Lazarenko M. The influence of the limited mobility in chain molecule systems on their phase transition parameters and relaxation processes

The manuscript discusses the influence of limited mobility in systems of long-chain aliphatic molecules on phase transition parameters and relaxation processes therein. The existence of local topological defects (topological solitons) in long-chain molecular systems with limited mobility is established. The topological solitons largely determine relaxation processes and phase transitions in such systems. It was found that the effect of confined space on phase transitions in a system of long-chain molecules cannot be described by the well-known Gibbs-Thomson model. The temperature and the latent heat of fusion of the aliphatic nanocrystals in the solid porous matrices were shown to be linear decreasing functions of the ratio of the total surface area of the nanocrystals to their total volume. At thermodynamic limit, where this ratio approaches zero, the above-mentioned temperatures and latent heats were found to be different from those obtained for bulk crystals. The density of nanocrystals, the magnitude of the structural stresses in it, as well as the difference in surface energies and surface entropies were determined using the equilibrium thermodynamic model of melting nanocrystals in rigid open porous matrices. The existence of a two-dimensional crystal structure in 1-octadecene nanocrystals is established. The phase transitions in this structure are caused by the formation and motion of topological solitons. The chemical modification of the pore surface by non-polar groups of different structures leads to the different signs of changes in the temperatures of phase transitions. In particular, the modification of the pore surface of silica gel with -TMS groups leads to the decrease in the melting temperature of 1-octadecene nanocrystals. At the same time, the modification of silica gel pores with nonpolar -C18H37 groups leads to the increase in the melting point of nanocrystals due to the ability to embed and “to sew” 1-octadecene nanocrystals with the silica gel surface. The process of dielectric relaxation in triacylglycerides is established to be caused by the correlated motion of topological solitons in aliphatic chains of molecules. The topological solitons can reflect from the molecules' branching and double bonds, failing to translate the molecules off the crystals. The motion of topological solitons in rigid molecules of microcrystal cellulose during relaxation process is absent. The relaxation process in the microcrystal cellulose was shown to be due to conformational transitions in the surface methylene groups of cellulose.