Inclusion/exclusion behavior of comonomer units in isodimorphic crystallization of biobased copolyamides PA6/PA56

A series of semi-crystalline bio-based copolyamides PA6x56y were synthesized via melt polycondensation from caprolactam (6 units) and bio-based Pentamethylene adipamide units (56 units). DSC results show that the melting and crystallization temperature depend on the comonomers composition, and exhibit isodimorphic behavior. Combined with the analysis of WAXD, it is found that the 56 units are more tend to be excluded from rather than incorporated into the PA6 crystals. However, it does not imply a complete exclusion. Unexpectedly, there is a significant relationship between the content of 56 units on the polymorphic crystallization behavior of PA6-type crystals. Moreover, the pseudo-eutectic point appears when 56 units content is 30 mol %, and the spherulite morphology exhibits a strong correlation with the pseudo-eutectic point. In addition, the excess free energy of defects (ε) is calculated by applying the Wending-Suter model, and it is found that the ε-value caused by the insertion of 56 units into the crystal formed by PA6 is higher than that of 6 units into a crystal formed by PA56. Therefore, 6 units are more easily inserted into crystals formed by PA56, i.e., 6 units have better compatibility with PA56 crystals. In summary, this work reveals the involved mechanisms of comonomer units in the crystallization process and provides a potential strategy to guide the development of high-performance copolymers in the future.