What is the primary reason that a polymer becomes flexible and rubbery above its glass transition temperature (Tg)?

Above the glass transition temperature the polymer chains have enough thermal energy that their segments can move past each other; this increased mobility allows the chains to slide and rotate. Because the chains can now rearrange, the material behaves like a viscous rubber rather than a rigid glass. The key idea is that thermal energy overcomes the barriers that keep the chains locked in place, creating free volume for motion. For example, a plastic toy that is stiff at 10 °C becomes soft and bendable when heated to 50 °C because the chains can now flow. Thus, the primary reason for flexibility above Tg is the enhanced segmental motion that turns the material into a rubbery state.