**4. Preparation of composite materials based on a polyimide matrix modified by inorganic nanofillers**

There are two basic mixing mechanisms: simple and dispersive.

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 polymer [23].

When creating polymer nanocomposites with an already prepared nanofiller, three main methods are used:


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].

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].

temperature [22].

ites for various applications.

92 Characterizations of Some Composite Materials

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

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 nanocompos-

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.

Since the polyimide matrixes used are insoluble in organic solvents and their softening temperature exceeds 300°C, it is advisable to use in-situ polymerization.
