Preparation and characterization of microcellular foams processed in supercriticaln carbon dioxide.

摘要

The aim of this research program was to elucidate the process-structure-property relationships that occur in the fabrication of microcellular foams using supercritical (SC) CO₂. The first goal was to develop an understanding of the microcellular foaming process for a homogeneous system (polystyrene was chosen as a model). Rapid decompression of a SC CO₂-saturated substrate at sufficiently high temperatures (above the depressed T_g) yields expanded microcellular foams. Foam structure and density can be controlled by manipulating processing conditions such as temperature, pressure, depressurization profile and vessel geometry. The foams were found to have either isotropic or transversely isotropic monodisperse cells ranging from 0.5 to 100 μm in diameter. The foamed samples either retained the geometry of the initial substrate or were expanded into the shape of the vessel in which they were made, depending on the conditions.; The compressive behavior and microcellular collapse mechanisms of the polystyrene foams produced in SC CO₂ were evaluated. The effects of cell geometry on the compressive strength were determined, and a buckling model was used to explain the results. The foams were found to have yield strengths exceeding those of conventional foams of equivalent density. The microcellular buckling mechanisms have been identified and it was found that collapse proceeds in a heterogeneous, progressive fashion. By analysis of the collapse behavior as a “reverse necking” phenomenon, a model was developed, using energy balance arguments, that describes the energy required for microcellular collapse. Additional studies were performed that explored the effects of material heterogeneity, constrained boundary conditions, temperature, and strain rate on the mechanical properties of the foams.; Polymer blends having kinetically trapped morphologies were made via the supercritical CO₂-assisted infusion of styrene monomer into and subsequent free-radical polymerization within solid polymer substrates. Blend composition and phase morphology were controlled by varying monomer concentration, reaction time and reaction temperature. Annealing studies were performed to evaluate the stability of the blends. Attempts to expand poly(tetrafluoroethylene- co-hexafluoropropylene (FEP)/polystyrene blends into composite foams were unsuccessful due to large scale phase separation. Expansion of poly(4-methyl-1-pentene) (PMP)/polystyrene blends was successful, and experiments were carried out to determine the effects of blend composition and phase morphology on foam structure.