Block copolymer micelle formation was studied by a combination of fluorescent probe and quasi-elastic light scattering (QELS) techniques. The polymers, polystyrene-poly(ethy1ene oxide) diblock and triblock copolymers, with M. values ranging from 8500 to 29 000, form spherical micelles in water over the entire concentration range over which QELS signals can be detected. Pyrene (Py) in water (6 X IO-' M) partitions between the aqueous and micellar phases, accompanied by three changes in the pyrene spectroscopy. There is a red shift in the excitation spectrum, a change in the vibrational fine structure of Py fluorescence (11113 decreases from 1.9 to 1.2), and an increase in the fluorescence decay time (from 200 to ca. 350 ns) accompanying transfer of Py from an aqueous to a hydrophobic micellar environment. From these data, critical micelle concentrations (range: 1-5 mg/L) and partition coefficients (3 x lo6) can be calculated.Block copolymers of polystyrene (PSI and poly(ethy1ene oxide) (PEO) form spherical micelles in water when the length of soluble PEO is significantly longer than that of the insoluble PS portion of the m o l e~u l e .~~~ This behavior is common to both PS-PEO diblock and PEO-PS-PEO triblock copolymers. In analogy with low molecular weight surfactants, one defines the onset of intermolecular association as the critical micelle concentration (cmc), and the theories of polymer mi~ellization~ predict that in the presence of micelles, the concentration of free, unassociated block copolymers is close in magnitude to that of the cmc.There are relatively few studies devoted to determination of cmc values for block copolymer micelles. Scattering techniques, which are very powerful for determining the size and shape of the micelles, are able to detect the onset of association only if the cmc occurs in a concentration region where these techniques are sensitive. For block copolymers in water, this is often not the case. For the examples considered here, the cmc values lie well below the smallest concentrations detectable by either Rayleigh or quasi-elastic light scattering (QELS).Fluorescence techniques have been used with great success in the study of low molecular weight surfactant micelles.6 They are useful not only for cmc determination but also for measuring the aggregation number of the micelles. By comparison, aqueous block copolymer systems have received scant attention7 Several years ago Ikema et al. reported very interesting results using l-anilinonaphthalene fluorescence to probe micelle formation in a water-soluble block copolymer.'* Because the change in the fluorescence signal they observed occurred in a concentration region too small for a corresponding change to be observed by light scattering, the authors chose not to interpret this signal as an indication of the onset of polymer association. This absence of clear-cut results seems to have discouraged others from applying these methods. It isonlyrecently that there has been an outburst 0024-9297/91/2224-1033$02.50/0 of activity in the stu...
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.
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.
Abstract:Self-assembly of molecular and block copolymer amphiphiles represents a well-established route to micelles with a wide variety of shapes and gel-like phases. We demonstrate an analogous process, but on a longer lengthscale, in which amphiphilic P-H-P and H-P-H cylindrical triblock comicelles with hydrophobic (H) or polar (P) segments that are monodisperse in length are able to self-assemble side-byside or end-to-end in non-solvents for the central or terminal segments, respectively. This allows the formation of cylindrical supermicelles and 1D or 3D superstructures that persist in both solution and the solid state. These assemblies possess multiple levels of structural hierarchy in combination with existence on a multimicron length scale, features that are generally only found in natural materials.
Main Text:Amphiphiles such as molecular surfactants and block copolymers have been shown to form a rich variety of self-assembled nanoscopic structures, including spherical micelles, cylinders, nanotubes, bilayers, and vesicles as well as gel-like phases (1, 2). The construction of hierarchical colloidal materials on a longer length scale using spherical nanoparticles (3, 4), branched nanocrystals (5), nanorods (6), and nanocubes (7) has also recently been the subject of intense investigation. Control over the size, shape and composition of these nanoscopic building blocks has enabled the formation of superstructures with significant structural diversity (3, 7). Self-assembly of Janus and patchy nanoparticles formed by surface modification (8, 9) or from block copolymers (10), including diblock (11) and star (12) or linear triblock copolymers (13-15), has further broadened the range of superstructures that can be prepared.Nevertheless, despite these impressive recent advances, the use of anisotropic amphiphilic building blocks derived from soft-matter remains limited: examples include polymer-based (16) and polymer-metal hybrid nanorods (17, 18) and self-assembled nanotubes and cylinders (19,20). These approaches
Fluorescence probe experiments were carried out on aqueous solutions of urethane-coupled polyethylene oxide) polymers containing CieHasO end groups. These HEUR polymers associate in water, giving rise to a sharp increase in zero-shear viscosity with increasing concentration above 0.2-0.5 wt % polymer and a pronounced shear thinning at modest shear rates. At very low concentrations (a few ppm), the hydrophobic end groups of these polymers come together to form micelle-like structures. We are interested in the mechanism of the polymer association and in determining the number of hydrophobic groups Nr that come together to form the micellar core. Fluorescence decay studies of pyrene excimer formation give values of Nr close to 20, independent of polymer concentration. This Nr value is a factor of 3 smaller than that found for typical nonionic micelles but larger than that inferred indirectly from different measurements on similar HEUR polymer systems. Steady-state fluorescence studies of intramolecular excimer formation in bis(l-pyrenyl)methyl ether (dipyme) solubilized in these polymers indicate that the micellar core is much more rigid than that of traditional surfactant micelles, with an estimated "microviscosity" an order of magnitude larger than that of sodium dodecyl sulfate micelles. A model is developed to accommodate these observations. In this model, the polymers form rosette-like micelles comprised of looped chains. At higher concentrations, larger structures are formed from aggregation of these micelles, held together by chains which bridge the micelles. The influence of dilution and of shear is to induce a bridge-to-loop transition objects, micelles and smaller micelle aggregates.
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