Morphology Development and Mechanical properties of Drawn Single-site Polyethylene Blends
Gandara Amarasinghe, Robert Shanks
RMIT University
Australia
Keywords: Metallocene, Polyethylene, Drawing
During the blown film extrusion process, the film undergoes biaxial orientation along longitudinal and transverse directions. The degree of deformation of a polyethylene film primarily depends on its crystal structure, which is determined by various factors including, molecular weight, crystallinity and branch structure. The presence of branches significantly affects the deformation of polyethylenes, because a branch on a tie molecule hinders the disentanglement. It has been shown that branch length is not important, but the amount of branch in controls the properties. The aim of this work is to investigate the morphology and mechanical behaviour of blends of low density polyethylene (LDPE) with high density polyethylene (HDPE), very low density polyethylene (VLDPE) and ultra low density polyethylene (ULDPE) after drawing. HDPE has virtually no branches, whereas the VLDPE (4.2% comonomer) and ULDPE (12.5% comonomer) are single-site catalysed ethylene-octene copolymers having uniformly distributed short and long branches.
Blends of LDPE-HPDE, LDPE-VLDPE and LDPE-ULDPE were prepared as extruded tapes by melt extrusion. Tensile drawing experiments were performed using an Instron universal test instrument. Samples were prepared at various draw ratios. The effect of uniaxial deformation was studied by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis, scanning electron microscopy (SEM) and X-ray scattering. The crystallinity and unit cell parameters were determined by wide angle X-ray diffraction (WAXD), and the crystal long period was determined by small angle X-ray scattering. WAXD measurements showed the increased crystallinity of the drawn samples. SEM revealed changes to the crystal structure along the direction of drawing. Depending on the draw ratio, amorphous and lamellae regions can be organised and thereby provide changed properties to the film. As a result, increases in stiffness and mechanical properties were observed upon deformation of the blends. The change of thermal and mechanical properties in relation to the structural development is interpreted.