## 5 Results of the Foamed Poly(Lactic Acid)
### Differential Scanning Calorimetry  
We examined the morphology that formed after foaming process, using differential scanning calorimetry, the results of which can be seen in Figure 11 and Table 8. Figure 11 shows the DSC curves of the single-processed reference poly(lactic acid) (extruded granules, processed similarly to foamed samples 2003_ref and 8052_ref) and the foamed samples in the case of the first heating curve. In the case of foams manufactured with the same processing conditions, the glass transition temperature of the polymer (T\({}_{\rm g}\)), which is the first registered endothermic change in the DSC curve, was not different from the reference value in the case of the Tracel4200 foaming agent and the 8052_3170 recipe. On the other hand, in the case of the other foaming agents T\({}_{\rm g}\) decreased by 2-4 \({}^{\circ}\)C (Table 8). A change in the second registered heat flow is the exothermic phenomenon of cold crystallization, which, in the case of foam structures, shifts considerably to lower temperatures. The foaming agent causes cold crystallization to happen at a much lower temperature than in the case of neat PLA. The second, bigger endothermic peak is the crystalline fraction melting range. It can be observed that the extruded foam samples are capable of cold crystallization during the DSC test, which is an exothermic reaction. This is because after processing, due to fast cooling, the crystalline fraction cannot fully form, since the molecular chains do not have time to get ordered. During the DSC test, due to the relatively slow temperature rise (5 \({}^{\circ}\)C/min), the molecular chains can become more ordered, which is accompanied by the release of heat. These crystalline fractions melt later, at a higher temperature and the polymer melts. In the case of 8052D, two crystalline melting peaks can be observed. [PERSON] et al. [PERSON] et al. (2017) provided the explanation that crystals with lower melting temperatures are smaller and formed during cold crystallization, while, crystal types with a higher melting temperature formed during the primary crystallization process, therefore, in our case, they are created by the technology. Therefore, foaming agents influence the crystalline structure that forms in the polymer. The crystalline melting range has two peaks and is composed of two different crystalline types: the \(\alpha\) and \(\alpha^{\prime}\) types. The \(\alpha^{\prime}\) type has a lower melting temperature, while the \(\alpha\) type has a higher melting temperature and is also thermodynamically more stable. The range between the melting of the two phases is called \(\alpha^{\prime}\)-\(\alpha\) phase transition [PERSON] et al. (2017). The crystalline fraction forming during manufacturing is typically low, due to the cooling of the polymer in open air (<5%). The foam structure with the highest population density and highest expansion has the highest crystalline fraction from manufacturing (4.3%). Crystalline fraction was typically higher than that of reference PLA; the nucleating agents in the CBAs probably had a beneficial effect on the forming of crystalline nuclei. Thus, the foaming agent affected the crystalline state and structure of the samples and also their thermal properties. In the presence of the foaming agent, transition temperatures typically shifted to lower temperatures compared to the reference poly(lactic acid).