Characteristic features of the molecular structure of polyethylene naphthalate (PEN) are the rigidity of the polymer chains and the occurrence of two trans conformations of the naphthalene ring, the so-called alpha- and beta-conformations. The rigidity of the molecular chains permits production of PEN tyre yarns by a simple high-speed melt spinning process without drawing steps. The yarns crystallize with the naphthalene ring in the beta-conformation. The crystal structure of this modification (PEN-beta) is monoclinic (spacegroup P 1 2(1)/a 1) with unit cell parameters a = 9.49, b = 13.31, c = 12.61 Angstrom, alpha = 90, beta = 135, gamma = 90 degrees. The melting points of the beta-crystals can be over 300 degrees C. Under the influence of stress, as occurs for instance when the yarns are dipped for application in tyres, the crystals transform irreversibly into the fully extended alpha-form with retention of a high melting point. The ultimate modulus of PEN yarns produced by different processes is substantially higher than that of polyethylene terephthalate. However, in the amorphous domains of PEN yams, rotation of the naphthalene ring induces conformational alpha double left right arrow beta and cis double left right arrow cis transitions which negatively influence the dynamic properties. More specifically, the work loss is four times as high as that of other tyre yarns, and, going from 20 to 60 degrees C, i.e. a moderate in-use temperature for tyres, the dynamic modulus decreases by 20%. (C) 2000 Elsevier Science Ltd. All rights reserved.