Analysis as a Function of Temperature of the Dynamic Linear Infrared Dichroism Spectra of Isotropic and Cold-Drawn High-Density Polyethylene

Abstract

A dynamic linear infrared dichroism (DLIRD) study was carried out as a function of temperature on isotropic and cold-drawn high-density polyethylene (HDPE) with the use of a rapid-scan setup. The dynamic in-phase dichroic spectrum and its components (parallel and perpendicular polarized spectra) were analyzed. From the results, it turned out that, in addition to molecular orientation effects, other effects such as frequency shifts are clearly identified in the dynamic signals, which furthermore were in agreement with previously reported results on polyethylene under stress from the use of conventional infrared spectroscopy. Moreover, the frequency shifts were more apparent at the lowest temperatures and in the cold-drawn material, i.e., with increasing sample moduli. A revision of the interpretation of the results published earlier, in which the dynamic signals were solely attributed to molecular orientations, is also carried out in view of our results. From our results it seems that indeed an orientation of the crystallographic b axis parallel to the applied strain is suggested from the –CH₂–bending and rocking ranges of the isotropic sample. However, an orientation of the crystallographic a axis of the unit cell perpendicular to the strain direction as suggested from previous studies is questioned, as well as the assignment of a certain dynamic signal to a reorientation of chains in the amorphous phase. Besides these results, the behavior of the –CH₂– wagging at 1176 cm⁻¹, assigned to the crystalline phase, suggests an orientation of the crystallographic c axis of the unit cell parallel to the strain direction. As a consequence, two conclusions are made: (1) a more complex orientation motion occurs in the crystals than has been reported so far, and (2) a more thorough study of the DLIRD signals has to be carried out in which other factors such as thickness variations and instrumental artifacts need to be addressed.