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.