Investigation of relaxation processes in linear polyethylene in the 10−9 sec region by means of high-resolution neutron spectroscopy

Results are presented of neutron incoherent scattering experiments on isotropic linear polyethylene samples of high (80%) and low (48%) crystallinity in the temperature range between ?180°C and +85°C for values of the scattering vector between 0.29 Å^?1 and 1.81 Å^?1 obtained with a high resolution backscattering spectrometer (Δ?ω = 0.25 ? 1.0 μeV) and between 0.57 Å^?1 and 2.4 Å^?1 with a time-of-flight spectrometer (Δ?ω = 420 μeV). From a comparison of the results on these samples one concludes that relaxation takes place predominantly in the noncrystalline regions. This motion cannot be adequately accounted for by any of the existing models for the γ-process. Therefore, a more liquidlike motion is suggested. Diffusion of shorter chain segments has also been ruled out since it is too slow to be observed. A simplified model of protonic jumps between equidistant sites located on the periphery of a circle of radius 2.5 Å reproduces the experimental results well. For the average time between successive CH₂-group reorientations one obtains τ₁ = τ₀ exp(E_actRT) with τ₀ = (2.0 ± 1.5) × 10^?13 sec and E_act = (4.5 ± 1.0) kcal/mole. The values join up well with those for the γ-process observed by NMR. It has been concluded that 60–90% of the protons in the noncrystalline regions participate in this motion.