Synergic Effect of TiO2 Filler on the Mechanical Properties of Polymer Nanocomposites

Nanocomposites with polymer matrix offer excellent opportunities to explore new functionalities beyond those of conventional materials. TiO2, as a reinforcement agent in polymeric nanocomposites, is a viable strategy that significantly enhanced their mechanical properties. The size of the filler plays an essential role in determining the mechanical properties of the nanocomposite. A defining feature of polymer nanocomposites is that the small size of the fillers leads to an increase in the interfacial area compared to traditional composites. The interfacial area generates a significant volume fraction of interfacial polymer, with properties different from the bulk polymer even at low loadings of the nanofiller. This review aims to provide specific guidelines on the correlations between the structures of TiO2 nanocomposites with polymeric matrix and their mechanical properties. The correlations will be established and explained based on interfaces realized between the polymer matrix and inorganic filler. The paper focuses on the influence of the composition parameters (type of polymeric matrix, TiO2 filler with surface modified/unmodified, additives) and technological parameters (processing methods, temperature, time, pressure) on the mechanical strength of TiO2 nanocomposites with the polymeric matrix.


Introduction
Polymer nanocomposites represent a new class of composite materials that generally exhibit better properties than traditional microcomposites, in terms of mechanical properties, thermal and dimensional stability, fire and chemical resistance, optical and electrical properties, etc. Polymer nanocomposites with inorganic fillers attracted significant an attention due to their unique properties and their numerous applications in modern technology. The properties of polymer nanocomposites are mostly a simple combination of incorporated inorganic nanoparticles and polymeric matrix.
Polymeric materials can be used as matrices in nanocomposites due to their good thermal stability, environmental resistance (durability), and electrical, chemical and mechanical properties [1]. However, it is well known that some polymers (e.g., epoxy resin) are highly brittle. This disadvantage limits the application of these polymers in products that require high impact and fracture strength. Inorganic filler added into polymer matrix improved the mechanical performance of the polymeric nanocomposites. Nanofillers have large surface areas, making them chemically active, and making them interact more easily with the matrix [2]. There are many methods to reinforce polymers with rigid fillers to reduce the cost of production, alleviate some of the polymers limitations and expand their applications [3]. How fillers influence the characteristics of these polymers depends on the polymer nature and the proportion of the filler. Fillers are used to modify many properties composition, surface morphology, and processing parameters. Polymers are a source of a wide variety of low-priced raw materials, which offer many advantages, such as the following [10]: low specific weight, high material stability against corrosion, good electrical and thermal insulation, ease of shaping and economical mass production, and attractive optical properties. However, they have some deficiencies in strength and stiffness. Fillers are integrated into polymer materials to make up for those deficiencies. These polymers can be epoxy resins, polyester fibreglass resins systems, PURs, PIs, urea, etc.
Theoretically, the associations that can be made between different polymers and the wide range of fillers are infinite. In practice, however, although numerous, the polymerfiller associations are limited. Among the thermoplastic polymers for the processing of which fillers can be introduced, the most important are as follows: polyolefin, polyamides, ABS polymers, polyesters, polycarbonates, and PVC. Elastomers are flexible polymers that comprise a low crosslink density and generally have low Young's modulus, and by incorporating the fillers, these matrices can be more resistant [11].

Matrix-Filler Interface
The nature of the interface between the matrix and filler is an essential factor influencing the nanocomposite properties. According to Sharpe [33], the interface is defined as an intermediate region of two phases in contact, whose composition, structure, and properties vary throughout the area and are generally different from the two phases. Such phases are rarely devoid of chemical interaction. The volume of material affected by the interface interaction forms a a three-dimensional zone, called the interphase. The term interphase is widely used in the adhesion community to indicate the presence of a chemically or mechanically altered zone between adjacent phases [34,35]. The interphase concept, according to Drzal [36], is schematically represented in Figure 1. Knowledge of the relationship between microstructure and properties in the interface region is essential for the correct use of composite materials. There is no simple quantitative relationship for interface optimization that combines polymeric matrix and fillers [16]; the physicochemical variation and the thermodynamic-mechanical principles are sources of information for the qualitative assessment of the interface phenomena. Numerous researches have been carried out to improve the properties of the composites, particularly the interface when the filling is inorganic, for example, TiO2 [17].
Studies have shown [37,38] that the interface has different properties from both the matrix and the filler material. This consists of several layers that can each affect the adhesion of the components. The filler-matrix bonding depends on the following physicochemical aspects of the interfaces of the composite: atomic arrangement, molecular conformation, the chemical constitution of the fillers, matrix and fillers morphological properties, and the diffusivity of the elements in each constituent. The adhesion between the polymeric matrix and the dispersed phase particles was explained in mechanical and thermodynamic adhesion, chemical compatibility, chemical reactions with new bonds, electrostatic attraction forces, and macromolecular interdiffusion, adsorption and watering, as shown in Figure 2 [39]. The mechanical coupling or interlocking adhesion mechanism Knowledge of the relationship between microstructure and properties in the interface region is essential for the correct use of composite materials. There is no simple quantitative relationship for interface optimization that combines polymeric matrix and fillers [16]; the physicochemical variation and the thermodynamic-mechanical principles are sources of information for the qualitative assessment of the interface phenomena. Numerous researches have been carried out to improve the properties of the composites, particularly the interface when the filling is inorganic, for example, TiO 2 [17].
Studies have shown [37,38] that the interface has different properties from both the matrix and the filler material. This consists of several layers that can each affect the adhesion of the components. The filler-matrix bonding depends on the following physicochemical aspects of the interfaces of the composite: atomic arrangement, molecular conformation, the chemical constitution of the fillers, matrix and fillers morphological properties, and the diffusivity of the elements in each constituent. The adhesion between the polymeric matrix and the dispersed phase particles was explained in mechanical and thermodynamic adhesion, chemical compatibility, chemical reactions with new bonds, electrostatic attraction forces, and macromolecular interdiffusion, adsorption and watering, as shown in Figure 2 [39]. The mechanical coupling or interlocking adhesion mechanism is based on the adhesive keying into the surface of the substrate [40] and locking the rough irregularities on the surface of the nanocomposites. In many studies, it was shown that the adhesion mechanism was due to interchain entanglement and not chemical bonding between the components of the composites. The mechanical adhesion primarily depends on the forces in the transition region between the non-contacting areas [41,42]. The thermodynamic Polymers 2021, 13, 2017 4 of 24 mechanism assumes that it does not require a molecular interaction for good adhesion, only an equilibrium process at the interface [43]. In neutral environments, such as air, the thermodynamics of the polymer system will attempt to minimize the surface free energy by orientating the surface into the non-polar region of the polymer. When the polymer surface is in contact with a polar substance, such as water, good adhesion requires that the interfacial tension be minimized [44]. The other theories mentioned above are explained based on the physico-chemical interactions between the components of the composites.
is based on the adhesive keying into the surface of the substrate [40] and locking the rough irregularities on the surface of the nanocomposites. In many studies, it was shown that the adhesion mechanism was due to interchain entanglement and not chemical bonding between the components of the composites. The mechanical adhesion primarily depends on the forces in the transition region between the non-contacting areas [41,42]. The thermodynamic mechanism assumes that it does not require a molecular interaction for good adhesion, only an equilibrium process at the interface [43]. In neutral environments, such as air, the thermodynamics of the polymer system will attempt to minimize the surface free energy by orientating the surface into the non-polar region of the polymer. When the polymer surface is in contact with a polar substance, such as water, good adhesion requires that the interfacial tension be minimized [44]. The other theories mentioned above are explained based on the physico-chemical interactions between the components of the composites.

Fillers and Surface Modifications
Composite materials with optimal performances are obtained if an optimal adhesion between the matrix and the filler is achieved. Optimal adhesion is realized between materials of close chemical nature. It is weak between very different materials, from a chemical point of view, such as between the polymer matrix and inorganic fillers. Thus, the adhesion can be improved by treating the surface of the dispersed particles with coupling agents. Surface modification of fillers is becoming more important because of its adhesion improvement on the stress transfer between polymer and filler, which leads to an increase in the dispersion degree [45]. The coupling agent diffusion and adsorption processes at the surface of the filler particles occur at the interface. The properties of the interface and the adhesion of the components can be modified by treating the surface of the fillers before introduction into the polymer matrix. These treatments either remove the weaker layers related to the filler surface of the material or introduce new functional groups capable of influencing the adhesion between the materials.
Surface treatment of the fillers can be achieved by [46,47] the following: − The chemical interaction of the fillers with compounds that possess functional groups; − Chemical absorption on the surface of the particles of the filling material of some modifying agents; − Coating the filler particles with a suitable coupling agent.

Fillers and Surface Modifications
Composite materials with optimal performances are obtained if an optimal adhesion between the matrix and the filler is achieved. Optimal adhesion is realized between materials of close chemical nature. It is weak between very different materials, from a chemical point of view, such as between the polymer matrix and inorganic fillers. Thus, the adhesion can be improved by treating the surface of the dispersed particles with coupling agents. Surface modification of fillers is becoming more important because of its adhesion improvement on the stress transfer between polymer and filler, which leads to an increase in the dispersion degree [45]. The coupling agent diffusion and adsorption processes at the surface of the filler particles occur at the interface. The properties of the interface and the adhesion of the components can be modified by treating the surface of the fillers before introduction into the polymer matrix. These treatments either remove the weaker layers related to the filler surface of the material or introduce new functional groups capable of influencing the adhesion between the materials.
Surface treatment of the fillers can be achieved by [46,47] the following: − The chemical interaction of the fillers with compounds that possess functional groups; − Chemical absorption on the surface of the particles of the filling material of some modifying agents; − Coating the filler particles with a suitable coupling agent.
These processes are generally laborious and increase the cost of the fillers, but they offer the possibility of considerably increasing the fillers content in mixtures without worsening their characteristics.
Modification of the surface of fillers is becoming more important because of its improvement in adhesion [48]. Hence, it is on the stress transfer between the polymer and filler, leading to an increase in the dispersion degree.

Size, Shape and Specific Surface Area of the Nanoparticles
Titanium dioxide (TiO 2 ) is the natural oxide of the element titanium. Titanium dioxide adopts four structures polymorphs found in nature rutile, anatase, brookite, and TiO 2 (B). An additional four high-pressure forms have been synthesized, as follows: TiO 2 (II) with the α-PbO 2 structure, TiO 2 (H) with hollandite, baddeleyite with ZrO 2 , and cotunnite with PdCl 2 [49]. Among the eight structures, rutile and anatase are mostly manufactured in the chemical industry as microcrystalline materials. Thermodynamically, rutile is the most stable phase at all temperatures and pressures below 60 kbar, when TiO 2 (II) becomes the favourable phase. Particle size influences surface energy and phase stability. Thus, anatase is most stable at sizes less than 11 nm, brookite at sizes between 11 and 35 nm, and rutile at sizes greater than 35 nm. Anatase and brookite are more stable than rutile at nano-size, due to the differences in surface energy. Anatase is more stable than brookite at even smaller sizes [50]. From a commercial point of view, titanium dioxide can be found in the following two common forms that differ in crystal structure: anatase and rutile [51][52][53].
Titanium dioxide can be prepared in the following various morphologies: nanoparticles, nanowires, nanotubes, and mesoporous structures. There are physical and chemical methods for synthesizing TiO 2 nanoparticles in the liquid phase, as follows: hydrothermal/solvothermal method, sonochemical method, electrochemical synthesis, solgel method, microwave field synthesis, and vapor phase, which includes spraying, atomic deposition of layers, pulsed laser deposition, chemical vapor deposition, physical vapor deposition, and pyrolysis spray [54,55]. The controllable synthesis of TiO 2 with unusual morphologies and dimensions can give the polymeric matrices with particular features and qualities.
The specific surface area of TiO 2 increases as the particle size decreases, meaning nanoparticles are attracted due to van der Waal electrostatic forces. With the decreasing particle size, the ratio of surface/volume increases. Therefore, the smaller the particles are, the more important the surface properties will be, influencing agglomeration behavior and interfacial properties as a result of interaction with the polymer matrix [56,57]. The formation of particle agglomerates and non-uniform dispersion has motivated research to better process polymer-TiO 2 nanocomposites. Several methods have been approached to minimize agglomeration and ensure better distribution. Such methods may be as follows: melt mixing, solution mixing in aqueous media or polymer matrices, particle surface modification involving polymer surfactant molecules or other modifiers, which must generate a strong repulsion between nanoparticles, mechanical stirring, and ultrasonic irradiation.

Surface Modification of TiO 2 Nanoparticles
TiO 2 nanoparticles can be directly added to the organic matrix, but due to the high surface area and high polarity, there is a strong tendency for them to aggregate. TiO 2 nanoparticles form agglomerates at higher concentrations due to their high surface energy. Surface modification of TiO 2 nanoparticles effectively reduces their surface energy and improves their dispersion properties in the organic matrix. Therefore, to improve the homogeneous dispersion of nanoparticles, many researchers have focused on the surface modification of nanoparticles and a new method for incorporating inorganic nanofiller into an organic matrix [58][59][60]. Several ways have been employed to modify the surface of nanoparticles [61,62].
The surface modification of TiO 2 nanoparticles is often conducted by either a physical or chemical method. The chemical method has attracted the attention of many researchers because the interactions between inorganic nanoparticles and the matrix are much stronger [63]. The surface modification of nanoparticles by chemical treatments is a useful method to improve the dispersion stability of TiO 2 nanoparticles and the development of interfaces between the organic and inorganic phase. In this regard, the concept of silane coupling agent was reported by Plueddemann and et al. [64]. Researchers found that organofunctional silanes are silicon chemicals that contain both organic and inorganic reactivity in the same molecule, and which can be used as coupling agents [65,66]. Coupling agents connect resin and fillers, and improve the physical, mechanical and electrical properties of composites. Moreover, they enhance the wetting of inorganic substrates, decrease the viscosity of the resin during mixing, and ensure smoother surfaces of composites [67,68].
The general formulation of the coupling agent molecule is as X-R, where X interacts with the filler and R is compatible with the polymer. Organosilanes are of the form R-Si-(OR') 3 , where OR' can be methoxy, ethoxy, acetoxy, and R can be alkyl, aryl or organofunctional group [56]. According to this structure, the following steps may take place, as shown in Figure 3: Hydrolyzation of alkoxy groups obtaining silanol, which reacts with the mineral surface; The condensation reaction between silanol molecules; Bond formation between TiO 2 nanoparticles and the organofunctional group.  Some coupling agent recommendations for the surface modification of TiO2 nano ticles is given in Table 1.  The choice of organosilane is established, taking into account the polymers chemical structure to be compatible. For example: for a phenolic and epoxy resin an epoxy silane, or an amino silane is recommended and for an unsaturated polyester resin a methacrylsilane. The reactivity of the thermosetting polymers should be close to that of organosilane. For a thermoplastic matrix, bonding occurs by diffusion of the organosilane network in the interphase region of the composite [66].
The surface modification of TiO 2 has been reported using different silane coupling agents, such as 3-aminopropyltriethoxysilane (APTES). The photocatalytic activity of TiO 2 has been shown to increase with increasing the concentration of APTES used [87]. For example, Mallakpour and Barati [88] reported the surface modification of TiO 2 nanoparticles by the reaction with APTES. The silane coupling agent was adsorbed on the surface of the nanoparticles at its hydrophilic end and interacted with the hydroxyl groups pre-existing on the surface of the nanoparticles. Thus, it was confirmed that the heat stability of the nanocomposite was improved. Shakeri et al. [89] studied the self-cleaning capability of surfaces covered TiO 2 nanoparticles, modified by APTES. They concluded that the surface could degrade the dye used as an organic pollutant due to the obtained coating being stable. Klaysri et al. [90] proposed a one-step synthesis method of APTES-functionalized TiO 2 surface. They showed that obtained nanomaterials are capable of the photocatalytic decolonization of methylene blue.
Modification of the surface of TiO 2 nanoparticles with silane coupling agents was obtained via reflux in an aqueous solution [75,91]. Chen et al. investigated the interactions between 3-aminopropyltrimethoxysilane (APTMS) and phenyltrimethoxysilane with commercially available TiO 2 nanoparticles (Degussa P-25) [91]. They obtained results showing that the silane coupling agents used bind covalently on the surface of the TiO 2 nanoparticles. In another study, Zhao et al. reported the cross-linking and chemical bonding mechanisms of APTMS and 3-isocyanatopropyltrimethoxysilane on TiO 2 nanoparticles [75].
To improve TiO 2 nanoparticles dispersion and enhance the interactions between the nanoparticles and polymeric matrix (polyamide/modified-TiO 2 nanocomposites), the surface of TiO 2 was modified with a 1,3,5-triazine based silane coupling agent [92].
Godnjavec et al. have coated TiO 2 nanoparticles by 3-glycidyloxypropyltrimethoxysilane (GLYMO) as an additive in a clear polyacrylic coating. According to their results, grafting GLYMO on the nanoparticles surface improved the dispersion, transparency, and UV protection of the clear acrylic coating [100].
Yang et al. [101] reported silanization of TiO 2 particles through a sol-gel method. Based on their results, vinyl triethoxysilane (VTES) as a surface modifier improved the stability of dispersion and suspension in tetrachloroethylene. Dalod et al. [50] modified TiO 2 nanoparticles with amino silane groups using a hydrothermal method and found that the nanoparticles shape and structure depends on the type of silane coupling groups.
Tangchantra et al. [102] investigated the effect of different silane coupling agents on the surface grafting of TiO 2 with hexadecyl trimethoxysilane (HTMS), triethoxyvinylsilane (TEVS), and aminopropyl trimethoxysilane (APS). The results showed that silane coupling agents could modify the surface of TiO 2 nanoparticles via the hydrolytic condensation of titanium isopropoxide. The TEVS agent improved the dispersibility of TiO 2 particles and showed optimum mechanical properties.
The appropriate surface modification on nanoparticles leads to better dispersion and compatibility in the polymer matrix. The formation of chemical and physical interactions with the polymer matrix could guarantee remarkable mechanical properties of polymeric nanocomposites.

Properties, Commercial Products and Applications
At the nanoscale size, the material properties may dramatically change and differ significantly from their bulk counterparts. Particular attention has been paid, in recent years, to obtaining TiO 2 with photocatalytic properties [103][104][105][106], optical properties [107], with applications related to the degradation of pollutants [108][109][110], and the realization of the photoelectrochemical cells [111]. Also of interest are titanium dioxide films deposited on various substrates to obtain special characteristics, such as surfaces with self-cleaning properties [112,113].
All applications of TiO 2 nanoparticles depend on their crystal structure, morphology, specific surface area, particle size, and form. TiO 2 has been widely used in the industry for many years for its numerous and diverse applications, as shown in Table 2. The applications that can be mentioned are sensors, photo-conductors, additives in plastics, catalysts, photo-/electrochromics and photovoltaics applications, dye-sensitized solar cells, sunscreens, paints, antimicrobial applications, water purification by photocatalysis processes, biosensing, and drug delivery [114]. TiO 2 nanoparticles incorporated into outdoor building materials, such as paving stones or paints, can reduce volatile organic compounds and nitrogen oxide concentrations.
TiO 2 is a material with multifunctional properties that can be incorporated in polymeric matrices as a filler to develop new nanocomposites with enhanced properties [115].

Preparation Methods
Polymeric matrix nanocomposites can be obtained using injection molding, compression molding, in situ polymerization, sol-gel, melt mixing and sintering.
In situ polymerization involves the dispersion of inorganic nanoparticles in a monomer phase as a first step, followed by bulk phase polymerization. This process is mainly used for thermosetting polymers. As a result, unstable nanocomposites can be transform into a different morphology than expected. The in situ polymerization method is a simple and inexpensive method. The nanocomposites with the polymer matrix, and inorganic filler with good filler distribution in the polymer matrix, can be obtained [116].
Most compression molding techniques require pre-treatment of the nanoparticles with curing, but injection molding is the most widely used process for obtaining nanocomposite materials. Injection molding can be used in a variety of applications, in both commercial and research fields [117]. Sintering, powder compaction and sol-gel are all alternative techniques to produce polymeric composites. However, the operating conditions (temperature, pressure, time, etc.) are far more than those of injection molding [118]. Some reports were found in the literature focusing on obtaining TiO 2 nanocomposites with the polymeric matrix, as shown in Table 3.
Studies on polymer-TiO 2 nanocomposites prepared by melt mixing have shown a slight improvement or no change in mechanical properties [119,120]. Somani et al. [121] received highly piezoresistive conducting polyaniline/TiO 2 composite by in situ deposition technique at a low temperature. Feng et al. [122] synthesized a composite of polyanilineencapsulating TiO 2 nanoparticles by in situ emulsion polymerization. They investigated and explained the interaction between polyaniline and nano-TiO 2 particles, and the nature of chain growth according to Fourier transform infrared (FTIR) spectra. Xia and Wang [123] prepared a polyaniline/nanocrystalline TiO 2 composite by ultrasonic irradiation. They think that ultrasonic irradiation provides a new way to prepare 0-3-dimensional conducting polymer/nanocrystalline composites.
Titanium dioxide has been used to reinforce polypropylene (PP) via extrusion, followed by injection molding, by Alghamdi [124]. There were presented to the mechanical and structural aspects of PP for different loading of TiO 2 filler (up to 30 wt. %). As the TiO 2 weight percent increases, the impact strength decreases. This behaviour is expected because the PP is incompatible with TiO 2. The PP phase is non-polar, hydrophobic and has low surface energy, while TiO 2 represent the polar phase, hydrophilic and high surface energy for TiO 2 . The highest resilience value was recorded for the sample with 20% TiO 2 (37.09 ± 5.3 J/m).
Mourad et al. [117] studied HDPE nanocomposites with 5% TiO 2 , obtained by injection molding under the following different processing parameters: temperature, pressure, injection velocity, and injection time. The results showed the influence of processing parameters on the mechanical and thermal properties of HDPE-TiO 2 nanocomposites. Mechanical testing revealed that the tensile strength varied from 22.5 to 26.3 MPa, while the Young modulus increased by 8.6% as the molding temperature increased.
Vladuta et al. [125] investigated the effect of the TiO 2 nanoparticles on the PETrubber interface in nanocomposites obtained from waste by compression molding. The modifications in surface energy, morphology and crystalline structure were discussed for samples kept under visible light and UV radiation. TiO 2 develops new physical interactions in the composite, but induces, even in visible light, oxidation processes. The results indicated that the optimum concentration for TiO 2 to the composites, for obtaining better interface properties, is 0.25 wt. %.
Regardless of the method of obtaining nanocomposites with the polymeric matrix, it is found that the nature of filler has a significant influence on mechanical properties.

Mechanical Properties
Titanium dioxide is used as a filler in many polymeric matrices because of the improved physical and mechanical properties it yields. Many studies showed improvements in the mechanical strength and modulus of TiO 2 -filled polymeric nanocomposites compared to the pristine-base matrix. The mechanical properties of the TiO 2 nanocomposites depend significantly on their internal structure. The poor compatibility of hydrophilic TiO 2 nanoparticles with a hydrophobic polymer matrix may lead to particle aggregates and/or agglomerates. The aggregates create defect sites in the nanocomposites, and the improvement in mechanical properties is not observed. More uniform dispersion of nanoparticles is recommended, using one-dimensional nanoparticles, i.e., nanorods, nanotubes or nanoribbons, particles with a high aspect ratio [46]. Several factors that may influence the mechanical properties of composites with a polymer matrix and inorganic fillers are presented in Figure 4. Titanium dioxide is used as a filler in many polymeric matrices because of the improved physical and mechanical properties it yields. Many studies showed improvements in the mechanical strength and modulus of TiO2-filled polymeric nanocomposites compared to the pristine-base matrix. The mechanical properties of the TiO2 nanocomposites depend significantly on their internal structure. The poor compatibility of hydrophilic TiO2 nanoparticles with a hydrophobic polymer matrix may lead to particle aggregates and/or agglomerates. The aggregates create defect sites in the nanocomposites, and the improvement in mechanical properties is not observed. More uniform dispersion of nanoparticles is recommended, using one-dimensional nanoparticles, i.e., nanorods, nanotubes or nanoribbons, particles with a high aspect ratio [46]. Several factors that may influence the mechanical properties of composites with a polymer matrix and inorganic fillers are presented in Figure 4. TiO2 fillers affect the basic mechanical properties of the polymer. The effect of TiO2 fillers on composites properties depends on the particle size and shape, concentration and the interaction with the matrix, as shown in Table 3. For example, to increase the modulus and hardness of polymers, micrometre-sized inorganic particles are frequently applied.

The Nature of the Filler
TiO 2 fillers affect the basic mechanical properties of the polymer. The effect of TiO 2 fillers on composites properties depends on the particle size and shape, concentration and the interaction with the matrix, as shown in Table 3. For example, to increase the modulus and hardness of polymers, micrometre-sized inorganic particles are frequently applied. However, a reduction in the material ductility may take place. By diminishing the particle size or by enhancing the particle volume fraction, the strength can be improved. Still, in some cases, the fracture toughness and modulus remain relatively independent of the particle size. The properties of TiO 2 that make it a good filler for composite materials are good dispersibility in the polymer system and good heat stability. Titanium dioxide has a relatively high elastic modulus, which can be frequently combined into various polymers to obtain the composites mechanical gain.
Mikešová and et al. [126] studied the effects of nanoparticles and the properties of the nanocomposites of polypropylene and filler TiO 2 . They used isotactic polypropylene (PP) as a matrix, and as fillers they used TiO 2 in the following different shapes: a commercial titanium dioxide micropowder (mTiO 2 ; a mixture of anatase and rutile), a commercial titanium dioxide nanopowder (nTiO 2 ; anatase modification), and titanate nanotubes (TiNT). More series of samples were obtained with PP unmodified and with PP modified by electron beam irradiation (PP*), resulting in PP*/TiX composites (i.e., PP*/mTiO 2 , PP*/nTiO 2 , and PP*/TiNT). These were prepared by melt mixing of PP* with 5 wt. % of TiX. The stiffness and microhardness properties of PP*/TiX systems are improved in the order PP*/mTiO 2 , PP*/nTiO 2, PP*/TiNT, due to the specific surface of the TiX particles.
Nano-sized TiO 2 was further studied in starch/(poly[vinyl alcohol]) blends by Sreekumar et al. [127]. The nano-sized TiO 2 could provide the composite with superior mechanical properties because of good interfacial adhesion between the polymer matrix and filler.
Bora et al. [128] studied the effect of TiO 2 particle concentrations (up to 25 wt. %) on the properties of polyphenylenesulphide (PPS)-TiO 2 composites. The increase in TiO 2 particle concentrations in the PPS matrix improves the stiffness of the composite. High values of flexural and residual flexural strength were obtained at 10 wt. % TiO 2 particle concentrations. Saluja et al. [129] obtained polyester composites filled with TiO 2 concentrations up to 25 wt. %. This study shows that the addition of TiO 2 particles improves the effective thermal conductivity of polyester-TiO 2 composites, the glass transition temperature (Tg), and the reduction in the coefficient of thermal expansion (CTE).
The mechanical properties of nanocomposites depend significantly on their internal structure. In the nanocomposite, TiO 2 nanoparticles can appear as agglomerations due to their low compatibility with the hydrophobic polymer matrix.
In this case, the large surface area of the nanowires decreases rapidly, the aggregates create defect sites in the nanocomposites, and no improvement in the mechanical properties is observed. A more uniform dispersion of nanoparticles, using one-dimensional (1D) nanoparticles, i.e., nanorods, nanotubes, or nanoribbons, would improve these properties. Compared with the isometric nanoparticles, a large surface-to-volume ratio of the 1D nanoparticle generally improves the nanocomposites properties. Contrary to the anatase polymer nanocomposites, only a few papers concerning polymers filled with titanate nanotubes have been found in the literature [130][131][132].
The majority of nanoparticle fillers added in the polymer matrix improve mechanical properties such as flexibility, ductility, hardness, and strength and stiffness, even in small amounts.

The Nature of the Polymer Matrix
Polymer-TiO 2 nanocomposites have been successfully synthesized in different polymer matrices such as the following thermoplastic polymers: polyacrylate, poly (methyl methacrylate), polyimide, polystyrene, and polyolefines; the following thermosetting polymers: polycarbonate, polyamide 6, epoxy, unsaturated polyester; and silicone elastomer [77,132].
Saritha et al. [133] studied the incorporation of TiO 2 in rubber composites. The tensile strength, modulus, and tear strength increased with increasing TiO 2 loading. More recently, processing techniques were developed to allow the size of TiO 2 to decrease to the nanoscale. Manap et al. [134] demonstrated that TiO 2 and multi-walled carbon nanotubes (MWCNT) as filler reinforcements could address the agglomeration issue, by exhibiting even distribution of particles in the TPU matrix. The combination of MWCNT and TiO 2 in the TPU matrix enhanced the mechanical and thermal properties significantly, this being a good heat insulator.
In the function of the matrix nature, the percentage by weight of the inorganic filler introduced can remain very low (on the order of 0.5% to 5%) due to the incredibly high surface area-to-volume ratio of the particles. This area can generate a new material behavior, which is widely determined by interfacial interactions, offering unique properties and an entirely new class of materials. Several important types of research in this regard are presented in Table 3.
When designing new polymer-TiO 2 nanoparticle composites, the following aspects should be considered: − Nature of filler and polymer matrix; − Amount of filler; − The distribution of filler, this should not form agglomerates in the samples; − Concentration of coupling agent for modifying of filler surface; − The method of obtaining, which is an essential factor.
The impact resistance of polymer matrices with TiO 2 filled is of particular interest to researchers, as long as it represents the weak point of most composite materials. Hardening of thermoplastics by modification with elastomers could be a new way to solve this problem. It is recommended to study new cheaper and more efficient polymer matrices to produce composites with predetermined properties. In this case, we recommend using of polymeric waste as a matrix for obtaining nanocomposites with TiO 2 filler.
Polymer nanocomposites give a new way to overcome the limitations of pure polymers or their traditional composites. Nowadays, polymer nanocomposites with TiO 2 filled represent an area of interest for many researchers. This article contains information on the nature of the polymer matrix (thermoplastic, thermosetting, elastomeric) and the type of TiO 2 filler, processing methods, possible surface modifications of the filler and how they influence the mechanical properties of nanocomposites, thus completing the areas of knowledge for many researchers.

Advantages, Limits and Applications
Polymeric materials can be used as matrices in TiO 2 nanocomposites due to their good thermal stability, environmental resistance (durability), and electrical, chemical and mechanical properties. However, it is well known that some polymers (e.g., epoxy resin, polyamides) are highly brittle. This disadvantage limits the application of these polymers in products that require high impact and fracture strength. TiO 2 filler added in the polymer matrix improves the mechanical performance of the polymeric nanocomposites over conventional polymer composites, as shown in Table 4. Finally, typical existing and potential applications are shown in Figure 5.   The incorporation of TiO 2 nanoparticles into the epoxy resin improved flexural stiffness, flexural strength, and fracture toughness of the polymer. [144] epoxy-TiO 2 nanocomposites Matrix: epoxy resin Filler: TiO 2 (5-40 nm, 0.5-2 wt.%); Obtaining: mixing of matrix with filler; Characterization: thermal properties, mechanical properties, morphology, viscoelastic properties.
The tensile strength of ABS/TiO 2 and ABS/ATO nanocomposites increased by 7% at the 2.5 wt. % TiO 2 filler, respectively, by 9.2% at 0.50 wt. % ATO filler. The modulus of elasticity increases up to 5 TiO 2 wt. % and then decreases.

Polymer Nanocomposites Conventional Polymer Composites
Fillers separation are in nm, and properties will be affected by size effects of nanofiller; Small amounts of TiO 2 filler are enough (less than 10%) to achieve desired properties; Properties are obtained without sacrificing the inherent properties of the polymer or adding excessive weight; Improvements in properties even in low amount is due to nanosized of TiO 2 filler and interphase region; Using nanosized particles can reduce the likelihood of finding defects, such as grain boundaries, voids, dislocations, and imperfections.
Fillers are separated in µm, and there is not that much of size effect; High concentrations should be needed as compared to nanofiller case; Fillers can unfavorably impact other benefits of polymers, such as appearance, ductility and toughness; There is not that much improvement in properties even for a large amount of fillers; It is difficult even observed in conventional polymer composites.

Conclusions
An essential characteristic of polymers is modifying their inherent physical properties by adding fillers, while retaining their characteristic processing ease. By adding inorganic fillers into the polymers matrix, composite materials become stronger, stiffer, electronically conductive, magnetically permeable, flame retardant, more challenging, and more wear-resistant.
After reviewing part of the existing literature on polymeric composites with TiO2 fillers, it is found that the interfacial connection between the filler and polymer matrix is an important element for determining the mechanical properties of the composite.
The addition of TiO2 nanoparticles into the polymeric matrix demonstrates their ability to significantly improve important mechanical properties (tensile modulus, tensile strength, toughness and fracture toughness, fracture energies, flexural modulus, flexural strength, elongation at break, fatigue crack propagation resistance, abrasion, pull-off strength, and fracture surface properties), even at low filler contents.
From the literature, one can conclude that the mechanical properties of the composites with the polymer matrix depend on the particle size, and particle-matrix interface adhesion and loading (type, quantity, filler distribution and orientation, and void content). Along with those properties, the interfacial bonds and the interphase load mechanisms also play an essential role.
Studies performed on polymeric matrix nanocomposites filled with TiO2 nanoparticles were performed to verify the influence of several variables (shape, size, % loading,

Conclusions
An essential characteristic of polymers is modifying their inherent physical properties by adding fillers, while retaining their characteristic processing ease. By adding inorganic fillers into the polymers matrix, composite materials become stronger, stiffer, electronically conductive, magnetically permeable, flame retardant, more challenging, and more wearresistant.
After reviewing part of the existing literature on polymeric composites with TiO 2 fillers, it is found that the interfacial connection between the filler and polymer matrix is an important element for determining the mechanical properties of the composite.
The addition of TiO 2 nanoparticles into the polymeric matrix demonstrates their ability to significantly improve important mechanical properties (tensile modulus, tensile strength, toughness and fracture toughness, fracture energies, flexural modulus, flexural strength, elongation at break, fatigue crack propagation resistance, abrasion, pull-off strength, and fracture surface properties), even at low filler contents.
From the literature, one can conclude that the mechanical properties of the composites with the polymer matrix depend on the particle size, and particle-matrix interface adhesion and loading (type, quantity, filler distribution and orientation, and void content). Along with those properties, the interfacial bonds and the interphase load mechanisms also play an essential role.
Studies performed on polymeric matrix nanocomposites filled with TiO 2 nanoparticles were performed to verify the influence of several variables (shape, size, % loading, surface change, etc.) and also to propose various areas of applicability of these nanocomposites.