Molecular Dynamics Study On The Interaction Mechanism Of Ionic Liquids And Polymer

The large amount of carbon dioxide?CO₂?emissions lead to global warming and bring a series of serious environmental problems,thus,high-efficient CO₂ capture and separation has great significance.In recent years,ionic liquids?ILs?-polymer composite membranes have attracted extensive attention due to their better CO₂separation performance.However,the main challenge is the wide variety of ILs and polymers,and it is difficult to select the optimal ILs and polymer to fabricate a composite membrane within a limited time and cost.In this work,molecular dynamics?MD?simulation was adopted to study the composite systems of different kinds of ILs with poly?vinylidene fluoride??PVDF?and with CO₂.The research content and conclusions mainly include the following aspects:?1?MD simulations were carried out for[bmim][PF₆]-PVDF composite systems with ILs content of 0%,10%,20%,30%,40%,50%and 100%.Simulation results show that ILs aggregate in the PVDF matrix and form the heterogeneous aggregation regions,and the aggregation region continuously expands and finally becomes continuous ionic channels for CO₂ diffusion with increasing ILs content.The stronger interactions between PVDF and cations allow ILs to be immobilized in the PVDF matrix,forming composite membranes.With the increase of ILs content,hydrogen bond?HB?networks among PVDF chains were gradually broken up,which accelerates the movement of PVDF chains and facilitates the transportation of CO₂ among PVDF chains.?2?MD simulations were performed for the composite systems of[bmim][Tf₂N]-PVDF and[bmim][B?CN?₄]-PVDF containing different kinds of anions,respectively,and the simulated results were compared with[bmim][PF₆]-PVDF composite system.Simulation results show that the strongest interaction of PVDF-[B?CN?₄]and the weakest aggregation of[bmim][B?CN?₄]are favorable for forming the continuous ionic channels in PVDF matrix for CO₂ diffusion.Moreover,the free energy of solvation,Henry's law constant and self-diffusion coefficients for CO₂ in three ILs-PVDF systems containing different kinds of anions suggest that[bmim][B?CN?₄]-PVDF composite membrane possesses better CO₂ separation performance.?3?MD simulations were carried out for[C_nmim][B?CN?₄]-PVDF?n=2,4,6,8,10?composite systems containing different alkyl side chain length in cations.Simulation results show that with the increase of cationic alkyl side chain length,the electrostatic interactions between PVDF and cations gradually decrease.Therefore,the[C₁₀mim][B?CN?₄]ILs can better disperse in PVDF matrix,which facilitates the formation of continuous ionic channels for CO₂ diffusion.In addition,the free energy of solvation,Henry's law constant and self-diffusion coefficients for CO₂ in five ILs-PVDF systems containing different cationic alkyl side chain length suggest that[C₁₀mim][B?CN?₄]-PVDF composite membrane possesses superior CO₂ separation performance.