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Polymers exhibit viscoelastic behavior: they respond to stress with both elastic (spring-like, recoverable) and viscous (dashpot-like, time-dependent) components. The glass transition temperature Tg marks the transition from a rigid glassy state to a rubbery plateau where chain segments gain mobility. Above Tg, modulus drops dramatically. Rubber elasticity arises from entropic recoil of crosslinked network chains. Creep, stress relaxation, and time-temperature superposition (the WLF equation) are key concepts for predicting long-term polymer performance in applications.
Measure the storage modulus (E') of a polymer as a function of temperature (via DMA) and identify the glassy, transition, and rubbery regimes. Apply time-temperature superposition to shift data from different temperatures onto a master curve.
From your study of polymer structure, you know that a polymer chain is a long, flexible molecule that can adopt an enormous number of different shapes (conformations) by rotation around backbone bonds. This structural feature — long, entangled chains rather than the rigid crystal lattices of metals — is the root cause of everything unusual about polymer mechanics. When you learned stress-strain behavior for metals, the elastic response came from stretching atomic bonds: stretch a metal slightly, and the bond energy acts like a spring restoring it. Polymers have a completely different source of elasticity, and they also have a component of behavior that metals lack entirely: viscous flow that makes response time-dependent.
The key conceptual model is the viscoelastic solid — a material that behaves simultaneously like a spring (elastic: stores energy, responds instantly, fully recovers) and a dashpot (viscous: dissipates energy, responds slowly, does not recover). At short time scales or low temperatures, chain segments cannot rearrange fast enough to keep up with the applied deformation, so the material behaves rigidly like a glass. At long time scales or high temperatures, chain segments have time to flow, and the material behaves like a viscous liquid or a soft rubber. The glass transition temperature Tg marks the boundary: below Tg, segmental motion is frozen out and modulus is high (~GPa); above Tg, segments become mobile and modulus drops dramatically (by 3 orders of magnitude for an amorphous polymer). This is not a sharp melting transition — it is a kinetic phenomenon where the time scale of segmental motion matches the observation time scale, so Tg shifts with measurement rate.
Rubber elasticity deserves special attention because its origin is entropic rather than energetic. An unstretched crosslinked rubber network has chains in their most probable, coiled configurations — maximum entropy. Stretching the rubber forces chains toward extended, less probable conformations — lower entropy. The rubber pulls back not because you are stretching chemical bonds, but because the Second Law of Thermodynamics drives systems toward higher entropy. This is why rubber stiffens as temperature rises (unlike metals, which soften) — higher temperature makes the entropic driving force stronger, a prediction confirmed by experiment and derivable from the statistical mechanics of polymer chain statistics.
Creep and stress relaxation are the two signatures of viscoelasticity in practice. In creep, a constant stress produces strain that increases with time as chains slowly rearrange. In stress relaxation, a constant strain produces stress that decreases with time for the same reason. The time-temperature superposition principle (the WLF equation) connects these: data measured at higher temperatures can be shifted horizontally on a log-time axis to overlap data at lower temperatures, producing a master curve that spans many decades of time. This is enormously useful in engineering — you can measure properties over hours in the lab at elevated temperature and predict behavior over decades at service temperature. Understanding these concepts is prerequisite to predicting whether a plastic part will deform under sustained load, how a rubber seal will behave over its service life, or why polymer films creep and wrinkle over time.