Size of voids in polyethylene

When analyzing the positron lifetime spectra of several lamellar polyethylene samples of different crystallinities and structural characteristics, in terms of three components the longest lifetime, associated with positronium annihilation, increases in a sigmoidal way in proportion to the amorphous phase. In a similar way as previously done for other molecular crystals, the data derived from a four-component analysis of the spectra are well fitted by using a simple model, involving positronium transition and trapping from the crystal to the disordered region. Density measurements of the samples and a theoretical model of molecular crystals, relating positronium lifetime to cavity volume, lead to mean values of voids in the amorphous phase ranging from 0.119 nm3, in the most crystalline specimen, to 0.176 nm3 in the most defective one. A density of voids larger than 1020 cm−3 is found and estimations of the positronium diffusion coefficient in the amorphous phase give values of about 10−6 cm2 s−1. It is found that the positronium formation probability is nearly 0.15 in the crystal, and reaches 0.43 in the disordered region, a value similar to that measured in liquid aliphatic hydrocarbons.