Hani E. Naguib

Effect of Nanoclay on the Mechanical Properties of PMMA/Clay Nanocomposite Foams

In this study, the effects of nanoclay on the mechanical properties of poly(methylmethacrylate) (PMMA)/clay nanocomposite foams are investigated. Intercalated PMMA/clay nanocomposites have been prepared through a solvent co-precipitation method. PMMA/clay nanocomposites with only 0.5 wt% of well-dispersed montmorillonite nanoclay showed considerable improvement of mechanical properties; specifically in elastic modulus, tensile strength, and elongation at break. However, with an increased load of clay in the nanocomposite, the mechanical properties decreased due to the agglomeration of excessive nanoclay. Microcellular foams have been processed with PMMA/clay nanocomposite material using a subcritical gas foaming method. When a short foaming time is used, the increased amount of nanoclay induced a greater amount of heterogeneous nucleation during the foaming process and therefore decreased the density of the foam. In contrast, when a longer foaming time is used, foam density increased with a larger nanoclay load due to the higher diffusivity coefficient of CO2 blowing agent. Nanoclay, as a nucleation agent and reinforcement filler, changed the foaming behavior and mechanical properties of the PMMA microcellular foams. The microcellular foams made of PMMA/clay nanocomposite with 0.5 wt% exhibited an optimized mechanical response under tensile experiments. It is observed that the mechanical properties of nanocomposite foams are greatly related to the mechanical properties of unfoamed material and foam density. The nanoclay dispersion quality is a very important factor for the mechanical properties of both foamed and unfoamed polymer/clay nanocomposites.

Design and control of a shape memory alloy based dexterous robot hand

Modern externally powered upper-body prostheses are conventionally actuated by electric servomotors. Although these motors achieve reasonable kinematic performance, they are voluminous and heavy. Deterring factors such as these lead to a substantial proportion of upper extremity amputees avoiding the use of their prostheses. Therefore, it is apparent that there exists a need for functional prosthetic devices that are compact and lightweight. The realization of such a device requires an alternative actuation technology, and biological inspiration suggests that tendon based systems are advantageous. Shape memory alloys are a type of smart material that exhibit an actuation mechanism resembling the biological equivalent. As such, shape memory alloy enabled devices promise to be of major importance in the future of dexterous robotics, and of prosthetics in particular. This paper investigates the design, instrumentation, and control issues surrounding the practical application of shape memory alloys as artificial muscles in a three-fingered robot hand.

Effect of Talc Content on the Volume Expansion Ratio of Extruded PP Foams

This paper elucidates the effects of cell density on the volume expansion behavior of polypropylene (PP) foams blown with butane in extrusion. The cell density was controlled by varying the talc content, and foam expansion was observed at a fixed blowing agent content while varying the melt and die temperatures. As observed in our previous studies, the curve of the final expansion ratio of PP foam versus temperature showed a typical mountain shape for each talc content, confirming that the two governing expansion mechanisms were gas loss and stiffening of melt. As the talc content increased, the expansion curve skewed towards the lower temperature, which showed that the expanded foams with a high talc content were more susceptible to gas loss at elevated temperatures. This indicates that the processing temperature should be decreased to have a large expansion ratio from the extruded PP foams at a high talc content. In other words, the optimum temperature to maximize expansion decreased at a higher talc content. It is believed that the shift of the expansion curves was caused by the promoted expansion rate of extruded foams with a greater talc content because of the reduced diffusion distance of gas molecules to the nearest stabilized nucleus. On the other hand, the increased cell density at a high talc content increased the number of cell layers in the cross section of the extruded foam, and thereby the gas loss was localized to the cells on the surface which acted favorable for the final expansion ratio to a certain degree.

Comparison of morphology and mechanical properties of PLGA bioscaffolds

In this study, bioscaffolds using poly(DL-lactide-co-glycolide) acid (PLGA) were fabricated and studied. The gas foaming/salt leaching technique in a batch foaming setup was employed, and the effects of material composition of PLGA on the morphology and mechanical properties using this process were investigated. Two material compositions of PLGA 50/50 and 85/15 were used, and characterization of scaffolds fabricated with these materials showed that a lower relative density can be achieved with an increasing poly(DL-lactide) acid (PDLLA) content; however, higher open-cell porosity was obtained with lower PDLLA content. Furthermore, the effect of PLGA composition on modulus of the scaffolds was minor.

Middleware Support for Context-Aware Multimedia Applications

We describe QoSDREAM, a middleware framework for the construction and management of context-aware multimedia applications. The contributions of QoSDREAM include (1) a novel approach to the handling of location data derived from sensors in the physical world which integrates sensor data from a variety of sources into streams of application-relevant events and (2) a component-based architecture for the construction of real-time multimedia and other context-aware applications. The component architecture supports the construction of application models that are used for quality of service analysis and management purposes. Working distributed applications are derived from the models.

Extruded open-celled ldpe-based foams using non-homogeneous melt structure

This paper presents an open-celled foaming extrusion process with low-density polyethylene (LDPE) and LDPE/polystyrene (PS) blends. The basic strategy for achieving a high open-cell content was to induce a hard/soft melt structure with crosslinking and to foam this non-homogeneous melt structure. The hard sections formed by crosslinking assist in maintaining the shape of each cell and the overall foam structure, while the soft sections easily open up the cell walls during cell growth. Since too hard a polymer matrix would adversely affect cell opening, an optimum amount of crosslinking was observed for cell opening. The effect of the processing temperature on cell opening was also investigated in this study. A large expansion ratio of foam at a low temperature was favorable for cell opening with thin cell walls, but too low a temperature was not desirable because of the increased melt strength. This optimum processing temperature for cell opening was experimentally verified at various crosslinking agent contents. Blending with a small amount of PS turned out to be effective for cell opening; it yielded a higher cell density, which caused thinner cell wall thickness. Optimizing the material compositions and processing temperature successfully achieved a high open-cell content up to 99%.

Study of Shear and Extensional Viscosities of Biodegradable PBS/CO2 Solutions:

The purpose of this research is to study the pressure drop profiles of biodegradable polybutylene succinate (PBS)/CO2 solutions in a slit die and to measure the rheological properties of the solutions as a function of the blowing agent concentration. A slit die with four pressure transducers has been designed to describe the effects of shear rate, temperature, pressure, and gas content on the shear viscosity and extensional viscosity of the flowing PBS/CO2 solutions. The low shear rate viscosity of the pure polymer was measured using a cone and plate rheometer. Extensive experiments were conducted to investigate the polymer/gas solution viscosities at five different shear rates, three temperatures and five gas contents. Cross-Carreau model and generalized Arrhenius equation were used to describe the shear-viscosity behaviors of PBS/CO2 solutions. The extensional viscosity of solution was modeled based on Cogswell’s equation.

Bi-cellular Foam Structure of Polystyrene from Extrusion Foaming Process

This article investigates the foaming process of bi-cellular polystyrene foams blown with n-butane and water in extrusion. The bi-cellular foam structure has two types of cells: large cells ranging from 0.1 to 1.2 mm and small cells ranging in size from about 5% to about 50% of the average large cell size, which constitute more than 90% of the total cell volume. A bi-cellular structure has outstanding heat insulation property. In order to generate a bi-cellular structure, a water-blowing technology was used. This technique is environmentally benign and economical since butane and water are used as blowing agents. Despite these advantages, the foaming process and mechanism of bi-cellular foams have not been identified in detail. Therefore, in this article, an attempt has been made to enhance our knowledge and understanding of the foaming behavior of bi-cellular polystyrene foam. The effects of n-butane, water, and silica (a nucleating agent) on the foam cell morphology are presented.

Piezoresistance characterization of poly(dimethyl-siloxane) and poly(ethylene) carbon nanotube composites

This study examines the piezoresistive behavior of polymer–carbon nanotube composites. Piezoresistive composites of poly(dimethyl-siloxane) (PDMS) and poly(ethylene) (PE) filled with multiwall carbon nanotubes (MWNTs) were prepared. The morphology and the electrical conductivity of the composites were characterized at various MWNT compositions. The percolation threshold was found to be 3 wt% for PDMS composites and 2.2 wt% for PE composites. The piezoresistive behavior under compression was measured using a setup comprised of a mechanical tester and a digital sourcemeter. Negative piezoresistive behavior was observed, signifying a reducing mean interparticulate distance in the composites. The PE–MWNT composites were found to be more sensitive than the PDMS composites (97% versus 78% change in resistance), which was attributed to the dissimilar morphologies as a result of difference in processing. Increasing the MWNT concentration in the PE composites resulted in decreasing the sensitivity to stress. The results were found to fit well to a modified version of a piezoresistance model. PDMS and PE composites were found to have different piezoresistance behavior during stress relaxation and cyclic loading. The resistance of PE, in comparison to PDMS, was less prone to changes in stress during stress relaxation and exhibited greater sensitivity and less drift during cyclic loading.

Fabrication and characterization of melt-blended polylactide-chitin composites and their foams

This study details the fabrication and foaming of melt-blended polylactide (PLA) and chitin composites. The chitin used for compounding was as-received, as chitin nanowhiskers and as chitin nanowhiskers with a compatibilizing agent. The chitin nanowhiskers were produced by an acid-hydrolysis technique and their morphology was examined with transmission electron microscopy. The composite morphology was characterized with scanning electron microscopy and was related to the observed thermal, rheological, and mechanical behaviors of the composites. Chitin was found to decrease the thermal stability of the composites. Addition of chitin was also found to reduce the viscosity of the composites, which is believed to be because of the hydrolysis of PLA during melt blending of chitin in suspension. The stiffness of the composites was found to increase with increasing chitin content while the strength was found to decrease. Porous PLA—chitin composites were produced by a two-step batch-foaming technique, and the expansion behavior was correlated with the visco-elastic observations. The statistical significance of chitin type and composition dependence on the mechanical properties and foam morphologies were evaluated.