جستجوی مقالات مرتبط با کلیدواژه « nanostructure » در نشریات گروه « مهندسی شیمی، نفت و پلیمر »

With the increase of offshore drilling for oil extraction, exploitation and transportation of oil and oil products, on one hand, the risk of oil spills and oily waste water in the marine environment has increased significantly, on the other side, easy, controllable and economical modern methods for oil-water separation has become necessary. Oil spills and oily waste water have a disastrous effect on the ecosystem and environment, and the separation of crude oil associate water by traditional methods also requires the use of expensive chemicals. During the past few years, the development of special wetting materials for the separation of water - hydrocarbons has attracted much attention from researchers, and many researchers have created materials with selective wettability that are hydrophobic/oleophilic or hydrophilic/oleophobic, which can be used to separate a selective phase from an oil (hydrocarbons, petroleum, fats)-water mixture. In the current research, the possibility of using nonostructure tungsten oxide as a coating on stainless steel mesh is investigated in order to check the possibility of changing the wettability of the mesh and using it to selectively separate one of the water or hydrocarbon phases from their mixture. For this purpose, both sides of the 5 micron stainless steel mesh were coated with nanostructured tungsten oxide by hydrothermal method. The measurement of the contact angle of water and oil on this mesh showed that the wet coated mesh is hydrophilic in air and becomes oil-repellent when submerged. Due to such wetting behavior, this mesh can be used to separate oil and water mixtures. A piece of stainless steel mesh was placed in an autoclave with an inner Teflon coating containing 250 mL of an aqueous solution of hydrated sodium tungstate, hydrochloric acid, and oxalic acid. The autoclave was placed in an oven at 180°C for 3 hours. After cooling to room temperature, washing with ethanol and Water and drying, it was roasted in an electrical furnace of 350°C for about an hour. The wettability of the hydrophilic and underwater oleophobic filter was also evaluated by measuring the static contact angle. The wetting behavior mechanism of the constructed filter is mainly related to the hierarchical structure of the nano-spherical coating of the surface and its relative roughness, and hence the hydrophilicity of the surface increased. The EDS analysis of the coating formed on the mesh and the FE-SEM and TEM images of the fabricated filter all indicate the formation of WO3 nanostructured coating on both sides of the stainless steel mesh.

Molecular self-assembly is the spontaneous aggregation of molecules or macromolecules into supramolecular structures with non-covalent interactions. This phenomenon is an interdisciplinary research topic that has a lot of potential applications in various fields. One of the main driving forces of molecular self-assembly is the existence of molecular amphiphilicity in the system which can cause microphase separation and create complex and stable nanostructures. Self-assembling peptides are one of the most important classes of molecules with the ability to self-assemble. The rich self-assembly behavior is observed in systems of peptides, due to the simultaneous presence of different interactions (such as electrostatic interaction, hydrophobicity and hydrogen bond) in systems consisting of them and the diversity of their molecular configuration. Better understanding of peptides self-assembly enables the better design of peptides to form functional nanostructures. In this review article, at first, peptide self-assembly and its importance are stated. Then, some examples of self-assembling peptides which have attracted the interest of scientists for various reasons, such as cyclic peptides, amphiphilic peptides, ionic complementary peptides and some other examples, are explained. Also, some important applications and benefits of peptides self-assembly, which include nanoscale construction, tissue engineering, drug delivery, applications in biosensors, and the study of conformational diseases, are reviewed.

Amphiphilic polymers are polymers composed of hydrophilic and the hydrophobic structural units. The hydrophilic groups can be carried by azo chromophores or more commonly attached on the other parts of the polymers. Amphiphilic azo polymers include homopolymers, random copolymers, block and grafted copolymers, star-like polymers, tadpole-shaped polymers and dendritic polymers. Amphiphilic polymers can be considered as synthetic counterparts of ubiquitous amphiphilic compounds in nature such as lipids and proteins. Amphiphilic polymers, especially block copolymers, can form a variety of order aggregates, such as micelles and vesicles. When compared to micelles from traditional surfactants, polymeric self-assemblies have recently been recognized for advantages such as superior stability, toughness and micellization depending on selective solvents. In many instances, the morphology of the polymeric self-assembly and its application are closely related. Thus, controlling the morphology of the assembled structures from block copolymers is of great practical value. Self-assembled polymeric materials with well-defined structures such as spheres, rods, vesicles, lamellas and other nanostructures have attracted increasing interests recently due to their potential applications in biomedical engineering, electronics and optics. The self-assembled structures of azo polymers can undergo structural changes both in solution and in the solid state when triggered by light or other external stimuli. Understanding the self-assembling processes can lead to the development of photo responsive materials with new functions for future applications. In this review the self-assembly of amphiphilic azo polymers in solutions are reported.

Metals have been replaced by polymers in many applications due to the favorable mechanical and chemical properties of polymers. But flammability and combustibility of polymeric materials are their major shortcoming that limiting their applications. Beside these concerns, toxic gases produced during combustion of polymeric materials increase the fire hazards. To overcome these problems, numerous attempts have been made to improve the flame retardancy of polymeric materials. Pyrolysis of polymer by thermal treatment leads in the formation of highly reactive •O, •H and •OH radicals. The first two species are mainly converted to the hydroxyl radical (•OH) and the •OH radical affords the required heat for fire propagation in an exothermic reaction. Thus, •OH prevention would result in avoiding fire spreading. Halogenated flame retardants produce halogen radicals or volatile phosphate retardants releasing •HPO, •PO or •PO2 radicals, which can inhibit •OH radical formation. For reducing flammability of the polymers, it is possible to either change the polymer structure or create a protecting layer on the surface of polymers and textiles using surface coating. This paper aims to give a short overview on fundamentals of polymeric materials combustion, modes of action of flame retardants in both vapor and condensed phase including: heat sink, barrier layer, intumescent effect, prevention of flame propagation, as well as, recent developments in nanostructure flame retardants. We also highlight the applications of flame retardants in polymeric materials and composites, toxicity of flame retardant and the fire retardancy tests, which have been used to describe fire behavior, nature and modes of flame retardant materials.

In this paper, zinc oxide nanowires with a thin layer of tin(II-IV) oxide p-n multilayer Core–Shell structure are synthesized on the silicon substrate. Synthesized nano-structures were characterized by XRD and FE-SEM methods. Studying X-ray diffraction pattern of the multilayer core–shell structure shows that Sn(II) and Sn(IV) are formed in this composition. FE-SEM image of the core–shell structure shows that a thin film of tin oxide with 50 nm thickness is formed on the ZnO nanowires. In addition, the sensing behavior of the synthesized nano-structures to ethanol vapors and CO is studied. Results reveals that Core-Shell structure shows up to 3.5 times higher sensitivity to 500 ppm of ethanol vapors in compared with the simple ZnO nanowires. Moreover synthesized Core-Shell structure has negligible responses to CO.

In recent years, much attention has been paid to aerogel materials (especially carbon aerogels) due to their potential uses in energy-related applications, such as thermal energy storage and thermal protection systems. These open cell carbon-based porous materials (carbon aerogels) can strongly react with oxygen at relatively low temperatures (~ 400°C). Therefore, it is necessary to evaluate the thermal performance of carbon aerogels in view of their energy-related applications at high temperatures and under thermal oxidation conditions. The objective of this paper is to study theoretically and experimentally the oxidation reaction kinetics of carbon aerogel using the non-parametric kinetic (NPK) as a powerful method. For this purpose, a non-isothermal thermogravimetric analysis, at three different heating rates, was performed on three samples each with its specific pore structure, density and specific surface area. The most significant feature of this method, in comparison with the model-free isoconversional methods, is its ability to separate the functionality of the reaction rate with the degree of conversion and temperature by the direct use of thermogravimetric data. Using this method, it was observed that the Nomen-Sempere model could provide the best fit to the data, while the temperature dependence of the rate constant was best explained by a Vogel-Fulcher relationship, where the reference temperature was the onset temperature of oxidation. Moreover, it was found from the results of this work that the assumption of the Arrhenius relation for the temperature dependence of the rate constant led to over-estimation of the apparent activation energy (up to 160 kJ/mol) that was considerably different from the values (up to 3.5 kJ/mol) predicted by the Vogel-Fulcher relationship in isoconversional methods

In the present study, potato starch nanobiocomposites films plasticized with glycerol and containing four different montemorrillonite loading (0, 1, 3 and 5 wt% starch) were prepared via solution casting method. Differential scanning calorimetry (DSC) confirmed that the melting point and glass transition temperatures were increased and heat stability of the nanocomposites were improved. Upon 5% MMT loading, the glass transition and melting point of PS film were increased from 185.6 and 282.9 to 203.5 and 304.6, respectively. Colorimetry and UV-Vis spectroscopies were employed to evaluate the UV and visible-shielding efficiency of the PS-MMT nanocomposite films. Incorporationof 5% MMT did not have significant effect on color parameters (L*, b* and a*), color differences (E) and whitness index, but decrease yellow index of the films. Presence of 5% MMT in film formulation successfully blocked more than 82, 38 and 11% of UV-A, UV-B and UV-C lights, while opacity and transparency of the films were unchanged. Investigation of chemical structure of nanobiocomposite films by Fourier-transform infrared spectroscopy (FT-IR) and Atomic force microscopy (AFM) and revealed the hydrogen bonds interactions between MMT and starch and uniform dispersion of MMTplatelets in the starch matrix, respectively. Also, AFM topography images were used to study the surface morphology and roughness of starch films. Plasticized starch (PS) film had smoother surfaces and a lower roughness parameter. Adding MMT to starch matrix caused to increase the average roughness (Ra) and the most frequently quantitative parameters of roughness (Rq) from 32.4 nm and 39.9 nm in PS film to 119 nm and 147 nm in PS-5%MMT, respectively. The AFM phase images described the uniformity of the starch-MMT mixtures.

Nowadays polymeric materials are an important part of human life around the world. Polymer materials are used to manufacture different substances from commonly used instruments to complex and precise scientific tools. Additives are commonly used in polymer industry to improve the properties of polymer materials and reduce their costs. Nano-scaled additives، due to their small dimensions and enormous surface area، produce improved properties compared to larger structures. Zinc oxide (ZnO) is an n type inorganic semiconductor with a band gap energy of 3. 37 eV which due to good stability، environmental friendly feature، high ultraviolet absorption and low cost are commonly used in polymer industry as inorganic additive. Therefore، ZnO nanostructures can produce various hybrid، blends or combination of composite materials in polymer matrixes and improve their properties. In this study، applications and the role of various nanostructures of zinc oxide in polymer productions are reviewed.

The main objective of this study was synthesis of proper ion sieves for selective extraction of lithium from solutions. Therefore, in the first stage, MnO2 nanostructure oxidizer, spinel type Li-Mn-O precursor and lithium ion sieve of MnO2 nanorods were synthesized under two different conditions of a controlled hydrothermal method and the structure characteristics and ion exchange ability were studied by XRD and SEM analysis. Then, the Li+ adsorption isotherm, kinetics and selectivity measurements were investigated, separately. The produced MnO2 nanorods, with the size about 30–60 nm in diameter and 0.5–2 μm in length, were found to have a remarkable lithium ion-sieve property. In addition, the study on equilibrium distribution coefficients (Kd) of synthesized ion sieve indicating high selectivity for Li+ ions in the presence of other coexisting ions such as Na+, K+ and Mg2. In addition, adsorption isotherm followed the Langmuir equation and the maximum monolayer uptake capacity of Li+ ions was 4.45 mmol/g estimated in the optimized condition. Consequently, it appears that the synthesized ion sieves have the adequate potential for extraction lithium from brines or seawater.

Aerogels consist of low density open-cell foams with large void spaces of nanometer pore size and they are composed of sparsely semi-colloidal nanometer sized particles forming open porous structures. The high porosity and nanometer pores and particles of aerogels have interesting properties such as low thermal conductivity and low sound transmission. The main disadvantages of conventional method for preparation of aerogels are high cost of resorcinol، long preparation time and high cost of supercritical drying device. The previous research showed that sol-gel polymerization lower than temperature of normal boiling point of solvent، and a 50°C difference in temperature between the novolac resin synthesis and the temperature of curing novolac resin is the main reason for long sol-gel polymerization time. A low temperature polymerization is used for prevention of solvent evaporation and foam blowing process. The objectives of this research were to reduce time and cost in the production process of organic aerogels. To reduce the cost of raw materials، the reaction of a low cost commercial novolac resin and HMTA were used for preparation of the gel. To reduce gelation time، the polymerization was performed in an atmosphere of solvent saturated-vapor as a novel method to restrict sol-gel polymerization of curing novolac resin at appropriate temperature. The investigation showed that the polymerization carried out in solvent-saturated vapor did help to reduce polymerization time to 5 h from 5 days duration in conventional reactions without any visible shrinkage under ambient pressure and temperature. Furthermore، the compressive strength increased by one order of magnitude besides other achievements. Aerogels comprise class of low density open-cell foams with large void space، nanometer pore size and they are composed of sparsely semi-colloidal nanometer sized particles forming open porous structures. The high porosity and nanometer pores and particles of aerogels results interesting properties such as low thermal conductivity and low sound propagation speed. The main disadvantages of conventional method for preparation of aerogels are high cost of resorcinol، long preparation time and high cost of supercritical drying device. The previous research showed that sol-gel polymerization lower than temperature of standard boiling point of solvent، also 50 °C differences between appropriate temperatures for curing novolac resin and the performed temperature of curing novolac resin is the main reason for long time of sol-gel polymerization. The low temperature polymerization used for prevention of solvent evaporation and solution blowing. The objectives of this research are reducing time and cost in the production process of organic aerogels. To reduce the cost of raw material، the reaction of low cost commercial novolac resin and HMTA were used for preparation of gel. To reduce gelation time، the polymerization in saturated atmosphere of vaoer of solvent as a novel method for domination of restriction sol-gel polymerization temperature curing novolac resin in appropriate temperature is proposed. The investigation shows that the polymerization in saturated atmosphere of solvent vapor، reducing time of polymerization from 5 days to 5 hours without any observable shrinkage in ambient atmosphere. Furthermore، the compressive strength increased one order and it’s other important achievements of this research.