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.

The polymer blend of polyethyleneoxide (PEO) and polymethylmethacrylate (PMMA) constitutes a miscible blend of high dynamical asymmetry; that is, the fully miscible components exhibit a large difference in their glass-transition temperatures, which are 200 K apart. To get a deeper understanding of the unusual PEO dynamics in this system, we have performed a fully atomistic MD simulation. Here we present all information and results obtained on the chain self-motion. We present the mean square displacements and the associated non-Gaussian parameters as a function of temperature. The associated self-correlation function is compared thoroughly with experiments. We display a Rouse analysis and find strongly modified mode friction coefficients but restoring forces that are identical to the pure melt. Thereby, the Rouse correlators are strongly stretched, and the mode number, p, dependence of the relaxation times deviates strongly from the p −2 Rouse behavior. We have also carried out simulations of a simple bead−spring blend, which exhibits the same qualitative dynamic features of the PEO/PMMA system. This suggests that such features are not specific of the PEO/PMMA system, but they are generic in real polymer blends with strong dynamic asymmetry. A further important issue was the test of different models that have been invoked to explain the anomalous PEO dynamics. We compare with a generalized Langevin equation (GLE) approach and with a random Rouse model dealing with a random distribution of friction coefficients. In all aspects, the GLE model agrees qualitatively very well with the results of the fully atomistic simulations. The random Rouse model may be considered to be a phenomenological instantaneous approximation valid for the case where the density fluctuations of the slow PMMA components are relaxing much slower than the relevant PEO dynamics.

We studied unentangled poly(ethylene-alt-propylene) (PEP) in a composite with hydrophobic silica particles as a function of the filler concentration. Our neutron spin echo (NSE) experiments cover both the internal dynamics as well as the center of mass diffusion beyond the Rouse time. The key experimental results are (i) all of the chains are equally mobile, (ii) the basic segmental (Rouse) relaxation rate is unaffected even at highest filler concentrations, and (iii) apparently the obstacles reduce significantly the translational center of mass motion. This happens, even in the case when the particles do not significantly confine the polymer. (iv) A transition from regular to anomalous diffusion in the Rouse regime at the highest particle fractions is clearly evidenced. In order to understand the microscopic mechanisms underlying the experimental observations, we performed coarse grained simulations. We demonstrate that the geometrical confinement only affects the dynamics at a long time scale outside the experimental window and therefore it is not able to explain the results found in the NSE experiments. The consideration of inter-chain interactions, however, results in a significant influence even at shorter times and a quantitative agreement between the experiments and simulations was found. The simulations clearly demonstrate that the interfaces cause a deceleration of the chains in their close vicinity. Then the interchain interactions carry this slowing down to the other chains at a time-scale of the Rouse relaxation time. Hence, in the experimental datasets an overall slowing down is observed.

Combining neutron diffraction with polarization analysis on isotopically labeled samples and fully atomistic molecular dynamics simulations, we have unravelled the structural features of the poly(alkylene oxide)s (PAOs) series. The experimental results show clear signatures of nanosegregation of main chains and side groups leading to the presence of alkyl nanodomains, as previously reported for other comb-like polymers like poly(n-alkyl methacrylates) (PnMAs). Comparison with polyethylene (PE) data shows that the atomic arrangements of side groups within the nanodomains in PAOs are more similar to bulk PE than those in PnMAs. After validating the simulations by direct comparison with the diffraction results on deuterated and protonated samples, we have exploited them to unveil the origin of the structure factor peaks and predict the outcome of potential neutron diffraction experiments on partially labeled samples. The simulated structures undoubtedly confirm the nanosegregation scenario in PAOs.

We have investigated the dynamically asymmetric polymer blend composed of short (M n ≈ 2 kg/mol) poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) chains focusing on the collective dynamics of the fast PEO component. Using neutron spin-echo (NSE) spectroscopy, the single chain dynamic structure factor of PEO was investigated and compared to results from molecular dynamics simulations. After a successful validation of the simulations, a thorough analysis of the RPA approximation reveals the composition of the experimentally measured total scattering signal S(Q,t). Using the simulations, we show and calculate two contributions: (1) the relaxation of hydrogenated PEO against deuterated PEO, yielding the single chain dynamic structure factor of PEO, and (2) the relaxation of the PEO component against the PMMA matrix. For the short chains presented here the second contribution shows a significant decay at higher temperatures while it was previously shown that, in the case of long chains, no relaxation is found. This difference is related to a decrease of the glass transition temperature which takes place with decreasing chain length. In a second step we analyze the approximations that are used when calculating the single chain dynamic structure factor using the Rouse model. For a system like pure PEO, where the dynamics follow the predicted Rouse behavior, excellent agreement is achieved. In the case of PEO in PMMA, however, the slow PMMA matrix strongly influences the PEO dynamics. As a result, the distribution functions show a strong non-Gaussianity, and the calculation of S(Q,t) using the Rouse approximation fails even considering nonexponential Rouse mode correlators.