Effect of electron-irradiation on cross-link density and crystalline structure of low- and high-density polyethylene

Low- and high-density polyethylenes (LDPE and HDPE) were cross-linked in solid state by electron beam irradiation. Molar mass between cross-link joints, M_c, and cross-link density,ν, were calculated using rubber elasticity theory and hot set data. The results showed that the ν and creep modulus increased and creep strain and M_c decreased with increasing irradiation dose. As compared to HDPE, the LDPE had higher ν and lower M_c values at a similar irradiation dose. X-ray analysis and differential scanning calorimetry investigation of first heating cycle revealed no changes in crystalline structure of the irradiated samples. This was attributed to immobilization of radicals frozen in the crystalline phase. As a result of hindered mobility of the polymeric chains, these radicals were not able to cross-link the chains in the crystalline region. However, after melting of the crystals and during subsequent re-solidification process, different levels of crystallinity were developed depending on the applied irradiation dose. The irradiated samples with higher dose had lower crystallization and melting temperatures with reduced crystallinities. These confined crystallization behaviors, observed after a series of cooling and heating cycles, could be attributed to the decrease in the M_c values. The length of chain segment needed for usual crystallization by chain folding is decreased due to formation of cross-link joints and hence the crystallization process was hindered.