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COMMENTARYTime,temperature,andload:TheflawsofcarbonnanotubesRodney S. Ruoff*Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208-3111he ‘‘mechanics of nanostruc-tures’’ is of intrinsic and practicalinterest. An acorn turning intoTan oak tree can lead one to con-sider the (often unknown) mechanicalforces exerted by, and acting on, nano-structures present in the tree. A mantra ofnanotechnology [which may ultimatelyoutpace (1) ‘‘natural’’ evolution] is having‘‘a place for every atom and every atom inits place’’ (www.foresight.orgnanowhatismm.html). What level of perfectionmight be achieved considering the knownlaws of physics and the constraints ofchemistry? In principle, there is no limita-tion to achieving essentially perfect cova-lent bonding in material structures. Withincreasing atom number, a size is eventu-ally reached where the defect-free struc-ture is not the most stable (consider therole of entropy) (2), but it may be kineti-cally stable if there are high barriers tothe nucleation of defects. In a recent issueof PNAS, Dumitrica et al. (3) considercarbon nanotubes (CNTs) and, buildingon prior theoretical work by themselvesand others, present the pathways to failurecaused by tensile load as a function oftime and temperature. Because CNTs canhave different chiralities, the issue of theorientation of the COC bonds in the dif-ferent CNTs is treated and shown to criti- Fig. 1. The Young’s modulus of the ...