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The nonlinear rheology of three selected commercial low-density polyethylenes (LDPE) is measured in uniaxial extensional flow. The measurements are performed using three different devices including an extensional viscosity fixture (EVF), a homemade filament stretching rheometer (DTU-FSR) and a commercial filament stretching rheometer (VADER-1000). We show that the measurements from the EVF are limited by a maximum Hencky strain of 4, while the two filament stretching rheometers are able to probe the nonlinear behavior at larger Hencky strain values where the steady state is reached. With the capability of the filament stretching rheometers, we show that LDPEs with quite different linear viscoelastic properties can have very similar steady extensional viscosity. This points to the potential for independently controlling shear and extensional rheology in certain rate ranges.

22 páginas, 18 figuras, 5 tablas, 17 esquemas.-- et al., As part of a continuing scientific challenge, a substantial effort during the past few decades has been devoted towards altering the selectivity of chemical transformations by arranging the potential reactants in a number of organized and confining media. Such systems, having features significantly different from those of isotropic solutions, include, for example, micelles, microemulsions, molecular aggregates, liquid crystals, and zeolites. Among these materials, stimuli-response gels constitute another important class of nanostructured and dynamic systems with high active surface areas and remarkable diffusion properties. Within this group, polymer gels have been traditionally used to obtain catalytic and reactive soft materials. Moreover, gels made of low-molecular-weight compounds represent a major novelty in this area as potential soft-vessels to carry out chemical reactions with control on product selectivity. In addition, the possibility of integrating switchable catalytic functions in both organo- and hydrogels shall accelerate the development of robust platforms for the ‘bottom–up’ tailor-fabrication of more sophisticated functional materials. The present critical review reports on the most important results published during the last decade regarding the use of ‘smart’ gels that has displayed promising properties as selective soft-nanoreactors and/or heterogeneous recyclable catalysts (152 references)., D.D.D. thanks Universität Regensburg and the Alexander von Humboldt Foundation for financial support.

The influence of flow rate and die geometry on the observable flow rate/pressure relationship of a linear high density polyethylene is investigated using a capillary rheometer. The experimental results are applied to an adapted version of the relaxation–oscillation model proposed by Molenaar and Koopmans for describing the oscillating flow regime. The current model allows for a quantitative description of the hysteresis cycle in the oscillating flow regime in terms of the main experimental variables, such as imposed flow rate, reservoir (barrel) volume, and material compressibility.

Two high density polyethylene (HDPE) resins–samples 801 and 802–both nominally the same material, as they are taken from successive batches of the same commercial grade, are characterized for their molecular structure and rheological properties. Gel permeation chromatography (GPC) and low angle laser light scattering (LALLS) results must be interpreted in combination with rheological data to show the presence of somewhat more high molecular weight material in 802 that in 801. Small amplitude oscillatory shear, steady shear, and capillary shear measurements performed in different laboratories show consistently higher shear viscosity values at low shear rates for sample 802. Extensional viscosity measurements show similar results. The interpretation of rheological data in terms of molecular structure could be complicated by the possible presence of long chain branching (LCB). The zero shear viscosity and discrete relaxation spectrum is estimated for both samples. The small rheological difference between 801 and 802 forms the basic information for understanding their time dependent extrudate swell behavior, as will be described in Part II.

Two high density polyethylene resins—801 and 802— are examined with regard to their isothermal, time-dependent, and nonisothermal swelling properties when emerging from two annular and three diverging dies. The short time swelling characteristics of samples 801 and 802 are very important for these dies, resulting in a lower diameter swell for the latter, independent of the die geometry or flow rate. Output variations have much less impact on the swelling behavior than small changes in the geometry of the die mandrel. Accordingly, shear stress and shear rate parameters alone cannot be used to explain the swelling properties of a HDPE resin in the different die geometries. Straight annular dies induce higher diameter swelling than diverging dies.

For two high density polyethylene resins, the isothermal time dependency of extrudate swell has been measured. Very minor differences in the large molecular weight part of the molecular weight distribution, hardly detectable with gel permeation chromatography and low angle laser light scattering techniques, dramatically influence the time dependency of extrudate swell as well as the maximum swell attainable. The presence of larger molecules in sample 802 than in 801 is reflected in a lower short time (after seconds) and a larger long time (after minutes) or maximum extrudate swell value. Extruding the polymers through a capillary die L:D = 30:2 mm into air at ambient temperature allows only the short time swelling behavior to be observed, because cooling and sagging of the strand.

The low bioavailability of albendazole affects the therapeutic response in patients with echinococcosis. Cimetidine co-administration is reported to improve bioavailability. To analyze the assumed dose-dependent bioavailability of albendazole, we administered 5 to 30 mg/kg albendazole to 6 male volunteers in a randomized cross-over study. To assess the effect of cimetidine (10 mg/kg twice daily), the drug was given with albendazole (20 mg/kg). A dose-dependent bioavailability was not observed. This was due to inter-individual variability of the maximal concentration (Cmax 38%-72%) of albendazole sulphoxide (ABZSX), the active metabolite of albendazole. Cmax was 0.21+/-0.14 mg/L after 5 mg/kg and 0.39+/-0.19 mg/L after 30 mg/kg albendazole (P = 0.217). Cimetidine tended to decrease Cmax by 52% (P = 0.109) and significantly inhibited ABZSX breakdown as indicated by the prolongation of ABZSX elimination half-life from 7.4+/-3.3 hr to 19.0+/-11.7 hr (P = 0.028). Remarkably, the inter-individual variability of Cmax was significantly lower during cimetidine co-administration: 14% versus 72%.

Die swell is a complex Theological phenomenon [1]. It can be observed as an extrudate with a cross-section (Dex) which is greater than the die cross-section (Do). This effect, also known as extrudate swell, Barus effect, or % memory, is defined as the ratio D ex /D 0 = B and is a feature of polymer melt flow. Die swell is associated with the viscoelastic nature of polymer melts as it exceeds the swelling of constant viscosity (Newtonian) fluids. Accordingly, for laminar flow situations, the swelling due to velocity profile rearrangements or mass balance considerations accounts for only 10–20% and cannot explain the 50–300% increase in extrudate cross-section of the polymer emerging out of a die.

Molten polymers, when forced or extruded through dies at high rates, will show extrudate distortions. Such phenomena have been described since the advent of thermoplastic polymeric materials. Over the years, a very large number of publications has been devoted to this subject in efforts to define the nature and origin of these extrudate distortions, commonly referred to as ‘melt fracture’ [1]. Many observations have been made regarding the varying degrees in which extrudate distortions can manifest themselves [2]. Visual observations range from smooth-glossy extrudates (no distortions), over extrudates showing degrees of surface irregularities defined as matt, loss of gloss, ‘orange peel’, ‘’shark skin’ , wavy, screw thread, to extrudates which are volumetrically distorted as a consequence of their oscillating emergence from the die (often referred to as’ spurt’) or their very irregular,‘chaotic’ shape. For most polymers, such a succession of extrudate distortions can be observed with increasing flow rates. Certainly, the common terminology, melt fracture, has a too narrow connotation to capture all observed phenomena as described above. It only suggests that the melt emerging from the die fractures. Therefore, the term ‘extrudate distortion’ seems more appropriate to refer to all these phenomena. The long-standing academic and industrial interest in this subject may be associated with several driving forces. One, which poses a continuing intellectual challenge, is related to the elusiveness of finding the origin of extrudate distortions in relation to the molecular composition of the polymer. The complexity of the issue is related to the many melt processing variables (e.g. temperature, die geometry and alloy) and polymer-type variables (e.g. molecular weight, molecular weight distribution and branching) which need to be considered. The effect of each variable on extrudate distortion is difficult to determine experimentally. Often contradictory findings are reported. Another driving force is the evolution to faster melt processing equipment and a shift to higher flow rates. Accordingly, the practical barrier to extrudate distortion, which is considered to limit industrial melt processing operations, needs to be shifted. Existing solutions to alleviate or reduce extrudate distortion such as the use of extra additives or changing die geometry all tend to have a direct (negative) impact on the economics of the industrial practice of melt processing.

The long-standing problem of polymer melt-flow instabilities is looked upon from a new and quite general mathematical viewpoint. A single mathematical model is developed allowing the macroscopic description of measurable state variables characterizing the different melt-flow instability regimes. Based on the theory of relaxation oscillations, a set of two coupled first-order ordinary differential equations, relating state variables—pressure and flow rate—provides sufficient degrees of freedom to describe all the relevant flow regimes in shear flow. By means of this model, the appearance of extrudate distortions can be expressed in terms of pressure and output fluctuations without the need to invoke a microscopic theory. The analysis contains the potential for practical melt-flow instability predictions. ¿

Bioinspired molecular self-assembly is a popular route to novel functional materials for industrial applications. Here we explore, for the first time, the possibility of using organogel and hydrogels of short self-assembling beta-sheet-forming peptides, as starting states for the creation of nanostructured peptide aerogels. The effects of supercritical fluid drying (SCF) and freeze-drying (FD) on the nanofibrillar peptide gel network were investigated. SCF processing was found to cause collapse of peptide organogel networks, presumably because of peptide insolubility in carbon dioxide (CO 2 ). Freeze-drying of peptide hydrogels proved a more efficient method of removing the solvent without destroying the self-assembled fibrillar network, leading to a microscopic aligned lamellar structure consisting of thousands of stacked peptide nanofibrils. These chiral, nanostructured, low-density aerogels are characterised by chemical versatility and regular display of functional groups on their surface. Appropriately designed peptides can also be triggered to self-assemble in situ inside other porous structures and impart biological-like functionality for catalysis, sensing, separation and filtration applications, or tissue engineering.

The various methods for the determination of the vinylacetate (VA) content in any ethylene vinylacetate copolymer are examined critically. Each available method is described briefly, together with its relative accuracy. All methods give comparable results and are equally valid, but IR spectrometry can be considered the most flexible and accurate VA content measuring technique.

The comprehensive patent list concerned with the acetoxyhydroxide transformation of ethylene vinyl acetate Copolymers is examined. The possible reaction mechanisms and processing techniques for this kind of transformation are described. Finally, a survey of about 55 of the most interesting patents is given, together with some general features of the acetoxyhydroxide exchange.

The physical properties of high-pressure ethylene vinylacetate (EVA) copolymers having an average melt index of 8.5 g/10 min and a mol% of VA less than 20 are studied. A comparison is made with the properties of their saponified derivatives, the ethylene vinylalcohol (EVAl) co- and ethylene vinylalcohol vinylacetate (EVAlVA) terpolymers. A melt-index effect is noted. Density, thermal, and mechanical properties of EVA copolymers are determined by the degree of crystallinity, which depends on the mol% VA and on the degree of alkyl shortchain branches. EVAlVA terpolymer properties depend on the residual mol% VA. EVAl copolymers proved to have some properties similar to low-density polyethylene. The hydrogen-bonding effect via hydroxyl groups was negligible for this level of vinylalcohol incorporation.