X-ray diffraction analysis demonstrated that a PEN film annealed from 160 to 260 degrees C crystallizes In the a crystal form while crystallization of PEN directly from the melt results in the beta crystal form when T-o > 200 degrees C and in the alpha form at lower T-c. Infrared spectra from PEN samples containing alpha, beta, and amorphous phases were obtained, and digital subtraction of the amorphous contribution from the semicrystalline spectra provided characteristic spectra of amorphous, alpha, and beta crystalline spectra. The bands at 1004, 1332, and 814 cm(-1) were attributed to the all-trans conformation in the alpha crystalline phase, and the bands at 975 and 1348 cm(-1) were attributed to the beta crystalline phase. Our infrared spectroscopic investigation suggests that the alpha crystal form adopts an all-trans conformation while the beta crystal form adapts a conformation with appreciable gauche character. Conformational changes occurring in PEN not only are due to the rotation of the ethylene glycol unit but also arise from the rotation of the naphthalene ring. This is different from PET, where conformational changes have been observed to occur only in the ethylene glycol unit. The normalized absorbances of the band at 824 and 814 cm(-1) were correlated to polymer density, showing that these bands can be used unambiguously to represent the amorphous and alpha crystalline phases, respectively. Using drawn PEN film, the assignment of the amorphous 1338 cm(-1) band to the trans conformation was explored.