Transport properties of polystyrene above and below the glass transition temperature
Fike, Lawrence Robert
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In the production of expandable polystyrene (EPS) foam, it is desirable to be able to predict the density of the final product by some means other than empirical estimates. Therefore, a mathematical model was developed to predict the density of EPS during a prepuffing expansion process. Use of the mathematical model requires some knowledge of the fundamental properties of viscosity, shear modulus, and mass diffusion coefficients for water and n-pentane through polystyrene. Since expansion is induced by thermal stimulus, these properties must be known as a function of temperature. The theoretical and experimental development in measuring these parameters constitute the bulk of the text in this thesis. A very simplistic Maxwell model for viscoelastic behavior was used as the basis for the theoretical development in the determination of the viscosity and shear modulus of polystyrene. Mass diffusion coefficients for water and n-pentane in polystyrene were determined by a solution to Fick's second law with appropriate initial and boundary conditions. Once the parameters had been determined, they were fit to an Arrhenius type model in order to determine temperature dependence. The transport properties were observed to be strongly affected by the polymer's glass transition. Finally, a computer program was developed and used to predict the density of EPS as a function of time during the expansion process.