25 research outputs found

    Convenient Quantification of Accessible Surface-Attached ATRP Initiators and RAFT Chain Transfer Agents on Cross-Linked Polystyrene Nanoparticles

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    The exact quantification of surface-attached initiators for grafting-from radical polymerization on micro- and nanoparticles is still a challenging task. Here we demonstrate that, by combining UV/vis spectroscopy and an efficient derivatization protocol, the originally nearly invisible ATRP initiators and RAFT chain transfer agent entities can be quantified easily and precisely. Application of an isorefractive dispersion medium for the nanoparticles moreover provides reliable information about the amount of chemically really “accessible” surface-grafted initiators, that is, those that are not hidden in the particle’s shell interior. To qualify the developed procedure further, nanoparticles of different grafting densities were generated, and the values determined afterward for the initiator concentration were in good agreement with expectations

    Poly(2-hydroxyethyl methacrylate)-Based Amphiphilic Block Copolymers for High Water Flux Membranes and Ceramic Templates

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    Microphase separation drives the structure formation in block copolymers (BCP), and it is a promising and convenient way for the preparation of well-defined hierarchically structured materials. In this work, we focus on the synthesis of amphiphilic block copolymers, i.e., polystyrene-<i>block</i>-poly­(2-hydroxyethyl methacrylate) (PS-<i>b</i>-PHEMA), via sequential anionic polymerization. Molecular weights of the prepared BCPs were in the range of 103–308 kg mol<sup>–1</sup> with low polydispersity values (<i>Đ</i> = 1.03–1.25) and a PHEMA content of 17–25 vol % as proven by size exclusion chromatography (SEC) and <sup>1</sup>H NMR spectroscopy measurements. For the first time, the PS-<i>b</i>-PHEMA BCPs were subjected to the self-assembly and non-solvent-induced phase separation (SNIPS) process for the generation of integral asymmetric films on tailor-made porous paper substrates. After water immersion, the final BCP membranes reveal an isoporous open structure at the surface featuring adjustable pore diameters of 20–60 nm. It is demonstrated that a high water flux of up to 2750 L m<sup>–2</sup> h<sup>–1</sup> bar<sup>–1</sup> in the fully swollen state of the membrane can be obtained. Additionally, the hydroxyl moieties of the porous BCP membranes and tailor-made paper support are further exploited for postmodification strategies by using sol–gel chemistry. For this purpose, titanium­(IV) bis­(ammonium lactato) dihydroxide solution (TiBALDH) is used followed by thermal treatment at 600 °C in a nitrogen atmosphere. Noteworthy, the pristine BCP porous membrane structure can be maintained having a titanium-functionalized open porous structure

    One-Step Anionic Copolymerization Enables Formation of Linear Ultrahigh-Molecular-Weight Block Copolymer Films Featuring Vivid Structural Colors in the Bulk State

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    Ultrahigh-molecular-weight (UHMW) tapered block copolymers (BCPs) consisting of polyisoprene-<i>block</i>-poly­(4-methylstyrene) featuring overall molar masses in the range of 1101–2033 kg mol<sup>–1</sup> (<i>M</i><sub>w</sub>) are synthesized via a convenient one-step anionic copolymerization protocol. The obtained UHMW BCPs are investigated by differential scanning calorimetry, size exclusion chromatography, and <sup>1</sup>H NMR spectroscopy. Microphase separation for the UHMW BCPs in the bulk state is investigated by transmission electron microscopy (TEM) measurements and scanning electron microscopy (SEM), revealing well-ordered lamellar and spherical domains with large domain sizes in the range of 100–200 nm. Excellent order and periodicity are observed for lamellar morphologies over large film areas of 90 × 120 μm. Because of this high order of the underlying domains and the different refractive indices of the block segments, vivid structural colors can be observed in the bulk state. Structural colors of BCP films are investigated by angle-dependent UV/vis measurements, revealing intensive reflection colors according to Bragg’s law of diffraction. The optical characteristics are directly correlated to TEM and SEM results. Moreover, colored BCP films featuring spherical domains for one block segment with domain sizes of 97–122 nm revealed blue structural colors stemming from disordered particle scattering

    Single-Source Magnetic Nanorattles By Using Convenient Emulsion Polymerization Protocols

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    A novel strategy to achieve easily scalable magneto-responsive nanoceramics with core/shell and nanorattle-type or yolk/shell architectures based on a ferrocene-containing polymer precursor is described. Monodisperse nanorattle-type magnetic particles are obtained by using convenient semicontinuous emulsion polymerization and Stöber process protocols followed by thermal treatment. The particles are characterized by TGA, TEM, WAXS, DLS, XPS, and Raman spectroscopy. Herein, established synthetic protocols widen opportunities for the convenient bottom-up strategies of various ferrocene-precursor-based spherical architectures for advanced ceramics with potential applications within fields of sensing and stimuli-responsive nanophotonics

    Water-Soluble Poly(vinylferrocene)-<i>b</i>-Poly(ethylene oxide) Diblock and Miktoarm Star Polymers

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    We describe the synthesis of water-soluble diblock and miktoarm star polymers consisting of poly­(vinylferrocene) (PVFc) and poly­(ethylene oxide) (PEO) blocks. First, end-functionalized poly­(vinylferrocene) was generated by end-capping the living carbanionic PVFc chains with benzyl glycidyl ether (BGE) or ethoxy ethyl glycidyl ether (EEGE). Acidic hydrolysis of the EEGE-terminated PVFc partially oxidized the PVFc backbone. However, the dihydroxyl end-functional PVFc was obtained in quantitative yields by hydrogenolysis of the BGE-terminated PVFc. A series of block copolymers and AB<sub>2</sub> miktoarm star copolymers was obtained in a second polymerization step, utilizing the respective end-functionalized PVFc as a macroinitiator for the ring-opening polymerization (ROP) of ethylene oxide. All polymers were analyzed in detail, using NMR spectroscopy and size-exclusion chromatography (SEC). Online SEC-viscosimetry as well as MALLS was carried out, confirming the formation of miktoarm structures. Quantitative functionalization and subsequent removal of the acetal and benzyl protective groups, respectively were confirmed by MALDI–ToF mass spectrometry. Molecular weights of the end-functionalized PVFcs range between 1000 and 3600 g mol<sup>–1</sup>, and block copolymers with 10 000 to 50 000 g mol<sup>–1</sup> overall molar masses were synthesized. In addition, the water-soluble block copolymers were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). For characterization of the morphology in aqueous solution, transmission electron microscopy (TEM) was performed, showing micelles and multicompartment micellar structures

    Pressure Induced Structure Formation in Langmuir Monolayers of Amphiphilic Metallocene Diblock Copolymers

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    We present in situ structural investigations of a metal-containing diblock copolymer on a water surface. Monolayers of poly­(vinylferrocene-<i>b</i>-(2-vinylpyridine)) (PVFc-<i>b</i>-P2VP) block copolymers are studied in a wide range of compositions by varying molar masses of P2VP with two different molecular weights of PVFc. We focus on the role of the respective block partners, PVFc and P2VP, when compressing the layer on the water surface. Compression isotherms are presented and interpreted in terms of the classical gaseous, expanded, and condensed phases. We calculate isothermal compressibilities, which reveal a minimum value independent of the molar masses of the respective block partners. We find the isotherms to be dominated by P2VP while PVFc barely contribute to the compression behavior due to its rather compact coil structure. We consider the diblock copolymer monolayers as a two-dimensional model system, which is reflected by two-dimensional scaling behavior in the semi dilute and condensed regime. By X-ray reflectometry (XR), we monitor in situ the monolayer structure change with increasing surface pressure Π and observe the PVFc-<i>b</i>-P2VP separation at high Π

    Hydrophobic Nanocontainers for Stimulus-Selective Release in Aqueous Environments

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    The preparation of nanocontainers with a hydrophilic core from water-in-oil emulsions and their subsequent transfer to aqueous medium is crucial because it enables the efficient encapsulation of hydrophilic payloads in large quantities. However, major challenges are associated with their synthesis including low colloidal stability, leakage of encapsulated payloads due to osmotic pressure, and a demanding transfer of the nanocontainers from apolar to aqueous media. We present here a general approach for the synthesis of polymer nanocontainers that are colloidally stable, not sensitive to osmotic pressure, and responsive to environmental stimuli that trigger release of the nanocontainer contents. Additionally, the nanocontainers can selectively deliver one or two different payloads upon oxidation and changes of pH or temperature. Our approach uniquely enables the synthesis of nanocontainers for applications in which aqueous environments are desired or inevitable

    Redox-Responsive Polymer Brushes Grafted from Polystyrene Nanoparticles by Means of Surface Initiated Atom Transfer Radical Polymerization

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    Well-defined poly­(2-(methacryloyloxy)­ethyl ferrocenecarboxylate) (PFcMA) brushes were synthesized by surface-initiated atom transfer radical polymerization (SI-ATRP) on cross-linked polystyrene particles. The ATRP of FcMA monomer was reinvestigated leading to molar masses up to 130 kg mol<sup>–1</sup> with a good reaction control and high monomer conversion (91%). The SI-ATRP was done with different amounts of initiator in the PS particle shell leading to PFcMA surface conformations from “mushroom-like” to dense “brush-like” polymers, which could be confirmed by dynamic light scattering (DLS) experiments. Redox-responsive behavior of the PFcMA shell was investigated by DLS and cyclic voltammetry (CV) measurements indicating a tremendous increase in the hydrodynamic volume of the ferrocene-containing shell. The transformation of PFcMA-grafted PS particles to magnetic iron oxides after thermal treatment could be investigated by SQUID magnetization measurements showing the typical hysteresis for ferromagnetic material

    Polyvinylferrocene-Based Amphiphilic Block Copolymers Featuring Functional Junction Points for Cross-Linked Micelles

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    The synthesis of high-molecular-weight, well-defined poly­(vinylferrocene)-<i>block</i>-poly­(ethylene glycol) (PVFc-<i>b</i>-PEG) diblock copolymers (<i>M</i><sub>n</sub> = 13 000–44 000 g mol<sup>–1</sup>; <i>Đ</i> = 1.29–1.34) with precisely one allyl group at the junction point is introduced. Allyl glycidyl ether (AGE) was used to end-functionalize PVFc, resulting in hydroxyl functional macroinitiators for the oxyanionic polymerization of ethylene oxide. The self-assembly behavior of the amphiphilic PVFc-<i>b</i>-PEG copolymers in water has been investigated in a detailed manner, using dynamic light scattering (DLS) and transmission electron microscopy (TEM). The redox activity of the PVFc block was confirmed by UV/vis spectroscopy, while cyclovoltammetry (CV) measurements were carried out to support the stability and full reversibility of the ferrocene/ferrocenium redox couple. Both formation and dissociation of the macromolecular self-assemblies in aqueous solution via oxidation and reduction of the PVFc segments were evidenced by TEM and DLS. The dye Nile Red was used as model compound to investigate the stabilization of a water-insoluble molecule in aqueous solution by the block copolymers via encapsulation inside micellar structures. Oxidation of the PVFc segments lead to instantaneous and quantitative release of the dye. Furthermore, incorporation of the allyl moiety at the block junction point was used to cross-link the shell of the compartments. By this strategy a stable incorporation of the dye was achieved while triggered release via oxidation led to quantitative liberation

    Thermo-cross-linked Elastomeric Opal Films

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    An efficient and convenient thermal cross-linking protocol in elastomeric opal films leading to fully reversible and stretch-tunable optical materials is reported. In this study, functional monodisperse core–shell particles were arranged in a face-centered cubic (fcc) lattice structure by a melt flow process. A problem up to now was that un-cross-linked films could not be drawn fully reversibly and hence lost their optical and mechanical performance. After thermal cross-linking reaction, the obtained films can be drawn like rubbers and the color of their Bragg reflection changes because of controlled lattice deformation, which makes the cross-linked films mechanochromic sensors. Different techniques were developed for the cross-linking of the films a posteriori, after their preparation in the melt flow process. A photo-cross-linking approach was reported earlier. This study now deals with a very efficient thermo-cross-linking approach based on the chemistry of hydroxyl- and isocyanate-functionalities that form urethane bridges. The focus of the present work is the mechanism and efficiency of this cross-linking process for elastomeric opal films with excellent mechanical and optical properties
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