High-Temperature Polymer Nanocomposites Based on Heterocyclic Networks from Nitrile Monomers

The results of physical studies of the structure, dynamics, mechanical, and thermal properties of CER/epoxy-POSS nanocomposites based on CE monomers (DCBE, DCBA, 6F-DCBA) or oligomer (PT-30) published during the last decade are presented. The nanoparticle content in the nanocomposites varied from 0.01 wt.% to 10 wt.%, and their analysis was performed by TEM, EDXS, mid-IR, far-IR, XRD, SAXS, DSC, DMA, CRS, and TGA methods. The effective molecular dispersity of nanoparticles in the amorphous matrix and the most uniform POSS distribution in the composite nanovolumes were achieved only at their contents < 1 wt.%. This and the covalent “embedding” of POSS units into the matrix led to significant suppression of matrix (cycles) dynamics and to a significant improvement in the thermal and mechanical properties of the nanocomposites. The extremal dependencies of T_g on POSS content, with its increase from 240–250° to 280–300 ℃ at low POSS contents and the appearance of additional interfacial dynamics transition at 375 ℃, and the dynamic heterogeneity around T_g were found. The superiority of oligomer-based nanocomposites with T_g ~ 400 ℃, total thermal stability up to this temperature, regardless of the nature of the environment, and the increase of the modulus from ~ 2 GPa to ~ 4 GPa at 20 ℃ and from 0.1 GPa to 2 GPa at 300 ℃ are shown.

The results of physical studies of nanostructure, dynamics, mechanical and thermal properties of DCBE-based CER/amino-MMT nanocomposites are presented. The 2D MMT nanolayers content in these composites varied from 0.01 wt.% to 5 wt.%, and their analysis was performed by TEM, EDXS, mid-IR, far-IR, XRD, DSC, DMA, CRS, and TGA methods. The process of MMT exfoliation increased with decreasing MMT content in the composite, down to the formation of individual nanolayers at 0.1 wt.%. The ambivalent influence of MMT nanolayers on matrix dynamics, T_g, and thermal stability is shown, with the most positive effect at small and ultra-low MMT contents (0.025–0.5 wt.%). Anomalous composition and mechanical properties of the subsurface micron layers in the CER-based nanocomposites were detected.

The results of physical studies of the hybrid CER-silica composites with 0.01–10 wt.% silica prepared via a sol–gel technology are presented. HAADF-STEM, EDXS, DMA, DSC, and TGA were used for their characterization. It was found that the greatest positive effect of introducing covalently embedded silica units into the matrix is observed at their ultra-low content of 0.02–0.1 wt.%. The possibility of creating subnanometer-sized silica nodes in the matrix network in the absence of clustering was revealed in this case, and the term “subnanocomposites” was introduced into the literature. The effect of the quasi-regular distribution of silica subnanounits or molecularly dispersed nanoparticles in nanovolumes of amorphous matrix was detected. The superiority of the properties of polymer subnanocomposites over those of the corresponding composites with SiO₂ nanoclusters was shown. Certain correlations between the nanostructure, matrix dynamics, and the properties of the investigated composites were traced. Additional calculations based on the obtained structural data were performed, which made it possible to establish the nature of the above effects. It turned out that the dynamics of the matrix as a whole transformed into the state of constrained interfacial dynamics in this case.

This chapter considers the structure and properties of CER-based nanocomposites containing mesoporous silica particles, graphene oxide (GO), carbon nanotubes (CNTs), or “unzipped” carbon nanotubes (uCNTs). It is shown that by introducing glycidyl silane functionalized mesoporous silica FMCM-41, the thermal stability increases, whereas the dielectric constant and the dielectric loss tangent of the material decrease from 3.2 to 1.98 and from 0.023 to 0.009, respectively. GO produced a strong catalytic effect on the matrix curing process, and the addition of 1% GO significantly improved its mechanical properties. Of particular interest are the results of studying nanocomposites with functionalized uCNTs. In addition to significantly improving the mechanical and thermal properties of the matrix, uCNTs significantly enhanced its luminescence properties and absorption in the UV region.

The results of the physical study of a series of phthalonitrile nanocomposites with 0.5 wt.% of functionalized POSS nanoparticles are of four types. Their nanostructure, dynamics, relaxation, elastic and thermal properties were examined by TEM, mid-IR, far-IR, EDXS, DMA, DSC, and TGA techniques. Polymerization at temperatures not exceeding 300 °C resulted in the completion of the process by ~70–80% only, and post-curing at higher temperatures increased the degree of polymerization to 90–95%. Far-IR spectra confirmed the main contribution of phthalocyanine cycles to the matrix structure. TEM images and histograms of Si content in the nanovolumes of the composites (EDXS data) indicated a quasi-regular, at the first approximation, distribution of POSS nanoparticles in the amorphous matrix. The introduction of POSS nanoparticles and post-curing led to some suppression of the matrix dynamics and increasing T_g from 380–410 °C to 446 °C for neat matrix and to 520–560 °C for the nanocomposites, with the manifestation of dynamic heterogeneity in the glass transition. A complete suppression of the glass transition as well as the constancy of the dynamic modulus E′ ≈ 3.2 GPa over the range from 20° to 560 °C was observed after heating nanocomposites in an inert atmosphere. The total thermal stability of the nanocomposite up to ~ 400 °C and the “switching on” of thermo-oxidation processes in an air medium from 550 °C were registered by TGA.

The results of physical studies of molecular structure, nanostructure, relaxation, elastic and thermal properties of a group of hybrid phthalonitrile nanocomposites with different contents, from 0.03 to 5 wt.%, of functionalized MMT nanolayers, using TEM, EDXS, DMA, DSC, TGA, and mid-IR/far-IR spectroscopy are shown. Far-IR spectra confirmed the main contribution of phthalocyanine heterocycles to the molecular structure of nanocomposites and the effect of constraining matrix dynamics after “embedding” MMT. The degree of exfoliation of MMT blocks in nanocomposites increased with decreasing their content: at 0.03–0.1 wt.% MMT single nanolayers were formed. For cured materials, neat matrix and nanocomposites, T_g = 380–390 °C but after post-curing T_g of the matrix increased to 446 °C, whereas the nanocomposites exhibited dynamic heterogeneity in the glass transition and their T_g's reached 540° and 570 °C. After treatment at high temperatures in an inert medium, the complete suppression of the glass transition and constant modulus E’ = 3 GPa in the range from 20° to ~ 600 °C were observed for the nanocomposites. The nanocomposites were stable when heated up to 400 °C, regardless of the nature of the environment. The incorporation of MMT nanolayers led to a substantial improvement in the resistance to thermo-oxidative degradation in the air environment. The char residue at ~ 700 °C increased from 7% for the neat matrix to 45% for the nanocomposite with 2% MMT.

The structure and properties of three types of cured PhN/metal oxide nanocomposites with Al₂O_3, TiO₂, or ZnO nanoparticles of an average diameter of 60 nm, which were treated with aminopropyltrimethoxysilane GX-540 as coupling agent, are described. In all cases, a satisfactory dispersion of nanoparticles was achieved. A significant improvement in the thermomechanical properties of nanocomposites with respect to the properties of the neat matrix was shown. The experimental data obtained for the elastic modulus of the studied Al₂O₃-containing nanocomposites were compared with the Series, Halpin–Tsai, and Kerner theoretical models; the best match was shown for the Halpin–Tsai model. The barrier properties of the TiO₂-containing nanocomposites as anticorrosive protective coatings were shown. The UV–visible transmittance spectra of the ZnO-containing nanocomposites showed that adding ZnO nanoparticles improved the UV-shielding properties of the neat matrix.

The structure and properties of six types of cured PhN nanocomposites containing functionalized silicon nitride, boron nitride, MAX phase, tungsten nanoparticles, MXene nanosheets, and graphite nanoplatelets are described. In all cases, the introduction of nanoparticles led to a significant improvement in the thermomechanical properties of the matrix. The most striking examples of their positive influence are as follows. For PhN/BN nanocomposites, the storage modulus increased from 1.94 GPa up to 7.2 GPa at 50 °C and from ~ 0 to 4 GPa at 300 °C. The flexural modulus and strength at 20 °C increased from 1.8 GPa and 75 MPa to 8.5 GPa and 190 MPa, respectively, and a 26-fold increase in thermal conductivity was registered. The nano-MXene phase (3 wt.%) resulted in increasing mass loss temperature T_10% from 493 °C to 649 °C for the nanocomposite. For nanotungsten-containing composites, the effectiveness in shielding against gamma radiation was shown. Adding graphite nanoplatelets decreased the electric resistivity of the matrix from ~10¹⁴ Ω cm down to 10⁷ Ω cm. The data of X-ray photoelectron spectroscopy, IR spectroscopy, and other information allowed us to conclude that the main reasons for the ultra-high thermal properties of phthalonitrile nanocomposites are the leading role of uniquely heat-stable phthalocyanine heterocycles in their structure and constraining matrix dynamics by the covalently embedded nanoparticles.