**4. Mechanical Properties**

The dynamic mechanical properties of the multiscale composites were explored by DMA, technique that provides information about the viscoelastic behavior of the matrix, indicating changes in the stiffness and the relaxation processes that occur as a function of temperature. The influence of the IF-WS2 on the dynamic mechanical behavior of polymer/IF-WS2 nanocomposites has also been investigated [16–18]. In particular, it was observed that the improvements in the storage modulus values of PPS/IF-WS2 nanocomposites are noticeably higher than those achieved in other thermoplastic nanocomposites based on IFs (e.g., iPP, nylon-6, PEEK), suggesting the presence of specific polymer-filler interactions in the case of PPS. The molecular nature of these interactions are still not understood, but they may be associated with the presence of outer S atoms on the IF nanoparticles, and more work is required in order to explain this phenomenon. Figure 5 presents the storage modulus (*E*') and loss tangent (tan į) at the frequency of 1 Hz for PPS- and iPP-based composites incorporating 1.0 and 2.0 wt.% IF-WS2, and the glass transition temperature (*T*g) *vs.* nanoparticle content is shown in the inset of the Figure. Different behavior is also observed depending on the polymer matrix. Regarding PPS/CF laminates, the addition of very low IF-WS2 loadings (*i.e.*, 0.1 wt.%) leads to a slight drop in *E*' (~7% at 25 °C), probably related to the decrease in the crystallinity found for this sample, as revealed by DSC analysis, since the crystalline regions enhance the modulus of semicrystalline polymers. The laminate incorporating 0.5 wt.% IF-WS2 exhibits similar *E*' to that of PPS/CF, since the reinforcement effect of the IF-WS2 should compensate for the slight decrease in crystallinity. In contrast, the incorporation of nanoparticle contents > 0.5 wt.% leads to significant *E*' increments, by up to 22% for 2.0 wt.% nanoparticle content at 25 °C. On the other hand, the gradual addition of IF-WS2 to iPP/GF results in progressive *E*' increases, by about 27% at 2.0 wt.% loading. This behavior is associated with the increase in crystallinity caused by heterogeneous nucleation, combined with an effective reinforcement effect arising from a very homogeneous nanoparticle dispersion. For both types of composites, the reinforcement effect is more pronounced at temperatures below *Tg*, in agreement with the behavior reported for PP/nanoclay/GF composites [30], where significant *E*' enhancements were found at low temperatures whereas the differences in modulus among the samples became insignificant at temperatures above the glass transition.

**Figure 5.** Evolution of the (**a**) storage modulus *E'* and (**b**) tan į as a function of temperature for the neat polymers and some multiscale laminates. The inset shows the glass transition temperature *T*<sup>g</sup> *vs.* IF-WS2 content.

The evolution of tan į (ratio of the loss to storage modulus, a measure of the damping within the system) as a function of temperature (Figure 5b) exhibits an intense peak, named Į relaxation that corresponds to the *Tg*. Further, the iPP/GF laminates show a peak at about 88 °C related to the relaxation of the crystalline phase (Į*c*). In an unfilled system, the polymer chain segments are free from restraints. The incorporation of fillers decreases the free volume and restricts the mobility of the matrix chains, which is reflected in higher *Tg* values (see inset of Figure 5). Once again, different trend is found depending on the nature of the matrix. Thus, in the case of PPS/CF, the incorporation of low IF-WS2 contents ( 0.5 wt.%) led to a downshift in *Tg*, while the addition of higher concentrations resulted in an upshift. As mentioned above, the addition of low nanoparticle loadings slows down the crystallization rate of PPS, leading to the formation of a more amorphous phase that provokes a slight drop in *T*g. However, the incorporation of higher contents has a nucleation effect,

thereby raising the crystallinity of the polymer, which combined with a larger IF-WS2-matrix interfacial contact area results in an effective immobilization of the polymer chains, consequently an increase in *Tg* of up to 18 °C at 2.0 wt.% IF-WS2. In contrast, the *Tg* progressively increases upon addition of these nanoparticles to iPP/GF, the increment being about 6 °C for the same nanoparticle loading. In the same way, the presence of IF-WS2 causes an increase in the crystalline relaxation temperature Į*c* of iPP, since the strong nucleation effect of these nanoparticles accelerates the crystallization of iPP in the nanocomposites.

The magnitude of the tan į peak is indicative of the filler-matrix interactions. For both types of composites, the height of the tan į peak decreases with increasing IF-WS2 content, indicative of a strong nanofiller-matrix interfacial adhesion. Moreover, this reduction probably arises from a synergistic effect between the micro- and nano-fillers on restricting chain mobility, in agreement with the behavior reported for other GF-reinforced hierarchical composites [52]. The incorporation of both reinforcements has a strengthening effect, leading to a lower degree of molecular motion, hence, lower damping characteristics. It also noteworthy that the width of the tan į peak becomes broader with increasing nanoparticle loading, phenomenon that can be interpreted as improved nanofiller-matrix interactions, and is another indication of the larger nanoparticle-matrix interfacial area. The IF-WS2 and microscale fibers disturb the relaxation of the neighbour polymer chains, which would behave differently from those situated in the bulk matrix, resulting in a wider maximum. This behavior was also observed in IF-WS2 reinforced iPP [16] and PEEK [18] nanocomposites, attributed to a more inhomogeneous amorphous phase in the composites in relation to the pure matrix.

The static mechanical properties of iPP and PPS based hybrid laminates have been investigated by tensile tests [22,23], and the Young's modulus (*E*), tensile strength (ıy), elongation at break (İb), and toughness (*T*) as a function of nanofiller loading are plotted in Figure 6. The trends observed are similar to those described previously for the storage modulus. *E* and *ıy* rise progressive with increasing nanoparticle loading in the case of iPP/GF composites, while they decrease slightly at low loadings and then grow in PPS/CF laminates, behavior that is directly related to the crystallinity of the samples, as discussed previously. Interestingly, both parameters only rise marginally upon addition of the IF-WS2, the maximum increments being ~14% and 11% at 2.0 wt.% nanoparticle content, respectively, in the case of PPS/CF, and even smaller for iPP/GF composites (Figure 6). However, considerably larger increases were observed for the binary iPP/IF-WS2 nanocomposites [49], where *E* and ıy improved by around 42 and 31%, respectively, for the indicated loading. For multiscale composites, it is expected that the nanofillers predominantly influence the properties that are matrix-dominated; consequently, only small increases are observed in the Young's modulus and tensile strength of the hybrids, since the tensile properties are more fiber-dominated. These results are consistent with the behavior reported for other thermoplastic-based hybrids [53], where *E* and ı<sup>y</sup> of the fiber reinforced polymer only improved marginally upon incorporation of the nanoscale fillers due to the dominating role of the fibers.

With regard to the strain at break (İb), the trend found is very similar for both composite series. A moderate increase is found at low nanoparticle loadings, followed by a sharp reduction at higher concentrations. This indicates that higher amounts of IF-WS2 hinder the ductile flow of the matrix. This tendency is in contrast to that typically reported for CNT-reinforced multiscale laminates [53], where İb systematically decreases upon addition of the carbon nanofillers, attributed to the presence of aggregates that produces stress concentrations at the filler-matrix interface, leading to premature failure. Similarly, Rahman *et al.* [30] found around 50% reduction in tensile strain upon incorporation of 6.0 wt.% nanoclay to PP/GF (30 wt.%), also ascribed to the poor nanoclay dispersion that strongly limits the plastic deformation of the matrix. The surprising behavior observed for the composites filled with IF-WS2 is probably related to the lubricant character and more uniform dispersion of these inorganic nanoparticles combined with their spherical shape that reduce the stress concentration sites, thereby improving the matrix ductility. However, for IF-WS2 concentrations higher than 1.0 wt.%, a stiff hybrid network of micro- and nano-fillers could be formed that acts very effectively as a barrier for the mobility of the polymer chains, thus limiting the ductile deformation. A qualitatively similar behavior is found for the toughness, measured as the area under the tensile curve, that increases considerably at low IF-WS2 loadings (*i.e.*, by 35% at 0.1 wt.% content compared to iPP/GF) while drops moderately at concentrations higher than 1.0 wt.% (around 20% decrease at 2.0 wt.% loading compared to PPS/CF). The small aggregates contribute to increase the brittleness under high strain rates, since they nucleate secondary cracks and favour the formation of dimples.

**Figure 6.** (**a**) Young's modulus (*E*), (**b**) tensile strength (ıy), (**c**) elongation at break (İb) and (**d**) toughness (*T*) as a function of IF-WS2 loading. Solid and open symbols correspond to PPS/IF-WS2/CF and iPP/IF-WS2/GF systems, respectively.

The influence of the IF-WS2 on the flexural properties of iPP/GF and PPS/CF has also been investigated [22,24]. In this case, maximum increments in the flexural modulus Ef and flexural strength ıfM of iPP/GF up to 26 and 22%, respectively, have been attained at 2.0 wt.% loading. Similarly, enhancements of 25 and 15% have been found in PPS/CF composites for the same nanofiller loading. The comparison of the results with those obtained for the corresponding binary nanocomposites [22,24] reveals a synergistic effect of both fillers on enhancing the flexural properties of the matrix.

**Table 2.** Comparison of the increment in static mechanical properties (in %) for different polypropylene (PP) and polyphenylene sulfide-(PPS) based hierarchical laminates. MWCNT: multi-walled carbon nanotubes; MMT: montmorillonite; Woll: Wollastonite; *E*: Young's modulus; ıy: tensile strength at yield; *G*: impact strength; *E*f: flexural modulus; ıfM: flexural strength.


Table 2 compares the improvements in static mechanical properties reported for various PP and PPS-based hierarchical composites [30,46,54–57]. Clearly, the highest improvements are attained upon addition of multi-walled carbon nanotubes (MWCNTs) to fiber-reinforced PP composites [54], which is reasonable taking into account the very high modulus of these carbon nanofillers. Nevertheless, among the various inorganic fillers, IF-WS2 lead to larger stiffness and strength improvements than montmorillonite [30], wollastonite [55], or nanosilica [46], and comparable to those of CaCO3 [56,57].

In the same way, the incorporation of INTs can also lead to improvement in the mechanical properties of polymer/INTs [25,27]. As an example, the characteristic mechanical data (e.g., Young's modulus, *E*, tensile strength, ıy and strain at yield, İy) for the PP nanocomposites incorporating

nanoreinforcing fillers with different morphologies are summarized in Table 1 [31–42]. It can be observed that the addition of INT-MoS2 progressively enhances the Young's modulus of the matrix, with increments of 15, 28, and 40% for loading fractions of 0.1, 0.5, and 1.0 wt.%, respectively. The improved *E* obtained in this work is ascribed to the very uniform dispersion of the INT-MoS2 and their high aspect ratio, which results in larger nanofiller-polymer interfacial area. Qualitatively similar trends were found for the tensile strength, where the increments were around 13, 34, and 41% for the abovementioned nanofiller contents. On the other hand, the incorporation of the inorganic nanotubes leads to a slight decrease in İy. This is a typical behavior of nanofiller-reinforced polymer composites, since the nanofillers restrict the ductile flow of the matrix, and is in agreement with the results reported by Lopez-Gaxiola *et al*. [58] for carbon filler-reinforced PP composites. Table 1 also shows the percentage variations in the mechanical properties of iPP nanocomposites containing similar amounts (a1.0 wt.%) of various nanofillers. Remarkable improvements in the mechanical properties are observed for iPP/INT-MoS2, where the non-modified nanofillers were dispersed uniformly in the iPP matrix for all the compositions prepared [31]. The magnitude of increase in the modulus and strength is similar to that obtained for IF-WS2 nanoparticles [42] and far exceeds that reported for both modified HNTs [34] and CNTs [37]. However, silicon nitrides clearly provide the best reinforcement for PP matrix, which has been related to the alignment and exfoliation of rod-shaped Si3N4 particles [38]. These phenomena were also mainly responsible for the 95% enhancement in the tensile strength and 152% increase in the tensile modulus of PP using p-aminobenzoic acid modified-clay with PP-g-MA as a compatibilizer [40]. On the other hand, Reddy *et al*. have reported that the high rigidity of INT-WS2 and the effective load transfer from the matrix to the INT-WS2 were responsible for the improved mechanical properties of PMMA/INT-WS2 nanocomposites [27]. In particular, it was observed that the elastic modulus of PMMA fiber meshes was increased by 10 and 22 times upon incorporation along the fiber axis of 1.0 and 2.0 wt.% INT-WS2, respectively. Analogously, the tensile strength of the composite fibers increased by 35 and 32% for the indicated nanoparticle loadings. However, the toughness of the sample with 2.0 wt.% INT-WS2 was lower than that of the neat PMMA fiber, since nanofiller aggregation started to take place. Overall, experimental results point out the advantages of using these environmentally friendly and cheap inorganic fullerenes and nanotubes instead of conventional nanoparticles for improving the mechanical performance of thermoplastic composites.
