Open-cell, porous microcellular foams with nanofibrillated structures were prepared from high tacticity isotactic polypropylene (i-PP) with a crystal nucleating and gelling agent. The 1,3: 2,4 bis-O-(4-methylbenzylidene)-D-sorbitol gelling agent (Gel-all MD) was used as the crystal nucleating and gelling agent, which enhanced the crystallization and gelation of i-PP with a three-dimensional network of highly connected nanofibrils. The core-back foam injection molding technique was employed to foam the i-PP with nitrogen (N-2) at a high expansion ratio, where the crystal nucleating agent induced bubble nucleation and bubble growth in the inter-lamella region and opened the cell walls with a nanoscale-fibrillated structure. The effects of the nucleating agent on the open cell content (OCC), density and crystallinity were thoroughly investigated. We prepared open-cell micro/nanocellular foams with an average cell size of microscale voids of < 5 mu m. Nanometerscale fibrillated structures were formed on the cell wall of the microscale void, the expansion ratio was five-fold and the open cell content was over 90%. (C) 2013 Society of Plastics Engineers