September 200214

Polymer nanotubes from porous templates

RESEARCH NEWS

New work has revealed the 20chart-toppers for the moststable four component alloys[Phys. Rev. Lett. (2002)88(25), 255506]. Scientistsat the Technical University ofDenmark have used densityfunctional theory and anevolutionary algorithm tosearch possible alloycombinations for desiredproperties. As well as knownalloys, new possibilities forsuperalloys were identified.There are 192 016 possiblecombinations of the 32transition, noble, and simplemetals for four componentalloys in either fcc or bccstructures. The Danish groupsearched all of these to findthe 20 structures that aremost stable. This task is only

possible because of theincreased speed with whichcalculations can be performedusing density functional theory.The other necessary elementis a suitable algorithm. In thespirit of Darwin, the groupdeveloped an algorithm thatselects only the fittest or moststable alloys to survive from aninitial random population ofalloy combinations. These are'bred' together, mixing thecombinations, to give a newgeneration of alloys. Thismethod quickly converged togive the Top 20 chart. All themost stable configurationsprefer the bcc structure andtend to involve an equalnumber of early and latetransition metals. The authors,however, were not satisfied:

"The bcc metals are usuallyless interesting than fcc or hcpmetals." They then included therequirements that the alloysmust be more stable in the fccrather than the bcc structureand excluded Si and the mostexpensive noble metals. Thisgave a diverse list of alloys withNi3Al at the top, the bestexisting superalloy. New andinteresting candidates forsuperalloys also appeared inthe list, making furtherinvestigation worthwhile. "Apurely theoretical design ofnew materials is well into thefuture," the authors admit, butthey hope computationalapproaches will be able toidentify possible candidates fornew materials with desiredproperties.

Survival of the fittest

Researchers at the Philipps-University and Max-Planck-Institute of MicrostructurePhysics in Germany havecreated a simple technique forfabricating oriented and

monodisperse polymernanotubes [Science (2002)296, 1997]. They claim thatthe technique "should have anoutstanding potential inproviding customized

nanotubes for a broad range ofapplications in nanoscience."The group drops polymer meltsor solutions onto poroustemplates. The liquid polymercovers the pore walls in a thinwetting film in a few minutes.By using a template withaligned pores of amonodisperse size distribution,the resulting polymernanotubes are also orientedand uniform in size. Finally, thetemplate is removed.Templates with pore diametersbetween 300 and 900 nmproduced polymer nanotubeswith lengths of up to 100 µmand wall thicknesses ofbetween 20 and 50 nm. The authors suggest thesimplicity of this techniqueshould lend itself to thefabrication of functionalizedpolymer nanotubes.

Ordered polymer nanotubes fabricated using porous templates. [Copyright Science.]

The key tolong lifeIonic liquids may hold the keyto realizing practical, long-livedπ-conjugated polymerelectrochemical devicesaccording to researchers fromSanta Fe Science andTechnology, New Mexico, andthe universities of Wollongongand Monash, Australia.While a number of different π-conjugated polymer materialshave been developed forapplication in batteries,actuators, photovoltaic cells,and electrochromic displays,little attention has been paidto the electrolyte in thesesystems. Wen Lu and co-workers find that use of roomtemperature ionic liquidelectrolyte systems (ILES) canovercome the problems ofpoor environmental stabilityand short lifetimes, problemsthat can plague these devices[Science (2002)10.1126/science.1072651]."Ionic liquids are salts that arefluid over a large temperaturerange, including roomtemperature," explain theauthors.A polypyrrole actuator in anILES was stable over severalthousand cycles, showing nopolymer degradation. Apolyaniline electrochromicdisplay showed a fast cycleswitching speed of 100 msand exceptional stability, withno observable decrease inperformance over one millioncycles.The group believes the use ofroom temperature ionic liquidsholds promise for the wideapplication of π-conjugatedpolymers in electrochemicaldevices.