**2. Polyimides: the structure of polyimide matrices**

directional changes in structure and properties under physicochemical effects. To reduce the

Most of the composite materials are developed for the aerospace industry, which has always been and still is the most high-tech branch of modern production. At the same time, these materials and technologies for their production are also innovative drivers in many other

To produce materials with increased rigidity, impact, and tribological properties, during the last decade a lot of research has been devoted to the modification of polymers by nanoparticles. These composites exhibit unique properties that combine the advantages of inorganic fillers, such as stiffness, high thermal stability, and mechanical properties with processability, flexibility, and plasticity of organic polymers. Due to the influence of nanosized fillers on the bulk properties of polymer nanocomposites, it is possible to achieve such unique properties

Polymeric nanocomposites representing a new class of materials have unique barrier properties, electrical conductivity, thermal conductivity, increased strength, heat resistance, and thermal stability, as well as reduced combustibility. It is known that the addition of nanodispersed layered silicates and various forms of carbon nanofillers to polymeric matrices can significantly affect the mechanisms of thermal and thermooxidative destruction and burning of nanocomposites.

One of the most important issues facing nanotechnology is how to get molecules to group themselves in a certain way, to organize themselves, in order to eventually obtain new materials or devices capable of long-term preservation of their performance properties under the action of high and very low temperatures, chemical agents, increased radiation, and other factors. One of the main ways to solve this problem is the creation of composite materials based on a polyimide (PI) matrix. Adding different amounts of nanoparticles at different stages of polymerization of the matrix, increasing the number of available monomers (dianhydrides acid), and reversibility of the imidization reaction (the second stage of the synthesis reaction) will allow to vary the molecular and molecular mass characteristics and, as a consequence, their thermal resistance and solubility to processing, deformation-strength, and other properties of future composites.

In the world literature, examples of nanocomposite materials based on polyimide matrix filled with carbon nanotubes (CNTs), carbon fibers, and nanostructured silicon carbide (SiC) are known, and their thermal and deformation strength and other properties have been measured. Expanding the diversity of polyimide matrices and varying the content of nanostruc-

Thus, creation of new composite materials with high performance characteristics and technol-

During the last decade, a lot of research has been devoted to the combination of polymers with nanoparticles to produce materials with increased rigidity, impact, and tribological properties [1]. The growing demand for nanomaterials is due to the fact that new chemical and physical properties are achievable with the addition of nanosized fillers to the polymer matrix, even if the same material without a nanofiller does not have such advantages. This is due to the influence of the unique nature of the nanosized filler on the bulk properties of nanocomposites on

tured materials allow us to obtain a huge variety of composite materials.

ogy for their production is a very urgent and important task.

cost of material and give it special properties, various fillers are actively used.

sectors, such as construction, engineering, energy, instrumentation, medicine.

by adding small amounts of nanofillers to the polymer matrix.

86 Characterizations of Some Composite Materials

Polyimides are one of the most interesting polymers, which have increased heat resistance and are widely used in the manufacture of high-temperature plastics, adhesives, dielectrics, and other materials.

Currently, polyimide resins are used as matrices to create reinforced composites based on lightweight carbon fibers, as a replacement for metal parts in the aerospace industry and airframe, due to their outstanding thermal and mechanical resistance, as well as resistance to the action of ionizing radiation. Polyimide resins are widely used in such areas as microelectronics, aerospace, gas separation, and the production of fuel cells. They are used in the cable industry for the production of electrical insulating varnishes and enamels, which have high heat resistance, elasticity, and good dielectric properties.

As is known, polyimides (PI) can be aliphatic, alicyclic, or aromatic, depending on the chemical structure. Depending on the structure of the chain, polyimides can be linear or three-dimensional [6]. There are polyimides with aliphatic links in the main chain of the macromolecule and purely aromatic polyimides. The first are solid, readily crystallizable substances of white or yellow color. Polypyromellitimides based on aliphatic diamines containing less than seven carbon atoms in the molecule have high melting points that are higher than the temperatures of their onset of decomposition (above 350°C) and do not dissolve in known organic solvents. Polypyromellitimides based on aliphatic diamines containing, in the chain, more than seven carbon atoms or having a branched hydrocarbon chain (at least seven carbon atoms), as well as polyimides of other aromatic tetracarboxylic acids and various aliphatic diamines, soften at temperatures of 300°C.

Aromatic polyimides are characterized by high heat resistance, and the most heat-resistant polyimides based on pyromellitic acid (**Figure 1**) and 1,4,5,8-naphthalenetetracarboxylic (**Figure 2**) acids, practically not softening before the onset of thermal decomposition, have a glass transition temperature of 500°C.

The heat resistance of other polyimides is well regulated by varying the nature of the monomers and is usually 300–430°C [7]. Most aromatic polyimides, especially high-heat-resistant,

which the mass of the residue remains practically unchanged up to 3000°C. Aromatic polyimides are also stable under conditions of prolonged isothermal heating; there is a decrease in the mass of poly-4,4′-diphenylene oxide-pyromellitimide after heating in an inert atmosphere for 15 hours at 400, 450, and 500°C by 1.5, 3.0, and 7.0%, respectively. Significantly more intensive polyimides decompose during thermal oxidation. The main products of destruction

Nanocomposite Polyimide Materials http://dx.doi.org/10.5772/intechopen.79889 89

To date, a wide range of polymers have been obtained by changing the chemical structure of the dianhydride (Q) and diamine (R) fragments of macromolecules (**Figure 3**), differing in

By chemical structure and physical properties (softened and melted), they are divided into

• Group A: polyimides consist only of aromatic groups and imide cycles. These polyimides are non-softening, Tm > Td, hard, brittle with maximum heat resistance (**Figure 4**).

• Group B are the PI with hinges (connections around which a chain turn is possible) in the dianhydride fragment. These are non-softening, rigid polymers, with some elasticity

• Group C are the PI with hinges in the diamine fragment. These are rigid, strong, and elastic polymers that do not have a clearly defined range of softening temperatures (**Figure 6**). • Group D are the PI with hinges in the diamine and in the dianhydride fragment. These

So, all the cyclic structures can be divided into several types, the thermostability of which will decrease in the following order: conjugated carbocycles (carbon) ≥ ladder structures >

As noted above, the stability of polyimides at high temperatures is naturally determined primarily by their chemical structure. The data on the dependence of thermal stability on the chemical structure are of great importance both in determining the ways of further synthesis of thermostable polyimides and in selecting among them the most practically promising ones. A huge number of polyimides obtained on the basis of a large number of dianhydrides of tetracarboxylic acids have been synthesized and characterized. The main practical application

polymers are elastic and have a clear region of softening and melting (**Figure 7**).

condensed heterocycles > isolated heterocycles > non-conjugated carbocycles [6].

.

of aromatic polyimides are CO and CO2

**Figure 3.** General structural formula of polyimide.

structure and properties.

four groups.

(**Figure 5**).

**Figure 1.** Structural formula of pyromellitic acid.

**Figure 2.** Structural formula of 1,4,5,8-naphthalenetetracarboxylic acid.

are insoluble in known organic solvents and are inert to the action of oils, and also hardly change under the action of dilute acids.

With the introduction of various substituents into the side chain, especially card groups (phthalide, phthalimidine, fluorene, anthrone), the solubility of polyimides is substantially improved. Thus, polypyromellitimide aniline phthalate is soluble in dimethylformamide (DMF), m-cresol, symtetrachloroethane, and hexafluoro-2-propanol. Polyimides based on 3,3′,4,4′-benzophenone tetracarboxylic acid or 3,3′,4,4′-diphenyloxide tetracarboxylic acid and anilinfluorene polyimides are also dissolved in methylene chloride and chloroform [8]. Under the influence of alkalis and superheated steam, aromatic polyimides hydrolyze. However, the propensity for hydrolysis depends significantly on their nature. Thus, polyimides with five-membered imide cycles are much less hydrolytically stable than analogous polyimides with six-membered rings.

Aromatic polyimides are distinguished by high radiation resistance [9]. Thus, poly(4,4′ diphenylene oxide pyromellitimide) films retain good mechanical and electrical characteristics after irradiation with high-energy electrons at a dose of 102 MJ/kg (films of polystyrene and polyethylene terephthalate become brittle after irradiation with a dose of 5 MJ/kg). Polyimides are resistant to the action of ozone, i.e., retain 50% strength after exposure to 3700 hours in air with an admixture of 2% ozone; they are also resistant to UV radiation. An important feature of aromatic polyimides is their high thermal stability.

The most heat-resistant are polyimides containing only imide rings and aromatic rings. In a vacuum and an inert atmosphere, aromatic polypyromellitimides are resistant to 500°C, and a significant reduction in mass (up to ~65% of the initial one) occurs above this temperature, after which the mass of the residue remains practically unchanged up to 3000°C. Aromatic polyimides are also stable under conditions of prolonged isothermal heating; there is a decrease in the mass of poly-4,4′-diphenylene oxide-pyromellitimide after heating in an inert atmosphere for 15 hours at 400, 450, and 500°C by 1.5, 3.0, and 7.0%, respectively. Significantly more intensive polyimides decompose during thermal oxidation. The main products of destruction of aromatic polyimides are CO and CO2 .

To date, a wide range of polymers have been obtained by changing the chemical structure of the dianhydride (Q) and diamine (R) fragments of macromolecules (**Figure 3**), differing in structure and properties.

By chemical structure and physical properties (softened and melted), they are divided into four groups.


So, all the cyclic structures can be divided into several types, the thermostability of which will decrease in the following order: conjugated carbocycles (carbon) ≥ ladder structures > condensed heterocycles > isolated heterocycles > non-conjugated carbocycles [6].

As noted above, the stability of polyimides at high temperatures is naturally determined primarily by their chemical structure. The data on the dependence of thermal stability on the chemical structure are of great importance both in determining the ways of further synthesis of thermostable polyimides and in selecting among them the most practically promising ones.

A huge number of polyimides obtained on the basis of a large number of dianhydrides of tetracarboxylic acids have been synthesized and characterized. The main practical application

**Figure 3.** General structural formula of polyimide.

**Figure 2.** Structural formula of 1,4,5,8-naphthalenetetracarboxylic acid.

change under the action of dilute acids.

**Figure 1.** Structural formula of pyromellitic acid.

88 Characterizations of Some Composite Materials

are insoluble in known organic solvents and are inert to the action of oils, and also hardly

With the introduction of various substituents into the side chain, especially card groups (phthalide, phthalimidine, fluorene, anthrone), the solubility of polyimides is substantially improved. Thus, polypyromellitimide aniline phthalate is soluble in dimethylformamide (DMF), m-cresol, symtetrachloroethane, and hexafluoro-2-propanol. Polyimides based on 3,3′,4,4′-benzophenone tetracarboxylic acid or 3,3′,4,4′-diphenyloxide tetracarboxylic acid and anilinfluorene polyimides are also dissolved in methylene chloride and chloroform [8]. Under the influence of alkalis and superheated steam, aromatic polyimides hydrolyze. However, the propensity for hydrolysis depends significantly on their nature. Thus, polyimides with five-membered imide cycles are

much less hydrolytically stable than analogous polyimides with six-membered rings.

important feature of aromatic polyimides is their high thermal stability.

Aromatic polyimides are distinguished by high radiation resistance [9]. Thus, poly(4,4′ diphenylene oxide pyromellitimide) films retain good mechanical and electrical characteristics after irradiation with high-energy electrons at a dose of 102 MJ/kg (films of polystyrene and polyethylene terephthalate become brittle after irradiation with a dose of 5 MJ/kg). Polyimides are resistant to the action of ozone, i.e., retain 50% strength after exposure to 3700 hours in air with an admixture of 2% ozone; they are also resistant to UV radiation. An

The most heat-resistant are polyimides containing only imide rings and aromatic rings. In a vacuum and an inert atmosphere, aromatic polypyromellitimides are resistant to 500°C, and a significant reduction in mass (up to ~65% of the initial one) occurs above this temperature, after

There are two general methods for obtaining composites. One is the mixing of CNTs with a polymer matrix in a molten form to produce a composite. The other is the dispersion of CNTs

Park et al. have reported on a method of effective dispersion of single-walled carbon nanotubes (SWNTs) in a polyimide matrix [11]. The obtained SWNT polyimide films are electrically conductive and optically transparent. A jump in the conductivity was observed between 0.02 and 0.1 vol.% SWNT; during this process the nanocomposite was converted from a capacitor to a conductor. Appending 0.1 vol.% SWNT increased the conductivity by 10 orders of magnitude, which exceeds the antistatic criterion for thin films for space applications (1 × 10−<sup>8</sup> S cm−<sup>1</sup>

polyimide film containing 1.0 vol.% SWNT still passed 32% of visible light at 500 nm, while the film obtained by direct mixing passed less than 1%. A dynamic mechanical test showed that elastic modulus increases by 60% by addition of 1.0 vol.% SWNT; also the thermal stabil-

Connel et al. [12] have reported on a synthesis of alkoxysilane polyamide acid (PAA), with SWNTs added to the previously prepared polyamide acid solution. When the loading was 0.05 wt.% SWNT, the percolation barrier was reached, which is evident from the sharp drop in the surface resistance of the material. Surface resistance and bulk resistance indicate that the SWNT polyimide composite is conductive. However, the presence of SWNT in polyimide has very small influence on Tg and the tensile strength of the polymer [12]. Increase in ionic strength

ing 0.03 wt.% SWNT led to films, reduced by four orders of magnitude of the surface and bulk resistance [13, 14]. There is an increased electrical conductivity of nanocomposite films; however, electric percolation occurs at larger loads than those that are commonly used in SWNT polyimide nanocomposites. The parameters of the film modulus slightly increase with growth of SWNT content. Electrospun fibers were obtained from the same SWNT polyimide suspensions used for the preparation of films. Images obtained by high-resolution scanning electron microscopy showed that SWNT are inside fibers and can have an orientation parallel to the fiber axis [15]. Sun and coworkers [16] have reported on the production of functionalized CNT by using polyimides with side hydroxyl groups. It was found that the obtained polyimide functionalized CNT are soluble in the same solvents as the original polyimide. A significant advantage of this method is that these functionalized nanotubes can be used directly to produce

Electrically conductive polyimide composites are made from corresponding polyimides and electrically conductive fillers, such as carbon nanotubes, graphite, and acetylene black. The polyimide precursor (polyamide acid) was synthesized from 3,4,3′,4′-biphenyl tetracarboxylic dianhydride and 4,4′-diaminodiphenyl ether by means of intensive mechanical stirring at −5°C. The results of the experiments showed that the electrically conductive composites based on carbon nanotubes and polyimide possess better electrical, mechanical, and adhesive

In addition to carbon nanotubes, one of the promising fillers is nanostructured silicon carbide (SiC). SiC nanoparticles are chosen because of their unique physical properties, such as

). A

91

) led to the formation of an SWNT

into a composite contain-

Nanocomposite Polyimide Materials http://dx.doi.org/10.5772/intechopen.79889

in the polymer solution, curing the resulting solution and removing the solvent.

ity of polyimide improves in the presence of SWNT.

of the polyimide matrix by adding an inorganic salt (CuSO4

network sufficient for conductivity. The addition of 0.014 wt.% CuSO4

polyimide-CNT composites with relatively high content of nanotubes.

properties than the other two composites [17].

**Figure 4.** Benzidine polypyromellitimide (Group A).

**Figure 5.** 3,3′,3,4′-Benzophenone tetracarboxylic acid dianhydride and p-phenylenediamine polyimide (Group B).

**Figure 6.** Polyimide based on pyromellitic dianhydride (Group C).

**Figure 7.** Polyimide based on 3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride (Group D).

was found for polyimides made of pyromellitic acid dianhydrides; 3,3′,4,4′-diphenyloxide tetracarboxylic acid; and 3,3′,4,4′-benzophenone tetracarboxylic acid.

#### **3. Composite materials containing nanosized filler**

The introduction of small amounts of modern fillers, such as carbon nanotubes, carbon fibers, and nanostructured silicon carbide, increases the thermal stability, which makes it possible to obtain a composite material with high performance properties.

Recently, carbon nanotubes (CNTs) have attracted attention, since nanocomposites based on them will have improved mechanical properties. In addition, they can provide a certain type of electrical conductivity. It is expected that the combination of CNTs and polyimides will play an important role in the development of new, highly effective nanocomposites [10]. There are two general methods for obtaining composites. One is the mixing of CNTs with a polymer matrix in a molten form to produce a composite. The other is the dispersion of CNTs in the polymer solution, curing the resulting solution and removing the solvent.

Park et al. have reported on a method of effective dispersion of single-walled carbon nanotubes (SWNTs) in a polyimide matrix [11]. The obtained SWNT polyimide films are electrically conductive and optically transparent. A jump in the conductivity was observed between 0.02 and 0.1 vol.% SWNT; during this process the nanocomposite was converted from a capacitor to a conductor. Appending 0.1 vol.% SWNT increased the conductivity by 10 orders of magnitude, which exceeds the antistatic criterion for thin films for space applications (1 × 10−<sup>8</sup> S cm−<sup>1</sup> ). A polyimide film containing 1.0 vol.% SWNT still passed 32% of visible light at 500 nm, while the film obtained by direct mixing passed less than 1%. A dynamic mechanical test showed that elastic modulus increases by 60% by addition of 1.0 vol.% SWNT; also the thermal stability of polyimide improves in the presence of SWNT.

**Figure 5.** 3,3′,3,4′-Benzophenone tetracarboxylic acid dianhydride and p-phenylenediamine polyimide (Group B).

was found for polyimides made of pyromellitic acid dianhydrides; 3,3′,4,4′-diphenyloxide

The introduction of small amounts of modern fillers, such as carbon nanotubes, carbon fibers, and nanostructured silicon carbide, increases the thermal stability, which makes it possible to

Recently, carbon nanotubes (CNTs) have attracted attention, since nanocomposites based on them will have improved mechanical properties. In addition, they can provide a certain type of electrical conductivity. It is expected that the combination of CNTs and polyimides will play an important role in the development of new, highly effective nanocomposites [10].

tetracarboxylic acid; and 3,3′,4,4′-benzophenone tetracarboxylic acid.

**Figure 7.** Polyimide based on 3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride (Group D).

**3. Composite materials containing nanosized filler**

obtain a composite material with high performance properties.

**Figure 6.** Polyimide based on pyromellitic dianhydride (Group C).

**Figure 4.** Benzidine polypyromellitimide (Group A).

90 Characterizations of Some Composite Materials

Connel et al. [12] have reported on a synthesis of alkoxysilane polyamide acid (PAA), with SWNTs added to the previously prepared polyamide acid solution. When the loading was 0.05 wt.% SWNT, the percolation barrier was reached, which is evident from the sharp drop in the surface resistance of the material. Surface resistance and bulk resistance indicate that the SWNT polyimide composite is conductive. However, the presence of SWNT in polyimide has very small influence on Tg and the tensile strength of the polymer [12]. Increase in ionic strength of the polyimide matrix by adding an inorganic salt (CuSO4 ) led to the formation of an SWNT network sufficient for conductivity. The addition of 0.014 wt.% CuSO4 into a composite containing 0.03 wt.% SWNT led to films, reduced by four orders of magnitude of the surface and bulk resistance [13, 14]. There is an increased electrical conductivity of nanocomposite films; however, electric percolation occurs at larger loads than those that are commonly used in SWNT polyimide nanocomposites. The parameters of the film modulus slightly increase with growth of SWNT content. Electrospun fibers were obtained from the same SWNT polyimide suspensions used for the preparation of films. Images obtained by high-resolution scanning electron microscopy showed that SWNT are inside fibers and can have an orientation parallel to the fiber axis [15].

Sun and coworkers [16] have reported on the production of functionalized CNT by using polyimides with side hydroxyl groups. It was found that the obtained polyimide functionalized CNT are soluble in the same solvents as the original polyimide. A significant advantage of this method is that these functionalized nanotubes can be used directly to produce polyimide-CNT composites with relatively high content of nanotubes.

Electrically conductive polyimide composites are made from corresponding polyimides and electrically conductive fillers, such as carbon nanotubes, graphite, and acetylene black. The polyimide precursor (polyamide acid) was synthesized from 3,4,3′,4′-biphenyl tetracarboxylic dianhydride and 4,4′-diaminodiphenyl ether by means of intensive mechanical stirring at −5°C. The results of the experiments showed that the electrically conductive composites based on carbon nanotubes and polyimide possess better electrical, mechanical, and adhesive properties than the other two composites [17].

In addition to carbon nanotubes, one of the promising fillers is nanostructured silicon carbide (SiC). SiC nanoparticles are chosen because of their unique physical properties, such as excellent chemical stability, heat resistance, high electron mobility, excellent thermal conductivity, outstanding mechanical properties. They are used to produce high-performance composites and are used in electronics [18, 19]. These properties make SiC nanoparticles a suitable material for the production of polymer nanocomposites with a reinforced structure [20].

**4. Preparation of composite materials based on a polyimide matrix** 

Simple mixing is the process in which a random distribution of the particles of the initial components in the volume of the mixture occurs without changing their initial dimensions. Dispersive mixing is a process of mixing, which is accompanied by a change (decrease) in the initial particle sizes of the components, due to their fragmentation, aggregate destruction, deformation, and disintegration of the dispersed phase, etc. The main task of dispersive mixing is to destroy aggregates of solid particles and distribute them in the volume of a liquid

Nanocomposite Polyimide Materials http://dx.doi.org/10.5772/intechopen.79889 93

When creating polymer nanocomposites with an already prepared nanofiller, three main

Since the polyimide matrixes used are insoluble in organic solvents and their softening tem-

Carbon nanotubes have excellent mechanical, electrical, and magnetic properties, as well as a nanometer scale with a high length-to-diameter ratio, which makes them an ideal reinforcing agent for high-strength polymer composites. However, CNTs usually form bundles stabilized by van der Waals forces, which are extremely difficult to disperse in the polymer matrix. The biggest problem in the production of reinforced CNT composites is the efficiency of the dispersion of CNTs in the polymer matrix. There are several methods for dispersing nanotubes in a polymer matrix, such as mixing in solution, melt mixing, electroforming, in-situ

With this approach, the dispersion of carbon nanotubes in a suitable solvent and the polymer are mixed in solution. The CNT/polymer composite is formed by precipitation or by evaporation of the solvent. It is well known that it is very difficult to efficiently disperse nanotubes in a solvent by simple mixing. Processing with high-power ultrasound is more effective in the formation of the dispersion of CNTs. Ultrasonic treatment is widely used for dispersing, emulsifying, crushing, and activating particles. With the help of ultrasound, it is possible to

**modified by inorganic nanofillers**

• Melt mixing (for thermoplastic polymers)

polymer [23].

methods are used:

• In-situ polymerization

*4.1.1. Mixing in solution*

There are two basic mixing mechanisms: simple and dispersive.

• Mixing in solution (for solubles in organic solvents of polymers)

perature exceeds 300°C, it is advisable to use in-situ polymerization.

polymerization, and chemical functionalization of carbon nanotubes.

effectively destroy aggregates and coils of carbon nanotubes.

**modified with carbon nanotubes and silicon carbide**

**4.1. Methods for producing composite materials based on a polyimide matrix** 

There are reports of the nanocomposite films' properties, which were obtained by two simple methods from a new polyimide and nanoparticles of silicon carbide, SiC.In the first method, the SiC nanoparticles were initially functionalized with epoxy end groups using 3-glycidoxypropyltrimethoxysilane (mSiC); then, this solution was mixed with polytriazoles. A homogeneous solution for preparation of the film based on polytriazoles and mSiC was heated in vacuo. In the second method, a new diamine containing the 1,2,4-triazole ring, and the commercially available dianhydride (4,4′-(hexafluoroisopropylidene) diphthalic dianhydride) reacted in situ in the presence of SiC nanoparticles to form a homogeneous mixture of polyamide acid and silicon carbide (PAA/SiC). Next, after a high-temperature process in a vacuum, the mixture turned into a film based on polytriazoles and SiC. The research results showed that a strong chemical bond between the SiC nanoparticles and the polymer matrix leads to an increase in the glass transition temperature Tg from 300°C to higher than 350°C, the tensile strength from 108 MPa to 165 MPa, and the temperature of 5% weight loss (Td5%) from 380 to 500° C. The intensity of photoluminescence also increased, and moreover, with an increase of the SiC content, a shift in the blue region of the spectrum can be observed [21].

A highly effective composite material based on silicon carbide (SiC) and bismaleinimide, modified with allylic novolak for abrasive tools and wear-resistant elements, was developed and characterized. The research results showed that the residual strength at 440°C (1 hour) decreased to 64%, and the thermooxidative stabilities, compared to SiC/polyimide composites, which were made in a similar way, were also better. The ratio of polymer in the composite affects the mechanical properties—its flexural strength increases with the increase of bismaleinimide ratio. However, the excess content of bismaleinimide results in the formation of bubbles in the composite structure. The best composite with a flexural strength of 82.4 MPa was obtained by using 13 wt.% bismaleinimide. After treatment at 280°C for 1 hour, the flexural strength increased by 34% because of the further polymer cross-linking at a higher temperature [22].

It is expected that the combination of polyimides and other organic/inorganic compounds will play an important role in the development of innovative high-performance nanocomposites for various applications.

One of the main problems in obtaining nanocomposites is the prevention of aggregation of particles. It is quite difficult to obtain a monodisperse distribution of nanoparticles in a polymer matrix. This problem can be solved by modifying the nanoparticle surface, which allows improving the interaction of the inorganic modifier and the polymer. There are two main versions of modification. The first is carried out by adsorption or reaction of the surface layer with small molecules (for example, with silanizing agents). The second option is based on grafting polymer molecules through covalent bonds to hydroxyl groups existing on the surface of nanoparticles. The second method has the advantage that it allows one to obtain particles with necessary and predictable properties, due to the possibility of fine selection of the type of particles, grafted monomer, and process conditions.
