High temperature materials are divided into two main categories—semi-crystalline and amorphous—based on their difference in molecular structure.
Semi-crystalline materials have a highly ordered molecular structure with sharp melt points. They do not gradually soften with a temperature increase, instead, semi-crystalline materials remain solid until a given quantity of heat is absorbed and then rapidly change into a low viscosity liquid.
These materials are anistropic in flow, shrinking less in the direction of flow vs. transverse to flow. They have excellent chemical resistance. Semi-crystallines exhibit substantial improvement in their HDTs when reinforced and retain useful levels of strength and stiffness well beyond their glass transition temperature (Tg).