فصلنامه بسپارش
سال ششم شماره 1 (پیاپی 18، بهار 1395)

In the last two decades, the application of lignocellulosic fibers, as reinforcing agents of thermoplastic composites, in automobile, packaging and building industries have significantly grown due to recent environmental issues. Lignocellulosic fibers with desirable properties such as low density, specific strength, high modulus, renewability, biodegradability, high availability and low cost are appropriate alternatives to synthetic fibers. Therefore, due to these unique characteristics, in this study, we have investigated the properties and fabrication methods of nanocellulose fibers.

Poly(vinylidene fluoride) (PVDF) has received great popularity because of their excellent mechanical properties, chemical stability and ferroelectricity. It has a simple chemical structure and is crystallized in five different phases including α, β, γ, δ and ε. Since β-phase has a greater dipole moment, its pyro and piezoelectric activity is very high. To date, various techniques including mechanical stretching, applying high pressure, cooling, use of polar solvents, injection molding, polarization under high strain and electrical field, its copolymers derivatives, addition of nano fillers and electrospinning have been discovered and provided in order to increase the β-phase proportion in poly(vinylidene fluoride). The purpose of this review article is to collect and study numerous research reports performed to prepare and characterize of PVDF/multi-walled carbon nanotubes, PVDF/nanoclay and PVDF/other nanofiller polymeric nanocomposites. For updating and helping the readers in order to understand the importance of nanotechnology, it has been tried to study and gather data from authentic journals in the field of PVDF nanocomposites. Moreover, the focus of the present review article is how it is possible to increase the amount of β-phase of PVDF and consequently its piezoelectric property.

Shape-memory polymers ()SMPs) are an important class of intelligent materials which have attracted much attention in recent years, due to their prominent characteristics such as light weight, high recovery strain, low cost and ease of fabrication compared to shape-memory alloys. Shape-memory polymers can be deformed and fixed into a temporary shape, and recover their original permanent shape only upon asmall variation in environmental conditions such as heat, electric field, magnetic field, pH, light, ions, solvent, sonic field, enzyme, water and etc. This capability results from a combination of the polymer structure and the polymer morphology together with the applied processing and programming technology which is simply termed asshape-memory effect ()SME). Polymers that exhibit SME are an important class of materials in medicine, especially for minimally invasive surgery deployment of devices. These unique properties make SMPs suitable and useful materials for various technological applications, including the manufacturing of smart biomedical devices. In this review, recent developments and common limitations in the design of SMPs especially for biomedical applications are discussed.

The use of triglycerides to produce adhesive and composite materials is taken into consideration, instead of fossil-based feedstock due to their economic and environmental benefits. Several methods have been studied and Investigated for production of resins from triglycerides and epoxidation of modified resins. The reactive sites on the triglyceride structure such as double bonds, allylic carbons, ester groups and etc. are able to enter into chemical reactions. Triglycerides can be converted into resins by reactions such as, free radical polymerization, ring opening polymerization, poly-condensation and modification by anhydrides, polyisocyanates and diacids. Also, various materials such as percarboxylic acid, organic or inorganic peroxides and halohydrins can be used for epoxidation of triglycerides. Epoxidation of triglycerides resins lead to branching and cross-linking of chains. Consequently, the bond sresulting from epoxidized triglyceride resins are more resistant. The results showed that epoxidized triglyceride resins can be used to produce more resistant composite and wood panels compared to those produced from inorganic resins such as urea formaldehyde.

Epoxy systems have many industrial applications because of their excellent adhesion, high chemical and thermal resistance, interesting mechanical and physical properties and so on. Generally, epoxy systems are categorized as two-component and one-component systems. In this report, different kinds of one-component systems, hardeners and accelerators are studied. In single-component epoxy systems, the resin and curing agent have already been mixed together and are supplied as a single product. There is no significant reaction between the resin and curing agent, unless environmental conditions are favorable to trigger a reaction. Several new methods have been developed over the years to provide single-component systems with a long shelf life and good performance characteristics. Among these methods, the most effective and widely used method is the use of latent curing agents especially, dicyandiamide, which providesnot only high mechanical properties, but also a shelf life longer than any othersingle-component curing agents. Curing reaction takes place at high temperatures, when latent curing agents are used. Hence, to achieve a desired curing system which can be activatedat lower temperature, one or more compatible accelerators are required with achosen curing system. The selection of an accelerator depends on the desired final properties. Typically, for a single-component system with the dicyandiamide curing agent, substituted ureas are the most appropriate accelerators. Among substituted urea, 4,4-methylenebis()phenyldimethyl urea A) ()grade isomer) with commercial name of Omicure U-25 has the best performance, since it provides a set of optimized properties.

The formulation, gun and safety of the sticky foams as a non-lethal weapon are studied. Sticky foams are one of the polymeric materials which have specific properties such as low density, high adhesion and suitable tensile strength. Sticky foams are a type of pressure-sensitive adhesive ()PSA) which are mixed with a foaming agent. Sticky foams are prepared by blending different compounds such as thermoplastic elastomers, tacky resin, surfactant and foaming agent. The foams are stored in tanks under pressure and released to atmospheric pressure at a desirable time. In addition, increase in the amount of the tacky resin and softener will lead to enhancement in the adhesion properties. The weapon needs a pressure vessel for storing the sticky foam formulation in order to release the sticky foam. Weight, throwing power and sticky-foam storage capacity are important parameters associated with asticky foam gun. Contacting the skin surface of the bodywith a sticky foam leads to irritation and local inflammation. Long-term continual exposure may lead to cancer due tooil derivatives and heavy hydrocarbons present in the sticky foam formulation. Sticky foams have various applications in obscuring the military sites, weakening the visibility of the military tools such as tanks and combatting insurgency in prisons and street demonstrations.

To date, by development of inorganic polymer technology and the use of these materials in electronics industry, medical, aerospace, military and other fields, the researchers have shown a growing interest in the synthesis of novel inorganic polymers having specific physical, chemical and electrical properties in their laboratories and industry research and development centers. Polyphosphazenes are one of the most wide-ranging groups of inorganic polymer families, until nowat least seven hundred variations of these polymers have been synthesized. These polymers with unique properties such as self-extinguishing, hydrophobicity, and biocompatibility have attracted the attention of many researchers in polymer science. All these unique properties are related to the main chain of polyphosphazene which has two side groups in each repeating unit; that they can be altered with a wide range of diverse organic compounds simply to indicate various properties. This study aims to contribute to this growing area of research by outlining the structure, synthesis methods and applications of polyphosphazenes.

Recently, nanostructured polymers have attracted much attention. One category of these nanostructured polymers is polymersomes, also termed as polymer vesicles. Due to their unique properties such as mechanical stability, tunable permeability and elastic properties, polymersomes have potential applications in designing and synthesis of drug delivery systems, nanoreactors, smart catalysts, and etc. In this article, polymersomes and their essential aspects have been introduced and their broad applications have been addressed.

The photo-electronic devices are widely used in electronic industries. These devices are made from specific organic and inorganic materials with unique properties. Most of these materials are highly sensitive to atmosphere conditions such as moisture and oxygen.We need to protect these devices from harmful atmospheric conditions. Encapsulation layer with suitable barrier properties against oxygen and moisture and proper mechanical properties to protect photo-electronic devicesand to improveits longevity. The photo-electronic devices emit or absorb light. For this reason the encapsulation layer should be transparent in the visible wavelength range to support a photo-electronic device with its satisfactory performance. Therefore, the polymeric materials are the best candidates for this application to afford the required properties. In this article, to find a suitable polymer for use as an encapsulation layer, the properties of different kinds of polymers are investigated.