CO2-induced Rejuvenation in Polyetherimide: a New Key to Understand the Brittle-to-Ductile Transition in Mechanical Behavior of Nanocellular Polymers

Abstract

Nanocellular polyetherimide exhibits significant improvements in mechanical properties like toughness and impact resistance, commonly associated with the presence of nanoporosity. However, this work demonstrates these enhancements, often measured directly after processing, cannot be fully explained solely by the cellular structure but also originate from a modification of the polymer matrix induced by the CO2-saturation process. Through a systematic study involving thermal treatments and saturation-desorption processes without foaming, it is shown that CO2 exposure, even in the absence of pore formation, induces an apparent rejuvenation of the polymer, as evidenced by a reduction in the yield stress, which persists after complete CO2 desorption and in the absence of residual gas during mechanical testing. Therefore, the observed ductile response is not associated with the presence of CO2 during deformation, but with a permanent modification of the polymer matrix induced by prior gas exposure. This structural state can be thermally reversed by activating the beta relaxation of the polymer. For nanocellular polymers, the presence of residual gas within the matrix during foaming restricts thermal relaxation and helps preserve the CO2-modified state. As a result, the mechanical response of the solid phase reflects the intrinsic properties of the saturated polymer and the architecture imposed by the cellular structure. This work demonstrates for the first time that CO2 saturation can permanently alter the mechanical state of a high-Tg amorphous polymer, providing a new framework to interpret the brittle-to-ductile transition in nanocellular PEI.