Materials Science Forum Vols. 745-746

Abstract: The surface instability of trilayer films consisting of a fluid layer sandwiched in between the two thin elastomer capping layers was studied. The solid-liquid-solid sandwiched films will form well-defined periodic surface buckling spontaneously. In the present study, the flow of the sandwiched liquid layer was approximated by the theory of lubrication. The elastic capping films was modeled with the nonlinear theory of a thin plate. A linear stability analysis identified the growth rate and the critical wave number of the surface undulation of trilayer films. The analysis showed that applied deformation in the capping layers regulated the surface buckling and resulted in well-defined periodic surface corrugation with tunable wavelength. The result of this study may provide a mechanism to control the morphology of the films in a mechanical way.

Abstract: In order to realize the opto-electronic integration of optic gyro, the quasi-planar ridge SOI waveguide resonator is proposed due to its low loss and small bend radius. First, the single mode condition and smallest bend radius were simulated by using the 3-dimension finite difference beam propagation method (3-D BPM). Then the relationship between the sensitivity of gyro and the resonator characteristics, such as radius and coupling coefficient, was analyzed by applying the model of the integrated optical gyro. Based on these simulations, the ridge height and width of the waveguide were 220 nm and 1.2 μm respectively and the smallest bend radius was 600μm. It is shown that the theoretical detection limit of the gyro was 0.031 °/s when the resonator diameter was 5cm and the couplers splitting ratio was 46.7:53.3. This gyro system is suitable for tactical and civil applications.

Abstract: Study of the effect of dielectrophoresis (DEP) parameters is important in high-precision DEP assembly of carbon nanotubes (CNTs). The DEP parameters usually considered in the literature include the magnitude and frequency of the applied voltage, the assembly time, the concentration of the CNT suspension, and the geometry of the electrodes. This paper reviews the current progresses on both numerical and experimental study of the CNT assembly by DEP, especially the influence of the DEP parameters on the assembly process and results. The review shows that the magnitude of the applied voltage affects the DEP force and the number of deposited CNTs. The assembly time and CNT concentration influence the density of deposited CNTs. Different electrode geometries have an effect on the distribution of the electric field. The electrode spacing changes the field strength and the direction of the field gradient. The related discussion is presented as well.

Abstract: Biodegradable composites were prepared by adding hyperbranched poly (ester amide) (HBPs) into poly (3-hydroxybutyrate-co-3-hydrovalerate) (PHBV) through melt blending method. It was found that the tensile strength and toughness of PHBV were simultaneously enhanced by the addition of HBPs. Compared with neat PHBV, the tensile strength of the composite increased about 23% from 20.96 to 25.87 MPa for the content of 2.5 wt.% HBPs, and more remarkable enhancement in tensile elongation at break can be achieved by about 88% for 5 wt.% HBPs. The influences of HBPs on crystallization, thermal and fracture morphologies of PHBV were further evaluated by using differential scanning calorimeter and scanning electron microscope, respectively. The decrease in the crystallinity of PHBV and high dispersion of the HBPs in PHBV matrix were observed, which should contribute to the improvement on the mechanical properties of PHBV.

Abstract: 2,2-bis [4-(2-hydroxy-3-methacryloyloxypropoxy) pheny propane (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) have been commonly used as a viscous monomer and a reactive diluent in the organic phase of dental restorative composites, respectively. The purpose of addition of TEGDMA is mainly to decrease the high viscosity of Bis-GMA caused by hydrogen bonding between hydroxyl groups. However, some adverse effects will accompany with increased amounts of the TEGDMA, such as higher values of polymerization shrinkage, which is not undesirable for the clinical application. Therefore, substituting hydroxyl groups of Bis-GMA might be an appropriate and effective way to reduce the amount of diluents and weaken the accompanied adverse effects. This work focuses on the synthesis of a novel Bis-GMA derivate, substituting acetyl groups for hydroxyl groups in Bis-GMA. The viscosity of Bis-GMA characterized with rotational rheometer was significantly decreased from 820 Pa.s to 11 Pa.s by substitution of acetyl group, leading to the low amount of TEGDMA in resin matrix. Differential Scanning Calorimeter (DSC) was used for investigating the reaction kinetics of this novel monomer with different mass ratios of TEGDMA. The results suggested that the maximum conversion of the Ac-Bis-GMA can reach 88% while the corresponding value for Bis-GMA is 75%. Dental composites were prepared from 2,2-bis [4-(2-acetyl-3-methacryloyloxypropoxy) pheny propane (Ac-Bis-GMA) or Bis-GMA resin mixtures with TEGDMA filled with 70 wt% silica co-fillers. The results presented that dental composites prepared from new resin matrixes exhibited adequate mechanical properties.

Abstract: This paper reviews the recent progress on the research of contact resistance of carbon nanotubes (CNTs), including the contact resistance between CNTs and electrodes/metals and that between CNTs. In particular, the current research advances in improving CNT contact resistance are presented. The challenges in improving CNT contact resistance are also addressed from a practical point of view. A thorough understanding of the electrical contact properties of CNTs helps to explore new methods to improve their contact resistance for full application of this unique group of materials.

Abstract: Hydrogels had been prepared by blending four types of surfactants such as alkyl betaine (zwitterion surfactant), tween 60 (non-ionic surfactant), sodium fatty alcohol ether sulfate (anionic surfactant) and silicone quaternary ammonium salt (cationic surfactant) with bombyx mori silk fibroin (SF) solution. Surfactant molecules would interact with SF molecules, resulting in lower critical micelle concentration (CMC). Gelation time measurements showed surfactants had significantly accelerated the hydrogelation process of SF solution, which could be controlled by the type or blend ratio of surfactants. Fourier transform infrared (FTIR) indicated that the addition of surfactants affected the molecular secondary of SF. The results from X-ray diffraction indicated that surfactants and SF were only blended but phase separation with two kinds of crystalline structure. SEM images showed that the surfactants had significantly changed the morphology of hydrogels according to certain rules, especially non-ionic and cationic surfactants could induce silk fibroin solutions into porous and nanofibrous hydrogels. These results indicated that a novel and potential method not only can be used to accelerate hydrogelation process of SF solution but also can be used to alter the structural and morphological of SF hydrogels. Furthermore, the porous and nanofibrous hydrogels of SF induced by surfactants provided a novel strategy to mimic the nanofibrous structure of collagen in extracellular matrix (ECM).

Abstract: Hierarchical porous TiO₂ fibre was successfully synthesized using cotton as template. Microstructures of product were characterized by Field Emission Scanning Electron Microscope (FESEM), nitrogen absorption-desorption technique and transmission electron microscopy (TEM). Titanium oxide material was characterized by repetitious networks consisting of the fibres with diameter of ca. 3-8 μm. The results showed that the products were composed 2-5 nm piled pores stacked by polycrystalline TiO₂ nanoparticles. The small crystallite diameter (6-10 nm) and the high specific surface area (83.7 m²·g^-1) of sample was believed to be resulted from wide-angle X-ray Diffraction (XRD), high resolution TEM (HRTEM) and the BET method. While the concentration of methylthionine chloride solution was 20 mg/L, catalyst amount 0.1 g/L, the reaction had a higher decoloring rate following irradiation with a UV light source, the decoloring rate can reach over 100% after 250 min.

Abstract: Hydroxyapatite (HAP), as an essential component of bone and tooth, is one of the promising bioactive materials. The purpose of this study was to investigate the effects of incorporation of nanoHAP and SiO₂ particles on the properties of bioactive dental composite. Silanized HAP and SiO₂ were mixed with BisGMA/TEGDMA (1/1 wt/wt) monomers at various HAP/SiO₂ mass ratios of 0:7 (no HAP), 5.25/1.75, 3.5/3.5, 1.75/5.25, 7:0 (no SiO₂) to yield a series of pastes all containing 70 wt% filler loading. The results showed that flexural strength, elastic modulus and compressive strength gradually decreased with increasing amount of HAP particles, as well as curing depth and polymerization shrinkage, which resulted from the nature of HAP particle material. Water sorption and solubility in water increased slightly with addition of HAP particles. Degree of conversion test could support results above well. Remineralization test in SBF demonstrated dental composite with HAP particles had a good apatite-forming ability. Consequently, dental composite filled with lower amount of nanoHAP (HAP/SiO₂ around 0.25) particles would possess better comprehensive performances.

Abstract: Biodegradable polymeric nanofibers produced by electrospinning have been used as scaffolds for tissue engineering. As one kind of biodegradable material, electrospun polylatic acid fibers have an important application on biomedical materials. For better application of this kind of material, the properties of annealed electrspun poly (L-lactic acid) (PLLA) was studied in this investigation. The material properties of as-spun and annealed nanofibers were studied using differential scanning calorimeter (DSC), X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM). Thermal and XRD analysis revealed that PLLA as-spun fiber exhibited amorphous phase and began to crystalline when annealing temperature was above its glass transition temperature. SEM measurements of annealed fibers showed that fusion among fibers occurred when annealing temperature was too high.