Permeability of Polymer/Clay Nanocomposites
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Permeability of Polymer/Clay Nanocomposites Georgios A. Choudalakis and Alexandros D. Gotsis Department of Sciences, Technical University of Crete, GR-73100, Chania, Greece ABSTRACT Because of their large interfacial area, the presence of nanoplatelets in the polymeric matrix decelerates the process of diffusion of gases through the material. The particles are impermeable barriers to the diffusing molecules, forcing them to follow complicated paths, increasing, thus, the diffusion length. The barrier properties of the nanocomposites depend on the properties of the polymeric matrix, the volume fraction of the nanoplatelets, their aspect ratio, their orientation, and their interactions with the matrix. The mobility of the molecules is hindered by the crystallinity but it is facilitated by the free volume within the material. The size and shape of the free volume holes in the polymer affect, thus, the rate of diffusion. Interactions between the nanoparticles and the matrix may lower the barrier properties because they may increase the free volume in the material. The estimation of free volume in the nanocomposite is important for the proper choice of components and the manufacturing of nanocomposite coatings with optimum barrier properties. Detailed information about the diffusion mechanisms at atomic and molecular levels can be obtained using the approach of free volume. INTRODUCTION Polymer/clay nanocomposites have attracted much attention in recent years due to their potential performance advantages on mechanical and barrier properties. Nanocomposites as barrier materials are used primarily in food, pharmaceuticals and electronics packaging, in protective coatings and in gas storage tanks. Oxygen and water are the most unwanted penetrants for the food industry, as they affect the quality and shelf-life of the products. Water can also cause cracking of silica or corrosion on metal surfaces. The carbon dioxide influences the quality of several drinks, while atmospheric pollution gasses, such as sulfur dioxide, causes serious degradation of exposed marble surfaces. The permeation of light gases, e.g. hydrogen, should be prevented in order to keep them in a tank for a long period. Although some of the above requirements are fulfilled by multilayer structures of conventional materials, high cost and, often, environmental factors, such as, e.g., organic solvents, create the need of alternative solutions. Nanocomposite materials are two-phase systems that consist of a polymeric matrix and dispersed inorganic particles of nanometer scale. The most common inorganic particles belong to the family of 2:1 phyllosilicates. The layer thickness of each platelet is around 1 nm, and their lateral dimensions may vary from 30 nm to several microns. The layers are located on top of each other like the pages of a book. Van der Waals gaps exist between the layers, called galleries. Two particular characteristics of layered silicates play an important role in the creation of nanocomposites: The first is the ability of silicate sheets to d
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