Quasielastic Neutron Scattering and Molecular Dynamics Simulation Study on the Structure Factor of Poly(ethylene-alt-propylene)

Abstract

By combining neutron scattering and fully atomistic molecular dynamics simulations, we have investigated the static [S(Q)] and dynamic [S(Q,t)] structure factor of poly(ethylene-alt-propylene) in a wide momentum-transfer Q range (0.2 < Q < 3 Å−1) spanning over inter- and intramolecular length scales and including also the region of intermediate length scales. The experiments consist of diffraction with polarization analysis and neutron spin-echo measurements on a fully deuterated sample. These results have been used to thoroughly validate the simulations, which have been subsequently exploited to unravel the different contributions to S(Q) and S(Q,t) and provide insights into real space. We have first disentangled the short-range order in this polymer. At the first peak of S(Q) which is clearly dominated by the interchain contribution of the main-chain/main-chain correlations, the dynamics above the glass transition reveal the genuine α-relaxation. The direct observation of the α-relaxation in this way has allowed establishing the viscosity scaling, the stretching, and the deGennes narrowing. An in-phase modulation with S(Q) is observed for the amplitude and shape parameter of the slow decay of S(Q,t) around the first peak. The corresponding time scale also displays a maximum that shifts with temperature in a much stronger way than that of S(Q). Coherency effects in the slow decay are lost above ≈1.5 Å−1, where intramolecular correlations dominate. Interestingly, the apparent activation energy of the characteristic time mirrors the structure factor in the whole Q range investigated.