Highly oriented single-phase blend films of high- and low-density polyethylene

Abstract

The microstructures of highly oriented drawn films of blends of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) were investigated by transmission electron microscopy, electron diffraction, X-ray diffraction, and differential scanning calorimetry (DSC). The average crystal size, as well as long period, crystalline content, and melting endotherm peak, decreased as LDPE was added to the blend. When the LDPE content exceeded ∼ 50%, the film texture changed from a single crystal texture to fibre symmetric. Segregation of the two polyethylenes was not detected at low LDPE contents in as-drawn or melted and recrystallized films. In the as-drawn films, a low temperature tail began to appear on endotherm melting peaks at LDPE contents ⩾70%, indicating the onset of segregation. In meltcrystallized films, however, two distinct melting endotherm peaks were visible for LDPE contents ⩾50%. An equilibrium melting point of 141° C and end surface free energy of 101 erg cm⁻² (101 × 10⁻⁷ J cm⁻²) were determined by use of the Thomson equation. The close agreement between these values and literature values for HDPE suggested that the crystals present in HDPE/LDPE blends were thermodynamically equivalent to HDPE crystals of equal size, implying that branches were excluded from the crystalline phase.