فصلنامه بسپارش
سال هفتم شماره 4 (پیاپی 25، زمستان 1396)

Carbon materials and specially graphene, due to their unique electrical, thermal, mechanical, optical and electrochemical properties have been very popular to combine with other materials and formation of nanocomposites in recent years. Graphene-based polymer nanocomposites are one of the most common polymer-based nanocomposites. They have much better thermal, electrical, mechanical, optical and electrochemical properties than pure substances i.e., polymer and graphene. Polyaniline is a useful conducting polymer that has been widely used in electronic devices, optical and electrochemical applications owing to its low cost, good environmental stability, interesting electroactivity, good electrical conductivity and easy preparation. Carbon-based polyaniline nanocomposites have attracted a great deal of interest due to their new properties or enhanced performance during the past few years. In recent years, synthesis and application of polyaniline/graphene nanocomposites is one of the most important strategies to improve organic solar cell functions. In this review, after a brief introduction on organic solar cells, the properties of polyaniline, carbon materials and graphene, references have been made to the most common types of carbon-based polyaniline nanocomposites, specially polyaniline/graphene. Some of the research done in the last few years is with the aim of designing organic solar cell in order to improve their performance in the past few years.

Nanosponges are generally porous materials with nanometer-sized pores that are capable of loading molecules in their cavities. Physical-chemical properties of cyclodextrins in the late 19th century were fundamental. Cyclodextrins have been used as practical and economical advantages to improve the physico-chemical properties and medicinal properties such as increased solubility, stability and bioavailability of the drug molecules. Cyclodextrin-based nanosponges are porous and insoluble in aqueous solution which can be in crystalline or amorphous structure and spherical shape and can be formed by different types of cyclodextrins and their derivatives. Dimension and the polarity of porous polymers are affected by types of cyclodextrins, cross-linkers and degree of cross-linking. In addition, depending on the type of cross-linker, neutral or acidic polymeric nanosponges can be synthesized. These polymeric nanosponges have hydrophilic and hydrophobic parts that induce capability to entrap wide range of lipophilic or hydrophilic molecules by forming inclusion and non-inclusion complexes. These complexes are formed between molecules without making any coordination bond and the driving force to induce electrostatic, Van der Waals and hydrophobic interactions, release of conformational strains and charge-transfer interaction. Characterization of these types of polymeric nanosponges are performed by different methods like microscopy, solubility studies, zeta potential, DSC and FT-IR spectroscopy. Various factors such as type of polymer and guest, temperature and method of preparation can affect the formation and the performance of nanosponges. These nanosponges are used in a variety of fields such as pharmaceutical, textile, catalysts, cosmetics, agriculture and other areas.

Various energetic binder materials are being investigated in an attempt to increase the energy level of explosives and propellant formulations. The nitrate ester prepolymer poly(glycidyl nitrate) (PGN) is an energetic prepolymer that is used as a binder in propellant and explosives formulations. PGN is the most energetic polymer among the energetic polymers. Three steps in PGN synthesis from glycerol are nitration, cyclization and polymerization. Glycerol is a by-product of biodiesel industry that needs purification before any chemical reaction. In this paper various preparation methods of PGN have been investigated and optimum methods are introduced. The compatibility of PGN with energetic materials in propellants or explosives is the most important property of PGN in practical applications. PGN is compatible with energetic plasticizers. Therefore, the maximum decrease in glass transition temperature (Tg) has been seen by the addition of BuNENA energetic plasticizer and maximum increase in flow behavior (lowering of viscosity) has been observed by the addition of DEGDN energetic plasticizer. The cured PGN undergoes self-decomposition or de-curing (caused by chain scission) at room temperature due to the proximity of the terminal hydroxyl groups of the polymer to the nitrate ester groups. Research works showed that the most effective way of removing the de-curing of PGN after curing is to modify the end groups of PGN with potassium carbonate. In this paper, different formulations of PGN-based propellants are studied and their combustion, ballistic and mechanical properties have been compared with typical propellants. Finally, a PGN-based propellant formulation, mixing and preparation method is introduced.

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.

The term "physical aging" was first coined by Struik to separate the relaxation of glassy polymers from those of other time-dependent phenomena such as chemical aging. Glassy amorphous polymers are in non-equilibrium states below their glass transition temperature (Tg). A glassy polymer gradually moves toward its equilibrium state with time when it is kept below Tg. The amorphous polymer can be heated again, known as thermal rejuvenation, to erase its memory and return the polymer to its non-equilibrium state. Many experimental and mathematical simulations have been conducted so far to investigate the rejuvenation of glassy polymers by applying mechanical stresses, i.e., mechanical rejuvenation. However, to turn a glassy polymer to its non-equilibrium state by imposing mechanical stress on the polymer is still under debate. Some scientists agree and others disagree with the idea. It was found that small strains can over-age an amorphous polymer, however, large strains can cause mechanical rejuvenation. In pre-yield regime, straining a polymer will not change its state and mechanical rejuvenation does not occur. In post-yield regime, straining will change the state of the system, but, it is totally different from that of an unaged sample. In this article, our aim is to present some information on physical aging and also discuss a controversial topic of thermal and mechanical rejuvenation of glassy amorphous polymers by reviewing the recently published papers.

Oligomerization of higher α-olefin monomers for production of fuels, synthetic lubricants and oils has attracted a lot of interest in recent years. The Poly(a-olefin)(PAO) fluids are usually produced from 1-dodecene (C12) and 1-decene (C10) as starting materials. Various synthetic protocols are employed in oligomerization processes. They are mostly based on (1) conventional Lewis acid such as AlCl3 and BF3/donor systems, (2) metallocene catalysts based on Ti, Zr, and Hf metals and cyclopentadienyl and indenyl ligands, (3) Cr/silica catalysts known as Phillips catalysts and (4) ionic liquids as active precursors. PAOs are graded and differentiated for different applications by their kinematic viscosities at 100 °C (or KV100). The most usual grades have kinematic viscosity between 2 cSt to 100 Sct. Due to the importance of PAO lubricants in industry, this paper is dedicated to this material. In this regard, after a short introduction on conventional lubricants, we mainly focus on PAO based synthetic oils. In this regard, first conventional catalysts in α-olefin oligomerization will be introduced in brief. Then, PAO properties, uses, and analysis methods will be reported and aimed to further raise interest in PAO research in the country

In the last decades, nanostructured materials, especially carbon nanoparticles, have attracted much attention because of their widespread applications in different areas. Spherical shape, chemically active surface as well as remarkable mechanical properties make nanodiamond an ideal candidate for improving polymers properties. Poor dispersion and agglomeration are major problems in achieving improved properties in polymer nanocomposites. To deal with this problem, various methods are considered for surface modification of nanodiamond and wet chemical method is used in most cases. Nanodiamond, due to its remarkable properties, is incorporated in many polymer nanocomposites with thermoplastic and thermoset matrix. Using nanodiamond has improved nanocomposites properties such as mechanical and wear properties. Using as-received nanodiamond and surface modified nanodiamond in thermoplastic matrix are reported and the mechanical properties are improved in both cases. Most of the works on thermoset matrices are focused on epoxy. Incorporation of high weight fractions (35 wt%) and low weight fractions (below the 1 wt%) of nanodiamond are investigated, showing improvements in mechanical and tribological properties.

Electrofusion is among several techniques to join polymer pipes using special fittings. In electrofusion, polymer pipes and fittings are joined together through wires that are installed in their structures. The main difference between the two techniques of common heat fusion and electrofusion is in heat transfer mechanism. In heat fusion, a heating device is used to melt interfaces of the pipes and polymer fitting. In electrofusion, there is internal melting through a conductive substance inside the weld interface or melting by a conductive polymer. The heat is produced through electric current contacting conductive materials installed inside the fittings. In order to have desirable materials and useful products, design and installation is based on proper joints having passed good welding testing conditions. Methods of welding and joining are different and depend on parameters such as the type of polymer pipe, polymer structural changes, requirements imposed on interior or external pressure of the pipe, decreasing leakage, prohibition of strokes and longitudinal impact loads, performance and operational conditions, construction and installation operations and types of products that have to be joined together. In this paper a survey of electrofusion process on dimensional strength, parameters accounted for in joint's integrity and stability, mechanisms related to macromolecular magnitude of joint formation, diffusion of molecules in polymer structures, and formation or rupture of chemical bonds through different experimental and computational methods are reviewed based on researcher's findings.