Calculations point to novel carbon networks Calculations by scientists at AlliedSignal, Morristown, N.J., indicate the possibility of crystalline networks of carbon atoms that expand laterally when stretched (have a negative Poisson's ratio) and contract when heated (have a negative coefficient of thermal expansion) [Nature, 365, 735 (1993)]. Most substances have a positive Poisson's ratio and and a positive coefficient of thermal expansion. If the AlliedSignal substances could be made, these properties would give them superior toughness and physical damage resistance. And because these are networks of conjugated double and/or triple bonds, the resulting substances might show useful color changes in response to mechanical deformation (piezochromism) or temperature change (thermochromism). The conjugated, unsaturated networks might also be dopable to metal-like electrical conductivities. Yet because thev are three-dimensional networks, the doping and undoping that would occur during charging and discharging of a battery, for example, might be reversible over a very large number of cvcles. This is because the substances would resist mechanical damage from the physical motion of dopant molecules in and out of their networks. Materials scientist Ray H. Baughman of AlliedSignal and physics professor Douglas S. Galvào, on leave from the State University of Campinas, Brazil, employ computer-aided molecular mechanics calculations to study the postulated networks. Molecular mechanics uses experimentally determined bond lengths, angles, and force constants of actual compounds, together with their heats of formation, to estimate energies and structures of various conformations of new substances. These shearable networks may be likened to such expandable-contractible structures as the pantographs used on electric trains to contact overhead wires, or accordion gates installed across staircase tops to prevent infants from tumbling down. Baughman and Galvào themselves use the analogy of a collapsible wine rack. A wine rack has two conformations: opened to dilate the diamond-shaped cavities for bottles, and closed flat for
Hinged carbon network moves between two phases Adjacent polydiacetylene chains are shown as horizontal sequences of red or black atoms. Polydiacetylene chains are linked by vertical polyacetylene chains (alternating single and double bonds). Single bonds in polyacetylene chains serve as hinges on which the network can close in two different directions from an open transition state.
storage. But the Baughman-Galvào network has two collapsed conformations, resulting from collapse of the open polydiacetylene structure in either of two directions. These collapsed conformations are produced by either coiling or uncoiling of polyacetylene chains during stretching. The two collapsed structures differ in approaching a cisoid or transoid conformation of chains. Each collapsed conformation is a local energy minimum, separated by only a small energy barrier. On a macroscopic scale, these molecular changes result in changes in dimensions and densities. For example, if the material were a long strip, then stretching it along the length might make it wider, thinner, and denser. The slightly increased width would represent the negative Poisson's ratio for that dimension; the much decreased thickness would come from natural contraction owing to stretching in that dimension; and the .increased density would result from the overall reduction in volume. Stephen Stinson
ι. Polyester
Rιζζαζζ
Ifyou 're looking for a highly reactive source of the C-6 moiety for conden sation polymers, look no further than DBE-6 (dimethyl adipate). You'll enjoy low reaction temperatures. Minimal side reactions. And, because it's a non-hygroscopic liquid, easy handling. What's more, DBE-6 excels in Dieckmann condensations. Anything else? That's up to you. Call 1-800-231-0998 for your sample. II
