Segmental and chain dynamics of isotactic polypropylene melts
GE Logotheti and DN Theodorou, MACROMOLECULES, 40, 2235-2245 (2007).
The properties of isotactic polypropylene (iPP) melts are investigated via atomistic molecular dynamics (MD) simulations in the isothermal- isobaric (NPT) ensemble. A fully flexible model is developed and validated by comparing predicted volumetric and thermodynamic behavior with available experimental data. Atomic-level packing in the simulated polymer melt is examined through the calculation of an X-ray diffraction pattern. Segmental dynamics is investigated through the reorientation of the methylene C-H bonds and the decorrelation function of torsion angles over a wide range of temperatures and pressures. Predicted correlation times are in reasonable agreement with experimental values derived from (13)C NMR, QENS, and dielectric spectroscopy measurements. The temperature (T) and pressure (P) effects on the relaxation times are compared by calculating the E(V)*/H* ratio, which provides a quantitive measure of the relative importance of P and T on the dynamics. By consistently mapping the atomistic trajectories onto the Rouse model, the dynamical behavior of the polymer on the chain level is investigated. Estimates of the segmental friction factor are derived from both the self-diffusion coefficient and the relaxation times of the first Rouse modes. The zero-shear viscosity is calculated from the friction factor through the Rouse model analysis.
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