Bubble Nucleation in Polymer­–CO₂ Mixtures

Using a PC-SAFT (perturbed-chain statistical associating fluid theory) based density functional theory (DFT) in combination with the string method, we investigate bubble nucleation in CO₂-supersaturated polymers in both binary and ternary systems. Below the critical temperature of CO₂, bubble nucleation can have two kinds of nuclei: vapor-like and liquid-liquid, with the liquid-like nuclei generally having a lower nucleation barrier.  Nucleation in a two-component (polymer+ CO₂) system generally requires very large supersaturation (high initial CO₂ pressure).  However, the addition of a third, volatile component (physical blowing agent) can significantly lower the nucleation barrier in the vicinity of a liquid–liquid–vapor 3-phase coexistence.  We have also examined a polymer–CO₂–surfactant ternary mixture using polypropylene glycol (polyol) as the polymer and polypropylene–poly dimethyl siloxane (PPO–PDMS) diblock copolymer as the surfactant. Adding CO₂ can significantly decrease the critical micelle concentration of the surfactant, and adding surfactant to the polyol–CO₂ can significantly reduce the bubble nucleation barrier, with complex nucleation pathways.