The physical state and morphology of a polymer have a strong influence on its mechanical properties. A simple measure of the differences produced in mechanical behaviour is the elongation that occurs when a plastic is loaded (stressed) in tension. A glassy polymer such as polystyrene is quite stiff, showing a high ratio of initial stress to initial elongation. On the other hand, polyethylene and polypropylene, two highly crystalline plastics, are usable as films and molded objects because at room temperature their amorphous regions are well above their glass transition temperatures. The leathery toughness of these polymers above Tg results from the crystalline regions that exist in an amorphous, rubbery matrix. Elongations of 100 to 1,000 percent are possible with these plastics. In PET, another semicrystalline plastic, the crystalline portions exist in a glassy matrix because the Tg of PET is above room temperature. This gives the material a stiffness and high dimensional stability under stress that are of great importance in beverage bottles and recording tape.
Almost all plastics exhibit some elongation on being stressed that is not recovered when the stress is removed. This behaviour, known as “creep,” may be very small for a plastic that is well below its Tg, but it can be significant for a partly crystalline plastic that is above Tg.
The most commonly specified mechanical properties of polymers include stiffness and breaking stress, quantified in the table of properties and applications as flexural modulus and tensile strength. Another important property is toughness, which is the energy absorbed by a polymer before failure—often as the result of a sudden impact. Repeated applications of stress well below the tensile strength of a plastic may result in fatigue failure.
Most plastics are poor conductors of heat; conductivity can be reduced even further by incorporating a gas (usually air) into the material. For instance, foamed polystyrene used in cups for hot beverages has a thermal conductivity about one-quarter that of the unfoamed polymer. Plastics also are electrical insulators unless especially designed for conductivity. Besides conductivity, important electrical properties include dielectric strength (resistance to breakdown at high voltages) and dielectric loss (a measure of the energy dissipated as heat when an alternating current is applied).