Papers by Keyword: Free Volume

Abstract: The temperature dependence of the diffusion coefficient in metallic glass-forming systems do not follow the Arrhenius behavior over a wide temperature range. Instead, it exhibits a kink behavior at around the glass transition temperature. Some researchers associate this behavior to the difference in the diffusion mechanism operating in the glassy and the supercooled liquid state, whereas others do not support this view. In addition, usually, the temperature dependence of the diffusion coefficient is analyzed by splitting the temperature range into two regions, above and below the glass transition temperature. In the present study, we developed an analytical theory that describes the continuous variation of the diffusion coefficient across a temperature where the kink behavior is observed. According to the theory, the kink behavior arises from the freezing of free volume available for diffusion by lowering the temperature. A connection to the vacancy mechanism of diffusion has been also pointed out.

Abstract: The Zr-Cu-Al bulk amorphous (BA) alloys, which have no long-range ordered structure, possess various properties such as high strength and toughness with compositional dependence. In the present study, in order to estimate the natures of local structure change and relaxation behavior of hyper-eutectic BA alloys during annealing, positron annihilation measurements and density measurement have been performed for hyper-eutectic Zr-Cu-Al BA alloys with annealing. The enthalpy relaxation and structural relaxation was also measured by differential scanning calorimetry (DSC) measurement. These results show that the relaxation process of free volume containing in hyper-eutectic BA alloys is different from that in hypo-eutectic alloys.

Abstract: Positron annihilation lifetime spectroscopy (PALS) is a recognized instrument for the studies of size-distribution of nanopores (intrinsic free volume holes FVH) in polymers, particularly membrane materials. The limits of this application in the case of “alien” pores, produced by swelling of poly (hexafluoro propylene) PHFP in the gas (CO₂) in super-critical (sc-) state are discussed. The obtained conclusions are controlled by measurements of low temperature gas (N₂) sorption (LTGS) and by comparison of the data with the results on permeation of various gases through the PHFP membrane. Attention is payed to relaxation with time of the newly created FVH in the PHFP membrane.

Abstract: The formation conditions of strained (non-equilibrium) triple junctions of grain boundaries were studied by the method of molecular dynamics. It is shown that strained triple junctions, containing excess free volume, mainly forms during crystallization process in the result of "locking" of the liquid phase density at a meeting of the three crystallization fronts and, as a consequence, of the concentration of excess free volume in the triple junction after solidification.

Abstract: Polymers always show time-dependent mechanical properties. In order to use polymers in engineering applications, long-term mechanical propertes should be characterized. Free volume theroy is the mostly used theory to predict and model the mechanical properties of polymers. The effect of temperature is modelled thorugh William-Landel-Ferry (WLF) equation, whereas, the combined effect of temperature and pressure is modelled by Filler-Moonan-Tschoegl (FMT) equation. Both of the models are based on free volume theory. A set of expermentations were performed to investigate the validity of free volume concpet for one of the most important engineering polymer; i.e. Polyamide-6.

Abstract: In this paper we have studied changes in the structure and properties of amorphous metallic alloys in annealing. We have identified the regularity of plasticity decreasing. We have determined that processes decreasing plasticity are thermally activated. The ratio of the alloy components affects the shape of the plasticity-temperature curves. Based on experimental results we have proposed an energy model of the plasticity formation in alloys in annealing, We have determined the relation between a crystallization temperature and the concentration of cobalt in ribbon metallic glasses. We have constructed diagrams of the time-temperature stability which allow establishing acceptable exploitation modes of amorphous metallic alloys and products made from them.

Abstract: Polymers are a class of materials widely used in different fields of applications. With imminent applications of polymers, the study of radiation induced changes in polymers has become an obvious scientific demand. The bombardment by ion beam radiations has become one of the most promising techniques in present day polymer research. When the polymers are irradiated, a variety of physical and chemical changes takes place due to energy deposition of the radiation in the polymer matrix. Scissoring, cross-linking, recombination, radical decomposition, etc. are some of the interesting changes that are obvious in polymers. The modification in polymer properties by irradiation depends mainly on the nature of radiation and the type of polymer used.Polymer electrolytes are obtained by modifying polymers by doping, complexing, or other chemical processes. In general, they suffer from low conductivity due to high crystallinity of the matrix. The effect of radiation on polymer electrolyte is expected to alter their crystalline nature vis-a-vis electrical properties. This review article shall elaborate modifications in the physical and chemical properties of polymer electrolytes and their composites. The variations in properties have been explored on PEO based polymer electrolyte and correlated with the parameters responsible for such changes. Also a comparison with different types of the polymers irradiated with a wide range of ion beams has been established.

Abstract: The theoretical method of molecular dynamics was applied to study H₂O molecular diffusion behavior in PAM/PVA Polymer blends, investigated the effects of component ratios, temperatures and water contents on diffusion coefficient. Results show that with the increase of PVA content in blend systems, the H₂O molecule diffusion coefficient first increased and then decreased. This indicates there is an optimum component ratio to make the H₂O molecule diffusion coefficient maximum. There is a certain influence of temperature on H₂O molecular diffusion in PAM/PVA blend system, the higher the temperature, the bigger the H₂O molecular diffusion coefficient. When the less H₂O molecules contained in system, there is a less impact on the diffusion coefficient. As the number of H₂O molecules reaches a certain amount, the effect on the diffusion coefficient is more obvious.

Abstract: A calculation method for prediction of water permeability through polymers is suggested. An appropriate equation for calculating the activation free energy of permeability is proposed. The method is based on a set of atomic constants associated with the polymer-water interaction energy. The chemical structure of polymers as well as the degree of crystallininty, temperature, and free volume are taken into account. The method is also applicable for polymeric nanocomposites.

Abstract: Positron Annihilation Lifetime Spectroscopy (PALS) is used to study the nanoporosity and fractional free volume in Ultra High Molecular Weight Polyethylene (UHMWPE) and composites with the addition of Martian Regolith (UHMWPE-MR) as-made and irradiated with ⁵⁶Fe heavy ions at an energy of 600 MeV/u to three different doses (10, 32, 64 Gy). The positron lifetime spectra were obtained using ²²Na positron source and the spectra were analyzed to two lifetime components using POSFIT program. First short lifetime component around 0.28 ns is related to positron annihilation in material including vacancy defects and the second long lived component around 1.7 ns is due to Positronium formation in free volume pores. UHMWPE-MR composites were shown to be less porous with much lower nanopores concentration compared to the UHMWPE polymer. The average size of the nanopores is around 0.5 nm (obtained from a simple model). Larger variations in positron lifetime parameters are observed with increasing irradiation dose for UHMWPE polymer compared to UHMWPE+MR composites. The 3-point bend test results also showed larger variations with increasing irradiation dose for the UHMWPE polymer. The variations in PALS parameters may indicate an increasing competition between two processes at higher irradiation doses: 1) vacancy defects aggregation and 2) escape of vacancy defects as the local temperature increases at higher doses resulting in increased vacancy defects mobility. Present results clearly indicate a qualitative inverse relationship between nanoscale porosity measured by positron life time and mechanical properties of UHMWPE and its composite with MR.