Competition between CO₂-philicity and Mixing Entropy Leads to CO₂ Solubility Maximum in Polyether Polyols

In carbon dioxide-blown polymer foams, the solubility of carbon dioxide (CO₂) in the polymer profoundly shapes the structure and, consequently, the physical properties of the foam. One such foam is polyurethane-commonly used for thermal insulation, acoustic insulation, and cushioning which increasingly relies on CO₂ to replace environmentally harmful blowing agents. Polyurethane is produced through the reaction of isocyanate and polyol, of which the polyol has the higher capacity for dissolving CO₂. While previous studies have suggested the importance of the effect of hydroxyl end groups on CO₂ solubility in short polyols (<1000 g/mol), their effect in polyols with higher molecular weight (≥1000 g/mol) and higher functionality (>2 hydroxyls per chain)-as are commonly used in polyurethane foams-has not been reported. Here, we show that the solubility of CO₂ in polyether polyols decreases with molecular weight above 1000 g/mol and decreases with functionality using measurements performed by gravimetry-axisymmetric drop-shape analysis. The nonmonotonic effect of molecular weight on CO₂ solubility results from the competition between effects that reduce CO₂ solubility (lower mixing entropy) and effects that increase CO₂ solubility (lower ratio of hydroxyl end groups to ether backbone groups). To generalize our measurements, we modeled the CO₂ solubility using a perturbed chain-statistical associating fluid theory (PC-SAFT) model, which we validated by showing that a density functional theory model based on the PC-SAFT free energy accurately predicted the interfacial tension.