Showing papers in "Advanced Materials in 1993"

Hybrid nanocomposite materials―between inorganic glasses and organic polymers

Abstract: The sol-gel process, with its associated mild conditions, offers a new approach to the synthesis of composite materials with domain sizes approaching the molecular level. Transparent organic-inorganic composites can be prepared by dissolving preformed polymers into sol-gel precursor solutions, and then allowing the tetraalkyi orthosilicates to hydrolyze and condense to form glassy SiO, phases of different morphological structures. Alternatively, both the organic and inorganic phases can be simultaneously formed through the synchronous polymerization of the organic monomer and the sol-gel precursors. Depending upon such factors as the structures of the organic and inorganic components, the phase morphology, the degree of interpenetration, and the presence of covalent bonds between the phases, the properties of these composites can vary greatly and range from elastomeric rubbers to high-modulus materials.

Advances in negative Poisson's ratio materials

Abstract: Poisson’s ratio is defined as the lateral contraction strain in a solid divided by the longitudinal extension strain measured in a simple tension experiment. In almost all materials Poisson’s ratio, usually denoted by v, is positive. For isotropic materials (those in which the properties are independent of direction), energy arguments in the theory of elasticity[’] may be used to show that 1 2 v I 112. The relationship between shear modulus G, bulk modulus K, and Poisson’s ratio v for isotropic materials is given by Equation (1).

Organic Materials for Third‐Order Nonlinear Optics

Abstract: The current status of organic low-molecular weight and polymeric materials for third-order nonlinear optics is reviewed. The importance of organic materials lies in their promise of large nonlinear optical figure of merit, high optical damage thresholds, ultrafast optical responses, architectural flexibility, and ease of fabrication. Organic materials exhibiting interesting third-order nonlinear optical properties are discussed to illustrate the importance of structure–property correlations. Results on emerging organic materials that include liquids, dyes, fullerenes, charge-transfer complexes, π-conjugated polymers, dye-grafted polymers, organometallic compounds, composites, and liquid crystals are presented. Organic nonlinear optical materials seem promising for a wide range of applications and their potential for integrated optics should be further explored.

Tuning of the photo‐ and electroluminescence in multi‐block copolymers of poly[ (silanylene)thiophene]s via exciton confinement

Abstract: The combination of wide‐ and narrow‐band‐gap polymers in a block copolymer can provide a means of increasing the luminescence efficiency of the materials. This occurs because the excitons that are formed are trapped within specific units in the polymer chains and therefore cannot migrate to quenching sites caused by the presence of dopants. Color‐tunability is also possible through adjusting the band gaps of the components. Devices based on silanylene'thiophene lock copolymers are reproted. Copyright © 1993 Verlag GmbH & Co. KGaA, Weinheim

Large changes in optical response through chemical pre‐ordering of poly(p‐phenylenevinylene)

Abstract: Conjugated polymers such as poly(p-phenylenevinylene), (PPV, see Figure), show real promise in optoelectronic applications A new synthetic route to improved PPV is presented, involving the thermal conversion of a precursor polymer containing rigid rod conjugated segments joined by flexible spacer groups A high degree of interchain ordering results which influences the optical response of the material

Fabrication of organic nanocrystals for electronics and photonics

Abstract: We have presented a simple technique for the fabrication of nanocrystals of organic molecules and polymers and have shown that it is possible, using the liquid-phase technique, to fabricate organic nanocrystals ranging in size from 10 nm to 1 μm by manipulating the preparative conditions. In particular, nanocrystals of poly(4-BCMU) ranging from 20 nm to 350 nm were prepared by controlling the preparation conditions. The main advantages of the liquid-phase technique are the practicality and suitability of the technique for a wide range of materials. The fabrication of organic nanocrystals, though at a very early stage, seems a promising approach for producing low-dimensional organic materials and, like inorganic nanocrystals, another important objective for future study would be to incorporate organic nanocrystals into a variety of inorganic and organic media. It is hoped that extensive work will be done on organic nanocrystals to evaluate their potential for electronics and photonics.