Synthesis and characterization of photopolymerizable hydrogels based on poly (ethylene glycol) for biomedical applications

Hydrogels are polymeric materials widely used in medicine due to their similarity with the biological components of the body. Hydrogels are biocompatible materials that have the potential to promote cell proliferation and tissue support because of their hydrophilic nature, porous structure, and elastic mechanical properties. In this work, we demonstrate the microwave-assisted synthesis of three molecular weight varieties of poly(ethylene glycol) dimethacrylate (PEGDMA) with different mechanical and thermal properties and the rapid photo of them using 1-hydroxy-cyclohexyl-phenyl-ketone (Irgacure 184) as UV photoinitiator. The effects of the poly(ethylene glycol) molecular weight and degree of acrylation on swelling, mechanical, and rheological properties of hydrogels were investigated. The biodegradability of the PEGDMA hydrogels, as well as the ability to grow and proliferate cells, was examined for its viability as a scaffold in tissue engineering. Altogether, the biomaterial hydrogel properties open the way for applications in the field of regenerative medicine for functional scaffolds and tissues.     

Long-term effect of diesel degradation on polyoxymethylene at different temperatures

Diesel is an important fuel, partly because of the longevity and cleanliness of diesel engines. Often, polymers are in direct contact with diesel and understanding compatibility is critical. Polyoxymethylene (POM) is a thermoplastic used to manufacture automotive fuel pump gears and rotors due to its low coefficient of friction and thermal and dimensional stability. In this study, tensile tests were performed on plain and glass fiber reinforced (POM and POMGF) after immersion in diesel at different temperatures (−10°C, 23°C, and 60°C) for 1000, 2000, 3000, 5000, and 10 000 hour. A mathematical model was developed using data from just three tensile stress-strain curves obtained at two different fluid temperatures and three different immersion times. Model and experimental results show good agreement with one another for all conditions tested.     

Chemical,thermal, and mechanical properties and ultraviolet transmittance of novel nano-hydroxyapatite/polyethylene terephthalate milk bottles

Polyethylene terephthalate (PET)/nano-hydroxyapatite (nHAp) composite granules were obtained using twin-screw extruder. Preforms were prepared by injection molding and then PET/nHAp bottles were produced by blow molding. For PET bottles with nHAp, the migration amounts of carboxylic acid (COOH), acetaldehyde (AA), diethylene glycol (DEG), and isophthalic acid (IPA); glass transition temperature (T_g); melting temperature (T_m); and the maximum crystallization temperature (T_cry) were measured. The load-carrying capacity, burst strength, stress cracking, and regional material distribution tests were carried out on the bottles. X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, and ultraviolet transmittance analyses were conducted to explain the changes in mechanical, chemical, physical properties, and light transmission of bottles. It was found out that the COOH amount increased and the AA content decreased with increasing nHAp amount. On the other hand, no change was observed in the amounts of DEG and IPA. Although the mechanical properties such as load-carrying capacity and burst strength of the bottles have improved, it has been determined that the standard environmental stress crack resistance test procedure cannot be applied to such a composite. Experimental findings indicate that nHAp disrupts the chemical structure of PET and it isolates harmful chemicals such as AA by forming intermolecular bonds. Moreover, with the addition of up to 0.8% nHAp, PET bottles block the light transmission approximately 80% within 400–700 nm wave length zone. The study demonstrates that the PET/nHAp composite bottles can be used in the food industry, particularly in the packaging of milk and milk products which are vulnerable to light exposure.     

Thermal dehydrochlorination of pure PVC polymer: Part I—thermal degradation kinetics by thermogravimetric analysis

Thermal degradation of PVC occurs in two stages, with each stage subdivided into two substages. The first refers to the dehydrochlorination, where hydrochloric acid is formed, and giving polyene structures. Hitherto, the degradation mechanism and action of hydrochloric acid as a catalyst during the dehydrochlorination stage are poorly known. Recently, the importance of the tacticity has gained attention for its influence on the dehydrochlorination mechanism. The present work focused on the dehydrochlorination stage, studying the molecular structure by FTIR analysis and the kinetic parameters by TGA analysis in Nitrogen atmosphere, based on three mathematical methods: Friedman, Kissinger, and Flynn-Wall-Ozawa. The sample was a pure homopolymer obtained by suspension polymerization. The dehydrochlorination kinetics follows a first order reaction model and occurs by nucleation and growth. The dehydrochlorination begins with the loss of very labile chlorine atoms present in defective and isotactic molecular segments. The formed HCl acts as a catalyst in the degradation. Following 40% conversion, a drop in Ea is observed. After that, chlorine atoms present in syndiotactic and atactic sequences, are released and, added to the large number of polyene chain sequences, and an increase in Ea is observed up to 60% conversion, where the dehydrochlorination stage is concluded.     

SiC MOSFET短路失效与退化机理研究综述及展望

SiC MOSFET可以大幅提升变流器的效率和功率密度,在高频、高温、高压等领域有较好的应用前景。但是,由于其短路耐受时间短、特性退化现象严重以及失效机理模糊等因素,致使SiC MOSFET的普及应用受到了限制。因此,探究SiC MOSFET短路失效与特性退化的机理,可以为SiC MOSFET器件的应用及其保护电路的设计提供指导,具有重要的研究价值。该文首先归纳SiC MOSFET的短路故障类型,并针对其中一种典型的短路故障进行详细的特性分析。在此基础上,论述SiC MOSFET单次短路故障后存在的两种典型失效模式,综述其在两种失效模式下的失效机理以及影响因素。其次,对SiC MOSFET经历重复短路应力后器件特性退化机理的研究现状进行系统的总结。最后指出当前SiC MOSFET短路失效与特性退化的研究难点,展望SiC MOSFET短路特性研究的发展趋势。     

Influence of starting powder choice on structure property relations in gadolinia stabilized zirconia 3Gd-TZP manufactured from co-milled starting powders

Two 3Gd-TZP materials were manufactured from powders produced by intense mixing and milling of unstabilized zirconia starting powders and gadolinia as stabilizer oxide by hot pressing at 1250 °C – 1400 °C. The materials show a combination of high toughness and moderate strength. In detail depending on starting powder the two TZP showed distinct differences concerning transformation characteristics, sintering temperature dependence of mechanical properties and the tendency to develop R-curve related deformation features such as non-linear stress strain curves and formation of transformation bands prior to fracture.     

用偏钛酸作原料制备纳米TiO2粉末以及颗粒相结构与光催化性能关系的研究

米用常温氨水水解TiOSO4过程中加入自制的表面活性剂制备了超细纳米TiO2粉末,用XRD,TEM和比表面仪对不同煅烧温度下的纳米TiO2粉末结构、粒径大小和比表面等进行了表征,研究了纳米TiO2粉末对甲基橙的光催化降解能力。研究表明煅烧温度在400℃～800℃时得到的纳米TiO2粉末呈锐钛矿结构,粒径约为5．5nm～9．6nm,比表面积高达189．45m^2/g;在850℃煅烧后所得的纳米TiO2粉末为锐钛矿与金红石型混晶结构;在l100℃时得到的纳米TiO2粉末为金红石型纳米TiO2粉末,同时微粒出现团聚且粒径变大。光催化实验表明：纳米TiO2粉末的光催化活性与煅烧温度密切相关,850℃煅烧1．5h所制得的混晶结构纳米TiO2粉末表现出较高的光催化活性。与国产商品纳米锐钛矿型TiO2相比,其降解甲基橙的速率约为国产商品纳米TiO2的1倍。     

Thermal stability of lamellar structure of PST crystal and lamellar boundary design for creep resistant TiAl alloy

The stability of lamellar structure is crucial for the creep resistance of TiAl alloys, but degradation of the lamellar structure is unavoidable at high temperatures. The degradation of the lamellar structure in PST crystals of Ti-48mol.%Al was studied during high temperature exposure (annealing and creep testing) to examine how to make a stable lamellar structure with high creep deformation resistance. Since the six orientation variants of γ lamellae are nucleated independently of the adjoining lamellae, pseudo twin and 120° rotational fault boundaries are most frequently observed at the initial stage of lamellar formation. The preferential removal of high energy (pseudo twin and 120° rotational fault) boundaries during the evolution of lamellar structure results in the highly probable appearance of a true twin boundary at a later stage of lamellar evolution. The coarsening of lamellar spacing and the spheroidization of the lamellae are the major degradation events occurring during creep deformation, and the migration of the lamellar boundaries brings both of them about. The lamellar structures of TiAl alloy contain four types of lamellar boundaries. The stability of the four types of boundaries decreases in the following order: γ/α₂ > true twin > pseudo twin > or=120° rotational fault boundaries. The γ/α₂ boundary has the highest stability (lowest mobility), and the high density of γ/α₂ boundaries is proposed to make a stable lamellar structure with good creep resistance. A material having the high density of γ/α₂ boundaries was produced through the heat treatment of a PST crystal in the α+γ two-phase regime. The excellent creep properties of the material were proven through creep tests of hard oriented PST crystals made of the material. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

ZnO／Ag纳米复合材料光催化性能研究

通过XRD、TEM测试研究了自制ZnO／Ag纳米复合材料的结构和形貌，通过UV检测确定了以该纳米复合材料为光催化剂，在不同条件下对甲基橙的光催化降解率。结果表明：与空气煅烧相比，真空煅烧所得纳米复合材料的光催化降解效果更好，且光催化降解率随纳米复合材料用量增加而增大：甲基橙溶液的pH在5左右时，光催化降解率最高：H2O2浓度为0．9g／L时，光催化降解率可达100％。     

Thermal degradation of foamed polymethyl methacrylate in the expendable pattern casting process

The thermal degradation of foamed polymethyl methacrylate (PMMA) patterns in the expendable pat-tern casting process has been studied. Various physical transitions that may occur during the degradation of PMMA have been determined using scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analysis, and the effects of polymer density on the degradation characteristics have been investigated. The results indicate that, when exposed to elevated temperatures, the polymer beads collapse at about 140 to 200 °. The collapsed beads melt at 260 ° and begin to volatilize. Peak volatilization temperatures are on the order of 370 °. The end temperature for volatilization is between 420 and 430 °. The initial density of the polymer does not have a significant effect on the transition tem-peratures associated with degradation.