Preparation And Performance Of PEG （Ethers）-PVDF Blend Membranes For CO₂Separation

Membrane technology is widely used in the field of gas separation due to its characteristics of flexibility, simple equipment and safety. Membrane technology has reduced the energy consumption and the exhaust emission, which is to the great interest of energy-saving and emission-reduction strategy in our country. The core of membrane technology is the material. Poly(ethylene oxide)(PEO) is of great potential since ether groups have an special affinity for CO2due to the interaction between them. However, PEO with low molecular weight can hardly form membrane on its own for its poor mechanical property. Blending PEO of low molecular weight with another polymer provides an effective way to prepare membranes for gas separation.In this thesis, Polyvinylidenefluoride (PVDF) was chosen as membrane material, porous polypropylene (PP) was selected as supporting membrane and solvent evaporation method was used to prepare blend composite membrane. Three blend systems have been studied:(Polyethylene glycol) PEG-PVDF,(Polyethylene glycol methyl ether) PEGME-PVDF and (Polyethylene glycol dimethyl ether) PEGDME-PVDF. Among these three blend systems, PEGDME-PVDF performed the best. Under the same condition, PEG-PVDF/PP composite membrane can get CO2permeability of13.0GPU and CO2/N2selectivity37.5; PEGME-PVDF/PP composite membrane can get CO2permeability of47.4GPU and CO2/N2selectivity42.6; while PEGDME-PVDF/PP composite membrane can get CO2permeability of71.9GPU and CO2/N2selectivity42.8. According to research, the blend system of PEGDME-PVDF provides the best results.The orthogonal test method was adopted to explore the influences of different factors in PEGDME-PVDF system. The effects of PEGDME content, PVDF concentration and the solvent evaporation temperature on the membrane morphology and gas separation performance of PEGDME-PVDF blend composite membrane have been investigated according to the orthogonal test results.With the increase of PEGDME content in the blend membrane, both CO2permeability and CO2/N2selectivity tend to rise. When the content of PEGDME reached50wt.%, CO2permeability is42.9GPU and CO2/N2selectivity is47.5. The existence of PEGDME decreased the melting point of PVDF, which led to an increase to the flexibility of the polymer segments and the free volume within the membrane. Accordingly, with the decrease of solvent evaporation temperature, PVDF crystallization decreased. The proper evaporation temperature lays on30to60℃. When the evaporation temperature is30℃, CO2permeability can reach84.7GPU and CO2/N2selectivity47.2.The concentration of PVDF has a great effect on the viscosity of casting solution. The suitable concentration of PVDF is from20wt.%to24wt.%when preparing blend membrane. With the increase of PVDF concentration, CO2permeability tends to decrease while CO2/N2selectivity rises slowly. When PVDF concentration is24wt.%, PEGDME content is50wt.%, under solvent temperature of40℃, CO2permeability reached30.3GPU and CO2/N2selectivity55.4.CO2permeability of blend composite membranes was simulated by Maxwell model. Results show that the values predicted with PEGDME as continuous phase are similar to the experimental data, especially when the volume fraction of PEGDME approaches50%.