Microcellular Foams from Some High–Performance Thermoplastics and Their Composites
Hongliu Sun, James E. Mark, Seng C. Tan, Narayanan Venkatasubramanian, Marlene D. Houtz, Fred E. Arnold and Charles Y-C. Lee
Microcellular foams were successfully processed from several high–performance thermoplastics by a two-stage batch process using CO2 as foaming agent. These microcellular foams had well-defined closed-cell structures with average cell sizes in the range 2–15 µm, and cell densities on the order of 109–1012 cells/cm3. Their microstructures were controlled through choice of foaming temperature and foaming time. Compression and tension tests were performed on these foams to evaluate their mechanical properties. In order to improve the thermostabilities and mechanical properties of these foams, composites were prepared by incorporation of rigid-rod polymers or nanodispersed clay layers in the thermoplastics to form molecular composites, or nanocomposites. These composites were also processed into microcellular foams. In these cases, variables in addition to foaming temperature and foaming time were found to be important, including (i) composition of the blend, (ii) extent of phase separation in the composite, (iii) amount of any residual solvent present, (iv) thickness of the sample, and (v) CO2 content upon saturation. As a result, the foaming behavior and morphologies of the resulting composite foams were much more complicated than those of the pure polymers. For example, the initial foaming temperature had to be higher and the foaming time shorter for the composites. Also, the cell sizes became smaller but the size distributions became broader, and the composite foams prepared from the solution-cast films usually had partially-open cell structures. The thermostabilities and some mechanical properties of the composite foams were significantly enhanced relative to those of the pure matrix polymers. These improvements could be due to nanoscale or even molecular dispersions of the rigid-rod chains in the polymer matrices, including having these chains aligned along the foam struts or within the foam walls in unique reinforcing arrangements.