 Thermoplastics , by contrast , form long molecular chains , which are often coiled or intertwined , forming an amorphous structure without any long @-@ range , crystalline order . Some thermoplastics may be fully amorphous , while others have a partially crystalline / partially amorphous structure . Both amorphous and semicrystalline thermoplastics have a glass transition , above which welding can occur , but semicrystallines also have a specific melting point which is above the glass transition . Above this melting point , the viscous liquid will become a free @-@ flowing liquid ( see rheological weldability for thermoplastics ) . Examples of thermoplastics include polyethylene , polypropylene , polystyrene , polyvinylchloride ( PVC ) , and fluoroplastics like Teflon and Spectralon . 
