Atomistic investigation of segmental mobility in atactic poly(propylene)

An atomistic model previously developed for atactic poly(propylene) has been analyzed through molecular dynamics simulation to study the variation of static and dynamical properties across a temperature range centered around the experimental glass transition temperature. Although few effects are seen in structural measures such as chain conformation, shape, and packing, characteristic features associated with the onset of segmental motions are revealed in the simulation results on local dynamics. The mechanism by which the vinyl polymer chain undergoes thermally activated motion is found to involve a series of spatial displacement events (SDE) occurring topologically, spatially, and temporally in a discrete fashion. An attempt was made to correlate the motions of the active chain segments with structural and mechanical properties such as local volume and rotational stiffness. The results indicate that the ability of the segmental mobility to diffuse in space and time, in a dynamically percolative manner, is a significant feature of atomic motions in glassy polymers.