Measurement of the equibiaxial elongational viscosity of polystyrene using lubricated squeezing

The equibiaxial elongational viscosity of polystyrene was determined using a lubricated squeezing technique. Constant strain rates up to Hencky strains of 4.5 could be maintained by a newly constructed instrument. Test results from controlled stress and controlled strain rate measurement were consistent and yielded well-defined steady-state viscosities. Measurements appeared to be unaffected by sample geometry, although proper lubrication is important in achieving steady state. The measured biaxial viscosity appeared to be strain rate thinning above a biaxial strain rate of ≈ 0.01 s⁻¹ at 160°C. As anticipated in the Newtonian region, biaxial elongational viscosity was approximately six times the shear viscosity. Thinning indices of both shear and biaxial elongational viscosities were 0.75. Data obtained at various temperatures were shifted following the timetemperature superposition principle. The resulting master curve could be fitted by a Carreau model with n ≈ 0.3 and a time constant of 110 s.     

Elongational viscosity of polymer melts: A lubricated skin-core flow approach

Previous work by this research group has shown that the use of a lubricated skin/core flow of polymer melts and a hyperbolic converging die results in an essentially pure elongational flow at a constant elongational strain rate in the core. The previous work was carried out on a laboratory-scale coextrusion system in a planar slit die; tracer particles and an image analysis system were used to confirm the predicted behavior. In this work, the technique was implemented first on the coextruder assembly, as a planar elongational rheometer, and then on a commercial capillary rheometer, as a uniaxial elongational rheometer for polymer melts. The later is achieved by replacing the standard capillary die with a hyperbolic axisymmetric die. A two-laycred billet is prepared for placement in the rheometer barrel by completely encapsulating the core polymer (the polymer to be analyzed) with a low-viscosity polyethylene skin. Commercial grades of polypropylenes, syndiotactic polystyrene, and nylon-66 were analyzed using this technique. Elongational viscosity at high extensional rates can be determined with this method; values in excess of 500 s⁻¹ have already been achieved. © 1996 John Wiley & Sons, Inc.     

Shear and extensional flow of a thermotropic liquid crystalline polymer

The flow of a thermotropic liquid crystalline polymer (unfilled and glass fiber filled) was studied using a capillary rheometer and an instrumented injection molding machine. Despite different thermal histories, the techniques gave similar results. From 330 to 350°C, viscosity was independent of temperature. At 340°C, where most measurements were carried out, pronounced shear-thinning occurred and the shear flow curves were nonlinear, the power law exponent decreasing from 0.51 at a shear rate of 10 s⁻¹ to 0.35 at 10⁴ s⁻¹. A previously reported model was used to derive elongational flow curves from die entry pressure data. Because of the nonlinearity of the flow curves, quadratic log-stress vs. log-strain rate plots were needed to model behavior over the strain rate region studied. The elongational flow curves were similar in shape to the shear flow curves, with an effective Trouton ratio of 30. Despite orientation and structure present in the melt, the extensional viscosities and Trouton ratios were within the range found with normal thermoplastic melts. The results suggest that extensional flow may be inhomogeneous, the flowing units possibly being partially ordered domains.     

Eine rheo-optische Apparatur für die Durchführung homogener,einfacher Dehnungen an Polymerschmelzen mit simultaner Aufzeichnung der Deformation,der mechanischen Spannung und der Doppelbrechung

The present paper describes a rheo-optical apparatus consisting of an elongational rheometer (with rotary clamps) and a device to measure birefringence. This equipment was designed to perform homogeneous deformations on polymer melts in the simple elongational mode. Elongational experiments were performed on polyisobutylene samples at room temperature using different strain rates (from \documentclass{article}\pagestyle{empty}\begin{document}$\dot \varepsilon$\end{document} 0.01 s^?1 to 1 s^?1). In typical experiments homogeneous elongations up to λ ≈ 200 and stresses up to 2 · 10⁶ Pa were achieved. The stress, the birefringence, and the deformation were obtained by measuring the force, the sample thickness, the optical retardation (the frequency of the sampling was 20 Hz) and the width of the sample (the frequency of the sampling was 3 Hz). It was found that the stress-optical coefficient was constant. The homogeneity of the deformations was carefully controlled because inhomogeneities cause serious errors in the measurements of elongational viscosity and birefringence.     

Determination of steady-state elongational viscosity for rubber compounds using bell-mouthed dies

A bell-mouthed die geometry was designed to cause convergent flow at a constant, uniform, elongational strain rate. An equation was derived, which showed that steady-state elongational viscosity could be calculated from a plot of pressure drop due to elongation against a simple function of die length. To obtain values of pressure drop due to elongation, it was necessary to correct the total pressure drop measured across the bell-mouthed dies for the contribution from shear occurring near the die wall. For this purpose, a simplified shape for the bell-mouthed dies was assumed, comprising several parallel sided segments. Applying a formula to pressure drop data measured across straight dies corresponding to these segments gave an estimate of the pressure drop due to shear across the bell-mouthed dies. Pressure drops due to elongation were determined by subtracting the pressure drop due to shear from the total pressure drop measured across the bell-mouthed dies. Measurements were also carried out with lubrication to validate the shear correction method. The results indicate that for the compound used in this study, a combination of bell-mouthed and straightsided dies can be used in a conventional capillary rheometer to determine steady-state elongational viscosity. An elongational viscosity of 190 kPa s at 90°C and at a strain rate of 10 s⁻¹ was determined for a simple styrene-butadiene rubber compound. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1139–1150, 1997     

Elongational behavior of short glass fiber reinforced polypropylene melts

The Rheometrics Elongational Rheometer was employed to study the uniaxial extensional flow of glass fiber filled polypropylene melts, in which the fiber concentration, c, varied between zero and 40 weight percent. The constant strain rate mode was used for strain rates, \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^. $\end{document}, between 0.003 and 0.6 s⁻¹. Steady state elongational viscosities were observed in most cases for fiber filled polypropylene melts, even at rates at which the stress continued to increase for unfilled polypropylene. The rate of relative stress growth increased with \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^. $\end{document} and was affected by the addition of fibers. The steady elongational viscosity of the fiber reinforced melts was found to decrease with increasing \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^. $\end{document} and to increase with increasing c. Yield stresses were observed in elongational flow at high concentrations, although there was no clear evidence of yield in steady shear.     

Oxidation of cyclohexanone to adipic acid with a cobalt acetate/oxygen/acetic acid system

The liquid phase oxidation of cyclohexanone to adipic acid at 378 K using oxygen as the oxidising agent and cobalt acetate as the catalyst in an acetic acid medium was investigated both at atmospheric pressure and at a pressure of 0.5MN m⁻². The effects of catalyst concentration, solvent concentration and partial pressure of oxygen, were studied at a constant stirrer speed of 535 rev min⁻¹. Increasing the solvent concentration and decreasing the catalyst concentration (up to 0.113×10⁻³ kmol m⁻³) had positive effects on the overall first order reaction rate constant. It was also found to vary linearly with gas flow rate.     

Inversion of sucrose solution by ion exchange: evaluation of reaction rate and diffusivity

The inversion of sucrose solution was studied in a packed bed of strongly acidic cation exchange resins in order to determine the values of the reaction rate constant, effective diffusivity and distribution coefficients for 20, 30 and 40 wt.% solution concentration. Typical values of specific rate constant for 30 wt.% sucrose solution varied from 0.0115 to 0.7391 min⁻¹ in the temperature range from 30 to 70 °C. The intrinsic values of activation energy and Arrhenius constant were 24.93 kcal mol⁻¹ and 7.657 × 10¹⁵ min⁻¹ respectively. The effective diffusivity varied from 3.826 × 10⁻⁸ to 18.951 × 10⁻⁸ cm² s⁻¹ for the above concentration and temperature intervals. The distribution coefficient was about unity. It was found that liquid film mass transfer resistance was important at low flow rates, although the solution concentration was high.     

Steady-state behaviour of an aerobic mixed culture growing on phenol in a continuous stirred reactor

The kinetic behaviour of an aerobic mixed culture was studied in a well-stirred reactor at various dilution rates with phenol as the sole carbon source and limiting factor. In the range of dilution rates investigated (up to 0.51 h⁻¹), biomass washout from the reactor was not observed. Analysis of steady-state data using the inhibitory kinetic parameters obtained in a previous batch growth study gave a very poor prediction of the reactor's performance. Since microbial growth occurred on the reactor walls, the experimental data were analysed using two steady-state equations (one with Monod, the other with Haldane growth kinetics) together with the Topiwala-Hamer model to describe the wall attachment. The mathematical model using Monod kinetics gave a good fit to the experimental data and the values of the kinetic parameters obtained using this model were as follows: maximum specific growth rate, 0.340 h⁻¹; saturation constant, 1.61 mg l⁻¹; wall growth constant, 2.84 mg l⁻¹. Furthermore, the yield and maintenance coefficients were calculated to be 0.47 mg mg⁻¹ and 0.007 mg mg⁻¹ h⁻¹ respectively.     

Studies on the Non‐isothermal Crystallization Behavior of Aluminum Nanopowder‐Filled Poly(3,3‐bis‐azidomethyl Oxetane)

Non‐isothermal crystallization of aluminum nanopowder‐filled poly(3,3‐bis‐azidomethyl oxetane) (PBAMO) from melt at various constant cooling rates from 1 to 10 °C min⁻¹ was studied. Ozawa's approach was used to analyze non‐isothermal crystallization kinetics. It was found that Ozawa's approach was effective at medium constant cooling rate. The crystallization activation energy derived from Kissinger's equation was 80.8 kJ mol⁻¹. The experimental results showed that the aluminum nanopowder acted as a nucleating agent to raise the peak crystallization temperature and to accelerate the crystallization rate at higher constant cooling rates. Uniform crystals which showed one melting peak could be obtained at a constant cooling rate of 1 °C min⁻¹.     

Electrochemical study of the interaction between glutathione and carbenicillin

The interaction of glutathione with carbenicillin was investigated by ac polarography. From measurements of the peak potential at different antibiotic concentrations, an interaction constant of 3.61 ×10³ M⁻¹ at physiological conditions of temperature and pH was calculated. The potential peak of glutathione was shifted by binding of the antibiotic. Polarographic measurements allowed us to calculate the interaction parameters and to evaluate the thermodynamics of the binding. The interaction constant varied from 1.32 to 8.58 × 10³ M⁻¹ at different pH from 6.0 to 9.0 corresponding with free energy changes from −4.41 to −5.55 kcal mole⁻¹ indicating that the interaction is favoured. The reaction is exothermic with a ΔH°= −24.82 kcal mole⁻¹ at pH 7.4. Theoretical equations involved in order to describe the interaction are strongly consistent with the experimental data.     

Viscoelastic flow in packed beds or porous media

An analysis of viscoelastic flow in packed beds or porous media is presented based on a capillary hybrid model of the flow which incorporates a viscous mode and an elongational mode. The development includes modelling of the elongational mode of the flow to obtain the elongational flow contribution to the potential drop for a viscoelastic fluid. A general expression describing viscoelastic flow in porous media is developed which utilizes the viscous response determined by the fluid model equation and an elongational flow response characterized by an elongational viscosity difference for the fluid. The expression applies to all three traditional bed models employing the tortuosity and Kozeny constant. The relationship yielded extensions of Darcy's law applicable to viscoelastic flow in porous media and an expression representing the flow of a viscoelastic fluid in a packed bed or porous core of length L. The relationship of the friction factors and respective Reynolds numbers is also presented.     

In situ fiber‐reinforced composites from blends containing polypropylene and polycaprolactone

Our study was focused on the presupposition that morphology control in immiscible polymer blend could give rise to reinforcement in composites. To investigate the effects of shear and elongational flow in polymer processing, observation of the mechanical properties and the morphology of the polypropylene/polycaprolactone (PP/PCL) blend system was performed. PP/PCL sheets were fabricated by means of a single‐screw extruder equipped with a slit‐type die to which high shear and elongational stresses were applied. For the sake of comparison, a second series of composites of identical composition was compression molded with a hot‐press machine that transmits lower shear and elongational stresses. The results indicate that the extruded sheets have better mechanical properties than those of the compression‐molded sheets, a result attributed to the generation of in situ dispersed long fiber minor phases and cocontinuous phases in the extruded composites. The differences in the crystallization behavior of the fibrous and spherically shaped components were indicated clearly by DSC curves. A PP crystalline peak indicative of in situ PP fiber formation is conspicuous around 980 cm⁻¹ (PP crystalline band) in the FTIR spectrum. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 833–840, 2004     

The correlation between surface conductivity and adsorbate coverage on diamond as studied by infrared spectroscopy

A high-resolution analysis of CH vibrational modes on a single crystal diamond(100) surface using Fourier-transform infrared (FTIR) spectroscopy in combination with conductivity measurements is reported. On a plasma-hydrogenated diamond(100) surface, the IR spectra measured in the multiple internal reflection mode reveal three absorption lines. Two of them at 2921 and 2854 cm⁻¹ vanish in air at an annealing temperature of 190°C and are assigned to the antisymmetric and symmetric CH₂ stretching modes of a physisorbed hydrocarbon species, respectively. The third band at 2897 cm⁻¹ has a width of 16 cm⁻¹, is stable up to 230°C and is associated with the stretching frequency of C₂H₂ monohydride units on the C(100) 2×1:2H surface. Upon annealing in air at temperatures lower than 200°C, the surface conductivity is reversibly reduced by up to five orders of magnitude. After cooling down to room temperature, it recovers the value of 1×10⁻⁵ Ω⁻¹ measured immediately after the plasma hydrogenation with a time constant of several days. Annealing at 230°C destroys the surface conductivity irreversibly and yields conductance values below the measurement limit of 5×10⁻¹² Ω⁻¹. We show that the chemisorbed hydrogen in the C₂H₂ configuration, together with at least one physisorbed species, is responsible for the surface conductivity of hydrogen-terminated diamond(100).     

Molecular orientation of polydimethylsiloxane induced by elongational and shearing strains

A study examining the molecular orientation of poly(dimethylsiloxane) for different combinations of elongational and shear strains is presented. Three different cases were studied: (1) pure elongational strain; (2) increasing shear and decreasing elongational strains; (3) increasing shear and increasing elongational strains. The experiments were performed in a converging flow cell (at room temperature), where elongational and shearing strain rates achieved values of 370 s^?1 and 640 s^?1 respectively. Values of the Hermans orientation function were obtained from measurements of birefringence and polarization angles while strain rates were estimated from laser Doppler anemometry velocity measurements. Prospects for predicting molecular orientation from the stress-optical laws and rheological flow models are outlined and commented on.     

Intrinsic kinetics of low-temperature oxidation of oil shale kerogen

The kinetics of oxidation of kerogen in the Colorado oil shale were measured using a thermogravimetric analysis technique in a continuously increasing temperature mode. The rate data were analysed based on the assumption that, at the relatively low temperatures at which the kinetics were measured, the oxidation reaction takes place on the surface of solid kerogen and that the decomposition of kerogen is not significant. The oxidation rate was determined to be of first order with respect to oxygen partial pressure. The activation energy of reaction was 11.0 ± 2.3 kJ mol⁻¹, and the pre-exponential factor was (6.8 ± 2.5) × 10⁻⁶ m(kPa · s)⁻¹.     

Analysis of impedance measurements performed on the platinum hexacyanoferrate (II) and (III) system in phosphate media

Impedance measurements of Fe(CN)³⁻₆ and Fe(CN)⁴⁻₆ solutions were taken in a potentiostatic mode in phosphate media at pH 7.2 in contact with a polished platinum electrode. Complementary data required to demonstrate the autocoherence of the results were deduced from thin layer spectroelectrochemistry measurements and from experiments carried out on a rotating disc electrode. The standard heterogeneous transfer rate constant was 0.1 cm s⁻¹ and decreased, in the presence of oxygen, when the contact time between the electrode and the solution increased. In addition, we confirm the accuracy of the impedance method for the characterization of an electrode surface before an experiment.     

Achieving 360 NL h−1 Hydrogen Production Rate Through 30‐Cell Solid Oxide Electrolysis Stack with LSCF–GDC Composite Oxygen Electrode

A 30‐cell solid oxide electrolysis (SOE) stack consisting of 30‐cell planar Ni–YSZ hydrogen electrode‐supported single cell with La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ–Ce_0.9Gd_0.1O_1.95 (LSCF–GDC) composite oxygen electrodes, interconnects, and sealing materials was tested at 750 °C in steam electrolysis mode for hydrogen production. The direction of gas flow in the stack was a cross‐flow configuration, and the stack configuration was designed to open gas flow channels at the air outlet. The electrolysis efficiency of the stack was higher than 100% at 90/10H₂O/H₂ ratio under <0.5 A cm⁻² current density. During hydrogen production, the stack was operated at 750 °C under 0.5 A cm⁻² constant current density for more than 500 h with 4.06% k h⁻¹ degradation rate. Up to 73% steam conversion rate and 91.6% current efficiency were obtained; the net hydrogen production rate reached as high as 361.4 NL h⁻¹. Our results suggested that the SOE stack that was designed with LSCF–GDC composite oxygen electrode could be used to conduct large‐scale hydrogen production.     

Effective promotion of tetrahydrofuran polymerization initiated with heteropolyacid

Tetrahydrofuran was polymerized using the heteropolyacid H₃PW₁₂O₄₀ as the initiator and ethylene oxide as the promoter, which effectively increased the rate and conversion of the polymerization. Water and butylene glycol were used to control the molecular weight of the product in the range of 1000–3000. The polymer was found to be polyether glycol containing 10–22 mol % oxyethylene moieties with hydroxyl groups at both chain ends. The melting point was ∼ 10°C lower compared to polytetramethylene ether glycol having the same molecular weight. The concentration of active species remained unchanged in the main period of the polymerization, indicating the absence of chain termination. The values of the chain propagation rate constant of tetrahydrofuran polymerization at 0 and 20°C were found to be 3.78 × 10⁻³ and 1.98 × 10⁻² L mol⁻¹ s⁻¹, respectively, which are close to the rate constant of chain propagation of tetrahydrofuran on ionic active species. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2303–2308, 1999     

Dynamic tensile mechanical properties and failure mode of zirconium diboride-silicon carbide ceramic composites

Dynamic indirect tension experiments were performed on zirconium diboride-silicon carbide (ZrB₂−20%SiC) ceramic. Flattened Brazilian disc specimens of ZrB₂−20%SiC were prepared to conduct dynamic tensile tests using the modified Split Hopkinson pressure bar system. The tensile experiments were completed at the range of loading rates from 7.53 to 74.71 GP s⁻¹. The tensile experimental results revealed that the zirconium diboride-silicon carbide ceramic composite is rate-sensitive in terms of the tensile strength and failure mode. The dynamic tensile strength increases linearly with the loading rate and changes from 195 MPa at 7.53 GP s⁻¹ to 654 MPa at 74.71 GP s⁻¹. Moreover, the dynamic tensile strength decreases with the increase in critical fracture time, which conforms to Tuler and Butcher's fracture criterion. In dynamic experiments, a high-speed camera was used to examine the tensile failure process, and fragments were collected to analyze the dynamic tensile failure mechanism. The tensile fracture mode of ZrB₂−20%SiC obviously showed the sensitivity of the loading rate. The fragment size of ZrB₂−20%SiC ceramic decreased but the quantity of fragments increased as the loading rate increased.