To improve the interface adhesion between high‐strength‐high‐modulus polyimide (PI) fibers and epoxy resin (EP) matrix, waterborne epoxy resin (WEP) sizing agents are used to modify the surface of PI fibers. The sizing treatment of PI fibers was carried out by the self‐made sizing machine, and the effect of WEP sizing concentrations on the structure and properties of PI fibers were discussed in detail. The surface roughness, activated carbon percentage, and surface energy of WEP‐sized PI fibers increased with increasing WEP sizing agent contents. While, the mechanical and thermal properties of WEP‐sized PI fibers decreased slightly with the increment of WEP sizing agent concentrations. The optimum WEP sizing concentration was recognized to be 2 wt%. The interfacial shear strength and the interlinear shear strength values of PI fibers and epoxy resin (PI/EP) composites were improved by 175.22 and 25.31%, respectively. Moreover, the mechanism of surface characteristic improvement with WEP sizing treatment were proposed, contributing to the enhancement of mechanical interlocking and chemical interaction between fibers and matrix.

The weak interface adhesion between high‐strength‐high‐modulus polyimide (PI) fibers and epoxy (EP) resin matrix has seriously hampered the application of PI/EP composites. Herein, a waterborne polyamide (WPA) sizing agent was used to modify PI fibers to enhance the interface adhesion strength between the PI fibers and EP resin matrix. The surface characteristics of PI fibers were investigated to determine chemical composition, morphology, roughness, wettability, interfacial shear strength (IFSS) and interlinear shear strength. The results indicated that the WPA sizing agent significantly enhanced the interface adhesion properties between the PI fibers and EP resin matrix with a 132% and 22.35% increment obtained for IFSS and interlaminar shear strength (ILSS) compared with unsized PI/EP composites. Then, the hybrid sizing agent generated by WPA and waterborne EP sizing agent exhibited the highest interface adhesion with 168.65% and 28.62% increment obtained for IFSS and ILSS compared with unsized PI/EP composites. Based on these results, a sizing mechanism with chemical interaction and mechanical interlocking was proposed, which provides an effective and feasible method to enhance the interface adhesion between the PI fibers and EP resin matrix.

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The novel copolymerization polyimide (PI) hollow fibers (HFs) of 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) containing –CF3 groups were prepared and investigated through both simulation and experiment. To demonstrate the alteration attributable to the introduction of fluoromonomers, the condensed states of pyromellitic dianhydride (PMDA)/4,4′-oxybisbenzenamine (ODA), PMDA/6FDA/ODA, and 6FDA/ODA PI were constructed by Material Studio, and we simulated the mobility of molecular chain, free volume fraction, and O2/N2 dissolution–diffusion process. The molecular dynamics simulation results demonstrated that the properties of the copolymerized PI system with 6FDA were significantly improved, while the selectivity remained almost unchanged. Then, the films of copolymerized PI and HFs were prepared by the two-step method, and O2/N2 permeability of the PI copolymer films was characterized, indicating that although the gas permeation performance was greatly improved, the selectivity was not so satisfactory. However, the selection factor increased heavily after polydimethylsiloxane coating.

To prepare PIs (polyimides) with desirable thermal and mechanical properties is highly demanded due to their widespread applications in flexible optoelectronic devices and printed circuit boards. Here, the PI films of BPDA/4,4′-ODA, BPDA/3,4′-ODA, PMDA/4,4′-ODA, PMDA/3,4′-ODA systems were prepared, and it was found that the PIs with 3,4′-ODA always exhibit a high modulus compared with the PIs with 4,4′-ODA. To disclose the mechanism of high-modulus PI films with 3,4′-ODA, amorphous PI models and uniaxial drawing PI models were established and calculated based on MD simulation. The PI structural deformations at different length scales, i.e., molecular chain cluster scale and repeat unit scale, under the same stress were detailed and analyzed, including the variation of chain conformation, bond length, bond angle, internal rotation energy, and torsion angle. The results indicate that PIs with 3,4-ODA have higher internal rotation energy and smaller deformation with the same stress, consistent with the high modulus.