Shing Chung Josh Wong

Electrical conductivity and dielectric properties of PMMA/expanded graphite composites

Abstract PMMA/expanded graphite (EG) composites were prepared by direct solution blending of PMMA with the expanded graphite filler. Electrical conductivity and dielectric properties of the composites were measured by a four-point probe resistivity determiner and a dielectric analyzer (DEA). Interestingly, only 1 wt.% filler content was required to reach the percolation threshold ( φ ) of transition in electrical conductivity from an insulator to a semiconductor using PMMA/EG. The thickness of the interlayer of the expanded graphite was shown to be close to the nanometer scale. The reported filler content was much lower than that required for conventional PMMA/carbon black (8 wt.% carbon) and PMMA/graphite (3.5 wt.% graphite) composites. The improvements in both electrical conductivity and structural integrity were attributed to the difference in filler geometry (aspect ratio and surface area) and the formation of conductive networks in the composites.

Processing Methodologies for Polycaprolactone-Hydroxyapatite Composites: A Review

ABSTRACT Biodegradable implants have shown great promise for the repair of bone defects and have been commonly used as bone substitutes, which traditionally would be treated using metallic implants. The need for a second surgery exacerbated by the stress shielding effect caused by an implant has led researchers to consider more effective, synthetic biodegradable graft substitutes. The hierarchical structures commonly designed are inspired by nature in human bones, which consist of minerals such as hydroxyapatite, a form of calcium phosphate and protein fiber. The bone graft bio-substitutes should possess a combination of properties for the purpose of facilitating cell growth and adhesion, a high degree of porosity, which would facilitate the transfer of nutrients and excretion of the waste products, and the scaffold should have high tensile strength and high toughness in order to be consistent with human tissues. Blending of polycaprolactone and hydroxyapatite has demonstrated great potential as bone substitutes. It is essential to identify a standardized processing methodology for the composite, which would result in optimum mechanical property for the biocomposite. In this study, biocomposites made of polycaprolactone (PCL) and hydroxyapatite (HAP) are reviewed for their applications in bone tissue engineering. The processing methodologies are discussed for the purpose of obtaining the porosity and pore size required in an ideal tissue scaffold. The properties of the composite can be varied based on the change in pore size, porosity, and processing methodology. This paper reviews and evaluates the methods to produce the hydroxyapatite-polycaprolactone scaffolds.

Effect of rubber functionality on microstructures and fracture toughness of impact-modified nylon 6,6/polypropylene blendsPart II. Toughening mechanisms

Abstract Toughening mechanisms in blends containing 60 parts nylon 6,6, 20 parts polypropylene (PP) and 20 parts styrene–ethylene/butylene–styrene (SEBS) grafted to different levels of maleic anhydride (MA) were investigated. The sequence of events was carefully characterised using different microscopic techniques. It was found that under triaxial constraint interfacial cavitation followed by multiple crazing and subsequently massive shear yielding of the matrix contributed to an enormous toughening effect in core-shell microstructures observed in 0.92%-maleated blend (0.74% in Part I paper [Wong SC, Mai Y-W. Polymer, 1999;40:1553], should be 0.92% as corrected in this paper). The core-shell structure was formed when the spherical domains of PP were surrounded by SEBS rubber in a nylon-rich matrix.. In this composition, miscibility between the dispersed SEBS- g -MA and the nylon phase was maximised as revealed by thermal-mechanical analysis. The SEBS was most effective in toughening the nylon/PP blends when it cavitated to introduce ligament bridges between debonded PP particles at the crack tip. Interfacial cavitation and multiple crazing served to relieve the hydrostatic tension ahead of crack growth and subsequently enhanced the shear-yielding component of stresses in the matrix material. Other blend compositions that did not show controlled cavitation resulted in little plastic flow surrounding the crack tip and reduced fracture toughness. These results reinforced the notion that cavitation of SEBS at the nylon–PP interface was an essential mechanism to promote toughening in materials subjected to high crack tip triaxiality.

Modelling of mechanical properties of electrospun nanofibre network

Electrospun nanofibres are widely investigated as extra-cellular matrix for tissue engineering and biomedical applications. Little is understood on the deformation mechanics of spun fibre mats. A model is developed to predict the deformation behaviour of randomly-oriented electrospun nanofibre network/mats with the fibre-fibre fusion and van der Waals interaction. The nanofibres in the mat are represented by chains of beads; the interactions between the beads are described by bonded (stretch, bending and torsion) and non-bonded (van der Waals) potentials. Stress-strain curves and dynamics fracture are predicted by this model. The results show that the fibre-fibre fusion has a significant effect on the tensile strength of the mats. Increasing the number of fusion points in the mat results in an increase in strength, but over-fusion may lead to lower fracture energy. The predicted stress-strain relationships are consistent with the experimental results.

Fracture and Impact Toughness of Syntactic Foam

In this study we assessed the fracture toughness, KIc, and the impact resistance of syntactic foam reinforced with glass microspheres of different densities and polymer binder as a function of microstructures. The results showed that both KIc and the linear elastic energy release rate, GIc, increased with increasing volume fraction of glass microspheres, and the increase was higher for microspheres possessing a lower density. The impact resistance of syntactic foam decreased with the inclusion of hollow microspheres.

Measurement of Adhesion Work of Electrospun Polymer Membrane by Shaft-Loaded Blister Test

The work of adhesion at the interface of electrospun membrane and rigid substrate is measured by a shaft-loaded blister test (SLBT). Poly(vinylidene fluoride) (PVDF) were electrospun with an average fiber diameter of 333 ± 59 nm. Commercial cardboard with inorganic coating was used to provide a model substrate for adhesion tests. In SLBT, the elastic response PVDF was analyzed and its adhesion energy measured. The average value of the adhesion work is 206 ± 26 mJ/m(2). Elastic modulus of electrospun membrane obtained by SLBT is found to be 23.42 ± 2.69 MPa, which is consistent with the value obtained from standard tensile tests. The results show SLBT presented a viable methodology for evaluating the adhesion energy of electrospun polymer fabrics.

Characterization of microstructures and toughening behavior of fiber-containing toughened nylon 6,6

The toughening behavior of short glass fiber reinforced toughened polymers was studied using fracture mechanics and microscopic techniques. The essential work of fracture (EWF) analysis shows that the inclusion of short glass fibers not only provided a stiffening effect but also a toughening influence. It was observed that rubber-related toughening and fiber-related toughening were competitive in nature for the reinforced, toughened nylon 6,6. When the matrix stress was substantially reduced by the presence of short fibers via the load-shedding mechanism, rubber toughening was severely curtailed. At higher fiber volume fractions, fiber pull-out work contributed significantly to the enhancement of the specific essential fracture work. Fiber-end plasticity was evident under microscopic examination.

The effect of fiber inclusions in toughened plastics—part I: fracture characterization by essential fracture work

The effect of fiber inclusions on toughened plastics was studied by means of the technique of essential fracture work. The major advantage of the technique for fiber-reinforced toughened polymers is attributed to its potential to distinguish fiber-related toughness from fiber-induced matrix toughness. Strictly speaking, the essential fracture work is the specific energy required to create two new surfaces and that consumed in the fracture processes involved. The essential work was only proportional to the ligament area whereas the work dissipated outside the process zone was dependent on the volume of plastically deformed region. The latter is not a material property. Fiber-related fracture work, such as fiber bridging, breaking and pullout, can scale with the ligament length but cannot scale with the ligament squared. With fiber inclusions, supertough Nylon 6,6 exhibited concomitant strengthening and toughening.

A Nano-Cheese-Cutter to Directly Measure Interfacial Adhesion of Freestanding Nano-Fibers

A nano-cheese-cutter is fabricated to directly measure the adhesion between two freestanding nano-fibers. A single electrospun fiber is attached to the free end of an atomic force microscope cantilever, while a similar fiber is similarly prepared on a mica substrate in an orthogonal direction. External load is applied to deform the two fibers into complementary V-shapes, and the force measurement allows the elastic modulus to be determined. At a critical tensile load, “pull-off” occurs when the adhering fibers spontaneously detach from each other, yielding the interfacial adhesion energy. Loading-unloading cycles are performed to investigate repeated adhesion-detachment and surface degradation.

Embrittlement mechanisms of nylon 66/organoclay nanocomposites prepared by melt-compounding process

Abstract Nylon 66 nanocomposites with different smectite clay loadings were prepared by conventional melt compounding process. The fracture toughness decreases with increasing clay content, which is a direct result of reduced plastic zone size at the crack tip region. The fracture mechanisms were studied using double-notched four-point-bending (DN-4PB) technique. A constraining effect from nanoclay fillers on plastic deformation of matrix is revealed by transmission electron microscopy (TEM). Micron-sized and submicron voids could be observed around the clay platelets. The voids coalesce and form premature cracks that promote crack propagation, thus reducing toughness.