Preparation And Performance Of Solvent Resistant PTFPMS Composite Membrane

Gas separation membranes with their low cost characteristics have broad application prospects in terms of energy conservation. As the core component of gas separation membrane, material used for selective layer and support along with their structure determine the performance of composite membrane. Nowadays, polydimethylsiloxane (PDMS) is widely used as selective layer material but it can be swollen heavily by organic gases, which leads to huge selectivity loss. Therefore, developing material of high solvent resistance is the key to fabricate composite membranes used for gas separation, which will widen their application fields also. Polyfluoropropylmethylsiloxane (PTFPMS) has similar backbone structure to PDMS and its unique trifluoropropyl group exhibits outstanding solvent resistance property. The main work of this dissertation was to fabricate a series of PTFPMS compoiste membranes and investigate their performance on simple and organic gas seperation, on the basis of molecular simulation which studies the interaction between polysiloxanes and gases.Firstly, polysiloxanes (PDMS, PPMS, PTFPMS and POMS) with same main chain but different side groups were selected as target coating materials. The interaction between polysiloxanes, solvents and gases were analyzed through free energy of mixing of the binary systems which were calculated by Materials Studio4.0software. Free energies of PTFPMS and different solvents was the largest among the four kinds of polysiloxanes, reflecting its highest resistance to non-polar solvents. Besides, the free energy of mixing between PTFPMS and PEG was low, indicating their good compatibility. Further, the diffusion coefficients of O2and N2molecules in PDMS and PTFPMS were calculated by molecular dynamic (MD) simulations. The diffusion of small molecules in PTFPMS showed inconsistency with main chains, which was different with the case of PDMS. The reason of this difference was the bulky trifluoropropyl hindered the diffusion of gases and the side groups in PTFPMS distributed asymmetrically which led to lower diffusion coefficients of gases. GCMC method was used to calculate solubilities of CH4, CO2, and C3H8in polysiloxanes and the results were in good agreement with reported experimental values. Also, the simulation revealed large pore volume can not be obtained in PTFPMS for its heavy distortion of side groups, which resulted in lower solubilities of organic gases.Secondly, PTFPMS was selected as selective coating material according to the simulation study. Cross-linking of PTFPMS was controlled in mild condition (crosslink temperature at80℃, crosslink time at0.5h) and coating parameters were determined through experiments. The CO2permeance of prepared PTFPMS/PEI composite membrane was193.7GPU. The selectivity of O2/N2and CO2/N2were2.25and16.38, respectively, which was enhanced by12.5%and48%compared with PDMS/PEI composite membrane. Based on the improved Henis transport model, the minimum thickness of coating layer was predicted. The calculation revealed that the thickness larger than15μm was necessary to guarantee a satisfied performance. For PTFPMS-Ⅲ(1100K g/mol), with water as pre-treatment solvent, one step coating at the concentration of10wt.%could decrease the thickness of insert layer by65%and increase the CO2permeance by43%compared to coating twice (5wt%). The selectivity for C3H6/N2of PTFPMS/PEI composite membrane is larger than that of PDMS/PEI composite membrane by18%. After soaking in isooctane and pentane for48h, ideal selectivities of C3He/N2for PTFPMS composite membrane decreased by1.4and2.3%, which were quite lower than that for PDMS composite membrane (24.5and27.8%), showing better solvent resistance of PTFPMS membrane.To further enhance the separation performance of CO2, polyethylene glycol (PEG) with different molecular weights were introduced into PTFPMS network to form a blend selective layer. FTIR spectrum of PEG/PTFPMS blend membrane showed no shift for functional groups and no new peak appeared in DSC result, whichmeant a good compatability between PEG and PTFPMS. The CO2permeation rate of the PEG-400/PTFPMS blend composite membrane with a blend ratio of0.2is65.6GPU, while the O2/N2and CO2/N2selectivities are improved by22%and17%, respectively. Maxwell’s model was applied to investigate the transport behavior of blend membrane and results indicated the transport path of gases consist of binary phase of PEG and PTFPMS.Finally, to improve the solvent resistance property of gas separation membrane, hydrophobic SiO2/PTFPMS hybrid composite membrane was fabricated by solution blending-crosslinking method. The effects of blend ratio of SiO2to PTFPMS in mass on the membrane morphology, solvent resistance, and gas separation performance of the hydrophobic SiO2/PTFPMS hybrid composite membrane were investigated. The swelling degree of hybrid membrane was0in isooctane and decreases by11.9%in ethyl acetate when the blend ratio is lower than0.018. Permeances of simple gases (H2, N2, O2) decrease with increased SiO2blend ratio, while permeance of CO2increases till the blend ratio of SiO2is0.012. The CO2gas permeation rate can reach as high as156.1GPU while selectivities of CO2/N2and were increased by28%and25%, respectively.