Structure and properties of 6/6.9 copolyamide series. II. The crystalline phase

Abstract The crystalline phase of a series of random 6/6.9 copolyamides was studied. The crystallization and fusion behavior, crystallography, and morphology were investigated. A decrease in melt crystallization temperature and rate, and an increase in cold crystallization temperature are caused by increasing either monomer content. The hydrogen bonding formation is interrupted by the comonomers, reducing the degree of crystallinity (17–41%). Melting temperature depression caused by the increase in comonomer 6.9 content is accompanied by a decrease in crystal size. The measured melting temperatures are lower than the predicted ones (according to Flory), due to comonomer incorporation in the crystal lattice. The copolyamides consist of α-form and 63–89% γ-form crystals. The γ crystal content increases with an increase in the comonomer 6.9 content. Polyamide 6 and the copolyamides rich in caprolactam exhibit sheaflike spherulites which become less perfect with an increase in the comonomer 6.9 content. The other copolyamides show lamellar morphology, however, without a spherulitic structure. In general, the morphological order in this series is higher than its crystallographic order.