Segmental Dynamics of Atactic Polypropylene As Revealed by Molecular Simulations and Quasielastic Neutron Scattering

Abstract

The dynamics of atactic polypropylene has been explored with quasielastic neutron scattering (QENS) measurements in the temperature range 4-460 K and momentum transfer range q) 0.2-2.25 Å-1. In parallel, molecular dynamics simulations of the same polymer have been conducted in the temperature range 260-600 K, using both a fully atomistic model and a model with united-atom methyl groups. In conjunction with the second model, a computational procedure for introducing the motion of methyl hydrogens a posteriori is proposed and tested against the fully atomistic simulation results. Simulated intermediate incoherent scattering functions I(q,t) reveal an initial exponentially decaying regime of duration ca. 1 ps, which is dominated by bond angle bending vibrations and torsional oscillations, as well as features attributable to torsional transitions of the methyl groups and to correlated conformational transitions of the backbone bonds at longer times. The time decay of I(q,t) beyond 1 ps is well-described by stretched exponential functions, the stretching exponent being around 0.5 at 600 K and decreasing with decreasing temperature. Analysis of the atomistic simulation trajectories yields distributions of relaxation times with a distinct log-Gaussian peak characteristic of methyl motion, from which a Gaussian distribution of activation energies for methyl torsional transitions with mean around 15 kJ/mol and standard deviation around 3 kJ/mol is extracted, in excellent agreement with QENS estimates. Torsional transitions of different methyls occur essentially independently of each other. QENS experiments reveal a nondecaying elastic contribution to the scattering over the time window of the measurement, which is not seen in the simulations. Apart from this, computed I(q,t) and incoherent dynamic structure factor S(q,ω) curves are in very favorable agreement with the measured QENS spectra and with earlier NMR data on atactic polypropylene.

Chain dynamics of poly(ethylene-alt-propylene) melts by means of coarse-grained simulations based on atomistic molecular dynamics

“…As W 0 eff ϳ N −0. 4 , the values estimated of Wb 4 slightly change with N ͑M w ͒. However, they perfectly agree in the average with the experimental value deduced from neutron scattering measurements at 492 K.…”

“…This figure also includes two low temperature values of Wb 4 estimated from incoherent neutron scattering results on a PEP sample of M w Ӎ 20 000 g / mol. 36 From the simulation results reported here, we can also calculate Wb 4 as the large N / p-limit of W eff b 4 . In that regime both W eff ͑W 0 eff ͒ and b are basically constant.…”

Neutron scattering investigations on methyl group dynamics in polymers

“…This assumption is confirmed (see e.g., [85][86][87][88]) by molecular dynamics simulations (MDS), which allow one to observe the motion of each individual methyl group in the system. Fig.…”

“…Though the RRDM was initially developed for polymer systems, it must be stressed that all its basic assumptions can, in principle, be also applied to non-polymeric disordered systems (see Section VII). In the framework of the RRDM [25,[31][32][33][34][35][36][88][89][90][91][92][93][94], it is assumed that the only effect of the structural disorder on methyl group dynamics is to introduce a distribution of rotational barriers g(V 3 ), originated from the different local environments felt by the individual methyl groups. A non-trivial assumption is that the distribution of distances between neighbouring methyl groups does not introduce significant coupling forces for the smallest distances, which, in particular, would modify the single-particle Hamiltonian (9) leading to a more complex energy level scheme [95][96][97][98][99][100].…”

Computer simulation of the heterogeneity of segmental dynamics in amorphous polymers

“…The distribution of relaxation times was determined by analysing segmental self-diffusion in a number of small regions of the model lattice, as explained below. These distributions have been calculated in other studies by means of Molecular Dynamics [7,8], but to our 4 knowledge this is the first time that this problem has been analyzed by the BFM. Model simulations showed that the distribution shifted toward longer times as the temperature decreased while broadening [9].…”