 Polysulfone (PSU), poly(ethersulfone) (PES) and polyetherimide (PEI) for example are amorphous; poly(phenylene sulfide) (PPS), polyetheretherketone (PEEK) and polyether ketones (PEK), however are semi-crystalline.Crystalline polymers (especially those reinforced with fillers) can be used even above their glass transition temperature.This is because semi-crystalline polymers have, in addition to the glass temperature Tg, the crystallite melting point Tm, which is mostly much higher located.