*3.1. Morphology*

Tailoring phase morphology of immiscible blends through the addition of nanoparticles is a universally accepted strategy of forming compatible nanocomposites polymer blends, and typically results in improved upon physical properties. The reason for this is that the added nanoparticles typically locate at the interface of the polymer domains and act as interfacial modifiers, strengthening the interfacial adhesion. Nanoparticles can induce the formation of fine dispersed phase particles from coalescence during melt processing, thus stabilizing the fine morphology and therefore maintaining the properties of the blend.

The cryogenically fractured surface morphologies using SEM of the PLLA/PVDF blends can be seen in Figure 1. For the 80/20 PLLA/PVDF mixture, a distinct twophase morphology was noted, with the PVDF phase dispersed evenly within the PLLA matrix. Furthermore, the mean diameter of the domains augmented with increasing PVDF content. When the PLLA content < 60 wt.%, the phase morphology was reversed and the PLLA phase became dispersed in the PVDF. For the ternary hybrid PLLA/PVDF/INT nanocomposites and individual PLLA/INT and PVDF/INT nanocomposites (data are not shown), it was found that the INTs were uniformly dispersed at the nanoscale without evidence of aggregates or agglomerates (see arrows pointing to individual INT-WS<sup>2</sup> tubes in the images), verifying the effectiveness of the melt extrusion conditions (Figure 2). The INT-WS<sup>2</sup> nanoparticles also improved the compatibility of the two phases, as demonstrated by an important reduction in the size of the dispersed PVDF domain. This can be attributed to the formed morphological structure in which INT-WS<sup>2</sup> was mainly dispersed in the PLLA matrix and at the PLLA/PVDF interface.

To achieve similar morphology, other authors employed more elaborate methodologies. For example, carboxyl functionalized multi-walled carbon nanotubes (MWCNTs) were used by Wu et al. to improve the compatibility of immiscible PLA/PCL mixtures [34]. In the case of PLA and PBS, Chen et al. employed double functionalized organoclay (TFC) to increase phase compatibility [35]. They also found that the concentration of TFC had a significant effect on the blend morphology, though when the TFC content < 0.5 wt.%, the PBS domain did not change in size and it was almost exclusively found within the PLA regions. Another nanoparticle that has been used to make immiscible PLA mixtures compatible is silica, as reported by Odent et al. in a mixture of PLA with a gummy copolyester based on 3-caprolactone, P[CL-*co*-LA] [36]. It was found that P[CL-*co*-LA] with spherical nodules dispersed regularly in the PLA matrix in a blank mixture containing 10% by weight of P[CL-*co*-LA], while surface treated (5 wt.% hexamethyldisilazane) spherical nodules disappeared. On the other hand, the compatibility of hydroxyl functionalized and poly(3-caprolactone)-*b*-poly(L-lactide) diblock copolymer grafted polyhedral oligomeric silsesquioxane (POSS-OH and POSS-PCL-*b*-PLLA, respectively) on PLA/PCL blends were analyzed by Monticelli et al. [37]. In both cases, the adhesion between PLA and PCL increased, and with POSS-PCL-*b*-PLLA, a nearly homogeneous microstructure was formed.

*Polymers* **2021**, *13*, x FOR PEER REVIEW 5 of 19

**Figure 1.** High-resolution SEM image for PLLA/INT-WS2, PVDF/INT-WS2, binary PLLA/PVDF, and ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites. **Figure 1.** High-resolution SEM image for PLLA/INT-WS<sup>2</sup> , PVDF/INT-WS<sup>2</sup> , binary PLLA/PVDF, and ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites. **Figure 1.** High-resolution SEM image for PLLA/INT-WS2, PVDF/INT-WS2, binary PLLA/PVDF, and ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites.

tube dimensions (thicknesses) of as received INT-WS2. **Figure 2.** High-resolution SEM image for ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites; the inset shows the nanotube dimensions (thicknesses) of as received INT-WS2. **Figure 2.** High-resolution SEM image for ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites; the inset shows the nanotube dimensions (thicknesses) of as received INT-WS<sup>2</sup> .

**Figure 2.** High-resolution SEM image for ternary PLLA/PVDF/INT-WS<sup>2</sup> hybrid nanocomposites; the inset shows the nano-

#### *3.2. Thermal Stability 3.2. Thermal Stability*

The thermal stability conditions the processing limits of this type of system, and as such, the study of the effect of blend composition on the degradation temperatures is of utmost importance. Figure 3 shows the variation of the integral and differential thermogravimetric curves, TGA and DTG, of PLLA/PVDF blends as a function of the blend composition. The initial weight loss of PLLA started at around 326 ◦C, and in the case of PVDF, at a temperature about 100 ◦C higher. The differential curves show a single peak for each polymer which implies that the degradation processes occurred in a single step. The values of the characteristic degradation temperatures *T*<sup>i</sup> (temperature for 2% weight loss), *T*<sup>10</sup> (temperature for 10% weight loss), *T*max (temperature corresponding to the maximum rate of weight loss), and *R*max (rate of maximum decomposition) are summarized in Table 1. The thermal stability conditions the processing limits of this type of system, and as such, the study of the effect of blend composition on the degradation temperatures is of utmost importance. Figure 3 shows the variation of the integral and differential thermogravimetric curves, TGA and DTG, of PLLA/PVDF blends as a function of the blend composition. The initial weight loss of PLLA started at around 326 °C, and in the case of PVDF, at a temperature about 100 °C higher. The differential curves show a single peak for each polymer which implies that the degradation processes occurred in a single step. The values of the characteristic degradation temperatures *T<sup>i</sup>* (temperature for 2% weight loss), *T<sup>10</sup>* (temperature for 10% weight loss), *Tmax* (temperature corresponding to the maximum rate of weight loss), and *Rmax* (rate of maximum decomposition) are summarized in Table 1.

**Figure 3.** (**a**) Thermogravimetric (TGA) and (**b**) derivative thermogravimetric (DTG) curves of PLLA/PVDF blends. **Figure 3.** (**a**) Thermogravimetric (TGA) and (**b**) derivative thermogravimetric (DTG) curves of PLLA/PVDF blends.

It is clear that the presence of PVDF influenced the degradation behavior of PLLA and vice versa. The addition of PVDF to PLLA hardly increased the thermal stability of

served. Although the change in *Tmax* values was small (Figure 4a), the change in the


**Table 1.** TGA parameters of different PLLA/PVDF/INT-WS<sup>2</sup> blend nanocomposites based on PLLA, PVDF, and INT-WS<sup>2</sup> .

*T*i : temperature for 2% weight loss; *T*10: temperature for 10% weight loss; *T*max: temperature corresponding to the maximum rate of weight loss; and *R*max: rate of maximum decomposition.

It is clear that the presence of PVDF influenced the degradation behavior of PLLA and vice versa. The addition of PVDF to PLLA hardly increased the thermal stability of the PLLA biopolymer (*T*<sup>i</sup> and *T*10), and the thermal degradation of both components took place via different mechanisms, since two well-defined decomposition stages can be observed. Although the change in *T*max values was small (Figure 4a), the change in the 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 *R*max 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).
