The foaming of polystyrene using supercritical (SC) CO2 has been studied to better understand the microcellular foaming process, as we plan future studies that involve the creation of composite microcellular foams. Rapid decompression of SC CO2-saturated polystyrene at sufficiently high temperatures (above the depressed T g) yields expanded microcellular foams. The resulting foam structures can be controlled by manipulating processing conditions. Experiments varying the foaming temperature while holding other variables constant show that higher temperatures produce larger cells and reduced densities. Structures range from isotropic cells in samples retaining their initial geometry to highly expanded foams recovered in the shape of the foaming vessel and having oriented, anisotropic cells and limited density reduction. Higher saturation pressures lead to higher nucleation densities and hence smaller cells. Decreasing the rate of depressurization permits a longer period of cell growth and therefore larger cell sizes. Foams having a bimodal distribution of cell sizes can be created by reducing the pressure in two stages.

Polystyrene/polyethylene composites have been prepared by the heterogeneous radical polymerization of styrene within supercritical carbon dioxide−swollen high density polyethylene (HDPE) substrates. Composition of the composites can be controlled with reaction time and initial ratio of styrene to HDPE. The polystyrene produced within the substrate is of high molecular weight. Differential scanning calorimetry and wide-angle X-ray diffraction indicate that the crystalline portion of the HDPE substrate is unaffected by the procedure used in this investigation. Scanning electron microscopy indicates that the polystyrene resides in the noncrystalline domains and permeates throughout the spherulitic structure of the HDPE substrates. This morphology is very different from the morphology of polystyrene/polyethylene blends produced by conventional melt/mixing techniques. The strengthening of the spherulitic structure of HDPE produces an efficient enhancement in the modulus of the overall composite. The tensile strengths of the composites are dramatically enhanced over conventionally produced blends. The addition of brittle polystyrene to extremely tough HDPE substrates decreases the overall fracture toughnesses of the composites.

A new class of organic−inorganic hybrid thermoset copolymers has been prepared by ring-opening metathesis polymerization catalyzed with bis(tricyclohexylphosphine)benzylideneruthenium(II) dichloride (1). Dicyclopentadiene (DCPD) and norbornenylethyl polyhedral oligomeric silsesquioxane (1NB-POSS) and tris(norbornenylethyl)-POSS (3NB-POSS) with isobutyl pendent groups have been copolymerized at 60 °C over a range of POSS loadings. These copolymers contain small aggregates of 1NB-POSS, three to four molecules, at high loadings and uniform dispersions over all loadings of 3NB-POSS. The pendent group of 1NB-POSS decreases the cross-link density of the PDCPD matrix while 3NB-POSS increases the cross-link density. POSS incorporation has little effect on the glass transition temperature (T g). Addition of 20 wt % 1NB-POSS decreases the T g from 128 °C for PDCPD to 114 °C. Addition of 3NB-POSS has little effect on the T g over the range of POSS loadings. The stiffness, in tension and compression, is observed to decrease with increasing POSS loading. Although the yield stress for both systems decreases, the toughness also decreases. The decrease in toughness of the 1NB-POSS copolymers is attributed to a loss of irreversible damage with increased POSS loading. Although a similar decrease in toughness is observed for the 3NB-POSS copolymers, the change in toughness is attributed to a decrease in cohesive strength with 3NB-POSS loading.

Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP)/polystyrene blends were prepared by the heterogeneous free-radical polymerization of styrene in supercritical (SC) CO2-swollen FEP substrates. Volume incorporations of up to 50% polystyrene were achieved, and the composition and phase morphology of the blends were controlled by varying the styrene monomer concentration and reaction time. The crystallinity and glass transition temperature of the FEP substrate are unaffected by the addition of the polystyrene component, indicating that polymerization occurs exclusively in the amorphous phase and that the polymers are immiscible. The molecular weight of the polystyrene formed within the FEP substrate is significantly higher than that which forms in the SC CO2 phase outside of the substrate. Attempts were made to prepare composite foams by saturation of the blends with SC CO2 and subsequent rapid depressurization. At lower temperatures (conditions under which polystyrene foams) the crystalline domains of FEP prevent expansion. At higher temperatures, in addition to expansion, large-scale phase segregation of the blends occurs.

Supercritical carbon dioxide (SC CO2) was used as an aid in fabricating polymer/polymer composites. Using a two-stage process, ethyl 2-cyanoacrylate (ECA) monomer was anionically polymerized within poly(tetrafluoroethylene-co-hexafluoropropylene) substrates. The composite fabrication process involved first infusing triphenylphosphine (the initiator) into the substrate using SC CO2. In the second step, monomer was introduced (again using SC CO2) to the substrate. As the monomer absorbed into the initiator-containing substrate, it polymerized. The composite surfaces were characterized using surfaceselective techniques. The mechanical performance of the composites was determined by measuring the adhesive fracture toughness of the composites. The locus of failure of fractured interfaces of composites with epoxy was determined by X-ray photoelectron spectroscopy.