Yu. A. Ol’khov

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.

Properties of nanocomposites based on crosslinked elastomeric polyurethane and ultrasmall additives of single-wall carbon nanotubes

It is shown for the first time that the addition of ultrasmall amounts of single-wall carbon nanotubes leads to a significant increase in the main mechanical characteristics of the crosslinked poly(urethane urea) elastomer. The elastic modulus and the tensile strength pass through maxima as the nanotube concentration is increased from 0 to 0.018 wt %; at a nanotube concentration of 0.002 wt %, the maximum values of the modulus and strength are higher by factors of 2.5 and 1.5, respectively, than the corresponding values for the unfilled polymer. The thermomechanical, spectral, and structural characteristics of nanomodified elastomers are investigated, and possible causes of change in their mechanical parameters are discussed.

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.

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.

Effect of gamma radiation on the molecular-topological structure and properties of poly(vinylidene fluoride) and the matrix in a PVDF-based glass-reinforced plastic

The amorphous-topological structure of poly(vinylidene fluoride) (PVDF), the pseudo-network structure of which is formed by polyassociative entities of the cluster type and persists upon γ-irradiation up to 250 kGy, has been investigated. As a result of the processes involved in the fabrication of the glass-reinforced plastic (GRP), five crystalline modifications arise in the polymer matrix instead of cluster branching points in the initial PVDF, and the temperature of the molecular flow of the polymer matrix in the GRP rises from 410 to 593 K in comparison with the initial PVDF. The strength of GRP made of untreated glass cloth (110 MPa) is higher than that of the initial PVDF (86 MPa). However, when using glass cloth treated with a process sizing agent, the strength of the GRP increases up to 286 MPa. After γ-irradiation of the latter, upon which a sharp drop in the strength is observed immediately with the onset of GRP irradiation, the exposure of PVDF and GRP made of untreated glass cloth results in a drop in the strength only after 50 kGy. The irradiation eliminates the properties acquired by the polymer matrix during the fabrication of GRP, and the properties of the GRP polymer matrix approach those of the initial PVDF with an increase in the radiation dose.

Molecular-topological structure of γ-irradiated ftorlon polytetrafluoroethylene

Six topological structures (an amorphous and five crystalline blocks) have been detected in a polytetrafluoroethylene film with a pseudo-network structure. During the Γ-irradiation of the polymer in air, the crystalline fractions degrade and gradually convert into amorphous and cluster states with the increasing radiation dose. After irradiation at a dose of 90 kGy, the polymer loses its capability for crystallization and forms a completely amorphous topological structure. However, regardless of dose in the range of 3–90 kGy, the topological structure of the polymer irradiated in a vacuum remains unchanged and consists of amorphous, cluster, and crystalline blocks.

Molecular-topological structure of γ-irradiated polytetrafluoroethylene

The molecular-topological structure of polytetrafluoroethylene (PTFE) has been studied in the range of −100 to +450°C by thermomechanical spectrometry. Revealed in this temperature range is a fourblock topological structure composed of one amorphous (Tg = 16°C) and three crystalline (low-melting (Tm = 315°C), intermediate (Tm1 = 355°C), and high-melting (Tm2 = 388°C)) polymorphs. At a dose of 1 kGy, the long-range orientation of chains in the intermediate and high-melting crystalline blocks of PTFE is replaced by short-range orientation of the cluster association structure. At doses of 100–500 kGy, the latter structure transitions to the amorphous state and the irradiated samples acquire a semicrystalline structure of the two-block type. The molecular-mass distribution function of interjunction chains of the pseudo-network of the amorphous block is bimodal in character and its maxima are noticeable shifted toward lower masses with an increase in the radiation dose. As the dose increases, the crystallinity decreases and the molecular mobility of amorphized chains is enhanced. As a result, both the glass transition and the molecular flow onset temperatures of the polymer are reduced.

Effect of γ-irradiation on the molecular-topological structure of Viton® fluoroelastomers

The molecular–topological structure of a terpolymer based on vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene has been studied for the first time with the use of thermomechanical spectrometry. A five-block topologically amorphous and crystalline pseudo-network structure with crystallites, which have different initial melting temperatures, as branching points has been detected in the terpolymer at temperatures from–100 to 250°C. When γ-irradiated at a dose of 30 kGy, the crystalline blocks of high-temperature modifications assimilated into one cluster block with the formation of a pseudo-network with a 1.5fold increase in the block-average molecular weight and a decreased initial molecular flow temperature.