Characterization of local dynamics and mobilities in polymer melts - a simulation study

The local dynamical features of a PEO melt studied by MD simulations are compared to two model chain systems, namely the well-known Rouse model as well as the semiflexible chain model (SFCM) that additionally incorporates chain stiffness. Apart from the analysis of rather general quantities such as the mean square displacement (MSD), we present a new statistical method to extract the local bead mobility from the simulation data on the basis of the Langevin equation, thus providing a complementary approach to the classical Rouse-mode analysis. This allows us to check the validity of the Langevin equation and, as a consequence, the Rouse model. Moreover, the new method has a broad range of applications for the analysis of the dynamics of more complex polymeric systems like comb-branched polymers or polymer blends.Comment: 6 pages, 5 figure

Microscopic understanding of the complex polymer dynamics in a blend —A molecular-dynamics simulation study

Within polymer blends composed of two species with largely different glass transition temperatures like PEO/PMMA, the dynamics of the fast PEO component is severely affected by the rather immobile PMMA, reflected by a breakdown of the typical Rouse scaling. The phenomenological random Rouse model (RRM), in which each monomer has an individual mobility obeying a broad log-normal distribution, has been applied to these blends. Using a newly developed method, we extract the distribution of friction coefficients from MD simulations of a PEO/PMMA blend, thereby testing the RRM explicitly. In our simulations we observe that the distribution is much narrower than expected from the RRM. Here, rather, the presence of additional forward-backward correlations of intermolecular origin is responsible for the anomalous PEO behavior

Dynamics of Poly(butylene oxide) Well above the Glass Transition. A Fully Atomistic Molecular Dynamics Simulation Study

We present fully atomistic molecular dynamics simulations on poly(butylene oxide) (PBO) at a temperature well above the glass transition. The cell is validated by direct comparison of the calculated intermediate incoherent scattering functions of the hydrogens with those obtained from quasielastic neutron scattering (QENS) measurements [Gerstl, C.; Macromolecules 2012, 45, 7293]. The information contained in the simulated atomic trajectories is first used to check the validity and explore the limits of the model used to interpret the QENS results of that work, which extended also to other members of the series of poly(alkylene oxide)s. Furthermore, the details of the segmental dynamics and the localized motions undergone by the side groups are unveiled from the simulations. Full rotations of the pendant groups take place during the decaging of the main-chain segments

Confined Polymer Dynamics on Clay Platelets

The structure and dynamics of poly(ethylene oxide) adsorbed on dispersed clay platelets are investigated by small-angle neutron scattering and neutron spin-echo spectroscopy. The intermediate scattering function has a mobile contribution described by the Zimm theory and an immobile contribution that is constant within the time window. The immobile fraction as a function of the scattering vector Q is described by a Lorentz function, from which a localization length is determined. The relaxation rates grow with polymer concentration in agreement with dielectric measurements but contrary to pure polymer gels

Single Chain Dynamic Strucutre Factor of Poly(ethylene oxide) in Dynamically Asymmetric Blends with Poly(methyl methacrylate). Neutron Scattering and Molecular Dynamics Simulations

We have investigated the dynamically asymmetric polymer blend composed of short (M-n approximate to 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(Qt). Using the simulations, we show and calculate two contributions: (1) the relaxation of hydrogenated PEO against deuterated PLO, yielding the single chain dynamic structure factor of P:EO, 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(Qt) using the Rouse approximation fails even considering nonexponential Rouse mode correlators