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Polymeric materials are characterized by long chains of repeated molecule units known as "mers".
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. Examples include
acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile copolymer (SAN), polyvinyl chloride (PVC),
polycarbonate (PC), and polystyrene (PS).
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. Examples of crystalline
plastics include acetal, polyamide (PA; nylon), polyethylene (PE), polypropylene (PP), polyester (PET, PBT), and
polyphenylene sulfide (PPS).
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.
In thermoplastics,
the polymer chains are only weakly bonded (van der Waals forces). 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. When heated, these cross links prevent the
polymer chains from slipping past one another. As such, thermosets cannot be reflowed once they are
cured (i.e. once the cross links form). Instead, thermosets can suffer chemical degradation (denaturing) if reheated excessively.
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