Synthesis and properties of soluble cross-linkable fluorinated co-polyimides

A series of high molecular weight soluble fluorinated cross-linkable co-polyimides (Co-PIs) were synthesized from 1,3-bis(2-trifluoromethyl-4-aminophenoxy)-5-(2,3,4,5-tetrafluorophenoxy)benzene (6FTFPB), 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB), 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-2-(3′-trifluoro-methylphenyl)benzene (m-3F-6FAPB) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA). The weight-average molecular weights (Mws) and polydispersities of the co-polyimides were in the range 5.53-7.24 and 1.71-2.04, respectively. By adjusting the feed ratio of diamines, the refractive indices of Co-PIs films were exactly tuned in the range of 1.5203-1.5574 at 633 nm. The co-polyimides showed outstanding solubility in organic solvents and high thermal stability (5% thermal weight-loss temperature beyond 525 °C). Meanwhile, the films had excellent mechanical properties with high tensile strength of 100-108 MPa, Young's modulus of 3.3-4.2 GPa and elongations at break of 8-15%. Polymers in the form of cast and spun films could be cross-linked by thermal curing at 260 °C. After curing, the Co-PIs films showed excellent chemical resistance, high glass transition temperatures above 270 °C, higher mechanical properties and thermal stability (5% thermal weight-loss temperature beyond 540 °C) and low dielectric constant in the range of 2.37-2.70.