*3.4. Melting Behavior*

The melting of semicrystalline thermoplastics is a very complex process significantly influenced by the crystallization conditions. In some circumstances, two peculiarities are observed in the DSC heating scans of semi-crystalline PLLA [40,41]. One is the emergence of a small exothermic peak just before the melting peak and the other is the occurrence of a double melting peak, which has usually been interpreted in terms of a pre-exiting morphology and/or reorganization [30,40]. These two phenomena can be well explained by taking into consideration the crystallization conditions in parallel with the α and α' crystal formation requirements [41]. When PLLA is crystallized at temperatures corresponding to α crystal formation, the small exotherm appearing just before the single melting peak is due to the transformation of disordered α' crystals to the ordered α-form. On the other hand, a double melting behavior appears when the crystallization temperature is situated in the region of simultaneous α´ and α type formation. For high crystallization temperatures, only α crystals are produced leading to a single melting peak.

other specific nucleating agents or nano-sized fillers.

can occur on the crystalline surface of the other polymer [5]. Based on this, it is suggested that the PVDF presence in the immiscible blends promoted PLLA crystallization via two routes, interface-assisted and heterogeneous epitaxial nucleation. The interface between the two phases reduce the surface free energy, facilitating crystal nuclei to form via heterogeneous nucleation. In addition to PVDF crystallization, phase separation can bring about the molecular ordering, alignment, and/or orientation of PLLA chains at the PLLA/PVDF domain interface via interdiffusion, further aiding crystal embryo development [5]. Regarding the samples containing INT-WS2, ∆*H<sup>c</sup>* was observed to increase dramatically as a result of the nucleating effect of the nanoparticles, reaching 54.6 J/g (58.7% crystallinity) for 80/20-INT, the dual-additive system was more important for nucleation than of PVDF alone. Figure 8 illustrates the WAXS profiles of neat PLLA, 80/20, and 80/20- INT recorded during cooling from the melt to room temperature. The characteristic peaks of α-form of PLLA in WAXS patterns (see the following part) appeared as soon as the material attained an appreciable degree of crystallinity. The appearance of these peaks relates well to the crystallization temperature calculated from DSC curves. These results also indicate that the presence of INT-WS<sup>2</sup> accelerated the crystallization rate of PLLA in the PLLA/PVDF-INT blend nanocomposites. This led to the appearance of the characteristic of the crystalline diffraction of PLLA at higher temperature. Nucleating effects due to the presence of nanofillers have previously been reported for PLLA filled with inorganic nanotubes, nano-calcium carbonate, nano-zinc citrate, graphene oxide and fullerenes (C60), nanoclay, and carbon nanotubes [30]. In particular, it was shown that INT-WS<sup>2</sup> exhibited much more prominent nucleation activity on the crystallization of PLLA than

**Figure 8.** WAXS diffractograms of the dynamic crystallization of PLLA, PLLA/PVDF (80/20), and PLLA/PVDF/INT-WS<sup>2</sup> (80/20-INT) nanocomposites. **Figure 8.** WAXS diffractograms of the dynamic crystallization of PLLA, PLLA/PVDF (80/20), and PLLA/PVDF/INT-WS<sup>2</sup> (80/20-INT) nanocomposites.

Figure 9 compares the heating thermograms after cooling from the melt of binary (PLLA/PVDF) and ternary (PLLA/PVDF/INT) hybrid nanocomposites with those of neat PLLA and PVDF. Because the melting peaks of PLLA and PVDF converge in the temperature range of 155−175 ◦C, the effects of blending on the melting behavior of PLLA cannot be easily identified. It is seen that PLLA presented a maximum endotherm at 166.1 ◦C of melt after the described exothermic cold-crystallization process. From Figure 10 of the WAXS diffractograms, it can be observed the PLLA sample did not show polymorphism when crystallized under the same conditions as those used for the DSC, with only the main (200)/(110) diffraction peak at 16.7◦ clearly visible, relating to the PLLA α-form [5,30]. This was similar for the high PLLA content blends, that also principally exhibited the PLLA α-phase characteristic diffraction. In the case of PVDF and high content PVDF blends, diffraction planes of (100), (020), (110), and (021) relating to 2θ = 17.7◦ , 18.5◦ , and 20.0◦ diffraction peaks, respectively were seen, and are typical of the PVDF α-phase [42,43]. In the case when both polymers or polymer blends with nanofiller (INT-WS<sup>2</sup> [30]) were present, all representative diffraction peaks were observed. From Figure 9b, it can be seen that INT-WS<sup>2</sup> addition to the mixed PLLA/PVDF blends significantly affected the melting behavior due to the suppression of the cold-crystallization processes as a result of heterogeneous nucleation and the consequences of this as noted earlier [30]. It is also important to note that the ∆*H*<sup>m</sup> values of the binary (PLLA/PVDF) and ternary (PLLA/PVDF/INT) hybrid nanocomposites were higher than those of neat PLLA and PVDF, which was more distinct for the PVDF-rich blends. This is ascribed to the positive effects of both PVDF and INT-WS<sup>2</sup> on PLLA crystallization.

(PLLA/PVDF/INT) hybrid nanocomposites were higher than those of neat PLLA and PVDF, which was more distinct for the PVDF-rich blends. This is ascribed to the positive

effects of both PVDF and INT-WS<sup>2</sup> on PLLA crystallization.

**Figure 9.** DSC thermograms of melting of (**a**) binary PLLA/PVDF and (**b**) ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites obtained during heating at 10 °C/min after cooling from the melt to room temperature at 10 °C/min. **Figure 9.** DSC thermograms of melting of (**a**) binary PLLA/PVDF and (**b**) ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites obtained during heating at 10 ◦C/min after cooling from the melt to room temperature at 10 ◦C/min.
