Don't balloons expand in the day and contact at night a bit due the sun heating the gas inside? Couldn't this stretch/release weaken the material a little and result in potential gas leaks?

To counteract this, I recall reading about people trying to send balloons across the Atlantic and needing to use ballast to stabilize the altitude and handle the gas losses. I wonder how DARPA can go without this requirement, as Helium really likes to escape whatever it's trapped inside of.