Advanced Materials

Polymer electrolytes for lithium ion batteries is the theme of this review. Two major strategies for the application of ion-conducting polymers as separators in lithium batteries are identified: the use of gel electrolytes and solid polymer electrolytes. The Figure illustrates the mechanism of cation transport in a lithium salt/poly(ethylene oxide) solid polymer electrolyte.

The conformation of poly(aniline) (PANI) within a MoO₃, nanocomposite is investigated by solid-state ²H NMR (see also cover). This technique is demonstrated to be highly suited to providing information about the structure of polymers intercalated within transition metal oxide hosts that cannot be obtained by other methods. Through a series of 1D and 2D experiments it is shown that PANI is organized within the interlayer regions of the inorganic host and that order within the composite is not determined solely by the oxide layers.

Friction force imaging of microphase-separated copolymers using scanning force microscopy (SFM) is demonstrated to be a viable technique for distinguishing between polymer blocks. The Figure is a friction force image of a cross section of a film of a polystyrene-poly(2 vinylpyridine) (PS-PVP) star copolymer measured with a COOH-modified tip, revealing the morphology of the copolymer.

The metallic state of conducting polymers is a subject of continued interest and controversy. By means of a series of high-precision reflectance measurements on PF₆-doped polypyrrole (PPy-PF₆), the processes occurring as the system passes through the metal–insulator (M–I) transition are clarified. Evidence is presented that metallic PPy-PF₆ is a “disordered metal” and that the M–I transition is disorder induced. It is concluded that the electronic properties of PPy-PF₆ result from weak localization and an Anderson-like M–I transition.

The ability to sense metal ions is a sought-after property in the design of chemical sensors. The synthesis of a new metal-ion-sensitive poly(p-phenylenevinylene) that contains terpyridyl ligands in the side chain (see Figure) is reported, and the changes in its fluorescence behavior—quenching or a shift—induced by the addition of several kinds of transition metals are described.

A humidity-sensing material based on C₆₀ is described whose conductivity is mediated by the amount of water vapor to which it is exposed. Its response is quick enough to follow the breathing of a person from a distance of 30 cm. The characterization of the material by DC polarization, AC admittance, and near-IR photoacoustic response measurements as well as UV-vis spectra is reported and the preparation of both bulk and thin-film specimens described. Work to resolve the composition and the sensing mechanism of the material in more detail is in progress.

A polymeric precursor route to non-oxide ceramics that involves pyrolyzing an organic/inorganic hybrid polymer containing alternating metal alkoxide and diacetylene units is examined. The Figure is a transmission electron microscopy image showing the 20–50 nm TiC nanoparticles that result from processing the titanium polymeric precursor at 1400°C for 2 h in argon.

The biomimetic self-assembly of an operating electrical circuit is described. The circuit, self-assembled via the hydrophobic effect from two different, but shape-complementary, non-functional subunits, contains a light-emitting diode (LED), a gold cathode, and a magnesium anode. It is shown that the system, when immersed in a suitable electrolyte (potassium ferricyanide), constitutes an electrochemical cell—the LED is powered through a reaction involving the dissolution of magnesium(0) at the anode and the reduction of ferricyanide to ferrocyanide at the cathode.

Controlled formation of nanoparticles in inverse diblock copolymer micelles by hydrolysis of titanium alkoxide precursors is described. Coagulation of the resulting TiO₂-loaded micelles occurs on casting if the sample is prepared by conventional heating, allowing strings of TiO₂ particles to form (see Figure). It is demonstrated that this can be prevented by microwave heating.

Self-assembled monolayers (SAMs) with terminal hydrogen bonding groups can have remarkably different structure and dynamics to those of the more familiar alkenethiol SAMs. The effect of carboxylic acid terminal groups on the structure and dynamics of gold nanoparticle SAMs is investigated by ¹³C NMR, 2D WISE NMR, and FTIR spectroscopies. A markedly increased conformational order is reported, compared to that of methyl- and hydroxy-terminated SAMs, along with a strong thermal hysteresis of the chain order, speculated to be due to a change from intra- to interparticle hydrogen bonding.

Control of the optical properties of SiO₂ colloidal photonic crystals through thermal treatment is reported. The optical properties, studied by light transmission and reflection, are shown to vary through structural and physicochemical modification of the material, without loss of order. The Figure is a scanning electron micrograph of the fcc crystalline {111} facet of a sample treated at 950°C.

The ability of organofunctional silicon alkoxides to bind metal ions is taken advantage of to produce copper-substituted mesoporous silica. Evidence is presented that the Cu²⁺, which is distributed homogeneously throughout the structure, is actually incorporated into the framework. Through X-ray diffraction, electron spin resonance, transmission electron microscopy, and surface area measurements it is demonstrated that, apart from having thicker walls, this new mesoporous material has similar structural characteristics and physical properties to those of pure hexagonal mesoporous silicas.

The importance of drying nanostructured semiconductors, such as porous silicon, in a controlled manner is explained. The mechanical instabilities that can arise during drying—resulting in cracking (see Figure) or peeling of the film—are discussed, as are methods that reduce or completely suppress these drying stresses, for example, supercritical drying, freeze drying, and “drying with pentane”.

Muscles convert chemical energy to mechanical energy plus heat at constant temperature. The search for artificial muscles that do the same is described, beginning with polymer gels—which were restricted to slow movements and had high energy consumption—and ending with the bilayer devices that have been constructed from conducting polymers more recently. The mechanism of operation of these devices is explained, showing how the electrochemically stimulated conformational movements of the conducting polymer chains are translated into macroscopic movements.

Colloid monolayer lithography is briefly reviewed and demonstrated to be a powerful alternative technique for the nanostructuring of surfaces. The Figure shows a colloid monolayer viewed through a TEM grid used to transport the monolayer to the desired substrate. The monolayer and grid together form a lithographic mask that can later be removed to leave, for example, a pattern of deposited gold dots.