Is there any new breaktrough development of a material for better containment of helium in weather balloons ?

 

Beyond mylar, with more durable containment, perhaps from nanothechnology ?

 

Or,.... is there any new larger molecule gas, lighter than air which will not escape trough current porous materials in balloons ?

Polyester (Mylar) isn't bad! I used natural rubber which is way worse, but my balloon had just to climb for 2-3h, burst and fall down. Generally, metal and ceramics are far better than polymers, but a metal layer in polyester is probably too thin to make a difference. At the thickness covering a space blanket, the metal film isn't even continuous. If you give it a try, take a thicker metal layer.

 

You could search for

helium diffusivity polymer

but few polymers have the other good properties of polyester.

 

A gas lighter than air weighs less than 29g per mole, so a check through the elements tells it...

http://www.webelements.com/

http://www.chemicool.com/elements/

Hydrogen (boom and leaks), neon (expensive and lifts little), N2 (lift nothing), CH4 (I used it. Boom, lifts little), NH3 (yuk), H2O (I used it : inconvenient on Earth, keep for Venus), HF (yuk)

-> That's all. No discovery possible.

 

So either you accept CH4, Ne, H2 - or you're reasonable and go to He.