Structural investigation of orthorhombic-to-hexagonal phase transition in polyethylene crystal: The experimental confirmation of the conformationally disordered structure by X-ray diffraction and infrared/Raman spectroscopic measurements

Structural changes in the orthorhombic-to-hexagonal phase transition of polyethylene crystal has been investigated by measuring the differential scanning calorimetry, X-ray diffraction, and infrared and Raman spectra for the geometrically-constrained ultradrawn samples in the course of heating up to the melting temperature. The infrared and Raman spectral patterns characteristic of the hexagonal phase have been confirmed. In particular, the bands characteristic of the disordered short trans segments (shorter than five CH2 units) and the bands of the kink or double gauche linkages have been detected definitely. The degree of orientation of the averaged chain axis, as detected from the temperature dependence of the X-ray fiber diagram, was reserved well enough even in the hexagonal phase, while the orientational degree of the methylene units, as detected from the polarized infrared spectral measurement, was found to be lowered appreciably. This finding, combined with the observation of the trans and gauche bands, has confirmed experimentally and definitely the existence of the conformationally disordered chains in the hexagonal phase.