Polymeric foam with the bimodal structure,contains a relatively larger cell size and a relatively smaller cell size,has attracted a great deal of interest in industry and academia because of its potential good mechanical performance and thermal insulation properties.As we all know,carbon particles as additives exhibit not only substantially higher nucleation efficiency in the foaming process but also better absorbent and refractive properties of IR radiation to reduce the thermal conductivity of foams.In this study,the preparation process of the bimodal polystyrene(PS)/carbon particle nanocomposite foam using supercritical carbon dioxide(SC-CO2)as the blowing agent via a two-step depressurization approach was investigated.The results showed that the volume ratio of the small cells to the large cells was mainly controlled by the degree of depressurization in the first step.When the degree of depressurization of the first step is large,the bimodal foam may not be obtained because there will be no unfoam space available for the second nucleation to occur.Moreover,comparing pure PS foam systems,the introduction of carbon particle will not change basically the size of larger cell but decrease obviously the size of smaller cell.