Computer modelling studies of the diffusion of low moleculer weight cyclic PDMS oligomer in PDMS polymer

Abstract

Molecular dynamics simulations have been carried out in order to examine the mechanism of diffusion of molecules in amorphous polymer matrix. PDMS model was folded in to a periodic cell, generated by rotational isomeric state (RIS) method at a prescribed temperature and density. Molecular dynamics was used to study transport properties of cyclic PDMS oligomers (hexa-methylcyclotrisiloxane (D3), octa-methylcyclotetrasiloxane (D4) and deca-methylcyclopentasiloxane (D5) using Dreiding and COMPASS force fields. Diffusion coefficients were calculated from the Einstein relation. Only D3 penetrant reached the long time limit from which the Einstein relation is satisfied. Analysis of displacement versus time for all the penetrants in PDMS matrix indicates that the penetrant motion is characterized by relatively long periods interspersed with fairly long and small jumps. Transport of solvent molecules occurs by jumps between individual sections of free volume (cavity/hole) through temporarily open channels.