Magnetic resonance imaging studies of diffusion in polymers

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Abstract
For a number of polymer/penetrant systems, for example fatty foods in direct contact with plastic wrapping, the migration of substances from the polymer is governed by the amount of penetrant entering the polymer. For food packaging this means that the rate of migration of substances into the food can be governed by the uptake of food into the packaging itself. To develop predictive models of migration under various conditions there is therefore a need to understand the mechanism of the penetration of the food into the packaging. In this paper a summary of recent Magnetic Resonance Imaging (MRI) studies is reported. Uptake of simulant, as measured by MRI, is quantitative and agrees well with gravimetric uptake data. Data are shown for a comparison of olive oil and isooctane penetration into low density polyethylene at various temperatures. Further, the rate of ingress of isooctane into a variety of commercial polyethylene plaques has been shown to differ widely. These data also allow us to probe the molecular interactions between polymer and penetrant. Finally MRI is combined with a Pulsed Gradient Spin Echo (PGSE) technique to provide spatially resolved measurements of penetrant diffusivity within a polymer. Diffusivity as a function of volume fraction of penetrant can also be measured. These data provide invaluable insights into diffusion in polymers which will aid development of more accurate models of polymer/penetrant interactions and small molecule mobility.