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Block Copolymer Assembly via Kinetic Control

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Science  03 Aug 2007:
Vol. 317, Issue 5838, pp. 647-650
DOI: 10.1126/science.1141768

Abstract

Block copolymers consist of two or more chemically different polymer segments, or blocks, connected by a covalent linkage. In solution, amphiphilic blocks can self-assemble as a result of energetic repulsion effects between blocks. The degree of repulsion, the lengths of the block segments, and the selectivity of the solvent primarily control the resultant assembled morphology. In an ideal situation, one would like to be able to alter the morphology that forms without having to change the chemistry of the block copolymer. Through the kinetic manipulation of charged, amphiphilic block copolymers in solution, we are able to generate different nanoscale structures with simple block copolymer chemistry. The technique relies on divalent organic counter ions and solvent mixtures to drive the organization of the block copolymers down specific pathways into complex one-dimensional structures. Block copolymers are increasingly used as templating materials; thus, the ability to control the formation of specific patterns and structures is of growing interest and applicability.

The broader development of nanoscale technologies requires methods to fabricate and manipulate material at the nanometer-length scale. This includes techniques that manipulate individual atoms or clusters, as well as materials that will self-assemble into organized patterns, often through solution-based processes. Molecules with varying chemical interactions are needed to drive the assembly, and this has been accomplished in block copolymers (17), surfactants (8), proteins (9), DNA (10, 11), peptides (12, 13), peptide amphiphiles (14), and polypeptides (15). This broad range of molecules was needed to tailor nanostructures with potential impact on disparate, emerging fields such as nano-medicine (14), organic photovoltaics (16, 17), and self-assembled spintronics (18, 19) or optoelectronic devices (11, 20).

Linear triblock copolymers, produced with common polymerization techniques, provide an opportunity to develop self-assembly strategies for complex nanostructure formation that do not necessarily require the altering of the molecule chemistry to create a wide range of structures. The chemical tunability of amphiphilic block copolymers has been used previously to produce micelles and nanostructures in solution (17, 21, 22). Polymeric micelle size and shape can be designed through monomer selection, chain architecture design, and variation of solution conditions (e.g., solvent mixtures, pH manipulation, salt concentration, and temperature) (17, 21, 23). However, the slow kinetics of block copolymers in solution, due to the slow exchange of chains between micelles because of the higher molecular weight of the molecules, hinders assembled structures from reaching global equilibrium states (21, 2426). We specifically take advantage of this lack of global equilibrium in amphiphilic charged block copolymers to produce complex, one-dimensional nanostructures. The block copolymers are controllably forced down specific assembly pathways through a combination of solvent mixing and the complexation of a charged, hydrophilic block with divalent, organic counterions. The resultant assemblies are kinetically trapped but stable because of the inability of the system to thermodynamically equilibrate.

The system we used consists of a linear poly (acrylic acid)-block-poly(methyl acrylate)-block-polystyrene (PAA-b-PMA-b-PS) triblock co-polymer, tetrahydrofuran (THF)/water mixed solvents, and organic diamines (Fig. 1A). Various micelles with different packing geometries, such as disks and toroids, have been constructed using this system (4, 22, 27, 28). This multicomponent assembly system allows control over the thermodynamics and kinetics of block copolymer assembly in the following ways. The selectivity of water for PAA allows the manipulation of interfacial curvature between the hydrophilic corona and hydrophobic core within a micelle, thus providing a means to control local micelle geometry. Organic diamine complexation with chargable PAA corona blocks influences both intermicellar interactions and the intramicellar PAA corona block conformation. By the use of different solvent-mixing protocols, the pathway through which polymer assembly occurs can be manipulated.

Fig. 1.

(A) Molecular structures of triblock copolymer, PAA-b-PMA-b-PS, and organic diamine (EDDA). (B and C) TEM images of one-dimensional assembled structures of PAA94-b-PMA103-b-PS44 at 67% THF/water solution in the presence of EDDA (molar ratio of amine groups: acid groups = 1:1). The samples were stained with uranyl acetate aqueous solution. (B, insert) Schematic drawing of cross section of one-dimensional assembled structures. PMA-PS stripes are illustrated as gray and dark blue bands. Light blue bands denote PAA concentrated area. EDDA, which is complexed with the PAA block, is not drawn for clarity.

The controlled assembly pathway generally begins with diamine complexation with the PAA block in pure THF solution producing PAA-diamine aggregates. Subsequent addition of water has the combined effect of aggregating the hydrophobic PMA and PS blocks while concurrently swelling and eventually solubilizing the PAA-diamine complexes into micelle coronas. The combination of the PAA-diamine complexation with subsequent solvent mixing produces a unique block copolymer assembly pathway resulting in the nanostructures displayed in Figs. 1, B and C. The cylindrical nanostructures consist of PAA94-b-PMA103-b-PS44 with 2,2'-(ethylenedioxy)diethylamine (EDDA) as a diammonium counterion by interaction with the acrylic acid residues of the PAA [see table S1 and (29) for block copolymer molecular details]. The periodic stripes perpendicular to the cylinder axes indicate the alternating layers of hydrophilic PAA complexed with EDDA and hydrophobic PMA-PS domains. The dark stripes are PAA layers that are positively stained as a result of uranyl cations interacting with carboxylic acid side chains of the PAA. The light stripes are composed of PS and PMA hydrophobic segments with a thickness of ∼20 nm (Fig. 1B, inset). It is notable that this assembly is not a typical hydrophobic core/hydrophilic corona micelle but rather a cylinder with alternating layers of hydrophilic and hydrophobic components arranged perpendicular to the cylinder axis.

The specific assembly pathway to produce the striped cylinders is as follows. PAA94-b-PMA103-b-PS44 was first dissolved in THF to form a 0.1 weight percent homogeneous solution. Next, EDDA was added to give a molar ratio of amine group:acid group = 1:1. The EDDA complexed with the PAA block affording PAA-diamine aggregates. Water was then added slowly (∼8 ml water per hour added to 20-ml THF block copolymer solution with a syringe pump) both to initiate aggregation of the hydrophobic blocks and to solubilize the PAA-diamine complexes. This slow addition of water first caused polymer phase separation into polymer-rich domains with a local lamellar nanostructure due to phase segregation of unlike blocks (22). Once sufficient water was added, the phase-separated polymer droplets were solublized into discrete micelles through the segregation of the hydrophobic blocks into hydrophobic core domains and the hydration of PAA-diamine into the corona. At high water content (THF:water = 1:4), stable, spherical micelles formed (Fig. 2A). THF was then pipetted into the spherical micelle solution to produce a final volumetric ratio of THF:water = 2:1. During the original slow addition of water to the block copolymer/diamine/THF solution, a 2:1 THF:water ratio produced block copolymer droplets with local lamellar structure (22). By forcing the system back to this solvent composition, the spherical block copolymer micelles were forced to aggregate into a locally ordered lamellar nanostructure. Transmission electron microscopy (TEM) images taken immediately after the THF addition demonstrated that all of the micelles polymerized along a preferred growth axis (Fig. 2B). Further aging for several hours allowed additional one-dimensional growth of these structures into long (up to microns in length) structures with uniform widths as shown in Fig. 1, B and C.

Fig. 2.

(A) Spherical micelles of PAA94-b-PMA103-b-PS44 formed at the 1:4 ratio of THF to water in the presence of EDDA (molar ratio of amine groups:acid groups = 1:1). (B) TEM image of one-dimensional aggregation of spherical micelles immediately after introducing THF into original solution to reach a 2:1 final ratio of THF to water. Further growth of these short structures led to a giant one-dimensional supra-assembly, as shown in Fig. 1, B and C. (C) Growth mechanism of spherical micelles. Sphere-disk transition occurred first as THF was introduced. Anisotropic shape of disk-like micelles allows for one-dimensional preferred growth. Inserted schematic illustrates proposed chain packing of spherical micelles, disklike micelles and one-dimensional packing structures. (D) Separate disklike micelles marked as black arrow. (E and F) Branches appear as growth defect. The samples were stained with uranyl acetate aqueous solution.

In amphiphilic block copolymer dilute solutions, two kinetic processes are possible in response to solvent compositional changes. One is a relatively fast intramicellar process of obtaining local preferred interfacial curvature in isolated micelles through fast local chain adjustment. The second is the relatively slow process of reaching a global equilibrium by means of intermicellar interactions [through infrequent intermicellar single-chain exchange (21, 24, 25) or micelle fusion or fission]. When combined, the mismatch of the two kinetic processes can produce the well-defined hierarchical structure in Fig. 1, B and C, and Fig. 2C schematically demonstrates the proposed mechanism. Upon quick introduction of THF, the local packing geometry of isolated micelles changes before intermicellar aggregation takes place. Because local chain adjustment is a much faster process than intermicellar interactions, oblate spheres or discoidal micelles form with the addition of THF (Fig. 2C, step 1). Although the local flat interfacial curvature is desired in the system, the resultant dispersed structures are not stable in the low-water-content solution and undergo aggregation. However, the aggregation is one-dimensional because the disklike micelles have PAA-diamine faces that experience long-range, attractive electrostatic interactions with other diamine-rich PAA faces in high THF content solution. [See figs. S1 to S4 and (29) for data and accompanying discussion about the electrostatic nature of the interactions between the PAA blocks and the multivalent amine counterions.] Direct visualization of several separate disklike micelles in the intermediate assembly stage can be seen in Fig. 2D, as marked by black arrows, supporting the concept that these segmented cylinders are formed by one-dimensional collapse of discoidal micelles. In addition, the diameter and volume of each cylindrical segment is comparable to the dimensions of the separate spherical micelles before aggregation, further supporting the transition mechanism. Branching appears as growth defects, as observed in Fig. 2, E and F. Theoretical prediction has shown that branching could occur in the one-dimensional aggregation of dipolar fluids when construction of a branch provides a lower free energy than the formation of a free chain end (30). In the current work, branching probably occurs because of polydispersity in size and shape of assembling micelle units. As intermicellar interaction proceeds, there is a chance that some spherical micelles do not have enough time to adjust their packing geometry before assembling into a growing cylinder. This curved surface could then allow two disklike micelles to attach, forming a branch. An alternative branching mechanism could be polydispersity in the sizes of disks, with slightly larger disks having more PAA-diamine surface area, so that two additional disks could assemble, thus forming a branch.

The periodic spacing of PAAwas then used as a template to interact with oppositely charged inorganic nanoparticles to construct periodic hybrid materials. Hybrid superstructures were created by immersing assembled one-dimensional structures into primary amine-coated gold nanoparticle aqueous suspension for several minutes. Dark stripes in Fig. 3, A and B, are due to the high electron density of gold nanoparticle–rich PAA regions. Lattice fringes of gold nanoparticle single crystals can be clearly seen in high-resolution TEM imaging (Fig. 3C). In high-angle annular dark-field (HAADF) imaging, the gold stripes are visualized as parallel bright lines (Fig. 3, D and E).

Fig. 3.

TEM images of directed gold nanoparticle assembly in the charged PAA region. (A and B) Bright-field images. Dark stripes are concentrated gold nanoparticle areas. Insert shows proposed structures. Yellow dots denote gold nanoparticles. (C) High-resolution TEM (HRTEM) imaging of lattice structure of gold single crystals. (D and E) high-angle annular dark field (HAADF) imaging of periodic gold stripes. Gold particles appear as bright stripes. (F) TEM image of periodic gold stripes when polyamine functionalized gold particles are used as counterions.

Another advantage of using charged corona blocks is that multivalent counterions, such as functionalized inorganic nanoparticles, can be used to influence local micelle structure and act as the stimulus for the formation of one-dimensional segmented nanostructures. This approach may be used in concert with, or as an alternative to, the addition of solvent. If the block copolymer is designed correctly, when spherical micelles come into contact with functionalized inorganic nano-particles, they should transform into disks that attractively collapse into segmented cylinders. This concept has been implemented with the addition of positively charged gold nanoparticles into a 50% THF/water suspension of PAA94-b-PMA103-b-PS130 spherical micelles, assembled without added diamine. Each gold nanoparticle had, on average, six primary amine groups on the surface and functioned as a multivalent counterion to complex with PAA. When they were added to the block copolymer spheres, the spheres collapsed into one-dimensional structures with alternating stripes of gold nano-particle–rich layers and hydrophobic layers, both perpendicular to the primary axis of the assembly. The distance between PAA gold-laden stripes was then 40 nm, as compared to approximately 20 nm between PAA layers for the sample in Fig. 1, B and C. This increased inter-layer spacing was due to the increase of the PS block length to 130 from 44 monomer repeat units. Apparently, the structures shown in Fig. 3F were not a consequence of pure one-dimensional growth of spherical micelles, because the diameter of the final cylindrical structures is about twice as large as the original spherical micelles. However, the addition of charged gold nanoparticles was able to influence the assembly of spheres in a preferred direction. The multivalent functionalized nanoparticle can be chosen independently, and the spacing between inorganic-rich layers of the final one-dimensional assembly can be tuned by choosing different relative polymer block lengths.

By taking advantage of slow kinetics of block copolymer chains in solution and the complexation of charged blocks with multivalent counterions, one can also produce complex micelles containing multiple hydrophobic blocks within the same micelle core that can undergo local, intramicellar phase separation. To obtain a single polymeric micelle geometry, such as cylinders, with each micelle core constituted by multiple hydrophobic blocks, at least two different, linear triblock copolymers are required, with similar overall molecular weight and relative block ratios but different core block chemistry. The key point for choosing the different chemistries of the two hydrophobic blocks is that the two blocks experience a high degree of mutual immiscibility. In the current experiment, polystyrene (PS) and poly(2,3,4,5,6-pentafluorostyrene) (PPFS) were employed as the different, third hydrophobic blocks in the two triblock copolymers (PAA94-b-PMA103-b-PS117 and PAA93-b-PMA99-b-PPFS100) (29). Equal molar amounts of the two triblock copolymers with different respective third blocks were dissolved in pure THF. EDDA was then added to reach a final 1:1 molar ratio of amine groups to acid groups. The diamines underwent complexation with the PAA blocks, thereby forming aggregates with PAA-diamine cores. Notably, these aggregates contained each of the triblock copolymers with both PS and PPFS hydrophobic blocks because of the simple trapping of unlike hydrophobic blocks in the same aggregate by PAA-diamine complexation. Next, introduction of water into the THF solution to a final ratio of THF:water = 1:2 provided for the formation of cylindrical micelles. However, the existence of the original mixed triblock copolymer aggregates, as a result of PAA and diamine complexation, forced the local co-assembly of unlike third hydrophobic blocks into the same micelle core. In addition, the lack of chain exchange in solution that disallows global chain migration and maintains nonequilibrated micelle structures, combined with the fact that the PAA chains in the corona of the newly formed micelles were still complexed with diamines and were not freely mobile within the micelle, guarantee the stability of the mixed-core micelle. The immiscibility of the two different hydrophobic blocks, PS and PPFS, eventually resulted in internal phase separation on the nanoscale, producing multicompartment micelles. The images shown in Fig. 4, A to D were taken after 4 days of aging a solution of mixed hydrophobic core cylinders. Internal phase separation is clearly indicated by the strong undulations along the cylinder surfaces and the TEM contrast variation along the cylinders. The larger, darker, and more spherical regions within the cylinders are hypothesized to be regions that are concentrated in PAA94-b-PMA103-b-PPFS100 triblock copolymer. First, there is a higher interfacial energy between PPFS and PMA, relative to PS and PMA, causing more chain stretching within PPFS-rich core domains so as to limit PPFS interactions with surrounding PMA blocks. Second, the greater electron density of the PPFS block provides a greater ability to scatter electrons and produce darker images in the TEM. The thinner region of the undulating cylinder would then be occupied primarily by PAA93-b-PMA99-b-PS117 (Fig. 4G). This internal cylinder phase separation only occurred at relatively higher amounts of water in the mixed solvent solutions. Cryo-TEM showed uniform cylinders without undulation on the surface at only 40% water/THF solution after 4 days (Fig. 4E). However, multicompartment cylinders could be observed as the water percentage increased to 67% (Fig. 4F). Reports in the literature have shown similar undulating cylinder morphologies through polymer blending, but with, at most, only three periods of undulation that always started from semispherical end caps (21). Clearly, the undulations shown here are not exclusively correlated with the spherical end caps and are obvious throughout the length of the cylinders. Safran et al. have demonstrated that the curvature energy of a cylinder with undulations could be lower than that of a nonundulating cylinder (31). However, the undulations observed here, although locally induced by unfavorable energetic interactions between PPFS and PS, are only possible kinetically because of the forced mixing of unlike hydrophobic core blocks as a result of PAA complexing with diamines and a specific solvent-mixing pathway.

Fig. 4.

Nanostructured multicompartment cylinders. (A and B) Bright-field TEM images. Dark regions present polypentafluorostyrene-chain rich area, (C and D) HAADF images of cylindrical micelles with internal phase-separated cores. (E) Cryogenic TEM (cryo-TEM) image of uniform cylindrical micelle at 40% water/THF solution. (F) Cryo-TEM image of cylindrical micelles with internal phase-separated cores at 67% water/THF solution. (G) Schematic illustration of formation of multicompartment cylinders.

Both the multicompartment cylinders with phase-separated cores and the cylindrical nano-structures with alternating layers of chemistry perpendicular to the cylinder axis are results of a solution assembly strategy to create structures with increased complexity with standard linear block copolymer architectures and chemistries. The key parameters are the combination of charged block interactions with multivalent counterions to influence both intra- and intermicellar interactions and solvent mixing to control the assembly pathways.

Supporting Online Material

www.sciencemag.org/cgi/content/full/317/5838/647/DC1

Materials and Methods

Figs. S1 to S4

Table S1

References

References and Notes

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