The foaming of thin (∼100 μm) polysulfone (PSU), poly(ether sulfone) (PES), and cyclic olefin copolymer (COC) films using carbon dioxide as a physical blowing agent has been studied. Microcellular foam morphologies were obtained by saturating the polymer with carbon dioxide and heating the sample above the glass transition temperature of the polymer/gas mixture after releasing pressure. The temperature range at which foaming took place was examined in detail, and the physical processes fixing the final foam morphologies were discussed. We find that the ease of plasticization of the different polymers and the CO2 diffusion coefficient under foaming conditions determine the morphology of the foams. Nucleation and growth of cells starts at the T g of the polymer/gas mixture; however, this process is severely inhibited by enhanced diffusion of gas from the films. The maximum cell density attainable is not determined by the ease of nucleation. Instead, CO2 loss by diffusion to the exterior of the sample determines the maximum number of cells in the polymer. This effect, which at sufficiently high temperatures results in decreasing cell densities with increasing temperature of the foaming bath, becomes stronger for the more readily plasticized system (COC > PSU > PES) and causes an upper temperature limit where foaming stops.

We investigate the physical foaming process of glassy poly(ether imide) and poly(ether sulfone) using carbon dioxide and report temperature−concentration diagrams (“foam diagrams”) marking out the foaming envelope in which dense CO2-saturated films expand and microvoids are introduced. Two types of porosities are observed. Closed microcellular structures occur at carbon dioxide saturation levels below 50 cm3 (STP)/cm3 (polymer); nanoporous bicontinuous (open) structures with pore sizes as small as 40 nm occur above this CO2 concentration threshold, which is identical for both polymers. The cellular-to-bicontinuous transition is characterized in detail on the basis of gas permeation measurements and is represented as a separate window inside the foaming diagram. In this paper, the transition to bicontinuous structures is reported for the first time, and its generic physical basis is critically reviewed.

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We report the formation of open nanoporous polymer films composed of homogeneous polysulfone/polyimide blends. Porosity is introduced by expansion of carbon dioxide-saturated films at elevated temperatures. To interpret details of the porous morphologies in terms of the experimental conditions during expansion, the glass transition temperature and carbon dioxide solubility of the dense film were examined at various blend compositions. We find that above a critical threshold of the carbon dioxide concentration the porous structure obtained changes from microcellular into open nanoporous. This critical carbon dioxide concentration is independent of the blend composition. Remarkably, it resembles the value previously reported on different polymers.

Novel MCO high-flux membranes for hemodialysis have been developed with optimized permeability, allowing for filtration close to that of the natural kidney. A comprehensive in vitro characterization of the membrane properties by dextran filtration is presented. The sieving profile of pristine membranes, as well as that of membranes exposed to blood for 40 minutes, are described. The effective pore size (Stokes-Einstein radius) was estimated from filtration experiments before and after blood exposure, and results were compared to hydrodynamic radii of middle and large uremic toxins and essential proteins. The results indicate that the tailored pore sizes of the MCO membranes promote removal of large toxins while ensuring the retention of albumin.