Rotational vibration of poly(tetrafluoroethylene) chains by the electron spin resonance method

Temperature‐dependent electron spin resonance spectra of main‐chain free radicals, ? CF₂(β)? C · F(α)? CF₂(β)? , in poly(tetrafluoroethylene) (PTFE) were analyzed by the change in the hyperfine splitting due to β‐fluorines, which was a decreasing function of the observation temperature. The results were interpreted in terms of the rotational vibration around the C_α? C_β bond. The amplitude of the vibration was estimated on the assumption of its harmonic oscillation. The vibration of the PTFE chain was found to have a large amplitude in comparison with that of a polyethylene chain in single crystals. The vibration of the large amplitude was caused by a weak interchain interaction in the PTFE matrices. The amplitude of the vibration in crosslinked PTFE was much larger than that in noncrosslinked PTFE. This result indicated that the free radicals in crosslinked PTFE were trapped in the amorphous region, which had the disordered sites of crosslinking, whereas the free radicals in noncrosslinked PTFE were mainly trapped in the paracrystalline region. The decay reaction of the free radicals in the PTFE matrices was also related to the heterogeneity in the structure and the rotational vibration. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1539–1547, 2004     

Cross-linking of polytetrafluoroethylene during room-temperature irradiation

Exposure of polytetrafluoroethylene (PTFE) to α-radiation was investigated to determine the physical and chemical effects, as well as to compare and contrast the damage mechanisms with other radiation types (β, γ, or thermal neutron). A number of techniques were used to investigate the chemical and physical changes in PTFE after exposure to α-radiation. These techniques include: Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and fluorescence spectroscopy. Similar to other radiation types at low doses, the primary damage mechanism for the exposure of PTFE to α-radiation appears to be chain scission. Increased doses result in a change-over of the damage mechanism to cross-linking. This result is not observed for any radiation type other than α when irradiation is performed at room temperature. Finally, at high doses, PTFE undergoes mass-loss (via small-fluorocarbon species evolution) and defluorination. The amount and type of damage versus sample depth was also investigated. Other types of radiation yield damage at depths on the order of mm to cm into PTFE due to low linear energy transfer (LET) and the correspondingly large penetration depths. By contrast, the α-radiation employed in this study was shown to only induce damage to a depth of approximately 26 μm, except at very high doses.     

Melt-processable poly(tetrafluoroethylene)—compounding, fillers and dyes

Recently, a window of molar mass has been identified where poly(tetrafluoroethylene) (PTFE) homopolymer can be processed from the melt, but at the same time has good mechanical properties in the solid state, so called HD-PTFE^®. This research evaluates the use of conventional melt-compounding with a co-rotating twin-screw to introduce a variety of fillers in this material. It was found that melt-compounding is indeed an efficient way to achieve a filler distribution of excellent homogeneity in HD-PTFE^®.     

Optimization of the two-phase separation process in flow-injection analysis with an electron-capture detection system

A liquid-gas-phase tubing separator was designed and its operation studied in comparison with a PTFE tape-based separator. The relative efficiencies of the two separators were compared after injection into the carrier solution of a sample containing free chlorine using a chromatographic electron-capture detector for detection in the gaseous phase. The membrane separator contained porous PTFE tape whereas the tubular separator consisted of microporous PTFE membrane tubing. The two separators were compared with respect to sensitivity and reproducibility, simplicity of design and liability to interferences. Linear ranges and free chlorine detection limits were established for each of the two separators, that of the PTFE tape-based separator being 100 μg 1^?1 and that of the tubing device 50 μg 1^?1.     

Study on the influence of PTFE particle size on the polymer thermal behavior: I. Melting

The influence of the size of poly(tetrafluoroethylene) (PTFE) particles, obtained from the formed polymer (bar) by cutting on the fusion was studied in order to get information on the polymer breaking consequence.Three fractions of particle diameters between 1.62–0.42 mm were submitted to DSC in nitrogen, DTA in air and DRX analyses and the results compared to those obtained with the non-processed formed polymer and PTFE mentioned in literature.The study on the heating rate influence on the melting temperature range (T) and the specific heat of fusion (Q/J g^–1) carried out with the fraction of 0.82 mm diameter afforded a method for estimating the polymer specific caloric capacity by means of DSC data.This revised version was published online in November 2005 with corrections to the Cover Date.     

粘结剂特性对填充树脂复合材料摩擦学性能的影响

采用酚醛树脂、丁腈橡胶改性酚醛树脂和聚四氟乙烯（PTFE）作为粘结相,通过填充一定配比的石墨、焦炭及碳黑制备了3种树脂基复合材料电刷试样,并在MM-200型摩擦磨损试验机上对比考察了复合材料试样与铜对摩时的摩擦磨损性能,结果表明,与未改性的酚醛树脂基复合材料相比,改性酚醛树脂基复合材料由于韧性提高和硬度降低,因而磨损加剧;但相应的偶件铜环的磨损有所减轻,PTFE基复合材料具有良好的综合性能,偶件铜环的磨损亦较小,因此是一种潜在的高性能电刷复合材料。     

Solid phase extraction for the speciation and preconcentration of inorganic selenium in water samples: A review

Selenium is an essential element for the normal cellular function of living organisms. However, selenium is toxic at concentrations of only three to five times higher than the essential concentration. The inorganic forms (mainly selenite and selenate) present in environmental water generally exhibit higher toxicity (up to 40 times) than organic forms. Therefore, the determination of low levels of different inorganic selenium species in water is an analytical challenge. Solid-phase extraction has been used as a separation and/or preconcentration technique prior to the determination of selenium species due to the need for accurate measurements for Se species in water at extremely low levels. The present paper provides a critical review of the published methods for inorganic selenium speciation in water samples using solid phase extraction as a preconcentration procedure. On the basis of more than 75 references, the different speciation strategies used for this task have been highlighted and classified. The solid-phase extraction sorbents and the performance and analytical characteristics of the developed methods for Se speciation are also discussed.     

The effect of temperature-induced phase transition of PTFE on the dynamic mechanical behavior and impact-induced initiation characteristics of Al/PTFE

PTFE is a semi-crystalline polymer which can undergo two phase transitions at 19°Ϲ and 30°Ϲ. To investigate influence of temperature-induced phase transition of PTFE on the mechanical behavior and impact-induced initiation characteristics of Al/PTFE, the dynamic mechanical analysis, split Hopkinson pressure bar test and drop-weight test were conducted at different temperatures. The correlation between fracture modes and ignition mechanism were analyzed associated with microstructures and reaction phenomena. The results show that with the temperature elevated, the mechanical behavior and reactive characteristics of Al/PTFE manifested a bilinear temperature dependence. Al/PTFE went through a gradual transition from brittleness to ductility. The ignition energy declined, explosion flare enhanced and reaction time prolonged consistent with the change of mechanical properties. The shock pressure and ignition time under drop-weight tests were far from the requirement of shock-induced reaction, indicating that it is fracture-induced reaction dominates the ignition of Al/PTFE at low strain rate.     

Load-displacement behavior for double-edge cracked plate of polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) has been employed in many engineering applications, mainly due to its special properties such as high electrical resistivity, high melting temperature, chemical inertness, corrosion resistance and very low friction. Although there are many works on PTFE, very few attempts have been made to understand the fracture behavior of this material. For this reason, the load-displacement behavior of double-edge cracked specimens of PTFE was examined and modeled and is reported in this paper. Specimens were tested under monotonic tensile load in quasi-static conditions at constant temperature. Images of the region around the crack were captured with a high-resolution camera and then processed by digital image correlation to obtain the displacement fields. Using these data, values of crack tip opening displacement and crack extension were estimated. To model the behavior of PTFE, a constitutive phenomenological model based on saturation and power law expressions combined with a damage evolution equation is proposed. The predictions are in good agreement with the experimental data.