Structural and Conformational Characterization of Poly(ethylene 2,6-naphthalate) by Infrared Spectroscopy

X-ray diffraction analysis demonstrated that a PEN film annealed from 160 to 260 °C crystallizes in the α crystal form while crystallization of PEN directly from the melt results in the β crystal form when T c > 200 °C and in the α form at lower T c . Infrared spectra from PEN samples containing α, β, and amorphous phases were obtained, and digital subtraction of the amorphous contribution from the semicrystalline spectra provided characteristic spectra of amorphous, α, and β crystalline spectra. The bands at 1004, 1332, and 814 cm -1 were attributed to the all-trans conformation in the α crystalline phase, and the bands at 975 and 1348 cm -1 were attributed to the β crystalline phase. Our infrared spectroscopic investigation suggests that the α crystal form adopts an all-trans conformation while the β crystal form adopts 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 α crystalline phases, respectively. Using drawn PEN film, the assignment of the amorphous 1338 cm -1 band to the trans conformation was explored.