*3.2. Effect of ECO in PLA on the Thermomechanical Properties of PLA*

Storage modulus (G') and damping factor (tan δ) were assessed by dynamic mechanical response. In Figure 6, the viscoelastic behavior of PLA\_ECO formulations were exposed. Two characteristic changes in the storage modulus could be distinguished. The first change, between 50 and 70 ◦C, was the drop of storage modulus, which was related to glass transition temperature (Tg) at around 60 ◦C, as reported by Yong et al. [47]. The second change, between 80 and 100 ◦C, was recognized as the beginning of cold crystallization process. The addition of ECO to PLA resulted in a loss of storage modulus at lower temperatures. This was due to the plasticizing effect that ECO exerts on the PLA matrix, which increases the free volume between PLA chains before a saturation effect, decreasing the interaction between them [48]. In addition, at room temperature, neat PLA showed a storage modulus value of 1300 MPa, while the plasticized PLA formulations showed a decrease of this modulus up to 1000 MPa as a consequence of the plasticization effect. On the other hand, the beginning of cold crystallization decreased as ECO content increased, obtaining a shift from 87 up to 84 ◦C for plasticized PLA. This effect was due to plasticizer enabling the rearrangement in packed structure under lower energetic conditions. *Polymers* **2021**, *13*, 1283 10 of 17

**Figure 6.** Plot evolution of dynamic mechanical thermal analysis (DMTA) of PLA with different ECO contents: (**a**) storage modulus (G'); (**b**) damping factor (tan δ). **Figure 6.** Plot evolution of dynamic mechanical thermal analysis (DMTA) of PLA with different ECO contents: (**a**) storage modulus (G'); (**b**) damping factor (tan δ).

The temperature of the tan δ peak was a great manner to obtain an accurate value of Tg which was moved to lower temperatures as ECO content increased. Specifically, the Tg values were reduced from 64.2 (PLA neat) up to 61.9 and 59.5 *°*C for PLA\_5%ECO and PLA\_10%ECO, respectively. Regarding tan delta peak magnitude, it is related to their molecular mobility. As Chieng et al. reported, the addition of plasticizer content led to increase the intensity of tan delta due to higher molecular mobility caused by the plasticizing effect [35]. Then, a significant increase in respect to neat PLA in the magnitude of the damping factor was observed with higher ECO content. On the other hand, Silverajah et al. observed that increasing the content of epoxidized palm oil plasticizer in PLA led to decrease the temperature of tan δ peak [49]. The temperature of the tan δ peak was a great manner to obtain an accurate value of T<sup>g</sup> which was moved to lower temperatures as ECO content increased. Specifically, the T<sup>g</sup> values were reduced from 64.2 (PLA neat) up to 61.9 and 59.5 ◦C for PLA\_5%ECO and PLA\_10%ECO, respectively. Regarding tan delta peak magnitude, it is related to their molecular mobility. As Chieng et al. reported, the addition of plasticizer content led to increase the intensity of tan delta due to higher molecular mobility caused by the plasticizing effect [35]. Then, a significant increase in respect to neat PLA in the magnitude of the damping factor was observed with higher ECO content. On the other hand, Silverajah et al. observed that increasing the content of epoxidized palm oil plasticizer in PLA led to decrease the temperature of tan δ peak [49].

Table 2 shows a summary of values obtained for Vicat softening temperature (VST) and heat deflection temperature (HDT) of neat PLA and PLA plasticized with ECO. These thermomechanical parameters are directly related to mechanical resistant properties. For this reason, the decreasing trend was the same that had been observed previously in tensile strength and modulus. Regarding VST, a remarkable decrease was detected when ECO content increased. Neat PLA had a VST of 56.6 *°*C, while samples with 7.5 and 10 wt.% ECO, respectively, reduced this value 4.4 *°*C. A similar trend was observed in HDT values, obtaining a difference of 2.6 *°*C between neat PLA and PLA\_10%ECO sample. The Table 2 shows a summary of values obtained for Vicat softening temperature (VST) and heat deflection temperature (HDT) of neat PLA and PLA plasticized with ECO. These thermomechanical parameters are directly related to mechanical resistant properties. For this reason, the decreasing trend was the same that had been observed previously in tensile strength and modulus. Regarding VST, a remarkable decrease was detected when ECO content increased. Neat PLA had a VST of 56.6 ◦C, while samples with 7.5 and 10 wt.%ECO, respectively, reduced this value 4.4 ◦C. A similar trend was observed in HDT values, obtaining a difference of 2.6 ◦C between neat PLA and PLA\_10%ECO sample. The addition

addition of ECO to PLA samples in different percentages facilitated the sliding of the polymeric chains, being also favored by the slight increase in temperature that takes place in

between the PLA and the macromolecules, obtaining the plasticization of the materials

**Table 2.** Vicat softening temperature (VST) and heat deflection temperature (HDT) of PLA with

**Reference VST (°C) HDT (°C)**  PLA 56.6 ± 1.5 52.8 ± 0.5 PLA\_2.5%ECO 55 ± 1.2 51.8 ± 0.6 PLA\_5%ECO 53.2 ± 1.3 50.4 ± 0.4 PLA\_7.5%ECO 52.2 ± 1.1 50.4 ± 0.6 PLA\_10%ECO 52.2 ± 1.4 50.2 ± 0.4

different epoxidized chia seed oil (ECO) content.

[50].

of ECO to PLA samples in different percentages facilitated the sliding of the polymeric chains, being also favored by the slight increase in temperature that takes place in the VST and HDT tests. The ECO molecules decreased the intermolecular attraction forces between the PLA and the macromolecules, obtaining the plasticization of the materials [50].

**Table 2.** Vicat softening temperature (VST) and heat deflection temperature (HDT) of PLA with different epoxidized chia seed oil (ECO) content.

