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-CO 2 mixture systems, poly(methyl methacrylate) (PMMA)-CO 2 and polystyrene (PS)-CO 2 . Nucleation is initiated by saturating the polymer liquid with high pressure CO 2 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 CO 2 pressure (P 0 ), as a result of an underlying metastable transition from a CO 2 -rich-vapor phase to a CO 2 -rich-liquid phase. The nucleation barrier is generally higher for PS-CO 2 than for PMMA-CO 2 under the same temperature and pressure conditions, and both higher temperature and higher initial pressure are required to lower the nucleation barrier for PS-CO 2 to experimentally relevant ranges. Classical nucleation theory completely fails to capture the structural features of the bubble nucleus and severely underestimates the nucleation barrier.

Density-Functional Theory for Mixtures of AB Random Copolymer and CO2

“…4 In several recent publications, we proposed a density functional theory (DFT) for mixtures of a homopolymer and carbon dioxide, 5 and systematically studied bubble nucleation in these mixtures. 6,7 The theory yields results for the bulk phase behavior and interfacial tension in good agreement with experiments, and predicts the mechanistic pathways for bubble nucleation in these mixtures. In particular, we find that the presence of a metastable transition between a CO 2 -rich vapor and a CO 2 -rich liquid leads to a discontinuous drop in the nucleation barrier as a function of the CO 2 supersaturation.…”

CO2‐blown nanocellular foams

“…These trends are expected to hold to some extend at the nanoscale, and mathematical foaming models using a form of the CNT in which is used as an adjustable parameter have shown promising results in predicting foaming at the nanoscale . However, the CNT performs poorly in quantitatively predicting the absolute nuclei production rate, the absolute free energy barrier or the maximum cell density due in part to failure to capture the polymer/CO 2 interactions at the interface of nanoscale bubbles. Additional limitations have been reviewed by Lubetkin and Tomasko et al…”

Polymer nano-foams for insulating applications prepared from CO2 foaming

“…Unlike SFCT based on statistical mechanical calculations, the classical nucleation theory overestimates surface energies of nano-sized cells and hence the energy barrier for nucleation. Other theories, such as density-functional theory (DFT), have been developed to offset the shortcomings of the classical nucleation theory and show that the latter fails to capture the structural features of the cell nuclei [98,99].…”