Cell Nucleation in Solid-State Polycarbonate-CO2 Foams: Evidence of a Triaxial Tensile Failure Mechanism

The mechanism for nucleation phenomenon in solid state microcellular foams is identified as a solid state failure process. This process originates at internal flaws within the gas-polymer matrix where it is induced by the presence of a state of hydrostatic tensile stress within the polymer matrix. The hydrostatic tensile stress is caused by the presence of the saturating gas within the polymer. The nucleation phenomenon is thermally activated at the effective glass transition temperature of the gas-polymer mixture. At this critical temperature, the hydrostatic tensile stress within the gas-polymer mixture is sufficient to cause the polymer matrix to fail, thereby creating a foam cell nucleus. In general, the nucleation sites are observed to be flat, approximately circular, fracture sites. After the appearance of the initial fracture, gas diffuses from the gas-polymer matrix into the fracture. The fracture seam inflates during the growth process where growth begins with the appearance of a disk shaped fracture and concludes with an approximately spherical cell. The results and conclusions presented here are distinct from previous attempts to address the nucleation phenomena in solid state microcellular plastics, and suggest a new avenue to explain the cell nucleation phenomena observed in this process.