Study of the electronic and vibrational properties of poly(ethylene terephthalate) and poly(ethylene naphthalate) films

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Abstract
We investigate the optical properties of biaxially stretched poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) polymer films by spectroscopic ellipsometry (SE) in a wide spectral region, from the infrared (IR) (900−3500 cm−1) to the vis-far UV (vis-fUV) (1.5−6.5 eV), in terms of their optical, electronic, and vibrational response. The stretching procedure during the fabrication of the films leads to the rearrangement of the macromolecular chains parallel to the stretching direction (or machine direction), resulting in an optical anisotropy of the films. For the deduction of valuable and accurate information about the films’ electronic and vibrational response, the analysis of the measured SE spectra has been realized by approximating the PET and PEN films as uniaxial materials with their optic axes parallel to the surface. In the vis-fUV spectral region, the characteristic features corresponding to the n→π* electronic transitions of the carbonyl −C=O and the A1g1→B1u1 electronic transition due to the π→π* excitation of the π-electron structures were identified and analyzed. Also, based on the assumption of the optical behavior of a uniaxial material, we have determined the principal components ε||(ω) and ε⊥(ω) of the dielectric function, parallel and perpendicular to the stretching direction of the polymer films. Furthermore, the study of the Fourier transform infrared (IR) SE spectra allowed the assignment of the characteristic features of the measured dielectric function ε(ω) to the vibrational modes of the different chemical bonds of the PET and PEN monomer units. Finally, the above analysis resulted in a detailed calculation of the bulk dielectric function of the PET and PEN polymer films in an extended spectral region from the IR to the vis-fUV.

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