## 5 Results of the Foamed Poly(Lactic Acid)
### The Morphology of Foam Structures  
We graded foam structures by cell population density, expansion, void fraction and density. The results can be found in Figures 9 and 10. The exothermic Tracel3170 produced more cells in unit volume in the case of the 8052_3170 recipe recommended for foaming (4.82 \(\times\) 10\({}^{5}\) cells/cm\({}^{3}\)) than in  
Figure 8: Scanning electron microscopy (SEM) images of samples foamed with different foaming agents (manufactured at 190 \({}^{\circ}\)C) (**a**) 8052_3170, (**b**) a 2003_3170, (**c**) 8052_4200, (**d**) 2003_4200, (**e**) 8052_Luv_9537, (**f**) a 2003_Luv_9537, (**g**) 8052_Hyd_3168, (**h**) 2003_Hyd_3168 20\(\times\) magnification. the case of 2003_3170 (4.17 \(\times\) 10\({}^{5}\) cells/cm\({}^{3}\)). The 8052D polymer not only produces a higher number of cells but the expansion of the cells is also higher, 2.36, as opposed 2.18 in the case the 2003_3170. As a result, density with 8052_3170 was lower, 0.53 g/cm\({}^{3}\), while in the case of 2003_3170, it was 0.57 g/cm\({}^{3}\). This means that a void fraction of 57.61% was achieved in the case of the 8052_3170 recipe (considering that the density of PLA is 1.24 g/cm\({}^{3}\). The good results can be attributed to the nucleating agent Tracel3170 contained based on the manufacturer's data and the TGA results (residual material content of 40.96% at 800 \({}^{\circ}\)C). The endothermic Tracel4200 produced fewer cells per unit volume in the case of the 8052_4200 recipe recommended for foaming (5.58 \(\times\) 10\({}^{4}\) cells/cm\({}^{3}\)) than in the case of the 2003_4200 recipe (1.91 \(\times\) 10\({}^{5}\) cells/cm\({}^{3}\)). In the case of the 2003D the expansion of the cells is also better, 1.53, as opposed to 1.24 in the case of the 8052_4200. This is because the decomposition products of Tracel4200 include water, which probably hydrolys the PLA, causing its melt pressure, viscosity and melt strength to decrease during the foaming process. Since the M\({}_{\rm w}\) of 8052D is lower (153,235 Da) than that of 2003D (180,477 Da), it resisted degradation by hydrolysis and produced better cell population density and expansion than the 8052D type PLA. The density of foam structures manufactured with Tracel4200  
Figure 10: (**a**) Cell population density and (**b**) porosity in the case of the different foaming agent types (manufacturing temperature 190 \({}^{\circ}\)C). Figure 9: Foamed sample (**a**) expansion values (**b**) density values (manufacturing temperature 190 \({}^{\circ}\)C). was 1.00 g/cm\({}^{3}\) in the case of the 8052_4200, while 0.81 g/cm\({}^{3}\) in the case of the 2003_4200, which is less than the density reduction achieved with Tracel3170. The endothermic Luv_9537 foaming agent produced more cells per unit volume in the case of the 8052_Luv_9537 recipe recommended for foaming (1.67 \(\times\) 10\({}^{5}\) cells/cm\({}^{3}\)) than in the case of the 2003_Luv_9537 recipe (5.51 \(\times\) 10\({}^{4}\) cells/cm\({}^{3}\)). Although the cells expanded, expansion in the case of the 2003D polymer was 1.22 and in the case of the 8052_Luv_9537 recipe, it was 1.23, which is quite little in both cases. The SEM images show that the size and shape of the cells in the case of the 8052_Luv_9537 are not homogeneous. The densities of PLA foams made with the Luv_9537 foaming agent are about 1 g/cm\({}^{3}\) in both cases. In the case of 2003_Luv_9537, density is 1.02 g/cm\({}^{3}\), while in the case of 8052_Luv_9537, it is 1.01 g/cm\({}^{3}\). We did not characterize the 2003_Hyd_3168 and 8052_Hyd_3168 recipes with the methods used for the characterization of foam structures, since no foam structure was formed during manufacturing. Only the PLA matrix formed a blend with the carrier of the CBA without the effective gases being able to form a gas phase and nucleate cells. In the case of exothermic foaming, PLA foamed to a much greater degree than in the case of endothermic foaming and so densities were lower.