Abstract
Phenylethynyl-terminated aromatic polyimides meet requirements of resin transfer molding (RTM) and exhibits high glass transition temperature (Tg) were prepared. Moreover, the relationship between the polyimide backbones structure and their melting stability was investigated. The phenylethynyl-terminated polyimides were based on 4,4′-(hexafluorosiopropylidene)-diphthalic anhydride (6FDA) and different diamines of 3,4′-oxydianiline (3,4′-ODA), m-phenylenediamine (m-PDA) and 2,2′-bis(trifluoromethyl)benzidine (TFDB) were prepared. These oligoimides exhibit excellent melting flowability with wide processing temperature window and low minimum melt viscosities (<1 Pa·s). Two of the oligoimides display good melting stability at 280–290 °C, which meet the requirements of resin transfer molding (RTM) process. After thermally cured, all resins show high glass transition temperatures (Tgs, 363–391 °C) and good tensile strength (51–66 MPa). The cure kinetics studied by the differential scanning calorimetry (DSC), 13C nuclear magnetic resonance (13C NMR) characterization and density functional theory (DFT) definitely confirmed that the electron-withdrawing ability of oligoimide backbone can tremendously affect the curing reactivity of terminated phenylethynyl groups. The replacement of 3,4′-ODA units by m-PDA or TFDB units increase the electron-withdrawing ability of the backbone, which increase the curing rate of terminated phenylethynyl groups at processing temperatures, hence results in the worse melting stability.
Highlights
Characterization of Oligoimides End-Capped with Phenylethynyl Groups
A series of phenylethynyl end-capped aromatic oligomers derived from 6FDA was prepared via one-pot high-temperature polymerization (Scheme 2)
The molecular weights of oligoimides were measured by Gel permeation chromatography (GPC)
Summary
Phenylethynyl-terminated PETI-5 derived from 3,30 ,4,40 biphenyltetraboxylic dianhydride (s-BPDA) prepared by researchers from NASA, with a designed molecular weight of 5000 g/mol, exhibits a good combination of high fracture toughness, high heat-resistance and good processability [16] It has minimum melting viscosity of 1000 Pa·s at 371 ◦ C and cured Tg of 270 ◦ C. Synthesized 4-Phenylethynyl phthalic anhydride (PEPA) end-capped imide oligomers derived from 4,40 -(2,2,2-trifluoro-1-phenylethylidene) diphthalic anhydride (3FDA), 4,40 -(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3,30 ,4,40 biphenylene dianhydride (s-BPDA), and founded 3FDA and 6FDA could endow imide oligomers better melting stability It seems that ethynyl groups with decreased electron density show lower reactivity due to the influence of fluorine, which eventually leads to good melting stability.
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