A spectroscopic ellipsometry study of PET membranes from IR to Vis‐FUV

A detailed study of Spectroscopic Ellipsometry (SE) spectra of uniaxially stressed, 12μm thick poly(ethylene terephthalate) (PET) membranes were realized in both IR (900‐3500 cm−1) and Vis‐FUV (1.5 to 6.5 eV) spectral regions and in various orientation angles ∅︁between the plane of incidence and the machine direction (MD) of the PET membranes. In the IR spectra, the three main modes of the ester group at ∼1126, 1256 and 1721 cm−1are observed. The first two of these peaks show identical polarization dependence, with preferential orientation ∅︁≈0° (i.e. the plane of reflection parallel to the MD), while the peak at ∼1721 cm−1is excited in the perpendicular direction, ∅︁≈90°. In the Vis‐FUV SE spectra, the dependence of the characteristic electronic absorption bands on the orientation angle ∅︁is studied. Besides the well known benzene band centered at ∼5 eV, a doublet of less intense peaks appear just above the absorption edge between 4.1 and 4.3 eV with the most striking characteristic that they show polarization selection rules opposing to that of the benzene band. The above characteristics of the electronic Vis‐FUV spectra of PET are discussed in a manner comparative to the corresponding IR spectra. The discussion becomes in the context of determining the molecular orientation of PET using a non‐destructive technique such as SE.