bluuebunny/crossref_metadata_2025 huggingface dataset:
Stabilized Blue Emission from Polyfluorene‐Based Light‐Emitting Diodes: Elimination of Fluorenone Defects
Authors: Gong X. , Iyer P.K. , Moses D. , Bazan G.C. , Heeger A.J. , Xiao S.S.
Published: April 2003

Polyfluorene (PF)‐based light‐emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low‐energy emission band (at ∼ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low‐energy emission band originates from fluorenone defects which are introduced by photo‐oxidization, thermal oxidation, or during device fabrication. X‐ray photo‐emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron‐transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.
Infiltrating Semiconducting Polymers into Self‐Assembled Mesoporous Titania Films for Photovoltaic Applications
Authors: Coakley K.M. , Liu Y. , McGehee M.D. , Frindell K.L. , Stucky G.D.
Published: April 2003

Interpenetrating networks of organic and inorganic semiconductors are attractive for photovoltaic cells because electron transfer between the two semiconductors splits excitons. In this paper we show that films of titania with a uniform distribution of pore sizes can be made using a block copolymer as a structure‐directing agent, and that 33 % of the total volume of the film can be filled with a semiconducting polymer.
Ultra‐Low Surface Energy Polymers: The Molecular Design Requirements
Authors: Tsibouklis J. , Nevell T.G.
Published: April 2003

In this Research News contribution, the molecular design requirements for the synthesis of polymeric materials that can be used for the fabrication of readily accessible film structures with ultra‐low surface energy characteristics are described.
Metal Nanostructures with Hollow Interiors
Authors: Sun Y. , Mayers B. , Xia Y.
Published: April 2003

We have recently developed a simple and versatile route to the large‐scale synthesis of metal nanostructures with well‐defined hollow interiors. The key step of this process involves a replacement reaction between the surface of a nanoscale template and the solution of an appropriate salt precursor. The capability and feasibility of this method have been demonstrated by preparing hollow nanostructures of gold with a range of different morphologies (e.g., triangular rings, prism‐shaped boxes, cubic boxes, spherical capsules, and tubes). In addition to gold, this method also worked well for other metals such as platinum and palladium (see Figure for an SEM image of Pd nanotubes). These metal nanostructures with hollow interiors are useful as fillers for generation of ultralight composites; they are also interesting for new types of applications that include use as components for highly sensitive colorimetric sensors and as effective, recoverable catalysts.
Fullerene‐like WS2 Nanoparticles: Superior Lubricants for Harsh Conditions
Authors: Rapoport L. , Fleischer N. , Tenne R.
Published: April 2003

WS2 nanoparticles with closed‐cage structure (fullerene‐like IF) are already being synthesized in macroscopic amounts from the respective oxide nanoparticles. They have been studied as superior solid lubricants under harsh conditions in recent years. Under severe contact conditions both fluids and greases are squeezed out from the contact area and consequently do not provide adequate lubrication conditions. Addition of even a small amount of IF nanoparticles to the oil was found to reduce the friction coefficient and wear rate, and increase the load‐bearing capacity of the friction pairs. Furthermore, IF nanoparticles were impregnated into polymer and metal coatings and into self‐lubricating porous metal parts, and were found to alleviate both friction and wear remarkably well. In another set of experiments, IF nanoparticles were shown to provide excellent tribological behavior for the contact between a ceramic alumina block and silicon nitride ball.The mechanism for the impro
ved tribological behavior of the IF nanoparticles is being investigated. In addition to the rolling friction, gradual exfoliation of the IF onions and transfer of monomolecular WS2 sheets onto the metal surface (third body transfer) is shown to play a major role in alleviating friction and wear. This work suggests numerous applications for this new solid‐state nanolubricant.
Oxide‐Assisted Growth of Semiconducting Nanowires
Authors: Zhang R.‐Q. , Lifshitz Y. , Lee S.‐T.
Published: April 2003

In this contribution, we outline oxide‐assisted growth (OAG) (distinct from the conventional metal‐catalytic vapor–liquid–solid (VLS) process) for the growth of nanostructured materials. This synthesis technique, in which oxides instead of metals play an important role in inducing the nucleation and growth of nanowires, is capable of producing large quantities of high‐purity silicon nanowires with a preferential growth direction, uniform size, and long length, without the need for a metal catalyst. The OAG 1D nanomaterials synthesis is complementary to, and coexistent with, the conventional metal‐catalyst VLS approach, and can be utilized to produce nanowires from a host of materials other than Si including Ge nanowires, carbon nanowires, silicon and SnO2 nanoribbons, and Group III–V and II–VI compound semiconductor nanowires.
A New Synthetic Approach to Silicon Colloidal Photonic Crystals with a Novel Topology and an Omni‐Directional Photonic Bandgap: Micromolding in Inverse Silica Opal (MISO)
Authors: Míguez H. , Tétreault N. , Yang S.M. , Kitaev V. , Ozin G.A.
Published: April 2003

High refractive index contrast silicon colloidal photonic crystals having a different topology to those previously reported have been synthesized. A new process of colloidal‐crystal templating based on micromolding in inverse silica opals (MISO) is used (see Figure), and photonic crystals so built present a full photonic bandgap, as indicated by band structure calculations.
Meso/Macroporous Inorganic Oxide Monoliths from Polymer Foams
Authors: Maekawa H. , Esquena J. , Bishop S. , Solans C. , Chmelka B.F.
Published: April 2003

Low‐density bimodal inorganic oxide monoliths (see Figure) have been prepared via a two‐step procedure using macroporous polystyrene foams and amphiphilic block copolymer structure‐directing agents. The resulting meso/macroporous inorganic framework structures could be obtained with different 3D bulk shapes and for a range of inorganic oxides, as demonstrated here for silica, zirconia, and titania.
Growth Direction and Cross‐Sectional Study of Silicon Nanowires
Authors: Li C.‐P. , Lee C.‐S. , Ma X.‐L. , Wang N. , Zhang R.‐Q. , Lee S.‐T.
Published: April 2003

Cross‐sectional samples of silicon nanowires (SiNWs) are examined using transmission electron microscopy. The cross‐sections are bounded by well‐defined low‐index crystallographic facets of various shapes (e.g., the square section in the Figure, 50 nm edge) and characterized by shape‐dependent growth directions, with 〈112〉 and 〈110〉 predominating. Both shape and growth direction are consistent with surface energy considerations and growth mechanisms.
Solid‐State Rechargeable Magnesium Batteries
Authors: Chusid O. , Gofer Y. , Gizbar H. , Vestfrid Y. , Levi E. , Aurbach D. , Riech I.
Published: April 2003

The development of all solid‐state rechargeable magnesium battery systems is reported, with components that are environmentally friendly and relatively simple in their structure and preparation. As anodes, magnesium alloys containing Zn and Al are used, and the cathode is the chevrel phase, Mo6S8, which can insert two magnesium atoms per unit (Mg2Mo6S8, 122 mA h g–1). The solid electrolyte is a gel comprising polyvinylidene difluoride, Mg(AlCl2EtBt)2 complex salt, and tetraglyme as a plasticizer. These batteries are found to function well in a temperature range of 0–80 °C with a voltage range of 1.3–0.8V.
Novel Synthesis of Pt6Si5 Nanowires and Pt6Si5–Si Nanowire Heterojunctions by Using Polycrystalline Pt Nanowires as Templates
Authors: Luo J. , Zhang L. , Zhu J.
Published: April 2003

Pt6Si5 nanowires and Pt6Si5–Si nanowire heterojunctions (see Figure) have been synthesized by electrodeposition using polycrystalline Pt nanowires as templates. In the synthesis, the polycrystalline nature of the Pt nanowires played an important role. A dependence between the diameters of the products and the Pt nanowires was detected. These junctions would find applications in future nano‐electronics.
High‐Performance, Quantum Dot Nanocomposites for Nonlinear Optical and Optical Gain Applications
Authors: Petruska M.A. , Malko A.V. , Voyles P.M. , Klimov V.I.
Published: April 2003

The preparation of CdSe nanocrystal quantum dot (NQD)–titania nanocomposites (see Figure) exhibiting large NQD packing densities and high photoluminescence quantum efficiencies is described. These new materials show both large third‐order nonlinear susceptibilities and optical gain. Furthermore, micro‐ring lasing and efficient dynamic holographic gratings using these nanocomposites are demonstrated.
High Permittivity from Defective Multiwalled Carbon Nanotubes in the X‐Band
Authors: Watts P.C.P. , Hsu W.‐K. , Barnes A. , Chambers B.
Published: April 2003

Defective multiwalled carbon nanotubes display higher real permittivity than graphitic carbon nanotubes (CNTs). That is the conclusion of the study presented here in which the permittivity of Fe‐filled and boron‐doped CNTs in the X‐band is compared with that of graphitic CNTs made by arc discharge. However, the imaginary parts of the permittivity are still very low in all three cases. This finding indicates that defective CNTs are excellent dielectric materials and can be used as supercapacitor components, i.e., high‐energy storage systems.
Spin‐on Nanostructured Silicon–Silica Film Displaying Room‐Temperature Nanosecond Lifetime Photoluminescence
Authors: Cohen Y. , Hatton B. , Míguez H. , Coombs N. , Fournier‐Bidoz S. , Grey J.K. , Beaulac R. , Reber C. , Ozin G.A.
Published: April 2003

A yellow transparent mesoporous silica film has been achieved by the incorporation of silicon nanoclusters into its channels. The resulting nanocomposite—fabricated using a combination of evaporation induced self‐ assembly and chemical vapor deposition—emits light brightly at visible wavelengths and has nanosecond radiative lifetimes at room temperature when excited by ultraviolet light (see Figure).
Universal Route to Cell Micropatterning Using an Amphiphilic Comb Polymer
Authors: Hyun J. , Ma H. , Zhang Z. , Beebe Jr. T.P. , Chilkoti A.
Published: April 2003

Long‐term, spatially resolved attachment and growth of mammalian cells in a biologically relevant milieu on a variety of substrates is possible using the micropatterning techniques presented here (see cover). Two general methods, both involving microcontact printing of an amphiphilic comb polymer (see Figure) and incubation with a protein‐containing solution are reported.
Single‐Crystalline In2O3 Nanotubes Filled with In
Authors: Li Y. , Bando Y. , Golberg D.
Published: April 2003

Single‐crystalline In2O3 nanotubes with cubic structure were synthesized via a physical vapor depostion process. The nanotubes grew along the 〈111〉 direction and were continuously filled with metallic In (see Figure). Photoluminescence measurement results displayed strong orange emission. Indium‐filled In2O3 nanotubes represent an exciting system and offer great potential for application in electronic devices or “nanothermometers”.
Light‐Driven Side‐On Nematic Elastomer Actuators
Authors: Li M.‐H. , Keller P. , Li B. , Wang X. , Brunet M.
Published: April 2003

A range of monodomain nematic azo side‐on elastomers, containing different proportions of photoisomerizable azo mesogenic moieties, were prepared for the first time photochemically using a radical photoinitiator working at wavelengths above 600 nm. These elastomers exhibit a photomechanical contraction of up to 20 % upon irradiation with UV light. The Figure shows an elastomer film in its contracted form under UV irradiation.
Nanoscale Ultraviolet‐Light‐Emitting Diodes Using Wide‐Bandgap Gallium Nitride Nanorods
Authors: Kim H.‐M. , Kang T.W. , Chung K.S.
Published: April 2003

The fabrication of p–n junctions in individual GaN nanorods has been realized using hydride vapor phase epitaxy. Application of the resulting heterostructures as wide bandgap current rectifiers with a high breakdown voltage and for near UV light‐emitting diodes is demonstrated. The Figure is a luminescence image of the light emitted from a forward‐biased nanorod p–n junction at 3 V.
Excellent Wave Absorption by Zirconium‐Based Bulk Metallic Glass Composites Containing Carbon Nanotubes
Authors: Bian Z. , Wang R.J. , Pan M.X. , Zhao D.Q. , Wang W.H.
Published: April 2003

Bulk metallic glass composites containing carbon nanotubes (CNTs) are reported (see Figure). The composites are found to have strong ultrasonic wave‐absorption abilities, which increase dramatically with increasing volume fraction of CNTs. This ability implies that such composites, besides their excellent mechanical properties, may also have a significant potential of application in shielding acoustic sound or environmental noise.
Field‐Induced Formation of the Polar SmCP Phase above the SmCP–Isotropic Transition
Authors: Weissflog W. , Schröder M.W. , Diele S. , Pelzl G.
Published: April 2003

The field‐induced formation of an antiferroelectric “banana mesophase” (SmCPA), which can be observed at up to 9 K above the transition SmCP → isotropic, is reported. This behavior may be due to the existence of ferroelectric clusters already in the isotropic liquid that are aligned by means of the external electric field. The Figure displays the field‐induced nucleation of the SmCP phase within the isotropic liquid.
Extreme Changes in the Electrical Resistance of Titania Nanotubes with Hydrogen Exposure
Authors: Varghese O.K. , Gong D. , Paulose M. , Ong K.G. , Dickey E.C. , Grimes C.A.
Published: April 2003

Extremely sensitive, drift‐free, and robust hydrogen sensors based on titania nanotube arrays (see Figure) are reported. Very large changes in the electrical conductivity of the nanotubes upon exposure to hydrogen gas have been measured, over one order of magnitude greater than the previous record. Dependence of the electrical conductivity on both temperature and nanotube pore size is found.
Tunable Inverse Opal Hydrogel pH Sensors
Authors: Lee Y.‐J. , Braun P.V.
Published: April 2003

The synthesis, swelling kinetics, and optical diffraction of inverse opal hydrogel sensors (see Figure) are demonstrated. Colloidal crystal templating is used to prepare methacrylate/acrylic acid (AA) copolymers that have an interconnected pore structure, allowing facile diffusion and a relatively rapid response. The system shows good mechanical stability and pH‐dependent shifts in optical diffraction regulated by the AA concentration.
Multiple Ordering Transitions: Hierarchical Self‐Assembly of Rod–Coil Block Copolymers
Authors: Park J.‐W. , Thomas E.L.
Published: April 2003

Organized structures over length scales differing by 103are reported, i.e., crystals of rod block (unit cell ∼ 1 nm), block copolymer microdomains (layer periodicity ∼ 50 nm), and Néel domain walls (wall periodicity ∼ 1000 nm)—see Figure and inside front cover. The result is an example of combined dynamic and static self‐ assembly, which creates an unconventional morphology with multiple length scales in a single‐step process.
Infrared Detectors with Poly(3,4‐ethylenedioxy thiophene)/Poly(styrene sulfonic acid) (PEDOT/PSS) as the Active Material
Authors: Meskers S.C.J. , van Duren J.K.J. , Janssen R.A.J. , Louwet F. , Groenendaal L.
Published: April 2003

Fast‐response large‐area plastic infrared detectors (see Figure (Wheatstone bridge configuration)) made from a conducting PEDOT/PSS film achieve a detectivity close to 105 cm √Hz W–1 at 1.18 eV. The nanosecond response arises from the direct absorption of infrared light by the charge carriers on the polymer chain. The recovery of the device after illumination is much slower and is governed by heat transport from the polymer.
Biomimetic Formation of Porous Single‐Crystalline CaCO3 via Nanocrystal Aggregation
Authors: Zhan J. , Lin H.‐P. , Mou C.‐Y.
Published: April 2003

Hexagonal CaCO3 vaterite particles with intracrystalline gelatin were obtained from calcium nitrate and urea in the presence of gelatin. The particles have the crystallographic nature of vaterite (see Figure: an SEM image). Detailed analysis indicates that the crystals grow from vaterite nanocrystals by self‐oriented aggregation growth, with gelatin playing a role in both the stabilization and the oriented attachment of the nanocrystals.
Focused Microwave‐Assisted Synthesis of 2,5‐Dihydrofuran Derivatives as Electron Acceptors for Highly Efficient Nonlinear Optical Chromophores
Authors: Liu S. , Haller M.A. , Ma H. , Dalton L.R. , Jang S.‐H. , Jen A.K.‐Y.
Published: April 2003

Highly efficient and tunable electron acceptors composed of a diverse family of 2,5‐dihydrofuran derivatives have been synthesized using single‐mode focused microwave irradiation. A very large electro‐optic coefficient and long‐term temporal stability (see Figure) have been demonstrated in a host/guest system composed of polyquinoline and the nonlinear optical chromophore shown in the Figure.
Directed Growth of Polymethylene Films on Atomically Modified Gold Surface
Authors: Guo W. , Jennings G.K.
Published: April 2003

Gold surfaces patterned with underpotentially deposited copper and/or silver monolayers are used to direct the growth of highly insulating polymethylene films. Copper adatoms function as active catalytic sites to enhance polymer film growth over that of unmodified gold surfaces (see Figure). In contrast, a silver monolayer is inactive toward the polymerization and can be patterned to locally prevent film growth.
Synthesen mit anionischen Templaten
Authors: Vilar Ramón
Published: April 2003

Im Unterschied zu den gut untersuchten templatgesteuerten Synthesen mit kationischen und neutralen Templaten sind nur wenige Beispiele von Synthesen mit anionischen Templaten bekannt. Dies wurde einigen der intrinsischen Eigenschaften der Anionen wie der diffusen Elektronenhülle, der Empfindlichkeit gegen pH‐Änderungen und ihrer relativ hohen Freien Solvatationsenergien zugeschrieben. Allerdings zeigt die zunehmende Zahl von Aggregaten, die mit anionischen Templaten erhalten und im Laufe der letzten Jahre beschrieben wurden, dass diese Einschränkungen nicht so entscheidend sind wie zunächst gedacht. Dieser Aufsatz fasst die wichtigsten Ergebnisse im Bereich anionengesteuerter Templatreaktionen zur Synthese einer breiten Vielfalt anorganischer und organischer Aggregate (z. B. Makrocyclen, Käfige, Helicate, Rotaxane und kontinuierliche Strukturen) zusammen. Ziel ist es, einen Anstoß zu weitergehenden Forschungen in dieser jungen Teildisziplin der Supramolekularen Chemie zu geben.
Phosphaorganische Chemie: Panorama und Perspektiven
Authors: Mathey François
Published: April 2003

Die gesamte phosphororganische Chemie wurde mit Beginn der siebziger Jahre infolge der Entdeckung stabiler phosphororganischer Derivate mit Koordinationszahlen des Phosphors von 1 oder 2 aufgefrischt. Bezüglich ihrer Chemie gleichen diese Verbindungen ihren reinen Kohlenstoff‐Analoga. Dieser Aufsatz widmet sich der Analogie zwischen den Phosphorderivaten und den entsprechenden Alkenen, Alkinen und Carbenen, wobei jede Verbindungsklasse anhand ihrer Synthese, ihrer Reaktivität und ihres Koordinationstyps kurz vorgestellt wird. Des Weiteren werden besondere Elektronenkonfigurationen der Phosphorverbindungen behandelt, darunter P‐P‐Einelektronenbindungen, gespannte Bindungen und aromatische Systeme. Abschließend wird auf mögliche Anwendungen dieser Chemie zur Herstellung molekularer Materialien und in der homogenen Katalyse verwiesen.
Gemini‐Tenside: neue synthetische Vektoren zur Gentransfektion
Authors: Kirby Anthony J. , Camilleri Patrick , Engberts Jan B. F. N. , Feiters Martin C. , Nolte Roeland J. M. , Söderman Olle , Bergsma Mark , Bell Paul C. , Fielden Matthew L. , García Rodríguez Cristina L. , Guédat Philippe , Kremer Andreas , McGregor Caroline , Perrin Christele , Ronsin Gaël , van Eijk Marcel C. P.
Published: April 2003

Die oberflächenaktiven Eigenschaften kationischer Gemini‐Tenside werden zur Lösung des komplexen Problems der Gentransfektion in Zellen verwendet. Von nahezu 250 getesteten Substanzen, denen etwa 20 verschiedene Struktur‐Typen zugrunde liegen, zeigten viele eine gute In‐vitro‐Transfektionsaktivität. Das Tensid bindet und komprimiert DNS wirkungsvoll. Strukturelle und rechnerische Untersuchungen liefern ein brauchbares Modell des geformten Lipoplexes. Der Lipoplex kann durch die äußeren Membranen verschiedener Zelltypen dringen und so ins Cytoplasma gelangen. Die Freisetzung aus dem Endosom – ein Schlüsselschritt der Transfektion – könnte durch Veränderungen im Aggregationsverhalten des Lipoplexes bei fallendem pH‐Wert kontrolliert werden. Dies deutet auf ein Loslösen der DNS aus dem Lipoplex vor dem Eintritt in den Zellkern hin, wo das neue Gen mit hoher Effizienz exprimiert werden kann.
Titelbild: Transition‐Metal‐Free Suzuki‐Type Coupling Reactions (Angew. Chem. 12/2003)
Authors: Leadbeater Nicholas E. , Marco Maria
Published: April 2003

Biarylverbindungen entstehen in einer Kupplungsreaktion vom Suzuki‐Typ in Wasser auch ohne einen Übergangsmetallkatalysator. Die Suzuki‐Reaktion (palladiumkatalysierte Kupplung von Arylhalogeniden mit Boronsäuren) hat sich zur Methode der Wahl für die Synthese von Biarylverbindungen entwickelt. Es lassen sich jedoch auch Bedingungen finden, unter denen katalysatorfreie Kupplungen vom Suzuki‐Typ möglich sind. Einzelheiten dazu finden sich in der Zuschrift von N. E. Leadbeater und M. Marco auf S. 1445 f.
Ungewöhnliche Komplexchemie der Seltenerdelemente: große Ionenradien – kleine Koordinationszahlen
Authors: Dehnicke Kurt , Greiner Andreas
Published: April 2003

Seltenerdelemente weisen wegen ihrer großen Ionenradien bei relativ niedrigen Oxidationszahlen in ihren Komplexen im Allgemeinen große Koordinationszahlen mit schwachen Metall‐Ligand‐Bindungen auf. Die Instabilität der Konfiguration dieser Komplexe in Lösung macht sie für die homogene Katalyse vielfach ungeeignet. Abhilfe versprechen Komplexe mit niedrigen Koordinationszahlen an den Metallatomen. Diese lassen sich auf klassischem Weg durch Verwendung voluminöser Liganden oder neuerdings durch Ligandengruppen mit besonderen Metall‐Ligand‐Bindungseigenschaften herstellen. Anhand von Komplexen mit dem sperrigen Bis(trimethylsilyl)amido‐Liganden und mit den “schlanken” Phosphaniminato‐Liganden (NPR3−) wird dieses Konzept vorgestellt und die Eignung dieser Komplexe als Katalysatoren für die Ringöffnungspolymerisation von Lactonen beschrieben.
Titelbild: Cyanide‐Bridged Iron(III)–Cobalt(II) Double Zigzag Ferromagnetic Chains: Two New Molecular Magnetic Nanowires (Angew. Chem. 13/2003)
Authors: Lescouëzec Rodrigue , Vaissermann Jacqueline , Ruiz‐Pérez Catalina , Lloret Francesc , Carrasco Rosa , Julve Miguel , Verdaguer Michel , Dromzee Yves , Gatteschi Dante , Wernsdorfer Wolfgang
Published: April 2003

Magnetische Nanodrähte gehören zu den neuen Objekten, die Chemiker auf dem Weg zur magnetischen Informationsspeicherung entwerfen. Das Titelbild zeigt Ansichten einer geordneten Dimetalldoppelkette aus Eisen(III)‐ und Cobalt(II)‐Ionen, die durch Cyanidgruppen verbrückt sind. Jede Kette hat bemerkenswerte anisotrope magnetische Eigenschaften (die Achsen einfacher Magnetisierung sind angegeben) und zeigt eine langsame Relaxation der Magnetisierung. Mehr über diese Nanodrähte erfahren Sie in der Zuschrift von M. Julve und M. Verdaguer et al. auf S. 1521 ff.
Titelbild: Absolute Asymmetric Photocyclization of a Photochromic Diarylethene Derivative in Single Crystals (Angew. Chem. 14/2003)
Authors: Yamamoto Satoshi , Matsuda Kenji , Irie Masahiro
Published: April 2003

Absolute asymmetrische Photochromie in chiralen Kristallen ist hier an 1,2‐Bis(2‐methyl‐5‐(3‐formylphenyl)‐3‐thienyl)perfluorcyclopenten visualisiert, das in einer chiralen Raumgruppe kristallisiert. Nach UV‐Bestrahlung der Diarylethen‐Einheit erfolgt im Einkristall eine Cyclisierung, wobei jede der beiden enantiomeren Kristallformen ausschließlich ein Enantiomer ergibt. Bei abwechselnder Bestrahlung mit UV‐ und sichtbarem Licht wird ein Photocyclisierungs/ringöffnungs‐Zyklus durchlaufen. Mehr hierzu enthält die Zuschrift von M. Irie et al. auf S. 1674 ff.
Anion‐Templated Synthesis
Authors: Vilar Ramón
Published: April 2003

In contrast to the well‐studied templating properties of cationic and neutral species there are relatively few examples of anion‐templated syntheses. This is attributed to some of the intrinsic properties of anions such as their diffuse nature, pH sensitivity, and their relative high solvation free energies. However, the increasing number of anion‐templated assemblies reported over the past few years demonstrates that these limitations are not as critical as first thought. This review summarizes the most important results on the use of anions as directing agents for the syntheses of a wide range of inorganic and organic assemblies (such as macrocycles, cages, helicates, rotaxanes, and extended structures). It is hoped that this will stimulate a closer look into this emerging area of supramolecular chemistry.
Phospha‐Organic Chemistry: Panorama and Perspectives
Authors: Mathey François
Published: April 2003

Since the beginning of the seventies, organophosphorus chemistry has been completely rejuvenated by the discovery of stable derivatives in which phosphorus has the coordination numbers one or two. The chemistry of these compounds mimics the chemistry of their all‐carbon analogues. In this Review article this analogy is discussed for the phosphorus counterparts of alkenes, alkynes, and carbenes. In each case, the synthesis, reactivity, and coordination modes are briefly examined. Some special electronic configurations are also discussed, which include one‐electron PP bonds, strained bonds, and aromatic systems. To conclude, some potential applications of this chemistry in the areas of molecular materials and homogeneous catalysis are presented.
Gemini Surfactants: New Synthetic Vectors for Gene Transfection
Authors: Kirby Anthony J. , Camilleri Patrick , Engberts Jan B. F. N. , Feiters Martin C. , Nolte Roeland J. M. , Söderman Olle , Bergsma Mark , Bell Paul C. , Fielden Matthew L. , García Rodríguez Cristina L. , Guédat Philippe , Kremer Andreas , McGregor Caroline , Perrin Christele , Ronsin Gaël , van Eijk Marcel C. P.
Published: April 2003

The superior surfactant properties of cationic gemini surfactants are applied to the complex problem of introducing genes into cells. Of almost 250 new compounds tested, of some 20 different structural types, a majority showed very good transfection activity in vitro. The surfactant is shown to bind and compact DNA efficiently, and structural studies and calculations provide a working picture of the “lipoplex” formed. The lipoplex can penetrate the outer membranes of many cell types, to appear in the cytoplasm encapsulated within endosomes. Escape from the endosome—a key step for transfection—may be controlled by changes in the aggregation behavior of the lipoplex as the pH falls. The evidence suggests that DNA may be released from the lipoplex before entry into the nucleus, where the new gene can be expressed with high efficiency.
Cover Picture: Cyanide‐Bridged Iron(III)–Cobalt(II) Double Zigzag Ferromagnetic Chains: Two New Molecular Magnetic Nanowires (Angew. Chem. Int. Ed. 13/2003)
Authors: Lescouëzec Rodrigue , Vaissermann Jacqueline , Ruiz‐Pérez Catalina , Lloret Francesc , Carrasco Rosa , Julve Miguel , Verdaguer Michel , Dromzee Yves , Gatteschi Dante , Wernsdorfer Wolfgang
Published: April 2003

Magnetic nanowires are among new objects that chemists can design with a view to magnetic information storage. The cover picture shows views of an ordered bimetallic double chain of iron(III) and cobalt(II) ions, bridged by cyanide groups. Each chain has remarkable anisotropic magnetic properties (axes of easy magnetization are shown) and shows slow relaxation of the magnetization. Find out more about these nanowires in the Communication by M. Julve, M. Verdaguer et al. on page 1483 ff.
Cover Picture: Absolute Asymmetric Photocyclization of a Photochromic Diarylethene Derivative in Single Crystals (Angew. Chem. Int. Ed. 14/2003)
Authors: Yamamoto Satoshi , Matsuda Kenji , Irie Masahiro
Published: April 2003

Absolute Asymmetric Photochromism in chiral crystals has enormous potential. The cover picture shows this phenomenon for 1,2‐bis(2‐methyl‐5‐(3‐formylphenyl)‐3‐thienyl)perfluorocyclopentene, which has no chiral resources, but forms chiral crystals. Upon UV irradiation the diarylethene unit undergoes the photochromic reaction both in the single crystals and in solution, and each chiral crystal gives one enantiomer. The photocyclization/cycloreversion cycle can be repeated by using UV and visible light alternately. Details of the study are reported by M. Irie et al. on page 1636 ff.
A viscosity–molecular weight relationship for polydimethylsiloxanes
Authors: Takimoto H. H. , Forbes C. T. , Laudenslager R. K.
Published: April 2003

The relationship of intrinsic viscosity [η] to the weight‐average molecular weight M̄w of polydimethylsiloxane in toluene at 25°C. has been determined. The modified Staudinger equation, [η] = KM̄wa, with K = 2.43 × 10−5 and a = 0.84, describes the relationship for the above polymers in the range of weight‐average molecular weight from 18,900 to 124,500 (as determined by light‐scattering measurements). These constants differ from the values reported by Barry, where K = 2 × 10−4 and a = 0.66 were obtained. The difference is attributed to the fact that Barry dealt with number‐average rather than weight‐average molecular weight.
Crosslinking efficiency of dicumyl peroxide in natural rubber
Authors: Thomas D. K.
Published: April 2003

The rate of decomposition of dicumyl peroxide in natural rubber has been measured at four temperatures and the results lead to the expression k = 3.9 × 1013e−32500/RT for the rate constant. The crosslinking efficiency is high and is constant over the range 110 to 140°C. The low values found for the ratio of acetophenone to cumyl alcohol in the reaction products indicate that hydrogen abstraction is brought about mainly by cumyloxy radicals, although it seems likely that methyl radicals present are also effective in producing crosslinks. No cumyloxy radicals become attached to the polymer during the crosslinking reaction.
The thermal diffusivity of polymer melts
Authors: Shoulberg Robert H.
Published: April 2003

A study of the thermal diffusivity of polymer melts as a function of temperature over the range 25–325°C. is described. Measurements were carried out with an apparatus especially designed and constructed for this purpose; this apparatus and the theoretical treatment of data obtained with it are treated in detail. The polymers examined include polystyrene, poly(methyl methacrylate), some of their copolymers, poly(vinyl chloride), linear polyethylene, polyacetal, polycarbonate, cellulose–acetate–butyrate, some “highimpact” polymer blends and terpolymers, and some higher methacrylates. The thermal diffusivities of the melts vary from about 11 × 10−4 cm.2/sec. (for linear polyethylene) to about 5 × 10−4 cm.2/sec. (for poly(sec‐butyl methacrylate)). In general the thermal conductivity of an amorphous polymer decreases as it passes from the glassy state to a melt, but remains quite constant over a wide temperature range in the melt state. On the basis of a limited amount of data one might tentatively conclude that the thermal diffusivities of the melts of vinyl polymers become lower as larger as longer groups are added to the carbon‐carbon backbone. As regards molecular weights equal to those common for molding powders, molecular weight seems to have no effect on the thermal diffusivity.
Thermal degradation of fluorine‐containing polymers. PART II. Degradation in oxygen
Authors: Cox J. M. , Wright B. A. , Wright W. W.
Published: April 2003

The thermal degradation in oxygen of a number of fluorine‐containing polymers has been studied by using a weight loss technique. The relative thermal stabilities of the different polymers have been determined and, where possible, the overall activation energy and frequency factors for their breakdown. The results have been compared with those for samples of the same polymers degraded in vacuum. There is little change in the relative thermal stabilities observed under both conditions, the fully fluorinated structures being the most stable. The changes in the rates of breakdown in the presence of oxygen are difficult to interpret in the absence of mechanistic information.
Effect of the chemical and physical properties of carbon black on its performance in polyethylene
Authors: Williams F. R. , Jordan M. E. , Dannenberg E. M.
Published: April 2003

A comprehensive study of the performance of various types of carbon black in polyethylene has been made. The physical and chemical properties of carbon blacks are discussed, and the effectiveness of various blacks as ultraviolet screening agents for polyethylene is assessed. It is seen as a first approximation that primary particle size is the property of the carbon black which largely governs the weatherability of a compound. The data suggest that this is due to that fact that the degree of ultraviolet protection is dependent on the presence of submicroscopic aggregates and their distribution throughout the polymer mass. It is proposed that the aggregate size and distribution are largely controlled by the surface energy arising from the dimensions of the primary particles. Studies of carbon black‐antioxidant systems gave results which were in agreement with those of Hawkins et al., who found that carbon blacks having a polar surface exhibit a synergistic behavior with organosulfur type antioxidant, greatly increasing the time required to absorb a given amount of oxygen or undergo thermal degradation. The present study shows that use of antioxidants also contributes to the ability of polyethylene compounds to resist accelerated aging. Outdoor aging of the same compounds are in progress, but more time is required to see if antioxidants are helpful in the manner indicated by the accelerated weathering studies. The effect of carbon blacks on the mechanical properties of polyethylene is discussed, and data are presented to show that higher loadings of carbon black than normally used for ultraviolet protection greatly increase the tensile yield (stress at draw point) at relatively fast rates of applied stress. It is suggested that the use of moderately high concentrations of carbon black would enhance the performance of polyethylene in applications such as pipe which often fails under rapidly applied stress.
Poly(vinyl alcohol)–iodine complexes
Authors: Zwick M. M.
Published: April 2003

The poly(vinyl alcohol)–iodine blue color reaction in dilute aqueous solution has been investigated, and extinctions at the absorption maximum have been measured as a function of the concentrations of polymer, iodine, iodide ions, and boric acid. Depending upon the reaction conditions, the main absorption maximum can be made to appear at any wavelength between 580 and 700 mμ, with the longest wavelengths and highest extinctions per vinyl alcohol group showing up at high boric acid concentrations. Poly(vinyl alcohol) samples from various sources displayed, under identical reaction conditions, great differences in staining intensities. This can be ascribed to differences in the regularity of the polymer chain structures. To account for the behavior of poly(vinyl alcohol) towards iodine under a wide variety of reaction conditions, as well as for a number of irreversible or only slowly reversible phenomena, a model for the mechanism of the PVA–iodine complex formation is proposed involving a dual process of helix formation and intramolecular helix association. There is evidence to indicate that at full iodine saturation of the polymer, 12 vinyl alcohol residues supported by one boric acid molecule form a single turn of a helix which enwraps one iodine atom, out of a long polyiodide chain nested in the interior of a poly(vinyl alcohol) helix.
Stress softening in rubber vulcanizates. Part I. Use of a strain amplification factor to describe the elastic behavior of filler‐reinforced vulcanized rubber
Authors: Mullins L. , Tobin N. R.
Published: April 2003

Measurements of Young's modulus of vulcanized rubbers containing thermal carbon black show the predicted dependence on the volume concentration given by relationships derived for a suspension of spherical particles, for example, that due to Guth, Simha, and Gold which gives E = E0 (1 + 2.5c + 14.1c2). A simple interpretation of the results is that the strain in the rubber is increased by the presence of filler so that the ratio of the average strain to the measured overall strain is given by the factor X = 1 + 2.5c + 14.1c2. This factor was used to analyze simple extension stress–strain data obtained at larger extensions. For this purpose the Mooney‐Rivlin relation was used to describe the behavior of the rubber phase. Values of C1 independent of the volume concentration and in close accord with measurements of the equilibrium volume swelling of the rubbers were obtained. Values of λ* were also consistent with those of C1. Analysis of stress–strain data obtained on rubbers containing smaller particle‐sized carbon blacks is more complex. For these materials the relation due to Guth, viz., E = E0 (1 + 0.67fc + 1.62f2c2), was chosen. By the choice of suitable values of f, good agreement with the Mooney‐Rivlin stress–strain relation was achieved at volume concentrations less than about 0.15.
A light‐scattering and membrane formation study on concentrated cellulose acetate solutions
Authors: Kunst B. , Škevin Đ. , Deželić Gj. , Pétres J. J.
Published: April 2003

An improved explanation of the mechanism of the porous membrane formation with asymmetric structure has been proposed. It emerged from the correlation of the results obtained by the light‐scattering measurements of the ternary membrane casting solutions and by the reverse osmosis and ultrafiltration testing of the asymmetric membranes prepared from these solutions. The results of anisotropic light‐scattering measurements indicated the extent of supermolecular structurization of the casting solution which affected the porous structure formation in the membrane surface region. The variation of the casting solution composition and, particularly, the role of the CA/F ratio, the formamide content, and their influence on the asymmetric membrane properties were investigated and explained using the improved concept of the membrane‐making mechanism.
Kinetics and energetics of a fast polyurethane cure
Authors: Lipshitz Stephen D. , Macosko Christopher W.
Published: April 2003

Methods for obtaining kinetic data on bulk polymerizing systems are reviewed, and adiabatic temperature rise is found to be the most useful for fast systems. An adiabatic method is described and used to study the polymerization of poly(ε‐caprolactone triol) with chain‐extended 1,6‐hexane diisocyanate catalyzed with dibutyltin dilaurate.
Thermal stability of phosphonitrilic fluoroelastomers
Authors: Valaitis J. K. , Kyker G. S.
Published: April 2003

The thermal stability of the polymer prepared by substituting poly(dichlorophosphazene) with an excess of a 64/36 mole‐% mixture of the sodium salts of 2,2,2‐trifluoroethanol/2,2,3,3,4,4,5,5‐octafluoropentanol were studied by thermogravimetric and gel permeation chromatography techniques. The primary mode of degradation of this polymer was shown to be via random scission. The overall degradation reaction corresponds to quasi‐zero‐order kinetics.
Laser marking in dye–polymer systems
Authors: Novotny V. , Alexandru L.
Published: April 2003

Light‐induced marking in dye–polymer systems based on dye diffusion was investigated. It was demonstrated that when a dye film coated on an appropriate polymer substrate is exposed to a laser beam, marked areas are obtained. In the exposed areas the dye melts and diffuses into the substrate and also flows laterally. The dye, when diffused into the polymer, shows altered spectral characteristics, and significant changes of optical densities can occur in the marked areas. A direct optical marking process based on these phenomena is described. Good spatial resolution and adequate marking sensitivities were demonstrated as submicron marks and energy density requirements below 100 mJ/cm2 were obtained.
Active transport membrane for chlorine ion
Authors: Ogata Naoya , Sanui Kohei , Fujimura Hideo
Published: April 2003

A specific functional group that could interact with ions was introduced in a synthetic membrane to achieve an active transport of ions. One way to synthesize the active transport membrane was to introduce a functional group which has a tautomerism upon pH changes in an aqueous solution. A polymer having pendent N‐hydroxyethyl amide groups was synthesized to form a membrane, and the membrane was fixed in a cell as a partition film, in which one side of the solution was adjusted to be acidic and the other side basic. It was then possible to transport chlorine ion through the membrane owing to the carrier functions caused by tautomerism of the N‐hydroxyethyl amide group from the acidic to the basic sides. The transport of the chlorine ion was not dependent on diffusion control.
The kinetics of hydrolytic polymerization of ε‐caprolactam. V. Equilibrium data on cyclic oligomers
Authors: Tai Kazuo , Tagawa Takashi
Published: April 2003

The concentrations of the cyclic oligomers (Ci; i = 3, 4, 5, and 6) in the polymeric products of ε‐caprolactam were determined by high‐performance liquid chromatography. The equilibrium data on the oligomers were obtained as a function of the polymerization temperature and initial water concentration. The concentration of each oligomer in the equilibrated polymer was found to increase with the temperature and/or initial water concentration. A set of the kinetic equations to express the oligomer formation during the polymerization was also proposed.
Effect of pH on the critical micelle concentration of sodium dodecyl sulphate
Authors: Rahman A. , Brown C. W.
Published: April 2003

Critical micelle concentration (CMC) of sodium dodecyl sulphate (SDS) at various pH's between 2 and 10 has been studied using three methods as dye absorption, conductance, and light scattering measurement. At low pH (below pH 4) the CMC decreases whereas at higher pH it remains constant.
Effect of grafted UV stabilizers on wood surface erosion and clear coating performance
Authors: Williams R. Sam
Published: April 2003

The critical element for a durable exterior clear coating on wood is photochemical stability of the wood surface beneath the coating. If this interface is not stabilized, even a durable coating will delaminate due to photoinduced wood degradation. A new method for stabilizing the wood surface was developed and evaluated. HEBP [2‐hydroxy‐4‐(2,3‐epoxypropoxy)benzophenone] was grafted to western redcedar (Thuja plicata). HEBP contains an ultraviolet (UV) stabilizer moiety that improved the xenon are accelerated weathering performance of western redcedar (WRC). The weathering performance of the HEBP‐modified WRC was compared with WRC treated with an unbound UV stabilizer of similar type and with untreated controls. The grafted stabilizer reduced the erosion rate (weathering) of unfinished WRC, and, as a pretreatment under clear finishes (spar varnish and exterior grade polyurethane), the stabilized surfaces improved coating performance and color retention.
Polyamide–polyether copolymers: A new family of impact‐resistant thermoplastics
Authors: Rasmussen Jerald K. , Smith H. K.
Published: April 2003

A new family of thermoplastic polyamide–polyether copolymers that display outstanding impact resistance and are especially useful as hot melt adhesives are described. These polymers are prepared by copolymerizationof polymerized fatty acid derivatives (dimer diamines) and poly(oxyalkylene) diamines with the typical polyamide monomers sebacic acid and piperazine. Impact resistance is apparently achieved through synergistic interaction between the dimer and the polyether portions of the polymers.
Mechanical behavior and dilatation of particulate‐filled thermosets in the rubbery state
Authors: Yilmazer U. , Farris R. J.
Published: April 2003

The stress–strain–dilatational response of glass‐bead‐filled, amorphous, network polyurethanes was investigated above their glass transition temperature. The mechanical–dilatational behavior was studied as functions of filler content, particle size, crosslink density of the polymer, and the surface treatment of the filler. It was found that the stress–strain properties are strongly affected by separation of the filler from the matrix. Measurements of the vacuole formation and growth processes enabled modeling the stress–strain response as well as understanding the ultimate behavior of the composites. Also, it was found that the dilatational response of the composites can be shifted on a single curve given by the second integral of the Gaussian function.
Moisture effects during cure of high‐performance epoxy matrices
Authors: Stark E. B. , Ibrahim A. M. , Munns T. E. , Seferis J. C.
Published: April 2003

The effects of absorbed moisture on the cure reactions and subsequent solid‐state properties of a high‐performance epoxy system were investigated in this study. The resin systems investigated were a model system, TGDDM–Novalac–DDS–BF3:MEA (TNDB), and its commercial analog, Hercules 3501–6. The samples were exposed to three environments: a liquid water environment at 50°C; an 85% relative humidity vapor environment at 50°C; and an evacuated, desiccated atmosphere at 22°C. Differential scanning calorimetry (DSC) thermograms were obtained for the Hercules 3501‐6 formulation as well as various compositions of the TNDB system for samples exposed to each of the three environments. Moisture was found to accelerate the cure especially for formulations exposed to the vapor environment. Dynamic mechanical experiments were also performed on cured, thin film samples. The “wet” samples were produced by adding water to the resin mixture prior to B‐staging. The moisture was shown to increase the extent of reaction, but produce a slightly lower degree of crosslinking as related to the sample's glass transition. This has been attributed to chain‐extension reactions, such as the hydroxyl–epoxide etherification reaction, which are favored in the presence of hydroxyl groups from water.
Primary molecular products of mechanical fracture of polymers
Authors: Enikolopian N. S. , Zarkhin L. S. , Prut E. V.
Published: April 2003

Primary molecular products of mechanical fracture were determined for polymers of different structure. For the first time the velocities of the products translation were measured, and kinetic energies and translation temperatures (2000–5400 K) were calculated.