Relaxation mechanism in several kinds of polyethylene estimated by dynamic mechanical measurements, positron annihilation, X-ray and 13C solid-state NMR

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 13C nuclear magnetic relaxation (13C NMR). From complex dynamic tensile modulus, the activation energies of α1 and α2 relaxations were determined to be 97–118 and 141–176 kJ/mol, respectively. The activation energy of β relaxation was 114–115 kJ/mol. These values were similar to those of α1 relaxation reported already. For γ relaxation mechanisms, there existed two mechanisms, γ1 and γ2, the activation energies being 9–11 and 23–25 kJ/mol, respectively. The values were independent of the molecular orientation and crystallinity. The two local motions indicate that non-crystalline phase composes of two regions of non-crystalline phase, rubber-like amorphous phase and interfacial-like amorphous phase. From 13C NMR measurements of 13C longitudinal relaxation time for the non-crystalline phase, the activation energy was 20.7 kJ/mol. This value is close to the activation energy (23–25 kJ/mol) of the γ2 relaxation estimated by the dynamic mechanical measurement. The result by 13C NMR did not provide two kinds of activation energy, indicating combined influence of the two correlation times. Even so, the activation energies obtained by 13C NMR indicated that the γ2 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 γ and β 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.