New fluorinated polyimides based on 1,2-bis(4-aminophenyl)-4,5-bis(4-trifluoromethylphenyl)-1H-imidazole

A new aromatic unsymmetrical diamine containing imidazolyl and trifluoromethyl groups, 1,2-bis(4-aminophenyl)-4,5-bis(4-trifluoromethylphenyl)-1H-imidazole was synthesized. A series of novel fluorinated polyimides (PIs) were prepared by polycondensation of this diamine monomer with various aromatic dianhydrides via one-step process. The resulting PIs bearing inherent viscosities of 0.45–0.61 dL/g were amorphous and readily soluble in organic polar solvents such as NMP, DMF, DMAc, THF, and CHCl3. These new PIs exhibited good thermal stability with 10% weight loss temperatures of 530–560°C and char yields of 61–65% at 800°C in nitrogen, as well as good flame retardancy with limited oxygen index value of 40.6–42.8, and outstanding hydrophobicity with the contact angle of 85.2–93.5°. Meanwhile, all the obtained PIs exhibited low water uptake of 0.30–0.53%, strong UV-vis absorption in the range of 267–310 nm in DMF solution, and their films presented high optical transparency with a cut-off wavelength in the 355–392 nm range. Except for the PI film derived from pyromellitic dianhydride, the other PI films had the tensile strength in the range of 53.2–74.1 MPa, elongation at breakage of 6–15%, and tensile modulus of 1.0–1.5 GPa, respectively.

Preparation and characterization of soluble heat-resistant polyimide films containing bis-N-phenyl-benzimidazole

To prepare soluble polyimides with high temperature resistance, two new diamine monomers, namely, 2,2′-(4,4′-oxybisphenylene)-bis(1-phenyl-5-aminobenzimidazole) (5a), and 2,2′-(4,4′-hexafluoroisopropylidene)-bis(1-phenyl-5- aminobenzimidazole) (5b), were synthesized and exploited to prepare three series of poly(benzimidazole imides)s (PBIIs) by a conventional two-stage synthesis. The resulting PI films were flexible and tough, possessing high glass-transition temperatures (Tgs = 311°C–390°C), improved optical transparency, and excellent solubility. Moreover, the effect of different configuration on performance was revealed, and these data provided a feasible method to enhance both Tg and solubility of PIs by incorporating N-phenyl benzimidazole and corresponding functional moieties.

Soluble copolyimides containing 4,4′-isopropylidenedicyclohexanol (HBPA) isomer units: Synthesis, characterization, thermal, mechanical, and optical properties

In this work, 4,4′-isopropylidenedicyclohexanol (HBPA)-based dinitro isomers mixture (H″BPBN and H′BPBN) was synthesized and separated, and the structures of dinitro isomers were confirmed by differential scanning calorimetry and proton nuclear magnetic resonance spectroscopy. A series of copolyimides were prepared from diamino monomers with different percentages of novel diamine H″BPDA and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride via a conventional two-step procedure. All the copolyimides could afford flexible, tough, and transparent films with transmittance no less than 73% at 450 nm, which was attributed to the fact that nonplanar alicyclic diamine and the bulky and weakly polarizable trifluoromethyl hampered the formation of charge-transfer complex. Moreover, all the copolyimides were soluble not only in polar solvents such as N,N-dimethylformamide but also in low-boiling-point solvents such as dichloromethane, which was related to the fact that the existence of alicyclic diamine and bulky trifluoromethyl decreased the intermolecular force. In addition, the conformation effects of H″BPDA and H′BPDA on the aspects of thermal, mechanical, optical, and soluble performance of copolyimides were investigated and their structure–property relationships were discussed in detail.

PEEK composites with polyimide sizing SCF as reinforcement: Preparation, characterization, and mechanical properties

In this work, the surface modification of short carbon fibers (SCFs) using polyimide (PI) as a sizing agent was conducted and fully characterized, and SCF-reinforced polyether ether ketone (PEEK) composites were obtained by extrusion and injection molding. The surface characteristics of the PI-coated SCFs were evaluated using scanning electron microscopy and X-ray photoelectron spectroscopy. The results indicated that a uniform PI sizing layer was formed on the surfaces of the SCFs. Thermogravimetric analysis results demonstrated that PI-coated SCFs had better thermal stability than commercial SCFs. The tensile strength and flexural strength of the PI-coated SCF/PEEK composites showed improvements of 11.8% and 16.6% compared with the commercial cases, which were attributed to the PI sizing treatment effectively improving the interfacial adhesion between the SCF and the PEEK matrix. Dynamic mechanical analysis and the morphologies of tensile fracture surfaces suggested better interfacial adhesion between the fibers and the PEEK matrix, which were in good agreement with the mechanical properties. Due to the convenient processing of PI sizing as well as the effectively improved mechanical properties of the composites, the PI-sizing methodology has great potential application in the field of fiber-reinforced high-temperature engineering plastics composites.

Soluble Polyimides Bearing (cis, trans)-Hydrogenated Bisphenol A and (trans, trans)-Hydrogenated Bisphenol A Moieties: Synthesis, Properties and the Conformational Effect

In this work, hydrogenated bisphenol A (HBPA) based dinitro mixed isomers (1a′ and 1a) were synthesized and separated via vacuum distillation under the monitor of DSC and ¹H NMR. Corresponding diamines (2a′ and 2a) were separately polycondensed with five commercial dianhydrides via a two-step thermal imidization to obtain PI-(1′-5′) and PI-(1-5). All the polyimides could afford flexible, tough, and transparent films, and most of them were readily soluble not only in common polar solvents like DMAc, but also in low boiling point solvents such as chloroform. ¹H NMR spectra of the polyimides demonstrated that HBPA moiety showed no conformation changes during the preparation of polymers. For a given dianhydride, PI-(1-5) exhibited better thermal stability than that of PI-(1′-5′), this can be attributed that the equatorial, equatorial C–O in PI-(1-5) promoted denser and more regular molecular chain stacking, as can be evidenced by the WAXD and geometric optimization results. Additionally, when the dianhydride was ODPA, BPADA or 6FDA, no apparent difference was found in either the transmittance or solubility between two series of polyimides, which could be attributed that twisted and flexible ether linkages, as well as bulky substituents, led to the “already weakened” inter- and intramolecular CT interaction and cohesive force. However, when it came to rigid and stiff dianhydride, e.g., BPDA, PI-3′ took an obvious advantage over PI-3 in transmittance and solubility, which was possibly owed to the larger molecular chain d-spacing imparted by equatorial, axial C–O. An overall investigation of PI-(1′-5′) and PI-(1-5) on aspects of thermal, mechanical, morphological, soluble and optical performance values was carried out, and the conformation effects of HBPA isomers on the properties of two series of polyimides were discussed in detail.