These long chains intertwine to form the bulk of the plastic. The nature by which the chains intertwine determine the plastic's macroscopic properties. Typically, the polymer chain orientations are random and give the plastic an amorphous structure.
Amorphous plastics have good impact strength and toughness.
If instead the polymer chains take an orderly, densely packed arrangement, the plastic is said to be crystalline . Such plastics share many properties with crystals, and typically will have lower elongation and flexibility than amorphous plastics. Most plastics can be classified as either thermoplastic or thermoset , a label which describes the strength of the bonds between adjacent polymer chains within the structure. The chains are free to slide past one another when sufficient thermal energy is supplied, making the plastic formable and recyclable. In thermosets , adjacent polymer chains form strong cross links.