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Thin polymer films are relevant for many technical applications and phenomena, such as packaging, barriers, membranes, medicine (implants, senors, drug release), tribology, or thin ion conducting polymers. The dynamical properties of polymers confined in thin films can differ significantly from that of bulk properties. For instance, the glass transition point can be altered drastically with respect to its bulk value, and hence, thin polymer films provide an excellent sample geometry for studying finite-size effects on glass forming materials.
The simulation group at the ICS as recently demonstrated these effects.[1] We do molecular dynamics simulations for supported (see panel on the left) and free-standing thin films of non-entangled polymers using a coarse grained (bead-spring) polymer model. The free-standing films (not shown) can be obtained in periodic simulation boxes (without walls). Our research is mainly concerned with the equilibrium properties of the films above the critical temperature Tc(h) of the mode coupling theory. We also determine the glass transition temperature Tg(h) by measurements of the film thickness h upon cooling.
We explore the influence of confinement on the structure and dynamics of the polymer films. We find that the dynamics in the films is accelerated compared to the bulk, i.e. the glass transition temperature increases with h !
The enhancement of the mobility in thin films originates from the surfaces, and this effect is larger at the free than at the supported surface. Thus, the films have lower Tc and Tg values relative to the bulk. As can be seen from the figure on the right, the film thickness dependence of our MD results can be well parametrized by
Tg(h) = Tg / (1+h0 / h),
(and similar for Tc) a function proposed in recent experiments on supported polystyrene (PS) films. The PS results are also shown as well as results from Monte Carlo (MC) simulations of a chemically realistic model for polypropylene (PP).