Effects of monomer structures on the foaming processes and thermal properties of polyimide foams

Four polyimide (PI) foams were prepared from polyamide acid precursors. The effects of monomer structures on the foaming processes and thermal properties of PI foams were investigated. The foaming processes of PI foams were observed by a self‐made visualization device. The thermal properties of four PI foams were studied by the methods of dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetry/differential thermogravimetry (TG/DTG) analysis. The results indicated that the inflation onset temperatures and maximum inflation degrees of four precursors increased from 123 to 171°C and decreased from 28 to 15 times with the increasing rigidity of the precursor molecule, respectively. The glass transition temperatures, the 5% weight loss temperatures, the decomposed activation energies, and pre‐exponential factors of PI foams increased with increase in the rigidity of monomer. Copyright © 2009 John Wiley & Sons, Ltd.     

Microcellular propagation at UHF and microwave frequencies for personal communications

In this paper we present theoretical models for microcellular communications at UHF and microwave frequencies. For rural microcells, a vectorial analysis of the two-ray model is carried out, and comparisons between the developed model and measurements reported in rural areas of New Jersey and San Francisco Bay are provided. For urban microcells, the street is three-dimensionally modeled as a deterministic multislit waveguide channel, according to its database, and vectorial analysis is also taken into account. For the 3D street model, comparisons in good agreement with measurements reported in New York City are performed. Comparisons provided with measured data show that the models proposed can be very useful tools toward increasing theoretical analysis in spite of empirical analysis in the field of prediction.     

Novel Ternary Molecular Composites Prepared by a Sol–Gel Process and Their Conversion into Microcellular Foams

Abstract

A carboxylated polysulfone (C‐PSF) was reacted with 3‐(glycidoxypropyl)trimethoxysilane (GPS) in a sol–gel reaction to form hybrid networks. Specifically, GPS was grafted to the C‐PSF chains at their epoxy ends by ring‐opening reactions and then the methoxy silane groups at their other ends were crosslinked through the usual hydrolysis and condensation reactions. These C‐PSF/GPS cross‐linked binary networks are of interest in their own right, but this type of network was also used in the incorporation of polybenzimidazole (PBI) rigid‐rod polymer to form a C‐PSF/PBI/GPS ternary composite. In this case, the network structures obtained by carrying out the same reaction in the presence of PBI chains successfully suppressed their undesirable segregation within the composite. The transparencies of the preparative solutions and the dried films indicated that the dispersion of PBI was maintained in these environments, presumably at the nano‐ or molecular level. The ternary composites were characterized using Fourier‐transform infrared (FT‐IR) spectroscopy, scanning electron microscopy (SEM), thermal mechanical analysis (TMA), and thermogravimetric analysis (TGA). Some of these composites were converted to closed‐cell microcellular foams, which were then characterized in a preliminary way with regard to their morphologies.     

The Modification of Calcium Lignosulfonate and Its Applications in Cementitious Materials

An azo calcium lignosulfonate (ACL) is synthesized by the coupling of a diazonium salt made from p‐aminobenzene sulphonic acid and calcium lignosulfonate (CL). The surface activity and foaming ability of ACL are evaluated; also the performances of ACL modified mortars such as fluidity and mechanical strength are investigated. Test results show that ACL has higher surface tension and lower foaming ability compared with CL, the disperse ability of ACL is stronger than that of CL at higher dosage than 0.5%, the flexural and compressive strength of ACL modified mortars are superior to that of CL modified mortars.     

伺服加速度计用于倾角测量的抗振动实验研究

针对伺服加速度计在振动状态下倾角传感测量出现偏差的问题，采用了两种减振方案来减少这个测量偏差。实验研究表明，使用软性发泡材料减振是一种行之有效的方法。     

Preparation of microcellular polypropylene/polystyrene blend foams with tunable cell structure

By using supercritical carbon dioxide (sc‐CO₂) as the physical foaming agent, microcellular foaming was carried out in a batch process from a wide range of immiscible polypropylene/polystyrene (PP/PS) blends with 10–70 wt% PS. The blends were prepared via melt processing in a twin‐screw extruder. The cell structure, cell size, and cell density of foamed PP/PS blends were investigated and explained by combining the blend phase morphology and morphological parameters with the foaming principle. It was demonstrated that all PP/PS blends exhibit much dramatically improved foamability than the PP, and significantly decreased cell size and obviously increased cell density than the PS. Moreover, the cell structure can be tunable via changing the blend composition. Foamed PP/PS blends with up to 30 wt% PS exhibit a closed‐cell structure. Among them, foamed PP/PS 90:10 and 80:20 blends have very small mean cell diameter (0.4 and 0.7 µm) and high cell density (8.3 × 10¹¹ and 6.4 × 10¹¹ cells/cm³). Both of blends exhibit nonuniform cell structure, in which most of small cells spread as “a string of beads.” Foamed PP/PS 70:30 blend shows the most uniform cell structure. Increase in the PS content to 50 wt% and especially 70 wt% transforms it to an irregular open‐cell structure. The cell structure of foamed PP/PS blends is strongly related to the blend phase morphology and the solubility of CO₂ in PP more than that in PS, which makes the PP serve as a CO₂ reservoir. Copyright © 2009 John Wiley & Sons, Ltd.     

Fabrication of open‐porous PCL/PLA tissue engineering scaffolds and the relationship of foaming process,morphology, and mechanical behavior

Tissue engineering scaffolds should provide a suitable porous structure and proper mechanical strength, which is beneficial for the delivery of growth factor and regulation of cells. In this study, the open‐porous polycaprolactone (PCL)/poly (lactic acid) (PLA) tissue engineering scaffolds with suitable porous scale were fabricated using different ratios of PCL/PLA blends. At the same time, the relationship of foaming process, morphology, and mechanical behavior in the optimized batch microcellular foaming process were studied based on the single‐factor experiment method. The porous structures and mechanical strength of the scaffolds were optimized by adjusting foaming parameters, including the temperature, pressure, and CO₂ dissolution time. The results indicated that the foaming parameters influence the cell morphology, further determine the mechanical behavior of PCL/PLA blends. When the PCL content is high, with the increase of temperature and time, the cell diameter and the elastic modulus increased, and the tensile strength and elastic modulus increased with the increase of the average cell size, and decreased as the increase of the cell density. While when the PLA content was high, the cell diameter showed the same trend, and the tensile strength and elastic modulus were higher, and the elongation at break was lower, and tensile strength and elastic modulus decreased with the increase of the average cell size and increased with the increase of cell density. This work successfully fabricated optimized porous PCL/PLA scaffolds with excellent suitable mechanical properties, pore sizes, and high interconnectivity, indicating the effectiveness of modulating the batch foaming process parameters.     

Biodegradable Foaming Material of Poly(butylene adipate-co-terephthalate) (PBAT)/Poly(propylene carbonate) (PPC)

A biodegradable blend foaming material of poly(butylene adipate-co-terephthalate)(PBAT)/poly(propylene carbonate)(PPC)was successfully prepared by chemical foaming agent and screw extrusion method.First,PBAT was modified by bis(tert-butyl dioxy isopropyl)benzene(BIBP)for chain extension,and then the extended PBAT(E-PBAT)was foamed with PPC using a twin(single)screw extruder.By analyzing the properties of the blends,we found that Young’s modulus increased from 58.8 MPa of E-PBAT to 244.7 MPa of E-PBAT/PPC 50/50.The viscosity of the polymer has a critical influence on the formation of cells.Compared with neat PBAT(N-PBAT),the viscosity of E-PBAT increased by 3396 Pa·s and E-PBAT/PPC 50/50 increased by 8836 Pa·s.Meanwhile,the dynamic mechanical analysis(DMA)results showed that the storage modulus(E’)at room temperature increased from 538 MPa to 1650 MPa.The various phase morphologies(“sea-island”,“quasi-co-continuous”and“cocontinuous”)and crystallinity of the blends affected the spread velocity of gas and further affected the foaming morphology in E-PBAT/PPC foam.Therefore,through the analysis of phase morphology and foaming mechanism,we concluded that the E-PBAT/PPC 70/30 component has both excellent strength and the best foaming performance.     

Effects of Hexane on Protein Profile,Solubility and Foaming Properties of Defatted Proteins Extracted from Tenebrio molitor Larvae

Inclusion of edible insects in human diets is increasingly promoted as a sustainable source of proteins with high nutritional value. While consumer acceptability remains the main challenge to their integration into Western food culture, the use of edible insects as meal and protein concentrate could decrease neophobia. The defatting of edible insects, mostly done with hexane, is the first step in producing protein ingredients. However, its impact on protein profiles and techno-functionality is still unclear. Consequently, this study compares the protein profiles of hexane-defatted and non-hexane-defatted yellow mealworm (Tenebrio molitor) meals and protein extracts, and evaluates the impact of hexane on protein solubility and foaming properties. Results showed that profiles for major proteins were similar between hexane-defatted and non-defatted samples, however some specific content differences (e.g., hexamerin 2) were observed and characterized using proteomic tools. Protein solubility was markedly lower for T. molitor meals compared to protein extracts. A large increase in the foaming capacity was observed for defatted fractions, whereas foam stability decreased similarly in all fractions. Consequently, although the hexane-defatting step was largely studied to produce edible insect protein ingredients, it is necessary to precisely understand its impact on their techno-functional properties for the development of food formulations.     

低含量聚乳酸对微孔发泡聚丁二酸丁二醇酯泡孔结构的改善

通过挤出制备了可生物降解聚丁二酸丁二醇酯(PBS)和3种聚乳酸(PLA)含量(7 wt%、15 wt%和20 wt%)的PBS/PLA共混物样品,采用超临界二氧化碳作为物理发泡剂对样品进行间歇发泡,研究发泡样品的泡孔结构,并分析其形成机理.在120oC发泡温度(Tf)下,借助PLA对PBS熔体黏弹性尤其是熔体强度的改善,获得了分布较均匀、形状较规则、直径较小(平均值约10μm)的微孔;共混物发泡样品的直径分布明显变窄,且符合高斯分布,这归因于细小的PLA相较均匀地分布于PBS基体中.进一步地,研究Tf对PBS和PLA含量为15 wt%的PBS/PLA共混物发泡样品泡孔结构的影响.结果表明,加入15 wt%的PLA使PBS的Tf下限从115oC降低至110oC,并显著改善了较高Tf(120和125oC)下制备的发泡样品内泡孔结构的均匀性.