Cylindrical Block Copolymer Micelles Research Articles

Scalable Preparation of Cylindrical Block Copolymer Micelles with a Liquid-Crystalline Perfluorinated Core by Photoinitiated Reversible Addition-Fragmentation Chain Transfer Dispersion Polymerization

One-dimensional block copolymer micelles have exhibited considerable promise in various fields. However, the large-scale preparation of one-dimensional block copolymer micelles is always challenging. Here, we report the preparation of cylindrical block copolymer micelles up to 30% w/w solids by photoinitiated reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of 2-(perfluorooctyl)ethyl methacrylate (FMA) in ethanol. Because of the liquid-crystalline ordering effect of the PFMA core-forming block at low temperatures, cylindrical micelles could be obtained over a wide range of degree of polymerization of PFMA. The length of cylindrical micelles could be roughly controlled by changing the molar mass of the macromolecular chain transfer agent (macro-CTA). Finally, the concept of seeded growth was successfully incorporated into photoinitiated RAFT dispersion polymerization, allowing the epitaxial growth of cylindrical micelles from seeds. We expect that this method will offer considerable opportunities for the large-scale preparation of well-defined cylindrical micelles that may find applications in some specific areas.

Seeded Photoinitiated Polymerization-Induced Self-Assembly: Cylindrical Micelles with Patchy Structures Prepared via the Chain Extension of a Third Block.

Cylindrical micelles formed by the self-assembly of block copolymers are of interest for a wide range of applications. In this study, aqueous seeded photoinitiated polymerization-induced self-assembly (photo-PISA) is developed for the preparation of cylindrical block copolymer micelles with patchy structures at high solids contents. Cross-linked cylindrical block copolymer micelles prepared by photo-PISA are used as seeds for further chain extension. Surface roughness of the patchy cylindrical block copolymer micelles can be controlled by varying degree of polymerization (DP) of the third block. The obtained patchy cylindrical micelles can be further functionalized via the modification of the third block. Due to the high solids content of patchy cylindrical micelles prepared by seeded photo-PISA (10% w/w or higher), we expect that this study will greatly expand the promise of PISA for the large-scale preparation of cylindrical micelles with well-defined structures.

Putting polymer micelles in their place

Nanomaterials Block-copolymer micelles generally have been used in solution, for applications such as drug delivery. Surface assembly methods that work with other nanosized objects that use electric or magnetic fields or shear tend to be inefficient with these materials. Gould et al. show that optical tweezers can be used to position cylindrical block copolymer micelles on a glass substrate. The micelles have a polyferrocenylsilane core with a high refractive index that enables optical manipulation. Total internal reflection fluorescence spectroscopy of a dye trapped in the poly(dimethylsiloxane) corona enabled positioning and orientation of the micelles on the surface. Arrays consisting of 100 objects were deposited in 10 minutes in a process that could be automated using dynamic holographic methods. ACS Nano 10.1021/acsnano.9b00342 (2019).

Manipulation and Deposition of Complex, Functional Block Copolymer Nanostructures Using Optical Tweezers.

Block copolymer self-assembly has enabled the creation of a range of solution-phase nanostructures with applications from optoelectronics and biomedicine to catalysis. However, to incorporate such materials into devices a method that facilitates their precise manipulation and deposition is desirable. Herein we describe how optical tweezers can be used to trap, manipulate, and pattern individual cylindrical micelles and larger hybrid micellar materials. Through the combination of TIRF imaging and optical trapping we can precisely control the three-dimensional motion of individual cylindrical block copolymer micelles in solution, enabling the creation of customizable arrays. We also demonstrate that dynamic holographic assembly enables the creation of ordered customizable arrays of complex hybrid block copolymer structures. By creating a program which automatically identifies, traps, and then deposits multiple assemblies simultaneously we have been able to dramatically speed up this normally slow process, enabling the fabrication of arrays of hybrid structures containing hundreds of assemblies in minutes rather than hours.

Microfibres and macroscopic films from the coordination-driven hierarchical self-assembly of cylindrical micelles.

Anisotropic nanoparticles prepared from block copolymers are of growing importance as building blocks for the creation of synthetic hierarchical materials. However, the assembly of these structural units is generally limited to the use of amphiphilic interactions. Here we report a simple, reversible coordination-driven hierarchical self-assembly strategy for the preparation of micron-scale fibres and macroscopic films based on monodisperse cylindrical block copolymer micelles. Coordination of Pd(0) metal centres to phosphine ligands immobilized within the soluble coronas of block copolymer micelles is found to induce intermicelle crosslinking, affording stable linear fibres comprised of micelle subunits in a staggered arrangement. The mean length of the fibres can be varied by altering the micelle concentration, reaction stoichiometry or aspect ratio of the micelle building blocks. Furthermore, the fibres aggregate on drying to form robust, self-supporting macroscopic micelle-based thin films with useful mechanical properties that are analogous to crosslinked polymer networks, but on a longer length scale.

An investigation into the hexagonal phases formed in high-concentration dispersions of well-defined cylindrical block copolymer micelles

ABSTRACTThis paper presents a detailed analysis of the structure of the hexagonal phase of poly(ferrocenylsilane) (PFS)-based cylindrical micelles found at concentrations above ca. 5 wt. % in non-polar solvents such as decane. Small-angle X-ray scattering indicated that the hexagonal order is not long-range. In all samples, deviations in the lower order peak positions were observed with respect to those expected for a perfect hexagonal lattice, with the degree of deviation correlating with micelle length. Furthermore, analysis of the peak shapes and peak widths suggests that the phase possesses intermediate translational order similar. to the hexatic phase. The observed features can be reproduced by amending Hosemann’s paracrystal theory to include a distribution of lattice parameters to model well and poorly condensed regions. It is proposed that this distribution arises due to the bending and intertwining of individual micelles in a hexagonal lattice, resulting in a kinetically trapped phase that is initially neither perfectly hexagonal nor canonically hexatic but which anneals over time towards a perfect hexagonal lattice.

Monodisperse Cylindrical Micelles and Block Comicelles of Controlled Length in Aqueous Media.

Cylindrical block copolymer micelles have shown considerable promise in various fields of biomedical research. However, unlike spherical micelles and vesicles, control over their dimensions in biologically relevant solvents has posed a key challenge that potentially limits in depth studies and their optimization for applications. Here, we report the preparation of cylindrical micelles of length in the wide range of 70 nm to 1.10 μm in aqueous media with narrow length distributions (length polydispersities <1.10). In our approach, an amphiphilic linear-brush block copolymer, with high potential for functionalization, was synthesized based on poly(ferrocenyldimethylsilane)-b-poly(allyl glycidyl ether) (PFS-b-PAGE) decorated with triethylene glycol (TEG), abbreviated as PFS-b-(PEO-g-TEG). PFS-b-(PEO-g-TEG) cylindrical micelles of controlled length with low polydispersities were prepared in N,N-dimethylformamide using small seed initiators via living crystallization-driven self-assembly. Successful dispersion of these micelles into aqueous media was achieved by dialysis against deionized water. Furthermore, B-A-B amphiphilic triblock comicelles with PFS-b-poly(2-vinylpyridine) (P2VP) as hydrophobic "B" blocks and hydrophilic PFS-b-(PEO-g-TEG) "A" segments were prepared and their hierarchical self-assembly in aqueous media studied. It was found that superstructures formed are dependent on the length of the hydrophobic blocks. Quaternization of P2VP was shown to cause the disassembly of the superstructures, resulting in the first examples of water-soluble cylindrical multiblock comicelles. We also demonstrate the ability of the triblock comicelles with quaternized terminal segments to complex DNA and, thus, to potentially function as gene vectors.

Formation of Periodically Arranged Nanobubbles in Mesopores: Capillary Bridge Formation and Cavitation during Sorption and Solidification in an Hierarchical Porous SBA-15 Matrix.

We report synchrotron-based small-angle X-ray scattering experiments on a template-grown porous silica matrix (Santa Barbara Amorphous-15) upon in situ sorption of fluorinated pentane C5F12 along with volumetric gas sorption isotherm measurements. Within the mean-field model of Saam and Cole for vapor condensation in cylindrical pores, a nitrogen and C5F12 sorption isotherm is well described by a bimodal pore radius distribution dominated by meso- and micropores with 3.4 and 1.6 nm mean radius, respectively. In the scattering experiments, two different periodicities become evident. One of them (d1 = 11.5 nm) reflects the next nearest neighbor distance in a 2D-hexagonal lattice of tubular mesopores. A second periodicity (d2 = 11.4 nm) found during in situ sorption and freezing experiments is traced back to a superstructure along the cylindrical mesopores. It is compatible with periodic pore corrugations found in electron tomograms of empty SBA-15 by Gommes et al. ( Chem. Mater. 2009, 21, 1311 - 1317). A Rayleigh-Plateau instability occurring at the cylindrical blockcopolymer micelles characteristic of the SBA-15 templating process quantitatively accounts for the superstructure and thus the spatial periodicity of the pore wall corrugation. The consequences of this peculiar morphological feature on the spatial arrangement of C5F12, in particular the formation of periodically arranged nanobubbles (or voids) upon adsorption, desorption, and freezing of liquids, are discussed in terms of capillary bridge formation and cavitation in tubular but periodically corrugated pores.

Conductive, Monodisperse Polyaniline Nanofibers of Controlled Length Using Well‐Defined Cylindrical Block Copolymer Micelles as Templates

Stable colloidal dispersions of polyaniline (PAni) nanofibers with controlled lengths from about 200 nm-1.1 μm and narrow length distributions (Lw/Ln < 1.04; Lw = weight average micelle length, Ln = number average micelle length) were prepared through the template-directed synthesis of PAni using monodisperse, solution-self-assembled, cylindrical, block copolymer micelles as nanoscale templates. These micelles were prepared through a crystallization-driven living self-assembly method from a poly(ferrocenyldimethylsilane)-b-poly(2-vinylpyridine) block copolymer (PFS25 -b-P2VP425). This material was initially self-assembled in iPrOH to form cylindrical micelles with a crystalline PFS core and a P2VP corona and lengths of up to several micrometers. Sonication of this sample then yielded short cylinders with average lengths of 90 nm and a broad length distribution (Lw/Ln = 1.32). Cylindrical micelles of PFS25 -b-P2VP425 with controlled lengths and narrow length distributions (Lw/Ln < 1.04) were subsequently prepared using thermal treatment at specific temperatures between 83.5 and 92.0 °C using a 1D self-seeding process. These samples were then employed in the template-directed synthesis of PAni nanofibers through a two-step procedure, where the micellar template was initially stabilised by deposition of an oligoaniline coating followed by addition of a polymeric acid dopant, resulting in PAni nanofibers in the emeraldine salt (ES) state. The ES-PAni nanofibers were shown to be conductive by scanning conductance microscopy, whereas the precursor PFS25-b-P2VP425 micelle templates were found to be dielectric in character.

Reversible Cross-Linking of Polyisoprene Coronas in Micelles, Block Comicelles, and Hierarchical Micelle Architectures Using Pt(0)–Olefin Coordination

Previous work has established that polyisoprene (PI) coronas in cylindrical block copolymer micelles with a poly(ferrocenyldimethylsilane) (PFS) core can be irreversibly cross-linked by hydrosilylation using (HSiMe(2))(2)O in the presence of Karstedt's catalyst. We now show that treatment of cylindrical PI-b-PFS micelles with Karstedt's catalyst alone, in the absence of any silanes, leads to PI coronal cross-linking through Pt(0)-olefin coordination. The cross-linking can be reversed through the addition of 2-bis(diphenylphosphino)ethane (dppe), a strong bidentate ligand, which removes the platinum from the PI to form Pt(dppe)(2). The Pt(0) cross-linking of PI was studied with self-assembled cylindrical PI-b-PFS block copolymer micelles, where the cross-linking was found to dramatically increase the stability of the micellar structures. The Pt(0)-alkene coordination-induced cross-linking can be used to provide transmission electron microscopy contrast between PI and poly(dimethylsiloxane) (PDMS) corona domains in block comicelles as the process selectively increases the electron density of the PI regions. Moreover, following the assembly of a hierarchical scarf-shaped comicelle consisting of a PFS-b-PDMS platelet template with PI-b-PFS tassels, Pt(0)-induced cross-linking of the PI coronal regions allowed for the selective removal of the PFS-b-PDMS center, leaving behind an unprecedented hollowed-out scarf structure. The addition of Karstedt's catalyst to PI or polybutadiene homopolymer toluene/xylene solutions resulted in the formation of polymer gels which underwent de-gelation upon the addition of dppe.

End-to-End Coupling and Network Formation Behavior of Cylindrical Block Copolymer Micelles with a Crystalline Polyferrocenylsilane Core

Cylindrical block copolymer micelles with a crystalline poly(ferrocenyldimethylsilane) (PFDMS) core and a long corona-forming block are known to elongate through an epitaxial growth mechanism on addition of further PFDMS block copolymer unimers. We now report that addition of the semicrystalline homopolymer PFDMS(28) to monodisperse short (ca. 200 nm), cylindrical seed micelles of PFDMS block copolymers results in the formation of aggregated structures by end-to-end coupling to form micelle networks. The resulting aggregates were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). In some cases, a core-thickening effect was also observed where the added homopolymer appeared to deposit and crystallize at the core-corona interface, which resulted in an increase of the width of the micelles within the networks. No evidence for aggregation was detected when the amorphous homopolymer poly(ferrocenylethylmethylsilane) (PFEMS(25)) was added to the cylindrical seed micelles whereas similar behavior to PFDMS(28) was noted for semicrystalline polyferrocenyldimethylgermane (PFDMG(30)). This suggested that the crystallinity of the added homopolymer is critical for subsequent end-to-end coupling and network formation to occur. We also explored the tendency of the cylindrical seed micelles to form aggregates by the addition of PI-b-PFDMS (PI = polyisoprene) block copolymers (block ratios 6:1, 3.8:1, 2:1, or 1:1), and striking differences were noted. The results ranged from typical micelle elongation, as reported in previous work, at high corona to core-forming block ratios (PI-b-PFDMS; 6:1) to predominantly end-to-end coupling at lower ratios (PI-b-PFDMS; 2:1, 1:1) to form long, essentially linear structures. The latter process, especially for the 2:1 block copolymer, led to much more controlled aggregate formation compared with that observed on addition of homopolymers.

Monodisperse cylindrical micelles by crystallization-driven living self-assembly

Non-spherical nanostructures derived from soft matter and with uniform size-that is, monodisperse materials-are of particular utility and interest, but are very rare outside the biological domain. We report the controlled formation of highly monodisperse cylindrical block copolymer micelles (length dispersity < or = 1.03; length range, approximately 200 nm to 2 microm) by the use of very small (approximately 20 nm) uniform crystallite seeds that serve as initiators for the crystallization-driven living self-assembly of added block-copolymer unimers with a crystallizable, core-forming metalloblock. This process is analogous to the use of small initiator molecules in classical living polymerization reactions. The length of the nanocylinders could be precisely controlled by variation of the unimer-to-crystallite seed ratio. Samples of the highly monodisperse nanocylinders of different lengths that are accessible using this approach have been shown to exhibit distinct liquid-crystalline alignment behaviour.

Nanopore Extrusion-Induced Transition from Spherical to Cylindrical Block Copolymer Micelles

Cylindrical micelles made of soluble and insoluble segments have found many interesting applications, for example, in templating the formation of inorganic nanocrystal arrays, controlled drug delivery, and bionanotechnology. Conventional methods for the preparation of cylindrical micelles are based on the judicious design of the molecular structures of the soluble and insoluble segments and the manipulation of the external parameters, such as the solvent, temperature, and guest ions/molecules. Here we report on the production of cylindrical micelles by extruding spherical micelles through nanoscale pores. The obtained cylindrical micelles and their transformation back into the spherical micelles are characterized using transmission electron microscopy imaging and dynamic laser light scattering. The extrusion process is continuous and simple and can be upgraded to large scales. We believe that our nanopore extrusion-based preparation method will provide enormous opportunities for the applications of block copolymer cylindrical micelles in nanotechnology and medicine.

Effect of Added Homopolymer on the Hexagonal Phase Formed by Cylindrical Block Copolymer Micelles in a Selective Solvent

The effect of addition of A homopolymer chains to HEX phases formed by cylindrical micelles obtained by dissolving an AB diblock copolymers in an A selective solvent was investigated. The following general trends were observed: (i) order-order transitions from HEX → Gyroid (G) → Lamellae (LAM); (ii) the inter-cylinder spacing of the HEX phase increased; (iii) when the homopolymer molecular weight and concentration was high, macrophase separation of the system occurred. The A homopolymer chains do not mix significantly with the A domain of the diblock due to excluded volume interactions, causing the corona chain brush to contract. As a result, order-order transitions that reduce the interfacial curvature are observed.

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

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 copolymer micelle nanolithography

Au-nanoclusters between 2 and 8 nm in diameter were deposited onto solid substratesin different pattern geometries. The basis of this approach is the self-assembly ofpolystyrene-b-poly[2-vinylpyridine(HAuCl4)]diblock copolymer micelles into uniform monomicellar films onsolid supports such as Si-wafers or glass cover slips. HAuCl4as metallic precursor or a single solid Au-nanoparticle caused by reduction of theprecursor are embedded in the centre of diblock copolymer micelles. Subsequenthydrogen, oxygen or argon gas plasma treatment of the dry film causes depositionof Au-nanoparticles onto the substrate by entire removal of the polymer. TheAu-dot patterns resemble the micellar patterns before the plasma treatment.Separation distances between the dots is controlled by the molecular weight of thediblock copolymers. The limitation of the separation distance between individualdots or the pattern geometry is overcome by combining self-assembly ofdiblock copolymer micelles with pre-structures formed by photo or e-beamlithography. Capillary forces of a retracting liquid film due to solventevaporation on the pre-structured substrate push micelles in the corners of thesedefined topographies. A more direct process is demonstrated by applyingmonomicellar films as negative e-beam resist. Micelles that are irradiated byelectrons are chemically modified and can hardly be dissolved from thesubstrate while non-exposed micelles can be lifted-off by suitable solvents.This process is also feasible on electrical isolating substrates such as glasscover slips if the monomicellar film is coated in addition with a 5 nm thickconductive layer of carbon before e-beam treatment. The applicationof cylindrical block copolymer micelles also allows for the formation of4 nm wide lines which can be 1–50 µmlong and also be organized in defined aperiodic structures.