Cell nucleation in solid-state polymeric 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 pr esence 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-po lymer mixture is sufficient to cause the polymer matrix to fail, thereby cr eating a foam cell nucleus. In general, the nucleation sites are observed t o be flat, approximately circular, fracture sites. After the appearance of the initial fracture, gas diffuses from the gas-polymer matrix into the fra cture. The fracture seam inflates during the growth process, in which growt h begins with the appearance of a disk shaped fracture and concludes with a n approximately spherical cell. The results and conclusions presented herei n suggest a new avenue to explain the cell nucleation phenomena observed in this process. (C) 1999 Kluwer Academic Publishers.