**1. Introduction**

Over the past few years, research interest in the field of thermoplastic composites has changed from "high-performance" advanced materials towards the development of "cost-performance" engineering composites. Especially, carbon fiber (CF) or glass fiber (GF) reinforced, thermoplastic-based composites have shown to offer design, processing, performance, and cost advantages compared to metals for manufacturing structural parts. Among the advantages provided by fiber-reinforced thermoplastics over metals and ceramics, that have been recognized for years, are improved fracture toughness, impact resistance, strength to weight ratio, as well as high resistance to corrosion and enhanced thermal and fatigue properties that have often been put in good use for practical applications in the aeronautic, automotive, and energy sectors [1–3]. Nevertheless, these applications require new properties and functionalities, especially superior mechanical performance, flame and chemical resistance, magnetic field and UV resistance, high electrical conductivity, environmental stability, low water absorption, and so forth. To address these issues, the integration of inorganic nanoparticles into a polymer matrix allows both properties from inorganic nanoparticles and polymer to be combined, thus, resulting in advanced polymer nanocomposites (PNCs) [4]. In particular, additional nanoscale fillers, such as carbon nanotubes (CNTs) [5] or inorganic nanoparticles [6], have been mixed with CFs to reinforce polymer matrices. Their high specific surface area enables the formation of a large interphase in the composite and strong filler-matrix interactions. In the same way, the addition of nanoclays to fiber-reinforced thermoplastic composites has been reported to improve damping properties, fatigue life, toughness, and wear resistance [7,8]. The synergetic effect of CFs with the inorganic nanoparticles is believed to be the major cause for the mechanical improvement achieved.

Recently, inorganic fullerenes (IFs) and nanotubes (INTs), based on layered metal dichalcogenides, such as WS2 and MoS2, have emerged as one of the most promising developments in the area of nanomaterials. These types of nanoparticles are currently the subject of intense research, summarized in these reviews that include synthetic methodologies, diverse properties of these new nanomaterials and their potential applications [9,10]. The first synthesis of such nanoparticles was reported by Tenne *et al.*, in 1992 and 1993 [11,12]. Since then, the synthetic technology has advanced considerably and almost pure materials (>99%) are currently synthesized in large amounts by ApNano Materials, Inc. (NanoMaterials, Ltd., Yavne, Israel) and employed in a wide variety of fields, such as aerospace, automotive, naval, defense, medical, energy, electronics, and various other industries. The physical properties of WS2 and MoS2 nanostructures (IF/INTs) have been studied in detail, both experimentally and by theoretical modeling. These properties are interesting, not only academically, but also because these kinds of nanostructures show substantial potential for becoming part of the ultrahigh-strength nanocomposite technology [13].

The objective of this article is to emphasize the most recent findings about the influence of IF nanoparticles and INTs on the structure, morphology and properties of thermoplastic polymer nanocomposites, in comparison with PNCs incorporating other nanofillers. Particular interest has been devoted to analyze the thermal, mechanical, and tribolological property enhancements attained in multiscale fiber-reinforced thermoplastic composites containing inorganic fullerene-like WS2 nanoparticles.

#### **2. Preparation and Dispersion of IF/INT into Thermoplastic Polymers**

The mixing of polymers and nanoparticles is opening new avenues of research and development of advanced engineering flexible composites that exhibit advantageous magnetic, electrical, optical, or mechanical properties. The main challenge in fabrication of these polymer nanocomposites for structural and functional applications is uniform dispersion of nanoparticles in the polymer matrix. However, good dispersion of nanoparticles in polymer composite materials is extremely difficult to achieve since nanoparticles have a strong tendency to aggregate due to their nano-size and high surface energy. In the case of organic–inorganic nanocomposites, the strength or level of interaction between the organic and inorganic phases is another important factor in improving the overall properties of the composites. Physical or simple mechanical mixing usually lead to a weak interaction between the phases via hydrogen bonding or van der Waals forces. In order to minimize interface energies between particles and polymer matrices, several surface modification/functionalization and stabilization techniques have been developed that are mainly used in chemical methods, such as sol-gel, *in situ* polymerization, *etc*. Owing to numerous papers published on polymer organic–inorganic composite materials, it is impossible to completely review this field. The reader is referred to the literature cited for a more detailed description of synthetic methods used for the processing of PNCs reinforced with different types of inorganic nanofillers [13–15].

Inorganic layered materials, such as transition metal dichalcogenides MS2 (M = Mo, W), are one of the most modern and the most promising development areas in the field of nanomaterials. Inorganic fullerene-like (IF) nanoparticles can provide significant advantages over other spherical nanoparticles for the preparation of advanced PNCs [13]. In particular, the incorporation of environmentally-friendly IF-WS2 nanoparticles has been shown to improve thermal, mechanical, and tribological properties of a series of thermoplastic polymers, including isotactic polypropylene (iPP) [16], polyphenylene sulfide (PPS) [17], poly(ether ether ketone) (PEEK) [18], and nylon-6 [19]. The efficient dispersion of IF-WS2 was achieved through simple melt-blending without using modifiers or surfactants. Moreover, the combination of inorganic fullerenes with other organic micro-particles (nucleating agents), micro-fibers (CFs) or nanofillers (CNTs) allows tailoring of more sophisticated hybrid materials with complex architectures, interactions, morphology, and functionality [20–24]. In the same way, the use of INT-WS2 (MoS2) offers the opportunity to produce novel advanced polymer nanocomposite materials with excellent nanoparticle dispersion. More specifically, since the beginning of 2011, we have successfully developed a new family of nanocomposites, which integrated MoS2 nanotubes into an isotactic polypropylene (iPP) matrix, one of the most widely investigated polymers in the preparation and application of nanocomposites, employing a simple and cost effective melt-processing route [25]. This strategy yields finer dispersion, with INT-MoS2 almost fully debundled into individual tubes or small clusters, which are randomly oriented in the iPP matrix. Additionally, well-dispersed WS2 inorganic nanotubes were efficiently incorporated into epoxy matrix, poly(methyl methacrylate) (PMMA), poly(propylene fumarate) (PPF), and poly(3-hydroxybutyrate) (PHB), using various processing techniques [26–29]. Figure 1 shows, as an example, typical SEM images of the fracture surfaces of composites containing inorganic fullerene-like nanoparticles or inorganic nanotubes obtained under optimal processing conditions. It has been demonstrated by statistical analysis of the surface density of IF-WS2 nanoparticles in the iPP nanocomposites, that the degree of dispersion strongly depends on the duration of melt blending [16]. For 1.0 wt.% IF-WS2 (Figure 1a), it can be seen that these nanoparticles are almost spherical, with an average diameter of around 80 nm, similar to that observed for the raw nanofiller, and are individually dispersed for mixing times between 5 and 20 min. However, for IF-WS2 contents 4.0 wt.%, 5 min is not enough time to attain single particle distribution, and for the highest concentration incorporated of 8.0 wt.% (not shown here), the influence of the mixing time on the degree of dispersion is even stronger. With increasing loading, the interparticle distance decreases, hence, flocculation of these nanoparticles can occur after the mixing is stopped. Thus, the crystallization rate, as well as the modulus of iPP, initially rise with increasing filler content and finally level-off at filler loadings of around 1.0 wt.% [16]. In the case of multiscale fiber-reinforced thermoplastic composites, the laminates were prepared by the film-stacking process. Four layers of GF or CF were alternatively stacked within five iPP/IF-WS2 (PPS/IF-WS2) films in a closed mold. Consolidation of the material was made at 210 °C in a hot-press (320 °C in the case of PPS matrix) [22,23]. The results obtained are very promising and suggest that the use of IF/INT can provide an effective balance between cost effectiveness and processability, making the resulting polymer nanocomposites highly suitable for a wide range of applications at a large scale.

**Figure 1.** SEM micrographs of novel polymer/IF(INT) nanocomposites. (**a**) iPP/IF-WS2 (1.0 wt.%); (**b**) PPS/IF-WS2 (1.0 wt.%); (**c**) iPP/INT-MoS2 (1.0 wt.%); (**d**) iPP/IF-WS2 (2.0 wt.%)/GF and (**e**) PPS/IF-WS2 (2.0 wt.%)/CF.
