Molecular Modification Of Aromatic Heterocyclic Liquid Crystalline Polymer And The Preparation, Characterization And Application Of Its High Performance Fiber

Poly（p-phenylene benzobisoxazole） （PBO） is one kind of aromatic heterocyclic rigid-rod polymer. PBO fiber produced via dry-jet wet-spinning technique has outstanding tensile strength and tensile modulus,high thermal stability and excellent flame retardant properties. These properties enable PBO fiber to be used as reinforcement materials and have great potential applications in advanced composites. However, PBO fiber has poor interfacial adhesion ability with matrix resin, poor resistance to ultraviolet radiation and poor compressive performance （0.2-0.4 GPa）. These defects greatly limit its application in advanced composites. Therefore. to improve the intermolecular interaction, axial compressive strength, ultraviolet stability and interfacial adhesion between PBO fiber and resin matrix present key problems for the application of PBO fibers in advanced composites field.In this thesis, towarding high performance PBO fibers, we systematically investigated the modification of aromatic heterocyclic polymer by molecular structure design and modified chemical copolymerization and obtained modified PBO fibers with much improved interfacial adhesion ability, axial compressive strength and ultraviolet stability.1. By introducing 2,5-dihydroxyterephthalic acid （DHTA） into poly（p-phenylene benzoxazole） （PBO） macromolecular chains, dihydroxy poly（p-phenylene benzobisoxazole） （DHPBO） was synthesized and then DHPBO fibers were prepared by dry-jet wet-spinning method. The tensile strength of DHPBO fibers was gradually decreased with the increasing of DHTA, but the tensile strength and tensile modulus of DHPBO fibers reached 5.05～5.24 GPa and 143.48～171.61 GPa. respectively, when the content of DHTA was 5～10 mol%. The testing result of the TGA indicated that the DHPBO fibers have excellent thermal stability.2. The effect of coagulation conditions during the dry-jet wet-spinning process (including coagulation bath temperature (T_Cb), coagulation bath concentration (C_Cb) and air gap distance （Dag）) on the morphology, microstructures and mechanical properties of DHPBO nascent fibers were investigated, which the coagulation bath system was polyphosphoric acid （PPA）/H₂O. The optimized preparation parameters of nascent DHPBO fibers were obtained. The results indicated that when the T_Cb was 25℃, C_Cb was 10% and Dag was 30 cm, highly crystallized DHPBO nascent fibers with fine crystallites, circular and smooth morphology, and excellent mechanical properties could be achieved.3. The surface properties of DHPBO fibers were investigated by X-ray photoelectron spectroscopy （XPS） and dynamic contact angle analysis system. XPS analysis showed an increased ratio of oxygen/carbon, which proved the successful incorporation of DHTA in polymer chains. The contact angles between fiber and water/alcohol were measured by an OCA 40 Micro dynamic contact angle analysis system. It was found that the contact angles of DHPBO to water and alcohol got smaller than that of PBO to either of them, and the surface free energy of DHPBO-10% fiber was increased to 42.89 mJ/m2,20% higher than that of PBO fibers.4. The interfacial shearing strength （IFSS） between DHPBO （10% mol content DHTA） fibers and epoxy resin is 18.87 MPa,90% higher than that of PBO fibers. SEM images indicate that the PBO/epoxy composite failure mode may change from fiber/matrix adhesive failure to partially cohesive failure.5. The compressive property of DHPBO fibers were investigated by single fiber axial compression bending test and elastic loop test methods. The results of single fiber axial compression bending test indicated that compared with PBO, DHPBO fibers showed improved axial compressive property. The equivalent bending modulus of DHPBO fibers showed increased to 19.37 GPa,26% higher than that of PBO fibers. Under same compression load, the kink band on PBO fiber surface is very clear, while the surface of DHPBO fiber still remains smooth and uniform. Meanwhile, the results of elastic loop test also showed that the compressive strength of DHPBO fibers were obviously higher than that of PBO fibers after introduction of hydroxyl groups into PBO macromolecular chains.6. Modified PBO fibers, which exhibits anti-ultraviolet properties, were prepared by chemical copolymerization and physical blend methods. Mechanical property test, intrinsic viscosity test, SEM and ATR-FTIR were used to characterize the fibers before and after UV accelerated aging. The results indicate that DHPBO/n-TiO₂ fibers show the best UV resistance. In addition, the UV resistance of nano-rutle-TiO₂ was obviously better than that of 2,2’-（1,2-Ethenediyldi-4,1-phenylene） bisbenzoxazole. Based on ATR-FTIR of PBO fibers before and after UV exposure, the UV aging degradation mechanism of PBO fiber was proposed.7. A series of fluorinated benzoxazole polymers （6FPBO） were synthesized by introducing 4’4-（hexafluoroisopropylidene） bis（benzoic acid） （BIS-B-AF） into PBO macromolecular chains. The polymerization was spun into fibers via dry-jet wet-spinning technique or casted into films. Basic performances of 6FPBO in field of dielectric materials were investigated. The results show that the 6FPBO fibers prepared in this experiment exhibit excellent mechanical properties, thermal stability, dielectric properties, swelling degree, water uptake and thermal expansion coefficient, which satisfyied the requirements for electronic packaging materials.8. Poly{[2,6-diimidazo（4,5-b:4’,5’-e）pyridinylene-1,4（2,5-dihydroxy）phenylen]. （PIPD）} （PIPD） was synthesized and PIPD fiber was produced via dry-jet wet-spinning technique. It was found that the contact angles of PIPD fiber to water and alcohol got smaller than that of PBO to either of them. Compared with PBO. PIPD fibers showed improved axial compressive properties, the equivalent bending modulus of PIPD fibers show increased to 22.54 MPa.47% higher than that of PBO fibers.9. DHPBO/GF/EP and PBO/GF/EP composite materials were prepared from PBO and DHBPO fibers’fabrics for the first time. Preliminary studies has been carried out on the machine weaved structure of PBO and DHBPO fabrics, interlaminar shear strength, impact resistance and interfacial bond property of composites. The result showed that the interlaminar shear strength, impact resistance and interfacial bond property of DHPBO/GF/EP composites were obviously better than that of PBO/GF/EP.