Conformational changes occurring during the melting of various polyethylene (PE) materials, including high-density PE (HDPE), linear low-density PE (LLDPE) and low-density polyethylene (LDPE), were investigated using near-infrared spectroscopy. The assignment of PE to the 1650-1900 nm spectral region was suggested, on the basis of computational and experimental data for normal alkanes. The present results suggested that the 1690 and 1710 nm bands should be assigned to the CH 3 groups in branched chains and chain ends, respectively. The HDPE and LDPE crystal lattices have very few defects, in contrast to LLDPE, which has more crystal lattice defects. The chain ends and junctions of LDPE are excluded from the crystal lattice and exist in a thick amorphous layer, whereas LLDPE includes comonomers in its crystalline lattice.

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We present a novel method for characterizing orientation behavior of typical polyethylene (PE) materials such as HDPE, LLDPE, and LDPE. The chain orientation to the stretching direction was examined under uniaxial deformation by near infrared spectroscopy. Then we obtained directly the orientation function of PE chain axis (c-axis) from the CH stretching vibration of NIR spectra as a function of extension time or strain. We compared the present method with the conventional infrared IR method where the orientation function of PE c-axis (chain-axis) has been indirectly obtained from the b-and a-axis’s assuming the orthogonal crystal form by using the CH2 rocking vibrations in IR spectra