# Thermal and Mechanical Assessment of PLA-SEBS and PLA-SEBS-CNT Biopolymer Blends for 3D Printing
## 3 Results  
### Results of PLA-SEBS Blends  
Figure 1 shows the results of the non-isothermal analysis, the effect of cooling rates on crystallization DSC curves at 0, 1, 3 and 5% SEBS contents. The samples are depicted in different colors. The results of the anisothermal DSC analysis performed on the extruded fibers depict that as the cooling rates increased, the crystallization shifted to a lower temperature and the crystallization interval also widened. In PLA, the crystallization peak appeared clearly only at the slowest cooling rate, but its size was not significant compared to the others. As a result of SEBS, the crystallization peaks became clearly visible at each cooling rate, and as the SEBS content increased, the shape of the curves also changed, suggesting an alteration in the crystal structure. To demonstrate the nucleating effect of SEBS, the increase in the initial temperature of crystallization and in the heat of crystallization must be investigated. DSC analysis on the printed specimens shows that the cooling during printing was so fast for the PLA that it was able to generate only a few crystalline phases (Figure 2). On the other hand, when it absorbed the heat from the heating, and passed through the glass transition temperature, the phenomenon of so-called cold crystallization started, i.e., the molecules began to rearrange, forming the missing crystalline phases. However, on further heating, melting occurred, where both the crystals originally formed and those formed during the heating melted. The analytical program subtracted the heat of cold crystallization from the heat of melting to obtain the number of crystals that the printed specimens had. The amount of crystalline phases has a serious effect on the mechanical properties of the finished product. The more crystals are formed, the higher the mechanical strength can be achieved, but the impact resistance decreases. In contrast, a decrease in the number of crystallites results in lower mechanical strength but better impact resistance. In the DSC curves obtained during melting, the peak represents the melting temperature of most crystals. During crystallization, the peak means reaching the maximum of the crystallization rate. In Figure 2, the heating rate was 20 \({}^{\circ}\)C/min. Figure 2 shows another peak around 60-70 \({}^{\circ}\)C, too. This area is the glass transition temperature of PLA; the peak here means the disappearance of the internal stress. Figure 1: Effect of cooling rates and SEBS content on crystallization curves. Figure 3 depicts the change in the initial extrapolated temperature of crystallization and the enthalpy changing as a function of SEBS content. According to ISO 11357-1, 'eic' is the extrapolated initial temperature of crystallization, which is the intersection of the extrapolated linear section of the rising peak with the apparent baseline below the peak. The enthalpy changing shows the heat of crystallization on the fibers, and the corrected heat of melting on the printed specimens. The samples are depicted in different colors. The change in the initial extrapolated temperature of crystallization occur on all cooling rates, which precludes the influence of cooling rate. It can be seen that increasing the SEBS content increases the initial crystallization temperature, and crystallization begins sooner at higher temperatures, which is one of the accepted proofs of nucleation [19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. As a result of the appearance of SEBS, the heat of crystallization increased at all cooling rates relative to PLA. A higher heat of crystallization means more crystalline phases. At the two slower cooling rates, although the SEBS content increases, the value of the heat of crystallization does not alter, but the shape of the curve changes (Figure 1). That is, as the initial temperature increases, the shape of the curve alters but the heat of crystallization does not, so it can be concluded that it is likely that crystallites of similar sizes but different shapes, increasingly perfect, will form.