Thermally stable and efficient polymer solar cells based on a novel donor-acceptor copolymer.

We report high photovoltaic performance of a novel donor-acceptor (D-A) conjugated polymer poly[2,6[4,8-bis(2-ethyl-hexyl)benzo[1,2-b;4,5-b']dithiophene-co-2,5-thiophene-co-4,7[5,6-bis-octyloxy-benzo[1,2,5]thiadiazole]-co-2,5-thiophene] (PBDTTBTZT) in bulk heterojunctions with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM). A power conversion efficiency (PCE) of more than 7% is obtained for optimized charge-extracting electrodes. Upon application of thermal stress via annealing, a superior thermal stability is demonstrated as compared to poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT).

Electrically induced structure formation and pattern transfer

The wavelength of light represents a fundamental technological barrier to the production of increasingly smaller features on integrated circuits. New technologies that allow the replication of patterns on scales less than 100 nm need to be developed if increases in computing power are to continue at the present rate. Here we report a simple electrostatic technique that creates and replicates lateral structures in polymer films on a submicrometre length scale. Our method is based on the fact that dielectric media experience a force in an electric field gradient. Strong field gradients can produce forces that overcome the surface tension in thin liquid films, inducing an instability that features a characteristic hexagonal order. In our experiments, pattern formation takes place in polymer films at elevated temperatures, and is fixed by cooling the sample to room temperature. The application of a laterally varying electric field causes the instability to be focused in the direction of the highest electric field. This results in the replication of a topographically structured electrode. We report patterns with lateral dimensions of 140 nm, but the extension of the technique to pattern replication on scales smaller than 100 nm seems feasible.

Self-assembly of nanoparticles into structured spherical and network aggregates

Multi-scale ordering of materials is central for the application of molecular systems in macroscopic devices. Self-assembly based on selective control of non-covalent interactions provides a powerful tool for the creation of structured systems at a molecular level, and application of this methodology to macromolecular systems provides a means for extending such structures to macroscopic length scale. Monolayer-functionalized nanoparticles can be made with a wide variety of metallic and non-metallic cores, providing a versatile building block for such approaches. Here we present a polymer-mediated 'bricks and mortar' strategy for the ordering of nanoparticles into structured assemblies. This methodology allows monolayer-protected gold particles to self-assemble into structured aggregates while thermally controlling their size and morphology. Using 2-nm gold particles as building blocks, we show that spherical aggregates of size 97 +/- 17 nm can be produced at 23 degrees C, and that 0.5-1 microm spherical assemblies with (5-40) x 10(5) individual subunits form at -20 degrees C. Intriguingly, extended networks of approximately 50-nm subunits are formed at 10 degrees C, illustrating the potential of our approach for the formation of diverse structural motifs such as wires and rods. These findings demonstrate that the assembly process provides control over the resulting aggregates, while the modularity of the 'bricks and mortar' approach allows combinatorial control over the constituents, providing a versatile route to new materials systems.

Ultrahigh-density nanowire arrays grown in self-assembled diblock copolymer templates.

We show a simple, robust, chemical route to the fabrication of ultrahigh-density arrays of nanopores with high aspect ratios using the equilibrium self-assembled morphology of asymmetric diblock copolymers. The dimensions and lateral density of the array are determined by segmental interactions and the copolymer molecular weight. Through direct current electrodeposition, we fabricated vertical arrays of nanowires with densities in excess of 1.9 x 10(11) wires per square centimeter. We found markedly enhanced coercivities with ferromagnetic cobalt nanowires that point toward a route to ultrahigh-density storage media. The copolymer approach described is practical, parallel, compatible with current lithographic processes, and amenable to multilayered device fabrication.

Photon correlation spectroscopy with high-energy coherent x rays.

We performed x-ray photon correlation spectroscopy on a model suspension of colloidal particles using x rays of three different energies, namely, 8 keV, 13.5 keV, and 19 keV. The observed reduction in the degree of coherence with increasing x-ray energy, as measured by the contrast of the correlation functions, is consistent with theoretical estimates. We show that it is well possible and under certain circumstances even advantageous to perform experiments with coherent x rays at these higher energies. We argue that the reduced absorption may not only allow for thicker samples but also for longer acquisition times because of the reduced radiation damage, thus outweighing in many cases the effect of the reduced coherent flux. The use of higher energy x rays for photon correlation spectroscopy can therefore lead to a substantial increase in the signal-to-noise ratio and constitutes a promising option for future experiments on samples of polymeric or biological origin.

Detection of Surface-Immobilized Components and Their Role in Viscoelastic Reinforcement of Rubber-Silica Nanocomposites.

Immobilized polymer fractions have been claimed to be of pivotal importance for the large mechanical reinforcement observed in nanoparticle-filled elastomers but remained elusive in actual application-relevant materials. We here isolate the additive filler network contribution to the storage modulus of industrial styrene-butadiene rubber (SBR) nanocomposites filled with silica at different frequencies and temperatures and demonstrate that it is viscoelastic in nature. We further quantify the amount of immobilized polymer using solid-state NMR and establish a correlation with the mechanical reinforcement, identifying a direct, strongly nonlinear dependence on the immobilized polymer fraction. The observation of a temperature-independent filler percolation threshold suggests that immobilized polymer fractions may not necessarily form contiguous layers around the filler particles but could only reside in highly confined regions between closely packed filler particles, where they dominate the bending modulus of aggregated particles.

Fabrication and characterization of a biomimetic composite scaffold for bone defect repair.

For successful bone tissue engineering, scaffolds with tailored properties are a basic requirement. The combination of different available materials not only appears to be desirable but also very challenging. In this study, a composite material consisting of hydroxyapatite and collagen was produced by a biomimetic precipitation method and characterized by X-ray diffraction (XRD) and thermogravimetry (TGA). Subsequently, a suspension-quick-freezing and lyophilization method was used to incorporate the hydroxyapatite into a polymeric matrix consisting of collagen and chitosan. Before physicochemical characterization, the highly porous scaffolds were consolidated by a dehydrothermal treatment (DHT). The main attention was focused on the particle size of hydroxyapatite, which should be in the nanometer range. This is relevant to achieve a homogeneous resorption of the material by osteoclasts. Small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), and environmental scanning electron microscopy (ESEM) were used to evaluate the outcome. The results suggest a successful polymeric embedding of nanoscaled hydroxyapatite particles into the matrix of the spongy construct. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.

Memory effect in isothermal crystallization of syndiotactic polypropylene - role of melt structure and dynamics?

The crystalline-memory effect on the crystallization of syndiotactic polypropylene is investigated by differential scanning calorimetry and solid-state NMR spectroscopy. The influence of several parameters in the thermal (pre-)treatment and the crystallization conditions is studied in detail. In agreement with previous reports, the power law behavior of the overall crystal growth rate is found to be remarkably different for melts with and without memory. This has previously been interpreted in terms of changes in the structure and/or the dynamics of the melt (disentangled state, local order), and a variety of NMR experiments is used to detect such potential changes. All our NMR results are identical for melts with and without memory, therefore excluding any large effect of the "memory" on melt structure or dynamics exceeding the percent level of the whole sample volume, and thus supporting more conventional interpretations in terms of persisting nuclei. Samples that were pre-crystallized at lower temperatures exhibit a larger memory effect, and the potential nuclei fraction is a non-equilibrium structure and is restricted to the 0.1% level if it is crystalline or highly ordered.

Direct visualization of random crystallization and melting in arrays of nanometer-size polymer crystals.

Using tapping mode atomic force microscopy, we visualized in direct space and time resolved the changes in viscoelastic properties during crystallization and melting of polyethyleneoxide in 12 nm spheres of a block copolymer mesophase. All spheres crystallized individually and independently, randomly distributed sphere by sphere. Melting of the confined crystals also proceeded in a stochastic manner. Not all spheres melted at the same temperature, indicating different degrees of order of the individual nanometer-size polymer crystals. The independence of the spheres opens the possibility to manipulate material properties of surfaces at the nanometer scale.

Size-selective electrochemical preparation of surfactant-stabilized Pd-, Ni- and Pt/Pd colloids.

A detailed study concerning the size-selective electrochemical preparation of R4N+Br- -stabilized palladium colloids is presented. Such colloids are readily accessible using a simple electrolysis cell in which the sacrificial anode is a commercially available Pd sheet, the surfactant serving as the electrolyte and stabilizer. It is shown that such parameters as solvent polarity, current density, charge flow, distance between electrodes and temperature can be used to control the size of the Pd nanoparticles in the range 1.2-5 nm. Characterization of the Pd colloids has been performed using transmission electron microscopy (TEM), small angle X-ray scattering (SAXS) and X-ray powder diffractometry (XRD) evaluated by Debye-function-analysis (DFA). Possible mechanisms of particle growth are discussed. Experiments directed towards the size-selective electrochemical fabrication of (n-C6H13)4N+Br- -stabilized nickel colloids are likewise described. Finally, a new strategy for preparing bimetallic colloids (e.g., Pt/Pd nanoparticles) electrochemically is presented, based on the use of a preformed colloid (e.g., (n-C8H17)4N+Br- -stabilized Pt particles) and a sacrificial anode (e.g., Pd sheet).

Long-range density fluctuations in orthoterphenyl as studied by means of ultrasmall-angle x-ray scattering.

The structure factor of a fragile glass-forming liquid orthoterphenyl was measured in the previously inaccessible intermediate q range between the conventional light scattering (LS) and small-angle x-ray scattering (SAXS) q ranges using the low-angle scattering beam line at the European Synchrotron Radiation Facility. At low q the structure factor exhibits an excess scattering and matches well the LS data. This excess scattering is due to long-range density fluctuations also observed in the isotropic component of scattered light. At high q the structure factor decays to a plateau corresponding to the isothermal compressibility in agreement with the conventional SAXS data. In the intermediate q range, the structure factor exhibits a power law q dependence which indicates that the excess scattering is due to fractal aggregates of denser domains.

Self-Assembly of Ionically End-Capped Diblock Copolymers.

The self-assembly of ionically end-capped, symmetric polystyrene-polyisoprene diblock copolymers (PS-b-PI) has been studied. Structural data obtained from small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were correlated with the aggregation behavior of charged chain ends as evidenced by a spin probe using electron paramagnetic resonance (EPR) spectroscopy. The resulting mesomorphic structures were shown to be determined by the chain end topology, i.e., the site where the ionic chain end has been introduced chemically: For omega-functionalized diblock copolymers (monofunctional species) microphase separation is significantly stabilized due to the presence of ionic aggregates within the respective phase separated homopolymer domains. In contrast, for salt-free alpha,omega-macrozwitterionic diblock copolymers a marked perturbation of the block copolymer superstructure was found. In this case, the formation of a network of mixed ionic aggregates creates an additional microdomain interface by joining the chemically distinct blocks at their chain ends. The alteration of the degree of microphase separation as observed for the different functionalities can be attributed to conformational changes of the copolymer chain. Chain end association in the present system is reminiscent of certain covalently joined star and graft copolymers.