On the separation between torsion-vibration and conformational relaxation processes in the incoherent intermediate scattering function of polyethylene

We present a detailed study of the local dynamics of short polyethylene (PE) chains in a wide temperature range (T=350, 450, 504 K) using molecular dynamics (MD) simulations based on a united atom model. Undercooled melt phases can be studied easily in simulations without any sign of crystallization in the sample. We focus on the interpretation of the incoherent intermediate scattering function in the range 0.375≤Q≤2.0 A-1 using a refined analysis of the time correlation functions in terms of a continuous linear combination of exponential decays weighted by a distribution of relaxation times (DRT). At 350 K, over the whole Q range investigated, the relaxation of the intermediate scattering function is due to a combination of a fast process occurring on the picosecond time scale and a slow process. The faster process is weakly Q- and T-dependent and is shown to originate from torsion-vibration motions. The DRT associated with the slow process, related to conformational jumps, changes quantitatively and qualitatively in the explored Q range and does not resemble a stretched exponential behaviour. The average global relaxation time related to the slow process evolves below Q=1 A-1 towards a Q-3.3 power law behaviour