Florian J. Stadler

Novel preparation of anatase TiO2@reduced graphene oxide hybrids for high-performance dye-sensitized solar cells.

An effective method was developed to prepare hybrid materials of TiO2 nanoparticles on reduced graphene oxide (RGO) sheets for application in solar cells. The morphology, size, and crystal phase of the TiO2 nanoparticles and TiO2@reduced graphene oxide (TiO2@RGO) hybrids were investigated in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), Raman, and UV-vis diffuse reflectance spectroscopy. A possible growth mechanism of TiO2@RGO hybrids is proposed based on observations of the TiO2 nanoparticles obtained from the hydrolysis process under different conditions. The effects of different reduced graphene oxide contents on the energy conversion efficiency of the dye-sensitized solar cells (DSSCs) based on J-V and incident photon-to-current conversion efficiency (IPCE) spectra are also discussed. DSSCs based on TiO2@RGO hybrid photoanodes with a graphene content of 1.6 wt % showed an overall light-to-electricity conversion efficiency of 7.68%, which is much higher than that of pure anatase nanoparticles (4.78%) accompanied by a short-circuit current density of 18.39 mA cm(2), an open-circuit voltage of 0.682 V, and a fill factor of 61.2%.

Rapid self-healing and triple stimuli responsiveness of a supramolecular polymer gel based on boron–catechol interactions in a novel water-soluble mussel-inspired copolymer

Marine and freshwater mussels secrete proteinaceous adhesive materials for adherence to the substrates upon which they reside. It is well known that 3,4-dihydroxyphenylalanine (DOPA) is the key to understanding these mussel adhesive proteins (MAPs). In order to gain a better understanding of their complex formation and quick recovery upon rupturing, novel water soluble copolymers of N-isopropylacrylamide and dopamine methacrylate were synthesized in such a way that they have 1, 2.5, and 5 mole percent dopamine monomer with respect to the NIPAM monomer on average. The statistical distribution of DOPA-functionalities along the chain makes the material a close synthetic equivalent of the byssal thread proteins of mytili. At acidic pH, the aqueous copolymer solution behaves like an unentangled copolymer solution, but at basic pH, these catechol functionalities form a dicomplex with H3BO3, thereby crosslinking two chains, proven by 11B-NMR and gelation. The polymer solution is thermosensitive with a pH-dependent lower critical solution temperature (LCST) between 21 and 33 °C, depending on the DOPA-content. If 2 or more functionalities per chain are present, a gel is formed that is self-healing with very quick recovery from sustained damage. The moduli of the gels depend on the concentration of functionalities. Hence, triple stimuli responsive copolymers were obtained.

Facile template-free and fast refluxing synthesis of 3D desertrose-like BiOCl nanoarchitectures with superior photocatalytic activity†

3D desertrose-like BiOCl nanoarchitectures were synthesized via a facile refluxing method in the polyol triethylene glycol (TEG) without templates or surfactants. They were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR), UV-vis diffuse reflectance (UV-DRS) and nitrogen adsorption analyses. On the basis of a series of contrast experiments, the probable growth mechanism and the fabrication process of the products were proposed. These BiOCl nanoarchitectures composed of numerous nanosheets exhibit higher photocatalytic performance than BiOCl nanoparticles, nanoplates and P25 under visible light irradiation. The enhancement of photocatalytic activity of the desertrose-like BiOCl nanoarchitectures compared to other nanostructures is mainly attributed to their larger surface area, higher dye loading, special hierarchitecture, and narrow band gap.

Connecting micelles by metallo-supramolecular interactions : towards stimuli responsive hierarchical materials

A novel strategy to link block copolymer micelles via metal–ligand interactions, leading to hierarchical supramolecular networks is presented. The mechanical properties of the obtained materials can be tuned easily by the choice of the metal ions used. The strong networks exhibit a chemical and mechanical stimuli responsive behavior, with almost instantaneous recovery in the latter case.

Terminal viscous and elastic properties of linear ethene∕α-olefin copolymers

Several linear ethene∕α-olefin copolymers with butene, hexene, octene, dodecene, octadecene, and hexacosene as comonomers were characterized in linear-viscoelastic shear flow. Creep and creep recovery measurements were used to obtain the zero shear-rate viscosity η0 and the steady-state elastic recovery compliance Je0. The correlation between the zero shear-rate viscosity and the weight average molar mass η0(Mw) established for metallocene catalyzed high density polyethylene (mHDPE) was found to be obeyed by all samples containing comonomer contents up to 29wt%. For the linear steady-state elastic compliance Je0 an increase with growing molar mass Mw or comonomer content wc was observed.

Characterization of branched ultrahigh molar mass polymers by asymmetrical flow field-flow fractionation and size exclusion chromatography

The molar mass distribution (MMD) of synthetic polymers is frequently analyzed by size exclusion chromatography (SEC) coupled to multi angle light scattering (MALS) detection. For ultrahigh molar mass (UHM) or branched polymers this method is not sufficient, because shear degradation and abnormal elution effects falsify the calculated molar mass distribution and information on branching. High temperatures above 130 °C have to be applied for dissolution and separation of semi-crystalline materials like polyolefins which requires special hardware setups. Asymmetrical flow field-flow fractionation (AF4) offers the possibility to overcome some of the main problems of SEC due to the absence of an obstructing porous stationary phase. The SEC-separation mainly depends on the pore size distribution of the used column set. The analyte molecules can enter the pores of the stationary phase in dependence on their hydrodynamic volume. The archived separation is a result of the retention time of the analyte species inside SEC-column which depends on the accessibility of the pores, the residence time inside the pores and the diffusion ability of the analyte molecules. The elution order in SEC is typically from low to high hydrodynamic volume. On the contrary AF4 separates according to the diffusion coefficient of the analyte molecules as long as the chosen conditions support the normal FFF-separation mechanism. The separation takes place in an empty channel and is caused by a cross-flow field perpendicular to the solvent flow. The analyte molecules will arrange in different channel heights depending on the diffusion coefficients. The parabolic-shaped flow profile inside the channel leads to different elution velocities. The species with low hydrodynamic volume will elute first while the species with high hydrodynamic volume elute later. The AF4 can be performed at ambient or high temperature (AT-/HT-AF4). We have analyzed one low molar mass polyethylene sample and a number of narrow distributed polystyrene standards as reference materials with known structure by AT/HT-SEC and AT/HT-AF4. Low density polyethylenes as well as polypropylene and polybutadiene, containing high degrees of branching and high molar masses, have been analyzed with both methods. As in SEC the relationship between the radius of gyration (R(g)) or the molar mass and the elution volume is curved up towards high elution volumes, a correct calculation of the MMD and the molar mass average or branching ratio is not possible using the data from the SEC measurements. In contrast to SEC, AF4 allows the precise determination of the MMD, the molar mass averages as well as the degree of branching because the molar mass vs. elution volume curve and the conformation plot is not falsified in this technique. In addition, higher molar masses can be detected using HT-AF4 due to the absence of significant shear degradation in the channel. As a result the average molar masses obtained from AF4 are higher compared to SEC. The analysis time in AF4 is comparable to that of SEC but the adjustable cross-flow program allows the user to influence the separation efficiency which is not possible in SEC without a costly change of the whole column combination.

Graphene Oxide/Carbon Nanotube Composite Hydrogels—Versatile Materials for Microbial Fuel Cell Applications

Carbonaceous nanocomposite hydrogels are prepared with an aid of a suspension polymerization method and are used as anodes in microbial fuel cells (MFCs). (Poly N-Isopropylacrylamide) (PNIPAM) hydrogels filled with electrically conductive carbonaceous nanomaterials exhibit significantly higher MFC efficiencies than the unfilled hydrogel. The observed morphological images clearly show the homogeneous dispersion of carbon nanotubes (CNTs) and graphene oxide (GO) in the PNIPAM matrix. The complex formation of CNTs and GO with NIPAM is evidenced from the structural characterizations. The effectual MFC performances are influenced by combining the materials of interest (GO and CNTs) and are attributed to the high surface area, number of active sites, and improved electron-transfer processes. The obtained higher MFC efficiencies associated with an excellent durability of the prepared hydrogels open up new possibilities for MFC anode applications.

Method for obtaining tube model parameters for commercial ethene/α-olefin copolymers

We propose a method of obtaining all three key “tube” model parameters, namely, the plateau modulus GN0, the entanglement molecular weight, Me, and the frictional equilibration time τe, from the molecular weight per backbone bond of ethene/α-olefin copolymers with longer comonomers, ranging from C4 (butylene) to C26 (hexacosene). GN0 is obtained from a correlation by Fetters et al. [Macromolecules 35, 10096–10101 (2002)] and Me is obtained from this using the standard tube-model formula, Me=(4/5)(ρRT/GN0). The equilibration time τe is obtained from a remarkable finding by Stadler and Munstedt [J. Rheol. 52, 697–712 (2008)] that, at fixed weight-average molecular weight, the zero-shear viscosity of linear ethene/α-olefin copolymers is independent of comonomer type and content over a wide range of α-olefin comonomers. From this observation, and from the values of GN0 and Me, we use the tube theory to construct a method for obtaining τe from the comonomer type and content. We show that these a priori values ...

Microfluidic‐Assisted Self‐Assembly of Complex Dendritic Polyethylene Drug Delivery Nanocapsules

Microfluidic platform for the synthesis of complex nanocapsules is presented via a controlled self-assembly. The monodisperse nanocapsules in the range of 50-200 nm consist of a dendritic polyethylene core and a Pluronic copolymer shell. The resultant nanocarriers encapsulate large amount of hydrophobic anticancer drug like paclitaxel while providing a low complement activation as well as sustained release profile with high tunability.

Revisiting the long-chain branch formation mechanism in metallocene catalyzed polyethylenes

The effect of polymerization parameters, such as pressure (ethylene concentration), temperature and comonomer type and concentration, on several long-chain branched (LCB) ethylene homopolymers and ethylene/α-olefin copolymers was investigated with respect to their degree of long-chain branching. Common long-chain branched metallocene polyethylenes (LCB-mPEs) and comb-like LCB-PEs, which have high viscosity and significantly differ from conventional LCB-mPEs, have been used to evaluate different proposed mechanisms for LCB-formation based on available characterization methods. The correlations between polymerization parameters and degree of long-chain branching were compared with the correlations predicted by different formation mechanisms (intermolecular auto-copolymerization by re-incorporation of β-hydride terminated chains, intramolecular insertion of stored vinyl terminated chains, chain walking and σ-bond metathesis reactions) to elucidate which of these mechanisms is the correct one. The intermolecular auto-copolymerization mechanism was found to be the predominant mechanism behind LCB-formation in slurry phase polymerization. However, σ-bond metathesis reactions cannot be entirely ignored.