Effects of amide comonomers on polyamide 6 crystallization kinetics

Amide comonomers are common in proteins, which bring chemical confinement to their beta-folding. Furthermore, co-polymerization is a common industrial way to enhance polymer properties. By means of fast-scanning chip-calorimetry (Flash DSC) measurement, we investigated how various amide comonomers (amide 6,6 and amide 12) influence the isothermal crystallization kinetics of polyamide 6. The results show that both amide-6-based random copolymers containing separately 18 mol% of amide 6,6 and 20 mol% of amide 12 crystallize slower than polyamide 6 due to the structural mismatching of amide comonomers in hydrogen-bonding interactions, reflecting the general chemical confinement of co-monomers in copolymer crystallization. However, in the low temperature region, the copolymer holding amide 12 crystallizes even slower than the copolymer holding amide 6,6, opposite to what we usually expected for a higher mobile comonomer of amide 12. We found that the anomalous behavior could be attributed to the temperature region where polyamide 12 crystallizes still slower than polyamide 6,6. Our wide-angle X-ray diffraction results revealed that those amide comonomers retain the habits of their corresponding homopolymers in copolymer crystallization due to their inclusion in the crystalline phase of polyamide 6.