Computer simulation of polymer permeability and segmental dynamics

The gas transport properties of cis-1,4-polybutadiene (PBD) [1], atactic poly- propylene (PP) [2], polyethylene terephthalate (PET) [3], and polyisobutylene (PIB) [4] have been investigated using molecular dynamics simulations. The two novel film cell methodologies of Kikuchi et al. were investigated, with KKF1 [5] being designed to probe the solubility directly, and KKF2 [6] probing per- meability. Both were also capable of estimating diffusivity. The results of these film cell simulations were compared to the results of traditional gas permeation simulations carried out using bulk samples of PBD, PP, and PET. Analysis of simulation results was mainly via inter-comparison between different polymers, gases, and simulation methodologies. The three different methods showed reasonable agreement in terms of their solubility and diffusivity estimates. Each simulation methodology was found to have its own benefits and shortcomings, with both of the novel film cells having specific challenges in their use. The KKF1 method was found to be simpler to apply than KKF2, and to provide solubility and diffusivity values with smaller uncertainties, meaning that it was the KKF1 model that was chosen to investigate the interesting gas transport properties of polyisobutylene. Using the KKF1 method, molecular dynamics simulations were able to reproduce the order of magnitude difference in the permeability of PIB when compared to PBD. In addition to the gas transport investigation, the local segmental dynamics of PBD, PP, and PET were investigated. Autocorrelation functions of chord vectors spanning different numbers of backbone bonds were calculated, with the CONTIN [7, 8] algorithm being used to find the corresponding distributions of relaxation times. All distributions of relaxation times showed a peak on short time scales, thought to correspond to short length scale segmental motion, and a long time scale peak, thought to correspond to global chain relaxation dynamics. As longer length scale chord vectors were investigated a third peak appeared, temporally located intermediate between the other two peaks. Investigating the temperature dependence of the peaks' characteristic time scale showed that the intermediate and long time scale processes shared identical activation energies, which could infer that they share a common source.EThOS - Electronic Theses Online ServiceGBUnited Kingdo