Thermodynamic limits on cell size in the production of stable polymeric nanocellular materials

The reduction of cell size to the nanometre scale is one of the main current focuses in polymeric cellular materials research. The recent achievement of cell sizes in the range of 10–200 nm naturally gives rise to the question of what are the main limiting factors in obtaining further improvements. This paper explores the theoretical limits of this reduction in scale size, and presents an application of atomistic modelling strategies for the study of nanofoams produced by the gas dissolution technique. The main conclusion is that, as the cell size scale becomes closer to molecular sizes, atomic-scale interactions should be considered, and conventional rules for foams usually applied to continuous media should be reassessed due to the increasingly discrete nature of the polymeric material. The gas-polymer equilibrium for a range of different polymers (HDPE, PP, PVDF, PEI, PS, PES, PC and PMMA) were calculated using molecular dynamics (MD), in order to generate structures in the nanometric range and to study their stability. The results show that the minimum stable cell size is governed by the behaviour of the internal pressure in the polymer within small gas-filled voids, which may explain the differences between cell sizes observed experimentally in different polymers. [Display omitted] •Theoretical limits in the production of nanofoams are explored.•For cell sizes close to molecular sizes, atomic-scale interactions become more relevant.•Gas-polymer interactions are explored with molecular dynamics simulations.•Internal pressure governs the minimum cell size that different polymers are able to achieve.