The effect of composition and electron beam irradiation on the free volume and crystallinity of linear low-density polyethylene (LLDPE)/ethylene-octene copolymer (EOC) blends: a positron annihilation lifetime spectroscopic study

Blends of linear low-density polyethylene (LLDPE) and ethylene-octene copolymer (EOC) are prepared in various compositions by melt mixing, followed by electron beam (EB) irradiation at dose ranging from 50 to 250 kGy. The effects of blend composition and EB irradiation on the free volume of LLDPE/EOC blends are investigated using positron annihilation lifetime spectroscopy (PALS). Two ortho-positronium (o-Ps) lifetime components extracted from the PALS spectra indicate the presence of two distinct phases in the LLDPE/EOC blends. These components provide insights into the free volume properties of the crystalline and amorphous phases of the polymers. Analysis of the variation in o-Ps annihilation parameters (lifetime and intensity) in these phases confirms a significant increase in the crosslinked three-dimensional network with increasing irradiation dose up to 150 kGy for the LLEO82 blend (LLDPE/EOC: 80 wt%/20 wt%) and 200 kGy for the LLEO28 blend (LLDPE/EOC: 20 wt%/80 wt%). The increase in crosslinking density with EB irradiation is further supported by gel fraction measurements. The results from PALS, gel fraction and thermal analysis are consistent with each other. The study shows that o-Ps parameters are highly effective for determining the free volume hole size and relative fractional free volume in both unirradiated and irradiated LLDPE/EOC blends.