The present disclosure provides polyamide 6 copolymers polymerized from caprolactam, tetramethylene diamine, and terephthalic acid monomers. The copolymers exhibit sufficiently high second heat melting points such that they are capable of being processed using traditional melt processing operations, while also being useful for high temperature applications. The copolymer may be used as polymeric base compositions useful for a variety of applications including the manufacture of metal-replacing articles.