Polysulfone Case Study

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4) Polysulfone (PSU)[6]

The foaming behavior of PSU foams is very similar to that of COC discussed above. PSU is potentially useful for high temperature applications. Cell nucleation density as high as 1015 cells/cm3 can be achieved with PSU foams. Thus cell sizes in the nanocellular range (20-30 nm) can be achieved.

Foaming is carried out by the solid state foaming process.

CO2 solubility in PSU

CO2 solubility in PSU increases with a decreasing foaming temperature. PSU samples were foamed in a temperature range of -10 to 600C. Figure 22 Solubility of CO2 in PSU as a function of saturation temperature (Courtesy to [6])

Relative density

Figure 23 Relative density as a function of foaming temperature for samples initially saturated
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This is evident from the following figure.

Figure 24 SEM images of (a) Sample with Tsat=200C, Tf=1100C and RD=84.1; (b) Sample with Tsat=00C, Tf=900C and RD=84.1. (Courtesy to [6])

Effect of foaming temperature on cell nucleation density and average cell size

Variation in nucleation densities with temperature can be observed in the following figure

Figure 25 Cell nucleation density as a function of foaming temperature for samples foamed at different saturation temperature. (Courtesy to [6])

The nucleation density increases with a decrease in foaming temperature. Cell nucleation density as high as 1014 cells/cm3 were obtained for saturation temperature of -100C for which CO2 concentration was 14.7% whereas for a saturation temperature of 600C the nucleation density was 109 cells/cm3 where CO2 concentration was 5%.
With an increase in cell nucleation density, the cell size is expected to decrease. This is seen from the following
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The cellular structures show some interconnectivity, indicated by the visibility of the structures below the fracture plane. For typically closed-cell structures, for example, Figure 28, cells have well-defined cell walls and the structures below the fracture plane are not visible. However, the cellular structures shown in figure 30 allow us to see the underlying structures through the cells. Therefore, the cells are interconnected. This type of structure is called as bicontinuous nanoporous structure: two continuous phases are the polymer matrix and air pores. Homogeneous PSU-CO2 mixture separates into two continuous phases: PSU-rich phase (i.e., polymer matrix) and CO2-rich phase (i.e., voids). Although both cellular structures are bicontinuous nanoporous, and cell size and relative density are also similar, they seem to have different morphologies. The left-side sample in figure 30 shows a more open porous and three-dimensional structure, where cells are very randomly distributed. The right-side sample shows a layered structure, where there seem to be many layers and cells distribute on each layer. The only difference in these two samples is the CO2 concentration. The different CO2 concentration can result in different levels of thermal instability, which may cause the formation of different

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