Research On The Preparation Of P（VDF-HFP）/IL Gel Membranes By Thermally Induced Phase Separation For CO₂/N₂Separation

Whether the carbon dioxide (CO2) can be separated efficiently or not has greatsignificance for the future of earth. It not only is very helpful in alleviating greenhouseeffect, but also can recycle the carbon resource. As we know, there are many methods toseparate the CO2in industry over the years. The membrane separation technology is oneof them. Compared with the traditional technology of Separating the CO2, it has theadvantage of energy-saving, high performance, simple operation and so on. Among them,gel membrane technology attract extensive attention. Better than other, it hashigh-permeation rate, separation factor and high stability. Thermally Induced PhaseSeparation (TIPS) technique is an appropriate method to prepare the gel membrane, andwill be introduced carefully in this paper. In this technique, the polymer is dissolved bydiluents in high temperature, and then is cooled to induce phase separation. Better thanthe tranditional gel membrane preparation methods, TIPS technology don’t have solventevaporation, has regulation of technological parameters, and is easier to control filmstructure and so on. As a result, preparating the gel membrane with the TIPS technology isnot a bad idea.This paper mainly focus on the preparation of P(VDF-HFP)/IL gel membranes by theTIPS technology. As the definition of TIPS is mentioned earlier, the P(VDF-HFP) and[CnMIM]PF6are selected for the polymer and diluent of TIPS in this paper. As last, weget the P(VDF-HFP)/IL gel membranes. Moreover, this paper also research on the effectof the ionic liquid content, length of ionic liquid cations alkyl chain and cooling rate inpreparation of membrane processes by TIPS technology on the Internal structure,mechanical properties and gas permeation performances of the gel membranes werestudied. Meanwhile, the non-isothermal crystallization kinetics and crystalline structure ofP(VDF-HFP)/[CnMIM]PF6gel systems were discussed. The main conclusions are listedbelow:First of all，P(VDF-HFP)/[CnMIM]PF6(n=4,6,8) gel films was prepared by TIPStechnology at50wt%of [CnMIM]PF6,[C4MIM] PF6can be written as [BMIM] PF6form.The gel membrane structure, crystallinity, mechanical properties, selectivity of CO2/N2and gas permeability of P(DF-HFP)/[CnMIM]PF6gel membranes with cationic alkylchain length of [CnMIM]PF6(n=4,6,8) were studied. The results showed that P(VDF-HFP)and [CnMIM]PF6interaction increased with increasing the length of cation alkyl chain, while spherulite size of the polymer backbone structure becomed small and thecrystallinity decreased. When cationic alkyl chain length of the [CnMIM]PF6increasesdfrom4to8, the CO2permeability coefficient improved from66.8to94.3Barrer, theCO2/N2ideal selectivity increased from23.9to38.9was mainly due to an enhancedeffective diffusion coefficient. The results show that three kinds of ionic liquids in thisexperiment, P(VDF-HFP)/[BMIM] PF6gel membrane have best overall performance.Secondly，the effect of the P(VDF-HFP)/[BMIM]PF6gel membrane structure,crystallinity, mechanical properties and gas permeation performances with different[BMIM]PF6addition were studyed. The results indicated that the dense polymer backbonestructure of gel membrane into loose spherulitic structure with the addition of [BMIM]PF6increased. Meanwhile, the presence of the [BMIM]PF6reduced the overall crystallinity,improving the flexibility of P(VDF-HFP) segment, thereby CO2and N2permeabilitycoefficient of the gel membranes significantly increased, whereas the permeabilityselectivity of CO2/N2showed an initial increase and then a gradual drop. When the[BMIM]PF6content increased from0wt%to60wt%, the CO2permeability coefficientimproved from0.2to94.3Barrer was mainly due to an enhanced effective diffusioncoefficient.Moreover, P(VDF-HFP)/[BMIM]PF6gel membranes containing50wt%of[BMIM]PF6content was prepared by TIPS technology. In preparation ofP(VDF-HFP)/[BMIM]PF6gel membranes processes, the effect of cooling bathtemperature of on the polymer backbone structure, crystallinity, mechanical properties andgas permeability of the P(VDF-HFP)/[BMIM]PF6gel membrane were characterized. Theresults showed that size of the polymer spherulite skeleton structure becamed larger andthe degree of crystallinity decreased with increasing the cooling bath temperature. Thecooling bath temperature from0℃to40℃, leading to CO2coefficient of the gelmembranes increased.Finally, The non-isothermal crystallization of P(VDF-HFP) in [CnMIM]PF6wasstudied by differential scanning calorimetry. The half-time t1/2of P(VDF-HFP)/[BMIM]PF6gel systerms increased with increasing [BMIM]PF6content and decreasingcooling rate, while crystallization rate constant decreased. Jeziorny,and Mo,equationcould fit the non-isothermalcrystallization kinetics of P(VDF-HFP)/[CnMIM]PF6gelsysterms. Avrami exponent was evaluated to be in the range of2.57~3.90at the primarystage of non-isothermal crystallization. It can be seen that the secondary crystallizationwas restrained at the higher [BMIM]PF6content. The activation energy of P(VDF-HFP)/[CnMIM]PF6gel systerms were reduced with increasing [BMIM]PF6content and the length of ionic liquid cations alkyl chain. Meanwhile，the crystaltransformation of vinylidene fluoride(VDF) segments were characterized by FT-IR andXRD, The results discovered that the blending with [BMIM]PF6were in favor of the αcrystalline phase to βcrystal phase transition.