Following a Theoretical Roadmap to Low Nucleation Barriers for CO₂ Nucleation in Polyol

The quest for insulation with thermal conductivity as low as stagnant air motivates production of ultra-low density foams with cells as small as 1μm across, requiring the controlled nucleation and growth of 10¹⁰ bubbles/(cm³ mother liquid). In relation to polyurethane foam, we seek high nucleation density guided by predictions from density functional theory (DFT) with the string method for the activation barriers. Predicted composition profiles across the interface suggest that cyclopentane (C5), a physical blowing agent, promotes CO2/polyol nucleation in multiple ways: C5 increases the density of the CO2-rich nucleus, and C5 preferentially concentrates at the interface---both tend to reduce the interfacial tension. To test theoretical predictions, we validated the thermodynamic properties of binary (CO2/polyol) and ternary (CO2/C5/polyol) systems. Experimental results for binary systems quantify the reduction of interfacial tension near the critical point and accord with predictions. In ternary systems, theory directed our attention to a range of composition and pressure in which three-phase coexistence may occur and preliminary results support the theoretical predictions. Then we investigated nucleation kinetics using a high-pressure microfluidic apparatus. Initial results obtained using optical microscopy will be presented, laying the foundation for future scattering measurements to quantify the number and size of incipient bubbles. This work was conducted in collaboration with Dow, including T Fitzgibbons, WJ Zhou, HK Chao, J Griffith, B Winniford, S Horsch and V Ginzburg; E DiMaio at Uni Naples and A Ylitalo, ZG Wang and RC Flagan at Caltech.