*3.3. Crystallization Behavior*

DSC was used to study the crystallization behavior of the biopolymers and it was found that PVDF, despite having a similar melting point to PLLA, crystallizes at a faster rate and has a higher crystallization temperature [5]. It can be understood therefore that the PVDF will separate from the melt and crystallize first during cooling. Any modification to the PLLA/PVDF domain interface would affect this process of phase separation, crystallization rate, and hence, crystal morphology.

Dynamic DSC cooling scans at a rate of 10 ◦C/min of the prepared PLLA/PVDF blends (Figure 6a) show that as the ratio of PVDF increased, the PLLA crystallization exotherms shifted to higher temperatures and had a higher enthalpy. It should be noted that the crystallization exotherm of pure PLLA with a crystallization enthalpy of only 8.0 J/g can be difficult to detect. For comparison, the samples containing only 0.5 wt.% INT-WS<sup>2</sup> are shown in Figure 6b, where it is seen that the crystallization exotherms shifted toward higher temperatures and the enthalpy increased, and in the case of neat PLLA, up to 41.6 J/g. To be able to compare this change in *T*<sup>c</sup> vs. PLLA concentration for the PLLA/PVDF blends and those containing INT-WS<sup>2</sup> nanoparticles, Figure 7a is presented. Regarding the blends, a strong increase in the crystallization temperature of PLLA was found upon increasing PVDF content, from 92.2 ◦C for neat PLLA to 133.1 ◦C for raw PVDF. A noticeable increase up to 111.3 ◦C was already found with the incorporation of only 20 wt.% PVDF, but no significant change in *T*<sup>c</sup> was observed with a further increase of PVDF content. The *T*<sup>c</sup> of PLLA increased dramatically after blending with PVDF, indicating that PVDF accelerated the melt-crystallization of PLLA. In the same way, the presence of INT-WS<sup>2</sup> caused an increase in the crystallization temperature, both in the neat polymers and in the blends, the rise being higher than 25 ◦C for the neat PLLA and about 2 ◦C for the

neat PVDF (see Table 2). With regard to the nanocomposites, this aspect was also observed, though it seems to be less dependent on the concentration of the polymers. As such, it would suggest that the nanofillers provoked nucleation in both polymeric components, with the effect being more pronounced for PLLA. In contrast, 2D-WS<sup>2</sup> nanosheets have been shown to slow down the crystallization rate of PLLA [25]. Such differences suggest that the nanoparticle shape plays a fundamental role in PLLA crystallization. This discrepancy is likely related to several factors, including the nanofiller geometry, its surface energy, roughness, and crystalline structure as well as on the filler ability to form the critical nucleus [17,22,23]. It should be noticed that, in the case of the 60/40-INT nanocomposite, a double crystallization exotherm was found (Figure 6b), with *T*<sup>c</sup> values of 114.1 ◦C and 134.5 ◦C, which is likely related to the presence of two distinct macrophases, one containing the majority of the 0.5 wt.% INT-WS<sup>2</sup> and the other encompassing very little. This tendency was also observed for the 40/60-INT and 20/80-INT nanocomposite samples. *Polymers* **2021**, *13*, x FOR PEER REVIEW 7 of 19 corresponding values of maximum rate of decomposition (*R*max) is representative of the variation in the composition of the blends, Figure 4b. In particular, the *Rmax* of PLLA/PVDF blends clearly fell dramatically with respect to the values observed of neat PLLA and PVDF. Analogously, the addition of 0.5 wt.% INT-WS<sup>2</sup> to the PLLA/PVDF blends slightly affected the thermal stability (Table 1), without apparent perturbation of the mechanisms of degradation of the blend components, as can be deduced from the presence of the degradation maxima corresponding to PVDF and PLLA (Figure 5).

**Figure 4.** (**a**) Variation of the temperature and (**b**) rate of maximum decomposition (*T*max/*R*max) of PLLA and PVDF in the binary PLLA/PVDF and ternary PLLA/PVDF/INT-WS2 hybrid nanocomposites with composition. **Figure 4.** (**a**) Variation of the temperature and (**b**) rate of maximum decomposition (*T*max/*R*max) of PLLA and PVDF in the binary PLLA/PVDF and ternary PLLA/PVDF/INT-WS2 hybrid nanocomposites with composition.

are shown in Figure 6b, where it is seen that the crystallization exotherms shifted toward higher temperatures and the enthalpy increased, and in the case of neat PLLA, up to 41.6 J/g. To be able to compare this change in *T<sup>c</sup>* vs. PLLA concentration for the PLLA/PVDF blends and those containing INT-WS<sup>2</sup> nanoparticles, Figure 7a is presented. Regarding the

**Figure 5.** DTG curves of PLLA/PVDF/INT-WS<sup>2</sup> nanocomposites. **Figure 5.** DTG curves of PLLA/PVDF/INT-WS<sup>2</sup> nanocomposites. tendency was also observed for the 40/60-INT and 20/80-INT nanocomposite samples.

*Polymers* **2021**, *13*, x FOR PEER REVIEW 9 of 19

otherms shifted to higher temperatures and had a higher enthalpy. It should be noted that the crystallization exotherm of pure PLLA with a crystallization enthalpy of only 8.0 J/g can be difficult to detect. For comparison, the samples containing only 0.5 wt.% INT-WS<sup>2</sup> **Figure 6.** DSC thermograms of the non-isothermal crystallization of (**a**) binary PLLA/PVDF and (**b**) ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites obtained during cooling from the melt to room temperature at 10 ◦C/min.

**Figure 6.** DSC thermograms of the non-isothermal crystallization of (**a**) binary PLLA/PVDF and (**b**) ternary PLLA/PVDF/INT-WS2 hybrid nanocomposites obtained during cooling from the melt to

room temperature at 10 °C/min.

**Figure 7.** (**a**) Variation of the crystallization temperature (*T*c) and (**b**) crystallization enthalpy (∆*H*c) of PLLA and PVDF in the binary PLLA/PVDF and ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites with composition. **Figure 7.** (**a**) Variation of the crystallization temperature (*T*c) and (**b**) crystallization enthalpy (∆*H*c) of PLLA and PVDF in the binary PLLA/PVDF and ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites with composition.

The previously mentioned nucleation effect that led to the increase in crystallization **Table 2.** DSC parameters of different PLLA/PVDF/INT-WS<sup>2</sup> blend nanocomposites based on PLLA, PVDF, and INT-WS<sup>2</sup> .


*T*c: crystallization temperature; ∆*H*c: crystallization entalphy; *T*cc: cold-crystallization temperature; ∆*H*cc: cold-crystallization entalphy; *T*m: melting temperature; ∆*H*m: melting entalphy.

The previously mentioned nucleation effect that led to the increase in crystallization temperature is highly important, particularly when evaluating the crystallization enthalpy tendency. This can be seen in Figure 7b where the variation of the crystallization enthalpy (∆*H*c) versus the PLLA wt.% of the blends and the nanocomposites is presented, with the parameters of crystallization detailed in Table 2. For the materials without nanoparticle addition, the value of ∆*H*<sup>c</sup> reduced from 48.4 J/g for PVDF, that is 47% crystalline (∆*H*100 PVDF = 103 J/g for perfect crystals [38]) to 8.0 J/g for PLLA, with 8.6% crystallinity (∆*H*100 PLLA = 93 J/g [39]). However, as can be seen, the increased presence of PVDF provoked a significant increment in the ∆*H*<sup>c</sup> of PLLA due to it helping to speed up PLLA's overall crystallization rate. This is reflected in both an increase the temperature of crystallization from melt and enthalpy of crystallization and cold-crystallization suppression (see next section). Using a similar PLLA/PVDF blend, the crystal nucleation was also investigated by Pan et al., where they found that transcrystallization of one polymer type 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 other specific nucleating agents or nano-sized fillers.
