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.     

A renaissance of color: New structures and building blocks for organic electronics

Natural dyes and pigments like indigo and its derivatives valued for their bright colors and photochemical stability has been used since antiquity. Recently, the need for better performing materials in the organic electronics field has inspired a resurgence of these historical molecules and their subsequent transformation into new families of π‐conjugated building blocks used to construct new (macro)molecular semiconductors. This Highlight will explore the renaissance of notable building blocks including diketopyrrolopyrrole, (iso)indigo, benzodipyrrolidone, and benzodifuranone, as well as nonfullerene acceptor structures 9,9′‐bifluorenylidene and quinacridone. In addition, as the organic electronics field continues to evolve, the design of molecules with precise structure and function embodies a new paradigm for the next generation of materials. Representative examples will be described that embrace this new model and point the direction for advanced technologies. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Roughness and anisotropy effects on wettability of polytetrafluoroethylene and sodium-treated polytetrafluoroethylene

The effect of roughness on wettability of skived polytetrafluoroethylene (PTFE or Teflon) and Na-treated PTFE film were studied by advancing contact angle measurements. The effect of an anisotropic force field of elongated Na-treated PTFE on the shear bond strength were also studied as a function of elongation. The results are analyzed in terms of London dispersion γs^d and Keesom polar γs^p contributions to surface energy γs. It was found that the roughness effect on wettability of PTFE is significant for untreated PTFE and negligible for Na-treated PTFE. Our shear bond strength σ_b analysis of elongated Na-treated PTFE showed that σ_b is influenced by an anisotropic force field and the σ_b increases with the fractional polarity p = γs^p/γs but decreases with the dispersion fraction d = γs^d/γs of solid-vapor surface tension γs = γs^d + γs^p     

Conformational transition of the core chain in radiation-modified polysilane micelles formed in selective solvents

Poly(methyl acrylate)-grafted poly(methyl-n-propylsilane) (PMPrS-g-PMA) and poly(acrylic acid)-grafted PMPrS (PMPrS-g-PAA) were synthesized by gamma-ray-induced graft polymerization, and the association behavior of these graft copolymers was investigated in selective solvents composed of good and poor solvents for the PMPrS main chain. Fluorescence spectroscopy with perylene as a fluorescent probe revealed that PMPrS-g-PAA in a water/THF mixed solvent self-assembles into micelles with a swollen core of PMPrS chains in the water content range of 50-95%. UV spectroscopy demonstrated that a further increase of the water content gives rise to the conformational transition of the PMPrS chains in the micelle core from the random conformation to the conformation that corresponds to that in the solid state at a water content of ca. 95%, independent of the grafting yield. Similar behavior was also observed in DMSO/THF solutions of PMPrS-g-PMA, for which the conformational transition occurred at the constant DMSO content of ca. 95%. These results indicate that solvatochromic behavior of polysilane, which is a characteristic feature of polysilane, proved to provide information on the inner structure of those micelles: PMPrS chains in the core undergo conformational transition as the content of the poor solvents for PMPrS increases, while maintaining the micelle structure.     

Sliding in polytetrafluoroethylene crystals

Stress–strain relationships are calculated for three models of the deformation of monocrystalline polytetrafluoroethylene, \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--} ({\rm CF}_{\rm 2} \rlap{--} )_n $\end{document}. The physical models comprise either sliding one molecule or one plane of molecules parallel to the molecular chain axis past its stationary neighbors. The potential energy is calculated for each stage of deformation by semiempirical methods by use of 6-exp and dipole–dipole interactions. Application of Eyring's activated complex theory leads to stress–strain relationships. These are compared with results of friction measurements on PTFE.     

Full emission color tuning in luminogens constructed from tetraphenylethene, benzo-2,1,3-thiadiazole and thiophene building blocks

Full color luminogens are constructed from tetraphenylethene, benzo-2,1,3-thiadiazole and thiophene building blocks. OLED fabricated using one of the luminogens exhibits orange-red electroluminescence with high luminance and efficiencies of 8330 cd m(-2), 6.1 cd A(-1) and 3.1%, respectively.     

Analysis of reversible melting in polytetrafluoroethylene

Summary The reversibility of crystallization and melting of polytetrafluoroethylene (PTFE) has been investigated as function of crystallization conditions and temperature by temperature-modulated differential scanning calorimetry (TMDSC). The total and average specific reversibility of the melt-crystallized PTFE is considerably larger than in case of as-polymerized powder. This experimental observation must be attributed to different coupling between crystallized sequences of the molecules within the globally semi-crystalline superstructure. The crystallinity of as-polymerized PTFE is close to 100%, and the crystals melt in a narrow temperature interval close to the equilibrium melting temperature. Melt-crystallized PTFE, in turn, shows a crystallinity of about only 40% and melts at lower temperatures. The morphology of the melt-crystallized PTFE allows molecule segments to melt and crystallize reversibly as a function of temperature. The extended-chain conformation, evident in as-polymerized powder, inhibits reversible melting due to required molecular nucleation after complete melting of a molecule. The experimental findings are discussed within the framework of a similar investigation on polyethylene of different crystal morphology and support both the concepts of lateral-surface activity and molecular nucleation.     

Thermal conductivity of polytetrafluoroethylene and polytrifluorochloroethylene

Summary Measurements of thermal conductivity were made on polytetrafluoroethylene and polytrifluorochloroethylene which were crystallized by various heat treatments. The variation of thermal conductivity with the degree of crystallinity can be explained qualitatively in terms of the difference of the mean free path. An approximate calculation of the mean free path for the amorphous polytrifluorochloroethylene was made. The calculation gives the value of the mean free path which is very close to the atomic distances in the polymer molecule.

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Zusammenfassung Die thermische Leitf?higkeit von Polytetrafluor?thylen und Polytrifluorchlor?thylen wurde an Proben gemessen, die durch verschiedene W?rmebehandlungen kristallisiert wurden. Die ?nderung der thermischen Leitf?higkeit mit dem Kristallisationsgrad kann qualitativ durch die Unterschiede der mittleren freien Wegl?nge erkl?rt werden. Eine absch?tzende Berechnung der mittleren freien Wegl?nge des Polytrifluorchlor?thylens wurde durchgeführt. Die berechneten Werte liegen nahe an den Atomabst?nden der Polymermoleküle.

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With 4 figures and 1 table

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This article is Part VI of the series “Studies on Thermal Conductivity of High Polymers”. Part I–V of this series were presented in High Polymer Chem., Japan (Kobunshi Kagaku).     

Electron-beam charging of polytetrafluoroethylene

The charging of polytetrafluoroethylene films by irradiation with fast electrons having an energy insufficient for the complete penetration through the entire thickness of the polymer was theoretically analyzed. The calculation results take into account the distribution of both the dose rate and the bulk electron thermalization rate over the film thickness in the approximation of prescribed radiation-induced conductivity of polytetrafluoroethylene. The influence of the parameters of radiation-induced conductivity on the charging behavior and on the time dependence of the transient current detected with the use of the split Faraday cup was examined. It was found that the observed transient current inversion is adequately explained with allowance for the specifics of radiation-induced conductivity in polytetrafluoroethylene.     

High-pressure phase of polytetrafluoroethylene

The effect of hydrostatic pressure on the crystal structure of PTFE has been studied up to pressures of ca. 25 kbar by x-ray diffraction. The experimental method uses opposed diamond anvils of small surface area as transmitters of pressure with MoKα x-radiation propagating through these anvils. A small specimen of oriented polymer is held in place with a molybdenum gasket. Pressures are measured by change in lattice spacing of sodium chloride included with some specimens. It is found that above 4.5 kbar the PTFE changes to a high-pressure phase in which the molecules have transformed from their normal helical arrangement to that of a planar zigzag and that the planes of the molecules all lie parallel to one another. Further changes in the diffraction pattern on increase in pressure are attributed to slip and twinning in the high-pressure phase rather than to another phase change.     

Laser ablation of polytetrafluoroethylene

Anomalous behavior of polytetrafluoroethylene (PTFE) under the action of CO₂ laser radiation in vacuum is discussed. A feature of the ablation process is formation of polymer fibers which quantity depends on the target preliminary treatment and the ablation conditions. Experimental results are set out and possible mechanisms of the polymer fibering are discussed. A conclusion is made concerning two dynamic polymer components differ by the resistance to the laser beam action, appearing in the ablation crater. A method is proposed for producing unique fiber-porous polytetrafluoroethylene materials and other useful products as well as for the polytetrafluoroethylene wastes recycling. The results of industrial application of these new fiberporous materials are discussed.     

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-thermal crosslinking of polytetrafluoroethylene

The formation of a three-dimensional network in polytetrafluoroethylene (PTFE) exposed to ionizing radiation at temperatures above the crystallite melting temperature—a phenomenon that has been revealed and studied in the last decade—is considered. A change in the structure and properties of PTFE during its radiation-thermal modification under the specified conditions was analyzed. It was shown that such modification imparts to PTFE a set of additional valuable properties including an enhanced modulus and breaking strength, a low creep, a high wear resistance, enhansed radiation resistance and transparency, thus opening new frontiers for practical application of this polymer.     

Chain folding in single crystals of polytetrafluoroethylene

Potential-energy calculations are performed in order to obtain satisfactory models for folding in single crystals of polytetrafluoroethylene crystallized in the phase stable below 19°C. The folds are assumed to be tight with adjacent reentry. Folds with a variable number of bonds are analyzed. Their conformation is deduced by a method which obtains closure between two semifolds into which each fold is decomposed. The allowed models are subsequently investigated by using an energy minimization program. Some possible models are proposed. The calculations show that the folds must be described by a number of rotational isomeric states higher than is usually assumed.     

Polymer radicals trapped in radical-initiated polytetrafluoroethylene

The electron spin resonance (ESR) spectra of polymer radicals found to be trapped in polytetrafluoroethylene (PTFE) polymerized with radical initiators were comparatively examined under various conditions and assigned. They are identified as the primary (propagating) radicals RCF₂CF₂·, which are transformed to primary peroxy radicals RCF₂CF₂OO· in the atmosphere. Studies of the rates of polymerization and postpolymerization and ESR measurements indicate that the radical content steadily increases during polymerization. The results are discussed in connection with the mechanism of polymerization of tetrafluoroethylene (TFE) and the unusual thermal stability of these radicals in PTFE prepared with initiator.     

Morphology of polytetrafluoroethylene before and after irradiation

Supramolecular structure and morphology of as-polymerized, sintered, and gamma-irradiated suspension PTFE were studied with scanning electron microscopy. Irradiation was performed both below and above melting point of crystal phase. Fibrillar supramolecular structure of as-polymerized PTFE is preserved after its sintering. In contrast to as-polymerized PTFE, in the sintered polymer some segments of fibrils form lamellae of thickness 100-300 nm and length up to several microns, with fibrils arranged perpendicularly to a lamella. Irradiation below the melting point (20 and 200 °C) does not change quantitatively PTFE morphology. In both cases and also in the case of pristine PTFE, dense and loose (porous) regions are present in its morphology. Dense regions are packages of irregular shape and consist of densely packaged fibrils. Loose regions consist of individual ribbons and fibrillar lamellae. Irradiation at 200 °C increases greatly the width of lamellae. PTFE structure rearrranges drastically under irradiation above the melting point. New morphology units, spherulites of size about 50 μm, are formed, the spherulites consisting of radially extending fibrils, and porosity decreases substantially. Formation of spherulites is ascribed to radiation-induced chain scission and decrease in molecular mass and viscosity of polymer.     

Radiation-modified polytetrafluoroethylene: Structure and properties

The scope offered by radiation technology for polytetrafluoroethylene modification was discussed. New modified materials possess enhanced antifriction and compressive parameters and are suitable for different commercial applications.     

The thermodynamic properties of polytetrafluoroethylene

Polytetrafluoroethylenes of different crystallinity were analyzed between 220 and 700 K by differential scanning calorimetry. A new computer coupling of the standard DSC is described. The measured heat capacity data were combined with all literature data into a recommended set of thermodynamic properties for the crystalline polymer and a preliminary set for the amorphous polymer (heat capacity, enthalpy, entropy, and Gibbs energy; range 0–700 K). The crystal heat capacities have been linked to the vibrational spectrum with a θ₃ of 54 K, and θ₁ of 250 K, and a full set of group vibrations. C_v to C_p conversion was possible with a Nernst–Lindemann constant of A = 1.6 × 10^?3 mol K/J. The glass transition was identified as a broad transition between 160 and 240 K with a ΔC_p of 9.4 J/K mol. The room-temperature transitions at 292 and 303 K have a combined heat of transition of 850 J/mol and an entropy of transition of 2.90 J/K mol. The equilibrium melting temperature is 605 K with transition enthalpy and entropy of 4.10 kj/mol and 6.78 J/K mol, respectively. The high-temperature crystal from is shown to be a condis crystal (conformationally disordered), and for the samples discussed, the crystallinity model holds.