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Cylindrical Block Copolymer Micelles and Co-Micelles of Controlled Length and Architecture

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Science  03 Aug 2007:
Vol. 317, Issue 5838, pp. 644-647
DOI: 10.1126/science.1141382

Abstract

Block copolymers consist of two or more chemically different polymers connected by covalent linkages. In solution, repulsion between the blocks leads to a variety of morphologies, which are thermodynamically driven. Polyferrocenyldimethylsilane block copolymers show an unusual propensity to forming cylindrical micelles in solution. We found that the micelle structure grows epitaxially through the addition of more polymer, producing micelles with a narrow size dispersity, in a process analogous to the growth of living polymer. By adding a different block copolymer, we could form co-micelles. We were also able to selectively functionalize different parts of the micelle. Potential applications for these materials include their use in lithographic etch resists, in redox-active templates, and as catalytically active metal nanoparticle precursors.

Block copolymers, which consist of two or more different polymers covalently linked together, form a range of structures when placed in a selective solvent. These assemblies include micelles with a solvent-insoluble core and a solvent-swollen corona (1). The shapes and sizes of the structures that form are typically determined by the lengths of the polymer blocks, their affinity for each other and for the solvent, and temperature (2). Long micelles with a narrow diameter are particularly useful (3). For example, cylindrical aggregates of a peptide-amphiphile have been used as a fibrous scaffold for the mineralization of hydroxyapatite (4). Polyethylene oxide–poly(ethylene-alt-propylene) (PEO-PEP) cylinders incorporated into epoxy resins greatly enhance the toughness of the resins (5). Cylindrical micelles with a polyferrocenylsilane (PFS) core and a cross-linked corona have been used as a template for creating linear arrays of silver nanoparticles (6). Whereas spherical block copolymer micelles are able to carry dyes and other drug analogs into cells (7), cylindrical micelles can orient and stretch in a flowing stream in a manner that is ideal for flow-intensive drug delivery applications (8, 9). Thus, cylindrical micelles are interesting in part because of their potential applications in nanotechnology and medicine.

Block copolymer micelles are normally formed under conditions close to thermodynamic control and are studied in a kinetically frozen state (2, 10). Most diblock copolymers form spherical micelles, whereas only a narrow range of compositions are able to form cylindrical micelles (2). In contrast, PFS block copolymers such as PFS-polyisoprene (PFS-PI), PFS-polydimethylsiloxane (PFS-PDMS), and PFS-polymethylvinylsiloxane (PFS-PMVS) form rod-like micelles for a broad range of block ratios. This process occurs in a variety of solvents that are selective for the complementary block and yields structures with a cylindrical organometallic core (1114). We previously provided evidence that the crystalline nature of PFS is a key feature that promotes the formation of cylinders for such a broad range of compositions (14). Here, we show that these micelles can undergo chain extension in ways that recall key features of living polymerization to form structures of controlled segment length and composition.

Living polymerization refers to a diverse class of systems in which molecules or other microscopic units form long chains by continuous addition to the reactive chain ends, without termination or transfer steps (15). Examples include the synthesis of linear macromolecules (16) and the growth of actin and microtubule filaments through self-assembly of the appropriate proteins (17, 18). The defining feature of this type of reaction is that after all the monomer units in solution have been consumed, the chain ends remain active and will continue to react once additional monomer is added. The chain length increases linearly with the amount of added monomer, and frequently the resulting product has a narrow contour length distribution (CLD). If the added monomer is different from the one initially polymerized, one obtains a block copolymer consisting of chemically different segments sharing a common junction. This is the method used to synthesize the PFS block copolymers examined here.

We begin by considering cylindrical micelles formed by a PFS53-PI320 diblock copolymer in hexane (the subscripts refer to the number-averaged degree of polymerization). [All of the polymers considered here have a narrow molar mass distribution (19) (table S1).] Undisturbed, the micelle structures were stable over a period of months. However, when aliquots of additional polymer dissolved in a common good solvent such as tetrahydrofuran (THF) or toluene were added, the micelles, by transmission electron microscopy (TEM), appeared to become longer (fig. S1). To enable us to follow changes in length distribution, we sonicated the solution to form shorter micelles with a mean length of ∼250 nm (Fig. 1A). Addition of more polymer dissolved in THF led to an increase in the apparent hydrodynamic radius Embedded Image as monitoredbydynamic light scattering (DLS) and by an increase in micelle length, as seen in Fig. 1. Two noteworthy features of these structures are their rigidity and their relatively narrow length distribution. The mean micelle length increased from 250 nm to ∼500, 750, and 2000 nm (Fig. 1, B to D, respectively), roughly proportional to the amount of PFS53-PI320 added. Simply adding 1 mg of PFS53-PI320 in 0.1 ml of THF to 1 ml of hexane, comparable to the amount of polymer present in the solution in Fig. 1B, gave a different result. Large aggregates of ill-defined shape could be seen by TEM. We infer that preexisting micelles are necessary initiators for controlled micelle growth.

Fig. 1.

TEM images for (A) sonicated PFS53-PI320 micelles in hexane (0.5 mg/ml); (B to D) elongated micelles after adding 0.5 mg (B), 1 mg (C), and 2 mg (D) of PFS53-PI320 in THF (0.1 ml) to 1.0-ml solutions of (A). Scale bars, 500 nm.

To test this idea, we examined micelles of a different polymer sample of similar composition (PFS48-PI264) in decane, using static and dynamic light scattering (SLS and DLS) experiments as well as TEM measurements. Direct formation of the micelles by heating and then cooling the solution led to long cylinders (>2 μm), which would make the study of their growth by TEM and light scattering difficult. After sonicating the solution, the micelles fragmented to give shorter structures. Sonication and dilution yielded rod-like micelles with a mean length (Lw) of 98 nm, an aggregation number of 310 molecules per micelle, an apparent radius of gyration Embedded Image = 38.3 nm (fig. S2), and a hydrodynamic radius Rh = 32.8 nm. For micelle growth experiments, approximately equal aliquots of the sonicated micelles (∼1.0 ml, 0.050 mg/ml; table S2) were transferred into six tared vials, and then five increasing amounts (samples B to F) of a solution of PFS48-PI264 in THF (2.316 mg/ml) were added. As a control (sample A), a similar amount of THF alone was added to one of the vials.

The cells were covered to prevent dust from entering the solutions, and the THF was allowed to evaporate as the solutions were aged at 21°C for 7 days. After the initial SLS and DLS experiments, the samples were carefully sealed and stored in the dark for an additional 20 weeks to test their long-term stability. The TEM and SLS data in Fig. 2 are taken from these samples aged for 5 months.

Fig. 2.

Properties of sonicated PFS48-PI264 micelles in n-decane (0.05 mg/ml). (A) Contour length distribution (CLD) obtained from TEM images after treatment with 0.03 ml of THF (sample A). (B) CLD for sample E. The lines correspond to the expression F(L)= L exp(–2L/Ln). (C) Plots of qRθM0Kc versus q for sample A, and after addition of 0.01, 0.02, 0.04, 0.07, and 0.12 ml of PFS48-PI264 in THF (2.316 mg/ml) (samples B to F, respectively). The lines represent the best fit to equations S3 and S4 (19) for thin, rigid rods with a Zimm-Schulz distribution of lengths. (D) Number-average length Ln versus total polymer concentration for the solutions in (C) deduced from TEM-CLD (♦) and SLS (▢) experiments. The dashed line is the predicted value for monodisperse rods assuming uniform growth of existing micelles as a function of polymer concentration. The samples were aged for 5 months before the SLS and TEM measurements.

The CLD of sample A is shown in Fig. 2A. The number-average length (Embedded Image = 105 nm) is consistent with that of the initial sonicated sample. The CLD of sample E is shown in Fig. 2B. Here the value of LTEMn has shifted to 510 nm with a Lw/Ln ratio of 1.6, similar to its value of 1.4 for sample A. In Fig. 2C we present Casassa-Holtzer plots of qRθM0Kc (where q is the scattering vector, Rθ is the Rayleigh ratio, M0 is the copolymer weight-average molecular weight, K is the optical constant, and c is polymer concentration), plotted as a function of q, for the six samples. The solid lines correspond to the best fit of the data to the form factor for thin rigid rods [(19), equations S3 and S4]. These plots highlight the presence of elongated structures in the solution, because such objects exhibit a plateau at high q. The magnitudes of the plateau values, from which the number of polymer molecules per unit length (Nag/L) can be calculated, were obtained as a fitting parameter for all of the samples, but the presence of the plateau is clearly evident in the two uppermost curves. Note that the samples approach their plateau values at lower q values as the amount of polymer added is increased. This behavior is expected for solutions containing rods of increasing length, and is mirrored in the upward curvature at low q for the longer micelles seen in the plots of Embedded Image as a function of q (fig. S3B). All samples are characterized by similar values of Nag/L ≈ 3 molecules/nm, and this property remained constant as the samples were subjected to long-term aging.

Values of Ln are plotted against c in Fig. 2D. These data exhibit three remarkable features. The first is the overall finding of these experiments that addition of polymer in THF solution leads to the growth of existing micelles while preserving the internal structure, as reflected in the essentially constant value of Nag/L. Values of Ln increase linearly with c, as one would expect for a living polymerization. The second feature is the concordance between Ln values determined by light scattering and by contour length analysis of the TEM images, with slightly larger values obtained by TEM (table S4). Finally, we note the long-term stability of the micelles (fig. S3). Even on a time scale of months, there is little evidence for unimer dissociation and new micelle formation, nor is there any indication of micelle fusion in dilute solution. The absence of micelle fusion suggests that the micelle growth happens via the addition of free chains on the preexisting micelle, rather than by “self-micellization” of the free chains followed by addition to the ends of preexisting micelles. It is interesting that most of the distributions (Fig. 2, A and B, and fig. S4) can be well fitted with the Zimm-Schulz expression F (L)= L exp(–2L/Ln). This is also the expression developed by Israelachvili (20) for the CLD of one-dimensional micelles formed under equilibrium conditions, but we note an important difference: Our finding that Ln increases linearly with concentration is different from the square-root dependence predicted by that model.

Another important characteristic of a living polymerization is that the chain ends will react with the addition of a different monomer. Starting with PFS53-PI320 cylinders in decane-THF mixtures, we prepared short rod-like seeds of PFS53-PI320 micelles in decane (Ln ≈ 200 nm, 0.5 mg/ml) by sonication. To a 1.0-ml aliquot of this solution, we added 0.1 ml of THF containing 1.0 mg of PFS48-PMVS300 and monitored the solution by DLS. The apparent Rh of the aggregates increased gradually until reaching a constant value. A TEM image of these micelles (Fig. 3A) shows that the addition of the second block copolymer did not disrupt the original shape of the cylinders, but their length increased to ∼1000 nm, which suggests that block co-micelles were formed.

Fig. 3.

(A and B) Bright-field (A) and dark-field (B) TEM images of triblock co-micelles M(PFS48-PMVS300)-b-M(PFS53-PI320)-b-M(PFS48-PMVS300) formed by adding PFS48-PMVS300 (1.0 mg in 0.1 ml of THF) to a solution (0.5 mg/ml) of sonicated PFS53-PI320 micelles in decane. (C) A TEM image of sample prepared in hexane/THF (10:1 v/v) of M(PFS40-PDMS330)-b-M(PFS53-PI320)-b-M(PFS40-PDMS330) chains by triblock co-micelles. (D) The PI corona were then cross-linked (XL) reaction with tetramethyldisiloxane and the sample was transferred to THF solution, which dissolved the PFS-PDMS chains but left the cross-linked Embedded Image structures intact.

A clearer indication of the micelle structure is provided by the dark-field TEM image in Fig. 3B. In a dark-field image, components containing heavy elements scatter more electrons and appear bright against the dark carbon film. The structures in Fig. 3B are characterized by a thin central line flanked by two thicker segments. Thethinlines can be attributed to the PFS cores of a central PFS53-PI320 block, whereas the thicker segments arise from the additional electron scattering of the silicon-rich PMVS corona that surrounds a PFS core. By TEM, we could not observe any micelles containing only PFS-PMVS copolymer, which indicates that micelle growth starts from both ends of the PFS53-PI320 micelle and leads to a triblock architecture, which we denote M(PFS48-PMVS300)-b-M(PFS53-PI320)-b-M(PFS48-PMVS300) (where M denotes micelle, b denotes block structure, and the sub-scripts are the components of each block).

The susceptibility of the PI chains of PFS-PI micelles to cross-linking provides another proof of the ability of the system to form triblock co-micelles. By adding a THF solution of PFS40-PDMS330 to a solution of PFS53-PI320 micelles in hexane, we produced co-micelles of M(PFS40-PDMS330)-b-M(PFS53-PI320)-b-M(PFS40-PDMS330) (Fig. 3C). The middle block of the co-micelles contains vinyl groups in the PI corona, whereas the PDMS corona chains of the two side blocks have no reactive functionality. We have shown that the vinyl groups of PI or PMVS in the corona of PFS micelles can be cross-linked using Pt(0)-catalyzed hydrosilylation (13). Using the same chemistry, with tetramethyldisiloxane as the cross-linking agent, we were able to cross-link the vinyl-containing PI blocks of the triblock co-micelles and fix the structure of the center block of the aggregates. When this sample was dried and then exposed to THF, the soluble PDMS chains dissolved, and only the cross-linked (Embedded Image) central block remained (Fig. 3D).

These findings show that the micelles in this hexane-THF mixture are sufficiently robust to support the hydrosilylation reaction to cross-link the PI corona chains. We also discovered that the semicrystalline PFS core of the micelle itself is accommodating to PFS chains of somewhat different length. The number-average degree of polymerization of the PFS portion of the polymers (PFS40-PDMS330) that grow off the ends of the PFS53-PI320 micelles is about 80% of that of the chains in the preformed micelles.

Micelle growth cannot occur, however, if the ferrocenylsilane block has a different chemical structure that prevents crystallization. Poly(ferrocenylmethylethylsilane) (PFMES) is an amorphous polymer, and PFMES-PMVS forms spherical micelles in hexane. When a THF solution of PFMES-PMVS is added to a hexane solution of sonicated PFS53-PI320 micelles, the product appears by TEM to be a mixture of spherical micelles and unchanged PFS53-PI320 micelles (fig. S6).

Figure 4 summarizes the nucleation and growth steps involved in the self-assembly of PFS-PI diblock copolymers in alkane solvents. The free chains formed upon heating the initial sample undergo homogeneous nucleation upon cooling and aging to form a discrete number of nuclei that serve as initiation sites for chain growth. Free chains deposit on the ends of the rod-like structures, accompanied or driven by epitaxial crystallization of the PFS blocks. Once this process is complete, growth stops. When more polymer, dissolved in a common good solventsuchas THF, is added to the solution, “chain growth” is reinitiated, leading to longer rod-like structures or triblock co-micelles.

Fig. 4.

A mechanism for the self-assembly of PFS-PI block copolymers in alkane solvents. Homogeneous nucleation followed by epitaxial growth yields cylindrical micelles with a semicrystalline core and a relatively narrow distribution of lengths. The rod ends remain active to further growth if additional polymer containing a PFS block is added to the system. As shown, the second polymer and PFS-PI have different soluble blocks.

The mechanism for block copolymer self-assembly in solution demonstrated here enables the production of rods with a narrow distribution of lengths, as well as the synthesis of segmented rod-like structures with control over segment length and composition. The blocks containing vinyl groups in their corona can be selectively cross-linked. Such cylindrical structures with controlled lengths and compositions are of interest as etch resists, reactive redox-active templates, conductive wires, and patternable precursors to magnetic and catalytic Fe nanoparticles.

Supporting Online Material

www.sciencemag.org/cgi/content/full/317/5838/644/DC1

Materials and Methods

Figs. S1 to S6

Tables S1 to S4

References and Notes

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