Synergistically Enhanced Interfacial And Wave-Transparent Properties By Constructing Self-Assembly COF On PBO Fiber

The inert and smooth surface of Poly-p-phenylene benzobisoxazole(PBO)fiber leads to poor interfacial compatibility and strong interfacial polarization between PBO fiber and resin matrix,which limits the mechanical and wave-transparent performance of composites.Therefore,it is significant to improve interfacial adhesive and wave-transparent properties of the composites by fiber surface modification.Based on the non-covalent assembly modification of carbon fiber with π-π conjugated structure,a method of non-covalent assembly modification of PBO fiber using three kinds of covalent organic framework(A-COF,B-COF and MCOF)with different dimensions was proposed in this paper.The assembly process and mechanism of COF with different dimensions on the surface of PBO fiber,and the influence of the constructed interphase on the interfacial compatibility and low dielectric properties of composites were studied,which achieved the synchronous enhancement of the interfacial and wide-band wave-transparent properties of composites.(1)Three kinds of COF with different dimensions were prepared by molecular design: 2D COF(A-COF,B-COF)and 3D COF(M-COF),and the physicochemical properties of the three COF were studied.It was proved that the three kinds of COF had higher specific surface area and porosity.2D COF showed strong crystal peak,and its microstructure was coral-like structure formed by lamellar accumulation.However,the crystallinity of 3D COF with irregular nanoparticle aggregates was poor.(2)A-COF@PBO-(24h,48 h,72h),B-COF@PBO-(24h,48 h,72h)and M-COF@PBO-(24h,48 h,72h)fibers were prepared by in situ assembly of three kinds of COF on Pristine PBO fiber surface.Additionally,the modified fibers with assembly time of 24 h and 48 h were prepared to explore the physicochemical characteristics and assembly mechanism of COF during the growth process on fiber surface.Compared with Pristine PBO fiber,surface roughness and surface energy of A-COF@PBO-72 h,BCOF@PBO-72 h and M-COF@PBO-72 h were improved.The assemblygrowth of 2D COF on PBO fiber surface was achieved through the balance between π-π interaction and covalent bond.The driving force of 3D COF assembly on fiber surface was π-π interaction,while the driving force in the growth process was mainly covalent bonding.(3)Pristine PBO,A-COF@PBO-72 h,B-COF@PBO-72 h and MCOF@PBO-72 h fiber bundle,monofilament and unidirectional composites were prepared to evaluate the interfacial properties,fracture morphology and wide-frequency(1 MHz-20 MHz and 12.4 GHz-18.0GHz)dielectric properties of the composites.Compared with Pristine PBO fiber composites,the TFBT and IFSS strength of A-COF@PBO-72 h,BCOF@PBO-72 h and M-COF@PBO-72 h fiber composites were all improved,and the interface enhancement mechanism of PBO fiber composite was proposed: 2D COF nanoparticles effectively increased the length of crack propagation path and enhanced substantial interfacial dissipation energy.The pore structure from 3D COF nanoparticles stacking on the fiber surface formed interlocking stereoscopic interphase structures via penetrating with resin matrix.Compared with Pristine PBO fiber composites,the wave-transparent properties of A-COF@PBO-72 h,B-COF@PBO-72 h and M-COF@PBO-72 h fiber composites were all enhanced in wide frequency range,and the wave-transparent enhancement mechanism of PBO fiber composite was proposed: in spite of the interlayer carrier flowing of 2D COF,COF@PBO-72 h fiber composites exhibited excellent wave-transparent performance during wide frequency range,attributing to the attenuating of interface polarization effect and the decrease of polarized molecule number per unit volume.