A new antifriction and sealing material based on radiation-modified polytetrafluoroethylene

The current situation with research into the effect of ionizing radiation exposure on the structure and properties of polytetrafluoroethylene was analyzed. New methods of radiation-induced modification, that improve the performance characteristics of polytetrafluoroethylene, were discussed. The results of research into properties of polytetrafluoroethylene irradiated at temperatures exceeding the melting point of its crystalline phase in media with different compositions were presented. It is shown that, under certain irradiation conditions, an extremely strong increase (up to 10 000-fold) in the wear resistance of polytetrafluoroethylene can be achieved, with the creepage decreased up to 100-fold. A conclusion was made that high-temperature radiation-induced modification can yield new modifications of the polymer, in which the advantages offered by untreated polytetrafluoroethylene are combined with high mechanical and triboengineering characteristics.     

Effect of irradiation on interfacial interaction and structure formation in filled PTFE composites

The supramolecular structure and tribological properties of filled polytetrafluoroethylene composites irradiated above the melting point of the crystalline phase of the polymer component and the phase transitions in them are investigated. In unirradiated composites, phase separation is observed, that is, the separation of the filler from the polymer matrix. The supramolecular structure of the polymer component does not depend on the nature and concentration of the filler, and it is characterized by the formation of lamellae during sintering and subsequent crystallization. Radiation exposure leads to the disappearance of the phase separation and the formation of axiolites with the radial orientation of fibrils, in the center of which the filler particles are located. Changes in the structure are explained by an increase in interfacial interactions through the radiation grafting of macromolecules (and low-molecular-weight products) to the surface of the filler particles. The linear wear rate of irradiated composites is 50 times lower relative to the unirradiated samples because of the transition from the delamination to abrasive wear mechanism.     

Effect of supramolecular structure on the morphology of the surface of carbonized polytetrafluroethylene

The morphology of the surface of polytetrafluoroethylene (PTFE), subjected to chemical etching in a sodium naphthalene solution in tetrahydrofuran, is studied by high-resolution scanning electron microscopy. It is shown that along with the molecular defluorination processes, etching of the polytetrafluoroethylene surface leads to surface fragmentation and crack formation. The emerging microstructure of the etched surface is associated with the bulk supramolecular structure and rheological properties of the polymer. The decreasing ductility of polytetrafluoroethylene after the pressing and sintering of polymerizate particles results in an increase in the number of cracks per unit area. The orientation of fibrils under uniaxial tension and the formation of spherulites, occurring upon radiation modification, is manifested in the preferred orientation of cracks. Increasing density and decreasing porosity of PTFE significantly lower the etching rate and the depth and length of cracks.     

Spectrophotometric and luminescent analysis of polytetrafluoroethylene treated by γ-irradiation near the melting point

Radiation-modified polytetrafluoroethylene (PTFE) that exhibited intense fluorescence in the visible spectral region was prepared. Radiation modification was performed with ⁶⁰Co γ rays at 330°C, a dose of 0.2 MGy, and an atmospheric air pressure ranging from 10⁻³ to 100 mmHg. The fluorescence and electronic absorption spectra of film (95 μm) specimens irradiated under the given conditions were studied. An important role of molecular oxygen in radiation-chemical processes responsible for the generation and buildup of fluorescing entities was revealed. The spectral characteristics of the four most intense fluorescent centers were determined. It was concluded that polyene systems of π bonds-CF₂-(CF=CF)_n-CF₂-(n=4−11) in the product are responsible for fluorescence. A scheme of radiation-chemical transformation resulting in π-conjugated structures was proposed. Original Russian Text ? S.A. Khatipov, R.N. Nurmukhametov, D.I. Seliverstov, A.M. Sergeev, 2006, published in Vysokomolekulyarnye Soedineniya, Ser. A, 2006, Vol. 48, No. 2, pp. 263–270.     

Polyhydroxybutyrate production from carbon dioxide by cyanobacteria

Genetic characterization and enhancement of polyhydroxybutyrate (PHB) accumulation in cyanobacteria were investigated for efficient PHB production from CO₂. The genome DNAs in the PHB-accumulating strains Synechococcus sp. MA19 and Spirulina platensis NIES46 retained the highly homologous region to phaC of Synechocystis PCC6803, whereas low homology was detected in the nonaccumulating strains Synechococcus sp. PCC7942 and Anabaenacylindrica NIES19. Synechococcus sp. MA19, which accumulates PHB up to 30% of dry cell weight from CO₂ as the sole carbon source, was mutated by insertion of transposon Tn5 to enhance the PHB accumulation. Genetic and physiological analysis of the mutant indicated that decreased phosphotransacetylase activity could trigger an increase of acetyl coenzyme A leading to enhancement of PHB accumulation. PHB synthase in Synechococcus sp. MA19 was probably attached to thylakoid membrane since PHB granules were associated with pigments. A genetically engineered cyanobacteria retaining soluble PHB synthase from Ralstonia eutropha accumulated pigment-free PHB granules, which is an advantage for the purification of PHB.     

Morphology of irradiated polytetrafluoroethylene

Supramolecular structures and morphologies of nonsintered, sintered, and radiation-modified suspension-polymerized PTFEs are investigated via scanning electron microscopy. Gamma irradiation is performed below and above the melting point of the crystalline phase. The fibrillar supramolecular structure of the nonsintered PTFE is preserved after its sintering. However, some regions of fibrils in the sintered PTFE form lamellas elongated perpendicularly to the orientation of fibrils. Irradiation of PTFE below the melting temperature at 20 and 200°C is not accompanied by a qualitative change in its morphology. Irradiation above the melting temperature results in reorganization of the PTFE structure, namely, formation of spherulites consisting of radially oriented fibrils.     

Radiation-Induced Conductivity of Polymer Composites Filled by Finely Divided Oxides

The radiation-induced conductivity of polymer composites based on polyisoprene rubber filled with finely divided oxides of various chemical natures was studied in the continuous irradiation mode. It was shown that the radiation conductivity is closely related to the molecular mobility of the rubber. It was found that a change in the character of thermal motion of fragments of rubber macromolecules, which is caused by interaction with the filler, could considerably enhance the radiation-induced conductivity of a polymer composite.     

Radiation-Induced Conductivity of Plasticized Flexible-Chain Polymers

The radiation-induced conductivity of cured and transformer-oil extended polybutadiene and polyisoprene rubbers was studied. It was shown that the extender ratio and degree of cure of a polymer system have a much greater effect on the radiation conductivity than the addition of donor–acceptor dopants trapping mobile charge carriers. It was concluded that the molecular mobility is the major factor determining the electron transport properties of the systems of interest.