| Poly-p-phenylene benzobisoxazole(PBO)belongs to the rod-like aromatic heterocyclic polymer family.The special rigid-rod structure contributes to its excellent mechanical properties,heat resistance and flame retardancy.However,the ultraviolet sensitivity,poor compressive strength and unsatisfactory interfacial adhesion performance of PBO fiber are unfavorable to its long-term use and seriously hinder its application in the composite field.In addition,abundant studies have pointed out that PBO fiber can be used as a new precursor for graphite fiber owing to its high thermal stability and easy graphitization.The primary purpose of this thesis is to improve these inferiorities of PBO fiber and furtherly expand its functional applications.For example,PBO is easy to graphitize and can be used as the precursor for high thermal conductivity carbon fiber.In this thesis,in order to solve the problems of poor mechanical property of PBO based carbon fiber and lower thermal conductivity of polyacrylonitrile(PAN)based carbon fiber,the strategy of preparing PAN/PBO composite carbon fiber was proposed.And the obtained carbon fiber s will meet the needs for novel carbon fibers and composites with high thermal conductivity and excellent mechanical properties in the aerospace field.Based on the studies about thermal decomposition of PBO fiber,structural transformation of PBO induced by heat treatment was revealed in this work.Under the heat treatments at the temperatures slightly lower than its thermal decomposition temperature along with proper axial drafting,heat-treated PBO(PBO-HT)fibers with purple thermal oxidation sheathes and excellent mechanical properties were prepa red.It has been proven that the peripheral UV sensitive oxazole groups were transformed into non-conjugated six membered rings after heat-induced gradient thermal oxidation.According to the ultraviolet-visible(UV-vis)adsorption spectra and ultraviolet(UV)accelerated aging tests,the absorbances in UV band of PBO-HT fibers were significantly reduced,while the visible band absorbances were substantially enhanced.Moreover,more than 90% of the PBO-HT fiber’s tensile strengths was retained and the surface defects after 800 h UV aging were also effectively decreased.However,the tensile strength of the control group(PBO-AS fiber)has reduced to 49.81% of its original strength.In a word,UV resistance s can be improved through creating thermal oxidized non-conjugated sheath on PBO fibers.There were also considerable improvements in mechanical properties of PBO fibers.For instance,the highest Young’s modulus and compressive strength of PBO fibers reached 221 GPa and 1.74 GPa,increased by 33.5 % and 74%,respectively.The inhomogeneous reductions in rigid-rod backbones resulted in the maintenance of internal microfibrils and decrease in peripheral microfibrils,and accordingly relieved the compressive damage.Meanwhile,PBO-HT fibers exhibited the highest interfacial shearing strength(IFSS)of 46.91 MPa,increased by 64% with the appearance of non-conjugated structure on its surface.Enhancements of their interfacial properties resulted from the increased surface roughness and decreased inert rigid-rod structures.Impressively,owing to its facile operation,high efficiency and consistent effect,heattreatment technique along with proper axial drafting was expected to be an industrialscale way to produce anti-UV PBO fiber.The structure evolutions in PBO fiber during graphitization showed that PBO could be transformed into highly oriented graphite microcrystals after high temperature heat treatment above 2000 ℃,which is an important structural feature of high thermal conductive carbon fiber.At present,polyacrylonitrile(PAN)is the common precursor for carbon fiber.However,it belongs to difficult graphitization material,so the thermal conductivity of PAN based carbon fiber(PAN-CF)is relatively low.In the present work,PBO macromolecules,which could contribute to the development of highly ordered graphite from conjugated aromatic backbone,were introduced onto PAN-CF to prepare coaxial PAN/PBO based carbon fiber(named as PAN/PBO-CF)after graphitization.Based on the Raman spectra results,the introduction of high crystallinity PBO layer could effectively repair the surface defects of PAN-CFs.And the higher crystallization and orientation of graphite crystallite was revealed by XRD and TEM results.Of interest,PAN/PBO-CF exhibited smaller interlayer spacing and higher crystallinity compared to P AN-CF,and PBO derived graphite crystals could also be perpendicular to fiber surfaces.These characteristics were expected to endow PAN/PBO-CF with enhanced radial and axial thermal conductivities.The thermal conductivity results showed that the axial thermal conductivity of PAN/PBO-CF fiber was as high as 102.37 W/(m·K)and its theoretical radial thermal conductivity was inferred to be 6.29 W/(m·K),44% and 56% higher than that of PAN-CF,respectively.However,the insufficient thickness(about 155nm)of PBO derived graphite layer greatly limited the through-thickness thermal conductivity enhancement of PAN/PBO-CF epoxy composites,which was 2.54W/(m·K)at the 55% carbon fiber loading.In order to further improve the through-thickness thermal conductivity of carbon fiber reinforced composites,three-dimensionally oriented PBO-derived graphite with increased thickness and thermal conductive hybrid network were assembled onto PAN-CF.Firstly,crosslinked Ni/CNT hybrid networks were pre-electrodeposited onto PAN-CF(named as CF@Ni/CNT)to introduce uniform coating of PBO layer with sufficient thickness.In order to further improve the through-thickness thermal conductivity of its composites,PAN/PBO-CF with three-dimensional heat conductive structures(named as CNC@PG-G)were constructed through introducing vertically oriented large PBO/GO layers on CF@Ni/CNT and graphitization.In addition,scanning electron microscopy(SEM)images showed intertwined Ni/CNT networks and vertical graphene-bridging structures on CF surfaces,which could provide efficient heat conduction pathways among CF filaments.Accordingly,the synergistic effect originated from directionally oriented PBO-derived graphite and highly thermal conductive Ni/CNT networks could considerably enhance the throughthickness thermal conductivity of CNC@PG-G/epoxy composite.The axial thermal conductivity of CNC@PG-G was increased to 127.19 W/(m·K),and its theoretical radial thermal conductivity was inferred to be 66.57 W/(m·K).In addition,it is easier for CNC@PG-G composites to form continuous heat conduction paths.Accordingly,the maximum thermal conductivity value of CNC@PG-2100 reinforced composites reached 5.39 W/(m·K)at the filler loading of 55%,with 573% enhancement compared to the PAN-CF counterpart(0.94 W/(m·K)).Furthermore,the infrared(IR)image results sufficiently demonstrated the significantly improved heat absorption and heat dissipation capability of composites. |
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