Cylindrical micelles of controlled length with a π-conjugated polythiophene core via crystallization-driven self-assembly

Solution self-assembly of the regioregular polythiophene-based block copolymer poly(3-hexylthiophene)-b-poly(dimethylsiloxane) yields cylindrical micelles with a crystalline P3HT core. Monodisperse nanocylinders of controlled length have been prepared via crystallization-driven self-assembly using seed micelles as initiators.     

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.     

Systematic study of the fluorescence decays of amino‐coumarin dyes in polymer matrices

We measured the fluorescence decays of seven different amino‐coumarin dyes in polymer films of poly(methyl methacrylate) (PMMA), poly(styrene) (PS), and ethylene‐butene rubber (EBR); as well as in the small molecule analogs ethyl acetate and toluene. Many of the dye‐solvent and dye‐polymer combinations exhibited single exponential decays with lifetimes ranging from 2.3 to 3.9 ns. Small deviations from single exponential behavior occurred for most of the dyes in EBR. Significant deviations from single exponential behavior occurred for 7‐(diethylamino)‐2‐oxo‐2H‐1‐benzopyran‐3‐carboxylic acid (coumarin‐3) in ethyl acetate and in all polymer matrices and 2,3,6,7‐tetrahydro‐11‐oxo‐1H,5H,11H‐[1]benzopyrano[6,7,8‐ij]quinolizin‐10‐carboxylic acid (coumarin‐343) in all of the polymer matrices. Time‐resolved fluorescence spectra indicated the presence of two different excited states for coumarin‐3 and coumarin‐343 in PMMA; these spectra were qualitatively different from the time‐resolved spectra of coumarin‐3 in ethyl acetate. We rationalize these results in terms of the chemical functionalities of the various dyes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2333–2343, 2007     

Poly(vinyl acetate‐co‐dibutyl maleate) latex films in the presence of grafted and post‐added poly(vinyl alcohol)

We describe the synthesis and characterization of a series of poly(vinyl acetate‐co‐dibutyl maleate) [P(VAc‐DBM)] latex particles (monomer molar ratio 10.6:1). One set of samples [high‐M and M_250k SDS‐P(VAc‐DBM), gel content 50% and 0%] was prepared in the presence of an anionic surfactant sodium dodecyl sulfate. The other two sets of samples [high‐M and M_250k PVOH–P(VAc‐DBM)] were prepared in the presence of poly(vinyl alcohol) (PVOH). These polymers differ in gel content (50 and 0%) and the extent of PVOH grafting (30 and 15%). Polymer diffusion across cell boundaries in the latex films was monitored by fluorescence resonant energy transfer (ET) experiments. First, we examined M_250k samples in the presence of grafted and post‐added PVOH. The presence of post‐added PVOH (5%) causes a small but detectable retardation on the rate of polymer diffusion, whereas the presence of grafted PVOH (degree of grafting: 15%) significantly promotes the polymer diffusion rate. For the high‐M P(VAc‐DBM), the presence of post‐added PVOH also retards the polymer diffusion. Strikingly, the presence of grafted PVOH (degree of grafting: 30%) in the high‐M PVOH‐P(VAc‐DBM) promotes the polymer diffusion to such an extent that the diffusion was complete in the freshly prepared films. Our data also suggest that under our experimental conditions, the rate of P(VAc‐DBM) diffusion increases with an increase of the degree of PVOH grafting. To confirm these results, we carried out fluorescence microscopy experiments to monitor the fate of PVOH in these latex films and found that in newly formed PVOH–P(VAc‐DBM) films, the PVOH was either uniformly distributed in the P(VAc‐DBM) matrix or the domains were too small to be resolved (i.e., < 0.5 μm). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5005–5020, 2004     

Synthesis,characterization, and emulsion polymerization of polymerizable coumarin derivatives

We describe the synthesis of a new polymerizable coumarin derivative, [6‐(β‐acryloxyethoxy)‐7‐isopropoxy‐4‐methyl coumarin ( 3 )], whose UV absorption spectrum significantly overlaps the emission spectrum of 9‐alkyl phenanthrene chromophore. This dye can serve several purposes in latex films. It can be a tracer for fluorescence microscopy experiments, or it can act as a donor or acceptor dye in nonradiative energy transfer experiments. Here we emphasize its role as an energy transfer acceptor in experiments with phenanthrene as the corresponding donor. Coumarin‐labeled poly(butyl methacrylate) latex dispersions could be synthesized by conventional batch emulsion polymerization with complete monomer conversion, complete dye incorporation, and uniform dye distribution. Attempts to extend this reaction to poly(vinyl acetate) copolymers failed because the dye inhibited monomer conversion. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3479–3489, 2004     

Elucidation of the structure of poly(γ-benzyl-l-glutamate) nanofibers and gel networks in a helicogenic solvent

The synthesis, characterization, self-assembly, and gel formation of poly(γ-benzyl-l-glutamate) (PBLG) in a molecular weight range from ca. 7,000–100,000 g/mol and with narrow molecular weight distribution are described. The PBLG is synthesized by the nickel-mediated ring-opening polymerization and is characterized by size-exclusion chromatography coupled with multiple-angle laser light scattering, NMR, and Fourier transform infrared spectroscopy. The self-assembly and thermoreversible gel formation in the helicogenic solvent toluene is investigated by transmission electron microscopy, atomic force microscopy, small-angle X-ray scattering, and synchrotron powder X-ray diffraction. At concentrations significantly below the minimum gelation concentration, spherical aggregates are observed. At higher concentrations, gels are formed, which show a 3D network structure composed of nanofibers. The proposed self-assembly mechanism is based on a distorted hexagonal packing of PBLG helices parallel to the axis of the nanofiber. The gel network forms due to branching and rejoining of bundles of PBLG nanofibers. The network exhibits uniform domains with a length of 200?±?42 nm composed of densely packed PBLG helices.      

Dimensional Control of Block Copolymer Nanofibers with a π‐Conjugated Core: Crystallization‐Driven Solution Self‐Assembly of Amphiphilic Poly(3‐hexylthiophene)‐b‐poly(2‐vinylpyridine)

With the aim of accessing colloidally stable, fiberlike, π‐conjugated nanostructures of controlled length, we have studied the solution self‐assembly of two asymmetric crystalline–coil, regioregular poly(3‐hexylthiophene)‐b‐poly(2‐vinylpyridine) (P3HT‐b‐P2VP) diblock copolymers, P3HT₂₃‐b‐P2VP₁₁₅ (block ratio=1:5) and P3HT₄₄‐b‐P2VP₁₁₅ (block ratio=ca. 1:3). The self‐assembly studies were performed under a variety of solvent conditions that were selective for the P2VP block. The block copolymers were prepared by using Cu‐catalyzed azide–alkyne cycloaddition reactions of azide‐terminated P2VP and alkyne end‐functionalized P3HT homopolymers. When the block copolymers were self‐assembled in a solution of a 50 % (v/v) mixture of THF (a good solvent for both blocks) and an alcohol (a selective solvent for the P2VP block) by means of the slow evaporation of the common solvent; fiberlike micelles with a P3HT core and a P2VP corona were observed by transmission electron microscopy (TEM). The average lengths of the micelles were found to increase as the length of the hydrocarbon chain increased in the P2VP‐selective alcoholic solvent (MeOH<iPrOH<nBuOH). Very long (>3 μm) fiberlike micelles were prepared by the dialysis of solutions of the block copolymers in THF against iPrOH. Furthermore the widths of the fibers were dependent on the degree of polymerization of the chain‐extended P3HT blocks. The crystallinity and π‐conjugated nature of the P3HT core in the fiberlike micelles was confirmed by a combination of UV/Vis spectroscopy, photoluminescence (PL) measurements, and wide‐angle X‐ray scattering (WAXS). Intense sonication (iPrOH, 1 h, 0 °C) of the fiberlike micelles formed by P3HT₂₃‐b‐P2VP₁₁₅ resulted in small (ca. 25 nm long) stublike fragments that were subsequently used as initiators in seeded growth experiments. Addition of P3HT₂₃‐b‐P2VP₁₁₅ unimers to the seeds allowed the preparation of fiberlike micelles with narrow length distributions (L_w/L_n <1.11) and lengths from about 100‐300 nm, that were dependent on the unimer‐to‐seed micelle ratio.     

Continuous and Segmented Semiconducting Fiber‐like Nanostructures with Spatially Selective Functionalization by Living Crystallization‐Driven Self‐Assembly

Fiber‐like π‐conjugated nanostructures are important components of flexible organic electronic and optoelectronic devices. To broaden the range of potential applications, one needs to control not only the length of these nanostructures, but the introduction of diverse functionality with spatially selective control. Here we report the synthesis of a crystalline‐coil block copolymer of oligo(p‐phenylenevinylene)‐b‐poly(2‐vinylpyridine) (OPV₅‐b‐P2VP₄₄), in which the basicity and coordinating/chelating ability of the P2VP segment provide a landscape for the incorporation of a variety of functional inorganic NPs. Through a self‐seeding strategy, we were able to prepare monodisperse fiber‐like micelles of OPV₅‐b‐P2VP₄₄ with lengths ranging from 50 to 800 nm. Significantly, the exposed two ends of OPV core of these fiber‐like micelles remained active toward further epitaxial deposition of OPV₅‐b‐PNIPAM₄₉ and OPV₅‐b‐P2VP₄₄ to generate uniform A‐B‐A and B‐A‐B‐A‐B segmented block comicelles with tunable lengths for each block. The P2VP domains in these (co‐)micelles can be selectively decorated with inorganic and polymeric nanoparticles as well as metal oxide coatings, to afford hybrid fiber‐like nanostructures. This work provides a versatile strategy toward the fabrication of narrow length dispersity continuous and segmented π‐conjugated OPV‐containing fiber‐like micelles with the capacity to be decorated in a spatially selective way with varying functionalities.