Polymer nanocomposites based on PA12 filled with different loading (0.1−10 wt%) of nanosized (average grain size of about 1−5 nm) chromium(III) oxide were prepared by in situ polymerization, and their mechanical and thermal properties were investigated. A homogeneous dispersion of Cr2O3 nanoparticles in the polyamide matrix was achieved at the level of isolated particles as a result of amphoteric nature of chromium (III) oxide. However, it was found that the tensile modulus and yield stress of the nanocomposites decrease. The reason for this is a change in the crystallization type of PA12due to the Cr2O3 nanoparticles (from α to γ type of crystal phase). The crystallization kinetic of PA12 matrix has been successfully studied and described using a two‐parameter Avrami model for isothermal crystallization. It is shown that nanosized chromium(III) oxide dioes not change the nucleation mechanism. The addition of Cr2O3 nanoparticles also does not have a significant effect on crystallization rate and energetic and kinetic parameters of crystallization. The presence of nanofillers in PA12 matrix decreases their crystallinity from 63 to 51%. Hence, the mechanical properties of nanocomposites can be influenced not only by the type and dispersion of nanofillers but also by their effect on the crystal structure and degree of crystallinity of the polymer matrix. POLYM. COMPOS., 36:999–1005, 2015. © 2015 Society of Plastics Engineers

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Broadband dielectric spectroscopy was employed to study polymer nanocomposites based on PA12 filled with different loading (0.1 -10 wt.%) of nanosized (average grain size of about 1 -5 nm) chromium (III) oxide. The experimental dielectric data were analyzed within the formalisms of complex permittivity and electric modulus. Three relaxation processes and Maxwell-Wagner-Sillars (MWS) interfacial polarizations were observed. It was found that all the relaxations were sensitive to filler contents. The presence of nanosized amphoteric chromium (III) oxide was shown to lead to the softening of the polyamide matrix that manifested in decrease of the activation energy of the α-and β-relaxation processes and glass transition temperatures. The softening of polymer matrix is the reason for the decrease in the mechanical properties of the polymer nanocomposites as compared to that of neat PA12.

Shear stress diagnostics in aerodynamic tests can be performed using thin-film coatings based on optically-active liquid crystals (LC) which are sensitive to shear stress. These coatings do not change the model geometry and can be easily applied on the investigated surface including metal models. The method of selective reflection of the white light falling on the liquid crystal with a planar texture was used in this work. Four series of tests have been performed on metal models at subsonic and transonic flow velocities in the wind tunnels (WT) T-128 and T-103 (TsAGI). The last one series of tests that will be observed in detail was performed at Mach numbers M=0.78-0.79 and Re numbers Re=3x106, 4.5x106 and 4.8x106 in T-128 transonic wind tunnel. Using the method of shear stress sensitive liquid crystals visualization of laminar-turbulent transition, flow separation and shock waves was successfully performed. Experiments showed that liquid crystals are a promise for panoramic shear stress diagnostics.