Two novel series of highly organosoluble and curable polyimides containing pendant nitrile groups PI (a–d) and phthalonitrile-terminated polyimides PN-PI (a–d) were prepared by solution polycondensation of isomeric bis(chlorophthalimides)s and 2,6-dichlorobenzonitrile with 4,4′-thiobisbenzenethiol. The inherent viscosities of these copolymers were in the range of 0.37–0.59 dL g−1 in N-methyl-2-pyrrolidone (NMP) at 30 °C. The phthalonitrile and pendant nitrile groups in isomeric poly(thioether-ether-imide)s were thermally crosslinked without a catalyst through thermal curing up to 280–360 °C, which led to the transformation from thermoplastic polymers to thermosetting polymers. All PIs before and after crosslinking were characterised by gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy. PN-PI (a–d) and PI (a–d) were found to be highly soluble in some solvents, such as chloroform, dimethylacetamide, m-cresol, dimethlyformamide and N-methyl-2-pyrrolidone, while the crosslinked polymers were insoluble in all tested solvents. The crosslinking results of pendant nitrile and terminal phthalonitrile groups are also compared. Both series of PIs showed high thermal stability based on 5% weight loss temperature (T5%) in the range of 471–509 °C and increased up to 35 °C after thermal curing. Polyimides showed high glass transition temperatures (Tgs) ranging from 191–213 °C as determined by DSC, but no detectable Tg was observed after thermal heating, indicating the crosslinked structure of PIs. The mechanical properties also enhanced after the crosslinking of the copolymers. The tensile modulus and strength of the cured PI films were in the range of 3.3–4.7 GPa and 68–122 MPa, respectively, which were up to 62% and 35% higher than the uncured films.
A novel soluble poly(aryl ether ketone) (PAEK) terminated with 4-nitrophthalonitrile was designed and synthesized, which was used to solve the problem of difficult solution processing. Then, a series of cross-linked PAEK films with different thermal treatment temperatures were fabricated by using 4,4-bis(3,4-dicyanophenoxy)biphenyl (BPH) as a novel cross-linking agent, improving the glass transition temperature ( T g ) of the traditional PAEK. Then, the fourier transform infrared (FT-IR) spectrum, X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA), dynamic mechanical analysis (DMA), mechanical testing machine and LCR meter were used to investigate the microstructure, thermal, mechanical and dielectric properties of PAEK films, respectively. The results indicated that the increase in heat-treatment time and temperature are beneficial for the improvement of the value of T g and T 5% , and the excellent thermal stability and mechanical properties of PAEK films are demonstrated. Compared to PAEK-1 film, the T g and T 5% both increased by 63 °C for PAEK-4 film. Significantly, the tensile strength of PAEK-3 film (42 MPa) reaches the highest value in comparison with other samples. In addition, the dielectric constants (ε) and loss tangent (tan δ) of PAEK-4 film is as low as 3.67 and 0.001 at 1 kHz, respectively. More importantly, the dielectric properties are found to be relatively stable to near the T g value, which can be widely used for applications in dielectric energy storage at high temperatures.
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