Synthesis and properties of soluble polyimides containing phenolphthalein structures
Aromatic diamine monomer 4,4'-(2,2'-trifluoromethyl)-diaminophenoxy-3,3'-dimethylphenolphthalein(Ⅱ)was prepared by aromatic nucleophilic substitution and oxidation-reduction reaction between o-cresolphthalein and 2-chloro-5-nitrotrifluorotoluene.Then,a series of copolymerized polyimides containing phenolphthalein trifluoromethyl and methyl structures were prepared from one-step polycondensation of Ⅱ and 2,6-diaminotoluene with 3,3',4,4'-diphenyl ether tetracarboxylic acid dianhydride with different feeding ratios,and further evaluated for structure and performance.The results showed that this series of copolymerized polyimides had excellent solubility,dissolving in high boiling point solvents,such as N-methylpyrrolidone,N,N-dimethylacetamide,N,N-dimethylformamide and dimethyl sulfoxide,as well as low boiling point solvents,such as chloroform,dichloromethane and tetrahydrofuran at room temperature.A series of high performance polyimide membrane materials could be conveniently prepared from solution casting of these copolymerized polyimides,with glass transition temperatures between 275～314℃,and temperatures at 10%mass loss in the range of 477～507 and 470～490℃in N2 and O2,respectively.Meanwhile,these polyimide membrane materials exhibited low dielectric constants and good thermal properties,with relative dielectric constants ranging from 2.69 to 2.92 at 1 MHz,and the tensile strength,elastic modulus as well as elongation at break of copolyimides ranging from 80 MPa to 92 MPa,1.2 GPa to 1.8 GPa,and 9.2%to 13.5%,respectively.
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