Orientation Anisotropy of the Mechanical α Relaxation of High-Density Polyethylene Films Having a Well-Defined Stacked Lamellar Morphology

Dynamic mechanical experiments were conducted on melt-extruded uniaxially oriented high- density polyethylene films having a well-defined stacked lamellar morphology. Samples were cut at different angles with respect to the original machine direction (MD) and tested by increasing the temperature from 20 to 130 °C at a heating rate of 0.5 °C/min in the frequency range from 0.01 to 10 Hz. Samples before and after the dynamic mechanical tests were also investigated by using WAXS, SAXS, and DSC. Oscillating parallel (0°) and perpendicular (90°) to the MD gave rise to a single tan δ dispersion peak, while oscillating at angles between the above two extremes generated a secondary dispersion peak at lower temperatures, with a maximum relaxation strength occurring at 45° orientation. It was concluded that, for the 0 and 90° orientations, the mechanical dispersion arises essentially from the crystalline phase, and it contains the contributions of earlier recognized intralamellar (α I ) and intracrystalline (α II ) relaxations. For the 45° orientation, the mechanical dispersion is believed to originate from an activated interlamellar shear motion that is related to the characteristics of the interface between the crystalline lamellar phase and the amorphous phase.