Could you make a balloon that would float in air out of carbon fibre composite?
The pressure is not really all that great if the interior was evacuated of air. However to make it buoyant, the structure would have to displace more than its own weight in air. Thus physical size would be required, requiring greater structural strength to withstand the pressure, meaning more weight. It would be like a dog chasing its tail! In addition, even if you achieve this, as the vessel ascends and the air density falls, the structure, being rigid, cannot expand itself like a flexible (say) helium- filled vessel. It thus would find an altitude at which it would reach equilibrium, and cease rising higher.
