Polymer materials can be divided in two different families depending on the level of molecular organization of the constitutive chains at the microscopic scale.
In semi-crystalline polymers, the molecular chains can locally fold and stack one of each other creating small crystals. The morphology of semi-crystalline polymers consists in crystalline domains surrounded by an amorphous phase, in which the polymer chains are tangled randomly. Due to the presence of crystals, semi-crystalline polymers are generally opaque and display non isotropic properties. They have two different thermal transition : i) the glass transition temperature corresponding to the softening of the amorphous phase and ii) the melting point corresponding to the melting of the different crystals and the flow of the material.
The performance of semi-crystalline polymers is directly related to the degree of crystallinity, i.e. the relative percentage of crystalline phase and amorphous phase and to the size of the crystalline domains. Those factors are intimately related to the polymer nature and to the used processing/molding parameters (applied shear, cooling rate, etc.).
Amorphous polymers present a glass-like structure in which the polymer chains are tangled randomly without any specific arrangement. Those polymers are generally transparent and present isotropic properties. They have no melting point or temperature but rather soften over a range of temperature around the glass transition temperature, Tg. Like glass, amorphous polymers are highly sensitive to stress cracking. Typical amorphous polymers are polystyrene (PS), polycarbonate (PC), polyetherimide (PEI) or sulfone polymers (PSU, PESU and PPSU).
Glass transition temperature Tg of PSU is equal to 190°C