A new foaming apparatus, which can depressurize the vessel that contains polymer + gas from 100 MPa to atmospheric pressure in 50ms, was developed to generate a high degree of supersaturation. The influences of the saturation conditions and the molecular weight of the polymer were studied with the new rapid decompression system using the polymethyl methacrylate (PMMA) - CO2 system as a model. It is found that the Sub-micron cellular structures could be produced by using the apparatus. The results confirm the effectiveness of the high degree of supersaturation achieved in decreasing the cell size and increasing the cell nucleation density of foams. Moreover, it was revealed that the cell size and the cell nucleation density do not always vary as expected as the saturation pressure is increased at the decompression rate and the saturation temperature conditions used in this work. (C) 2019 Elsevier B.V. All rights reserved.