Off-campus UMass Amherst users: To download dissertations, please use the following link to log into our proxy server with your UMass Amherst user name and password.

Non-UMass Amherst users, please click the view more button below to purchase a copy of this dissertation from Proquest.

(Some titles may also be available free of charge in our Open Access Dissertation Collection, so please check there first.)

Preparation and characterization of microcellular foams processed in supercritical carbon dioxide

Kelyn Anne Arora, University of Massachusetts - Amherst

Abstract

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 Tg) 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. ^

Subject Area

Chemistry, Polymer|Engineering, Materials Science

Recommended Citation

Kelyn Anne Arora, "Preparation and characterization of microcellular foams processed in supercritical carbon dioxide" (January 1, 1999). Doctoral Dissertations Available from Proquest. Paper AAI9932288.
http://scholarworks.umass.edu/dissertations/AAI9932288

Share

COinS