Diffusion in Poly(vinyl alcohol) and Polyethylene as Determined by Computational Simulations and Modeling

Poly(vinyl alcohol) and polyethylene polymer systems were built in order to study their transport properties (diffusion). First a verification of the AMBER force field was conducted for a poly(vinyl alcohol) system built from a chain with 145 repeating units. NPT-molecular dynamics simulations at temperatures between 400 and 527 K were performed. The results of the simulations were compared with pressure-volumetemperature data, solubility parameter, X-ray scattering pattern and data for the characteristic ratio. The fractional free volume distribution was computed and the diffusion characteristics of water in the polymer were studied. Further another poly(vinyl alcohol) system, with 600 repeating units, was used to study oxygen diffusion in dry and wet poly(vinyl alcohol). In these systems the focus was to investigate the oxygen paths relative to the backbone and also the effect of water on the diffusion coefficients. Jump maps and correlation function between the velocity of the oxygen was calculated. The water has a huge impact on the oxygen diffusion and the preferred paths. A larger molecule (limonene) was studied in a polyethylene matrix consisting of 6000 anisotropic united atoms. A 100 ns long trajectory was recorded and also shorter trajectories at different temperatures, which gave the temperature dependence of the diffusion coefficients. Correlation functions for the limonene molecule shows that it rotates and tumbles when moving thru the matrix. The main results from the molecular dynamics simulations showed that diffusion of larger molecules are possible and also that molecular dynamics simulations can predict plasticization effects. A new fast experimental method for determining diffusion coefficients with non iso thermal thermogravimetry were developed. The advantage is that the experiments only takes minutes instead of days with a small effect on the accuracy. .