Fick Diffusion Coefficients of Polystyrene Oligomers with Dissolved Blowing Agents by Dynamic Light Scattering and Molecular Dynamics Simulations

This work aims at establishing a fundamental understanding of mass diffusion in blowing-agent-loaded polymer melts. For this, dynamic light scattering (DLS) and molecular dynamics (MD) simulations were developed and applied for a comprehensive investigation of diffusive mass transfer in such systems. For this, for the first time, a full characterization of mixtures consisting of well-defined and nearly monodisperse oligomers with dissolved blowing agents was performed by the determination of their density and diffusivities as well as the study of their liquid structure. In this context, the influence of the molecular characteristics of the oligomer, including its chain length, and the solutes on the Fick diffusion coefficient D₁₁, self- and Maxwell-Stefan diffusion coefficients, and the thermodynamic factor, as well as the relationships between the different diffusion coefficients were investigated for the first time in mixtures of polystyrene (PS) oligomers with dissolved nitrogen (N₂), carbon dioxide (CO₂), or 1,1,1,2-tetrafluoroethane (R134a) at process-relevant conditions. For MD simulations, a new force field (FF) was developed particularly considering dihedral potentials, which can-in contrast to previously reported FFs for PS-accurately predict the influence of the chain length of oligomers on their thermophysical properties. For its validation, DLS was applied to determine D₁₁ in mixtures of oligomers with different blowing agents. Combining DLS and MD simulations a comprehensive understanding of the influence of the molecular characteristics of the solute and the solvent on diffusive mass transport was developed. It is demonstrated that the studied mixtures exhibit a nonideal behavior, indicating that the assumption of infinite dilution often adopted in the literature is not applicable. Moreover, for D₁₁, a characteristic asymptotic behavior with increasing chain length of the PS oligomers was revealed.