A comparison of the dynamic mechanical relaxation behavior of linear low- and high-density polyethylenes

The mechanical relaxation behaviors of oriented samples of a linear low-density polyethylene (LLDPE) and a high-density polyethylene (HDPE) were studied using tensile dynamic mechanical measurements. The anisotropy and activation energies of the relaxations were determined for several different samples to investigate why the alpha-relaxation in HDPE has characteristics similar in some respects to both the alpha- and beta-relaxations in LLDPE. It is concluded that the anisotropy of the alpha-relaxation in LLDPE is determined by c-shear, whereas in HDPE it relates to interlamellar shear. The activation energy measurements, however, show that the thermally activated process for the alpha-relaxation in both the LLDPE and HDPE is c-shear. It is proposed that, in HDPE, c-shear has to occur before there is enough mobility at the fold surface for interlamellar shear. It is concluded that the beta-relaxation is not observed in HDPE because the interlamellar regions are too constrained to allow interlamellar shear without c-shear.