ORIENTATION ANISOTROPY OF THE MECHANICAL ALPHA-RELAXATION OF HIGH-DENSITY POLYETHYLENE FILMS HAVING A WELL-DEFINED STACKED LAMELLAR MORPHOLOGY

Dynamic mechanical experiments were conducted on melt-extruded uniaxia lly oriented high-density polyethylene films having a well-defined sta cked lamellar morphology. Samples were cut at different angles with re spect to the original machine direction (MD) and tested by increasing the temperature from 20 to 130 degrees C at a heating rate of 0.5 degr ees C/min in the frequency range from 0.01 to 10 Hz. Samples before an d after the dynamic mechanical tests were also investigated by using W AXS, SAXS, and DSC. Oscillating parallel (0 degrees) and perpendicular (90 degrees) to the MD gave rise to a single tan delta dispersion pea k, while oscillating at angles between the above two extremes generate d a secondary dispersion peak at lower temperatures, with a maximum re laxation strength occurring at 45 degrees orientation. It was conclude d that, for the 0 and 90 degrees orientations, the mechanical dispersi on arises essentially from the crystalline phase, and it contains the contributions of earlier recognized intralamellar (alpha(I)) and intra crystalline (alpha(II)) relaxations. For the 45 degrees orientation, t he mechanical dispersion is believed to originate from an activated in terlamellar shear motion that is related to the characteristics of the interface between the crystalline lamellar phase and the amorphous ph ase.