Who: Wolfgang Paul, Institut für Physik, Martin Luther Universität, 06099 Halle, Germany
Place: Donostia International Physics Center
Date: Tuesday, 21 June 2016, 11:30
I will report on massively parallel Molecular Dynamics (MD) simulations of a melt of untentangled 1,4-polybutadiene chains confined between graphite walls, apolar walls confining an almost apolar polymer. The walls introduce positional and orientational ordering on the segment and the chain scale extending for about 5 chain diameters (2.5 nm) into the melt. The center of the confined films always is bulk-like. Relaxation processes are heterogeneous as well as anisotropic. I will present simulation results covering a broad range of temperatures from about 2 Tg to about 1.2 Tg. The van der Waals attraction to the walls also induces an additional relaxation process visible in NMR, dielectrical or scattering experiments as a third relaxation step beyond the vibrational part and the ?-relaxation. I will discuss the relaxation processes with respect to incoherent scattering and dielectric experiments. From the dielectric spectrum we would conclude that a shift of the glass transition is only observable within a 1 nm layer at the walls. The orientational ordering induced by the walls also induces a dielectric dipole layer of adsorbed chains. Interestingly, the resulting chain dipole is perpendicular to the end-to-end vector of the adsorbed chains, a behavior we call type A' chains.