A JUMP MOTION OF SMALL MOLECULES IN GLASSY-POLYMERS - A MOLECULAR-DYNAMICS SIMULATION

Motions of small molecules in an amorphous polymer below its glass transition temperature Tg have been studied using molecular dynamics simulation. An infinite length chain model having 600 interacting sites, each representing a CH2 unit, is used. The system, which is composed of the infinite length chain and ten spherical molecules representing oxygen, is examined at 30 deg below Tg. Although most of the molecules are trapped by cages created by the chain, a molecule reveals clear jump motions. During the jump, no significant change is found in the local free volume as well as in the nonbonded energy acting on the molecule. Potential energy surfaces for the molecules clearly show the cooperative process of jump and chain motions: a generation of a vacant cage and a channel which connects two cages. It is found that the molecule passes through the channel without significant energy barrier and concentration of free volume. © 1990 American Institute of Physics.