viscoelasticity

Silly Putty is the perfect teacher here. Roll it into a ball and throw it down and it bounces like a rubber ball (behaving as an elastic solid); leave the same ball resting on a table and within an hour it has slumped into a flat puddle (behaving as a thick liquid). Same material, opposite behaviour — and what decides which one you see is how fast you load it and how warm it is. Viscoelasticity is exactly this: polymers are part solid and part liquid, and their response depends on time and temperature.

The behaviour combines two ingredients: an instantaneous, springy elastic response (like a spring that stretches at once and pops back) and a slow, time-dependent viscous response (like a piston pushing through thick oil, called a dashpot). Load a polymer quickly or when it is cold and the springy side dominates — it feels stiff and glassy. Load it slowly or when it is warm and the viscous side dominates — it flows and yields. Two everyday consequences follow directly. Creep is when a polymer under a constant load keeps slowly stretching over time. Stress relaxation is the mirror image: hold a polymer at a fixed stretch and the stress needed to hold it there slowly fades away. Engineers model all this with combinations of springs and dashpots (the Maxwell and Voigt models).

It matters because polymers must be designed for time, not just for a single load. A plastic bookshelf sags visibly over months under the weight of books (creep); a plastic bottle cap or a nylon cable tie slowly loosens its grip (stress relaxation); a car tyre warms up and changes stiffness as you drive. One honest point that separates polymers from metals: a metal's elastic response is essentially instantaneous and its stiffness barely depends on how fast you load it, but a polymer's stiffness can change by a large factor depending on loading speed and temperature — so a single quoted modulus is an incomplete description of a plastic.