Search

A simplified theoretical model of the melting process, based on T admor' s analysis of the Maddock mechanism, is used to investigate the possibilities to increase the melting capacity of an extruder. Some screw constructions are discussed. The analysis leads to the conclusion that the best way to improved performance is to increase the heat flux to the melting front, which in turn is best achieved by increasing the dissipative heat generation in the melt film. This may be realized in a new type of melting element, the melttorpedo, of which the working is explained. Preliminary tests indicate that this element provides good possibilities to improve efficiency of single screw extruders.

High‐strength/high‐modulus structures such as fibres, tapes and rods can be produced currently on the basis of intrinsically flexible macromolecules. The prime example amongst these new developments is gelspinning of high‐molecular‐weight polyethylene resulting in fibrous structures possessing tenacities of 3–4 GPa and corresponding moduli up to appr. 150 GPa. The basic aspects of chain‐extension for polyethylene will be discussed in relation to the various routes toward oriented/extended PE structures as well as recent developments concerning the utilization of other flexible polymeric systems for the production of high‐strength/high‐modulus fibres.

The hydrogen storage in purified single-wall carbon nanotubes (SWNTs), graphite and diamond powder was investigated at room temperature and ambient pressure. The samples were sonicated in 5 M HNO3 for various periods of time using an ultrasonic probe of the alloy Ti-6Al-4V. The goal of this treatment was to open the carbon nanotubes. The maximum value of overall hydrogen storage was found to be 1.5 wt %, as determined by thermal desorption spectroscopy. The storage capacity increases with sonication time. The sonication treatment introduces particles of the Ti alloy into the samples, as shown by X-ray diffraction, transmission electron microscopy, and chemical analysis. All of the hydrogen uptake can be explained by the assumption that the hydrogen is only stored in the Ti-alloy particles. The presence of Ti-alloy particles does not allow the determination of whether a small amount of hydrogen possibly is stored in the SWNTs themselves, and the fraction of nanotubes opened by the sonication treatment is unknown.

No abstract.

We investigated chaotic advection induced by a magnetic chain in a two-dimensional circular cavity under the influence of a rotating magnetic field. Our focus is on the dynamics of the chain and the route to induce chaotic mixing. A direct simulation method based on the Maxwell stress tensor and a fictitious domain method is employed to solve magnetic and flow problems in a coupled manner. The motion of the chain is significantly affected by the Mason number, the ratio of viscous force to magnetic force. At a lower Mason number, the chain rotates like a rigid body following the field. Alternating break-up and reformation of the chain are observed within a limited range of the Mason number, which induce two typical flows: a single rotating flow and two co-rotating flows. From a series of deformation patterns of a blob, we found that the two alternating flows result in enhanced mixing, showing an exponential increase of the interfacial length.

Polypropylene/CaC03 composites were homogenized in a twin-screw compounder and then injection molded into tensile bars. Six different fillers were used in a wide range of average particle sizes between 0.08 and 12 pm. Tensile and flexural properties were measured by standard techniques, while impact resistance was determined by instrumented impact testing. Structure was characterized by light and electron microscopy, while failure initiation and propagation was studied with in situ high-voltage electron microscopy. The results showed that aggregation of particulate fillers occurs when their particle size is smaller than a critical value. This critical size depends on component properties and processing conditions. Strength and impact resistance usually decrease with increasing number of aggregates. The presence of aggregation can be detected by the use of a simple semiempirical model. Comparison of samples prepared by two different technologies showed that twin-screw extrusion and injection molding leads to relatively homogeneous composites, which was indicated by smaller deviations of the properties from theoretical predictions. In spite of the accept-able dispersion, impact resistance showed a large standard deviation, probably determined by the local variation of structure. In composites containing relatively large particles, the dominating micromechanical deformation process is debonding, while in the presence of extensive aggregation of small particles, cracks are initiated inside and propagate through aggregates. Mixed-mode failure may also occur at certain intermediate particle sizes.

Magnetic artificial cilia are bio-inspired micro-hairs covering a surface that can be actuated using a magnetic field to pump or mix fluids in microfluidic devices. This paper presents a novel fabrication method to realize magnetic artificial cilia using a roll-printing process. When integrated in a closedloop channel, the artificial cilia were shown to be capable of generating substantial microfluidic pumping using external magnetic actuation. The development of this cleanroom-free, high speed and potentially large area method of production of artificial cilia is another step towards their implementation in real-life applications.

RF MEMS capacitive switches and tunable capacitors have been realized in an industrialized thin-film process developed for manufacturing high-quality inductors and capacitors. Combining integrated passives with high-performance tuning and switching elements on the same die offers a potential for building a new generation of RF front-ends for hand-held mobile communication. Capacitive switches with an insertion loss of 0.4 dB and an isolation of 17 dB at 1 GHz have been demonstrated. Dual-gap relay type tunable capacitors have been fabricated that show a continuous and reversible tuning ratio of 12 together with a quality factor larger than 150 at frequencies higher than 0.5 GHz. These are the highest tuning ratio and quality factor reported to date. A 0-level packaging concept that is compatible with the fabrication technology has been adopted.

In this study, an in-house numerical model is employed to predict the time-to-failure of polycarbonate under long-term constant or cyclic loading. This elasto-viscoplastic model captures the intrinsic deformation behavior of the glassy polymer. The model parameters are determined from uni-axial compression testing. In the case of long-term loading, however, special attention should be given to the intrinsic deformation behavior, as this might change within the time-scale of the experiment. Therefore, an elaborate investigation is performed to implement so-called aging kinetics in this model. It is shown that the application of approach leads to accurate predictions of time-to-failure under both constant loading and cyclic loading conditions. Moreover, the current model is based on a single parameter set and is molecular weight independent.

We investigate the structural, dynamical, and rheological properties of colloid-polymer mixtures in a volume fraction range of φ=0.15-0.35. Our systems are density-matched, residual charges are screened, and the polymer-colloid size ratio is ∼0.37. For these systems, the transition to kinetically arrested states, including disconnected clusters and gels, coincides with the fluid-fluid phase separation boundary. Structural investigations reveal that the characteristic length, L, of the networks is a strong function of the quench depth: for shallow quenches, L is significantly larger than that obtained for deep quenches. By contrast, L is for a given quench depth almost independent of φ; this indicates that the strand thickness increases with φ. The strand thickness determines the linear rheology: the final relaxation time exhibits a strong dependence on φ, whereas the high frequency modulus does not. We present a simple model based on estimates of the strand breaking time and shear modulus that semiquantitatively describes the observed behavior. © 2010 The Society of Rheology.

We proposed an integrated and robust microfluidic platform to perform cell-based schedule dependent drug combination screening. Multiple manipulations of cells in droplets, including cell dispensing, culture, drug stimulation and viability test were performed based on a 2D microfluidic droplet array. The screening of flavopiridol (Fla) and 5-fluorouracil (5Flu) in different sequence was carried out in 342 addressable droplets, with a consumption of ∼3 μg drug for each experiment.

In this study poly(2,6-dimethyl-1,4-phenylene ether) (PPE) is introduced as a ductile thermoplastic matrix for high-performance carbon fibre-reinforced composites by using epoxy resin as a reactive solvent. An interesting feature of this reactive composite processing route is that the epoxy resin acts not only as an effective solvent, lowering the viscosity and processing temperature of the polymer matrix, but also provides an essential structural part of the final composite material. Upon curing and subsequent phase separation, a morphology of epoxy-coated fibres in a nearly pure PPE matrix results. This phenomenon is not only of great importance with respect to the possibility to drastically improve the adhesion between the fibres and the thermoplastic matrix, but also creates the challenge to tailor the mechanical properties of these in situ formed interlayers by changing the chemistry of the epoxy phase.

Chaotic mixing, induced by breakup and reformation of a magnetic chain under the influence of a rotating magnetic field, is studied. A direct simulation method combining the Maxwell stress tensor and a fictitious domain method is employed to solve flows with suspended magnetic particles. The motion of the chain is significantly dependent on the Mason number (Ma), the ratio of viscous force to magnetic force. The degree of chaos is characterized by the maximum Lyapunov exponents. We also track the interface of two fluids in time and calculate the rate of stretching as it is affected by the Mason number. The progress of mixing is visualized via a tracer particle-tracking method and is characterized by the discrete intensity of segregation. Within a limited range of Mason number, a magnetic chain rotates and breaks into smaller chains, and the detached chains connect again to form a single chain. The repeating topological changes of the chain lead to the most efficient way of chaotic mixing by stretching at chain breakup and folding due to rotational flows.

The effect of viscoelasticity on multilayer polymer extrusion is discussed. In these coextrusion processes predeterminedpatterns are created with a remarkable breadth of complexity even in geometrically simple dies viaelastic rearrangements caused by the second-normal stress differences. A computational method is offered,based on the mapping method, which quantitatively describes the flow-induced patterns. Besides that theresults are esthetically beautiful, they are also relevant for practice, since process and die design optimizationis now possible. Not only to minimize interface distortion, but potentially also to deliberately create new processesand products based on this flow-induced patterning of polymers.

A method to locate periodic structures in general three-dimensional Stokes flows with time-periodic boundary conditions is presented and applied to mixing cavity flows. Numerically obtained velocity fields and particle tracking schemes are used to provide displacement and stretching fields. From these the location and identification of periodic points can be derived. The presence or absence of these periodic points allows a judgement on the quality of the mixing process. The technique is general and efficient, and applicable to mixing flows for which no analytical velocity field is available (the case for all three-dimensional flows considered in this paper). Results are presented for three different mixing protocols in a three-dimensional time-periodic cavity flow, serving as an accessible test case for the methods developed. A major result is that periodic lines are obtained for these three-dimensional flows. These lines can be complex in geometry and their nature can change along a line from hyperbolic to elliptic. They can serve as practical criteria in the optimization of three-dimensional mixing processes.

In situ small- and wide-angle X-ray scattering (SAXS and WAXS) experiments combined with Raman spectroscopy and thermal analysis have been performed to study the phase behavior of syndiotactic polystyrene (sPS) solutions. Three different solvents were used, respectively, decalin, benzyl methacrylate, and cyclohexyl methacrylate. When sPS solutions are quenched, gel formation occurs. In these gels sPS adopts a helical conformation that is stabilized by the solvent molecules; in fact, compound formation occurs. From the combined experimental data it was concluded that two different structural modifications exist within the solvent-included helical -phase, respectively the -phase, in which the solvent molecules are intercalated and ordered between the phenyl rings of sPS, and a -phase, where the solvent ordering is lost. The so-called -phase, the solvent-free helical phase, was not observed in our studies. The transformation from the helical () phase to the planar zigzag () phase occurs via melting and recry...

From the thermodynamics with internal variables we will derive the temperature equation for viscoelastic fluids. We consider the type of storage of mechanical energy, the dissipation of mechanical energy, the compressibility of the fluid, the nonequilibrium heat capacity and thermal expansion, and deformation induced anisotropy of the heat conduction. The well-known stress differential models that fit into the thermodynamic theory will be treated as an example. Adapting a power-law scaling of the shear moduli on temperature and density, as is usual in rubber elasticity, we will derive an approximation of the temperature equation in measurable quantities. This equation will be compared with experimental results.

The present research work analyzes the space charge(S.C.) behavior and correlated dynamics in epoxy based nano-composites (NC) with hexagonal boron nitride (hBN) nano- particles.The importance of adopting an effective particle dispersion techniquefor producing such nano-composites is also experimentally validated.Three different dispersion techniques are contrasted in terms of thecorresponding space charge behavior exhibited, and the interplaybetween increasing filler content and dispersion techniques isinvestigated and reported. It is observed that an effective particledispersion technique results in a homogeneous distribution of nano-particles in the polymeric matrix, which in turn gives rise to evenlydistributed trap sites and affects the local mobility of charge carriers,effectively restraining them. More significantly, since most of thedeveloped trap-sites are located close to/in the interfacial zonebetween the injecting electrode and nano-composite specimen, aninitial, reasonably homogeneous distribution of nano-particles atrelatively lower loading (vol. %) is just as efficient in restricting bulkspace charge accumulation as higher loaded nano-compositespecimens – proving the importance of adopting an effective particledispersion technique. Achieving lower levels of bulk space chargeaccumulation (as demonstrated in this work) results in smallerinternal electric field distortion and stress in insulation material, thushaving the beneficial effect of prolonging operational life-time andincreased reliability.

The environmental degradation behaviour of flax fibres and their polymer composites are explored. New upgraded Duralin flax fibres, which have been treated by a novel treatment process for improved moisture and rot sensitivity were studied. Environmental studies showed that these upgraded Duralin flax fibres absorb less moisture than untreated Green flax fibres, whereas the mechanical properties of the treated fibres were retained, if not improved. The effect of this novel flax fibre treatment on the environmental behaviour of natural-fibre-mat-reinforced thermoplastics (NMTs) is investigated by monitoring the moisture absorption and swelling, and measuring the residual mechanical properties of the flax/polypropylene composites at different moisture levels. The moisture absorption and swelling of the upgraded flax fibre composites is approximately 30% lower than that of composites based on Green flax fibres.

In previous work we developed a micro-mixer based on artificial cilia, that was experimentally proven to be very effective. Here, we present numerical simulations of the mixing process carried out using an advanced numerical tool, in combination with OCT experiments, that explain the observed effectiveness. As opposed to what was expected, fluid inertia was found to be the key ingredient for effective mixing.

The objective of the current study was to determine the \textsl{in vitro} passive transverse mechanical properties of skeletal muscle with Dynamic Mechanical Thermal Analysis (DMTA) tests. The starting hypotheses was that the time-temperature-superposition principle could be used to expand the DMTA results to a 1 kHz frequency range. Experiments were performed with rat hind leg skeletal muscle tissue samples on a rotational rheometer using a parallel plate geometry. Because of the small size and low modulus of the samples, the standard test geometry was altered and the samples were shifted from the center to the edge of the plates. From strain sweep tests it became clear that for strains smaller than 0.003 the muscle tissue behaves linearly. In the linear region storage moduli ranged between 24 kPa (omega = 1 rad/s) and 42 kPa (omega = 100 rad/s) at T = 4 degrees C and 22 kPa and 33 kPa at 29 degrees C within the experimental frequency range. The loss modulus decreased with increasing frequency and ranged between 7 and 4 kPa at 4 degrees C and 4.5 and 3.5 kPa at 29 degrees C. Although the properties are clearly temperature dependent, a temperature shift in phase angle delta could not be detected, thus Time Temperature Superposition is not allowed for skeletal muscle in vitro.

Responsive polymeric, micron-sized materials are described that can locally manipulate fluids by reversible micro-actuation. Selected examples of external stimuli, such as the use of a magnetic field are shown.

The Polymers Editorial Office retracts the article "The Effect of Intrinsic Mechanical Properties on Reducing the Friction-Induced Ripples of Hard-Filler-Modified HDPE" [...].