Of particular importance in the design process for polymers is an allowance for their susceptibility to creep deformation.
Under a constant applied load and at a constant temperature the deformation of the polymer will continue to increase over time.
The higher the applied stress or the higher the temperature the faster the deformation. Of particular concern with polymers is their relatively high creep rates at room temperature.
After Engineering Materials: Properties and Selection, K. G. Budinski, 8th Edition, Pearson Education International.
Below the Tg of a polymer creep is fairly limited due to the resistance to molecular motion from secondary bonds. However, above Tg creep escalates sharply as polymers undergo a combination of elastic (recoverable) and plastic (permanent) creep deformation referred to as viscoelastic behaviour. In this temperature range the molecular chains are free to slide over one another in a manner known as viscous flow.