Study On Synthesis And Gas Separation Properties Of Novel Fluorinated Polyimides

Aromatic polyimides possess high rigidity and exhibit good gas selectivity and,because of that,the gas separation properties of a large number of polyimides have been investigated.A good gas separation membrane has to show excellent permeability,and this is usually attained by means of structural changes that make difficult the molecular chain packing.The introduction of fluorine-containing groups into polyimide molecular chain has been proven as an effective way to improve gas permeability while maintaining remarkable thermal properties and high selectivity of the polymer.To this end,three types of fluorinated polyimides with specific structures were designed and synthesized from the point of view of structural modification.The main contents are as follows:1.A new diamine monomer FPPAB containing trifluoromethyl,ester group and large side group was synthesized by three-step organic reaction with 4-hydroxybenzaldehyde,4-trifluoromethylacetophenone and ammonium acetate as starting materials.A series of polyimides were synthesized by polymerization of FPPAB,ODA and various commercial dianhydrides.The thermal stability,solubility,interchain spacing,mechanical properties and gas separation properties of polyimides were tested and compared.The results show that the prepared polyimides have excellent thermal stability,good solubility,remarkable mechanical properties and large interchain spacing.Compared with polyimide(ODA/BPDA)originated from commercial diamine and dianhydride,their gas separation performance is significantly improved.2.A new type of diamine monomer FPAP with fluorine atom and noncoplanar triphenylpyridine structure was obtained by two-step reaction using 3,5-difluorobenzaldehyde,4-nitroacetophenone and ammonium acetate.A series of polyimides were prepared by polycondensation of FPAP with various commercial dianhydrides.The thermal stability,solubility,mechanical properties,interchain spacing and gas separation performance of polyimides were characterized.The results show that the obtained polyimides have outstanding thermal properties,good solubility and mechanical properties,weak molecular chain packing,and improved gas separation performance.3.A new diamine monomer FPPSD with hexafluoroisopropyl and sulfonyl group was obtained by six steps of organic reactions,including addition elimination,thermal rearrangement,hydrolysis,nucleophilic substitution,oxidation and reduction,with 4,4’-(perfluoropropane-2,2-diyl)diphenol,N,N-dimethylaminothioformyl chloride and 4-chloronitrobenzene as materials.A series of polyimides were prepared by copolymerization of FPPSD,ODA and various commercial dianhydrides.The physical properties of polyimides,including thermal stability,solubility,mechanical properties,interchain spacing and gas separation performance,were tested and compared with each other.The results show that the polyimides have excellent thermal properties,good solubility,exceptional mechanical properties,large interchain spacing and improved gas separation performance.