Voronoi-based analysis linking microscopic void evolution to macroscopic swelling in supercritical CO2-saturated polymers

Abstract

This study investigates the expansion behavior of EPDM rubber in supercritical carbon dioxide (scCO₂) through molecular dynamics (MD) simulation, and employs Voronoi analysis to correlate the evolution of microscopic pores with macroscopic swelling. It systematically explores the effects of CO₂ density, temperature, and polymer density, revealing that Voronoi-based free volume characterization provides key insights into the underlying mechanisms. The main findings from the Voronoi analysis are that at high CO₂ densities, Voronoi void plots show a significant increase in larger void volumes and obvious spatial heterogeneity, which helps explain the apparent contradiction between localized CO₂ accumulation and inhibited macroscopic expansion; elevated temperatures promote the formation of larger and more uniformly distributed pores within the polymer matrix, thereby increasing CO₂ penetration depth and ultimately leading to structural degradation; and lower polymer densities are more likely to form interconnected channels that facilitate rapid CO₂ diffusion, while higher polymer densities result in more isolated and evenly spaced voids that restrict CO₂ diffusion. This paper elucidates the density-dependent expansion dynamics using the Voronoi method, explains the pressure-induced critical expansion phase transition, and establishes a link between microscopic free volume topology and macroscopic deformation. Overall, this work demonstrates that Voronoi analysis serves as a robust tool for studying multiscale polymer-fluid interactions under scCO₂ conditions.