Showing papers by "Jacky Wing Yip Lam published in 2000"

Nanocluster-Containing Mesoporous Magnetoceramics from Hyperbranched Organometallic Polymer Precursors†

Abstract: Pyrolysis of hyperbranched poly[1,1'-ferrocenylene(methyl)silyne] (5) yields mesoporous, conductive, and magnetic ceramics (6). Sintering at high temperatures (1000-1200 degrees C) under nitrogen and argon converts 5 to 6N and 6A, respectively, in similar to 48-62% yields. The ceramization yields of 5 are higher than that (similar to 36%) of its linear counterpart poly[1,1'-ferrocenylene(dimethyl)silylene] (1), revealing that the hyperbranched polymer is superior to the linear one as a ceramic precursor. The ceramic products 6 are characterized by SEM, XPS, EDX, XRD, and SQUID. It is found that the ceramics are electrically conductive and possess a mesoporous architecture constructed of tortuously interconnected nanoclusters. The iron contents of 6 estimated by EDX are 36-43%, much higher than that (11%) of the ceramic 2 prepared from the linear precursor 1. The nanocrystals in 6N are mainly alpha-Fe2O3 whereas those in 6A are mainly Fe3Si. When magnetized by an external field at room temperature, 6A exhibits a high-saturation magnetization (M-s similar to 49 emu/g) and near-zero remanence and coercivity; that is, 6A is an excellent soft ferromagnetic material with an extremely low hysteresis loss.

Synthesis and Properties of Liquid Crystalline Polyacetylenes with Different Spacer Lengths and Bridge Orientations

Abstract: A group of mesomorphic polyacetylenes with different lengths of alkyl spacer −{HCC[(CH2)m−OCO−Biph−OC7H15]}n− [1(m), m = 2, 3, 4, 9] and orientations of ester bridge −{HCC[(CH2)3−CO2−Biph−OC7H15]}n...

Structure−Property Relationships for Photoconduction in Substituted Polyacetylenes

Abstract: New photoconductive materials are explored from three groups of polyacetylenes: poly(phenylacetylenes) [HCC(C6H5-p-R)]n, poly(3-thienylacetylenes) [HCC(3-C4H2S-β-R‘)]n, and poly(1-alkynes) {HCC[(CH2)mR‘ ‘]}n, where R = CH3 (2), CO2(CH2)6OCO-Biph-OC7H15 (Biph = 4,4‘-biphenylyl; 3); R‘ = Si(CH3)3 (4), Br (5); and R‘ ‘ = CO2(CH2)6OCO-Biph-OC9H19 (m = 2; 6), 9-carbazolyl (m = 3; 7) and OCO-Biph-OC7H15 (m = 9; 8). Photoconduction in the polyacetylenes under illumination of visible light is investigated using photoinduced xerographic discharge technique. In the pure (undoped) state, all the polyacetylenes except 5 show higher photosensitivity than do poly(phenylacetylene) (R = H; 1), a well-studied photoconducting polyacetylene, and poly(9-vinylcarbazole), the best-known photoconducting vinyl polymer. Among the polyacetylenes, photoconduction performance of the polymers with electron-donating and/or hole-transporting moieties is superior to those with electron-accepting ones. The liquid crystalline polymer 6 e...

C60-Containing Poly(1-phenyl-1-alkynes): Synthesis, Light Emission, and Optical Limiting†

Abstract: While polymerizations of 1-phenyl-1-propyne (PP) and 1-phenyl-1-butyne (PB) initiated by WCl6−Ph4Sn at room temperature yield small amounts of polymers (0.05−5.3%) with low molecular weights (Mn 8000−15000) and high polydispersity indexes (PDI up to 109), addition of C60 into the initiator mixture dramatically boosts its catalytic activity, producing polymers with high Mns (up to 171 000) and low PDIs (down to 2.2) in high yields (up to 99.5%). The resultant polymers are soluble, stable, and film-forming. GPC, IR, NMR, UV, and XRD analyses reveal that the polymers consist of polyacetylene chains and covalently bound C60 cages (up to 9.1 wt %). Whereas photoluminescence of C60-containing polyPPs is faint, C60-containing polyPBs emit strong blue light (λmax = 456 nm) readily observable with the naked eye under normal laboratory lighting. Compared with the parent C60, both two groups of the fullerene polyacetylenes show better optical limiting performance, and their THF solutions, with higher linear transmit...

Transition Metal Carbonyl Catalysts for Polymerizations of Substituted Acetylenes

Abstract: Most of the existing metal carbonyl catalysts for acetylene polymerizations need to be preactivated by chlorine-containing additives or by UV irradiation in halogenated solvents In this work, we developed a series of “simple” metal carbonyl catalysts of general structure M(CO)xLy (M = Mo, W), none of which require additives or pre-photoirradiation, most of which are air- and moisture-stable, and some of which work well in nonhalogenated solvents The acetonitrile complexes M(CO)3(NCCH3)3 initiated polymerizations of a variety of mono- and disubstituted acetylenes at room temperature The arene and diene complexes W(CO)3(mes) and Mo(CO)3(nbd) (mes = mesitylene, nbd = 2,5-norbornadiene) are tolerant of polar groups and effected polymerizations of functional acetylenes containing ester, ether, and cyano groups The halogenated complexes MI2(CO)3(NCCH3)2 catalyzed polymerizations of phenylacetylene in toluene The chlorine-containing acetylene monomers ClC⋮CC6H5 and ClC⋮CC6H13 were readily polymerized by the

Liquid crystalline polyacetylenes with tunable luminescent properties

Abstract: Development of advanced polymeric materials with both liquid crystallinity and light emissivity is of scientific interest and technological importance. In this study, we studied light emission from tetrahydrofuran solutions of a liquid crystalline polyacetylene, poly(11-{[(4'-heptoxy-4- biphenylyl)carbonyl]oxy}-1-undecyne), in the electrical field. The field exerts little effect on the photoluminescence of the polymer solution with a low concentration (0.10 mM). The photoluminescence of a concentrated solution (11.3 mM) is, however, noticeably quenched under an electrical field with a field strength of > 300 kV/m. When the field strength is increased to >= 367 kV/m, the bimodal emission spectrum of the solution changes to a monomodal one. Thus, both the emission intensity and spectral profile of the luminescence of the concentrated solution can be tuned by the electrical field, which is probably caused by the aggregate dissociation and mesogen realignment induced by the external stimulus.

Origin of the blue emission from poly(1-phenyl-2-alkynes) and poly(phenylacetylenes)

Abstract: We have studied the electronic structure, absorption, and photoluminescence of poly(1-phenyl-2-alkynes) - [C 6 H 5 )C- C(C m H 2m+1 )] n -(m = 1, 2), poly(phenylacetylene)-[HC=C(C 6 H 5 ] n - and its derivatives -[HC=C(C 6 H 4 -p-R] n with various non- liquid crystal ring substitutes. For poly(1-phenyl-2- alkynes), the PL efficiency is very sensitive to the molecular structure of the alkyl pendant and can be enhanced up to 50 times as the alkyl side-chain increases in length. But for poly(phenylacetylenes), their luminescent efficiency can be improved several times only as the tail becomes bulky. Regardless of the types of the pendants, the emission color of the polymers is pineed at ~450 nm (2.7eV). The band structure of the polymers, which has been calculated using extended Huckel tight-binding method, is essentially an ensemble of the backbone (extended states) and the pendants (localized states), and the processes of optical absorption and blue emission are confined in the directly attached aromatic ring. The interaction between the pheny chromophore and its nearest neighbors is of vital importance in improving the emission efficiency. Although the band gap of the backbone can be enlarged by the pendant, its (pi) - (pi) * interband transistion is insignificant for the blue emission.