High-density polyethylene/paraffin wax (HDPE/wax) systems with adjustable density of the amorphous regions were prepared by a melt-blending process to optimize/control the final oxygen barrier properties. The introduction of paraffin wax (a low molecular weight modifier) is the key to tune the gas permeability properties of polyethylene-based materials. Density gradient column (DGC) measurements distinctly showed that the incorporation of modifier led to densification of the amorphous phase of semicrystalline HDPE consisting in a decrease in the average fractional free volume confirmed by positron annihilation lifetime spectroscopy (PALS). Polyethylene with "densified" amorphous phase exhibits lower oxygen permeability parameters compared to pristine polyethylene, but it is characterized by similar thermal and thermomechanical properties. An increase in the density of the amorphous regions of polyethylene by about 0.003 g/cm3, which corresponds to 0.3%, reduces the permeability of oxygen by up to 22%. For the first time, it has been proven that by controlling the density of the amorphous regions of semicrystalline polymers, it is possible to obtain materials with appropriate transport properties (without changing other properties) for applications meeting specific requirements.
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