Crystallization and melting of polyethylene under high pressure. II. Effect of pressure on melting behavior of various types of crystals

The effect of pressure on the melting point and volume of fusion of polyethylene was studied by high-pressure dilatometry. Starting materials were crystallized slowly from the melt under pressures of 1500, 3500, 5130 kg/cm², and 1 atm. It has been shown that the unusual behavior observed at pressures above 4000 kg/cm² is due to crystallization and melting of two kinds of extended-chain crystals differing in thermal stability. These are designated as ordinary extended-chain and highly extended-chain crystals, respectively. The relation between pressure P and melting temperature T_m of folded-chain, ordinary extended-chain, and highly extended-chain polyethylene was determined precisely. At pressures up to about 3000 kg/cm², plots of P against T_m for the crystal forms have almost the same curvature and then become parallel. But at pressures above 4000 kg/cm², ordinary extended-chain crystals show a linear increase of T_m with a constant slope of about 70 atm/deg. Curve for the highly extended-chain crystals changes in slope from 70 to 50 atm/deg at pressures between 3500 and 4300 kg/cm², and then show a sharp increase of T_m with increasing pressure. Experiments show that the meltingpoint curve of the highly extended-chain crystals overlaps that of the ordinary extended-chain crystals at pressures below 4000 kg/cm². Annealing experiments with folded-chain and ordinary extended-chain crystals have been made under high pressure. It is suggested that the formation of highly extended-chain crystals occurs stepwise through the formation and reorganization of ordinary extended-chain crystals from the original folded-chain crystals by a mechanism of partial melting and recrystallization at pressures above 4000 kg/cm².     

Phase behaviour of tri-n-butylmethylammonium chloride hydrates in the presence of carbon dioxide

The phase behaviour of the system water?Ctri-n-butylmethylammonium chloride (TBMAC)?CCO₂ was investigated by pressure-controlled differential scanning calorimetry in the range 0?C10?mol% TBMAC in water and at CO₂ pressures ranging from 0 to 1.5?MPa. In the absence of CO₂, an incongruent melting hydrate, which estimated composition corresponds to TBMAC·30H₂O, crystallizes at temperatures below ?13.6?°C and forms with ice a peritectic phase at approximately 3.9?mol% TBMAC. In the presence of CO₂ at pressures as low as 0.5?MPa, curves evidenced the presence of an additional phase exhibiting congruent melting at temperatures that are strongly pressure dependent and significantly higher than those of hydrates obtained without CO₂. This new phase, whose enthalpy of dissociation and CO₂ content increase slightly with CO₂ pressure, was identified as a mixed semi-clathrate hydrate of TBMAC and CO₂ of general formula: (TBMAC?+?xCO₂)·30H₂O.     

Effect of x-rays on extended chain crystals of polyoxymethylene

Extended chain crystals of polyoxymethylene were irradiated with x-rays of about 10⁵ r./min. An immediate decrease in superheating on melting was noted. After 90 min. of irradiation the melting point level, refractive indices, and density also decreased. After 480 min. the DTA melting peak at a heating rate of 20°C./min. had decreased 25°C., the melting point decreased about 18°C., and the density calculated from refractive indices decreased 0.031 g./cm.^?3. These effects are interpreted as indication of chain scission and formation of amorphous defects.     

Kinetics of the reaction between H atoms and allyl radicals

The reaction between H and C₃H₅ has been studied at 291 K. Exciplex laser flash photolysis at 193.3 nm of hexa-1,5-diene-He mixtures generated both H and C₃H₅ ([H] ? [C₃H₅]), which were detected in time-resolved mode by resonance fluorescence and absorption spectroscopy, respectively. Rate coefficients are presented at four pressures in the range 98 ? P/torr ? 400; no clear pressure-dependence is found in this range of pressures and the mean rate coefficient is (2.8 ± 1.0) × 10^?10 cm³ molecule^?1 s^?1. Calculations based on the Troe factorization method confirm that this reaction is near its high-pressure limit under the experimental conditions.     

The effect of pressure on the oxidation of benzhydrol by alkaline permanganate

The rate of oxidation of benzhydrol and benzhydrol-d to benzophenone by alkaline permanganate has been measured over a pH range 10 to 12.5 and pressures from 1 to 1000 bar. The pressure effect on the rate is small, Δ$\text{V}$^≠ = ?6 to ?8 cm³mol^?1. This is held to support a hydrogen atom transfer in the rate-determining step.     

Crystallization and melting of polyethylene under high pressure. I. Crystallization by slow cooling from the melt

Crystallization and melting behavior of linear polyethylene under high pressures up to 6000 kg/cm² has been investigated with a high-pressure dilatometer. Crystallization was carried out at a cooling rate of 1°C/min from the melt at each pressure. The samples were characterized by differential scanning calorimetry, density, and electron microscopy. Folded-chain crystals are formed in the low-pressure region below 2000 kg/cm². Crystallization in the intermediate-pressure region between 2000 and 3500 kg/cm² gives a mixture of folded-chain and extended-chain crystals. The extendedchain crystals are the more stable and predominate at increasing pressure. At high pressures above 4700 kg/cm², two stages of crystallization and of melting can be observed. The phenomenon suggests that the two kinds of extended-chain crystals with different thermal stability, i.e., the ordinary extended-chain crystals and “highly extended-chain” crystals form through individual crystallization processes from the melt at high pressure.     

Determination of specific heat capacity and standard molar combustion enthalpy of taurine by DSC

Using XRY-1C calorimeter, the standard molar enthalpy of taurine was determined to be ?2546.2?kJ?mol^?1 . The reliability of the instrument used was tested by using naphthalene as reference material; and through comparing the molar combustion enthalpy of naphthalene measured with its standard value found in literature, the absolute error and relative error were found to be 4.53?kJ?mol^?1 and 0.09%, respectively. The melting point and melting enthalpy of taurine were determined by Differential Scanning Calorimetry (DSC), which was found to be 588.45?K and ?22.197?kJ?mol^?1, respectively. Moreover, using the DSC method, the specific heat capacities C _p of taurine was measured and the relationship between C _p and temperature was established. The thermodynamic basic data obtained are available for the exploiting new synthesis method, engineering design and industry production of taurine.     

Kinetics of the II-III phase transformation of polytetrafluoroethylene at high pressure

The rate of transformation from helical to all-trans-planar polytetrafluoroethylene (PTFE) at high pressures has been determined by monitoring the Raman spectra of PTFE following pressure jumps from the stability field of PTFE II to pressures between 9 and 14 kbar at temperatures between 0 and ?30°C. The transformation kinetics can be described by Avrami's equation for nucleation and growth kinetics with an exponent of 0.5, although observations at lower temperatures suggest that even smaller values of the exponent may be appropriate. At 10 kbar and 0°C, the specific rate constant is 0.51 min^?1/2. The energy and volume of activation are 11 kcal mole^?1 and ?7 cm³ mole^?1, respectively. The values of these parameters suggest that the transformation mechanism involves propagation of helix reversal planes along the several adjacent chains leaving all-trans material in their wakes.     

Experimental determination of the logarithmic term in the density expansion of the viscosity coefficient of krypton at 25°C.

In order to examine the existence of the logarithmic term in the density expansion of the viscosity coefficient, the viscosity of krypton has been measured with high precision. The measurements are performed at 25°C and at pressures from 1 to 68 bar, using a capillary viscometer. It is found that, within the precision of 1 in 10000, the experimental data can be represented by a polynomial in the density ? as well as by a polynomial in ? plus the logarithmic term η₂?² 1n ?. When ? is taken as amagat density, the absolute value of the coefficient η′₂ is less than 2 x 10^?4 micropoise.     

Melting behavior of poly(ether ether ketone) in its blends with poly(ether imide)

We detail the melting behavior of poly(ether ether ketone) (PEEK) and investigate its melting behavior in miscible blends with poly(ether imide) (PEI). The determination of the equilibrium melting point (T_m⁰) of PEEK is discussed by considering its inhomogeneous morphology. T_m⁰ is obtained by a long extrapolation of a Hoffman–Weeks plot to 384°C. Hindrance of PEEK crystal reorganization induced by PEI during heating is observed over the blend composition investigated (20–75 wt % PEEK). This behavior is correlated with the incorporation of PEI in the interlamellar zones of PEEK crystals. The interaction parameter χ of PEEK/PEI blends is estimated by the equilibrium melting point depression. This gives the interaction density B = ?1.2 cal/cm³, and x = ?0.40 at 400°C. © 1993 John Wiley & Sons, Inc.     

Uniaxial Draw Of Poly(aryl-ether-ether-ketone) by solid-state extrusion

Poly(aryl-ether-ether-ketone) (PEEK) films and rods have been solid-state extruded at 154 and 310°C, respectively. The crystal orientation functions, melting behavior, density, and tensile properties of the drawn PEEK films (EDR ≤ 3.7) and rods (EDR ≤ 5.5) have been measured. As extrusion draw ratio (EDR) was increased, the c-axis orientation function, melting temperature, and tensile modulus and strength increased. Moduli up to 6.5 GPa and the strengths up to 600 MPa, 3 and 6 times the values of undrawn films, respectively, were obtained for the drawn films. The thermal expansivities along (α_‖) and perpendicular (α_?) to the draw direction of PEEK rods were measured from ?40 to +10°C. As EDR was increased, α_? increased, but α_‖ decreased. At EDRs of 3.8 and 5.5, α_‖ even exhibited negative values (about ?5 × 10^?6°C^?1), probably due to reversible contraction of elongated tie-molecules.     

Effect of molecular weight on the crystalline structure of polytetrafluoroethylene as-polymerized

Melting and crystallization behavior of polytetrafluoroethylene as polymerized in emulsion and suspension is shown to depend on molecular weight. DSC heating curves for virgin PTFE with low molecular weight below 3 × 10⁵ have a single peak, whereas curves for higher molecular weight samples have double peaks. With increasing heating rate the areas of higher melting peaks become larger than the lower melting peaks. The morphology of polymer exhibiting double melting peaks is mainly folded ribbons or granular particles. The phenomenon of double melting is explained on the basis of two different crystalline states which correspond to the “fold regions” and the “linear segments” in a folded ribbon. The melting temperature of virgin PTFE is almost constant at ca. 330°C for molecular weights below 1 × 10⁶, and rises as the molecular weight increases above 1 × 10⁶. The heat of melting of virgin PTFE is nearly independent of molecular weight. On the basis of these results, we propose a model for melting and crystallization of low and high molecular weight PTFE and for the crystal structure.     

Embedded atom model for liquid metals: Liquid iron

A method for calculating the embedded atom model potential suggested earlier for liquid Ga and Bi uses data on the structure and thermodynamic properties of metals close to their melting points. This method was applied to liquid iron at temperatures and pressures up to 5000 K and 360 GPa. Several iron models with the potential of the embedded atom model were constructed by the method of molecular dynamics at temperatures from 1820 to 5000 K and densities from 8.00 to 12.50 g/cm³. The thermodynamic, structural, diffusion, and viscosity properties of iron were calculated. The self-diffusion coefficients decreased almost linearly as the volume of the system became smaller. The conclusion is drawn that iron in the external region of the Earth’s core behaves as a liquid with self-diffusion coefficients of about ～10^-5 cm²/s and viscosity ～10^-3?10^-2 Pa s. At the boundary between the external and inner core regions, at densities of 11–12 g/cm³, iron has the properties of an amorphous phase and its self-diffusion coefficient becomes too low to be estimated by the method of molecular dynamics. Under the Earth’s inner core conditions, the embedded atom model of iron spontaneously crystallizes.     

Determination of thermophysical properties of high temperature alloy IN713LC by thermal analysis

The presented paper deals with the study of thermophysical properties of cast and complex alloyed nickel based on superalloy Inconel 713LC (IN713LC). In this work, the technique of Differential Thermal Analysis was selected for determination of the phase transformation temperatures and for the study of the effect of varying heating/cooling rate at these temperatures. The samples taken from as-received state of superalloy were analysed at heating and cooling rates of 1, 5, 10, 20 and 50?°C?min^?1 with the help of the experimental system Setaram SETSYS 18_TM. Moreover, the transformation temperatures at zero heating/cooling rate were calculated. The recommended values for IN713LC after correcting to a zero heating rate, are 1,205?°C (T _?á?,solvus), 1,250?°C (solidus) and 1,349?°C (liquidus). Influence of heating/cooling rate on shift of almost all temperatures of phase transformations was established from the DTA curves. Undercooling was observed at the cooling process. The samples before and after DTA analysis were also subjected to the phase analysis by scanning electron microscopy using the microscope JEOL JSM-6490LV equipped with an energy dispersive analyser EDAX (EDS INCA x-act). Documentation of the microstructure was made in the mode of secondary (SEI) and backscattered (BEI) electron imaging. On the basis of DTA analysis and phase analysis it may be stated that development of phase transformations of the alloy IN713LC will probably correspond to the following scheme: melting?????? phase; melting???????+?MC; melting????eutectics ??/?á?; melting???????+?minority phases (e.g. borides); and matrix ???????á?.     

Thermal properties of some cyclic disulfides: naphthalene disulfide and diphenylene disulfide

The specific heat, the melting heat and entropy, the vaporization heat of naphtalene disulfide (C₁₀H₆S₂) and of diphenylene disulfide (C₁₂H₈S₂) have been determined by differential scanning calorimetry (DSC).Over the temperature range examined the specific heat may be represented as follows:

where T is the temperature in degrees Kelvin, while melting heat, vaporization heat, melting entropy are for naphtalene disulfide: 3.10 kcal mol^?1, 6.42 kcal mol^?1, 7.87 cal deg^? mol^?1 and for diphenylene disulfide: 4.62 kcal mol^?1, 6.90 kcal mol^?1 and 11.87 cal deg^?1 mol^?1.     

Collisional focusing effects in radio frequency quadrupoles

The transmission of ions through a conventional two-dimensional radiofrequency-only (rf) quadrupole has been studied for comparatively high operating pressures between 5 × 10^?4 and 1 × 10^?2 torr. Measurements of signals from mass-resolved analyte ions and total ion currents show that, provided the initial injection ion energy is low (1–30 eV), the ion transmission observed through a small aperture at the exit of the rf quadrupole first increases as the gas pressure increases, reaching a maximum at ? 8 × 10^?3 torr before decreasing at higher pressures. This is in direct contrast to the expectations of classical scattering. This “collisional focusing” appears to be analogous to effects seen in three-dimensional ion traps. The collisional focusing increases with the mass of the ion (not mass-to-charge ratio) for masses up to at least 16,950 u. The collisional focusing of the ions is found to be accompanied by significant losses of axial kinetic energy. A Monte Carlo simulation of the energy loss process is reported that can provide agreement with the observed losses for reasonable collision cross-sections. The results suggest that operation of rf quadrupoles at relatively high pressure may find practical application in sampling ions from high (e.g., atmospheric) pressure ion sources.     

Thermodynamic modeling of redox equilibria in sodium silicate glasses with low iron content

The available literature data on the content of oxidized and reduced iron forms in molten sodium silicates at temperatures the 1573?C1873 K and partial oxygen pressures 10^?8?0.21 bar have been scrutinized. On the basis of this information, the temperature dependence of the Fe₂O₃ activity coefficient in glass melts containing up to 4 mol % Fe (??Henrian?? solution) has been suggested in the framework of the quasi-chemical model. The results are compared with previous data.     

The kinetic of the oxidation of InSn48

The oxidation of InSn48 has been investigated at partial pressures between 10^-8 Pa and 10⁺⁴ Pa over a temperature range from 22^°C to 250^°C with different analytical methods. The oxide film contains a mixture of several oxides, although indium oxide forms preferentially. Below the melting point a logarithmic growth, and above this, a parabolic growth of the oxide film has been observed. The oxide film formed in air at 250^°C does not become thicker than 50 nm in the first 5 min of oxidation.     

Characterization of crystalline and amorphous content in pharmaceutical solids by dielectric thermal analysis

Morphological and thermodynamic transitions in drugs as well as their amorphous and crystalline content in the solid state have been distinguished by thermal analytical techniques, which include dielectric analysis (DEA), differential scanning calorimetry (DSC), and macro-photomicrography. These techniques were used successfully to establish a structure versus property relationship with the United States Pharmacopeia standard set of active pharmaceutical ingredient (API) drugs. A distinguishing method is the DSC determination of the amorphous and crystalline content which is based on the fusion properties of the specific drug and its recrystallization. The DSC technique to determine the crystalline and amorphous content is based on a series of heat and cool cycles to evaluate the drugs ability to recrystallize. To enhance the amorphous portion, the API is heated above its melting temperature and cooled with liquid nitrogen to ?120 °C (153 K). Alternatively a sample is program heated and cooled by DSC at a rate of 10 °C min^?1. DEA measures the crystalline solid and amorphous liquid API electrical ionic conductivity. The DEA ionic conductivity is repeatable and differentiates the solid crystalline drug with a low conductivity level (10^?2 pS cm^?1) and a high conductivity level associated with the amorphous liquid (10⁶ pS cm^?1). The DSC sets the analytical transition temperature range from melting to recrystallization. However, analysis of the DEA ionic conductivity cycle establishes the quantitative amorphous and crystalline content in the solid state at frequencies of 0.10–1.00 Hz and to greater than 30 °C below the melting transition as the peak melting temperature. This describes the “activation energy method.” An Arrhenius plot, log ionic conductivity versus reciprocal temperature (K^?1), of the pre-melt DEA transition yields frequency dependent activation energy (E _a, J mol^?1) for the complex charging in the solid state. The amorphous content is inversely proportional to the E _a where the E _a for the crystalline form is higher and lower for the amorphous form with a standard deviation of ±2%. There was a good agreement between the DSC crystalline melting, recrystallization, and the solid state DEA conductivity method with relevant microscopic evaluation. An alternate technique to determine amorphous and crystalline content has been established for the drugs of interest based on an obvious amorphous and crystalline state identified by macro-photomicrography and compared to the conductivity variations. This second “empirical method” correlates well with the “activation energy” method.     

Die Wärmeleitfähigkeit von Polyäthylen im Temperaturbereich von 20 bis 200 °C

Zusammenfassung Die W?rmeleitf?higkeit von je einemPhillips-,Ziegler- und Hochdruckpoly?thylen mit hohem Molekulargewicht wurde im Temperaturbereich von 20 bis 200 ‡C gemessen. Von 20 ‡C bis zum Schmelzende zeigt sich bei den verschiedenen Poly?thylentypen ein um so gr?\erer Abfall der W?rmeleitf?higkeit, je h?her der Kristallisationsgrad bei 20 ‡C ist. Die W?rmeleitf?higkeit in der Schmelze betr?gt etwa 6 · 10⁻⁴ cal/‡C cm sec und steigt schwach mit der Temperatur an.

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Summary Thermal conductivity of three representative types of polyethylene —Phillips-, Ziegler- and high pressure-type — has been determined from 20 to 200 ‡C. The stationary method applied in this work limited the experiments to polyethylene with extremely high molecular weight. The decrease of thermal conductivity between 20 ‡C and the melting point is an increasing function of cristallinity at 20 ‡C, whereas in the liquid phase the value of about 6 · 10⁻⁴ cal/‡C cm sec is nearly the same for each of the three investigated substances; a slight increase with temperature can be observed.