Carbon nanotubes of uniform size may soon be available in macroscopic
quantities. Here we examine the ordered condensed phase of these tubes
. As the tube diameter varies, the structural properties show a clear
transition between two regimes with qualitatively different behavior.
Tubes 10 angstrom and less in diameter behave as rigid cylinders. For
diameters over 25 angstrom, the tubes flatten against each other under
the van der Waals attraction, forming a honeycomb structure. This str
ucture exhibits an anomalous rigidity, which does not decrease as expe
cted with increasing tube diameter. Based on reported tube sizes, both
regimes should be experimentally accessible.