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Institute
- Institut für Chemie (140) (remove)
Magnetic ionogels (MagIGs) were prepared from organosilane-coated iron oxide nanoparticles, N-isopropylacrylamide, and the ionic liquid trihexyl(tetradecyl)phosphonium dicyanamide. The ionogels prepared with the silane-modified nanoparticles are more homogeneous than ionogels prepared with unmodified magnetite particles. The silane-modified particles are immobilized in the ionogel and are resistant tonanoparticle leaching. The modified particles also render the ionogels mechanically more stable than the ionogels synthesized with unmodified nanoparticles. The ionogels respond to external permanent magnets and are therefore prototypes of a new soft magnetic actuator.
The switching kinetics of thin thermo-responsive hydrogel films of poly(monomethoxy-diethyleneglycol-acrylate) (PMDEGA) are investigated. Homogeneous and smooth PMDEGA films with a thickness of 35.9 nm are prepared on silicon substrates by spin coating. As probed with white light interferometry, PMDEGA films with a thickness of 35.9 nm exhibit a phase transition temperature of the lower critical solution temperature (LCST) type of 40 degrees C. In situ neutron reflectivity is performed to investigate the thermo-responsive behavior of these PMDEGA hydrogel films in response to a sudden thermal stimulus in deuterated water vapor atmosphere. The collapse transition proceeds in a complex way which can be seen as three steps. The first step is the shrinkage of the initially swollen film by a release of water. In the second step the thickness remains constant with water molecules embedded in the film. In the third step, perhaps due to a conformational rearrangement of the collapsed PMDEGA chains, water is reabsorbed from the vapor atmosphere, thereby giving rise to a relaxation process. Both the shrinkage and relaxation processes can be described by a simple model of hydrogel deswelling.
The scanning tunnelling microscope (STM)-induced switching of a single cyclooctadiene molecule between two stable conformations chemisorbed on a Si(100) surface is investigated using an above threshold model including a neutral ground state and an ionic excited state potential. Switching was recently achieved experimentally with an STM operated at cryogenic temperatures (Nacci et al 2008 Phys. Rev. B 77 121405(R)) and rationalized by a below threshold model using just a single potential energy surface (Nacci et al 2009 Nano Lett. 9 2997).
In the present paper, we show that experimental key findings on the inelastic electron tunnelling (IET) switching can also be rationalized using an above threshold density matrix model, which includes, in addition to the neutral ground state potential, an anionic or cationic excited potential. We use one and two-dimensional potential energy surfaces. Furthermore, the influence of two key parameters of the density matrix description, namely the electronic lifetime of the ionic resonance and the vibrational lifetimes, on the ground state potential are discussed.
The switching of single cyclooctadiene molecules chemisorbed on a Si(100) surface between two stable conformations, can be achieved with a scanning tunneling microscope [Nacci , Phys. Rev. B 77, 121405(R) (2008)]. Recently, it was shown by quantum chemical and quantum dynamical simulations that major experimental facts can be explained by a single-mode model with switching enforced by inelastic electron tunneling (IET) excitations and perturbed by vibrational relaxation [Nacci , Nano Lett. 9, 2997 (2009)]. In the present paper, we extend the previous theoretical work in several respects: (1) The model is generalized to a two-mode description in which two C2H4 units of COD can move independently; (2) contributions of dipole and, in addition, (cation and anion) resonance-IET rates are considered; (3) the harmonic-linear vibrational relaxation model used previously is generalized to anharmonic vibrations. While the present models highlight generic aspects of IET-switching between two potential minima, they also rationalize specific experimental findings for COD/Si(100): (1) A single-electron excitation mechanism with a linear dependence of the switching rate on tunneling current I, (2) the capability to switch both at negative and positive sample biases, and (3) a crossover temperature around similar to 60 K from an IET-driven, T-independent atom tunneling regime, to classical over-the-barrier isomerization with exponential T-dependence at higher temperatures for a bias voltage of +1.5 V and an average tunneling current of 0.73 nA.
A unified model for quantitative description of harmonic spectra of gases obtained by wavelength modulation spectroscopy (WMS) technique is presented. In the model, both intensity modulation (IM) and frequency modulation (FM) of the laser emission are taken into account using minimum number of parameters. For the first time, the static behavior of a laser is described as a limiting case of its dynamic response. Laser and its driver are considered as a single device converting applied bias to laser emission. This allows application of the model to any type of laser and the introduced parameters can be assigned to the corresponding laser and/or driver properties. The approach was tested using a distributed feedback (DFB) laser spectrometer. Correctness of the proposed model is justified by very good agreement between the measured and modeled/fitted spectra, which allowed evaluation of the setup performance and assessment of modulation parameters of the DFB laser. An algorithm to minimize the time of numerical calculation of harmonic spectra using numerically approximated Voigt lineshape function was developed. Absolute values of the absorption line parameters (line strength and line width) were obtained from a single calibration- and reference-free spectrum scan with accuracy better than 0.1%.
C-branched carbohydrates are of current interest for glycochemistry, are widely found in nature and serve as important subunits in many antibiotics, bacterial polysaccharides and macrolides. Among C-functionalized saccharides, 2-C-branched carbohydrates represent challenging structures for synthetic chemists, since in contrast to C-glycosides they are not easily accessible from glycosyl bromides or other simple precursors. In this perspective we want to summarize recent approaches to 2-C-branched carbohydrates over the past fifteen years. The two main strategies are based on ring-opening of 1,2-cyclopropanated carbohydrates by various reagents, as well as radical additions to glycals and further transformations, developed in our group. Both methods are characterized by high stereoselectivities and good yields and give access to a broad variety of functionalized carbohydrate 2-C-analogs.
Transparent, ion-conducting, luminescent, and flexible ionogels based on the room temperature ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl) imide [Bmim][N(Tf)(2)], a PtEu2 chromophore, and poly(methylmethacrylate) (PMMA) have been prepared. The thermal stability of the PMMA significantly increases with IL incorporation. In particular, the onset weight loss observed at ca. 229 degrees C for pure PMMA increases to 305 degrees C with IL addition. The ionogel has a high ionic conductivity of 10(-3) S cm(-1) at 373 K and exhibits a strong emission in the red with a long average luminescence decay time of tau = 890 mu s. The resulting material is a new type of soft hybrid material featuring useful thermal, optical, and ion transport properties.
Hydrogel systems based on hydroxyethyl starch-polyethylene glycol methacrylate (HES-P(EG)(6)MA) or hydroxyethyl starch methacrylate (HES-MA) were used to assess the protein release behavior. Here, we analyzed the in vitro release of FITC-anti-human antibodies incorporated in either HES-P(EG)(6)MA or HES-MA hydrogel delivery systems in PBS or human serum. In addition, hydrogel disks and microparticles prepared from the two polymers were subcutaneously implanted in BALB/c mice. The in vivo release of FITC-IgG was non-invasively monitored by an in vivo imaging system (IVIS 200) over a time period of up to 3 months. The imaging system allowed to asses individual animals over time, therefore only a small number of animals was required to obtain high quality data. The reduction in fluorescence intensity at the site of administration was compared to in vitro release profiles. These investigations demonstrated a sustained release from HES-MA hydrogel disks compared to rapidly degrading HES-P(EG)(6)MA disks and microparticles. The sustained release from HES-MA disks could be further optimized by using increased polymer concentrations. Human serum as in vitro release medium reflected better the in vivo release from HES-P(EG)(6)MA systems than PBS, suggesting that the presence of organic substances like proteins or lipids may play a significant role for the release kinetics.
The chemistry of water on alpha-alumina kinetics and nuclear quantum effects from first principles
(2012)
Water adsorption on an alumina (alpha-Al2O3) surface is studied here from first principles using periodic density functional theory in the generalized gradient approximation. Two different coverage regimes, low and high, are considered. For the low-coverage regime (with a coverage of 1/4 with respect to the number of coordinatively unsaturated Al sites), possible reactions at the surface such as dissociation, rotation, and diffusion of water and its fragments are investigated, using first principles thermodynamics and kinetics. A microkinetic model is set up with rates calculated from Eyring's transition state theory in order to cover a wide range of time scales. Special emphasis of this study is on the magnitude of quantum effects and on anharmonic corrections, particularly for reactions and dynamics. These have often been neglected in the past for water/alumina systems but can influence the system. This is particularly true for processes involving hydrogen atoms, where, for example, tunneling corrections to reaction rates are found to be important even at room temperature. For a higher-coverage regime (with a coverage of 2 ML), hydrogen dynamics becomes even more complex and is characterized, e.g., by concerted atom motion, strong anharmonicity, and delocalization. In this regime, classical molecular dynamics becomes questionable as well as quantum mechanical treatments based on the harmonic approximation.
Ionic liquids (ILs) on the basis of metal containing anions and/or cations are of interest for a variety of technical applications e.g., synthesis of particles, magnetic or thermochromic materials. We present the synthesis and the results of electron paramagnetic resonance (EPR) spectroscopic analyses of a series of some new potential ionic liquids based on tetrachloridocuprates(II), [CuCl4](2-), with different sterically demanding cations: hexadecyltrimethylammonium 1, tetradecyltrimethylammonium 2, tetrabutylammonium 3 and benzyltriethylammonium 4. The cations in the new compounds were used to achieve a reasonable separation of the paramagnetic Cu(II) ions for EPR spectroscopy. The EPR hyperfine structure was not resolved. This is due to the exchange broadening, resulting from still incomplete separation of the paramagnetic Cu(II) centers. Nevertheless, the principal values of the electron Zeemann tensor (g parallel to and g perpendicular to) of the complexes could be determined. Even though the solid substances show slightly different colors, the UV/Vis spectra are nearly identical, indicating structural changes of the tetrachloridocuprate moieties between solid state and solution. The complexes have a promising potential e.g., as high temperature ionic liquids, as precursors for the formation of copper chloride particles or as catalytic paramagnetic ionic liquids.
Building blocks for oligospiroketal (OSK) rods and evaluation of their influence on rod rigidity
(2012)
We report on the synthesis of three new sleeves and their incorporation in OSK rods. The structures of these sleeves are based on neo-inositol, terephthalaldehyde diacetals, and indacene. To quantify the influence of the sleeves on rod rigidity, we applied the worm-like chain (WLC) model on the new rods and found that this approach is rather disappointing. As the chief cause of this result, we assume that the rigidity of typical molecular rods largely exceeds the rigidity of polymers, which were successfully described by the WLC model. Alternatively, we suggest quantifying the rigidity of molecular rods by fitting an empirical function on the end-to-end distance distribution curve obtained by MD simulations. After checking various function types, the Levy-Martin function proved to be most suitable for this purpose. On the basis of this function, we defined the Levy-Martin parameter and suggest using this parameter for the characterization of the rigidity of molecular rods.
Random copolymers of 4-vinylbenzyl tri(oxyethylene) and tetra(oxyethylene) ethers, as well as alternating copolymers of 4-vinylbenzyl methoxytetra(oxyethylene) ether and a series of N-substituted maleimides, were synthesised by conventional free radical polymerisation, reversible addition fragmentation chain transfer (RAFT) and atom transfer radical polymerisation (ATRP). Their thermosensitive behaviour in aqueous solution was studied by turbidimetry and dynamic light scattering. Depending on the copolymer composition, a LCST type phase transition was observed in water. The transition temperature of the obtained random as well as alternating copolymers could be varied within a broad temperature window. In the case of the random copolymers, transition temperatures could be easily fine-tuned, as they showed a linear dependence on the copolymer composition, and were additionally modified by the nature of the polymer end-groups. Alternating copolymers were extremely versatile for implementing a broad range of variations of the phase transition temperatures. Further, while alternating copolymers derived from 4-vinylbenzyl methoxytetra(oxyethylene) ether and maleimides with small hydrophobic side chains underwent macroscopic phase separation when dissolved in water and heated above their cloud point, the incorporation of maleimides bearing larger hydrophobic substituents resulted in the formation of mesoglobules above the phase transition temperature, with hydrodynamic diameters of less than 100 nm.
The copolymerization of an excess of a functionalized styrene monomer, 4-vinylbenzyl methoxytetrakis(oxyethylene) ether, with various N-substituted maleimides yields tapered diblock copolymers in a one-step procedure, when applying reversible deactivation radical polymerization (RDRP) methods, such as ATRP and RAFT. The particular chemical structure of the diblock copolymers prepared results in reversible temperature-responsive two-step aggregation behavior in dilute aqueous solution. In this way, a double hydrophilic block copolymer is transformed step by step into an amphiphilic macrosurfactant, and finally into a double hydrophobic copolymer, as followed by turbidimetry and dynamic light scattering. Copolymers in which the maleimide repeat units bear short hydrophobic side chains are freely water-soluble at low temperature and form micellar aggregates above their cloud point. Further heating above the phase transition temperature of the second block results in secondary aggregation. Copolymers with maleimides that bear strongly hydrophobic substituents undergo two thermally induced aggregation steps upon heating, too, but show in addition intramolecular hydrophobic association in water already at low temperatures, similar to the behavior of polysoaps.
Small fluorescent organic molecules based on [1,3]dioxolo[4,5-f][1,3]benzodioxole (DBD) could be used as probes for lipophillic microenvironments in aqueous solutions by indicating the critical micelles concentration of detergents and staining cell organelles. Their fluorescence lifetime decreases drastically by the amount of water in their direct environment. Therefore they are potential probes for fluorescence lifetime imaging microscopy (FLIM).
Background: Magnetic composites of thermosensitive shape-memory polymers (SMPs) and magnetite nanoparticles (MNPs) allow noncontact actuation of the shape-memory effect in an alternating magnetic field. In this study, we investigated whether the magnetic heating capability of cross-linked poly(epsilon-caprolactone)/MNP composites (cPCLC) could be improved by covalent coating of MNPs with oligo(epsilon-caprolactone) (OCL).
Methods: Two different types of cPCLC containing uncoated and OCL-coated MNP with identical magnetite weight content were prepared by thermally induced polymerization of poly(epsilon-caprolactone) diisocyanatoethyl methacrylate. Both cPCLCs exhibited a melting transition at T-m = 48 degrees C, which could be used as switching transition.
Results: The dispersion of the embedded nanoparticles within the polymer matrix could be substantially improved, when the OCL-coated MNPs were used, as visualized by scanning electron microscopy. We could further demonstrate that in this way the maximal achievable bulk temperature (T-bulk) obtained within the cPCLC test specimen in magnetic heating experiments at a magnetic field strength of H = 30 kA.m(-1) could be increased from T bulk = 48 degrees C to T bulk = 74 degrees C.
The surface of single-walled carbon nanotubes (SWCNTs) was functionalized with azide-terminated poly(vinylidene fluoride) (PVDF). Functionalization was confirmed by dispersibility, Raman spectroscopy, and thermogravimetric analyses. Raman spectra show disordering of the SWCNTs, thus, strongly suggesting that PVDF was covalently attached to SWCNTs. Functionalized SWCNTs were mixed with commercially available PVDF in a twin-screw extruder and thin films were obtained by melt-pressing. Films containing 0.5 and 1 wt% PVDF-functionalized SWCNTs exhibited significantly improved electrical conductivity compared to PVDF films containing pristine SWCNTs.
The synthesis of block copolymers consisting of poly(vinylidene fluoride) (PVDF) and polystyrene (PS) is reported. Firstly, a propargyl-functionalized alkoxyamine initiator (PgOTIPNO) was prepared and subsequently used for the preparation of a propargyl-terminated PS homopolymer of different chain lengths with low dispersities via nitroxide-mediated radical polymerization. A tailored PVDF homopolymer with iodine end groups originating from iodine transfer polymerization was transformed to PVDF with azide end group. Then, alkyne-terminated PS with different molecular weights and azide-terminated PVDF were joined together via copper-catalyzed alkyne-azide coupling. The block copolymers were characterized using H-1-NMR, F-19-NMR, IR, SEC, and DSC.
Purpose: Previous investigations have shown that poly(ether imide) (PEI) membranes can be functionalized with aminated macromolecules. In this study we explored whether the characterization of PEI functionalized with oligo(ethylene glycol) (OEG) or linear, side chain methylated oligoglycerols (OGMe), by angle-dependent X-ray induced photoelectron spectroscopy (XPS) can be used to prove the functionalization, give insight into the reaction mechanism and reveal the spatial distribution of the grafts.
Methods: PEI membranes were functionalized under alkaline conditions using an aqueous solution with 2 wt% of alpha-amino-methoxy oligo(ethylene glycol) (M-n = 1,320 g.mol(-1)) or linear, side chain methylated monoamine oligoglycerols (M-n = 1,120, 1,800 or 2,270 g.mol(-1)), respectively. The functionalized membranes were investigated using XPS measurements at different detector angles to enable comparison between the signals related to the bulk and surface volume and were compared with untreated and alkaline-treated PEI membranes.
Results: While at a perpendicular detector angle the bulk signals of the PEI were prominent, at larger surface volume-related detector angles, the signals for OGMe and OEG were determinable.
Conclusion: The surface functionalization of PEI with OEG and OGMe could be verified by the angle-dependent XPS. The observations proved the functionalization at the PEI surface, as the polyethers were detected at angles providing signals of the surface volume. Furthermore, the chemical functions determined verified a covalent binding via the nucleophilic addition of the amine functionalized OGMe and OEG to the PEI imide function.
A polymer-clay based composite adsorbent was prepared from locally obtained kaolinite clay and polyvinyl alcohol. The composite adsorbent was used to remove lead (II) ions from aqueous solution in a fixed bed mode. The increase in bed height and initial metal ion concentration increased the adsorption capacity of lead (II) and the volume of aqueous solution treated at 50% breakthrough. However, the adsorption capacity was reduced by almost 16.5% with the simultaneous presence of Ca2+/Pb2+ and Na+/Pb2+ in the aqueous solution. Regeneration of the adsorbent with 0.1 M of HCl also reduced its adsorption capacity to 75.1%. Adsorption of lead (II) ions onto the polymer-clay composite adsorbent in the presence of Na+ and Ca2+ electrolyte increased the rate of mass transfer, probably due to competition between cationic species in solution for adsorption sites. Regeneration further increased the rate of mass transfer as a result of reduced adsorption sites after the regeneration process. The length of the mass transfer zone was found to increase with increasing bed height but did not change with increasing the initial metal ion concentration. The models of Yoon-Nelson, Thomas, and Clark were found to give good fit to adsorption data. On the other hand, Bohart-Adams model was found to be a poor predictor for the column operation. The polymer-clay composite adsorbent has a good potential for the removal of lead (II) ions from highly polluted aqueous solutions.
The chelating dithioether 1,2-bis(2-methoxyethylthio)benzene. a novel solvent extractant for Pd(II), is aimed to be utilised in the selective recovery of palladium from spent automotive catalysts. For that, the extraction system has been further customised, including the choice of an appropriate diluent (1,2-dichlorobenzene) as well as an effective stripping agent (0.5 M thiourea in 0.1 M HCl), which both have been selected from a number of potential agents. It is shown in batch experiments that the selectivity for Pd(II) is maintained when the organic phase (10(-2) M 1,2-bis(2-methoxyethylthio)benzene in 1,2-dichlorobenzene) is used several times to extract an oxidising leach solution. According to the McCabe-Thiele plot two theoretical stages are needed to extract more than 98% of the Pd(II) contained in that solution. The calculation of the thermodynamic quantities Delta H degrees. Delta S degrees and Delta G degrees reveals that the reaction is entropy driven - the temperature has only a slight influence on the extraction yield. It is demonstrated that the mono-oxidised extractant has a catalytic effect on the extraction kinetics when the aqueous phase contains highly concentrated hydrochloric acid. HPLC measurements prove the presence of small quantities of 1-(2-methoxyethylsulfinyl)-2-(2-methoxyethylthio) benzene in the organic phase.