garage chemist - 17-11-2004 at 12:32
In a book, I've read that when polystyrene is foamed to styrofoam with helium instead of air, the blocks float in air! 
That seems like a really nice experiment: producing a solid with a lower density than air.
The obvious problem is how to introduce the helium (or hydrogen) into the (molten?) PS as little bubbles, with a high enough gas/solid ratio so that
it flies.
I remember reading about a polyurethane plastic that consisted of two different liquids: when both are mixed, a polymer forms and CO2 is given off, so
that a low density foam is produced that has excellent thermal insulation properties and can be foamed into any space, just by mixing the two liquids
inside it. I think this is the standard way to insulate fridges (does anone know this for sure?).
Now, would it be possible to come up with two compounds that form a polymer when mixed and give off hydrogen in the process?
Or how to build a machine that can produce hydrogen- containig flying styrofoam?
BromicAcid - 17-11-2004 at 13:27
I would imagine that the gas to mass ratio in the styrofoam would have to be incredibly high, on par with an aerogel, which would be very difficult
indeed to produce in a home setting. Of course that would be neglecting the fact that by virtue of the bulk of the mass being filled with helium the
walls of the cells holding in helium molecules would be ridiculously thin and as such the helium would probably quickly diffuse out of the system
somewhat rapidly. But if it worked it would be neat, like a solid baloon.
chemoleo - 17-11-2004 at 13:39
For starters, it might be good to calculate how much a, i.e. litre of dry He or H2 can lift. This will be the amount of polymer (at the most) that is
to be used, per one litre volume. I imagine this to be very little. Possibly one'd have to find a polymer that can form very thin sheats, and
cures rapidly. THen you could simply bubble H2/He through it (FAST), while it hardens - alwyas making sure the 1 litre volume is filled with bubbles.
Hmm. Tricky.
I guess the polystyrene type idea might be better, i.e. the reaction producingthe gas H2 itself. Good luck in finding such a polymer!
Rosco Bodine - 17-11-2004 at 13:59
Some plants produce seed pods which burst open forcefully after they ripen and dry , and an extremely low density very fine fibrous
"balloon" structure expands
around each tiny seed . The material appears to be natures own aerogel material which is a cellulose fiber so light
and fine that it is almost neutral density in
cold air . When sunlight warms the entrapped air the structure actually becomes buoyant and floats upwards even in still air , just like a hot air
balloon
having solar energy for its burner . The
seed floats upward until it finds a current
of air and then floats away on the wind .
So you don't even necessarily need helium for a buoyant aerogel , unless of
course you fly by night 
neutrino - 17-11-2004 at 14:32
Hydrogen and helium would both lift about 1.1g/L. That would have to be some light styrofoam.