Design Of Poly（Vinyl Acetate）-Based Polymers As CO₂-Philies And Their Interactions With CO₂

As a environment-friendly solvents, supercritical CO2 (scCO2) has been extended more and more widely in many processes such as polymer reaction, CO2-assisted polymer processing and CO2 flooding. However, to a large extent, the lower solvating power of CO2 limits its industrial applications. The design of CO2-philic polymers as surfactants or solubilizers, will increase the solvating power of CO2 and expand the applications of ScCO2. Therefore, it is essential to deeply understand the interactions between low cost hydrocarbon polymer and CO2, which is conclusive to design new CO2-philes with higher solubility in CO2 and has important theoretical significance and application value.Focusing on the interactions between solute-solute, solvent-solvent and solute-solvent in the solution, this work studied the theory method to evaluate the compatibility between polymer and CO2 and the mechanism for enhancing their compatibility. For evaluating the effect of molecular chain structure on the solute-solute and solute-solvent interactions, PVAc with different molecular chain structures and molecular weights were designed to determine the CO2-philicity. For weakening the solute-solute interaction, the poly(VAc-alt-maleate) series copolymers were designed to evaluate the compatibility with CO2. Then the poly(VAc-co-alkyl vinyl ether) series copolymers were designed to enhance the compatibility with CO2 through favorable solute-solute and solute-solvent interactions. Moreover, the effect of cosolvent on the scCO2+cosolvent system was investigated to improve the solvent-solvent and solute-solvent interactions for the enhancement of the compatibility with polymer. The detail works are as follows:The density and solubility parameter of ScCO2 and PVAc at different temperatures and pressures was calculated by using molecular dynamics (MD) simulation, which verified that the feasibility of simulation method and the reliability of the COMPASS force field. The characterization method of the solubility of polymer in CO2 was established and confirmed the reliability of the equipment through the comparison with literatures. The results of MD simulations showed that the solubility parameter of pure CO2 at super/near critical conditions had a a linear relationship with the system density, and the van der Waals force was the dominated interaction between PVAc chain and CO2. The dissolution behavior measurement demonstrated that the cloud point pressure (Pc) of PVAc in CO2 increased with the temperature and the molecular weight (Mn).The effect of molecular chain structure on the CO2-philicity of PVAc was evaluated. The MD simulations were used to calculate the interactions and the Flory-Huggins parameter (χ12) between CO2 and PVAc with different molecular weights and molecular chain structures. The simulation results indicated that the variation trend of the χ12, the radial distribution functions had a good agreement with that of the cloud point experiments. At low Mn, the polymer-polymer interaction was the main driving force deciding the CO2-philicity due to the significant difference in different molecular chain structures. At high Mn, multi-armed structure enhanced the CO2-philicity due to the synergistic effect of moderate polymer-polymer interaction and large polymer-CO2 interaction.The maleate series monomers were introduced to reduce the solute-solute interactions. The ab initio calculated interaction energies of the candidate CO2-philic molecule models with CO2, including vinyl acetate dimer (VAc), dimethyl maleate (DMM), diethyl maleate (DEM) and dibutyl maleate (DBM), showed VAc was the most CO2-philic segment. Three kinds of alternating copolymer were built by using VAc and DMM, DEM, DBM for the MD simulations of the interactions with CO2. The simulation results indicated that poly(VAc-alt-DBM)(PVDBM) had the lowest solubility parameter and the lowest χ12 with CO2. The cloud point experiments also demonstrated PVDBM had the most CO2-philicity. Although copolymerization of DBM with PVAc reduced the polymer-CO2 interactions, the weakened polymer-polymer interaction increased the CO2-philicity of the copolymers.The alkyl vinyl ether series monomers, vinyl ethyl ether (VEE) and vinyl butyl ether (VBE), were introduced to reduce significantly the polymer-polymer interaction on the premise that the polymer-CO2 interaction was not weakened but enhanced. Three kinds of homopolymer (PVAc, PVEE and PVBE) and two kinds of alternating copolymer (PVAEE and PVABE) were built to calculate the interactions with CO2. From the multi-scale simulations, it could be seen that the interaction energy between PVAEE and CO2 was the largest one and PVAEE possessed the best CO2-philicity. The dissolution behaviors of the synthesized poly(VAc-co-alkyl vinyl ether) copolymers in CO2 showed the best CO2-phile had the VEE content of about 34 mole%. The MD simulations also indicated that the interaction of random poly(VAc-co-VEE) containing about 30 mole% VEE with CO2 was the strongest and the χ12 was the smallest in these polymer/CO2 systems, which was consistent with the experiment result. Not only could the VEE reduce the polymer-polymer interaction, but also could enhance the polymer-CO2 interaction with an optimized composition.The cosolvents (ethanol, acetone, n-heptane) were introduced to form CO2+cosolvent systems for improving the solvent-solvent and solvent-solute interactions and enhancing the the compatibility with polymer. The multi-scale molecular modeling and dissolution behavior measurement showed that the addition of cosolvent could enhance the compatibility between CO2 and PVAc to a certain extent, and the solubility of PVAc in scCO2+cosolvent increased with the mole fraction of cosolvent. The interaction between ethanol and CO2 was strongest due to the bigger solubility parameter of ethanol and the obvious hydrogen bond bewteen ethanol and CO2, which increased significantly the solubility parameter of solvent and improved the dissolution of polymers in CO2.