Preparation of polymeric foams with bimodal cell size: An application of heterogeneous nucleation effect of nanofillers

A new method was proposed for fabricating the polymer foams with bimodal structure. Briefly, nanoparticles and polymer powders were dry-mixed by ball-milling and then hot-pressed into the composites in which nanoparticles were selectively distributed at the interfaces between polymer multi-facets. Upon batch foaming using scCO(2) as blowing agent, bimodal structure was generated due to the selective distribution and heterogeneous nucleation effect of the nanoparticles. Using this method, several bimodal composite foams based on different nanoparticles, i.e. graphene, carbon nanotubes, and silica nanoparticles and different polymers, i.e. thermoplastic polyurethane (TPU) and polystyrene (PS) were successfully fabricated. Specifically, TPU/graphene system was systemically studied. The results showed that the foam morphologies could be easily tuned by changing nanoparticle content, particle size of polymer powder, hot-pressing temperature and pressure, as well as foaming conditions (e. g. foaming temperature and pressure), indicating the great simplicity and tune-ability of this method.