Study of the dynamics of poly(ethylene oxide) by combining molecular dynamic simulations and neutron scattering experiments

Abstract

We performed quasielastic neutron scattering experiments and atomistic molecular dynamics simulations on a poly(ethylene oxide) (PEO) homopolymer system above the melting point. The excellent agreement found between both sets of data, together with a successful comparison with literature diffraction results, validates the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field used to produce our dynamic runs and gives support to their further analysis. This provided direct information on magnitudes which are not accessible from experiments such as the radial probability distribution functions of specific atoms at different times and their moments. The results of our simulations on the H-motions and different experiments indicate that in the high-temperature range investigated the dynamics is Rouse-like for Q-values below approximately 0.6 A(-1). We then addressed the single chain dynamic structure factor with the simulations. A mode analysis, not possible directly experimentally, reveals the limits of applicability of the Rouse model to PEO. We discuss the possible origins for the observed deviations.

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

“…6 and 42͒, and more recently poly͑ethylene oxide͒. 8 These type of deviations have also been reported from theoretical treatments as well ͑see e.g., Refs. 43 and 44͒.…”

“…Such simulations have been performed only for medium length chains where large-scale dynamics and rheological properties seem to be well described in terms of the simple Rouse model. [5][6][7][8][9] In order to simulate dynamical and rheological behavior of long chains, one has to resort to coarse-grained models. Many different coarse-grained models have been reported in the literature, 10 being perhaps the most popular one that of bead-spring type, where the polymer chain is represented as a sequence of beads connected by springs.…”

Polymer Rheology by Dielectric Spectroscopy

“…On the contrary, a lower frequency cut-off would improve slightly the agreement between the normal mode data and the Rouse model prediction but would imply and higher coupling between the segmental dynamics and the whole chain motion. Small deviations from the Rouse model predictions have also been reported from numerical simulations, molecular dynamics calculations and detected by neutron scattering experiments, Brodeck et al (2009); Doxastakis et al (2003); Logotheti & Theodorou (2007); Richter et al (1999) although these deviations become evident only for relatively high p-values. Note, that dielectric experiments being mainly sensitive to the low-p modes can hardly detect such deviations.…”

“…the second mode contributing to the dielectric normal mode. All this might evidence the fact that the Rouse model approach ignores several aspects of the actual chain properties as that of the chain stiffness Brodeck et al (2009) or the lower friction expected to occur at the chain ends Lund et al (2009). In this context it is noticeable that the contribution from the α-relaxation extended considerably towards the frequency range where the normal mode is more prominent.…”

Collective dynamics of glass-forming polymers at intermediate length scales

“…dielectric or mechanical spectroscopy also allow covering such a low-Q limit. On the other extreme, neutron scattering has already been used to investigate the dynamic structure factor at the first structure factor peak Q max , reveling thereby the "genuine" α-relaxation, in a relatively large number of glassforming systems, and in particular in polymers [1][2][3][4][5][6][7][8][9][10][11][12][13]. In fact, the application of neutron scattering techniques to protonated and deuterated samples (addressing thereby the self-motions of hydrogens and the collective response, respectively, see Appendix A) has been key to characterize the dynamics of the structural relaxation at inter-and intra-molecular length scales [8,14,15].…”