S. R. Allayarov

Effect of γ-radiation on the molecular-topological structure of polytetrafluoroethylene

Four topological structures of polytetrafluoroethylene (PTFE): amorphous and three crystalline (high-melting, intermediate, and low-melting forms) blocks, were revealed by the technique of thermomechanical spectroscopy. The γ-irradiation of PTFE leaves both free volume and the molecular-mass characteristics in the pseudo-network structure of the amorphous block practically intact. In the crystalline phase, γ-irradiation decreases the molecular mass of crystallized chains of the low-temperature modification and leads to the disappearance of the intermediate and high-temperature crystalline phases as a result of their transformation into the amorphous phase.

Effect of accelerated protons on the molecular-topological structure and thermal stability of poly(vinylidene fluoride)

The bombardment of poly(vinylidene fluoride) (PVDF) with accelerated protons in vacuum results in the release of gaseous products and the destruction of cluster junctions in its pseudo-network structure, which pass to the crystalline ones of three modifications with different temperatures and rates of melting, the molecular mass of crystallized chains, and their weight fractions. The composition of gaseous products indicates that, upon proton bombardment, the detachment of fluorine or hydrogen atoms yielding H2 and HF occurs as the main process, and the scission of the carbon chain hardly takes place in this case.

The effect of continuous CO2 laser radiation on the thermal and molecular—Topological properties of polytetrafluoroethylene

The impact of high-intensity laser radiation on a polymer in vacuum is accompanied by the release of gaseous products of degradation and, in some cases, of clusters of the partially destroyed polymer. Polytetrafluoroethylene (PTFE) exhibits an abnormal behavior in this process: being exposed to continuous CO2 laser radiation, it degrades at a high rate and its clusters have a fibrous form. Depending on the irradiation conditions, the fibrous fraction forms two types of product, “cotton wool” and “felt”. Polytetrafluoroethylene and its laser-modified “cotton wool“ product have a semicrystalline topological structure. The preliminary γ-irradiation of PTFE enhances the laser ablation process.

Effect of gamma-irradiation on morphology of copolymer of tetrafluoroethylene with hexafluoropropylene as revealed by thermomechanical analysis and radiothermoluminescence

The morphologies of different commercial forms of the copolymer of tetrafluoroethylene with hexafluoropropylene (FEP) were investigated by thermomechanical analysis. Both granules and powders of FEPs have similar morphologies, but different temperatures for phase transitions. Irradiation reduces the crystallinity of the FEPs and results in a transformation from the crystalline to the amorphous form. After heating of various forms of FEP irradiated at 77 K, there are three intense maxima of radiothermoluminescence in the temperature ranges: 150–144, 174–167, and 210–206 K. The nature of the radiothermoluminescence and the phase transitions of the FEPs as a result of gamma—irradiation are discussed.

Anomalous behavior of dielectric permittivity of graphene-filled polymer composite

Dependences of the dielectric permittivity of a graphene-filled polymer composite and the ESR signal intensity in γ-irradiated samples of this composite on the concentration of graphene additive have been studied. Anomalous behavior of the permittivity correlates with the ESR signal intensity variation, which is indicative of the possible structural rearrangement in the epoxy resin-graphene filler system.

Radiolysis of n-perfluoroalkanes and polytetrafluoroethylene

Abstract The reason for radiolysis differences between linear perfluoroalkanes (PFAs) and their hydrocarbon analogues (namely, the lower by an order of magnitude radiation–chemical yield for PFAs in comparison with their hydrocarbon analogues) is due to the feasibility of stabilization the radicals of the type −CF 2 ⋅ CF 2 . The possibility of double bonds acting as a trap for atomic fluorine to protect the fluoropolymers from radiative damage has been demonstrated. It has been shown that the presence of a double bond in a perfluorocarbon molecule, on the one hand, increases the yield of stabilized radicals under radiolysis and, on the other hand, decreases the damage to the principal carbon chain in the molecule. Both these interrelated effects are the consequence of reaction of F atoms formed during radiolysis with the double bond.

Thermomechanical properties of polytetrafluoroethylene in different zones of impact of continuous CO2 laser radiation

The pseudo-network structure of the amorphous block of polytetrafluoroethylene is formed by branching points, the crystallites of low- and high-melting crystalline polymorphs and the cluster segments of macromolecules. The polymer treated by laser irradiation is amorphized, with the degree of amorphization and other changes depending on the radiation fluence. A depression of the molecular flow onset and initial melting temperatures of the crystalline polymorphs, a decrease in the molecular mass, and the disappearance of the crystalline branching points characteristic of the original polymer have been observed in the irradiated polymer. Areas with different degrees of modification of the molecular structure appear in the dynamic mode with moving boundaries during continuous CO2 laser irradiation. The difference in absorbance of the crystalline and amorphous portions of the polymer at the laser emission wavelength and a relatively high transmittance make laser-induced degradation differ from thermolysis in contact with a hot surface.

Effect of fast protons on the molecular—topological structure and surface properties of tetrafluoroethylene—hexafluoropropylene copolymer

The topologically diblock structure of a tetrafluoroethylene—hexafluoropropylene copolymer with a total weight fraction of crystalline structures of 0.07 is transformed into a fully amorphous matrix with a pseudo-network structure after irradiation with 1—4 MeV protons. Instead of crystalline branching points (as in the unirradiated copolymer), cluster structures have appeared in the matrix. Oxidative degradation processes that occur during the bombardment result in significant functionalization of the copolymer and a substantial increase in the surface free energy, its acid—base component, and surface polarity.

The effect of γ-irradiation on laser ablation of polyketone

Results of a pioneering study of the effect of laser radiation in vacuum on the surface of a polyketone (alternating terpolymer of ethylene, propylene, and carbon monoxide, POK) plate are presented. It has been found that laser beam irradiation leads to the surface heating of the plate, its melting, and the formation of a characteristic surface microrelief, an ablation crater, from which the gas flow of the ablation plume carries away products that are deposited on surfaces outside the laser beam area to form a coating with a chemical composition close to that of the substrate POK. A rim grows from molten POK around the crater. The melting point of the crystalline modification (377 K), the molecular flow temperature (427 K), and the molecular weight of the coating (25560) are much lower than those of the initial POK (464 K, 477 K, and 159200, respectively), thereby indicating laser-induced chain degradation of POK. The preliminary γ-irradiation of POK to a dose of 100 kGy enhances its laser ablation rate.

Analysis of the ESR spectra of the ~ CF2CFCF2~ macroradical trapped in a γ-irradiated polytetrafluoroethylene matrix at 77 K

The Β-fluorine atoms in the ~CF2CFCF2~ radical trapped in γ-irradiated polytetrafluoroethylene (PTFE) were found to be nonequivalent and, hence, responsible for a doublet-triplet-triplet ESR spectral pattern. The conformational angle between the axis of the unpaired-electron orbital and the projection of CΒ-Cγ bond is equal to 60‡. The hyperfine coupling constants for α- and Β-fluorine atoms were determined. The hyper-fine splitting constants for two equivalent Β-fluorine atoms were found to be 7.2 mT. The corresponding value for the two other equivalent fluorine atoms is 1.8 mT, whereas the hyperfine splitting due to α-fluorine atom is 23.8 mT.