# Partially-Perforated Self-Reinforced Polyurea Foams
## 3 Results and Discussion  
The SEM micrographs of a low-relative density of 0.21 (EML227) and high-relative density (EML350) of 0.33 polyurea foams are shown in Figure 2. The average cell diameter of the EML227 (\(\phi_{c,227}\)) foam was \(134.5\pm 108.5\) um, which is nearly 48% larger the cell diameter of EML350 (\(\phi_{c,350}=90.9\pm 48.9\) um) based on micrographic analysis of the SEM micrographs. The variation between the cell sizes of EML227 and EML350 is attributed to the interplay between the desired final sheet thickness as a function of the initial mixing ratio of the chemicals and the blowing agent. That is to say, the volume of the foam cavity was kept constant, while adjusting the amount of the poured foam slurry--which resulted in congregating the entrapped gas bubbles, yielding an overall smaller cell diameter. Lastly, the standard deviation of the cell size is associated with a hierarchal microcellular structure, since the cell walls separating adjacent large microcells tend to encompass cells with smaller diameters (circled in Figure 2a,d for EML227 and 350, respectively). Such a hierarchal microcellular structure plays a significant role in the quasi-static mechanical behavior of the foam [5, 6, 32]. Figure 1: Schematic of the mold and foam fabrication process. The generation of carbon dioxide gas bubbles based on the reaction between isocyanate and DI H\({}_{2}\)O continues to expand the cell walls resulting in the formation of the spherical microcellular foam structure, as shown in Figure 2. The carbon dioxide generation process is said to be a self-limiting chemical reaction, which impedes the transformation of the microcellular structure to a polyhedral geometry. Moreover, the relatively short curing time of the polyurea elastomer also limits the expansion process [33]. The large perforations around the cell surfaces are due to the interplay between the internal gas pressure and the proximity of adjacent cells, or shared walls between neighboring cells. A close examination of Figure 2 elucidates the perforation formation process. Portions of the cell walls appear to be much thinner sheaths, sites encircled in red in Figure 3, depicting areas where large perforations would have been created if the cell wall expansion had continued, due to CO\({}_{2}\) generation. The ruptured edges are also wrapped towards the cell with lower internal pressure, confirming the large perforation formation mechanism through a close examination of the periphery of existing perforations. The rupture process of the thin sheaths commences when the strength and stiffness of the viscous polymer sheath cannot resist the deformation induced by the applied tension in the cell wall, due to the difference in the internal gas pressure between adjacent cells. The mechanical properties of the curing polyurea can be determined through rheological studies, which are the focus of future research. Additionally, stretch marks (insets in Figure 3) on the cell walls reveal the effect of the gas pressure entrapped inside the cells during the curing process before setting. The pressure from the mold enclosure to control the dimensions of the sheets also enhances the applied load. The formation of the small holes is an independent process and is explained next. Figure 2: SEM micrographs of cured samples showing the cell perforation and deposits of microspheres of EML227 polyurea foam in the top panel (**a**–**c**) and EML350 polyurea foam in the bottom panel (**d**–**f**). Omnipresent small holes are preferentially located on one side of the cell in addition to the large perforations discussed above, where the nucleation of the holes is associated with the localized precipitation of dispersed isocyanate during the violent mixing process. That is, the formation of micro-holes implies a localized concentration of isocyanate that locally generates CO\({}_{2}\) when reacting with the entrapped DI H\({}_{2}\)O.