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Article

Mechanical and Thermophysical Properties of Epoxy Nanocomposites with Titanium Dioxide Nanoparticles

by
Yurii S. Bukichev
1,2,
Lyudmila M. Bogdanova
1,
Valentina A. Lesnichaya
1,
Nikita V. Chukanov
1,
Nina D. Golubeva
1 and
Gulzhian I. Dzhardimalieva
1,2,*
1
Federal Research Center of Problems of Chemical Physics and Medicinal Chemistry, Russian Academy of Sciences, Academician Semenov Avenue 1, Chernogolovka 142432, Russia
2
Department of Advanced Materials and Technologies for Aerospace Purposes, Moscow Aviation Institute (National Research University), Volokolamskoe Shosse, 4, Moscow 125993, Russia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(7), 4488; https://doi.org/10.3390/app13074488
Submission received: 9 March 2023 / Revised: 28 March 2023 / Accepted: 30 March 2023 / Published: 1 April 2023
(This article belongs to the Special Issue Advanced Polymers Synthesis, Analysis and Applications)
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Abstract

:
The introduction of nanoparticles and their homogeneous distribution in the polymer matrix, as well as their size, can have a significant effect on the mechanical properties of composite materials. In this work, we studied the mechanical characteristics of TiO2/epoxy nanocomposites with different contents and sizes of nanoparticles. The preparation of nanocomposites was carried out by a stepwise curing (at 90 and 160 °C) of ED-20 dianic epoxy resin in the presence of an aromatic hardener with the addition of titanium (IV) dioxide nanoparticles preliminarily synthesized by the plasma-chemical method. Ultrasonic dispersion was used to achieve a uniform distribution of nanoparticles in the polymer matrix. The chemical and phase composition, the structure of the as-synthesized TiO2 nanoparticles, and the resulting epoxy nanocomposites were characterized by elemental analysis, X-ray diffraction, transmission and scanning electron microscopy, and infrared spectroscopy. The mechanical properties of the nanocomposites were determined by the static tensile test, and the impact toughness was determined by the Charpy method. The glass transition temperature and thermal stability of the TiO2/epoxy nanocomposites were studied by thermal analysis methods. The formation of an interfacial layer between the TiO2 nanoparticles and an epoxy matrix has been shown for the first time by spectral methods. It is shown that the mode of curing and ultrasonic dispersion used, as well as varying the content and dispersity of the TiO2 nanoparticles, make it possible to obtain epoxy nanocomposites with simultaneously improved deformation-strength characteristics and impact strength values.

1. Introduction

Thermosetting polymers based on epoxy resins are widely used in the production of paints and various coatings [1,2], high-performance adhesives [3,4], and microelectronics [5,6]. The most promising area of application is polymeric construction materials in the automotive and aerospace industries [7,8]. Due to the large set of hardeners [9], it is possible to vary the curing temperature–time conditions and to obtain epoxy resins with a high strength and special properties including high adhesion to most materials. In addition, the advantage of epoxy polymer synthesis technology is low shrinkage of about 5–7% [10] and the absence of harmful and hazardous volatile compounds during synthesis.
However, the modern requirements of materials are increasing, so epoxy resins should be modified. Epoxy nanocomposites are finding vast applications in various sectors such as the aerospace, defense, and automobile industries, anti-corrosive coatings, high-voltage fields, and wind energy [11,12]. The physical and chemical properties of the epoxy systems are influenced by the processing techniques, clay modifier, and curing agents used for the preparation of nanocomposites [9]. Researchers and industries have adopted epoxy nanocomposites as suitable materials in designing the parts for use in sensors, electronics, electromagnetic interference shielding, and more [13]. To change and improve the properties, fillers of different chemical natures, shapes, and sizes, including fibers [14], nanotubes [15], and micro- and nanoparticles [16], are introduced into the epoxy matrix. Nanoscale fillers are characterized by a high specific surface area. Consequently, a very small amount of nanofillers in the matrix is enough to achieve functional properties similar to those of other types of microfillers.
Composite materials based on the epoxy matrix and inorganic nanoparticles and carbon nanofillers (CNFs) provide an opportunity to improve their performances. Inorganic nanoparticles such as ZnO, TiO2, SiO2, and Al2O3 are added to epoxy resin to improve its mechanical [17,18,19,20,21], dielectric [22,23], and other properties. The addition of CNFs can increase the fracture toughness, flexural strength, and electrical conductivity of epoxy resin [18,20]. The incorporation of Al2O3 nanoparticles into the epoxy resin improved the flexural stiffness, flexural strength, and fracture toughness of the polymer at the same time. Moreover, the Al2O3 nanoparticles improved the fatigue crack propagation resistance of epoxy [21]. The addition of graphene oxide and ZnO to epoxy resin can increase its tensile strength, flexural strength, and elasticity modulus [18]. Similarly, the addition of TiO2 and SiO2 nanoparticles can improve the hardness and wear resistance of epoxy resin [19].
Titanium (IV) dioxide nanoparticles (TiO2) are widely used as inorganic fillers due to the set of unique properties, such as photocatalytic [24], optical [25], electrical [26], and sensory [27] ones, as well as their chemical inertness, relatively low toxicity, low cost, etc. Titanium dioxide is widely used as a white pigment in the construction, pharmaceutical, and food industries, cosmetology, etc. [28], as a photocatalyst for purifying air and water from harmful impurities and as a component of self-cleaning [28,29,30] and reflective [31,32] coatings in space technology.
Epoxy nanocomposites are produced by the curing of epoxy oligomers in the presence of a filler (ex situ method) or the corresponding precursor of metal nanoparticles [33] (in situ method). These additives can affect the kinetics of the process and the properties of the resulting matrix, which was discussed in detail in the review by V. Irzhak [34].
The properties of polymer nanocomposite materials are mainly determined by the size and shape of the inorganic particles, their distribution in the polymer volume, and their interaction with the organic binder [35,36].
Most of the published works are related to the study of the photocatalytic properties of composites with titanium dioxide [32,37,38], and a much smaller number of publications are related to the studies of the physical and mechanical properties. The dependence of the mechanical properties on concentration and particle size was investigated by Hamad et al. [39] and Kusiak-Nejman [40]. The authors synthesized epoxy-matrix-based nanocomposites containing TiO2 of 1%, 3%, 5%, and 10% in weight, of 17 nm and 50 nm. Improved mechanical properties were found: tensile strain increments of 3% and 5% were shown for particles of 17 nm and 50 nm in size, respectively. It is assumed that the number of nanoparticles and their size affect the quality of the interface in the composite, which in turn affects the deformation properties of the matrix.
A comparative study of the TiO2 particle size effect between ~50 nm and ~50 μm on the mechanical properties of epoxy nanocomposites (ENC) was carried out by Al-Ajaj I.A. et al. [41]. The improvement of the mechanical properties up to the content of 4 vol. % TiO2 and their subsequent decrease due to TiO2 agglomeration were shown for ENC. In the case of microparticles, with an increase in their concentration, the modulus of elasticity increases and the strength characteristics, on the contrary, decrease.
The article [21] describes the physical and mechanical properties of epoxy nanocomposites based on the diglycidyl ester of bisphenol A and cycloaliphatic amine with commercial surface-functionalized TiO2 particles of 300 nm size, Kronos 2310 type, and specific surface area equal to 5 m2/g at concentrations up to 10 vol. %. The nanoparticles were dispersed in epoxy resin using a high shear energy device and a bead mill. The elasticity modulus and flexural strength were shown to increase by more than 30% and 10%, respectively, and strain reduction by 35%. A significant, up to 70%, increase in impact toughness was found. The mechanisms of hardening and cracking have been considered.
In [42,43,44,45], an analysis of the dispersing conditions of commercial TiO2 of 10–50 nm in size was shown for large volumes of suspensions and high (up to 20 wt. %) concentrations of TiO2 in a solution of bisphenol-A-based epoxy oligomer in methyl ethyl ketone. The effectiveness of double dispersing (ultrasonic) and mechanical high speed—up to 2000 rpm—is demonstrated. The optimal time of the ultrasonic dispersion was 15–20 min [37]. The extreme dependences of strength characteristics at concentrations up to 20 wt. % of TiO2 in composites based on bisphenol A diglycidyl ether and diamines—such as aliphatic, cycloaliphatic, and aromatic ones—are found. The mechanisms of fracture formation (cracking) in epoxy nanocomposites are discussed.
In some papers, the authors do not specify the hardeners used in the synthesis process, only indicating their structure (aliphatic, amine, cycloaliphatic, etc.) [45,46,47]. Perhaps the authors use hardeners from well-known manufacturers, and therefore the authors cannot disclose the synthesis completely. Unfortunately, this fact does not allow for a full assessment of the obtained results, since the use of different hardeners affects the final properties of the nanocomposite, including the physical and mechanical ones.
Despite the great interest in the photocatalytic properties of epoxy nanocomposites filled with TiO2, the physical and mechanical properties of the polymer matrix after the introduction of an inorganic nanofiller have not been sufficiently studied due to the large range of epoxy oligomers and hardeners. The aim of this work was to investigate the structural characteristics, and the mechanical and thermal properties of epoxy composites based on nanocrystalline titanium dioxide (TiO2/EP) with varying composition, content, and dispersity.

2. Materials and Methods

Epoxy oligomer based on ED-20 dian resin (GOST 10587-84) was produced by AO REACHIM, Moscow, Russia; the content of epoxy groups was 22.6 wt. %. 4,4’-diaminodiphenylmethane (DDM) manufactured by Sigma-Aldrich, which was used as a hardener without additional purification. The structural formulae are shown in Figure 1.
Two different types of TiO2 nanopowders obtained by the plasma-chemical method [48] have the following characteristics: (1) dav = 46 nm, specific surface area (SSA) = 42.5 m2/g, ρ = 4.2495 g/cm3, and phase composition is anatase—75% and rutile—25%; and (2) dav = 100 nm, SSA = 13.3 m2/g, ρ = 4.2700 g/cm3, and phase composition is anatase—60% and rutile—40%.
To improve the dispersion of nanoparticles in the matrix, Pluronic L61 (L61) surfactant was used. L61 is a nonionic surfactant belonging to the Pluronic family of block copolymers. It is composed of a hydrophilic polyethylene oxide (PEO) block and a hydrophobic polypropylene oxide (PPO) block, arranged in a PEO–PPO–PEO structure. The molar mass of L61 is 2000 g/mol, and it has a PEO/PPO ratio of 1:6.

2.1. Synthesis of TiO2/EP Nanocomposites

Film-shaped samples of 80–100 µm in thickness were synthesized for the study. TiO2 was added to the mixture of epoxy oligomer ED-20 and DDM. The ratio between functional groups was 1:1. To improve dispersing, the mixture was treated with ultrasonic processing (Sonorex Digital 10p ultrasound bath (35 kHz, 20 min)). Then, the mixture was heated a little and poured between two glasses pretreated with parting agent. The thickness of the resulting film was set by the thickness of the spacer between the glasses. The glasses were placed preliminarily in the special metal construction and preheated to the initial curing temperature. Then, the whole construction was placed in a Binder E28 heating oven for curing at a step-by-step temperature mode (for 3 h at 90 °C and then for 3 h at 160 °C), which ensured complete polymerization. In this work, two Systems synthesized by this method are considered: I—ED-20 + DDM + TiO2 (dav = 46 nm); and II—ED-20 + DDM + TiO2 (dav = 100 nm).
System III (ED-20 + DDM + TiO2 (dav = 46 nm) + L61) was synthesized with the addition of the surfactant Pluronic L61. L61 was added to a mixture of ED-20 and TiO2 (dav = 46 nm) and treated with ultrasonic processing for 20 min. Then, DDM was added to the mixture and the mixture was sonicated again for 20 min. Then, the mixture was heated, poured between the glasses, and cured in the same technique as in Systems I and II.

2.2. Characterization

Elemental analysis of C, H, and N was performed using a Vario EL cube elemental gas analyzer manufactured by Elementar GmbH (Langenselbold, Germany, 2016), using a combustion and reduction tube, and katharometer (thermal conductivity detector) as a detector. The gas separation takes place according to the classical “purge and trap” technology on three adsorption columns. To determine the Ti content in the original nanopowders, an atomic absorption spectrophotometer AAS-3 manufactured by VEB Feinmesszeugfabrik (Oberammergau, Germany, 1988) was used.
The morphologic characteristics of the as-synthesized nanoparticles were studied using microphotographs obtained on a JEOL JEM-1400 PLUS universal transmission electron microscope manufactured by JEOL (Akishima, Japan) at an accelerating voltage of 120 kV. Histograms of particle size distribution were calculated based on the results of image analysis.
The microstructure of the synthesized nanocomposites was studied using a Zeiss LEO SUPRA 25 by Carl Zeiss (Oberkochen, Germany, 2008) scanning autoemission microscope with application of carbon sputtering.
X-ray diffraction (XRD) phase analysis was performed using an Aeris X-ray diffractometer manufactured by Malvern PANalytical B.V. (Worcestershire, UK) equipped with a ceramic X-ray tube with a copper anode manufactured by PANalytical and a fast linear detector PIXcel1D. The average crystallite size dav was calculated from the line broadening in the spectrum using the Scherrer equation (Equation (1)):
d = kλ/(βcosθ),
where k is the dimensionless coefficient of particle shape (Scherrer constant), which is equal to 0.94; β is the reflex width at half the maximum intensity; λ is the wavelength of X-rays; and θ is the diffraction angle.
A Bruker ALPHA FT-IR spectrometer by Bruker (Ettlingen, Germany) was used to obtain infrared spectra of the obtained nanocomposites by the method of attenuated total reflection (ATR) in the range of 360 to 4000 cm−1. Tablets of TiO2/KBr were created to obtain the spectra of TiO2 powders. OPUS v6.5 and OriginPro 2018 v9.5.1 software were used to control the equipment and process the data, respectively. Changes in the spectra were evaluated by the difference between the spectra of the initial cured epoxy resin and the obtained TiO2/EP nanocomposites.
Mechanical tensile tests were performed on a Zwick/Roel Z010 TC-FR010TH (Ulm, Germany, 2002) universal testing machine for tensile, compression, bending, and fracture toughness of materials. Tests were carried out according to ASTM D882-10 [49] with loading speed of 1 mm/min.
Charpy impact toughness was determined on the Charpy impact testing machine by Zwick according to ISO 179-1 [50] on samples in the form of sticks with size of 4 × 6 × 50 mm.
Thermogravimetric analysis was performed using an analyzer for thermogravimetric and differential thermal analysis TGA/SDTA851e from Mettler-Toledo GmbH (Nänikon, Switzerland) in a range of 20 to 450 °C at a temperature rise rate of 10 deg/min. A differential scanning calorimeter DSC822e/200 from Mettler-Toledo GmbH (Nänikon, Switzerland) was used to determine the glass transition temperature. The STAR software package v15.00a was used to control the instruments and process the results.

3. Results and Discussion

3.1. Materials Characterization

As noted above, two types of TiO2 of dav = 46 nm and 100 nm in size and different relative contents of rutile and anatase polymorphs were studied as nanofillers. The elemental analysis of TiO2 nanopowders, as expected, confirmed their purity (Table 1). According to the calculations, the theoretical elemental Ti content in TiO2 should be 59.95%. This value is slightly below the theoretical content because, as is known, TiO2 nanoparticles have OH groups on the surface and inside the particles [51,52,53].
The TiO2 particle sizes were determined by a transmission electron microscope (TEM) examination. In the images for two types of nanoparticles (Figure 2a,c), subspherical particles and polyhedrons were observed (probably due to the presence of two TiO2 polymorphs). It can be seen that the nanoparticles with an average diameter of 46 nm are in a strongly aggregated state, in contrast to the larger ones. The distribution histograms plotted from the images (Figure 2b,d) show a fairly wide range of particle sizes, and their average diameter was 44 nm and 104 nm.
The selected area electron diffraction (SAED) patterns (Figure 3a,b) indicate the presence of two TiO2 polymorphs of anatase and rutile. The brookite signals are mostly close to anatase and rutile signals. The absence of signals of the atomic planes (hkl) 211, 221, and 302 of brookite, which do not match the atomic planes of other polymorphs, makes it possible to conclude that it is absent in the nanoparticle composition.
The study of the obtained TiO2/EP nanocomposites using a scanning electron microscope showed the formation of large aggregates in the case of the composite with TiO2 (dav = 46 nm) even at their low content (1 wt. %). As can be seen from Figure 4a, which shows an SEM image of particle aggregates in System I, such clusters can be several times larger than the original TiO2. This fact is described in more detail in a previous paper [54], and is consistent with other authors [45,55,56]. On the contrary, in System II (dav = 100 nm), the particles are uniformly distributed throughout the polymer and there are no large aggregates (Figure 4b). This difference can be explained by the active aggregation of smaller particles due to their higher surface energy compared to larger particles.
The phase composition of the synthesized TiO2/EP was investigated by XRD (Figure 5). A typical diffraction pattern was observed for the original nanopowders, which confirmed the presence of the anatase (JCPDS card no. 21-1272 or RRUFF ID R060277.9) and rutile (JCPDS card no. 21-1276 or RRUFF ID R110109.9) polymorphic modifications of TiO2 [57,58,59]. The observed difference in the intensities of the anatase (2θ = 25.3, 37.8, 48.0, 53.9, 55.0, and 62.7 deg.) and rutile (2θ = 27.5, 36.1, 41.6, 44.0, 54.3, 56.6, and 64.1 deg.) peaks are due to the different relative contents of anatase and rutile in the samples of dav = 46 and 100 nm in size. The third TiO2 polymorph, brookite (JCPDS card no. 29-1360 or RRUFF ID R130225.9), was not detected in the obtained diffraction patterns (2θ = 25.35, 25.69, 30.81, 32.81, 36.24, 37.28, 37.96, 38.36, 38.6, 39.25, 39.94, 40.16, 42.33, 46.07, 46.39, 48.03, 49.16, 49.68, 52.06, 54.22, 54.4, and 55.24).
For the cured ED-20, the appearance of the additional lane in the amorphous halo region was observed at 2θ = 22 deg., which corresponds to the ordered epoxy polymer structure, probably formed as result of the free volume relaxation by the diffusion mechanism [60,61]. The XRD patterns of the studied composite Systems showed the diffraction reflexes of the cured ED-20 (2θ = 16.3, 21.9 deg.) and TiO2. This observation suggested that the presence of TiO2 did not affect the epoxy’s supramolecular structure formed during curing. The calculated average crystallite sizes from the Scherrer equation are 36 nm and 95 nm for the nanoparticles in Systems I and II, respectively. It should be noted that these values are close to the results obtained by SEM; therefore, the distortions of the crystal lattices due to the microstrains around the defects do not play a significant role in the broadening of the reflexes in the powder XRD patterns.
The TiO2 nanopowders and synthesized TiO2/EP nanocomposites were further characterized by Fourier transform infrared (FTIR) spectroscopy (Figure 6). In the FTIR spectra of the TiO2 nanopowders, a broad peak of Ti–O stretching vibrations is observed in the range of 900–400 cm−1 with absorption maxima at 605 and 670 cm−1, which is typical for TiO2 polymorphs [62]. Actually, this peak is a superposition of the bands related to the antisymmetric stretching vibrations of the O–Ti–O (above 600 cm–1) and Ti–O–Ti (below 500 cm–1) fragments [63]. The bands at 1633 cm–1 and in the range of 3100–3650 cm–1 correspond to the bending and stretching vibrations of adsorbed water molecules [64,65]. In the range of 900–3100 cm–1 corresponding to the vibrations of organic groups, the spectra of the cured ED-20 and composites are very similar and show a complex pattern. The FTIR spectra of the synthesized TiO2/EP nanocomposites differ from that of the cured ED-20 by an additional absorption in the range of 900–400 cm–1 corresponding to the Ti–O bonds in TiO2. According to [53,66], the wavenumber of the band of C–O stretching vibrations of the Ti–O–C fragment in the composites, with the O atom belonging to the organic matter, is about 1040 cm−1.
To study the fine changes caused by the introduction of TiO2 into the matrix structure, the difference spectra obtained by subtracting the spectrum of the composite with 4 wt. % nanoparticles from the spectrum of the cured resin were constructed (Figure 7). The intensity of the difference spectrum is increased for clarity. The most prominent was a shift in the peak position of the ordinary C–N bonds and conjugated C–N from 1031 to 1011 cm−1 and from 1294 to 1274 cm−1, respectively. Moreover, the band of =C–O–C- at 1231 cm−1 shifts to 1211 cm−1. All of these groups are polar and the shift of a small portion of these peaks by ~20 cm−1 could be caused by the dipole–dipole interactions at the surface of the TiO2 nanoparticles as well as a change in the conformation organic groups. The absence of the band at 1607 cm–1 in the difference spectrum reflects the transformations of a part of the unsaturated hydrocarbon groups during the nanocomposite preparation.

3.2. Mechanical and Thermophysical Properties

The dependence of the tensile mechanical properties of the TiO2/EP nanocomposite on the concentration and size of the TiO2 particles is shown in Figure 8a–c. It is evident that the inclusion of a small amount of nanoparticles (1 wt. %) in the studied Systems leads to an improvement in all mechanical properties. The transfer of the mechanical loading from the matrix to the harder nanoscale filler is provided by interfacial interactions leading to the improvement of the mechanical properties [67]. The results obtained confirm the formation of sufficiently strong interfacial interactions, because their absence would lead to a sharp drop in tensile strength even at low concentrations of TiO2, as shown in the work by Chatterjee A. et al. [47].
Nevertheless, Figure 8a shows that the limiting value of the tensile strength for System I is reached in the range of 1–3 wt. % TiO2, while, for System II, the tensile strength continues to increase even at a concentration of 4 wt. % TiO2. This material behavior may be caused by the aggregation of nanoparticles, as noted earlier. It is known that larger Van der Waals forces act between smaller particles. The result is the reduced ability of the nanoparticles to disperse and the formation of aggregates. Smaller particles aggregate actively at lower concentrations than larger ones [39]. This fact explains the decrease in tensile strength at TiO2 concentrations exceeding 1–3 wt. % in System I and 5 wt. % in System II. It is logical that, at the maximum nanoparticle concentration, their interaction with each other prevails over their interaction with epoxy matrix, thus forming clusters of several hundred nanometers in size. It is important to note that the original nanoparticles have a close true density (4.2495 g/cm3 for dav = 46 nm and 4.2700 g/cm3 for dav = 100 nm) contrary to their bulk density (0.187 g/cm3 for dav = 46 nm and 0.457 g/cm3 for dav = 100 nm). Therefore, the less bulk density, the more volumetric filling with the same weight filling. A larger volume filling with smaller particles leads to a decrease in the distance between the particles, which also contributes to their aggregation. These aggregates act as defects of the nanocomposite materials leading to the loss of high mechanical characteristics. The addition of surfactants in System III improves the dispersion of nanoparticles, having a direct impact on the physical properties, including the tensile strength. In System III, the ultimate tensile strength is reached at 1 wt. % TiO2 content and is the maximum among the considered Systems. With a further increase in the TiO2 content, the tensile strength decreases to 68.5 MPa (5 wt. %). The necessity of a high degree of dispersion of nanoparticles and strong interfacial interactions in the synthesis of nanocomposites is demonstrated in the review by Irzhak and Uflyand [68].
As expected, the elasticity modulus of Systems I and II increases with the TiO2 concentration (Figure 8b). For System II, the increase in the elasticity modulus was 10–15% higher than for System I. Thus, the increase in the elasticity modulus additionally confirms the formation of sufficiently strong interactions between the nanoparticles and the matrix [69,70]. It should be noted that, in System I, the elasticity modulus continues to increase at concentrations of TiO2 above 3 wt. %, while the strength, on the contrary, decreases. A similar result has been published in the articles [47,71], in which the nanoparticle addition leads to an increase in the elasticity modulus while the strength decreases. In System III, due to the addition of a surfactant, the elasticity modulus is expected to decrease.
A distinction between Systems I and II was also observed when considering the relative elongation in the tensile test (Figure 8c). It was shown that, for System I, a maximum elongation was observed in the 1 wt. % TiO2 concentration, while, for System II, it was observed in the 4 wt. % TiO2 concentration. This behavior may be related to the specific surface area of the nanoparticles and, consequently, to the different content of the specific interface area in the composites. It seems that there is a limit of the interface specific area that ensures the best mechanical properties of nanocomposites to be achieved. This regularity may be related to the difference in the volumetric filling, which was mentioned above. A further decrease in the tensile strain can be explained by the fact that, with an increase in the concentration of nanoparticles and their aggregates, the mobility of the polymer chains decreases. The result is the more brittle behavior of the material. The elongation in System III does not have a pronounced maximum, contrary to Systems I and II. The addition of TiO2 and L61 increases the elongation to values close to the obtained values for Systems I and II, but when the concentration of nanoparticles increases in the studied range, the numerical value of the characteristic changes weakly.
The addition of TiO2 significantly increases the impact toughness of the epoxy polymer from 7 kJ/m2 to 22 kJ/m2 for System I and up to 34 kJ/m2 for System II, as shown in Figure 8d. The critical point for System I becomes the 2 wt. % TiO2, while, in System II, a sharp drop in the characteristic is observed above 4 wt. %. System II with larger nanoparticles showed the best impact toughness of the composites. The impact toughness of a material is related to the energy required to fracture a material and its numeric value the ability of the material to resist the impact. Usually, the impact properties of polymers are improved by smaller particles, because large particles can act as centers of stress concentration and, consequently, crack formation centers [21]. However, the greater activity of small particles facilitates their agglomeration and, consequently, hampers their dispersion. The agglomerated particles act as the centers of formation of large defects, which leads to a decrease in the impact toughness. Nevertheless, well-dispersed TiO2 nanoparticles can block crack propagation through the polymer and, as a consequence, increase the impact toughness of the nanocomposite.
The dependences of the Tg and thermal stability on the TiO2 concentration were investigated. The Tg value for the cured epoxy resin was 172 °C and practically did not change with the increase of TiO2 concentration (Table 2). The Tg value of the cross-linked polymers is closely related to the cross-linking density and mobility of the polymer chain segments. The inclusion of nanofillers usually leads to a decrease in the cross-linking degree and consequently decreases the Tg. The aggregation of nanofillers in the polymer matrix reduces the interface and produces an additional free volume that promotes the chain segments’ movement. This phenomenon was observed in earlier studies on epoxy composites [41,72]. In the studied TiO2/EP nanocomposites, nanoparticles also form large aggregates; however, Tg does not demonstrate a decreasing tendency. Perhaps the nanoparticles are involved in the cross-linking of the epoxy, which makes it possible to compensate for the increasing mobility of the matrix chain segments. A similar effect was observed earlier [73]. A good dispersion of the nanoparticles in the polymers is necessary to increase Tg in the composites [44,45].
The filling of epoxy polymer with TiO2 increases the thermal decomposition temperature from 347 °C to 369 °C and 374 °C for Systems I and II, respectively, at a TiO2 concentration of 1 wt. % (Table 3). At the same time, this characteristic practically did not depend on the TiO2 concentration in the range of 1–5 wt. %. Similar results were obtained by Sagar et al. [70]. Moreover, Parameswaranpillai et al. [74] showed that, regardless of the nanofiller content, the mass loss in the composites is almost the same.

4. Conclusions

Epoxy composites based on epoxy dian resin and nanocrystalline titanium dioxide were obtained and characterized by varying its content in the polymer matrix (1–5 wt. %) and particle size (dav = 46 and 100 nm). TiO2 nanoparticles were prepared by the plasma-chemical method and dispersed in the native epoxy resin using an ultrasonic treatment. The TiO2/EP nanocomposites were obtained by a curing reaction according to a two-step temperature regime in the presence of an aromatic diamine DDM.
According to the IR spectroscopy data, it was shown for the first time that mainly the polar (nonconjugated C–N and conjugated = C–N and = C–O–C–) groups as well as the unsaturated hydrocarbon groups of the polymer matrix are involved in the interaction with the TiO2 nanoparticles, indicating the interfacial layer formation and, consequently, improvement of the mechanical properties. The difference in IR spectra show that, in the interface interactions, mainly the conjugated and ordinary C–N bonds of the matrix as well as the aromatic groups are involved.
The study of the mechanical properties of the obtained epoxy nanocomposites showed that the introduction of TiO2 nanoparticles into the epoxy resin simultaneously improved the elasticity modulus, tensile strength, and fracture toughness of the TiO2/EP composites, with the nature of changes and quantitative parameters depending on both the content of the dispersed phase and the size of the TiO2 nanoparticles. It was noted that, in the case of System I (dav = 46 nm), the tendency for the nanoparticles to aggregate during curing is higher than for System II with larger particle sizes in the initial state, which is correspondingly reflected in the mechanical properties of the obtained epoxy composites. Using a Pluronic L61 surfactant in System III improved the nanoparticle distribution and prevented the formation of large aggregates, resulting in an improved tensile strength but lower elasticity modulus, contrary to Systems without the surfactant.
The addition of TiO2 nanoparticles significantly increases the impact toughness of the epoxy polymer (3.1 times for System I and 4.9 times for System II). This is probably due to a lower degree of nanoparticles agglomeration and their more homogeneous distribution in the polymer matrix in the case of System II. Thus, for the first time, the simultaneous improvement of the strength properties and impact toughness of the composites has been shown for the TiO2/EP system. Probably a necessary condition for such behavior is the absence of a tendency for the agglomeration of nanoparticles, without using, for example, surfactants or modifying agents preventing particle agglomeration, which was demonstrated in the example of System II. At the same time, the presence of nanocrystalline titanium dioxide does not lead to a significant change in the glass transition temperature of the composites as compared to the native epoxy polymer, which may indicate insignificant changes in the degree of cross-linking in the polymer matrix.
Additional research is needed to establish the mechanism of hardening and, simultaneously, improvement of the impact toughness of the obtained epoxy nanocomposites.

Author Contributions

Conceptualization, G.I.D.; Investigation, Y.S.B., L.M.B., V.A.L., N.V.C. and N.D.G.; Writing—original draft, Y.S.B. and L.M.B.; Writing—review & editing, N.V.C. and G.I.D.; Visualization, Y.S.B.; Supervision, G.I.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

This work was performed in accordance with the state task (state registration no. AAAA-A19-119101590029-0 and AAAA-A19-119032690060-9) using the equipment of the Multi-User Analytical Center of FRC PCP and MC RAS.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 04488 g001 550
Figure 1. Structural formulae: (a) ED-20 epoxy oligomer and (b) 4,4’-diaminodiphenylmethane.
Figure 1. Structural formulae: (a) ED-20 epoxy oligomer and (b) 4,4’-diaminodiphenylmethane.
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Applsci 13 04488 g002 550
Figure 2. TEM results of the as-synthesized TiO2 samples. For the dav = 46 nm sample: TEM image (a), and size distribution histogram (b). For the dav = 100 nm sample: TEM image (c), and size distribution histogram (d).
Figure 2. TEM results of the as-synthesized TiO2 samples. For the dav = 46 nm sample: TEM image (a), and size distribution histogram (b). For the dav = 100 nm sample: TEM image (c), and size distribution histogram (d).
Applsci 13 04488 g002
Applsci 13 04488 g003 550
Figure 3. SAED patterns of the as-synthesized TiO2 samples: dav = 46 nm (a), and dav = 100 nm (b). The following symbols are used: A—anatase, R—rutile, and B—brucite. The numbers in parentheses correspond to the hkl indices of the atomic planes.
Figure 3. SAED patterns of the as-synthesized TiO2 samples: dav = 46 nm (a), and dav = 100 nm (b). The following symbols are used: A—anatase, R—rutile, and B—brucite. The numbers in parentheses correspond to the hkl indices of the atomic planes.
Applsci 13 04488 g003
Applsci 13 04488 g004 550
Figure 4. SEM image of particle dispersion for (a) System I (dav = 46 nm, 1 wt. % TiO2); and (b) System II (dav = 100 nm, 1 wt. % TiO2).
Figure 4. SEM image of particle dispersion for (a) System I (dav = 46 nm, 1 wt. % TiO2); and (b) System II (dav = 100 nm, 1 wt. % TiO2).
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Applsci 13 04488 g005 550
Figure 5. XRD patterns: 1—TiO2 (dav = 46 nm); 2—TiO2 (dav = 100 nm); 3—cured epoxy; 4—TiO2/EP of System I (4 wt. % TiO2); and 5—TiO2/EP of System II (4 wt. % TiO2). The following symbols are used: A—anatase, R—rutile, and E—cured epoxy resin.
Figure 5. XRD patterns: 1—TiO2 (dav = 46 nm); 2—TiO2 (dav = 100 nm); 3—cured epoxy; 4—TiO2/EP of System I (4 wt. % TiO2); and 5—TiO2/EP of System II (4 wt. % TiO2). The following symbols are used: A—anatase, R—rutile, and E—cured epoxy resin.
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Figure 6. ATR-FTIR spectra: 1—TiO2 (dav = 46 nm); 2—TiO2 (dav = 100 nm); 3—cured epoxy; 4—TiO2/EP of System I (4 wt. % TiO2); and 5—TiO2/EP of System II (4 wt. % TiO2). AR denotes aromatic groups, and =C–N denotes conjugated C–N bonds.
Figure 6. ATR-FTIR spectra: 1—TiO2 (dav = 46 nm); 2—TiO2 (dav = 100 nm); 3—cured epoxy; 4—TiO2/EP of System I (4 wt. % TiO2); and 5—TiO2/EP of System II (4 wt. % TiO2). AR denotes aromatic groups, and =C–N denotes conjugated C–N bonds.
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Figure 7. IR spectrum of cured ED-20 (1); difference spectrum obtained as a result of subtraction of the spectrum of the TiO2/EP nanocomposite (3 wt. % TiO2 dav = 46 nm) from the spectrum of the cured ED-20 (2).
Figure 7. IR spectrum of cured ED-20 (1); difference spectrum obtained as a result of subtraction of the spectrum of the TiO2/EP nanocomposite (3 wt. % TiO2 dav = 46 nm) from the spectrum of the cured ED-20 (2).
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Figure 8. Mechanical properties: tensile strength (a), elasticity modulus (b), and elongation under maximum tension (c); and Charpy impact strength (d). Symbols: I—TiO2/EP of System I (dav = 46 nm), II—TiO2/EP of System II (dav = 100 nm), and III—TiO2/EP of System III (dav = 46 nm + L61).
Figure 8. Mechanical properties: tensile strength (a), elasticity modulus (b), and elongation under maximum tension (c); and Charpy impact strength (d). Symbols: I—TiO2/EP of System I (dav = 46 nm), II—TiO2/EP of System II (dav = 100 nm), and III—TiO2/EP of System III (dav = 46 nm + L61).
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Table
Table 1. Results of elemental analysis of TiO2 nanopowders.
Table 1. Results of elemental analysis of TiO2 nanopowders.
dav TiO2, nmTi, wt. % (Calc.)Ti, wt. % (Exp.)
4659.9559.54 ± 0.06
10059.9559.59 ± 0.01
Table
Table 2. Glass transition temperatures (°C) for TiO2/EP nanocomposites of Systems I and II.
Table 2. Glass transition temperatures (°C) for TiO2/EP nanocomposites of Systems I and II.
Particle Size, nmTiO2, wt. %
0123451015
46 (System I)172168169171171173--
100 (System II)--168168166--
21 (Rajabi L., 2013) [73]51.36160.5--63.560.5
48 (Goyat M., 2018) [44]62656871-758063
48 (Goyat M., 2018) [45]70778085-8910082
Table
Table 3. Decomposition onset temperatures for TiO2/EP nanocomposites.
Table 3. Decomposition onset temperatures for TiO2/EP nanocomposites.
Nanoparticle Size, nmTiO2 wt. %
012345
46 (System I)347369368367368368
100 (System II)374372372371371
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Bukichev, Y.S.; Bogdanova, L.M.; Lesnichaya, V.A.; Chukanov, N.V.; Golubeva, N.D.; Dzhardimalieva, G.I. Mechanical and Thermophysical Properties of Epoxy Nanocomposites with Titanium Dioxide Nanoparticles. Appl. Sci. 2023, 13, 4488. https://doi.org/10.3390/app13074488

AMA Style

Bukichev YS, Bogdanova LM, Lesnichaya VA, Chukanov NV, Golubeva ND, Dzhardimalieva GI. Mechanical and Thermophysical Properties of Epoxy Nanocomposites with Titanium Dioxide Nanoparticles. Applied Sciences. 2023; 13(7):4488. https://doi.org/10.3390/app13074488

Chicago/Turabian Style

Bukichev, Yurii S., Lyudmila M. Bogdanova, Valentina A. Lesnichaya, Nikita V. Chukanov, Nina D. Golubeva, and Gulzhian I. Dzhardimalieva. 2023. "Mechanical and Thermophysical Properties of Epoxy Nanocomposites with Titanium Dioxide Nanoparticles" Applied Sciences 13, no. 7: 4488. https://doi.org/10.3390/app13074488

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