Evolution of Structure in Solid-State Microcellular and Nanocellular Polyetherimide (PEI) Foams as a Function of Carbon Dioxide Concentration

Abstract

Evolution of Structures in PEI-CO2 System This study systematically investigates the evolution of structures in solid-state Polyetherimide (PEI) foams as a function of CO2 concentration across a saturation pressure range of 0.5 to 5.5 MPa at room temperature. The PEI-CO2 system produces a microcellular structure between the saturation pressure range of 0.5 (2.2 wt.% CO2) to 3 MPa (8 wt.% CO2), with cell sizes ranging from 1 to 5 μm. A rapid transition from a microcellular to a nanocellular (cell size ~ 10 nm) structure is observed between 3 and 3.5 MPa (10 wt.% CO2), which is unique to the PEI-CO2 system. The PEI-CO2 system produces a nanocellular structure between the saturation pressure range of 3.5 to 5.5 MPa (12.4 wt.% CO2) with average cell sizes ranging from 20 to 130 nm. At 5.5 MPa, the foams exhibit a uniform nanocellular structure with an average cell size consistently around 20 nm across various foaming temperatures. A probabilistic model based on cell nucleation density is presented to predict the size distributions of the critical radius of cell nuclei based on the Laplace Equation. The model explains the mechanism of the structural transition from microcellular to nanocellular through progressive activation of smaller flaws by increasing saturation pressure. Microcellular PEI foams exhibit a unique cellular structure, wherein secondary nanopores, ranging in size from 10 to 80 nm, develop on the cell walls of the primary microcells. This study experimentally characterizes the evolution of the secondary nanoporous structure as a function of primary cell expansion. The study comprises two parts: In Part I, the foaming temperature was varied independently at saturation pressures of 0.5 MPa, 2.5 MPa, and 3.3 MPa. In Part II, the foaming time was varied independently at different foaming temperatures under a saturation pressure of 1 MPa. The experimental results demonstrate that the growth of the primary cells directly influences the growth of the secondary nanopores. A modified classical nucleation theory framework based on strain energy due to primary cell expansion is presented to explain that the underlying nucleation mechanism of secondary nanopores is a coupled effect of polymer crazing and foaming. Investigating the Effect of Cell Size on the Toughness of PEI Foams This research is a collaborative effort involving Microcellular Plastics Lab, Meza Research Group in the Department of Mechanical Engineering, and the Multiscale Analysis of Materials & Structures (MAMS) Lab in the Department of Aeronautics and Astronautics. The title of the project is "Investigating Fundamental Toughness Mechanisms in Nanocellular Foams," which was funded by the National Science Foundation. The PI of the project is Dr. Lucas Meza from the Department of Mechanical Engineering. Dr. Vipin Kumar and Dr. Marco Salviato are the co-PIs from the Department of Mechanical Engineering and the Department of Aeronautics and Astronautics, respectively. This investigation was conducted in collaboration with Kush Dwivedi, a Ph.D. student in Mechanical Engineering. Complete details of all the tensile and fracture experiments, along with numerical simulations, are to be provided in Kush Dwivedi's Ph.D. dissertation. The author contributed to the development of process space to fabricate microcellular and nanocellular foams, the fabrication of foam specimens for mechanical tests, the preparation of mechanical test specimens, the SEM characterization of foams before mechanical testing, and the SEM characterization of fracture surfaces. Process space maps consisting of relative densities at different foaming temperatures were established for microcellular and nanocellular PEI foams. Process conditions were selected from the process space maps to fabricate microcellular PEI foams with cell sizes ranging from 3 to 5 μm and nanocellular PEI foams with cell sizes ranging from 15 to 40 nm. The relative densities of the foams ranged from 0.4 to 0.8. Uniaxial tensile and fracture tests were performed to investigate the effect of cell size on fracture toughness. Tensile behavior was characterized using true stress–strain curves, while fracture behavior was characterized using load–CMOD (Crack Mouth Opening Displacement) curves. True strain and CMOD measurements were obtained using Digital Image Correlation (DIC). In general, nanocellular foams demonstrated significantly higher toughness than microcellular foams at equivalent relative densities. These findings contradict the classical scaling law that predicts a reduction in fracture toughness with decreasing cell size. Notably, nanocellular foams with a relative density of 0.8 exhibited fracture toughness comparable to that of unprocessed PEI. Post-test fracture surface analysis was conducted using SEM. Regions of stable crack propagation exhibited cell wall stretching, indicative of ductile failure. Conversely, unstable crack propagation regions exhibited flat fracture surfaces, characteristic of brittle failure. Overall, nanocellular foams exhibited larger regions of stable crack growth with cell wall stretching. This indicates plastic deformation occurring locally at the nanoscale level. Therefore, ductility at the nanoscale level leads to toughness enhancement, which is the underlying reason for the disruptive fracture behavior of the nanocellular foams. Advanced Methods to Produce Skinless Solid-State Foams Previous studies have shown that nanocellular PEI foams have an open-celled porous structure. However, access to these porous structures is limited by the presence of the solid skin layer. A novel plasma treatment-based solid-state process was developed in collaboration with Ankush Nandi (Ph.D. student, ME) from the Vashisth Lab in the Department of Mechanical Engineering to produce skinless foams. The author contributed to designing, executing the experiments, and conducting SEM analysis. The author and Ankush Nandi contributed to tuning the plasma system parameters to achieve solid-state foaming. This process involves plasma treatment of CO₂-saturated polymer sheets (with thicknesses ranging from 0.5 to 1.6 mm) using a commercially available low-temperature air plasma system. The resulting foams exhibit a porous surface on the plasma-treated side, with a gradient foam structure extending into the cross-section. Skinless microcellular foams were successfully fabricated in the PC–CO₂ system at saturation pressures of 1 MPa and 5 MPa. Additionally, skinless nanocellular foams with an average cell size of 90 nm were produced in the PEI–CO₂ system. Selective foaming was achieved on the 15 μm-thick skin of a prefabricated open-cell porous nanocellular PEI sheet by partially re-saturating the skin with CO2 and subsequently treating it with plasma. The SEM images revealed pores within the skin region and on the treated surface. The acetone-based dye penetration test confirmed that the treated surface is permeable, and the skin is now porous and interconnected with the core. Thus, we successfully demonstrated a new technique to selectively make the solid skin porous, thereby enabling a new way to access the porous structure at the core. Therefore, the plasma treatment-based solid-state foaming process is a promising method for producing skinless solid-state foams to access the core.