Table 3 gathers the main results obtained after the mechanical characterization of the PLA pieces with different OLA contents. As expected, the progressive addition of OLA led to lowering σ
b values from 64.6 MPa, for the neat PLA, down to 37.4 MPa, for the PLA pieces containing 20 wt% OLA. This decreasing tendency was almost linear as it can be seen in the table for the other compositions. This tendency on mechanical strength is the typical a plasticizer produces on the base polymer. Some other OLA additives have demonstrated a similar effect on mechanical properties by increasing remarkably ductility, mainly in the film form, as reported by Burgos et al. [
45]. In the latter work, it was reported an increase in ε
b from 4% to 315% with an OLA content of 25 wt%, but it is worthy to note the OLA previously used was designed for plasticization of PLA films. Such dramatic increase in the ε
b was not observed when using OLA in this work since the primary use of this type of OLA was to improve impact strength, as indicated by the supplier and it will be discussed further.
Regarding mechanical ductility, the neat PLA piece showed a very low value of 7.9% and the addition of OLA did not promote an increase in ε
b, but a slight decrease down to values of 5%. It is not usual that a plasticizer promotes a decrease in ductility since the typical effect of a plasticizer is a decrease in the tensile resistant properties (σ
b and E
t) and an increase in ductile properties (ε
b). Nevertheless, it has been reported that some plasticizers promote a clear plasticization that is detectable by a decrease in T
g while no improvement in ductility occurs. This atypical behavior was reported by Ambrosio-Martín et al. [
54] in PLA films blended with different synthesized OLAs. A ε
b value of 5.25% was reported for neat PLA, while the addition of 25 wt% of a purified OLA yielded a ε
b of 2.52%. It was also reported a slight increase in E
t and an apparent decrease in σ
b, in a similar way as obtained in this work. It was concluded that, although there is clear evidence of the mechanical plasticization of OLA-containing PLA films, they were not more deformable, which is also in agreement with the work performed by Courgneau et al. [
55]. Concerning E
t, the neat PLA piece was characterized by a value of nearly 2.2 GPa and the values remained in the 2.2–2.4 GPa range after the addition of OLA. Thus, the main effect of this type of OLA on the tensile mechanical properties was a remarkable decrease in σ
b, which representative for some plasticization, but also a slight decrease in ε
b. Interestingly, as it can also be seen in
Table 3, the addition of OLA successfully increased the impact strength of PLA. Neat PLA showed an impact strength of 25.7 kJ m
−2, which indicates a brittle behavior, and the only addition of 5 wt% OLA provided a slight increase in the impact strength to 30.4 kJ m
−2. Nevertheless, the most remarkable changes were obtained for OLA additions of 10 wt% and 15 wt%, showing impact strength values of 54.2 kJ m
−2 and 69.7 kJ m
−2, respectively. Therefore, the PLA piece with 15 wt% OLA presented the maximum impact strength with a percentage increase of approximately 171% with regard to the neat PLA. It is also worthy to mention that the PLA piece containing 20 wt% OLA showed a decrease in impact strength in comparison with the other OLA-containing PLA pieces, thus suggesting certain OLA saturation in the PLA matrix. In this regard, Fortunati et al. [
56] reported the use of isosorbide diester (ISE) as plasticizer for PLA. It was observed plasticizer saturation at 20 wt% ISE and this was attributed to a limitation of T
g decrement. In addition, a noticeable decrease in ε
b was observed once the plasticizer saturation was achieved. Furthermore, Ferri et al. [
57] reported the plasticization of PLA by fatty acid esters, observing a remarkable decrease in impact strength above 5 parts per one hundred parts (phr) of PLA. Accordingly, a remarkable decrease in T
g was attained for contents of up to 5 phr whereas, above this, the T
g values did not change in a noticeable way, corroborating the relationship between the impact and thermal properties. Regarding hardness, one can observe that the Shore D values remained nearly constant after the OLA addition, showing values in the 78–82 range. Therefore, the most important feature this OLA can potentially provide to the mechanical properties of PLA is a remarkable improvement in impact strength while the elasticity can be slightly improved and ductility reduced. This particular mechanical behavior could be ascribed to an increase in the sample crystallinity and also to the presence of soft domains of OLA dispersed within the PLA matrix, simultaneously improving impact strength and reducing flexibility.
As previously indicated, one of the most widely used strategies to improve toughness in PLA-based formulations is blending with rubber-like polymers such as PCL [
58], PBS [
29,
59], or PBAT [
60,
61]. In these immiscible blends, the energy absorption is related to presence of finely dispersed rubber-like small polymer droplets embedded in the brittle PLA matrix. In some cases, a synergistic effect can be found when different reactive or non-reactive compatibilizers are used. In this work, OLA has the same chemical structure than PLA, thus leading to miscibility without the need of compatibilizers. In this regard, Burgos et al. [
45] have reported the similarity between the solubility parameters of both PLA and OLA, which plays an essential role in miscibility. According to this,
Figure 1 gathers the FESEM images corresponding to the fracture surfaces from the impact tests of the PLA pieces with the different OLA loadings.
Figure 1a shows the fracture surface of the neat PLA piece. As one can observe in this micrograph, the surface was smooth with multiple microcracks presence, which is an indication of a brittle behavior. As opposite,
Figure 1b shows that the fracture surface morphology of the PLA piece with 5 wt% OLA changed noticeably. Phase separation could not be detected due to the high chemical affinity between PLA and OLA and the microcracks were not observed but, in contrast, macrocracks were produced. Therefore, the presence of OLA seems to inhibit microcrack formation and growth and, therefore, the cracks could grow to a greater extent thus leading to a rougher surface that is responsible for higher energy absorption during impact.
Figure 1c,d show the FESEM images corresponding to the fracture surfaces of the PLA pieces with 10 wt% and 15 wt% OLA, respectively. In these images, the above-mentioned effect was more intense then showing rougher surfaces that are related to enhanced energy absorption. Finally,
Figure 1e shows that the PLA piece containing 20 wt% OLA presented a similar fracture surface than the other pieces and, despite there was a clear loss of toughness, its morphology did not allow identifying phase separation.