Theoretical Study Of Organic Molecule/Polymer Solubilities And Crystallizations In Ionic Liquid

Due to the unique physical and chemical properties, ionic liquids have been extensively applied as solvents for dissolution of inorganic molecules, organic molecules and polymers. In this regard, ionic liquids can be used in the development and preparation of polymer composites. In this paper, the microstructures, solubility equilibria and interfacial properties of organic molecules/ionic liquids and polymer/ionic liquids systems have been studied through constructing relevant theoretical models in cosideration of the specific interactions in each system. The main contents include:(1) A three-demional density functional theory model has been constructed to investigated the equilibria and interfacial properties of the ionic liquid/organic molecule binary systems, where the different chemical specificities and geometries of ionic liquids and organic molecules have been considered. The spatial distributions of thiophene, benzothiophene and dibenzothiophene around different ionic liquids have been calculated to evaluate the interfacial tensions and solubilities of organic molecules. Compared with the corresponding experimental data, the theoretical predictions show the average deviation of less than 10%. Meanwhile, the obtained structure provides a microscopic mechanistic illustration of the solubility of organic molecule in ionic liquid. According to the microscopic mechanism of the solubility, the investigation of different cation/anion combinations allows us to decipher the key issue in designing functional ionic liquids. Four imidazolium based ionic liquids have been designed, which exhibit high solubility for organic molecules.(2) A free-energy functional has been constructred to account for the contribution of polymer configuration entropy and integrated into the density functional theory. The predicted solubility equilibrium of [EMIM][BF4]-Polyethylene oxide binary system is in good agreement with the experimental data, indicating that the current model is capable to describe such systems. The microstructures of Polyethylene oxide around different groups of ionic liquid provide a explanation of microscopic mechanism of [EMIM][BF4] dissolved in Polyethylene oxide. In addition, the interfacial tensions have been obtained at different temperatures.(3) A tensor version has been empolyed to modify the free-energy functional of hard-sphere repulsion to study the polymer crystallization. The crystal-melt coexistence data have been calculated, and the structures of crystallites and interfaces have been optimized via a restricted and full free-energy minimizations. It has proven that the current theoretical model is reasonable by comparing the calculated data with the experimental data. With the crystal-melt coexistence data, the spatial density distributions and crystal lattice parameters of crystallites, and density distributions and interfacial tensions of different crystal-melt interfaces have been determined systematically. Accordingly, the nucleation free-energy barriers, and critical sizes of nuclei have been quantitatively predicted by minimalizing the free-energy of crystals and interfaces; These thermodynamic properties are expected to provide a solid basis to understand the polymer crystal growth from micro-level.(4) The above theoretical model has been extended to study the effect of ionic liquids on polyvinylidene fluoride crystallization. The microstructures of polyvinylidene fluoride crystal and crystal-melt interfaces have been characterized to analyze the morpholgy of crystal, and to obtain the interfacial free-energy at equilibrium state. The spatial density distributions of different ionic liquids at different concentrations on polyvinylidene fluoride crystal surface have been demined to evaluate their influences on polyvinylidene fluoride crystallization. In addition, the nucleation free-energy barriers and the critical nuclei of polyvinylidene fluoride in different ionic liquids with different contents at given supersaturation have been calculated to intepret the effect of ionic liquid on polymer crystallization.