 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 <unk> thermoplastics have a glass transition , above which welding can occur , but <unk> 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 <unk> weldability for thermoplastics ) . Examples of thermoplastics include polyethylene , polypropylene , polystyrene , <unk> ( PVC ) , and <unk> like Teflon and <unk> . 
