Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide II. Tensile and impact properties

Microcellular foams, closed-cell polymer foams with cells of order 10 μm, have been studied for over two decades. These foams have shown significant improvements in mechanical properties, such as strength-to-weight ratio, over conventional foams with cells on the order of 1 mm. Will an additional 100-fold reduction in cell size yield further improvement in properties? Here we answer this question in the solid-state PEI–CO2 system, a unique gas–polymer system in which cellular structures can be created throughout the polymer volume at either micro or nano scales. The tensile and impact behaviors of microcellular (cells in 2–5 μm range) and nanocellular (cells in the 50–100 nm range) structures are experimentally compared for foams with relative densities, to that of the starting solid, in the range of 0.75–0.90. We found that nanofoams show a significantly higher strain to failure, resulting in an improvement in the modulus of toughness by up to 350% compared to microcellular foams. Falling weight impact tests show evidence of a brittle-to-ductile transition in nanofoams resulting in impact energies that are up to 600% higher compared to microcellular foams. These results point to the promise of nanofoams as an important new class of materials.