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Subject classification: 72.20.Ee; 72.80.Ng; S12Percolation approach is used to study the dc hopping conductivity and thermopower in systems with a Gaussian density of localized states typical for disordered organic materials. It is shown that the theoretical methods developed earlier for the description of hopping transport in disordered inorganic solids, such as amorphous semiconductors, can also be successfully applied to description of hopping transport in organic disordered solids, such as conjugated or molecularly doped polymers. Calculations within the percolation approach give results in excellent agreement with those obtained by using a more transparent, though less rigorous approach based on the concept of the transport energy.Introduction In various disordered inorganic and organic materials, the transport of charge carriers at low temperatures is related to incoherent hopping between localized states. While the development of theory for transport in inorganic disordered solids, such as doped crystalline semiconductors, mixed crystals, semiconductor glasses, amorphous and microcrystalline semiconductors, was logically consistent, the situation is different for disordered organic solids, such as molecularly doped polymers, conjugated polymers and organic glasses. Efficient theoretical methods have been developed for inorganic systems. Among the most successful methods, one can note the percolation approach and the approach based on the concept of the transport energy. However, these methods are rarely applied to organic systems. On the contrary, description of hopping transport in organic materials is often based on the ensemble averaging of hopping rates, which is known to be quite inappropriate for the description of hopping processes. Such situation is unsatisfactory, since the physics of hopping processes in organic and inorganic solids and the basic models for their description are rather similar [1,2]. In both cases it is assumed that the hopping rate of a charged carrier G ij between two localized states i and j with energies e i and e j separated by the distance R ij is determined by the standard expression [1]

Conduction mechanism in ionic glasses is still considered one of the great challenges in physics and chemistry of glasses [A. Bunde, K. Funke, and M. Ingram, Solid State Ionics 105, 1 (1998)]. We show that consequent application of the routine percolation theory leads to the consistent description of most puzzling conduction effects for both direct current (dc) and alternating current (ac) conductivity. Moreover, comparison of the theoretical results with experimental data reveals the well-known random-energy model suggested a few decades ago for ionic transport in glasses as a very plausible model. The results provide a general basis for the study of transport phenomena in ionic glasses.

The effects of adding nanoclay to polyamide‐6 (PA‐6) neat resin, and the effects of processing parameters on cell density and size in microcellular injection‐molded components were investigated. In addition, the crystal sizes, structures, and orientation were analyzed with the use of x‐ray diffraction (XRD) and a polarized optical microscope. The standard ASTM D 638‐02 tensile bars for the analyses were molded according to a fractional four‐factor, three‐level, L9 Taguchi design of experiment (DOE) with varying melt temperature, injection speed, supercritical fluid (SCF) concentration, and shot size. It was found that the presence of montmorillonite (MMT) nanoclay greatly reduced the size of the cells and crystals, but increased their density in comparison with neat resin processed under identical molding conditions. In addition, at the sprue section downstream of the machine nozzle, cell size gradually decreased from the part center toward the skin for both the neat resin and the nanocomposite. It was also found that shot size was the most important processing parameter for both the neat resin and nanocomposite in affecting cell density and size in microcellular injection molding components. Weakly preferred crystal orientations were observed on the surface of microcellular injection‐molded PA‐6/MMT tensile bars. Finally, the addition of nanoclay in PA‐6 neat resin facilitated the formation of γ‐phase crystals in the molded components. Polym. Eng. Sci. 45:52–61, 2005. © 2004 Society of Plastics Engineers.

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