Electrothermal processes were studied in pyroresistive composites based on high-density polyethylene (HDPE) containing 8 vol.% carbon black (CB), 8 vol.% carbon fibers (CF), and their mixture 4 vol.% CB + 4 vol.% CF. It is shown that the kinetic heating curves of composites are well described by an exponential dependence with a certain heating rate constant k for each type of composite. After a short heating time, the equilibrium temperature T_e is reached in the sample. When the applied voltage exceeds a certain value, the T_e value decreases due to the presence of the positive temperature coefficient of resistance (PTC) effect. Due to the PTC effect, the composites exhibit a self-regulating effect relative to the T_e. Relations between the applied voltage, electric power, and equilibrium temperature are found, the T_e value depends on the applied voltage according to the quadratic law whereas there is a linear relationship between the T_e and electric power. A possible application of such pyroresistive composites is resistance welding of plastics using a heating element (HE) made of a pyroresistive material. The use of HDPE-CB composite to create HE for resistance welding is demonstrated and the welded joint of HDPE parts obtained using HE is shown.

The effect of particle size and oxidation degree of new carbon microfillers, based on coal pitch (CP) and petroleum pitch (PET) cokes, on the structure as well as thermal, mechanical, and electrical properties of the composites based on ultrahigh molecular weight polyethylene (UHMWPE) was investigated. The composites studied have a segregated structure of filler particle distribution in the UHMWPE matrix. It was found that composite with smaller CP grain fraction has the highest Young’s modulus and electrical conductivity compared to the other composites studied, which can be the result of a large contribution of flake-shaped particles. Additionally, conductivity of this composite turned out to be similar to composites with well-known carbon nanofillers, such as graphene, carbon black, and CNTs. Additionally, the relationship between electrical conductivity and Young’s modulus values of composites studied was revealed, which indicates that electrical conductivity is very sensitive to the structure of the filler phase in the polymer matrix. In general, it was established that the properties, especially the electrical conductivity, of the composites studied strongly depends on the size, shape, and oxidative treatment of CP and PET filler particles, and that the CP coke of appropriately small particle sizes and flake shape has significant potential as a conductive filler for polymer composites.

A method for the synthesis of protic polymeric ionic liquid (PIL) of a block oligomeric structure with ionic bonds in the main chain, which turns into a liquid state at temperatures below 50 °C, was developed. Ionic bonding was used to form a polymer chain. It was formed as a result of neutralization reaction between oligomers of telechelic structure with terminal acidic and basic groups. A new linear oligomer containing basic tertiary nitrogen atoms at the ends of the oligoether chain (PEO-2NEt2) was obtained by reaction of α,ω-diglycidyl ether of polyethylene glycol MW 1000 with N,N-diethylamine. A linear oligomer with terminal sulfonic acid groups (PEO-2SO3H), which is a product of the interaction of polyethylene glycol (MW 1000) with the cyclic anhydride of 2-sulfobenzoic acid, was used as an acidic linear oligomer. The synthesis of PIL [PEO-2H-2NEt2]2+ [PEO-2SO3]2- was carried out by neutralization of basic oligomer PEO-2NEt2 with the acidic oligomer PEO-2SO3H in their molar ratio of 1:1. The structure of the obtained compound was characterized by the methods of FTIR and 1H NMR spectroscopy. According to the DSC data, the synthesized PIL contains an amorphous phase with a glass transition temperature of -49.7 °С and a crystalline phase with a melting temperature of 34.5 °С. The decomposition onset temperature corresponding to 5% weight loss is 271 °C according to the TGA. The ionic conductivity of PIL studied by dielectric relaxation spectroscopy under anhydrous conditions in the range from 40 °C to 100 °C increases with increase in temperature and reaches 2.7·10-4 S/cm. The resulting compound is promising as a proton-conducting medium for various electrochemical devices.

In this work, the effect of the method of dispersion of a reactive N-phenylaminopropyl polyhedral oligomeric silsesquioxane (NPAP-POSS, 0.025 wt.%) with eight secondary amino groups, in dicyanate ester of bisphenol E (DCBE) on the chemical processes occurring in reactive DCBE/NPAP-POSS blends during dispersion, as well as on the chemical structure, viscoelastic, thermophysical, and thermal properties of heat-resistant organic-inorganic PCN/NPAP-POSS nanocomposites was investigated. The synthesis of the nanocomposite samples was carried out in two stages. In the first stage, to improve the efficiency of nanofiller dispersion, high-speed mechanical or ultrasonic mixing of NPAP-POSS with DCBE was used at different temperatures (T = 65 °C, T = 165 °C), which ensured the chemical interaction of the components. In the second stage, PCN/NPAP-POSS nanocomposites were synthesized by in situ high-temperature reactive molding by dynamic heating the samples in the temperature range of T = 20–300 °C. Using dynamic mechanical thermal analysis (DMTA) for PCN/NPAP-POSS nanocomposites synthesized by high-speed mechanical or ultrasonic mixing at a temperature of T = 65 °C, an unusually significant increase (by 26.5–28.5 °C compared to PCN) in the glass transition temperature (Tg) of the samples even at ultra-low NPAP-POSS content. This phenomenon demonstrates the so-called nanoscale effect. It was also found that the method of nanofiller dispersion affects the increase (compared to unfilled PCN) of the storage modulus (E’) and other viscoelastic properties, as well as the apparent network density (v) and the apparent average molecular weight (Mc) between crosslinks in the hybrid network matrix of the nanocomposites. Differential scanning calorimetry (DSC) also showed that the dispersion method changes the thermophysical properties of the synthesized PCN/NPAP-POSS nanocomposites. This effect is associated with the formation of additional organic-inorganic crosslinks due to the chemical embedding of NPAP-POSS nanoparticles and the formation of the hybrid PCN/NPAP-POSS network. Fourier transform infrared (FTIR), 1H NMR, and 13C NMR spectroscopy confirmed that during nanofiller dispersion in DCBE/NPAP-POSS reactive blends, a chemical interaction occurs between the –O–C≡N groups of DCBE and the secondary –NH groups of NPAP-POSS. This interaction is confirmed by the appearance of corresponding absorption bands and signals (chemical shifts) in the spectra indicating the formation of intermediate isourea fragments and triazine rings of polycyanurates. It was concluded that ultrasonic dispersion of the nanofiller is the most effective under these synthesis conditions for PCN/NPAP-POSS nanocomposites as it ensures the highest degree of cyanate group conversion in DCBE at the final stages of synthesis (confirmed by DMTA data), thereby extending the range of working temperatures within which the samples retain their mechanical and physical properties. Using thermogravimetric analysis (TGA), it was found that all nanocomposites exhibit high resistance to thermo-oxidative degradation (Td > 440 °C), which is largely unaffected by the method of nanofiller dispersion and is determined by the chemical structure of the densely cross-linked PCN/NPAP-POSS hybrid network.

Pyroresistive composites based on high-density polyethylene (HDPE) and polyvinyl chloride (PVC) were studied, which contained two types of filler, carbon black (C) or chopped carbon fibers (CC) in an amount of 8 vol.%. It was shown that when an electric voltage is applied to a composite sample, Joule heat is released, which heats the composite to an equilibrium temperature Te. The dependence of the equilibrium temperature on the voltage U is described by the quadratic equation Te = T0 + aU2, and on the electric power P supplied to the sample by the linear equation Te = T0 + b·P, where T0 is the initial temperature, a, b are the coefficients. However, when the voltage is increased above certain values, a deviation from the quadratic law is observed, and the achievable temperature Te is significantly lower than that calculated by the equation. The reason for the deviations is the manifestation of the PTC effect (positive temperature coefficient of resistance), as a result of which an increase in temperature due to an increase in voltage causes thermal expansion of the polymer matrix, which leads to the destruction of the conductive chains of the filler and an increase in the resistance of the sample. A comparison of the curves for composites based on HDPE and PVC demonstrates a significantly higher level of influence of the PTC effect for composites based on PE, and also a higher influence for composites containing carbon fibers compared to carbon black. A comparative calculation of the amount of energy Q required to heat a composite sample to Te and the amount of Joule thermal energy E released in the sample when electrical power is supplied to it showed that the energy Q is 20% of the Joule thermal energy E released in the sample, i.e. the main part of the Joule thermal energy (80%) is dissipated to the environment. The value of the E/Q ratio depends on the intensity of exchange with the environment, as well as on the geometry of the sample. The possibilities of contact welding of plastics using a heating element (HE) made of pyroresistive material are shown. This method demonstrated the maximum adhesive strength of the welded joint, which is comparable to the strength of the welded polymers, both for HDPE and PVC, and is promising for use in various industries.