Controlled Bimodal Cellular Structures in PS/CNT Nanocomposite Foams for Enhanced Electromagnetic Shielding and Mechanical Performance

Bimodal cellular foams featuring coexisting large and small pores at distinct length scales offer enhanced functional and structural performance. However, precise control over dual porosity remains challenging. In this work, a two-step supercritical CO2 foaming strategy was employed to fabricate polystyrene/carbon nanotube (PS/CNT) nanocomposite foams with well-defined and highly tunable bimodal cellular architectures. Key processing parameters, including first-step depressurization rate, holding time, intermediate pressure, and foaming temperature, were systematically investigated. The first-step depressurization rate was precisely controlled using a syringe pump. Notably, tuning the first-step depressurization rate within a low-rate regime (0.003–0.97 MPa/s) was found to be highly effective in controlling the large-cell population, enabling modulation of large-cell size from 19 to 116 μm and cell density over more than 2 orders of magnitude (4.7 × 105 to 1.6 × 108 cells/cm3), while exerting minimal influence on the small-cell structure. The bimodal cellular features remained stable even after prolonged holding times (>48 h), demonstrating the robustness of this approach for achieving controlled dual porosity. The resulting bimodal architecture synergistically integrates sparse large cells for density reduction with abundant small cells for enhanced electromagnetic wave attenuation and shielding. Furthermore, at a low foam density of 0.2 g cm–3, the bimodal foams exhibit 86% and 121% improvements in compressive strength and modulus, respectively, highlighting their strong potential for lightweight, high-performance composite applications.