Advanced nanocellular foams: Perspectives on the current knowledge and challenges

Abstract

Nanocellular polymers (i.e., cellular polymers with cells and walls in the nanometric range) were first produced in the early 2000s, with the works of Yokoyama et al. being the main precedents in this field, producing nanocellular structures by using supercritical carbon dioxide. However, it was not until a decade later that this research field started to grow significantly, attracting several international research groups in the quest to obtain cellular polymers with cells in the nanocellular range. From 2010 to 2014, the basis of bulk nanocellular foam production was established, and the CO2 gas dissolution foaming technique rapidly proved to be the most suitable production route for such materials (details and theoretical basis of this technique can be found elsewhere). Continuous technical advances (e.g., higher saturation pressures, lower saturation temperatures, faster pressure drop rates) and diverse nucleating agents, from inorganic nanoparticles to block copolymers, provided a broad collection of cellular polymers with submicrometric and nanometric cells. Although quite diverse polymers allowed the achievement of submicrometric cells, amorphous polymers such as polyetherimide (PEI), polystyrene (PS), and, notably, poly (methyl methacrylate) (PMMA) provided the best nanocellular structures, with cell sizes even below 100 nm and significant density reductions.