Dynamic infrared linear dichroism study of high-density and low-density polyethylene near the β-transition temperature

Abstract

High-density polyethylene (HDPE) and low-density polyethylene (LDPE) films under small-amplitude oscillatory strain were examined using time-resolved infrared (IR) linear dichroism spectroscopy. A negative dynamic dichroism at 1473 cm −1 and a positive dynamic dichroism at 1463 cm −1 were observed at +32°C (above the β-transition temperature, T β. This observation is interpreted to mean that above T β the orthorhombic crystallographic b axis reorients parallel to the direction of applied strain, while the crystallographic a axis reorients perpendicular to the strain direction for both HDPE and LDPE. Interestingly, the dynamic dichroism of both bands changes sign at −50°C (below T β), indicating that the dynamic reorientation directions for the crystalline a and b axes are shifted below T β. This result clearly shows that different deformation mechanisms operate above and below T β. Even though the IR peaks in this region are made up of overlapping bands from both crystalline and amorphous components, they are easily resolved in this experiment. The high spectral resolution was achieved because the electric dipole transition moments associated with individual bands reorient in different directions, at different rates, and to different extents under dynamic strain.