At what point does a polymer typically transform from a rigid state to a more flexible, rubbery form?

Polymers usually change from a stiff, glassy state to a rubbery, flexible state when the temperature reaches the glass transition temperature, or Tg. Below Tg the chains are frozen in place, so the material feels hard; above Tg the chains can move more freely, giving a soft, elastic feel. The exact Tg depends on the polymer’s structure, but it’s typically where the heat capacity jumps and the material’s modulus drops sharply. For example, ordinary nylon has a Tg around 50 °C, so it is rigid below that but becomes pliable once the temperature exceeds about 50 °C.