RESEARCH NEWS

March 200518

Air pollution and natural weathering

are damaging many historically

important buildings. While stone

preservation studies have mainly been

confined to the evaluation of

commercially available materials,

researchers at the University of

Messina in Italy have synthesized

hybrid silica-epoxy polymers as

consolidating or protective agents

(Cardiano et al., Polymer (2005)

doi:10.1016/j.polymer.2005.01.002).

Consolidation treatments reestablish

cohesion among stone grains without

compromising the material’s water

adsorption and mechanical properties,

while protective agents provide a

barrier that prevents water

penetration through the stone surface.

Cardiano and coworkers reacted a

primary amine (3-aminopropyl)

triethoxysilane (ATS) with the epoxy

derivatives 2-(3,4-epoxycyclohexyl)

ethyl-trimethoxysilane (ECET) and (3-

glycidyloxypropyl)methyldiethoxysilane

(GLYMS). The presence of a

cyclohexane ring and the absence of

aromatic unsaturation are thought to

provide ECET with good weathering

properties. Materials containing

GLYMS function as hydrophobic barrier

materials because of the presence of a

methyl group.

Tests indicate that ECET-ATS materials

are preferable to GLYMS-ATS for stone

preservation. Thermogravimetric

analysis indicates that ECET-ATS

materials exhibit higher degradation

temperatures, lower decomposition

rates, less weight loss, and, thus, are

more stable than GLYMS-ATS blends.

Capillarity absorption tests using

medium porosity stone indicate that

ECET-ATS blends provide an excellent

barrier against water penetration.

Neither type of blend is as effective on

less porous stone.

John K. Borchardt

New barrier tothe weatherPOLYMERS

The assembly of nanometer-sized ‘molecular wires’for directional, long-range electron transport isessential for molecular electronic devices. The useof molecular self-assembly to construct functionalmaterials is now well-established, and presents oneway of developing conducting materials. Phthalocyanines and tetrathiafulvalenes areattracting particular interest as novel conductingmaterials. Substituted phthalocyanines, containingcrown ether substituents bearing long alkyl chains,readily assemble into long fibers that show electronconduction, ion transport, and liquid crystallinity.Now, researchers at Radboud University Nijmegenin the Netherlands and Institut de Ciència deMaterials de Barcelona in Spain reportincorporating tetrathiofulvalene into substitutedphthalocyanine compounds to producetetra(thiafulvalene-crown-ether) phthalocyanines.These molecules self-assemble into helical tapesthat are nanometers wide and micrometers long [Sly et al., Chem. Commun. (2005), doi:10.1039/b416034g].

In a chloroform solution, these compounds can beinduced to form fibers when dioxane is slowly addedat 5°C. Transmission electron microscopy (TEM)reveals fibers several micrometers in length, theequivalent of stacking about 100 000 molecules.The fibers have an unusual, thin bilayer structurewith a width of ~20 nm. This is five times thecalculated width of an individual molecule. A secondtype of fiber also forms that is helical in nature and15-25 nm wide. Joseph Sly and coworkers suggest thattetrathiafulvalene-tetrathiafulvalene andtetrathiafulvalene-phthalocyanine interactionsdominate phthalocyanine-phthalocyanineinteractions, leading to the formation of both left-and right-handed helical tapes, which theresearchers call scrolled molecular architectures.The researchers are now attempting to introducechirality into the molecules to determine if this cancontrol whether the resulting helical structures areleft- or right-handed. John K. Borchardt

Tapes could wrap up molecular electronicsELECTRONIC MATERIALS

Getting tough on mollusksBIOMATERIALS

Researchers at the University of California,San Diego (UCSD) are studying the structureof mollusk shells to learn how to designtough materials for applications such as bodyarmor [Lin and Meyers, Mater. Sci. Eng. A(2005) 390 (1-2), 27].

The colorful oval shell of the red abalone isoften used for mother-of-pearl jewelry. Lesswell known is the fact that abalone shellsare able to withstand heavy impact blowswithout breaking. “In our search for a newgeneration of armors, we have exhausted theconventional possibilities,” says projectleader Marc A. Meyers. “We have turned tonature because millions of years of evolutionand natural selection have given rise in manyanimals to some very sturdy materials withsurprising mechanical properties,” Abalone shell is composed of thousands oflayers of CaCO3 ‘tiles’, each ~10 µm acrossand 0.5 µm thick, held together by a proteinadhesive. The positively charged proteinadhesive converts brittle CaCO3 into a toughmaterial by binding to the negatively chargedtop and bottom surfaces of the tiles. “Thetiles abutting each other in each layer arenot glued on their sides, rather they are onlyglued on the top and bottom, which is whyadjacent tiles can separate from one anotherand slide when a strong force is applied,”explains Meyers.John K. Borchardt

Red abalone shell. (Courtesy of UCSD.)