Beyond Nanoporosity: A CO₂-Conditioned Glassy Matrix Contributes to the Brittle-to-Ductile Transition of Nanocellular...

Nanocellular polyetherimide (PEI) exhibits enhanced toughness and impact resistance compared to the initial solid precursor, an effect usually attributed to the presence of nanometric cells within a confined glassy polymer matrix. In this work, we examine whether the CO₂ saturation step used in gas dissolution foaming can affect the mechanical response by modifying the thermodynamic state of the glassy polymer. Thermal annealing without CO₂ increases the yield stress and reduces ductility, whereas solid PEI saturated with CO₂ and subsequently fully desorbed shows lower yield stress, an increase in tensile ductility, and an improved impact response. Density, gas transport, WAXS, and PALS measurements were used to further analyze this state, showing that the effect of CO₂ leads to a subtle modification of the glassy matrix, changing the packing and gas accessible regions of the matrix. It is proposed that this matrix state, together with the nanocellular architecture, contributes to the brittle-to-ductile transition observed in nanocellular PEI. Our work highlights that optimization of the mechanical properties of nanocellular polymers should not only focus on the key characteristics of the final cellular morphology, but also on the conditions that define the state of the polymer matrix surrounding the pores before and during foaming.