Relaxation processes of several kinds of polyethylene films and fibers with different molecular orientational degrees and crystallinities were extensively investigated by the dynamic mechanical relaxation, positron annihilation and C-13 nuclear magnetic relaxation (C-13 NMR). From complex dynamic tensile modulus, the activation energies of alpha(1) and alpha(2) relaxations were determined to be 97-118 and 141-176 kJ/mol, respectively. The activation energy of P relaxation was 114-115 kJ/mol. These values were similar to those of alpha(1) relaxation reported already. For gamma relaxation mechanisms, there existed two mechanisms, gamma(1) and gamma(2), the activation energies being 9-11 and 2325 kJ/mol, respectively. The values were independent of the molecular orientation and crystallinity. The two local motions indicate that noncrystalline phase composes of two regions of non-crystalline phase, rubber-like amorphous phase and interfacial-like amorphous phase. From C-13 NMR measurements of C-13 longitudinal relaxation time for the non-crystalline phase, the activation energy was 20.7 kJ/mol. This value C-13 is close to the activation energy (23-25 kJ/mol) of the gamma(2) relaxation estimated by the dynamic mechanical measurement. The result by NMR did not provide two kinds of activation energy, indicating combined influence of the two correlation times. Even so, the activation energies obtained by C-13 NMR indicated that the gamma(2) relaxation mainly is due to the motion of the C-C central bond of a short segment (e.g. three to four CH2) within interfacial-like amorphous phase. The gamma and beta relaxation peaks by the dynamic mechanical measurements corresponded to the first and second lifetime transition of ortho-positronium indicating, in turn, a drastic change in free volume by local mode relaxation. (C) 2003 Elsevier Science Ltd. All rights reserved.