GENERATION OF MICROCELLULAR POLYMERIC FOAMS USING SUPERCRITICAL CARBON-DIOXIDE .1. EFFECT OF PRESSURE AND TEMPERATURE ON NUCLEATION

Supercritical carbon dioxide is known to swell and plasticize poly(met hyl methacrylate), PMMA. dramatically. We have employed a pressure que nch in a CO2-swollen PMMA sample to generate a microcellular core stru cture encased by a nonporous skin. Further. we have demonstrated that classical nucleation theory can be used to model the effects of satura tion pressure. temperature, and time on the cell density of the porous materials, provided that the effects of the CO2-diluent on the surfac e tension of PMMA are adequately taken into account. This is because o ur system is in a homogeneous liquid state at our operating conditions because of the plasticization. Both model predictions and data indica te that cell density uses sharply at a saturation pressure of approxim ately 14 MPa (at 40-degrees-C), leveling out above 27 MPa. By contrast , the effect of temperature on cell density in the range 40-degrees-C to 80-degrees-C is minimal.