Mutual and Thermal Diffusivities in Binary Mixtures of Polystyrene with Dissolved N2 or CO2 by Dynamic Light Scattering

In physical foaming processes of thermoplastics, a liquid mixture consisting of a molten polymer with a dissolved blowing agent is extruded through a die or injected into a mold. The morphology of the foam matrix strongly depends on the mutual diffusion coefficient D₁₁ of the liquid polymer-blowing agent mixture. For a better understanding of the underlying physical mechanisms during the foaming process and the development of corresponding models, accurate knowledge of D₁₁ is needed. This work reports on the simultaneous measurement of D₁₁ and the thermal diffusivity a of liquid mixtures consisting of polystyrene melts with dissolved nitrogen (N₂) or carbon dioxide (CO₂) at mass fractions w_solute up to 0.003 or 0.02 and temperatures T between (433 and 533) K using dynamic light scattering (DLS). The determined D₁₁ range is between (1 and 4) × 10⁹ m²⋅s⁻¹ and are slightly larger for the mixtures containing N₂ at a given T and w_solute. D₁₁ could be determined with an average expanded relative uncertainty of 18%. Considering all investigated state points and the achieved experimental uncertainties, both D₁₁ and a are independent on the amount of dissolved gas, despite relatively large mole fractions of the dissolved blowing agents x_solute of 0.997 and 0.93.