why does an balloon inflate

Rubber is an organic compund which by the virtue of cross-linking between it's molecules is blessed with this property and also the special propert that it has very small interstitial spaces between it's molecules.When you force air into a balloon the air tries to push hard and counter thye rubber's property to oppose tension and also tends to retain that air inflating it.

When it is inflated for long the the property of rubber to keep it's shape intact and simultaneously intermolecular spaces increase and therefore air slowly starts leaking out and consequently the balloon deflates out.