**3. Thermal Properties**

It is well known that the crystalline morphology and structure obtained during the thermoplastic processing plays an important role on the physico-mechanical behavior of the resulting polymeric material, conditioning its potential uses. In this way, the control of the crystallization process can be seen as a successful approach for improving physico-mechanical properties of polymers. Therefore, it is of great interest to investigate the nucleation, crystallization, and structural development of the matrix in IF/INT reinforced polymer nanocomposites [13]. This would help to optimize the manufacturing conditions in order to obtain high-performance nanocomposites and to fully exploit their potential in practical applications.

**Figure 2.** TGA thermograms under a nitrogen atmosphere for neat iPP, PPS and some hierarchical laminates. The inset shows the initial degradation temperature (*T*i) *vs.* nanoparticle loading.

The thermal stability of several polymer matrices reinforced with IF-WS2 nanoparticles was compared with that observed for other spherical inorganic nanofillers, organized by the nature of the matrix [13]. It was found that the incorporation of nanometer-sized k particles into a polymer enhances the thermal stability of the matrix inhibiting the formation and escape of volatile byproducts generated during the decomposition process. In the case of the hierarchical thermoplastic-based composites, the thermal stability of IF-WS2 reinforced iPP [22] and PPS [23] laminates has been investigated using TGA, and typical thermograms under a nitrogen atmosphere for the neat matrices, and composites reinforced with 1.0 and 2.0 wt.% IF-WS2 are shown in Figure 2. It is found that all the composites exhibit a single decomposition stage in a nitrogen environment, similar to that found for the neat polymers, indicating that the random scission of the polymeric chains is the predominant degradation process. The incorporation of increasing nanoparticle contents induces a progressive thermal stabilization of both matrices (see inset of Figure 2), the effect being more significant in the case of iPP, probably related to the lower thermal stability of this commodity plastic compared to high-performance PPS. Thus, an increase in the initial degradation temperature (*T*i) of 12 °C and 47 °C is attained at 2.0 wt.% loading in comparison to the reference PPS and iPP laminate, respectively. A similar trend is found for the temperature of 10% weight loss (*T*10) and maximum rate of weight loss (*T*max). This thermal stability enhancement has been ascribed to the barrier effect of the nanoparticles that effectively obstruct the diffusion of volatile products from the bulk of the polymer to the gas phase, therefore slowing down the decomposition process. Upon increasing IF-WS2 loading, the barrier effect becomes stronger, which is reflected in higher degradation temperatures. An analogous effect of thermal stability increase has been reported for PP/GF composites reinforced with other inorganic nanoparticles such as clays [30]. Nevertheless, for the same nanofiller loading, the improvements in thermal stability are larger in the case of IF-WS2, indicative of a more effective

heat barrier effect of the IF nanoparticles likely arising from their more homogenous dispersion and spherical shape, thus, larger specific surface area.

In the same way, the incorporation of INTs can also lead to an improvement in the thermal stability of polymer/INTs [25,27]. As an example, the characteristic weight loss temperatures for PP nanocomposites, filled with different nanoreinforcements in nitrogen, are summarized in Table 1 [31–42]. The data reveal that the concentration of non-modified INT-MoS2 has a dramatic effect on the thermal stability of the iPP nanocomposites. *T*10 of iPP/INT-MoS2 (1.0 wt.%) was almost 60 °C higher than that of neat iPP, suggesting that INT-MoS2 have outstanding properties for improving the thermal stability at low nanofiller content [31]. As a comparison, approximately the same increment was observed for iPP nanocomposites filled with 10 wt.% of silane-modified halloysite nanotubes (HNTs). In the case of iPP/HNTs, the thermal stability and flame-retardant effects are believed to result from the hollow tubular structure of HNTs, the barriers for heat and mass transport and the presence of iron in the HNTs [32–34]. Layered silicates, such as montmorillonite (MMT), also have important effects on the thermal stability of the PP matrix (Table 1). The dramatic improvement in thermal stability of around 90 °C was related to the confinement of the single nanoparticles in approximately 1 nm3 volume using sophisticated methods of modification/exfoliation [39–41].

The flammability behavior of PPS/IF-WS2/CF has been investigated by pyrolysis combustion flow calorimetry, in order to determine the heat release rate (HRR) at different nanoparticle contents [24]. The addition of IF-WS2 leads to a progressive drop in the average peak HRR, the reduction being about 17% for the laminate with 1.0 wt.% loading. Further, the onset temperature at which begins the release of heat and the temperature at peak HRR increase gradually with the nanoparticle loading, with maximum increments of 19 and 23 °C, respectively, at 2.0 wt.% IF-WS2. These improvements are probably related to the low degree of porosity and enhanced thermal stability of the hybrids. Moreover, there seems to be a synergistic effect of both micro- and nano-fillers on increasing the polymer resistance to fire. The coexistence of CFs and IF-WS2 in the laminates results in a more effective confined geometry that increases the barrier resistance to the evolution of flammable volatiles. Similar synergistic behavior has been described for different polymer/clay/carbon nanotube hybrids [43,44].

The degree of crystallinity is a key parameter in thermoplastic polymers because it has strong influence on both the chemical and mechanical properties. The crystalline phase improves the stiffness and tensile strength whilst the amorphous phase helps to absorb the impact energy. The influence of IF-WS2 on the crystallization behavior of PPS/CF [23] and iPP/GF [22] has been analyzed by DSC, and typical cooling thermograms for composites with 1.0 and 2.0 wt.% loading are shown in Figure 3. Moreover, the crystallization temperature (*T*p) as a function of IF-WS2 concentration is plotted in the inset of this Figure. Noticeable differences are detected depending on the thermoplastic polymer. In the case of PPS based composites, the addition of low nanoparticle contents (*i.e.*, 0.1 or 0.5 wt.%) results in a decrease in *Tp* and the degree of crystallinity (*Xc*), indicating the absence of a nucleating effect of the IF-WS2 on the polymer crystallization, and that the transport of macromolecular segments to the growing surface of PPS in the composite is hindered. However, the incorporation of higher nanoparticle contents leads to an increase in both *Tp* and *Xc*, by up to

9 °C and 14%, respectively demonstrating that higher nanoparticle contents act as nucleating agents for PPS. On the other hand, these nanoparticles effectively nucleate the iPP matrix in the concentration range of 0–4.0 wt.%, with increases up to 22 °C and 6% in *Tp* and *X*c, respectively, at the highest loading tested. These improvements are greater than those reported for binary iPP/IF-WS2 nanocomposites [16], pointing towards a synergistic effect of both fillers on promoting the crystallization of iPP. This behavior is in agreement with the reported for PP/ZnO/GF [45] and PP/SiO2/GF hybrids [46], where the combination of nano- and micro-fillers additionally increased the *Tp* of the matrix, albeit the increments found in those hybrids (~7 and 6 °C at 2.0 wt.% ZnO and 1.0 wt.% SiO2 content, respectively) are smaller than the increases found for the same amount of IF-WS2. Further, *Xc* of PP dropped upon incorporation of ZnO or SiO2 and GF, while the combined nucleating effect of IF-WS2/GF provoked a slight increase in crystallinity.

**Table 1.** Thermal stability, crystallization, and mechanical data for isotactic polypropylene (iPP) nanocomposites using nanoreinforcing fillers with different morphologies (e.g., tubular, spherical and laminar-like particles) taken from literature. ¨*T*10 = increment of degradation temperature for 10% weight loss, ¨*T*p = increment of crystallization peak temperature, G*E* = Percentage variations of Young's modulus, G ıy = Percentage variations of tensile strength and Gİb = Percentage variations of strain at yield.


**Figure 3.** DSC crystallization thermograms for neat iPP, PPS and some IF-WS2 reinforced multiscale laminates. The inset shows the crystallization peak temperature *T*<sup>p</sup> *vs.* IF-WS2 content.

In this way, the control of the crystallization behavior has been shown to be a successful approach for improving physico-mechanical properties of polymer/INT nanocomposites. Table 1 summarizes the findings of several studies on the nucleating efficiency (NE) of nanoreinforcing fillers, and data can be compared by analyzing the difference between the crystallization peak temperature (*Tp*) of each nanocomposite and that of the neat matrix (ÄTp). Clearly, the ÄTp value for INT-MoS2 far exceeds the values observed for montmorillonite nanoclay [39]androd-Si3N4 [38], and is comparable to that observed for MWCNTs [35]. However, the nucleation efficiency of INT-MoS2 is significantly lower in comparison to the value of 40% observed for inorganic fullerene-like WS2 nanoparticles at 1.0 wt.% [16]. The results obtained clearly show that the addition of INT-WS2 plays a remarkable role in accelerating the crystallization rate of iPP. In these systems, the crystallinity of iPP was found to rise up to 14% with increasing the INT-MoS2 content, from a value of 50% for iPP, to values of 54, 57 and 56% for the nanocomposites with 0.1 wt.%, 0.5 wt.% and 1 wt.%, respectively [25]. Furthermore, a new study on the crystallization behavior of biopolymer/INTs suggests that INT-WS2 exhibits much more prominent nucleation activity on the crystallization of PHB than other specific nucleating agents or nano-sized fillers [29]. An increment of 35 °C in the crystallization temperature of PHB was observed for as little as 0.1 wt.% INT-WS2. This corresponds to the highest value observed hitherto for PHB formulations using specific nucleating agents (e.g., talc, boron nitride lignin) or nano-sized fillers (e.g., CNTs, graphene oxide) [29].

**IF-WS2**

 **0.0 0.1 0.5 1.0 2.0 4.0**

 **(wt.%)**

**Figure 4.** Room temperature thermal conductivity of iPP and PPS-based laminates as a function of IF-WS2 concentration.

The addition of thermally conductive organic or inorganic nanofillers typically enhances the thermal conductivity (Ȝ) of polymers, which is interesting for applications that require effective dissipation of accumulated heat like connectors or thermal interface materials. It depends on several factors, namely the filler size, aspect ratio, concentration and state of dispersion, the nature, molecular weight and degree of crystallinity of the polymer, as well as the porosity of the material. The room temperature thermal conductivity of iPP- [22] and PPS- [24] based laminates has been measured in the transverse directions, and the results are shown in Figure 4. The incorporation of IF-WS2, which exhibit about twice the thermal conductivity of the neat matrices [47], results in significant Ȝ improvements in the case of iPP/GF laminates, up to 21% at 2.0 wt.% loading, whilst for PPS/CF composites the increments are smaller, about 9% for the same loading. This discrepancy is ascribed to the low thermal conductivity of the GF fabric (~0.05 W m<sup>í</sup>1 Kí<sup>1</sup> ) compared to that of CF (>200 W m<sup>í</sup>1 Kí<sup>1</sup> ). It seems that the CFs play a dominating role in the thermal conductivity properties and mask the effect of the IF-WS2, as can be deduced from the comparison with the results of binary PPS/IF-WS2 nanocomposites [48], where Ȝ rose by up to ~45% upon addition of 2.0 wt.% IF-WS2. However, for iPP-based samples, the improvements in the hierarchical laminates are comparable to those reported for the corresponding binary composites [49], indicating that effect of the nanoparticles predominates. An analogous behavior has been reported for other hierarchical laminates based on thermoplastic polymers, such as PEEK/CNT/GF laminates [50], where Ȝ increased by ~48% at 1.0 wt.% CNT, similarly to the enhancements found in the binary composites [51]. It is worthy to note that for the same nanofiller concentration, the increases in Ȝ upon addition of CNTs are only about double those achieved with the incorporation of the IF-WS2, while much higher differences would be expected considering the extraordinary high thermal conductivity of CNTs.The strong agglomerating tendency of CNTs, the small thermal conductance of the nanotube-polymer interface and the high interfacial thermal resistance between nanotubes within a bundle probably limits the property enhancement, whereas for composites incorporating IF-WS2 the large nanofiller-matrix interfacial contact area and the very homogeneous dispersion lead to experimental Ȝ values even higher than the theoretical predictions.
