Macromolecules, Vol.35, No.6, 2095-2104, 2002
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 arid 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 degreesC 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 H-1 NMR, dynamic mechanical tensile measurements, and FT-Raman spectroscopy.