Answer: Polymers show both elastic (spring-like, recoverable) and viscous (flow-like, time-dependent) behaviour, so their deformation depends on time and rate of loading: creep and stress relaxation.
- An ideal elastic solid deforms instantly and recovers fully (Hooke's law, \(\sigma=E\varepsilon\)). An ideal viscous liquid flows, and the strain keeps increasing with time (\(\sigma=\eta\,d\varepsilon/dt\)).
- Polymers combine both. When loaded, the chains first stretch (elastic part) and then slowly slide past each other and uncoil (viscous part).
- Creep: under a constant load the strain keeps increasing slowly with time (e.g. a plastic shelf sagging).
- Stress relaxation: at constant strain the stress needed falls with time (e.g. a plastic clip loosening).
- When the load is removed, part of the deformation recovers immediately, part recovers slowly, and some may be permanent.
- Viscoelastic behaviour is strongest near \(T_g\); it is modelled by a spring and dashpot in series (Maxwell model) or in parallel (Kelvin–Voigt model).
