Quasi-linear rheological behaviour of polymer melts: Comparison between mechanical and improved flow birefringence measurements

Summary The latest improvement of the rotor unit (cone- and plate-type) for the measurement of the flow-birefringence of polymer melts is described. As a consequence of this improvement, the influence of parasitic birefringences, as produced in the corners near the windows, is minimized. It is shown that the limitation of the range of available rates of shear, as experienced with previous versions of the apparatus, was due to those parasitic birefringences which predominated at rates of shear where the main birefringence caused an optical path difference of one, two or more whole wavelengths (fringes) and was, as a consequence, virtually zero. This explains why great troubles were experienced in particular with polymers possessing a high back-bone anisotropy. The validity of the linear stress-optical rule up to shear rates, where melt fracture occurs in capillary flow, is well understood in terms of the wriggling motion of chain molecules. Comparison with direct mechanical measurements was very satisfactory.

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Zusammenfassung Eine Beschreibung der jüngsten Verbesserung an der Rotorzelle (Kegel-Platte-Typ) für die Messung der Strömungsdoppelbrechung polymerer Schmelzen wird gegeben. Als Folge dieser Verbesserung wird der Einfluß der parasitären Doppelbrechung, die in den Ecken neben den Fenstern auftritt, minimalisiert. Es wird gezeigt, daß die Begrenzung des Bereiches der verfügbaren Schergeschwindigkeiten, wie sie in früheren Versionen des Apparates gefunden wurde, auf das Vorherrschen der parasitären Doppelbrechungen bei Schergeschwindigkeiten zurückzuführen ist, bei denen die Hauptdoppelbrechung optische Weglängen von einer, zwei oder mehreren ganzen Wellenlängen verursacht und daher effektiv null wird. Dies erklärt, warum große Schwierigkeiten gerade bei solchen Polymeren auftraten, die eine stärkere Kettenanisotropie aufweisen. Die Gültigkeit der linearen spannungsoptischen Regel bis zu Schergeschwindigkeiten, bei denen in der Kapillaren Schmelzbruch auftritt, kann man aufgrund der Ringelbewegung der Makromoleküle gut verstehen. Ein Vergleich mit direkten mechanischen Messungen ergab sehr befriedigende Ergebnisse.

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With 8 figures and 2 tables     

A soft molecular 2Fe–2As precursor approach to the synthesis of nanostructured FeAs for efficient electrocatalytic water oxidation

An unprecedented molecular 2Fe–2As precursor complex was synthesized and transformed under soft reaction conditions to produce an active and long-term stable nanocrystalline FeAs material for electrocatalytic water oxidation in alkaline media. The 2Fe2As-centred β-diketiminato complex, having an unusual planar Fe₂As₂ core structure, results from the salt-metathesis reaction of the corresponding β-diketiminato Fe^IICl complex and the AsCO⁻ (arsaethynolate) anion as the monoanionic As⁻ source. The as-prepared FeAs phase produced from the precursor has been electrophoretically deposited on conductive electrode substrates and shown to act as a electro(pre)catalyst for the oxygen evolution reaction (OER). The deposited FeAs undergoes corrosion under the severe anodic alkaline conditions which causes extensive dissolution of As into the electrolyte forming finally an active two-line ferrihydrite phase (Fe₂O₃(H₂O)_x). Importantly, the dissolved As in the electrolyte can be fully recaptured (electro-deposited) at the counter electrode making the complete process eco-conscious. The results represent a new and facile entry to unexplored nanostructured transition-metal arsenides and their utilization for high-performance OER electrocatalysis, which are also known to be magnificent high-temperature superconductors.

A molecularly derived FeAs has been used as an electro(pre)catalyst for an efficient alkaline OER for the first time and subsequently, its active structure has been determined by quasi in situ X-ray absorption spectroscopy and ex situ methods.     

Wall material and roughness effects on transmittable shear stresses of magnetorheological fluids in plate–plate magnetorheometry

A direct comparison of plate–plate magnetorheometry results for nonmagnetic (titanium/brass) and ferromagnetic plates is presented, using a modified Anton Paar magnetocell MRD180/1T. Necessary corrections to derive the true flux density in the magnetorheological fluid (MRF) from the online Hall probe reading and to account for the gap opening effect caused by normal forces on shear stress and flux density are addressed. The measured shear stress versus magnetic flux density characteristics agree in the low flux density regime <0.1 T but yield distinctly higher transmittable shear stresses for ferromagnetic plates at elevated flux densities (49% increase at 1 T for 90% by weight carbonyl iron powder (CIP) and 84% for 85% by weight CIP). Remarkably, the normal force, if corrected for its magnetostatic part, remains independent of the type of plates up to about 0.6 T. We address the role of normal forces, of magnetic interactions between CIP and wall, as well as the role of wall roughness in a solid body friction model. A systematic variation of wall properties and materials was achieved by introducing both a modular rotor and stator, which ease the variation of the walls in contact to the MRF. The transmittable shear stress of nonmagnetic plates (e.g., brass) may be increased up to the level of ferromagnetic disks by a higher wall roughness or by grooves. No shear stress increase is obtained for grooves in ferromagnetic plates, which is explained by the different local flux density modulation at the grooves for ferromagnetic compared to nonmagnetic plates. Finally, we address the effect of ferromagnetic and nonmagnetic coatings on brass and steel disks, and show that, e.g., a layer of CIP on brass efficiently increases the transmittable shear stress.     

Interfacial tension in a lower critical solution temperature blend: Effect of temperature,blend composition,and deformation of the interphase

Interfacial tension is a very important material parameter in two‐phase polymer blends. It determines the morphology development during processing, which is crucial for the end‐use properties of the material. Although different techniques for interfacial tension measurement give comparable results for immiscible polymers, the determination of the interfacial tension in lower critical solution temperature blends is not straightforward. This is illustrated for poly(α‐methyl styrene acrylonitrile)/poly(methyl methacrylate)(PαMSAN/PMMA), a slightly incompatible polymer pair. Interfacial tension has been measured with three different techniques: small‐amplitude oscillatory shear, recovery after elongation, and elongation of a multilayer sample. The large differences in these results can be attributed to the fact that most experimental techniques determine an apparent value, rather than the thermodynamic equilibrium value, of the interfacial tension. The latter is only obtained if the measurement is performed under quiescent conditions on a system that is composed of the coexisting PαMSAN‐rich and PMMA‐rich phases. The apparent interfacial tension depends on the actual composition of the phases and on the deformation of the interface. An order of magnitude approximation for such effects has been derived from theoretical considerations. Finally, each of these apparent values can be of practical importance. If a blend is prepared by melt mixing of the pure polymers, a high apparent value of interfacial tension should be considered. If, however, a blend is prepared by phase separation of a homogeneous mixture, the thermodynamic value is important. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 679–690, 2002     

Measurement modes of the response time of a magneto-rheological fluid (MRF) for changing magnetic flux density

The response of a magneto-rheological fluid (MRF) to a change of magnetic flux density is investigated by using a commercial plate–plate magneto-rheometer MCR501 (Anton Paar GmbH) at constant shear rate. The instrument was modified to allow an online determination of the transient flux density in the MRF. Both current and voltage imposition to the magneto-cell were applied by using a power operational amplifier to drive the electromagnet. Assuming a Maxwell behavior with switching time λ and a linear increase in shear stress with flux density, analytic relations for the transient shear stress are derived for sinusoidal and single exponential flux densities vs time. True switching times of a few milliseconds are only obtained if the low pass filter in the original MCR501 torque signal is surpassed by a firmware allowing a sampling rate of 0.1 ms. For a sinusoidal flux density, the switching time is derived from the modulation depth of the shear stress. An upper bound of λ < 3 ms for a flux density of 0.8 T was found. For step coil current imposition of 1 T magnitude, switching times of 2.8 ms (start-up) and 1.8 ms (shutdown) allowed to fit the transient torque signal more than 2/3 of the total change. Finally, the effect of a sigmoidal characteristic on the switching time determination is addressed. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27-29, 2006.     

Polymer melt rheology with a slit die

A piston driven high pressure slit die rheometer with three pressure holes along the die and one in the barrel was used to investigate viscosity, entrance and exit pressure losses, and pressure coefficient of viscosity of a LDPE melt. Hydrostatic calibration of melt pressure transducers can be performed in the rheometer. The slit die results are compared with measurements on circular dies assuming linear and parabolic pressure profiles in both cases. A simplified conversion from apparent to true viscosity, applicable for single point measurements, has been used. In spite of the different symmetries the Bagley correction from the linear pressure profile of circular dies was found to be equal to the sum of exit and entrance pressure losses in the slit. The magnitude and sign of the exit pressure loss depend on the type of pressure profile used. The influence of a pressure dependent viscosity and a temperature gradient along the die on the curvature of the pressure profile is discussed. To directly investigate the effect of pressure the pressure level at constant flow rate was increased stepwise by means of a valve attached to the exit of the slit die.     

Manifestation of phase separation processes in oscillatory shear: droplet-matrix systems versus co-continuous morphologies

Phase separation processes in mixtures of poly-α-methylstyrene-co-acrylonitrile (PαMSAN) and poly-methylmethacrylate (PMMA) with lower critical solution temperature (LCST) behavior have been studied, focusing on the manifestation of the interface in oscillatory shear measurements. By using blends of different composition, systems with a droplet-matrix morphology or a co-continuous structure are generated during the phase separation process. The feasibility of probing this morphology development by rheological measurements has been investigated. The development of a disperse droplet phase leads to an increase in the elasticity of the blend at low frequency, showing up as a shoulder in the plot of storage modulus versus frequency. Here, the droplet growth is unaffected by the shear amplitude up to strains of 0.2; therefore the resulting dynamic data are suitable for quantitative analysis. In contrast, for blends in which phase separation leads to a co-continuous structure, the storage modulus shows a power law behavior at low frequency and its value decreases as time proceeds. For the latter systems, effects of the dynamic measurement on the morphology development have been observed, even for strain amplitudes as low as 0.01. To probe the kinetics of morphology evolution in droplet-matrix systems, measurements of the time dependence of the dynamic moduli at fixed frequency should be performed (for a whole series of frequencies). Only from such measurements, curves of the frequency dependence of the moduli at a well defined residence time can be constructed. From fitting these curves to the emulsion model of Palierne, the droplet diameter distribution at that particular stage in the phase separation and growth process can be obtained. It is not appropriate to use a simplified version of the Palierne model containing only the average droplet size, because a morphology with too broad a size distribution is generated. Received: 15 February 1999 Accepted: 20 May 1999     

Reliable plate–plate MRF magnetorheometry based on validated radial magnetic flux density profile simulations

The apparent shear stress from plate–plate magnetorheometry, using the commercial magnetocell MRD180/1T (Physica/Anton Paar) in standard configuration, is distinctly overestimated. The effect is due to a flux density maximum near the sample rim and radial migration of iron particles towards the rim. Radial magnetic flux density profiles were investigated by finite element simulations using the Maxwell^®2D code and by direct Hall probe measurements. The reliability of the finite element method results, both for the empty magnetocell and with magnetorheological fluid (MRF) in the measuring gap, allows conclusions on the true flux density within the MRF, which cannot be accessed by Hall probe measurements. If the MRF sits on top of the bottom yoke (standard configuration), the flux density maximum reaches twice the plateau value (0.74 T for 3 A coil current and 0.3 mm gap height of the investigated MRF). This yields a higher effective flux density and causes radial iron particle migration, resulting in an additional magnetic flux increase near the rim due to augmented MRF magnetisation. As a result, the rotor torque at constant rotary speed increases with time. Reliable results are achieved by a modification of the magnetocell, such that the MRF sits on a non-magnetic Hall disc of 1.5 mm thickness, allowing an online flux density measurement by a FW Bell Hall probe. In this configuration, the radial flux density profile near the rim remains sufficiently flat and no iron particle migration is detected.     

Measurement of pressure coefficient of melt viscosity: drag flow versus capillary flow

The pressure coefficient of viscosity of poly(α-methylstyrene-co-acrylonitrile) was measured using a high-pressure sliding plate rheometer (HPSPR) and two types of capillary rheometer: a piston-driven device with a throttle at the exit [piston capillary rheometer with throttle (PCRWT)] operated at a fixed flow rate, and a counter-pressure nitrogen capillary rheometer (CPNCR) operated at a fixed pressure drop. In the HPSPR, the pressure, shear rate, density, and viscosity are all uniform throughout the sample, while the analysis of capillary data is complicated by the axial pressure gradient and the radial shear rate gradient. The polymer was found to be piezorheologically simple, and the HPSPR data indicated that the pressure coefficient of viscosity β ≡ dln(a _P)/dP decreased slightly with increasing pressure at high pressure. While β from PCRWT data from different laboratories and instruments agreed fairly well, the β values were on average about 2/3 of that from the HPSPR. The CPNCR yields β about 18% lower than that of the HPSPR.