Nanocellular polymer foams nucleated by core-shell nanoparticles

The synthesis of low surface energy polymer grafted silica nanoparticles is reported for the utilization as highly efficient cell nucleation agents to obtain nanocellular, CO2 blown polystyrene (PS) and poly(methyl methacrylate) (PMMA) films in a batch process. For nanoparticle surface functionalization hydroxyl-terminated perfluoropolyether and poly(dimethylsiloxane) (PDMS) were used. Their successful grafting to silica nanoparticles was confirmed by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). Following melt blending of the modified silica nanoparticles with PS or PMMA their dispersions were evaluated by scanning electron microscopy (SEM) analyses. We demonstrate that proper selection of the polymer grafts results in nucleation efficiencies of up to approximately 0.5 (i.e. 1 foam cell per 2 particles on average), which is the highest value reported so far for nanofillers as nucleation agents. This number was confirmed by the presence of only 2 to 4 nanoparticles per cell in nanocellular PS and PMMA foams containing SiO2 nanoparticles with a PDMS shell as was observed in cross sectional SEM images. The lowest density foam we obtained (∼0.32 g cm−3) had a nanocellular morphology with a cell size and cell density of ∼440 nm and 1.85 × 1013 cells cm−3, respectively. It is shown that the use of a low surface energy thin shell around silica nanoparticles is beneficial for cell nucleation compared to untreated particles.