Chi-Pong Tsui

Wear performance of ultrahigh molecular weight polyethylene/quartz composites

Ultrahigh molecular weight polyethylene (UHMWPE)/quartz composites were compression molded in the presence of organosiloxane, and then hydrolyzed. The used organosiloxane is vinyl tri-ethyloxyl silane. The gelation, the melting behavior, the crystallinity, the mechanical properties and the wear resistance of UHMWPE/quartz composites were investigated. The results showed that organosiloxane can act as a cross-linking agent for UHMWPE matrix and serve as a coupling agent for improving the bonding between the quartz particles and the UHMWPE matrix. The correlation between the various properties and the morphology of the composites has been discussed. At about 0.5phr organsiloxane while the degree of crystallinity of the composite is at the peak value of 57%, the mechanical properties and the wear resistance of UHMWPE/quartz composites reaches their maximum.

Finite element analysis of polymer composites filled by interphase coated particles

Abstract The effects of different interphase properties on Young’s modulus, maximum stress concentration factor and stress distribution in glass bead-filled polycarbonate (GB/PC) and interphase-coated glass bead-filled polycarbonate (GB/INT/PC) have been investigated using the finite element method. A three-dimensional unit cell model has been built for modeling both two-phase and three-phase composites. The results from the predictive model allow the deduction of the optimum interphase properties for reduction of the stress concentration and minimization of the drop in elastic modulus of the resulting composites.

Water-soluble graphene grafted by poly(sodium 4-styrenesulfonate) for enhancement of electric capacitance

Water-soluble poly(sodium 4-styrenesulfonate) modified graphene (PSSS-GR) was successfully synthesized via covalently grafting poly(sodium 4-styrenesulfonate) (PSSS) on the surfaces of graphene (GR) nanosheets. The structure of PSSS-GR was investigated with Fourier transform infrared, x-ray photoelectron and Raman spectroscopy, thermogravimetric analysis, transmission and scanning electron microscopy and atomic force microscopy. The PSSS chains made the GR nanosheets fully exfoliate into a single-layer structure, and the PSSS layer on GR reached 90 wt%. PSSS chains displayed mutually repulsive effects on promoting GR sheets that were more stable in water. The performances of supercapacitors made of PSSS-GR and unmodified GR electrodes were compared using cyclic voltammetry and galvanostatic charge/discharge techniques. The results showed that PSSS is an effective binder for graphene sheets and can increase the specific capacitance of PSSS-GR based supercapacitors and improve their rate capability. The maximum specific capacitance of the PSSS-GR based supercapacitor was 210 F g(-1) at 5 A g(-1), which was 166% higher than for one made of unmodified graphene electrodes. Electrochemical impedance spectroscopy demonstrated fast ion diffusion in the PSSS-GR electrode structure. PSSS-GR based supercapacitors can fulfil one of the essential requirements for potential electric energy storage applications.

PANI–PEG copolymer modified LiFePO4 as a cathode material for high-performance lithium ion batteries

The poor electronic conductivity and low lithium ion diffusion rate of a LiFePO4 cathode material are the two major obstacles for its commercial applications in the power lithium ion batteries. This article utilized an electroactive and ion conductive copolymer, polyaniline–poly(ethylene glycol) (PANI–PEG), to modify carbon-LiFePO4 (cLFP) by a facile in situ chemical copolymerization method. The structure and morphology of the cLFP/PANI–PEG composite were confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Compared with a cLFP/PANI composite, the cLFP/PANI–PEG composite exhibited a more uniform and full polymer coating layer. Furthermore, this cLFP/PANI–PEG cathode material exhibits excellent cyclic stability (95.7% capacity retention after 100 cycles at 0.1 C) and high rate capability (125.3 mA h g−1 at 5 C) as the PANI–PEG copolymer coating layer facilitated electron and ion transport within the electrode. Electrochemical impedance spectroscopy (EIS) proved that the lithium ion diffusion in the cLFP/PANI–PEG composite was increased significantly by one order of magnitude compared with cLFP, indicating its possibility to be served as a cathode material for high-performance lithium ion batteries.

Numerical Simulation of Dynamic Electro-Mechanical Response of Ionic Polymer-Metal Composites

Ionic Polymer-Metal Composites (IPMC) is an emerging class of Electro-Active Polymer (EAP) materials. IPMC has attractive features, such as high sensitivity and light weight, which are useful for developing novel designs in the fields of bionic actuators, artificial muscles and dynamic sensors. A Finite Element (FE) model was developed for simulating the dynamic electro-mechanical response of an IPMC structure under an external voltage input. A lumped Resistor-Capacitor (RC) model was used to describe the voltage-to-current relationship of a Nafion IPMC film for the computation of electric field intensity. Moreover, the viscoelastic property of the IPMC film was considered in the model and the non-uniform bending behavior was also taken into account. Based on the proposed model and the assumption that the thicknesses of the two electrodes are the same and uniform, the optimal coating thickness of the IPMC electrode was determined. It was demonstrated that the dynamic electro-mechanical response of the IPMC structure can be predicted by the proposed FE model, and the simulation results were in good agreement with the experimental findings.

Modified scheme based on semi-analytic approach for computing non-probabilistic reliability index

A new computation scheme proposed to tackle commensurate problems is developed by modifying the semi-analytic approach for minimizing computational complexity. Using the proposed scheme, the limit state equations, usually referred to as the failure surface, are obtained from transformation of an interval variable to a normalized one. In order to minimize the computational cost, two algorithms for optimizing the calculation steps have been proposed. The monotonicity of the objective function can be determined from narrowing the scope of interval variables in normalized infinite space by incorporating the algorithms into the computational scheme. Two examples are used to illustrate the operation and computational efficiency of the approach. The results of these examples show that the proposed algorithms can greatly reduce the computation complexity without sacrificing the computational accuracy. The advantage of the proposed scheme can be even more efficient for analyzing sophistic structures.

Tensile properties of polycaprolactone/nano-CaCO3 composites

Abstract The effects of nanometer calcium carbonate content and tensile rate on the tensile properties of the filled polycaprolactone (PCL) composites were investigated. There was a certain reinforcing effect of the filler on the PCL resin. The tensile modulus increased nonlinearly, and the tensile strength also increased with increase of the filler weight fraction. When the filler weight fraction was kept constant, the tensile modulus and tensile strength increased slightly with increasing tensile rates. By comparing the experimental results with those determined from the tensile yield strength theory, the interfacial adhesion between the filler and matrix was found to be relatively strong; it should be one of the reasons for the good reinforcing effect.

Flexural Properties of Poly-L-Lactide and Polycaprolactone Shape Memory Composites Filled with Nanometer Calcium Carbonate

The flexural properties of poly-L-lactide (PLLA) and polycaprolactone (PCL) shape memory composites filled with nanometer calcium carbonate (nano-CaCO3) were determined at room temperature. The results showed that with the increase of the nano-CaCO3 weight fraction the flexural moduli and strength of PCL/nano-CaCO3 composites increased roughly linearly and reached a maximum at the filler content of 2%, while the flexural strength of the composites decreased. The flexural moduli and strength of the composites decreased roughly linearly with increasing PLLA/PCL ratio for the PLLA/PCL/nano-CaCO3 composites.

Melt flow properties and morphology of polypropylene composites filled with microencapsulated red phosphorus

Microencapsulated red phosphorus (MRP)-filled polypropylene (PP) composites were prepared using a twin-screw extruder. The effects of load and temperature as well as the dispersion or distribution of the filler particles in the matrix on the melt volume flow rate (MVR) and melt density (ρm) of the PP/MRP composites were investigated using a melt flow indexer and a scanning electron microscope. The temperatures and loads were varied from 180 to 205°C and from 2.16 to 12.5 kg, respectively. The results showed that the MVR of the composites increased nonlinearly with increase in temperature and load. The sensitivity of MVR of the composite melts to temperature was significant. The MVR of the composites also decreased slightly with an increase in the MRP weight fraction. However, the values of ρm of the composites varied slightly with increase in load, temperature, and MRP weight fraction. The findings can provide useful information for optimum processing of these composites.

Flaw-induced plastic-flow dynamics in bulk metallic glasses under tension.

Inheriting amorphous atomic structures without crystalline lattices, bulk metallic glasses (BMGs) are known to have superior mechanical properties, such as high strength approaching the ideal value, but are susceptible to catastrophic failures. Understanding the plastic-flow dynamics of BMGs is important for achieving stable plastic flow in order to avoid catastrophic failures, especially under tension, where almost all BMGs demonstrate limited plastic flow with catastrophic failure. Previous findings have shown that the plastic flow of BMGs displays critical dynamics under compression tests, however, the plastic-flow dynamics under tension are still unknown. Here we report that power-law critical dynamics can also be achieved in the plastic flow of tensile BMGs by introducing flaws. Differing from the plastic flow under compression, the flaw-induced plastic flow under tension shows an upward trend in the amplitudes of the load drops with time, resulting in a stable plastic-flow stage with a power-law distribution of the load drop. We found that the flaw-induced plastic flow resulted from the stress gradients around the notch roots, and the stable plastic-flow stage increased with the increase of the stress concentration factor ahead of the notch root. The findings are potentially useful for predicting and avoiding the catastrophic failures in tensile BMGs by tailoring the complex stress fields in practical structural-applications.