Bulk and surface polymer composites prepared in supercritical carbon dioxide

This dissertation describes the use of supercritical carbon dioxide (SC CO₂) as an aid in fabricating polymer/polymer composites. Monomers and initiators were infused into solid polymer substrates using SC CO ₂. The monomers were subsequently polymerized within the substrates to form composites. CO₂ swells the polymer substrate and increases the diffusively of reactants within the substrate. The solvent strength of SC CO₂ is tunable allowing control over the degree of swelling and over the partitioning behavior of the reactants. CO₂ can be easily removed from the final products.; First, polystyrene/polyethylene bulk composites were investigated. Styrene and a radical initiator were infused into and reacted throughout the bulk of polyethylene substrates. The composite composition was controlled by controlling infusion time, reaction time and partitioning conditions. Characterization of the composites showed that the crystalline domains of the polyethylene were unaffected. Styrene infused into and polymerized within only the amorphous domains of polyethylene.; Polyethylene and polystyrene are immiscible; the semicrystalline nature of polyethylene frustrated gross phase separation of the polystyrene. The resulting “kinetically trapped” phase morphology gave the composites interesting mechanical properties. The phase morphology was characterized, and the polystyrene was found to reside within the interlamellar regions and the centers of the polyethylene spherulites. The polystyrene formed a continuous “scaffold” that reinforced the polyethylene. The reinforcement provided efficient and dramatic improvement in the composite modulus and strength. However, the composites fracture toughness decreased with increasing polystyrene content. The fracture behavior was correlated to the microstructural damage mechanisms in the composites.; Second, surface composites were investigated. Using a two-stage process, ethyl 2-cyanoacrylate (ECA) monomer was anionically polymerized in the surface regions of poly(tetrafluoroethylene-co-hexafluoropropylene) substrates. An investigation of the anionic polymerization of ECA in CO ₂ established the viability of that system. The composite fabrication process involved first infusing a basic initiator into the substrate using SC CO₂. In the second step, monomer was introduced (using SC CO ₂) to the substrate. As the monomer absorbed into the initiator-containing substrate, it would polymerize. The composite surfaces were characterized using surface-sensitive techniques. The mechanical performance of the composites were determined by measuring the adhesive fracture toughness.