CO ₂ foaming based on polystyrene / poly(methyl methacrylate) blend and nanoclay

Poly(methyl methacrylate) (PMMA) and nanoclay composite was blended as a dispersed phase with polystyrene (PS) in a twin-screw extruder. The mixture was then batch foamed with supercritical CO ₂. It was found that the cell density of foams based on the blend is higher than that based on the weight average of the two pure polymer components at the same foaming conditions. The cell size decreases and the cell density increases with the increase of the PMMA domain size. One explanation is that large PMMA domains serve as a CO ₂ reservoir and the nucleation in the PS phase is enhanced by the diffusion of CO ₂ from the PMMA phase to the PS phase. Very small PMMA domains can not function as CO ₂ reservoir, so they are not able to facilitate nucleation. A much higher cell density and smaller cell size were observed when nanoclay is located at the interface of the PMMA and PS domains due to the heterogeneous nucleation.