Showing papers by "Marc A. Hillmyer published in 2004"

Multicompartment micelles from ABC miktoarm stars in water.

Abstract: By combining three mutually immiscible polymeric components in a mixed-arm star block terpolymer architecture, we have observed the formation of a previously unknown class of multicompartment micelles in dilute aqueous solution. Connection of water-soluble poly(ethylene oxide) and two hydrophobic but immiscible components (a polymeric hydrocarbon and a perfluorinated polyether) at a common junction leads to molecular frustration when dispersed in aqueous solution. The incompatible hydrophobic blocks form cores that are protected from the water by the poly(ethylene oxide) blocks, but both are forced to make contact with the poly(ethylene oxide) by virtue of the chain architecture. The structures that emerge depend on the relative lengths of the blocks and can be tuned from discrete multicompartment micelles to extended wormlike structures with segmented cores.

Polymorph selectivity under nanoscopic confinement.

Abstract: Polymorph selectivity has been achieved during crystallization of anthranilic acid (AA) and 5-methyl-2-[(2-nitrophyenyl)amino]-3-thiophenecarbonitrile (ROY), both considered benchmarks of polymorphic behavior, within nanoporous glass beads and polymer monoliths. Whereas polymorph III of AA crystallizes from the melt on nonporous glass beads or within larger pores, the metastable polymorph II crystallizes in pores with diameters <23 nm, with the selectivity toward this form increasing with decreasing pore size. Of the six ROY polymorphs characterized by single-crystal X-ray diffraction, the yellow form (Y) crystallizes during evaporation of pyridine solutions imbibed by the 30-nm cylindrical pores of porous polycyclohexylethylene (p-PCHE) monoliths. Although both R and ON grow from the melt on the external surfaces of PCHE, only the red form (R) crystallizes in the pores. Amorphous ROY also forms in p-PCHE pores during evaporation from pyridine solutions, subsequently crystallizing to the R nanocrystals upon heating. Although heterogeneous nucleation on the pore walls may play a role, these observations suggest that nucleation and polymorph selectivity is governed by critical size constraints imposed by the ultrasmall pores. The ability to achieve polymorph selectivity in both glass and polymer matrices suggests wide-ranging compatibility with various organic crystalline solids, promising a new approach to controlling polymorphism and searching for unknown polymorphs.

Large area nanolithographic templates by selective etching of chemically stained block copolymer thin films

Abstract: Magnetic nanodot arrays were prepared from thin film templates of cylinder-forming polystyrene-polylactide diblock copolymers by first annealing the films to obtain PLA cylinders oriented perpendicular to the surface, then selectively staining the PS matrix with RuO4 vapors, followed by etching the cylinders by O2-reactive ion etching. Metal was then deposited by molecular beam deposition and finally the polymer mask was lifted off.

Access to the Superstrong Segregation Regime with Nonionic ABC Copolymers

Abstract: Block copolymers readily self-assemble into a rich variety of ordered phases, an attribute shared with other important soft materials such as surfactants and liquid crystals. The factors that govern the choice of ordered state symmetry, and the details of chain packing within a particular lattice, are rather well understood in the case of bulk, two-monomer “AB” diblock copolymers and also for analogous diblocks that undergo micellization in selective solvents. In both bulk and solution, the key free energy balance lies between minimizing interfacial area and satisfying the desire of the constituent blocks to be random coils. Thus, in the undiluted AB copolymer the interfacial tension scales as xø, where ø is the interaction parameter between A and B repeat units; the distortion (“stretching”) of the individual blocks away from the preferred Gaussian conformation varies as L2/Nb2, where L is the microdomain periodicity, N is the number of repeat units, and b is the statistical segment length.1 In a micelle formed from AB copolymers in a solvent favoring A, the interfacial tension γ can be related to an effective ø parameter between the B core and the solvated A corona. The conformational freedom of the A block in this case is dominated by the osmotic repulsion, i.e., excluded volume, between the well-solvated corona chains, which also leads to chain stretching away from the interface. The self-consistent mean-field theory (SCMFT) approach to describing the resulting phase behavior has become highly refined and capable of describing most experimental observations with near quantitative accuracy.2 The SCMFT now recognizes three distinct regimes, termed “weak”, “intermediate”, and “strong” segregation. The first imagines the individual coils to be at most weakly perturbed from the Gaussian state expected as ø f 0 and is not actually accessed within the ordered state. The last regime is obtained when the individual chains are large enough, and the interfacial tension high enough, that the inevitable interfacial zone between domains of pure A and B is negligible in thickness compared to the domain dimensions themselves. In practice, this regime corresponds to the dimensionless product øN > 50-100. However, a few years ago, Semenov, Khokhlov, and co-workers identified a new regime of phase behavior that they dubbed “superstrong segregation”, in which the interactions between A and B become so strong that the interfacial energy overwhelms the conformational entropy (in the bulk) or the coronal crowding (in solution).3,4 In this case flat interfaces are favored, and the minor block becomes nearly fully extended. To date, this regime has not been examined experimentally in a systematic fashion. Indeed, Semenov et al. suggested that it might only be experimentally accessible using ionic polymers to gain the requisite interaction strength.3 It is the purpose of this communication to demonstrate that, in fact, it is possible to access this superstrong segregation regime (SSSR) with nonionic, flexible coil block copolymers, by use of three-monomer ABC triblock copolymers and judicious choice of chemical groups A, B, and C. The key aspect of this strategy is that the choice of a triblock copolymer can lead to internally segregated micelles with two interfaces (C/B and B/A + solvent), thereby significantly augmenting the role of interfacial tension. Furthermore, the A, B, and C monomers should be selected to maximize their mutual repulsion. In the nonionic case, a hydrophilic A, a lipophilic B, and a hydrophobic and lipophobic fluoropolymer C offer the necessary strong 3-fold philicity. A prototypical experimental system that we propose affords access to the SSSR consists of a water-soluble poly(ethylene oxide) (O) block; a hydrocarbon, polystyrene (S); a fluoropolymer (F), obtained by grafting perfluorohexyl iodide groups to a 1,2-polybutadiene (B) backbone. A linear OSB triblock was prepared first by controlled anionic polymerization, with block molecular weights of 13 200 (NO ) 300), 4700 (NS ) 45), and 1300 (NB ) 24), respectively; the B block was then functionalized to produce OSF, with an F block molecular weight of 4600.5 The micelles formed by dispersing 1% of each copolymer in aqueous solution were characterized by dynamic light scattering, small-angle neutron scattering, and small-angle X-ray scattering. These several experiments provided a consistent and quantitative characterization of the micellar shape and size.5 The OSB micelles were spherical, consisting of a core of intermixed S and B segments within an O corona; the core radius Rc was about 20 nm and the overall micellar radius R about 50 nm. In contrast, the OSF micelles were flattened disks or oblate ellipsoids, containing an F core, an S shell, and an O corona. In this case the inner F core had principal radii of 25, 25, and 5 nm, and the S shell was about 5 nm thick. The overall hydrodynamic radius was also about 50 nm. As will be discussed below, the appearance of the disk shape for a micelle formed from flexible coil polymers is a signature of the SSSR. Note that this behavior is fundamentally distinct from disklike micelles formed as a result of core block crystallization6 or from rod-coil copolymers7 and that fluorination of the butadiene block does not generate a rodlike conformation.8 The OSF micelles have more recently been examined by cryogenic transmission electron microscopy (cryo-TEM), in which thin (<200 nm thick) free-standing films of aqueous solution are vitrified by rapid immersion in liquid ethane prior to imaging. Figure 1 shows a representative cryo-TEM image. The round objects correspond to micellar cores viewed along the minor axis. The uniform shade of gray suggests projections of an object of approximately constant thickness (as opposed to, say, † Department of Chemistry. ‡ Department of Chemical Engineering & Materials Science. * Author for correspondence: lodge@chem.umn.edu. 6680 Macromolecules 2004, 37, 6680-6682

Synthesis and Characterization of Triptych μ-ABC Star Triblock Copolymers

Abstract: We describe a general procedure for the synthesis of miktoarm star triblock copolymers with a hydrocarbon, a fluoropolymer, and a hydrophilic segment. Several μ-(polyethylethylene)(poly(ethylene oxide))(poly(perfluoropropylene oxide)) [μ-(PEE)(PEO)(PFPO)] star triblock copolymers were prepared using two successive anionic polymerization steps and one polymer−polymer coupling reaction. Initially, living polybutadienyllithium chains were end-capped with 2-methoxymethoxymethyloxirane forming a heterobifunctional 1,2-polybutadiene (PBD) precursor, with a hydroxyl group and a protected hydroxyl group at one chain end. Catalytic hydrogenation of this PBD gave the corresponding polyethylethyene (PEE) while preserving the end group structures. Transformation of the terminal hydroxyl group in the PEE precursor to a potassium alkoxide followed by addition of ethylene oxide and subsequent end-capping with ethyl bromide generated polyethylethylene−poly(ethylene oxide) (PEE−PEO) diblock copolymers with a protected hyd...

Large Area Nanolithographic Templates by Selective Etching of Chemically Stained Block Copolymer Thin Films

Abstract: Magnetic nanodot arrays were prepared from thin film templates of cylinder-forming polystyrene-polylactide diblock copolymers by first annealing the films to obtain PLA cylinders oriented perpendicular to the surface, then selectively staining the PS matrix with RuO4 vapors, followed by etching the cylinders by O2-reactive ion etching. Metal was then deposited by molecular beam deposition and finally the polymer mask was lifted off.

Template Syntheses of Polypyrrole Nanowires and CdS Nanoparticles in Porous Polymer Monoliths

Abstract: Template synthesis is a powerful method for the preparation of nanoscale materials with specific size and shape. We describe the template synthesis of polypyrrole nanowires and CdS nanoparticles within monolithic nanoporous polymer templates prepared from ordered block copolymer precursors. SAXS measurements confirmed retention of the host structure after growth of the nanomaterials within the pores of the template. SEM and TEM were used for direct visualization of the size and shape of the templated materials both in the monolith as well as after removal of the template through dissolution. The size of the templated nanomaterials matched that of the template pores and was modifiable by using templates containing different pore dimensions. For the case of CdS nanoparticles, WAXS was also used to verify particle size and crystal structure. In an effort to impregnate the nanoporous monoliths with CdS particles smaller than the pore dimensions of the template, a synthetic protocol utilizing a capping agent w...

An ordered nanoporous monolith from an elastomeric crosslinked block copolymer precursor

Abstract: An ordered nanoporous monolith was prepared from a poly(isoprene-b-dimethylsiloxane) (PI-PDMS) hexagonally packed cylinder precursor. The PDMS cylinders were macroscopically aligned, the PI was crosslinked with dicumyl peroxide, and the PDMS was removed using tetra butylaminonium fluoride. The stability of the pores depended on the modulus of the matrix. At high crosslink density the pore were stable, but they collapsed at a low crosslink density. The nanoporous materials could be swollen in toluene.

Mapping Large Regions of Diblock Copolymer Phase Space by Selective Chemical Modification

Abstract: A series of 4,1-polyisoprene-b-1,2-polybutadiene diblock copolymers of differing molecular weights and compositions were prepared by anionic polymerization. The polybutadiene blocks were selectively hydrogenated to poly(ethylethylene) using a homogeneous ruthenium catalyst. The double bonds in the polyisoprene blocks were subsequently modified to varying extents by the addition of difluorocarbene. The precursor polyisoprene−poly(ethylethylene) materials were disordered, but upon difluorocarbene (CF2) addition the effective degree of segregation in these materials increased markedly. Small-angle X-ray scattering was used to characterize the ordered state morphologies and domains spacings. Effective interaction parameters (χeff) were extracted from the temperature- and composition-dependent domain spacings, and these in turn were used to place the various samples on an experimental segregation vs composition phase map. The interaction parameter between poly(ethylethylene) and polyisoprene increases by a fac...

Melt chain dimensions of polylactide

Abstract: Melt chain dimensions of two polylactide samples were measured using small-angle neutron scattering. Three polylactides were synthesized: a deuterated polylactide containing 26% R-stereocenters (d-PLA-26), a hydrogenous polylactide with an R-content similar to the deuterated polylactide (PLA-28), and a hydrogenous polylactide that contained no R-stereocenters (PLA-0). The hydrogenous polylactides were each solution blended with the d-PLA-26 at a volume fraction of 0.2 for the deuterated polymer, and the melt chain dimensions of these polymers were determined. Small-angle neutron scattering experiments were performed at 30 °C for the d-PLA-26/PLA-28 blend and at 200 °C for both the d-PLA-26/PLA-28 and d-PLA-26/PLA-0 blends. Using three analysis methods, the average values for the statistical segment lengths, based on a C6 repeat unit, were found to be 10.0 ± 0.2 A for the PLA-28 at 30 °C, 8.9 ± 0.2 A for the PLA-28 at 200 °C, and 9.9 ± 0.4 A for the PLA-0 at 200 °C.

Synthesis of semifluorinated block copolymers by atom transfer radical polymerization

Abstract: Two sets of styrene-based semifluorinated block copolymers, one with a perfluoroether pendant group and another with a perfluoroalkyl group, were synthesized by atom transfer radical polymerization. Microphase separation of the block copolymers was established by small-angle X-ray scattering and differential scanning calorimetry (DSC). DSC measurements also showed that the perfluoroether-based polymer had a low glass-transition temperature (-44 °C Contact-angle measurements indicated that the semifluorinated block copolymers had low surface energies (ca. 13 mJ/m 2 ). These materials hold promise as low-surface-energy additives or surfactants for supercritical CO 2 applications.

Isotactic Polymers with Alternating Lactic Acid and Oxetane Subunits from the Endoentropic Polymerization of a 14-Membered Ring

Abstract: A new isotactic, perfectly alternating polymer of (S)-lactic acid and oxetane was synthesized by the entropically driven ring-opening polymerization of a 14-membered cyclic diester (S,S-3) mediated by a zinc alkoxide catalyst. The polymer (S,S-PMOD) was characterized by NMR spectroscopy, size exclusion chromatography, and matrix-assisted laser desorption ionization mass spectrometry. Under polymerization conditions the equilibrium concentration (0.17 ( 0.05 M) of the monomer in toluene was essentially independent of temperature, and the average thermodynamic parameters for the ring-opening polymerization were found to be ¢Hp° ) 0.2 ( 0.7 kJ mol-1 and ¢Sp° )+ 16 ( 2 J mol-1 K-1 (standard state (S,S-3) ) 1.0 M). Theoretical calculations of the standard state enthalpy (¢Hp°) for ring-opening polymerization of various oxocyclics, including S,S-3, were well-correlated with experimental values. Kinetic studies showed that the polymerization of S,S-3 was slower (kp ) 0.82 ( 0.04 M -1 s -1 and kdp ) 0.12 ( 0.04 s -1 at 25 °C) than that of lactide (kp ) 2.2 M -1 s -1 ) using the same zinc alkoxide catalyst. Differential scanning calorimetry of S,S-PMOD showed a glass transition temperature of -30 °C for samples with molecular weights between 5 and 72 kg mol -1 . In support of the potential utility of S,S- PMOD as a PLA plasticizer, the complete miscibility of the polymeric components was demonstrated by the observation of single Tg values for a series of blends of S,S-PMOD and atactic PLA.

Star Polymer Synthesis Using Hexafluoropropylene Oxide as an Efficient Multifunctional Coupling Agent

Abstract: Well-defined polyisoprene three-armed star polymers (PI3) were synthesized through a combination of living anionic polymerization and an efficient coupling process using hexafluoropropylene oxide (HFPO). Anionic polymerization of isoprene followed by termination with HFPO yielded three-armed stars with narrow molecular weight distributions. Under optimized conditions, at least 97% of the chains can be coupled. Molecular characterization of the star polymers (SEC, NMR and IR spectroscopy, and MALDI-TOF mass spectrometry) gave data consistent with the star structure and proposed mechanism of formation. The effects of HFPO concentration and the molecular weight of the arm precursors on the extent of star polymer formation were investigated. End-functionalized polyisoprene stars were also prepared and have potential application as trifunctional precursors to branched block copolymers.

Fluorinated amphiphilic block copolymers: Combining anionic polymerization and selective polymer modification

Abstract: We report the preparation of novel fluorinated block copolymers using a two-step modification sequence. We first prepared model polyisoprene-poly-tert-butylmethacrylate block copolymers by anionic polymerization. Exposing these materials to difluorocarbene (generated by the thermolysis of hexafluoropropylene oxide) resulted in modification of the polyisoprene block to the corresponding difluorocyclopropane repeating unit without compromising the integrity of the poly-tert-butylmethacrylate block. Hydrolysis of the difluorocarbene-modified materials gave the corresponding difluorocarbene-modified polyisoprene-polymethacrylic acid diblock copolymers. These amphiphilic materials are expected to exhibit interesting self-assembly behavior in aqueous solution.

Oxyfunctionalization of polyolefins

Abstract: The present invention is a method for oxyfunctionalizing, that is, introducing oxygen functionality to, a polyolefin such as polypropylene and poly(ethylene-alt-propylene). The polyolefin is contacted with an oxygen source such as a persulfate and catalytic amounts of a metal porphyrin complex under mild conditions to yield an oxyfunctionalized polymer that has a polydispersity that is very similar to that of the starting polymer.