An interpretation for the first- and second-order phase transitions in nylon 66 crystal based on the intra- and intermolecular energy calculation

Intra- and intermolecular interactions in nylon 66 crystal are calculated mainly using internal rotation and Lennard-Jones potential functions reasonably supposing that the amide planes are fixed and the hexamethylene moiety in the chain has an inversion center. Partition function is formularized on the basis of sum of the intra- and intermolecular interaction potentials in analogy with the Ising model on a rectangular lattice with degeneracy of interaction energy to show that the first-order phase transition occurs when the transition temperature T-t is lower than the critical temperature T-c and the second-order phase transition does when T-t = T-c, where T-c only depends on the intermolecular interaction and T-t on the intramolecular one mainly. The pressure effect on this phenomenon is qualitatively explained from temperature- or Delta d (d(100) - d(110,010)) dependence of T-c and T-t.