Molecular dynamics study on glass transitions in atactic-polypropylene bulk and freestanding thin films.

Molecular dynamics simulation has been adopted in investigation of different glass transition behaviors of the bulk and the freestanding thin films (about 8 nm) of atactic polypropylene (a-PP). For characterization of glass transition temperature (T(g)) of above systems, both the specific volume and the local conformational transition rate of the systems were examined. The T(g) characterization from the local conformational transition rate of polymer chains was recently developed. Our simulation results show that the films have lower T(g) than the bulk, and the descent of T(g) is in a range of 30 approximately 10 K. These are consistent with experiments. a-PP chain consists of meso-dyad and racemic-dyad, the above results are obtained from both dyads. Individual contribution of the stereoregular dyads to the conformational transition rate was further studied. It was revealed that T(g) obtained singly from the meso-dyads was almost the same with T(g) individually from the racemic-dyads, although the racemic-dyads have much lower transition rate and higher transition barrier than the meso-dyads. In the present study, the reason for the thin films having lower T(g) than the bulk is attributed to the novel behavior that the films have lower transition rate and higher barrier than the bulk when it is below T(g). Such behavior was discussed according to the "coupling rotation" of the dihedrals, which depends on the increase of free volume and the inhibition of kinetic energy.