فصلنامه بسپارش
سال چهارم شماره 3 (پیاپی 12، پاییز 1393)

Electrically conductive adhesives have recently received a lot of attention by the researchers as a potential substitute to lead-bearing solders. The two main classifications of electrical conductive adhesives based on conductive path are isotropically and anisotropically conductive adhesives. The main application of isotropically conductive adhesive is in electronic industry. Studies in the field of conductive adhesives have shown that these matrials have numerous advantages compared to traditional solder technology in better environmental operation، lower process temperature، more flexibility and simpler processability. To achieve electrical conductivity، the volume fraction of conductive filler in an ICA must be equal to or slightly higher than the critical volume fraction that is called percolation threshold. In this paper discussions are extended to electrical conductivity mechanism in conductive adhesives، different matrices of conductive adhesives and conductive fillers، their effects on the electrical conductivity of conductive adhesives، and developed methods in electrical conductivity processes.

Fabrication of polymeric biodegradable products has gained much attention in solving environmental pollution، safety and health problems. Furthermore، the biodegradable polymers have been developed in medicine and drug delivery fields to avoid post operation removal of the implants at the end of their functional life. This article introduces some conventional biodegradable natural and synthetic polymers in medical and drug delivery applications. In this respect، some factors which may affect degradation and drug delivery profiles and also، commercial devices based on these polymers are introduced and discussed. Finally، the physical and mechanical properties of biodegradable polymers are compared so to be able to choose the most suitable systems in relation to their specific applications.

Carbon foams based on polymeric resins and precursors، due to such properties as low thermal conductivity، good thermal stability، high specific strength and low density، are used in different industries، especially aerospace، as high temperature insulator، thermal shields and material protecting structures. In this study، current methods of preparing carbon nanostructures are introduced. By a polymeric template method polymeric foam and aerogel are produced using the foaming process of polymer precursors of phenolic resins and sol-gel formation. Then، the polymeric nanostructure transforms into carbon foams in a carbonization process. Nevertheless، structural destruction due to oxidation reaction has limited the application of these nanostructures at temperature higher than 400˚C in the air atmosphere. In the next step، the oxidation mechanism of carbon foams is studied. In addition to control and optimization of the process parameters such as temperature of carbonization process، the oxidation-resistant material such as silicon carbide compounds can be used to improve the oxidation resistance of carbon aerogel. Finally، the role of formation of a silicon carbide layer on the surface of carbon pores wall، and the formation of SiC bonds in the carbon aerogel structure by alkoxy silane polymers، as efficient methods that increase the oxidation resistance of the carbon foam، is investigated.

Controlled morphology of nanoclay layers within nanopolymer particles is a very important topic for most industries، especially in food and medical packaging. Films prepared from these nanocomposites showed improved physical and mechanical properties and high impermeability towards gases، vapors، water and even oils and hydrocarbons. One successful method to design these systems is by miniemulsion polymerization which is useful for encapsulation of all materials (solids، liquids and gases) within polymer particles، provided a suitable choice of surfactant is made with optimum HLB. Another method is «pickering» or soap free polymerization، which can also be performed in the form of miniemulsion. In this new method nanoclay is used instead of a surfactant، and therefore، final films will show honeycomb structures applicable in impermeable packaging purposes.

Oral route is the most preferred route by medical practitioners and manufacturer due to patient''s acceptability. Fast dissolving tablets have been developed because of low bioavailability and weak acceptability of pediatric and geriatric patients who experience difficulties in swallowing the traditional oral solid-dosage forms. As a result of low bioavailability of fast dissolving tablets، oral thin films have been developed. These films are the most advanced form of oral solid dosages due to more flexibility and comfort and include a polymer، an active ingredient، flavoring and coloring agents، and surfactants. Oral thin films are made mostly with solvent casting and hot melt extrusion. Oral strips are evaluated for various attributes such as thickness، disintegration and dissolution study and mechanical properties. These films have many applications such as taste masking، fast and sustainable drugs release. At the moment، film formulations with encapsulated drugs are commercially launched using the oral film technology. It is predicted that the value of the overall oral thin film market will grow significantly.

The population balance methods are employed to model sulfur vulcanization process of natural rubber (NR) compounds. The proposed primary models are so far empirical and/ or mechanistic and have various limitations that do not embody all reactions and formation of all the process products. In this work، the models are developed، using population balance methods that explicitly acknowledge the nature of all the formed species and the various types of reactions. The kinetic model can also accurately describe the complete cure responses including the scorch delay، cure time and the reversion for a wide range of compositions، using a single set of rate constants. In addition، the concentration profiles of all the reaction intermediates as a function of polysulfidic lengths are predicted. The population balance model provides a quantitative framework for explicitly incorporating mechanistically reasonable chemistry of the vulcanization process of different natural rubber compounds.

Polymer nanocomposites are very important materials of recent times، because of their significantly enhanced mechanical properties and thermal stability versus neat polymers or conventional polymer composites. A nanocomposite describes a two-phase material where one of the phases has at least one dimension in nanometer range (1-100 nm). The use of natural fibers as reinforcements in polymers and composites has attracted much attention due to the environmental concerns. Nanocellulose is one of the most abundant renewable natural polymers، exhibiting a number of well-known advantages، such as low cost، worldwide availability، biodegradability، high stiffness and good mechanical properties. Particular attraction is its low density that leads to high specific mechanical properties. Polyvinyl alcohol is water soluble، semi-crystalline، fully biodegradable، non-toxic، and biocompatible. Therefore، nanocellulose can be used in reinforcement of polymer nanocomposites.