Polymers can be divided into two types of thermoplastic and thermosetting. Is generally considered a linear polymer having a thermoplastic macromolecule, the plastic can flow, it becomes solid when cooled after heating. This process is a physical change is reversible. As between the bonded molecules when heated, become the body structure, compared with the thermosetting, this process is often irreversible chemical change.
However, this general narrative but does not fully applicable to the polyimide. Linear thermoplastic polyimide may not have, because some species even when heated to the decomposition temperature still will not flow, or even noticeable softening.
And as a typical thermoplastic polyimide at elevated temperatures, for example above 400 ℃, it will gradually lose liquidity, which is produced because the cross-linking between molecules. In fact, all of the aromatic heterocyclic polymers at high temperature will produce crosslinking. One of the most common mechanism is due to an aromatic ring radical dehydrogenation, when it complex to crosslink between molecules occur.
Thus, polyimide and other aromatic heterocyclic polymer is not like other common polyolefins or polymers that can repeatedly melt processed. Even the thermoplastic aromatic heterocyclic polymers also have only limited number of melt processing.
Therefore, such polymers have called "false thermoplastic polymer." Polyimide in the synthesis of chemical modifications on the larger. For example, a low-temperature solution polycondensation at low temperatures or chemical imidization. Monomer mixture can also be easily introduced in a manner macromolecule or oligomer at a high temperature-sensitive reactive groups to give a prepolymer. And then crosslinking. Thus, compared to other aromatic heterocyclic polymers, polyimides are more readily available.