Nanoscale Cellular Foams from a Poly(propylene)-Rubber Blend

Plastic foams with nano/micro-scale cellular structures were prepared from poly (propylene)/thermoplastic polystyrene elastomer (PP/TPS) systems, specifically the copolymer blends PP/hydrogenated polystyrene-block-polybutadiene-block-polystyrene rubber and PP/hydrogenated polystyrene-block-polyisoprene-block-polystyrene. These PP/TPS systems have the unique characteristic that the elastomer domain can be highly dispersed and oriented in the machine direction by changing the draw-down ratio in the extrusion process. A temperature-quench batch physical foaming method was used to foam these two systems with CO2. The cell size and location were highly controlled in the dispersed elastomer domains by exploiting the differences in CO2 solubility, diffusivity, and viscoelasticity between the elastomer domains and the PP matrix. The average cell diameter of the PP/TPS blend foams was controlled to be 200-400 nm on the finest level by manipulating the PP/rubber ratio, the draw-down ratio of extrusion and the foaming temperature. Furthermore, the cellular C structure could be highly oriented in one direction by using the highly-oriented elastomer domains in the polymer blend morphology as a template for foaming.