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

Abstract: Poly(vinyl alcohol) and polyethylene polymer systems werebuilt in order to study their transport properties (diffusion).First a verification of the AMBER force field was conducted fora poly(vinyl alcohol) system built from a chain with 145repeating units. NPT-molecular dynamics simulations attemperatures between 400 and 527 K were performed. The resultsof the simulations were compared withpressure-volume-temperature data, solubility parameter, X-rayscattering pattern and data for the characteristic ratio. Thefractional free volume distribution was computed and thediffusion characteristics of water in the polymer werestudied.Further another poly(vinyl alcohol) system, with 600repeating units, was used to study oxygen diffusion in dry andwet poly(vinyl alcohol). In these systems the focus was toinvestigate the oxygen paths relative to the backbone and alsothe effect of water on the diffusion coefficients. Jump mapsand correlation function between the velocity of the oxygen wascalculated. The water has a huge impact on the oxygen diffusionand the preferred paths.A larger molecule (limonene) was studied in a polyethylenematrix consisting of 6000 anisotropic united atoms. A 100 nslong trajectory was recorded and also shortertrajectories atdifferent temperatures, which gave the temperature dependenceof the diffusion coefficients. Correlation functions for thelimonene molecule shows that it rotates and tumbles when movingthru the matrix.The main results from the molecular dynamics simulationsshowed that diffusion of larger molecules are possible and alsothat molecular dynamics simulations can predict plasticizationeffects.A new fast experimental method for determining diffusioncoefficients with non iso thermal thermogravimetry weredeveloped. The advantage is that the experiments only takesminutes instead of days with a small effect on theaccuracy.