Molecular dynamics simulation of O2 diffusion in polydimethylsiloxane (PDMS) and end-linked PDMS networks

Molecular dynamics simulations are used to compute diffusion coefficients for O-2 molecules in polydimethylsiloxane (PDMS) and end-linked PDMS networks. The PDMS chains and penetrants are modelled using a hybrid interatomic potential which treats the Si and O atoms along the chain backbone explicitly while coarse-graining the methyl side groups and penetrants. In PDMS models with different molecular weights, diffusivity of the O-2 penetrants is found to modestly decrease with an increase in chain length. To match typical experimental conditions, the end-linked PDMS networks are constructed with a PDMS to crosslinking (CL) molecule mass ratio of 5:1 or 10:1, demanding that the number of CL molecules exceeds the number of PDMS chains in each model. Despite end-linking, the presence of non-bonded CL molecules promotes increased O-2 diffusivity in comparison with uncrosslinked PDMS. Temperature dependence is captured using the Williams-Landel-Ferry equation.