Bubble nucleation in polymer–CO2 mixtures

Abstract

We combine density-functional theory with the string method to calculate the minimum free energy path of bubble nucleation in two polymer–CO₂ mixture systems, poly(methyl methacrylate) (PMMA)–CO₂ and polystyrene (PS)–CO₂. Nucleation is initiated by saturating the polymer liquid with high pressure CO₂ and subsequently reducing the pressure to ambient condition. Below a critical temperature (T_c), we find that there is a discontinuous drop in the nucleation barrier as a function of increased initial CO₂ pressure (P₀), as a result of an underlying metastable transition from a CO₂-rich-vapor phase to a CO₂-rich-liquid phase. The nucleation barrier is generally higher for PS–CO₂ than for PMMA–CO₂ under the same temperature and pressure conditions, and both higher temperature and higher initial pressure are required to lower the nucleation barrier for PS–CO₂ to experimentally relevant ranges. Classical nucleation theory completely fails to capture the structural features of the bubble nucleus and severely underestimates the nucleation barrier.