Synergy of Hybrid Fillers for Emerging Composite and Nanocomposite Materials—A Review

Nanocomposites with polymer matrix provide tremendous opportunities to investigate new functions beyond those of traditional materials. The global community is gradually tending toward the use of composite and nanocomposite materials. This review is aimed at reporting the recent developments and understanding revolving around hybridizing fillers for composite materials. The influence of various analyses, characterizations, and mechanical properties of the hybrid filler are considered. The introduction of hybrid fillers to polymer matrices enhances the macro and micro properties of the composites and nanocomposites resulting from the synergistic interactions between the hybrid fillers and the polymers. In this review, the synergistic impact of using hybrid fillers in the production of developing composite and nanocomposite materials is highlighted. The use of hybrid fillers offers a viable way to improve the mechanical, thermal, and electrical properties of these sophisticated materials. This study explains the many tactics and methodologies used to install hybrid fillers into composite and nanocomposite matrices by conducting a thorough analysis of recent research. Furthermore, the synergistic interactions of several types of fillers, including organic–inorganic, nano–micro, and bio-based fillers, are fully investigated. The performance benefits obtained from the synergistic combination of various fillers are examined, as well as their prospective applications in a variety of disciplines. Furthermore, the difficulties and opportunities related to the use of hybrid fillers are critically reviewed, presenting perspectives on future research paths in this rapidly expanding area of materials science.


Introduction to Composites and Nanocomposites
The attention of material scientists has been drawn to composite materials since the middle of the last century.Composites were first used as far back as 1500 B.C. when early Egyptians and Mesopotamian settlers used a mixture of mud and straw to create strong and durable buildings, pottery, and boats.Around 25 B.C., composite materials were described by the Ten Books on Architecture on concrete, which distinguished them by various types of lime and mortars [1].Then, in 1200 A.D., Mongols invented the first composite bows made from a combination of wood, bamboo, bone, cattle tendons, horns, and silk, which were bonded with natural pine resin.Thereafter, from the 1800s through the 1900s to the 2000s, composite materials advanced through the chemical revolution with the development of new types of resins and are now used for different manufacturing innovations, such as filament winding.They have also become more common in mainstream manufacturing and construction industries because of their low cost and effective replacement of traditional materials like metal and engineered thermoplastics.Composite and nanocomposite materials have received a lot of interest in recent years because of their outstanding qualities and widespread use in a variety of industries.Composite materials have gained significant attention in research and attracted grants from governments, manufacturers, and universities because of the increasing demand for high-performance, smart, and quality materials.This has also led to the development of new hybrid composites [1][2][3].Composite materials comprise mainly two phases because they are heterogeneous (multiphase) in nature: the reinforcement and matrix, which offer improved performance compared to the isolated components.One significant strategy for improving the effectiveness of these materials is to incorporate fillers into a matrix material.Fillers influence the mechanical (e.g., improve the tensile, flexural, compressive, hardness, etc., strength of composite and nanocomposite materials), thermal (e.g., fillers improve the thermal stability of the composite and nanocomposite materials), and electrical properties (e.g., the addition of fillers to the composite and nanocomposite materials improves the electrical conductivity) of composites and nanocomposites.These fillers serve as the reinforcement, while the resins serve as the matrix material.Fillers improve the mechanical, thermal, and electrical properties of the composites and nanocomposites because they increase the surface adhesion of and interphase interaction between the filler and the matrix.The structure and properties of the composite material change and differ from the primary parent materials because of the chemical interactions and interphase changes that take place during mixing.
Though traditional fillers have been widely employed, the inclusion of hybrid fillers offers a promising way to further improve material qualities through synergistic interactions.Hybrid fillers mix selected organic and inorganic molecules to create innovative materials with their greatest qualities, which leads to synergic combinations with distinct properties and increased performance in a wide range of applications such as remotely operated vehicles (ROVs) in marine industries, vehicle parts in the automobile industry, aircraft wings in aerospace industries, etc.Also, a number of unique benefits of hybrid fillers over a single filler in composite and nanocomposite materials are balanced strength and stiffness, balanced bending and membrane mechanical properties, balanced thermal distortion stability, reduced weight and cost, improved fatigue resistance, reduced notch sensitivity, and improved fracture toughness.Composite materials have received a lot of attention for their practical and structural usage in sensors, actuators, micro-electro-mechanical components, aerospace, and automotive industries [3,4].Polymer nanocomposites became prominent through a group of researchers from Toyota in the early 1990s, who advanced the chemical development of plastic materials such as vinyl, polystyrene, phenolic, and polyester.These materials were used as reinforcement to provide strength and rigidity to the composite materials [5].
Nanocomposites are heterogeneous (multiphase) polymers made of a minimum of one nano-scale phase (known as nanofiller material) that is dispersed in a second phase (known as the matrix), combining the unique qualities of its parts [5].Carbon nanomaterials have been introduced as fillers to polymer nanocomposites, which has enhanced their property improvement.Carbon nanomaterials have exceptional features; the electrical, thermal, and mechanical properties of these extraordinary nanomaterials are attributed to their unique nanostructure, which has a high surface-to-volume ratio and strong surface contacts.As a result of their significant interactions, carbon nanomaterials tend to cluster into bundles and ropes.This improvement is owed mostly to the superior intrinsic characteristics of the carbon nanomaterials [5].However, creating composites with strong electrical conductivity and good mechanical characteristics with little filler content has been a significant problem because delamination will occur when they are heated, and they can easily wear out during friction as a result of inadequate reinforcement.However, recyclable polymeric materials are regarded as a promising alternative to non-biodegradable plastics.Poly(lactic acid) (PLA), one of the most frequently used recyclable polymeric materials, is a nontoxic and environmentally friendly polymer with high strength and good processing capacity, biocompatibility, and biodegradability [6][7][8][9].Composite materials reinforced with natural fibers have been considered as an alternative to synthetic fiber because of their easy degradability, cost-effectiveness, and reusability in transportation applications such as wind turbine blades, prosthetics, smart memory devices, ship structures, bridge construction, automobiles, railway coaches, aerospace, and structural applications.Polymer nanocomposites outperform traditional micro-composites in terms of mechanical capabilities (good strength), thermal and dimensional reliability (good thermal stability), fire and chemical resistance, and optical and electrical qualities, to name just a few examples.Polymer nanocomposites containing inorganic fillers have generated significant interest due to their different properties and numerous uses in modern technology in areas like packaging, electromagnetic shielding, transportation, defense systems, electrical sensors, and the information industry.The features of polymer nanocomposites are primarily a straightforward mix of integrated inorganic nanoparticles and the polymeric matrix.Polymeric materials serve as matrices in nanocomposite materials because of their excellent thermal stability, environmental resistance (durability), and mechanical, chemical, and electrical properties [10][11][12].
Foam materials such as microcellular foams have been manufactured from a combination of fillers and polymer matrices.These foams have notable advantages such as lightness, high surface area, reliable manufacturing, optimal flexibility under various environmental circumstances, and low cost [13].Because of the introduction of cellular structures, foam materials are useful for developing and improving strain or pressure sensitivity as well as wave interferences.Foaming procedures involve saturating or impregnating polymers with a foaming agent, creating a supersaturated polymer-gas mixture through temperature or pressure changes, and promoting cell growth and stability.In thermoplastic foaming procedures, it is critical to produce foams with a closed-cell structure and thin polymer cell walls covering each cell.To achieve this structure, cell development must be controlled using this method.Polymer-based nanocomposite foams with carbonic fillers have considerably contributed to academic investigations into electromagnetic interference (EMI) shielding and piezoresistive sensing systems.The use of hybrid fillers has contributed to an increase in the shielding effectiveness of EMI in composite and nanocomposite materials [13].
Polymer-based dielectrics are widely used to insulate high-voltage systems such as power transmission cables, motors, transformers, and pulse generators.They are widely used due to their high breakdown strength (EBD), ease of manufacture, and mechanical flexibility at low density.Future-oriented insulation dielectrics should have increased thermal conductivity, thermal stability, and resistance to electrical monitoring, weathering, and fatigue, in addition to outstanding insulating performance (high dielectric strength and EBD, and low dielectric loss) [14][15][16][17].This reduced EBD can be related to a permeability disparity between fillers and polymers, leading to strong amplification and sharp deformation in local electric fields, as well as dielectrically weak connections (i.e., more breakdown initiation sites, greater numbers of space charges, or lower-energy pathways for electrical failure), especially when micron-sized or high filler loadings are used.
A study by Keramati et al. [3] presented a micromechanical investigation of a hybrid smart nanocomposite in which continuous BaTiO 3 fibers are implanted into the graphene nanosheet (GNS) filler with epoxy matrix.A Mori-Tanaka model was utilized in a multi-step process to estimate the thermal expansion, elastic stiffness, and piezoelectric constants of BaTiO 3 fiber/graphene nanocomposites.The authors further examined some important microstructures like percentage, piezoelectric fiber volume fraction, interfacial region characteristics, agglomeration, and alignment of GNSs as performed by the micromechanical model.They observed that (i) the addition of the polymer matrix with uniform dispersed GNSs is a commonly utilized practice to improve the elastic stiffness, piezoelectric, and thermal expansion coefficients of piezoelectric fiber-reinforced hybrid nanocomposites, (ii) thick interphase with a higher stiffness than the polymer matrix is necessary for improving the structural and functional properties of hybrid smart nanocomposites, (iii) increasing the BaTiO 3 fiber volume percentage can improve the thermo-electromechanical performance of hybrid smart nanocomposites, etc.
Kuang et al. [6] described an example of mixed poly(lactic acid) PLA with one-dimensional (1D) carbon nanotubes.The CNTs form a structure that conducts electricity in the polymer matrix, improving the composite's electrical conductivity.This was achieved by mixing PLA with a large amount of CNTs and mechanically mixing it with CB.The mixture was segregated Olhan and Behera [18] investigated the improvement in the mechanical, thermal, and viscoelastic properties of textile structure-based nanocomposite components employing glass and basalt fibers packed with graphene nanoplatelets (GNPs) using vacuum-assisted resin transfer molding (VARTM) techniques.The GNP filler increased the composite tensile, impact, and flexural strength by 15%, 34%, and 26%, respectively.
Wu et al. [19] studied the effect of hybridized graphene/carbon nanofiber wax colloid for paper-based electronics and thermal control on electrically and thermally permeable surface nanocomposites that are eco-friendly in the electronics sector.The carnauba wax was emulsified in isopropyl alcohol.This nanocomposite is regarded as lightweight, flexible, and environmentally friendly.They reported that the flexible and electrically and thermally conductive materials are fabricated by functionalizing paper with nanocarbon-conducive inks.The resulting percolation of the hybrid samples was observed to be of lowered value compared with the pure GNP composites because of the increase in the filler aspect ratio.However, the hybrid samples demonstrated higher bending and folding stability compared with the pure GNP composites.
The present paper seeks to provide a complete overview of the synergistic effects of hybrid fillers and their advantages over single fillers in emerging composite and nanocomposite materials, as well as their applications, emphasizing recent advances since 2018 and future prospects in the field.The next section highlights the major fillers used.
indicated that the main typical band spectrum of absorption is between 500 and 4000 cm −1 for each of the components and the hybrid without any new peak or considerable shift in peak position.From the report by Kuang et al. [6], the electrical conductivity of PLA/xC composite changes curves with varying CNT content (Figure 2).As the CNT concentration rises from 0 (PLA/0C) to 0.592 vol% (PLA/IC), the electrical conductivity of PLA/xC composite improves dramatically and subsequently stabilizes with an increase in CNT content.
The use of GNP as filler in nanocomposite materials has shown great improvement in thermal conductivity compared to other nanofillers due to their capacity to limit the movement of polymer chains [18].It was also discovered that enhancing the durability of the matrix-NH2-GNP interfaces by generating covalent bonds increased the optimum stress transmission of the GNP nanoparticles and prevented the fiber pull-out room in the reinforced nanocomposites.Meanwhile, the triboelectric-hybridized piezoelectric composite device made of micro-patterned PDMS has a far higher voltage of =250 V and an output power of =0.5 mW, which had been used to power 300 light-emitting diodes.The results demonstrated that a novel class of paper-based and lead-free energy harvesting devices presents a great prospect for increasing the usefulness and capabilities of highpower predators in versatile portable electronics such as sensors and medical equipment [55].In the next section, various hybrid fillers and their synergy effects are discussed.The use of GNP as filler in nanocomposite materials has shown great improvement in thermal conductivity compared to other nanofillers due to their capacity to limit the movement of polymer chains [18].It was also discovered that enhancing the durability of the matrix-NH 2 -GNP interfaces by generating covalent bonds increased the optimum stress transmission of the GNP nanoparticles and prevented the fiber pull-out room in the reinforced nanocomposites.Meanwhile, the triboelectric-hybridized piezoelectric composite device made of micro-patterned PDMS has a far higher voltage of =250 V and an output power of =0.5 mW, which had been used to power 300 light-emitting diodes.The results demonstrated that a novel class of paper-based and lead-free energy harvesting devices presents a great prospect for increasing the usefulness and capabilities of high-power predators in versatile portable electronics such as sensors and medical equipment [55].In the next section, various hybrid fillers and their synergy effects are discussed.

Hybrid Filler Composites and Synergy Effect
Hybrid fillers have been used to improve the filler interfaces and strength performance of multifiller polymer composites by increasing their high field endurance.Dualfiller arrangements in composites are predicted to maintain the property advantages imparted by microfillers while also improving electric field endurance by shielding the dielectrically weak interfaces.In polymer composites, symbiotic phenomena caused by the mixture of different fillers, known as the synergistic effect, can significantly improve heat conductivity [115].Multiple fillers can be used to combine the benefits of all fillers into a single composite.Hybrid filler structures are tailored to multifiller polymer composites and nanocomposites in order to improve the filler interfaces.The CaCO3/MMT layered hybrid fillers are expected to maintain the property benefits imparted by the microfiller while also enhancing electric field endurance by shielding the dielectrically weak interfaces [14].The MMT layered nanosilicates were chosen because they have a high aspect ratio (pseudo-2D), excellent electrical insulating and barrier characteristics, and have been demonstrated to improve the breakdown strength of composite and nanocomposite materials.The summary of several hybrid fillers, along with their characteristics and properties, is presented in Table 2.

Matrix
Hybrid Fillers Characterizations Contributions Ref.

Polyolefins CaCO3/MMT TEM, SEM, XRD
Positive relationships between the polar particles and the negatively charged silicates of the hybrid fillers.

Hybrid Filler Composites and Synergy Effect
Hybrid fillers have been used to improve the filler interfaces and strength performance of multifiller polymer composites by increasing their high field endurance.Dual-filler arrangements in composites are predicted to maintain the property advantages imparted by microfillers while also improving electric field endurance by shielding the dielectrically weak interfaces.In polymer composites, symbiotic phenomena caused by the mixture of different fillers, known as the synergistic effect, can significantly improve heat conductivity [115].Multiple fillers can be used to combine the benefits of all fillers into a single composite.Hybrid filler structures are tailored to multifiller polymer composites and nanocomposites in order to improve the filler interfaces.The CaCO 3 /MMT layered hybrid fillers are expected to maintain the property benefits imparted by the microfiller while also enhancing electric field endurance by shielding the dielectrically weak interfaces [14].The MMT layered nanosilicates were chosen because they have a high aspect ratio (pseudo-2D), excellent electrical insulating and barrier characteristics, and have been demonstrated to improve the breakdown strength of composite and nanocomposite materials.The summary of several hybrid fillers, along with their characteristics and properties, is presented in Table 2.The DSC measurements show the temperature that occurred while the emulsion was transparent and yellowish.TGA provides an extra heating treatment that stabilizes the coating adherence of the paper.
The SEM study indicates that the morphology of pure paper substrates ranges from 10 to 100 µm. [19] Ecoflex matrix CNT/CB TEM, SEM Good homogeneous blending of hybrid carbon and polymer resulting in high electrical percolation of the nanocomposites.[114] Polyimide (PI) BNNSs/rGO SEM, FTIR, XPS, Raman Spectroscopy The SEM reveals the cross-section of the hybrid BNNS/rGO/PI films.FTIR and XPS were used to identify the functional groups that existed in the hybrid fillers. [16] Polyetherimide (PEI) BNNSs/TiO 2 SEM, TEM, HRTEM, XRD, FTRI The SEM and TEM indicated the successful preparation of the composite materials.The microstructure of the 2D hybrid structure BNNSs-TiO 2 was analyzed. [15] Polyvinyl alcohol (PVA) NCC/GO XRD, BET, FTIR, TEM, WCA, SEM, DSC XRD and FTIR show the major characteristic peaks of the hybrid filler.[65] Epoxy resin CNT/CB IS, FTT, SEM The conductive filler contributed to the high dielectric of epoxy matrix and the electrical impedance decrease in the entire system.Increased K ic values by 40% compared with the neat epoxy.The residual carbon rate of the hybrid filler increased to 83.81% after the temperature was raised to 800 • C. [57] Polypropylene (PP) oMMT/BN SEM, TEM, XRD Increased rotation speed from 150 rpm to 500 rpm improves the dispersion of the hybrid fillers.
[36] Polytrimethylene terephthalate (PTT) GO, f-MWCNT RS, SEM, FTIR, DSC, DMA The RS indicated an overlapped band with a band attributed to the polymer matrix, the D band, located at 1386 cm −1 . [43] Polyvinyl alcohol (PVA) NCF/GO SEM, FTIR, ZP, TGA The SEM revealed a smoother surface, which shows a little agglomeration in the composite.This shows a good interaction between the functionality groups of GO and NCF. [66] Silicone rubber (QM) FSiO 2 /rGO CF, Ws, FTIR, XRD, SEM, DMA Improved DMA properties while coefficient of friction (CF) and wear rate (Ws) were lowered due to better dispersion of the hybrid fillers. [58] Polystyrene MWCNT/sN-LDH XRD, FTIR, RS, The XRD pattern combines the involvement of all segments, the intensity of the relevant phase, and their peaks.The RS shows that the spectra have a low concentration in the D-bands of the fillers, preventing their high-intensity expression. [45] Poly(vinylidene fluoride-hexafluoropropylene) (PHP) BT/MWCNT SEM, FTIR, XRD SEM analysis shows compatibilization of the structure by adding the hybrid fillers, while FTIR and XRD results depicted the increase in β/α ratio of the nanocomposites.[46] Natural rubber (NR) GO/SiO 2 ; GR/SiO 2 SEM, TEM The SEM morphology shows that the surface of GO/SiO 2 is more wrinkled and crumpled compared to GR/SiO 2 [68] Polymers 2024, 16,1907  The dynamic analysis performed for α-relaxation revealed a Vogel-Fulcher-Tammann dependency on temperature. [80] Poly(methyl methacrylate) (PMMA) rGO/Ag XRD, FTIR, SEM, TEM, EDX, RS, Raman analysis prepared for the samples shows the presence of the characteristic D and G bands of the graphene-related materials.The D band is at 1343 cm −1 and G band at 1573 cm −1 . [59] Poly(vinylidene fluoride) (PVDF) NiO-BaTiO 3 XRD, FTIR The incorporation of the hybrid fillers in the nanocomposites leads to the formation of long-stabilized planar zigzag and all-trans conformations, while also inducing growth in the electroactive β-phase and enhancing crystallinity. [104] Linear low-density polyethylene (LLDPE) GO/HAP; CNT/HAP FTIR, SEM, IR, XRD The structural modification of the nanomaterial at lower loading revealed a distinction between CNT-HAP-and GO-HAP-based composites; however, with greater loading, the difference in impact strength increase was nearly identical for both types of composites.This revealed that the robust HAP-coated/modified CNT and GO can boost the impact strength due to the adhesion and involvement of the LLDPE chain with the nanoparticles. [27] Li et al. [14] stated that the dual-fillers and their distribution within the polymer matrix, as observed by transmission electron microscopy, show the local dispersion (Figure 3a-d) of organo-montrorillonite and calcium carbonate (oMMT and CaCO 3 ), as well as the X-ray diffraction (XRD; Figure 3e).This was achieved by modifying the composite morphology (by engineering the pseudo-two-dimensional nanoclays to preferred physisorb on the surfaces of calcium carbonates, hence changing the nature of the filler/polymer interfaces).According to Ribeiro et al. [37], a facile method for the preparation of MoS 2 /h-BN hybrid nanosheets by direct liquid-phase exfoliation with sonication was used, and it effectively produced few-layer hybrid structures.The SEM and TEM images for the MoS 2 /h-BN hybrid material are shown in Figures 4a and 4b, respectively.Figure 4c shows the HRTEM image of the MoS 2 /h-BN hybrid material's few-layer flakes, which correspond to the selected area in Figure 4d used for the atomic resolution.Figure 4e shows thin MoS 2 /h-BN flakes, with a thickness corresponding to three to five layers, which is an indication of the exfoliation process's success.The homogeneity of the MoS 2 /h-BN hybrid material, as indicated by EDS, is shown in Figure 4f-i.GNP-CNF hybrid fillers emulsified in isopropyl alcohol produced a flexible and multifunctional nanocarbon-coated paper that is a promising electrical conductor and thermally dissipative material and, at the same time, can increase the environmental sustainability of the electronics sector.From Figure 5, the schematic in Figure 5a illustrates the conductive ink production, while Figure 5b shows the SEM analysis of pure paper substrates.These substrates range from 10 to 100 µm thick and consist of cellulose fibers with sizing agents that optimize the printability and water resistance of the fibrous network.Figure 5c-g show that after spraying the GNPs and hybrid inks, the paper surface appears to be carpeted with randomly oriented GNPs or GNPs-CNFs webs [19].The internal structure of composites, as described by Song et al. [114], was found to influence the physical properties of the materials.This is an indication that a well-interconnected filler structure is essential for the composite to have a good electrical percolation property, with the microstructures that were analyzed by calculating the surface area   The internal structure of composites, as described by Song et al. [114], was found to influence the physical properties of the materials.This is an indication that a well-interconnected filler structure is essential for the composite to have a good electrical percolation property, with the microstructures that were analyzed by calculating the surface area The internal structure of composites, as described by Song et al. [114], was found to influence the physical properties of the materials.This is an indication that a wellinterconnected filler structure is essential for the composite to have a good electrical percolation property, with the microstructures that were analyzed by calculating the surface area per volume of the filler clusters.From Figure 6a-i, as reported by Guo et al. [16], the characterization reveals that the resulting rGO-PI/BNNS-PI hybrid structure remained strongly aligned with waviness in composite walls due to the insertion of flexible rGO sheets.The hybrid aerogel was hot pressed transversely to produce a dense and lay-ered rGO-PI/BNNS-PI nanocomposite film (Figure 6e,f).The SEM images of the crosssections of BNNS-PI films show that BNNS alignments are also retained following compression (Figure 6g,h).Furthermore, the Raman spectrum of rGO-PI/BNNS-PI nanocomposites supported and revealed the presence of rGO and BNNS through their respective features (Figure 6i).per volume of the filler clusters.From Figure 6a-i, as reported by Guo et al. [16], the characterization reveals that the resulting rGO-PI/BNNS-PI hybrid structure remained strongly aligned with waviness in composite walls due to the insertion of flexible rGO sheets.The hybrid aerogel was hot pressed transversely to produce a dense and layered rGO-PI/BNNS-PI nanocomposite film (Figure 6e,f).The SEM images of the cross-sections of BNNS-PI films show that BNNS alignments are also retained following compression (Figure 6g,h).Furthermore, the Raman spectrum of rGO-PI/BNNS-PI nanocomposites supported and revealed the presence of rGO and BNNS through their respective features (Figure 6i).According to the research of Chen et al., the FTIR spectra of PVA and its nanocomposites exhibit a large peak at 3600-3000 cm −1 in Figure 7a and the distinctive stretching vibration of the -OH group [65].The XRD patterns, as illustrated in Figure 7b, show the major peak of GO nanosheets at 2O = 9.43o, with equivalent graphite interlayer as 0.923 nm, which is proof that the graphite is oxidized by incorporation of oxygen-containing groups, while the XRD pattern of NCC indicates that the major magnitude peak is around 22.6o, which is related to the crystalline structure of cellulose [65].Figure 7c shows that NCC has a rod shape, the clusters are reasonably strong, and the PVA/NCC/GO composite can effectively separate on the surface of GO [65]. Figure 7d depicts the UV-vis (250-850 nm) transmittance spectra of the produced films.The spectra show that PVA/NCC/ GO films have greater transmittances of 85% and 69% at 500 nm [65].Figure 7e also shows the PVA/NCC/GO's water contact angle readings and water droplet photos.It demonstrates that the hydrophilicity of PVA is extremely high, which can be affected by the filler qualities.Figure 7f depicts the cryofracture morphologies of PVA-based films, indicating the existence of rod-shaped clusters at high NCC and GO levels [65].In the next section, the effect of different polymers on hybrid fillers is discussed.According to the research of Chen et al., the FTIR spectra of PVA and its nanocomposites exhibit a large peak at 3600-3000 cm−1 in Figure 7a and the distinctive stretching vibration of the -OH group [65].The XRD patterns, as illustrated in Figure 7b, show the major peak of GO nanosheets at 2O = 9.43o, with equivalent graphite interlayer as 0.923 nm, which is proof that the graphite is oxidized by incorporation of oxygen-containing groups, while the XRD pattern of NCC indicates that the major magnitude peak is around 22.6o, which is related to the crystalline structure of cellulose [65].Figure 7c shows that NCC has a rod shape, the clusters are reasonably strong, and the PVA/NCC/GO composite can effectively separate on the surface of GO [65]. Figure 7d depicts the UV-vis (250-850 nm) transmittance spectra of the produced films.The spectra show that PVA/NCC/GO films have greater transmittances of 85% and 69% at 500 nm [65].Figure 7e also shows the PVA/NCC/GO's water contact angle readings and water droplet photos.It demonstrates that the hydrophilicity of PVA is extremely high, which can be affected by the filler qualities.Figure 7f depicts the cryofracture morphologies of PVA-based films, indicating the existence of rod-shaped clusters at high NCC and GO levels [65].In the next section, the effect of different polymers on hybrid fillers is discussed.

Hybrid Fillers and Polymer Composites
Different polymer matrices have been employed in the fabrication of biocomposi and nanocomposite materials.Polymer nanocomposites containing 2D nanofillers are pr marily suitable for achieving outstanding temperature control capacities and unique di lectric characteristics.They also have superior capacitive energy densities, higher therm stabilities, and higher mechanical strength than the pristine polymers and nanocomp sites based on 0D or 1D nanomaterials, thereby rendering them excellent for high-energ
Other applications of fillers in composite and nanocomposite materials include improving electrical breakdown strength for high voltage dielectric materials [14][15][16], being used as surfactants and compatibilizers to avoid adverse effects on composite mechanical properties [34], improving material thermal stability and linear coefficient of thermal expansion [51], enhancing viscoelastic properties in automotive components [18], making paper-based films for flexible power generators [55], enhancing the sustainability of the electronic sector [19], producing electrodes that are usable in highly stretchable aqueous Li-Ion batteries [114], various packaging applications [7], structural and functional applications [95,117], energy storage capacity [80], gas sensing and power harvesting [23], energy generation devices [123], being used in power-efficient nonvolatile resistive memory materials [62], and providing friction resistance and flame retardancy [124].Remarkable thermal conductivity, exceptional electrical insulation capabilities, and minimal thermal expansion coefficients of polymer composite and nanocomposite materials are also achievable using hexagonal boron nitride (h-BN).But the inherent anisotropy in its thermal conductivity poses a challenge because it restricts the uniformity of multi-directional heat transfer and dissipation.However, the introduction of isotropic thermal conductivity in thermal management has effectively addressed this challenge [125].

Summary and Recommendations
This review paper discusses current attempts to improve the mechanical properties, thermal conductivity, dielectric properties, and electrical conductivity of polymer-based nanocomposites.The inclusion of single fillers like Al 2 O 3 , SnO 2 , TSP, CNT, GNP, GNS, metal nanoparticles, and hybrid fillers such as CaCO 3 /MMT, MoS 2 /hBN, rGO/MoS 2 , rGO/Fe 3 O 4 , BNNS/rGO, CNT/CB, PUCNTs/CCTO, oMMT/BN, FSiO 2 /rGO, MWCNT/ sN-LDH, and other conductive fillers improves the electrical conductivity, consistent stiffness, and coefficients of thermal expansion of composite and nanocomposite materials.These improvements are better than those achieved with single fillers alone, as expected due to the combination rules.Also, conductive hybrid fillers contribute to improved electrical breakdown strength (E BD ), bending stability, regulation of local electric field strength, interfacial polarization, enhancement in energy storage properties, and the electrical pathway for filler interconnections in composite and nanocomposite materials, more effectively than single fillers.
Another new development aimed at further improvement is the use of several hybrid nanofillers rather than single ones, which has the potential to achieve synergy in electrical conductivity.The current literature examines the effects of the permeation threshold, interface, hopping, and tunneling mechanisms.Given the conflicting findings, different research institutions are actively working to gain an improved comprehension of why synergy takes place when multiple hybrid fillers are employed.
Applications that could benefit from synergistic electrically conductive hybrid nanocomposite technology include supercapacitors, flexible conductive films, energy storage, EMI shielding, fuel cells, piezoelectric composites, lightning mitigation, and sensing.
However, there is a gap in hybridizing hollow glass microspheres (HGMs) with other fillers, such as CNT, CB, NC, GO, etc., for potential applications in electrical appliances, sensors, and structural purposes.This would be an interesting area of research for future consideration.

Figure 1 .
Figure 1.(a,d) shows TEM images of MoS2 and h -BN specimens; (b,e) show HRTEM images of MoS2 and h -BN;(c,f) show the atomic layer of HRTEM for the insert of (b,e) Adapted with permission[37].2019 ACS Applied Material Interface.

Figure 1 .
Figure 1.(a,d) shows TEM images of MoS 2 and h-BN specimens; (b,e) show HRTEM images of MoS 2 and h-BN; (c,f) show the atomic layer of HRTEM for the insert of (b,e) Adapted with permission [37].2019 ACS Applied Material Interface.

Figure 2 .
Figure 2. Modulation of the electrical conductivity of PLA/xC composite with increasing CNT concentration, as well as log-log plots of electrical conductivity vs. CNT content of the composites [6].Adapted with permission.2023 Advanced Composite Material.

Figure 2 .
Figure 2. Modulation of the electrical conductivity of PLA/xC composite with increasing CNT concentration, as well as log-log plots of electrical conductivity vs. CNT content of the composites [6].Adapted with permission.2023 Advanced Composite Material.
hybrid filler indicated that the characteristic peak is very strong at 26 • .TGA shows two stages of weight loss (i.e., at 70 • C and 200 • C)

Figure 4 .
Figure 4. (a) SEM and (b) TEM images of the hybrid MoS2/h-BN system.(c) HRTEM image acquired from the edge of a MoS2/h-BN flake.(d) Enlarged HRTEM image of the area shown in (c).(e) An HRTEM view of the edge of the MoS2/h-BN flake film, showing the thickness and number of layers; (f-i) displays the outcome of the EDS patterning of the elements Mo, S, B, and N, respectively [37].Adapted with permission.2019 ACS Applied Material Interface.

Figure 4 .
Figure 4. (a) SEM and (b) TEM images of the hybrid MoS2/h-BN system.(c) HRTEM image acquired from the edge of a MoS2/h-BN flake.(d) Enlarged HRTEM image of the area shown in (c).(e) An HRTEM view of the edge of the MoS2/h-BN flake film, showing the thickness and number of layers; (f-i) displays the outcome of the EDS patterning of the elements Mo, S, B, and N, respectively [37].Adapted with permission.2019 ACS Applied Material Interface.

Figure 4 .
Figure 4. (a) SEM and (b) TEM images of the hybrid MoS 2 /h-BN system.(c) HRTEM image acquired from the edge of a MoS 2 /h-BN flake.(d) Enlarged HRTEM image of the area shown in (c).(e) An HRTEM view of the edge of the MoS 2 /h-BN flake film, showing the thickness and number of layers; (f-i) displays the outcome of the EDS patterning of the elements Mo, S, B, and N, respectively [37].Adapted with permission.2019 ACS Applied Material Interface.

Figure 5 .
Figure 5. (a) A schematic representation of the ink with the conductivity preparation process.(b-d) SEM morphologies of pure paper, GNP-based material, and hybrid, respectively.(e-g) Cross-sectional of the samples that were reported in (b-d) [19].Adapted with permission.2020.Advanced Electronic Material.

Figure 5 .
Figure 5. (a) A schematic representation of the ink with the conductivity preparation process.(b-d) SEM morphologies of pure paper, GNP-based material, and hybrid, respectively.(e-g) Crosssectional of the samples that were reported in (b-d) [19].Adapted with permission.2020.Advanced Electronic Material.

Author Contributions:
Conceptualization, O.A.A.; methodology, O.A.A.; formal analysis, O.A.A.; investigation, O.A.A.; resources, N.N.; writing-original draft preparation, O.A.A.; writing-review and editing, O.A.A. and N.N.; visualization, O.A.A.; supervision, N.N.; project administration, N.N.; funding acquisition, N.N.All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by The University of South Africa (UNISA), grant number 90546210, and the APC was funded by UNISA.Data Availability Statement: Not applicable.
filler network structure with the PLA.The authors reported that the addition of 1phr CNTs and 1phr CB resulted in nanocomposites with high electrical conductivity, tensile strength, flexural strength, and impact toughness (98 × 10 −2 S/m, 70.1 MPa, 913 MPa, and 2.8 kJ/m 2 , respectively).

Table 1 .
Summary of different fillers, their fabrication methods, and results.

Table 2 .
Summary of hybrid fillers' characterizations and their respective contributions.

Table 2 .
Summary of hybrid fillers' characterizations and their respective contributions.