Thermal expansion of irradiated polyoxymethylene

The thermal expansion coefficient of gamma irradiated polyoxymethylene has been measured in the temperature range 80–340 K by using a three terminal capacitance technique. The radiation induced changes are measured by recording the i.r. spectra of the irradiated samples. The change in crystallinity caused by irradiation is measured by an X-ray technique. The thermal expansion coefficient increases with radiation dose below 170 K due to the predominant effect of degradation. Above 170 K, this trend reverses and the expansion coefficient decreases with radiation dose due to the increased crystallinity caused by irradiation.     

Thermal expansion of irradiated polyethylene from 10 to 340 K

The coefficient of thermal expansion of γ-irradiated polyethylene has been measured from 10 to 340 K by using the three-terminal capacitance technique. The samples are irradiated to 500 Mrad in steps of 100 Mrad in air at room temperature with γ-rays from a Co⁶⁰ source at a dose rate of 0.3 Mrad/h. The crystallinity of the sample is measured by x-ray diffraction. The crystallinity is found to decrease with radiation dose. The thermal expansion coefficient is found to be constant with radiation doses from 10 to 110 K and decreases with doses from 110 to 340 K. © Wiley & Sons, Inc.     

Specific volume studies of γ-irradiated polytetrafluoroethylene from 40 to 150°C

The effect of γ-irradiation and post-irradiation heat treatment on the specific volume versus temperature relationships of polytetrafluoroethylene (PTFE) samples (1/2-in. diameter rods) have been studied over the 40–150°C. temperature range for radiation doses up to 8.9 X 10⁸ rad. At low doses the specific volume at any temperature decreased with dose, but above about 10⁸ rad it increased with dose. Similarly, the rate of volumetric expansion initially decreased with dose, while, at very high doses (8.9 X 10⁸ rad) the rate of expansion at temperatures above 100°C. exceeded that of the unirradiated PTFE. Heating at 150°C. for 100 hr. produced a substantial decrease in the specific volume and a decrease in the rate of expansion for the irradiated samples. Irradiation effects in PTFE are considered to be a result of such factors as radiation-induced chain scission, increased crystallinity, and increased void content. Changes resulting from post-irradiation heat treatment can be attributed to increased crystallinity, decreased void content, and weight loss.     

Relaxation processes and structure of ultradrawn polyethylene

Solid-state extruded polyethylene fibers have been prepared, with a wide range of draw ratios and constant processing temperature. The draw ratios vary from 4 up to 30, and the processing temperature was always 398 K. The extruded material behaves anisotropically, owing to the high degree of chain orientation in the drawing direction. The modulus and linear expansion coefficients in the fiber axis direction have been measured, over a wide temperature range, from 140 K up to 320 K. These two properties are closely related to the degree of structural continuity of the fibers. A fibrous structure model is proposed to explain the temperature effects and the values obtained for the modulus and expansion coefficients, in terms of crystallinity and volumetric fraction of extended-chains structure. At least three relaxation processes can be identified which cause the structural continuity of the fibers to change with temperature.     

Quantum effects in molecular dynamics and fracture kinetics of polymers

Molecular dynamics in crystallites of a number of polymers, such as polyethylene, nylon-6, poly-(vinyl alcohol), was studied at temperatures ranging from 4.2 to 400 K using the X-ray diffraction technique. Thermal expansion of the lattice was measured by the temperature-induced displacements of diffraction maxima. The thermal expansion coefficient was found to grow from 0 to ∼10⁻⁴ K⁻¹, this fact being the evidence of the quantum statistics in the vibrational dynamics of straightened molecules over the whole temperature range. Characteristic temperatures of vibrations were estimated. The kinetics of fracture was studied using highly - oriented nylon-6 and KEVLAR-49 fibers. Tensile strength (σ_r) was measured as a function of loading rate σ and temperature (T) in the range from 4.2 to 400–600 K. The observed specific features of the σ_r(σ) and σ_r(T) curves at low temperatures correspond to tunnel scission of stressed chain molecules.     

X-ray and NMR measurements on irradiated polytetrafluoroethylene and polychlorotrifluoroethylene

The effects of ⁶⁰Co γ-radiation on polytetrafluoroethylene (PTFE) and polychlorotri-fluoroethylene (PCTFE) have been studied for radiation doses up to 940 Mrad. The dependence of per cent crystallinity upon irradiation level has been determined from x-ray analysis. An initial increase in crystallinity in PTFE, attributable to chain scission in the amorphous phase of the material, was found, followed (above 300 Mrad) by a gradual decrease associated with a disordering of the crystallites. No initial increase was observed for annealed samples of PCTFE due to the large initial value of the per cent crystallinity. Above 200 Mrad the crystallinity was found to decrease with accumulated dose. Nuclear magnetic resonance measurements on PTFE have indicated a radiation-induced broadening of the amorphous component of the NMR line appearing to maximize above 700 Mrad. Similar measurements of PCTFE have shown a narrowing of the crystalline component of the NMR line and subsequent appearance of the amorphous component at approximately 200 Mrad. The data indicate that the radiation-induced behavior of PTFE and PCTFE is similar above 200 Mrad.     

Fourier transform infrared investigation of the effects of irradiation on the 19 and 30°C phase transitions in polytetrafluoroethylene

Fourier transform infrared spectroscopy has been used in conjunction with differential scanning calorimetric measurements to investigate the nature of molecular degradation and its effect on the phase transition temperatures in irradiated polytetrafluoroethylene (PTFE). Both the 19 and 30°C transitions are observed to exhibit similar shifts to low temperatures upon irradiation. Infrared absorbance subtraction data from irradiated PTFE indicate a continual decrease in sample crystallinity accompanied by an increase in the number of free and bonded ? COOH groups with increasing dose consistent with molecular degradation by chain scission. By comparing infrared band intensities on a number of irradiated PTFE samples with those from short chain perfluoro n-alkanes, it was determined that the overall reduction in chain length caused by irradiation was primarily responsible for the observed reduction in both phase transition temperatures.     

Radiation-induced synthesis of low molecular weight of PTFE and their crosslinking in acetone medium

Polytetrafluoroethylene was obtained by radiation-induced polymerization of tetrafluoroethylene in acetone at 195 K. An average diameter of the products was very small compared with the commercial one; it was about 0.2 μm. The yield was reached to 100% with a dose of 4 kGy. The crystallinity of the products is decreasing with increasing dose. The crystallinity becomes low, for higher irradiation doses. It was found that PTFE obtained by irradiation of TFE in acetone at 195 K has Y-type crosslinking structure. This was demonstrated by means of ¹⁹F MAS NMR.     

Thermal expansion studies on some gamma-irradiated binary borate glasses

The thermal expansion, density and molar volume of some binary borate glasses were measured before and after exposure to a gamma-ray dose of 10³ kGy. The expansion curves for all glasses, which were measured from room temperature to above the softening temperature, displayed similar characteristics. Increase of the lead oxide content decreased the thermal coefficient of expansion, but the effects of different alkali metal cations were shown to depend on their ionic radii. The various proposed mechanisms of thermal expansion are dealt with. The experimental results could be explained by considering the bond strengths, the polarizing powers of the different cations and the damage produced by radiation. The possible compaction of the structure due to irradiation is also discussed.     

Cold crystallization of polytetrafluoroethylene by γ irradiation

Changes in density and in the corresponding degree of crystallinity with radiation dose are studied experimentally for γ-irradiated polytetrafluoroethylene (PTFE) in the dose range from 1 X 10³ to 1 X 10⁹ R. The relation between the amorphous fraction and the radiation dose is derived from a quantitative analysis of cold crystallization by scission of polymer backbone chains. The characteristic radiation dose, at which one break occurs on the average per initial molecule, is estimated as about 3 X 10⁴ R on the basis of a derived kinetic equation. The theoretical relation is modified by considering microvoids produced in the irradiated samples. The radii of microvoids in the form of spheres are evaluated as about 0.2 nm, and are also related to cage spheres relevant to the chain scission process. Good agreement between the modified theoretical relation and experimental data is attained over the entire range of radiation dose.     

Thermal expansion and Young's modulus of uniaxially drawn poly(tetrafluoroethylene) in the temperature range from 100 to 400 K

PTFE specimens with a crystallinity of 42 % were uniaxially stretched at 473 K resulting in draw ratios between 1. and 4. The degree of molecular orientation was obtained by X-ray wide angle and birefringence measurements. The thermal expansion coefficients and the Young's moduli both parallel and perpendicular to the draw direction were measured in the temperature range from 100 to 400 K. The thermal expansion behaviour turned out to be dependent upon the molecular orientation in a sensitive manner and could be explained by a simple model of structure changes caused by the deformation processes.     

Polytetrafluoroethylene: Effects of γ-radiation on fine structure

The structure of granular polytetrafluoroethylene has been studied by electron microscopy. On the basis of the texture of surfaces resulting from fracture a model of the structure is proposed which suggests that PTFE consists of extended chain crystals with both inter- and intra-lamellar noncrystalline regions. The effects of γ-radiation on the structure have been investigated by examining the texture of irradiated fracture surfaces and also the texture produced by post-irradiation fracture. The irradiations have been performed in vacuo and in oxygen. In both atmospheres PTFE undergoes degradation with a concurrent increase in crystallinity. However, the texture of the surfaces of high crystallinity PTFE, prepared by radiation, differs markedly to the texture of fracture surfaces of high crystallinity PTFE prepared by thermal annealing. It is proposed that radiation causes rupture of bonds in the interlamellar (chain fold) and intralamellar regions, resulting in the production of chain ends and interlamellar links. Due to scavenging of the free radicals, interlamellar linking is pobably a minor process with irradiation in oxygen. These chemical changes cause modifications to the extended chain lamellar crystals and consequently alterations to the physical properties of the polymer.     

Ab initio equation of state of an organic molecular crystal: 1,1-diamino-2,2-dinitroethylene

A complete equation of state for the molecular crystal 1,1-diamino-2,2-dinitroethylene has been calculated from first principles for temperatures between 0 and 400 K, and for specific volumes from 61 to 83 cm3/mol, corresponding to relative volumes from 0.78 to 1.06. The calculated 300 K isotherm agrees very well with the experimentally measured pressure-volume relation reported by Peiris et al. (Peiris, S. M.; Wong, C. P.; Zerilli, F. J. J. Chem. Phys. 2004, 120, 8060). The volumetric thermal expansion coefficient is calculated to be 140 ppm/K at 300 K and atmospheric pressure and varies considerably with specific volume as well as temperature. The Grüneisen parameter varies significantly with temperature, but its variation with specific volume is small. The calculated specific heat (160 J/mol/K at 300 K and atmospheric pressure) has only a very small dependence on specific volume.     

ESR experiments on quaternary calcites and bones for dating purposes

A series of experiments and measurements regarding sample preparation, peak intensity estimation, annealing, signal identification, dose determination and dose-rate estimation were carried out in order to explore the technique of ESR for dating speleothemes and bones. In addition, for the first time, the behaviour of peaks at low temperatures (room temperature to 4K) has been studied. Materials from the cave Apidima in Peloponnese, where anthropological remains were discovered, have been used for this study. The results indicate the existence of four peaks, two organic and two radiation induced. Their intensity is increased at 260K and therefore the sensitivity of the technique is enhanced at lower temperatures. The bone samples were found to have absorbed large amounts of uranium from the environment making their dating impossible. A range of ages 20–45·10³ years have been calculated for the travertines of the cave, taking into account all possible factors affecting the age.     

Rheo-optical studies of high polymers. XVII. Time-temperature superposition of time-dependent birefringence for high-density polyethylene

The strain-optical coefficient and relaxation modulus were simultaneously measured for high-density polyethylene at various temperatures ranging from 12 to 100°C. Not only horizontal but also vertical shifts were necessary to obtain smooth master curves by the application of time–temperature superposition. However, the relaxation modulus decreases with rising temperature while the strain-optical coefficient increases. This behavior indicates that the variation of the relaxation modulus and the strain-optical coefficient with time can not be explained by a decrease in crystallinity with rising temperature since a decrease in crystallinity usually causes a decrease in the strain-optical coefficient with time can not be explained by a decrease in crystallinity with rising temperature since a decrease in crystallinity usually causes a decrease in the strain-optical coefficient. It was emphasized that another explanation should be sought for the vertical shift in the time–temperature superposition of the time-dependence curves of the relaxation modulus and the strain-optical coefficient at various temperatures. The master curve of the strain-optical coefficient at various temperatures. The master curve of the strain-optical coefficient or the optical distribution function of relaxation times determined from it serve to distinguish the type and thermal history of the polyethylene.     

Micromachining of crosslinked PTFE by direct photo-etching using synchrotron radiation

Micromachining of crosslinked PTFE (polytetrafluoroethylene) using synchrotron radiation direct photo-etching method has been demonstrated. High aspect-ratio microfabrication was carried out. The etching rate of crosslinked PTFE was higher than that of non-crosslinked PTFE. Through the etching rate measurements of various samples, it was found that synchrotron radiation etching rate of crosslinked PTFE only depends on the degree of crosslinking, neither molecular weight nor crystallinity. The effect of molecular motion on etching process was discussed from temperature dependence data on etching rate. Furthermore, the surface region of synchrotron radiation irradiated sample was investigated by Fourier transform infrared spectroscopy and the experimental result showed that the modification induced by synchrotron radiation proceeded before desorption.     

The temperature dependence of the local free volume in polyethylene and polytetrafluoroethylene: A positron lifetime study

Positron lifetime measurements, performed in the temperature range 80–300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data processing routines LIFSPECFIT and MELT. Two long-lived components appear, which are attributed to pick-off annihilation of ortho-positronium in crystalline regions and at holes in the amorphous phase. The ortho-positronium lifetimes, τ₃ and τ₄, are used to estimate the crystalline packing density and the size of local free volumes in the crystalline and amorphous phases. The interstitial free volume in the crystals exhibits a weak linear increase with the temperature which is attributed to thermal expansion of the crystal unit cell. In the amorphous phase, the hole volume varies between 0.053 and 0.188 nm³ (PE) and between 0.152 and 0.372 nm³ (PTFE). Its temperature variation may be fitted by two straight lines, the intersection of which is used to estimate a glass transition temperature of T_g = 195 K for both PE and PTFE. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macroscopic volume change below and above the glass transition. From this comparison a fractional hole volume at T_g of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) × 10²⁷ holes/m³ is estimated. Finally, it is found that the intensity of o-Ps annihilation in crystals shows a different temperature dependence to that in the amorphous phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1513–1528, 1998     

Thermodynamic Study of Poly(vinyl pyrrolidone) in Water/Dimethyl Sulfoxide Solutions by Viscometry

In this work, the intrinsic viscosities of poly(vinyl pyrrolidone) with a molar mass of 58?kg?mol^?1 were measured in water?Cdimethyl sulfoxide solutions at temperatures from (298.15 to 318.15)?K. The expansion factors of the polymer chains were calculated from the intrinsic viscosity data. The thermodynamic parameters (entropy of dilution parameter, the heat of dilution parameter, theta temperature, polymer?Csolvent interaction parameter and second osmotic virial coefficient) were derived by the temperature dependence of the polymer chain expansion factor. The thermodynamic parameters indicate that mixtures of water/dimethyl sulfoxide become poorer solvents for poly(vinyl pyrrolidone) as the temperature is increased. Also, the thermodynamic parameters indicate that water/dimethyl sulfoxide mixtures become better solvents for poly(vinyl pyrrolidone) by increasing the volume fractions of dimethyl sulfoxide.     

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.     

Net Emission Coefficients of Radiation in Air and SF<Subscript>6</Subscript> Thermal Plasmas

Net emission coefficients of radiation were calculated for isothermal plasmas of air and SF₆ as a function of the plasma temperature 5,000–30,000 K and the arc radius (0.01–10 cm) at various plasma pressures. Calculations take into account continuum and line radiations, special attention has also been taken to influence of molecular species in case of the air plasmas. It has been found that the molecular bands of O₂, N₂, N₂ ⁺, NO and NO⁺ have very strong effect on the net emission coefficients at low temperatures (below 10,000 K). In case of SF₆, effect of PTFE admixture on the net emission coefficients was also studied. It follows from the calculations that the net emission coefficients vary very little with various admixtures of PTFE. Values of net emission coefficients if SF₆ plasma calculated for various spectral regions were compared.