The beneficial influence of ultrasound in the polymerization of epsilon-caprolactam to polyamide-6 (Nylon 6). Part I: primary experimental results

ε-caprolactam (CL) polymerization to polyamide-6 (Nylon 6) was studied at different contents of water in CL (0.01–2 wt%), with or without ε-amino-caproic acid (ACA) as an activator, applying to the mixture an initial treatment of Ultrasound (US) (17.5–20 kHz) at low temperatures (70–110 °C) and for short times (max 10 min). It was verified that polymerization at 260 °C produces a polymer having a much higher molecular weight (MW) when US is applied with respect to silent (SIL) conditions i.e. without the use of ultrasound. This constitutes a “pre-sonication effect”. The ratio (MW)_US/(MW)_SIL is inversely proportional to the initial content of water in CL. The action of US converts CL at very low temperatures (70–110 °C) and water content, in comparison with silent conditions where CL was unconverted.

Optimized conditions are studied with respect to nature and pressure of gas inside the reactor, temperature, time and frequency of US irradiation, energy consumption and nature of activator.     

The beneficial influence of ultrasound in the polymerization of epsilon-caprolactam to polyamide-6 (Nylon 6). Part II: additional experiment to understand the "pre-sonication effect"

Ultrasound (US) “pre-sonication effect” is the beneficial effect of US in the hydrolytic polymerization of ε-caprolactam (CL) mixtures with very low water concentrations (about 0.1–1 wt%). It appears after a mild initial treatment of the mixtures with US [17.5–20 kHz, short times (5–15 min), low temperatures (70–110 °C)] followed by heating at 220–260 °C. An explanation is proposed on the basis of the formation in mild conditions (100 °C) of low concentrations of cyclic oligomers never detected in the literature at those conditions. These, under US irradiation, produce linear amino acid oligomers, which are strong activators of polymerization when the mixture of CL and water, after US irradiation, is heated at the suitable polymerization temperature indicated above.     

Characterization of heterogeneous interaction in binary mixtures of ethylene glycol oligomer with water, ethyl alcohol and dioxane by dielectric analysis

The associating behaviour of the binary mixtures of ethylene glycol oligomer (EGO), i.e. ethylene glycol (EG), diethylene glycol (DEG) and poly(ethylene glycol)s (PEG200, PEG300, PEG400 and PEG600) with water (W), ethyl alcohol (EA) and 1,4-dioxane (DX) over the entire concentration range at 25 °C have been investigated through their accurately measured values of dielectric constant. The static dielectric constant ε_o, high frequency limiting dielectric constant ε_∞, dielectric relaxation strength Δε, excess dielectric parameters ε^E₀ and ε^E_∞, effective Kirkwood correlation factor g^eff and corrective correlation factor g_f of EGO–W, EGO–EA and EGO–DX mixtures were determined to obtain qualitative and quantitative information about the complex formation through H-bond in these systems. Most of the evaluated dielectric parameters of EG and DEG in different ‘cosolvents’ have different characteristics as compared to the PEG–cosolvent mixture. The observed linear and non-linear behaviour of Δε against EGO monomer unit mole fraction X confirms the variation in the homogeneous structures in their binary mixtures with concentration variation. Appearance of the maximum in ε^E₀ against X plots indicates that a complex stable adduct is formed in the EGO–W mixtures at stoichiometric ratio 1:1.7 for lower oligomers but this ratio seems to be 1.7:1 for higher EGO molecules, which confirms that the EGO size and chain flexibility affects the complex formation between EGO and W. In case of EG–EA mixture 1:1 stoichiometric ratio form stable adduct whereas for higher EGO–EA, it is 3:1, at EGO monomer unit level. The complex formation behaviour of DEG–EA has entirely different characteristics when compared to the other studied EGO–EA mixtures. Although, 1,4-dioxane has weak polar behaviour dielectric properties of EG–DX and DEG–DX confirm the formation of stable adducts at the stoichiometric ratio 2:1 of EGO monomer unit mole fraction to the DX. For the higher EGO–DX mixtures, stable adduct forms at the stoichiometric ratio 9:1. Except DEG–EA mixtures, the EGO–W and EGO–EA form the complex with reduction in the effective number of dipoles. In EG–DX mixtures, the heterogeneous species form with a large reduction in the effective number of dipoles, which changes as the effective number of dipoles increases with the increase in monomer repeat units of EGO. Further the net electronic polarization in these binary mixtures increases due to heterogeneous interaction over the entire mixing concentration range.     

Calorimetric and resistometric studies of La2CuO4+δ single crystals: Displasive character of phase transitions in 190–310 K

Measurements of heat capacity, resistivity, and magnetic susceptibility for the La₂CuO_4+δ single crystals in the range 0.001≤δ≤0.035 revealed that following four kinds of phase transitions appear between 10 and 320 K: superconducting transition around 33 K, a transition around 200 K with small heat anomaly, martensitic transition around at 250–270 K, and a second order transition at 295 K. These transition temperatures strongly depended on the amount of excess oxygen content. The heat capacity anomalies for the sample with δ=0.035 in the range of 190 to 310 K were interpreted as connected with cooperative displacements of ions involving no diffusion process of oxygen ion.     

Specific heat of Sm1+xBa2−xCu3Oysolid solution of orthorhombic and tetragonal structure

The specific heats of Sm_1+xBa_2−xCu₃O_y solid solution of orthorhombic and tetragonal structure were measured in the temperature range 80–300 K. The data were analyzed in the framework of the Debye model with dilatation correction. The tendency to lower the high-temperature limit of the Debye temperature, θ^HT_D, with the oxygen deficiency was noticed. In contrast, the increase of Sm substitution causes a rise of θ^HT_D. The temperature dependence θ_D(T) was calculated for each compound from the series for the whole temperature region investigated.     

Comblike polymer electrolyte with main chain glass transition near room temperature

A new amorphous comblike polymer (CBP) based on methylvinyl ether/maleic anhydride altering copolymer backbone and on oligooxyethylene side chain was synthesized. The dynamic mechanical properties of CBP and its Li salt complexes were investigated by means of DDV-ll-EA type viscoelastic spectrometry. Results showed that there were two glass transitions (-transition and β-transition) in the temperature range from − 100 to 100 °C. The β-transition was assigned to oligo-PEO side chains and the temperature of β-transition increases with increasing Li salt content. The -transition was assigned to the main chain of CBP. The temperature of the -transition (T) is also dependent upon the Li-salt content, but not monotonie. The value of T lies between 30–45 °C in the Li salt concentration range studied, near room temperature. It was found that the CBP-Li salt complexes showed an unusual dependence of ionic conductivity on Li salt content. There are two peaks in the plot of the ionic conductivity vs. Li salt concentration, which has been ascribed to the movability of the CBP main chain at ambient temperature. The temperature dependence of the ionic conductivity indicated that the Arrhenius relationship was not obeyed, and the plot of log σ against 1/(T − T₀) showed the unusual dual VTF behavior when using side chain glass transition temperature (T_β) as T₀.     

Cl nuclear quadrupole resonance studies of molecular motions of ClO3 and water diffusion in Ca(ClO3)2·2H2O

A ³⁵Cl nuclear quadrupole resonance (³⁵Cl-NQR) investigation of polycrystalline Ca(ClO₃)₂·2H₂O is described. The ³⁵Cl-NQR frequencies (ν_Q) for two resonance lines (ν_Q1 and ν_Q2), the spin lattice relaxation time (T_1Q) for ν_Q2 only and the line width δν_Q2 were measured in the temperature range 292–345 K, except for the frequency measured up to 455 K. The observed decrease in the resonance frequencies with increasing temperature permitted the determination of the frequencies of librations of the ClO₃⁻ ion about two axes perpendicular to the three-fold axis of the ion mainly responsible for this effect. The temperature dependence of the relaxation time T_1Q proved the occurrence of water diffusion and hindered rotation of ClO₃⁻ ions. The activation energies of these two molecular motions were determined, and their effect on the electric field gradient at the site of a chlorine nucleus was discussed. Temperature measurements of the line width δν_Q2 confirmed the conclusions drawn from the analysis of T_1Q(T).     

Optical parametric oscillator based difference frequency laser source for photoacoustic trace gas spectroscopy in the 3 μm mid-IR range

We report on the extension of the emission wavelength range of a nanosecond pulsed commercial optical parametric oscillator (OPO) towards the infrared. By difference frequency mixing the idler of the OPO with the fundamental of its Nd:YAG pump laser the wavelength range from 2.5–4.5 μm is covered at considerable pulse energies. This laser source is then applied for the first time to photoacoustic trace gas spectroscopy in the fundamental C---H stretch band located around 3.3 μm. Spectra of single- and multi-component gas mixtures of volatile organic compounds are recorded and analyzed with respect to composition and concentration. The detection limits for the individual substances within the gas mixtures are found in the lower parts per million (ppm) range.     

Experimental study and calculations of nitric oxide absorption in the γ(0,0) and γ(1,0) bands for strong temperature conditions

Absorption spectra of nitric oxide in the γ(0,0) and γ(1,0) bands have been measured for hard temperature conditions up to 1700 K in order to validate a model for the simulation of these two bands. The good agreement between experiments and calculations (relative errors of 2–5% for the γ(0,0) band and 10–15% for the γ(1,0) band) consolidates the two important assumptions concerning the intermediate Hund's case between (a) and (b) for the X²Π state of the γ(0,0) and γ(1,0) absorption bands and the use of collisional broadening parameters of γ(0,0) to simulate the γ(1,0) band. Using this simulation, a study of the Beer–Lambert law behavior at high temperature has been carried out. With the instrument resolution used for these experiments, it was shown that a correction of the Beer–Lambert law is necessary. To apply this technique for the measurements of NO concentrations inside the combustion chamber of an optical SI engine, a new formulation of the Beer–Lambert law has been introduced, since the modified form proposed in the literature is no longer applicable in the total column range of interest.     

Trajectory-property relationships in MOD-derived YBCO films

A study of the physical and chemical changes during processing in MOD-derived YBCO films was performed. Fully processed films were 70–85% of theoretical density. The sintering rate increased substantially in the compositional range F/Ba = 1.8–1.5. The activation energy for sintering decreased above a P(H₂O) dependent threshold temperature. XRD indicated this temperature/composition threshold also corresponded to YBCO nucleation, suggesting ex situ YBCO forms in contact with a melt. The ramp rate and P(H₂O) were used to control F/Ba trajectories, which were correlated to performance. The nucleation of YBCO was strongly dependent on processing conditions. Nucleation temperature was varied by at least 60 °C in the study. The optimal YBCO nucleation temperature in the 300–800 nm films was around 725 °C. a-axis grains dominated the microstructures of films where YBCO nucleated at <700 °C. Large second phases, but no a-axis grains, were found when the nucleation temperature was >750 °C.     

Electric conductivity and relaxation in ZnF2–AlF3–PbF2–LiF glasses

The electric conductivity of ZnF₂–AlF₃–PbF₂–LiF glasses has been studied in the frequency range 10 Hz–2 MHz and in the temperature range from 300 K to just below the glass transition temperature. The conductivity decreases with the increase in the LiF content in the composition, which results from the trapping of F⁻ ions by Li⁺ ions. Small values of the stretching exponent β are observed for the present glasses. The value of the decoupling index decreases with an increase in LiF content, consistent with the composition dependence of the conductivity.     

Oxygen diffusion and surface exchange in La1−xSrxFe0.8Cr0.2O3−δ (x=0.2, 0.4 and 0.6)

Oxygen tracer diffusion (D*) and surface exchange rate constant (k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La_1−xSr_xFe_0.8Cr_0.2O_3−δ (x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700–1000 °C) and oxygen partial pressure (0.21–10⁻²¹ atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV (x=0.2), 1.53 eV (x=0.4) and 1.21 eV (x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 °C range from 10⁻⁸ cm² s⁻¹ for x=0.2 to 10⁻⁷ cm² s⁻¹ for x=0.4 and 0.6. The activation energies determined at constant H₂O/H₂ ratio are 1.21 eV (x=0.2), 1.59 eV (x=0.4) and 0.82 eV (x=0.6).

The surface exchange rate constant of oxygen for the H₂O molecule is similar in magnitude to that for the O₂ molecule and both increase with increasing Sr concentration.     

Effects of composition and processing on properties of modified maghemite for high-density recording

The effects of composition and processing on the magnetic properties of Co, Mn and Zn modified maghemite particulates obtained by coprecipitation from aqueous solutions and heat treatment have been explored. It was found that the saturation (at 2 T) magnetization, of 63–75 emu/g and residual magnetization of 45–52 emu/g change little with the composition studied (6–14.4 wt% Mn and 8–15 wt% Co), while coercivity (1100–2200 Oe) does. The temperature coefficient of coercivity (TCC) was -0.24 to -0.39%/°C, depending only on composition. The Co content was found to dominate coercivity, while Mn led to a better heat treatment response of the particulates. Low-level Zn doping enhanced magnetization and reduced the TCC.     

Ultrasonic characterization of heavy metal TeO2–WO3–PbO glasses below room temperature

The longitudinal ultrasonic attenuation measurements have been made using pulse echo method at fundamental frequencies of 2, 4, 6 and 8 MHz in 20WO₃–(80−x) TeO₂–xPbO ternary tellurite glasses (x=10, 12.5, 15, 17.5 and 20 mol%) in the temperature range 160–280 K. The results showed the presence of a broad peak which shifts to higher temperature with increasing frequency. The ultrasonic attenuation peaks suggest that the experimental behavior is controlled by thermally activated structural relaxations. The internal friction, acoustic activation energy, deformation potential, relaxation strength, number of loss centers and density of state have been calculated both as a function of temperature and PbO content. The acoustic activation energy was found to decrease from 0.156 to 0.135 eV with the increase of PbO content. The results showed that both the number of loss centers and their activation energy decrease with the atomic ring size. An increase in the density of state is observed with addition of PbO content at the same frequency in the whole range of temperature which is associated with structural units formed when PbO is added.     

Viscoelastic investigation by ultrasonic shear waves of liquid eutectic mixture methylurea–ammonium sulfamate

Shear impedance of the liquid eutectic mixture CH₃NHCONH₂–NH₄SO₃NH₂ was studied in the temperature range 291.7–313.2 K by using ultrasonic shear waves in the frequency range 12–80 MHz. Ultrasonic technique allowed the determination of the real part of the complex mechanical impedance and of the complex rigidity modulus. The relaxation region was shifted towards higher frequency in comparison to the Maxwell model and was fitted by the modified BEL model.     

NMR study of solid C60(γ-cyclodextrin)2

A solid complex of C₆₀ with γ-cyclodextrin (γ-CyD) was examined with NMR spectroscopic methods in order to understand the dynamics of C₆₀, and the interaction between C₆₀ and γ-CyD. A ¹³C solid-state cross-polarization magic angle spinning (CP/MAS) NMR spectra shows C₆₀ resonance at 142.6 ppm. This provides the evidence of interaction between ¹³C spins in C₆₀ and ¹H spins in the γ-CyD host. Ambient temperature experiments on the ¹³C CP/MAS NMR, with varying contact time, shows that the water associated with γ-CyDs plays an important role in the nuclear relaxation processes. The dynamics of C₆₀ in γ-CyD was investigated using temperature and field-dependent ¹³C spin-lattice relaxation time measurements. The influence of water on the dynamics of C₆₀ was less significant below 250 K.     

Effects of microalloying with Cd and Ag on the precipitation process of Al–4Cu–0.3Mg (wt%) alloy at 200°C

The present work investigates the effects of individual and combined additions of Cd and Ag on precipitation processes in an Al–4Cu–0.3Mg (wt%) alloy. Analytical scanning transmission electron microscopy revealed that microalloying with Cd stimulates nucleation of θ′ phase on {001} planes and that Cd-rich particles form on the rim and broad facets of the θ′ platelets. We interpret these observations to suggest that Cd nucleates heterogeneously at the θ′– interface and that θ′ can also nucleate heterogeneously at the Cd– interface. In the quinary alloy, it was observed that Ag and Cd additions seem to work independently resulting in a fine and uniform dispersion of both Ω and θ′. Furthermore, the hardening effect of the {111} Ω phase appears to be more potent than other precipitates formed in this system since the hardness of the quinary alloy was intermediate between the Al–Cu–Mg–Ag and the Al–Cu–Cd alloys.     

Thermally stimulated acoustic splitting of an elastic wave in polycrystalline niobium subjected to electron-beam remelting

The effect of temperature on the Young’s modulus of electron-beam-remelted polycrystalline niobium is studied in the temperature range 20–1000°C. The impurity content in the material is Ta<0.5 wt% and O₂<0.1 wt%. The acoustic split of the resonant frequency is found in the temperature range 60–180°C, which makes the determination of the Young’s modulus uncertain. Mechanisms behind the thermally stimulated splitting of an elastic wave and the behavior of the Young’s modulus over a wide temperature interval are discussed.     

Effect of different oxidizing gases on the in-situ growth of YBa2Cu3O7−δ films by pulsed laser deposition

The effectiveness of oxygen (O₂), nitrous oxide (N₂O), and nitrogen dioxide (NO₂) as oxidizing agents during in-situ growth of YBa₂Cu₃O_7−δ (YBCO) films on (100) SrTiO₃ substrates by pulsed laser deposition has been studied as a function of deposition temperature (700–800°C), and laser wavelength (193,248 and 355 nm), for a wide range of oxidizer gas pressure (0.1–200 mTorr). In general, the superconducting transition temperature of the films has been found to increase with increasing oxidant pressure, with zero-resistance temperature ≈90 K only obtained in films prepared in a relatively high pressure (150–200 mTorr) of oxidizer gas. At lower pressures, the transition temperature while being depressed is quite sensitive to the nature of the oxidant, the laser wavelength and the deposition temperature. Nevertheless, independent of the oxygen source or other growth parameters, an almost linear decrease in transition temperature with a corresponding increase in the c-axis lattice parameter has been observed for all the film. YBCO films have also been deposited in a low pressure background (≤ 1 mTorr) using a combination of atomic oxygen and pulsed molecular oxygen. The results are discussed in terms of the oxygen requirement for kinetic and thermodynamic stability of YBCO during growth of the film by pulsed laser deposition.     

Conductivity relaxation in zirconium fluoride glasses: effect of substitution of Zr by Y ions

The electrical conductivity and the conductivity relaxation of (55−x)ZrF₄–15BaF₂–xYF₃–30LiF glasses were studied in the temperature range from 300 K to just below the glass transition temperature and in the frequency range from 10 Hz to 2 MHz. No large changes in the conductivity were observed with the substitution of Zr⁴⁺ by the Y³⁺ ions. The activation energy remained almost constant up to 20 mol.% YF₃ content and increased for higher YF₃ content in the glass compositions. The frequency dependent conductivity was analyzed in terms of modulus formalism. The distribution parameter for the conductivity relaxation times remained almost unchanged with the substitution of YF₃ with an increase for 40 mol.% YF₃ content. The distribution of relaxation times of the present glasses was much broader than that for the YF₃-free zirconium fluoride glasses. The glass decoupling index decreased and the modulus relaxation rate increased with the increase of YF₃ content in the glass compositions with an anomaly for the composition having 20 mol.% YF₃ content.