Polymorphism of even nylons revisited through periodic quantum chemical calculations

Density functional theory calculations with periodic boundary conditions have been carried out for the 3D crystal cells of alpha and gamma polymorphs of even polyamides (ranging from nylon-4 up to nylon-12) in order to investigate the relative stability of the two forms. The infinite 1D polymer chains possessing alpha and gamma conformations have been also simulated to shed light on the intramolecular effects taking place in these systems and to discriminate the intermolecular effects in terms of hydrogen bonding and packing of CH2 units. The results of this analysis allowed to give a detailed interpretation of the polymorphism properties observed in even nylons, predicting a larger stability for the cc form in nylon-4 and nylon-6 and a stabilization of the gamma phase from nylon-8 up to nylons having a larger number of CH2 unit in the chain.