Synthesis and characterization of UV-curable maleimide-terminated imide oligomers

Three maleimide-terminated imide (MTI) oligomers/polymers with varying molecular weight were synthesized and characterized by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy, and size exclusion chromatography (SEC). The MTI oligomers were formulated with two reactive diluents, N,N-dimethylacrylamide (DMAA) or N-vinylpyrrolidone (NVP), and were photopolymerized with and without a photoinitiator. The kinetics of film formation was investigated using real-time FTIR and photo-differential scanning calorimetry (DSC). Thermal and viscoelastic properties of UV cured films were studied by thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC). The reaction rate and conversion were improved by adding a photoinitiator. MTI oligomer with low molecular weight (M¯n ∼ 2 kg/mol) MTI-2k had the highest reaction rate and final conversion. More than 90% of conversion was achieved after exposure to UV light for 60 s. A wide α-transition (tan δ) for all UV cured films was observed, indicating a heterogeneous system. When no photoinitiator was used, the MTI/NVP formulations polymerized, while the MTI/DMAA formulations did not. It was proposed that the NVP could react with MTI oligomers via a donor/acceptor complex, and the DMAA could not due to the electron-poor double bond.