Molecular Orientation of Poly(ethylene naphthalate)/Poly(ethylene terephthalate) Copolymers Utilizing Polarized Raman Spectra

Polarized laser Raman microscopy was used to estimate the molecular orientation of uniaxially drawn films of mostly random copolymers of poly(ethylene 2,6-naphthalate) (PEN) and poly(ethylene terephthalate) (PET) as a function of the draw ratio. The Raman bands of the symmetric stretching modes of benzene and naphthalene rings at 1616 and 1636 cm -1 have been respectively used for PET and PEN segments orientation evaluation. Polarized Raman spectra obtained from PEN/PET films in the whole concentration range indicate that PET segments are better oriented in the copolymers than in the homopolymer attenuating the neck formation of PEN. Moreover, PEN/PET films stretched at temperatures 30 °C higher than the corresponding glass transition temperatures exhibit different degrees of segmental relaxation depending on the dominant, PEN or PET, number-average segment length. These relaxation discrepancies are accompanied by conformational changes of the ethylene moieties. In certain PEN/PET polymer mixtures, at specific window of stretching parameters, PEN segments exhibit comparable and even higher homopolymer molecular orientation. Isotropic and drawn type films were both characterized by DSC, solution 1 H NMR, dynamic mechanical tensile measurements, and FT-Raman spectroscopy.