Isomorphous replacement in copolyamide systems: Adipic and terephthalic acids

Abstract Copolyamides with mixtures of adipic and terephthalic acids in various mole rations and three types of diamine were prepared, and a melting point vs. composition curve for each system was constructed. The three types of diamine were: (1) straight‐chain aliphatic diamines with six to twelve carbon atoms, (2) m ‐xylylenediamine, and (3) 4,4‐dimethyl‐1,7‐heptanediamine. These curves for copolyamides containing straight chain aliphatic diamines had no minimum at the intermediate ranges of composition. The curve for m ‐xylylene diamine had a minimum at a point corresponding to 77 mole‐% of adipic acid. The copolyamide containing 4,4‐dimethyl‐1,7‐heptanediamine was not crystalline. Evidence suggested that isomorphous replacement of adipic and terephthalic acids in copolyamide systems was greatly influenced by the structure of the diamine, possibly through the bulk effect of its substituents on chain‐packing.