly enhances the viscosity, thus slowing macroscopic phase separation; a soft solid gel forms between 22 and 26 wt% block copolymer. We have obtained cryo-TEM images from 1% solutions of this material with mixing protocols that range from gentle stirring to vigorous sonication. In each case, an opaque, low-density phase reemerges after a quiescent period. Figure 2, B and C, illustrates two representative cryo-TEM images of the associated self-assembled structures captured before macroscopic phase separation. This macromolecular surfactant forms an extended three-dimensional network morphology dominated by Y-junctions. Detailed analysis of large pieces of network, like that found in Fig. 2B, is complicated by the density of network material spanning the vitrified film, which screens individual features. Nevertheless, the edges of the object captured in Fig. 2B reveal the same general structural elements evident in the smaller fragment presented in Fig. 2C. Decreasing the size of the PEO block (i.e., from wPEO = 0.42 to 0.39 to 0.34) increases the population of Y-junctions to the point of network formation and phase separation. We emphasize that a systematic search of NPB = 46 diblock copolymer solutions between the B and C states (Fig. 1) failed to uncover such phase behavior (18).

Increasing diblock copolymer molecular weight has another notable consequence. The equilibrium solubility in water drops exponentially with the degree of polymerization, resulting in extremely slow exchange dynamics between individual micelles (19). Hence, fragmentation of the network phase by stirring or sonication produces a nonergodic dispersion of particles. Until buoyancy forces compact and fuse these particles (smaller particles should survive longer under the action of thermal motion), each is subject to a localized free-energy optimization.

We have exploited this property to produce small, isolated micelles in the 1 wt% wPEO = 0.34 aqueous solution by agitation. Representative cryo-TEM images taken from numerous examples are illustrated in Fig. 3. These images tell us much about the self-assembly characteristics of this PB-PEO diblock copolymer. All of these complex micelles (along with the network fragments shown in Fig. 2, B and C) are constructed from only three elements: Y-junctions, spherical end caps, and cylindrical loops. (One of the most distinguishable differences between the wPEO = 0.34 and 0.39 dispersions is the nearly complete lack of linear cylinders in the former.) Y-junctions can be found with three spherical caps (Fig. 3A), two caps (Fig. 3, D, E, and H), one cap (Fig. 3, B to D, F to H, J to L, and N), and no caps (Fig. 3, I and M). With just three exceptions (Fig. 3, F, K, and L), the objects appear to be planar. (To some extent the planar appearance may be driven by confinement within a thin-film geometry.) Y-junctions exhibit a tendency to pair, and to coalesce into periodic arrays. This tendency is evident in micelles containing as few as 2 (Fig. 3, C and D) and as many as 23 (Fig. 3N) junctions.

Fig. 3.

(A to N) Representative cryo-TEM images of networkphase fragments obtained after agitating the wPEO = 0.34 solution. These micelles are constructed from three structural units: Y-junctions, spherical caps, and cylindrical loops. The high degree of mirror symmetry is speculated to derive from a tendency to balance the local free-energy through transport of diblockcopolymer molecules within the reticulated tubular structures.

A striking feature shared by all the micelles presented in Fig. 3 is a high degree of (mirror) symmetry, with the most compelling examples found in panels (A) to (D), (I), and (M). We believe that this symmetry reflects a tendency to balance the internal free-energy through the redistribution of diblock copolymer molecules within the particle after micelle formation by fragmentation (or fusion) (20). Because the local movement of PB-PEO molecules is unhindered (the glass transition for the unentangled PB blocks is about –12°C) (21), certain rearrangements of the tubular structure are feasible subject to the well-established rules governing microphase separation of monodisperse block copolymers (22). Optimization of the particle free-energy will pit the overall surface tension (proportional to the micelle surface area) against chain stretching inside (PB) and outside (PEO) the tubular structure (23). The uniformity in core diameters (34 nm on the basis of the cryo-TEM images) found within each micelle and across all micelles is evidence that these rules are operative. Accounting for the recorded shapes and topologies should prove a challenge to self-consistent field theorists working with block copolymers. Even if we ignore “isomerizations” involving topological transitions (i.e., breaking or collapsing loops) (24), Y-junctions can be created by sprouting a spherical cap on a cylindrical section with the required polymer drawn from loop sections. The prevalence of spherically capped Y-junctions in Fig. 2, B and C, and Fig. 3 suggests that this element is favored over looped cylinders. It is tempting to hypothesize that, given time, particle N (Fig. 3) likely would have added another capped Y-junction by tapping the reservoir of diblock copolymer contained in the large loop, rendering perfect mirror symmetry.

The number of spherical caps relative to closed loops appears to decrease with increasing network particle size (Fig. 2C), leading us to speculate that the equilibrium morphology may be hexagonally ordered perforated sheets constructed solely from Y-junctions. Realizing this morphology on a macroscopic scale may be kinetically impossible, although a perforated vesicle with soccer-ball–style struts seems feasible given the four-, five-, and sixfold coordinated loops found in panels (L), (M), and (N) of Fig. 3, respectively. Small-angle x-ray scattering patterns taken from the dense network phase (e.g., the gelled 26 wt% specimen) indicate that the local domain curvature remains nearly cylindrical in this limit (Fig. 4).

Fig. 4.

Small-angle x-ray scattering pattern obtained from a 26 wt% aqueous solution of the wPEO = 0.34 PB-PEO diblockcopolymer. The solid curve was obtained with the form factor (9) for an ensemble of randomly arranged, noninteracting, infinitely long, and radially nearly monodisperse (R = 19 ± 1 nm) solid cylinders. Close agreement between the broad periodic intensity oscillations in the calculated and experimental scattering patterns, and the mean core dimension extracted from the cryo-TEM images in Fig. 3 (R = 17 nm), corroborate a nearly solid cylindrical geometry for the networkmorphology. Better agreement between the calculated and experimental results requires a theoretical form factor for the Y-junctions.

Branching in cylindrical dispersions is not a new concept (25, 26). Tlusty and Safran (27) hypothesized several years ago that under appropriate conditions Y-junctions would appear in cylinder-forming three-component (surfactant/oil/water) microemulsions, leading to network formation and phase separation (28, 29). Recently Bernheim-Groswasser et al. (15, 30) presented elegant cryo-TEM images obtained with a low molecular weight nonionic surfactant (C12E5) mixed with water and n-octane that demonstrates such branching. Our results (Figs. 2 and 3) are strikingly similar to the theoretic predictions in several ways. Tlusty et al. (13) predict the same sequence of topological transitions that we report in Fig. 1 for NPB = 170. This comparison suggests that the quantity wPEO for the two-component diblock copolymer/water system is analogous to coR in the three-component surfactant/oil/water system, where co is the spontaneous curvature and R the domain radius. There appears to be a threshold molecular weight above which block copolymer surfactants can exhibit microemulsion phase behavior in water without the addition of a third component. We speculate that the conformational freedom associated with high molecular weight PB blocks facilitates domain packing in complex geometries like the network junction without the addition of hydrocarbon solvent. The short and relatively inflexible hydrocarbon portions of conventional surfactants (e.g., a C12 unit) require added oil to ensure a constant domain density under comparable conditions.

Even certain detailed morphological predictions appear to be supported by our results. For example, the bulbous spherical end caps found in Figs. 2 and 3 closely resemble those anticipated by Tlusty and Safran (31), and the notion that the distance between junctions is comparable with their size as co → 0 is illustrated dramatically by the junction coalescence evident in the cryo-TEM images in Figs. 2 and 3.

These findings offer insights into the origins of morphological complexity in block copolymer/water mixtures (68) along with new opportunities for designing soft materials with enhanced properties. Reduction in the number of components required to create junctions and networks is of great practical value. We anticipate that the reported morphologies will be weakly dependent on temperature, in contrast to nonionic surfactantbased systems where co is highly temperature dependent, resulting in narrow microemulsion windows (13). In addition, macromolecular surfactancy should be universal, with phase behavior described by molecular composition and the relative interfacial interaction energy. Finally, the dimensional control demonstrated here can be amplified through manipulation of the core-block physical properties—for example, glassy, semicrystalline, cross-linked–rubbery, and liquid-like—a degree of materials design flexibility not available with low molecular weight amphiphiles.

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