RESEARCH NEWS

September 200410

There is increasing interest in using

nanoscale reinforcing fillers to improve

polymer flammability properties.

Takashi Kashiwagi and coworkers at

the US National Institute of Standards

and Technology and the University of

Kentucky have measured the thermal

and flammability properties of

polypropylene/multi-walled carbon

nanotube (PP/MWNT) nanocomposites

[Kashiwagi et al., Polymer (2004) 45

(12), 4227].

The MWNT content of the

nanocomposite was varied from 0.5%

to 4% by weight and the flammability

properties measured. The peak heat

release rate was observed at an

MWNT content of 1%. In contrast, the

addition of carbon black powder to PP

did not reduce the heat release rate as

much. This suggests that the size and

shape of MWNTs is crucial to reducing

PP flammability. The radiative ignition

delay time of nanocomposites with

<2% MWNTs by mass is less than

that of PP because the radiant flux

absorptivity at infrared wavelengths

increases significantly with the addition

of MWNTs. Ignition delay time and the

peak heat release rate of the

nanocomposite increases with MWNT

content above 1% by weight. The

lowest heat release rate is observed

with the PP/MWNT (1%) sample.

Scanning electron and optical

microscopy studies indicate that the

PP/MWNT flame retardancy results

from formation of a relatively uniform

network-structured MWNT layer, which

covers the entire sample surface

without any cracks or gaps. This layer

re-emits much of the incident radiation

from its hot surface, thereby reducing

the transmitted flux to the PP layers

below. Formation of this nanotube

network layer is critical in improving

flammability properties. John K. Borchardt

Nanotubesretard fire COMPOSITES

A simple but effective synthesis route for polymernanowires is a major challenge for materialschemists. However, progress is being made andnow Erkang Wang and colleagues at ChangchunInstitute of Applied Chemistry in China report thelarge-scale room-temperature synthesis of uniform poly(o-phenylenediamine) nanobelts from an o-phenylenediamine-HAuCl4 aqueous solution withoutusing templates or surfactants to direct thesynthesis.The nanobelts are several hundred microns long,several hundred nanometers wide, and tens ofnanometers thick [Sun et al., Chem. Commun.(2004) 10, 1182]. Scanning electron microscopy(SEM) of the precipitate formed when aqueoussolutions of the two chemicals are combined revealsa large quantity of uniform one-dimensionalstructures. Higher magnification indicates that

these structures are transparent nanobeltsseparated from one another. Despite their length,the nanobelts are very straight, suggesting thatthey are quite rigid. Nanoparticles were alsoobserved in the precipitate.Elemental analysis of the nanobelts andnanoparticles using secondary electron SEManalysis reveals peaks from Au, C, and N. X-raydiffraction analysis confirms that that thenanoparticles are Au and the nanobelts are purelypoly(o-phenylenediamine).The researchers suggest that o-phenylenediamineacts as an electron donor for HAuCl4, which is apowerful oxidant with a high reduction potential.They propose that spontaneous nanobelt formationand growth is the result of Au nanoparticle catalysisof poly(o-phenylenediamine).John K Borchardt

Synthesizing polymer nanobelts POLYMERS

Growing magnetic cables on the nanoscaleNANOTECHNOLOGY

Researchers at the University of SouthernCalifornia have developed a generic synthesistechnique for the growth of composite‘nanocables’ [Han et al., Nano Lett. (2004),4 (7), 1241].The nanocables consist of dense arrays ofultrafine MgO wires coated with a uniform,precisely controlled layer of a transitionmetal oxide (TMO). First, Chongwu Zhou andcoworkers created arrays of MgO nanowiresby condensing MgO vapor onto MgO plates

using an Au catalyst, following a vapor-solid-liquid mechanism. The resulting nanowiresare 30-100 nm in diameter and 3 µm long.Next pulsed laser deposition was used todeposit a layer of a TMO onto the MgOcores, which act as a template. “The trick iswe can preserve the TMO composition duringthis technique,” says Zhou, “while othertechniques cannot.” The resulting structuresare single crystalline core-shell MgO/TMOnanowires with precisely controlled layerthickness. The researchers grew nanocablesof MgO with an outer layer of thesuperconductor YBa2Cu3O6.66,La0.67Ca0.33MnO3, which shows colossalmagnetoresistance, the ferroelectricPbZr0.58Ti0.42O3, and magnetic mineral formof magnetite, Fe3O4. The La0.67Ca0.33MnO3/MgO nanowires show that the metal-insulator transition and magnetoresistanceare preserved down to the nanoscale.The versatile synthesis method could allowthe production of many different nanowirecompositions. “We expect that these TMOnanowires may offer enormous opportunitiesto explore intriguing physics at nanoscaledimensions,” says Zhou. “Future work will bedirected toward in-depth understanding anddevice applications of these new materials.”Cordelia Sealy

Left: Schematic diagram of the pulsed laser deposition setup,

where MgO nanowires are used as templates for the coating of a

second layer, therefore rendering core-shell nanocables. Upper

right: Scanning electron micrograph of MgO/YBCO core-shell

nanocables. Lower right: Transmission electron micrograph

revealing the MgO core and the YBCO shell. YBCO is known to be

a high-transition-temperature superconductor.