Temperature and pressure dependence of molar volume in solid phases of ammonia near the melting point

Temperature and pressure dependencies of the molar volume are studied here along the transition curve between solid I and solid II phases near the melting point in ammonia. The molar volumes are calculated in the temperature range of 217 to 224  K and in the pressure range of 3 to 8 kbar with respect to the triple point (T_t = 217.34 K, P_t = 3.070 kbar) where the melting curves of solid I and solid II coincide with the transition curve in ammonia.     

Some simulations on crystallinity in a typical linear low-density polyethylene

Crystallinity in ethylene/1-hexene copolymers, a type of linear low-density polyethylene, was investigated by Monte Carlo simulations. The comonomer distributions generated in the simulated chains and the melting temperatures of real chains were used to estimate the minimum crystallite thickness, which is the critical quantity for simulating crystallization in any type of polymer. Simulated values of this thickness were in good agreement with values calculated from Raman longitudinal acoustic mode (LAM) spectroscopy, except for very low 1-hexene mole fractions, where there were presumably complications from high melt viscosities and chain entanglements. The use of this information in estimating properties of these copolymers is illustrated by some preliminary results on the effects of varying amounts of this comonomer on the sizes and numbers of the polyethylene crystallites.     

High Temperature-High Pressure Structural Studies of Cerium

Energy dispersive X-ray-diffraction with the large volume press MAX 80 at HASYLAB/DESY is used in the present study for the determination of the p - V - T -relations around the critical point of the f - n -phase transition in Cerium. Furthermore the f - l -phase boundary is studied experimentally to remove the last question marks from the phase diagram of Cerium in the accessible region of pressure and temperature.     

Radiation-Induced Polymerization of Methyl Methacrylate at High Pressure

Radiation-induced polymerization of methyl methacrylate (MMA) was studied up to 7500 kg/cm² at 20°C. The rate of polymerization increased to 3000 kg/cm² with overall activation volume ΔV_pol? of -23.6 cm³/mole, and then the pressure dependence of the rate was very small in the pressure range between 3000 and 3700 kg/cm². The rate of polymerization increased again above 3700 kg/cm² up to the crystallization pressure of MMA (5500 kg/cm²) with ΔV_pol? of -13.7 cm³/ mole with increasing pressure. The volume contraction by polymerization decreased with increasing pressure up to 3000 kg/cm² but hardly decreased with increasing pressure above 3000 kg/cm². The stereoregulzarity (triad probability) of PMMA changed slightly at 3000 kg/cm; above 3000 kg/cm², syndiotactic addition decreased and heterotactic addition increased. Marked change in P-V isotherms of MMA, however, was not observed about 3000 kg/cm². We concluded from these facts that an alignment of monomer molecules, which does not cause large volume change, was realized about 3000 kg/cm². Polymerization proceeded above the crystallization pressure by long time irradiation, and isotactic addition increased clearly in the solid-state polymerization.     

Use of Adsorptive Transfer Stripping Voltammetry for Analyzing Variations of Cytosine Methylation in DNA

The electrochemical behavior and thermal stability of double stranded oligonucleotides containing 5‐methyl‐cytosine and 7‐deaza‐guanosine as nucleotide analogues, as well as of Jurkat genomic DNAs methylated to different degree were studied by ACV and SWV and by thernal denaturation analysis. ACV and SWV combined with thermal denaturation analysis of the natural and modified oligonucleotides gave information regarding the presence of methylation and the concomitant conformational changes. ACV and SWV of Jurkat DNA mixtures methylated to different degrees revealed a decrease of the peak heights with increasing methylation, indicating an increase of structural rigidity, in agreement with the thermal denaturation data. These results verify the, possibly local, conformational changes introduced by DNA methylation. The results obtained in all cases were reproducible.     

Electrochemical Detection of DNA Melting Curves by Means of Heated Biosensors

This article reports about the detection of DNA melting curves at heated electrochemical biosensors. Osmium tetroxide‐bipyridine‐labeled target oligonucleotides are hybridized with probe oligonucleotides immobilized on gold electrodes. Then, the gold electrode is successively heated in order to measure a complete melting curve consisting of alternating current voltammetric signals. Melting temperatures ?_m, determined at various ionic strengths and in dependence on different numbers of base pair mismatches, have been compared with those obtained by means of UV spectrophotometry. The proposed method holds great promise for the fast and easy parallel detection of nucleic acids sequences on selectively heated electrode arrays. A stringent hybridization temperature can be easily adjusted in order to discriminate base pair mismatches.     

Effect of transesterification reactions on the crystallization behaviour and morphology of poly(butylene/diethylene succinate) block copolymers

The melting behaviour, the crystallization kinetics and the morphology of block poly(butylene/diethylene succinate) copolymers (PBSPDGS) were investigated by means of differential scanning calorimetry and hot stage optical microscopy. Multiple endotherms were evidenced in the PBSPDGS samples, due to melting and recrystallization processes, similarly to PBS. By applying the Hoffman-Weeks’ method, the of both the homopolymer and the copolymers was derived. The isothermal crystallization kinetics was analyzed according to the Avrami’s treatment. The copolymers with long PBS blocks are characterized by a very similar behaviour with respect to pure poly(butilene succinate), indicating that PBS macromolecular folding is not affected by the presence of non-crystallizable diethylene succinate blocks. On the contrary, the copolymers characterized by very short PBS block length were found to crystallize at a slower rate than the homopolymer. As a matter of fact, a higher value of the work of chain folding was also derived on the basis of Hoffman-Lauritzen nucleation theory. Anyway, in all cases the crystallization mechanism (heterogeneous nucleation and three-dimensional growth) was found to be the same.     

Melting heat transfer in boundary layer stagnation-point flow towards a stretching/shrinking sheet

An analysis is carried out to study the steady two-dimensional stagnation-point flow and heat transfer from a warm, laminar liquid flow to a melting stretching/shrinking sheet. The governing partial differential equations are converted into ordinary differential equations by similarity transformation, before being solved numerically using the Runge-Kutta-Fehlberg method. Results for the skin friction coefficient, local Nusselt number, velocity profiles as well as temperature profiles are presented for different values of the governing parameters. Effects of the melting parameter, stretching/shrinking parameter and Prandtl number on the flow and heat transfer characteristics are thoroughly examined. Different from a stretching sheet, it is found that the solutions for a shrinking sheet are non-unique.     

Influence of Cu substrate surface oxides and heating rates during reflow on melting point of Sn-3.5Ag solder

Three types of copper substrates, fresh, aged (kept for years in open atmosphere) and acid washed aged, were investigated for the reflow behaviour of a solder using different heating rates. Melting point of the Sn-3.5Ag solder was lowered on the aged Cu substrate. Reduction was found to be higher in high heating rate and declined with the decrease in the heating rate. Melting point was lowered from 221 °C to 175 °C with the heating rate of 180 °C/min, but recovered to 210 °C when aged Cu substrate was washed with sulphuric acid. XPS depth profile revealed the presence of Cu₂O up to the greater depth in the aged substrate compared to the fresh and acid washed aged substrates. Study showed the relation of reduction in melting point with the depth of Cu₂O on the surface of aged Cu substrates. It was proposed that lower dissipation of heat generated in high heating rates by the oxidation of the flux carbon during reduction of high Cu₂O amount in aged carbon was responsible for the variations in melting points.     

Theoretical study of melting curves on Ta, Mo, and W at high pressures

The melting curves of tantalum (Ta), molybdenum (Mo), and tungsten (W) are calculated using a dislocation-mediated melting model. The calculated melting curves are in good agreement with shock-wave data, and partially in agreement with wire explosion and piston-cylinder data, but show large discrepancies with diamond-anvil cell (DAC) data. We propose that the melting mechanism caused by shock-wave and laser-heated DAC techniques are probably different, and that a systematic difference exists in the two melting processes.