Synthesis, Structure And Properties Of Modified Poly (P-phenylene Benzoxazole) (DHPBO)

Poly（p-phenylenebenzobisoxzole）（PBO） fiber can be widely used in advanced composites due to its excellent tensile strength,modulus,thermal resistance and flame retardant properties.However,the interfacial adhesion between PBO fiber and resin matrix is not sufficient for practical uses.The poor adhesion is generally the result of the chemically inactive surface and/or smooth surface of PBO fiber.In addition,PBO has lower compressive strength and poor resistance to ultraviolet radiation.Therefore it is very important to improve the adhensive and anti ultraviolet properties.In this thesis hydroxyl groups was introduced to PBO molecular chain to improve its surface property and compressive strength.In this study,dihydroxy PBO（DHPBO） containing hydroxyl groups in macromolecular chains was obtained by co-polymerization from 4,6 -diamino resorcinol dihydrochloride（DAR） and terephthalic acid（TPA）,with addition of certain amounts of 2,5-Dihydroxyterephthalic acid（DHTA） in replace of TPA in polyphosphoric acid（PPA）.The FT-IR and element analysis results showed that DHTA was incorporated into polymer chains.DHPBO fiber was spun via dry-jet and wet-spinning technique,and the mechanical property,thermal property and aggregating structure of the fiber were investigated.The tensile strength of DHPBO-5%fiber and DHPBO-10%fiber were 5.24GPa and 5.05 GPa respectively.The aggregating structure of DHPBO was affected by the introduction of hydroxyl groups.However,the crystallinity of DHPBO fiber was changed a little.The thermal stability of DHPBO fiber was excellent and the decomposition temperature was about 667°C in nitrogen atmosphere.The surfaces of the fibers were carefully analyzed and quantified.The contact angles between fiber and water/ethyl alcohol were measured by an OCA 40 Micro dynamic contact angle analysis system.It was found the contact angle of DHPBO to water and alcohol got smaller than that of PBO to either of them.For DHPBO-5%and DHPBO-50%fiber the contact angle to water was 66.0°and 50.7°respectively,while the value for PBO fiber was 71.4°.The surface free energies of DHPBO-5%and DHPBO-50%were increased up to 39.57mJ/m² and 50.25mJ/m² respectively,while the value for PBO fiber was 35.5mJ/m².The interfacial shear strength（IFSS） was studied by microbond fiber pull-out test.The results showed that the IFSS of DHPBO fiber/ epoxy composite was improved obviously compared to PBO fiber/epoxy system.The IFSS of DHPBO-5%,DHPBO-10%,DHPBO-20%and DHPBO-50%/epoxy were 16.83MPa,18.87 MPa,17.43MPa and 15.32MPa and was increased by 71.73%, 92.55%,77.85 and 56.33%respectively compared to PBO fiber/epoxy system （IFSS=9.69MPa）.XPS analysis indicated that the surface oxygen-to-carbon ratio was increased and the ionization energy（or binding energy） of C_1s and O_1s was decreased. The single fiber compress-bending measurement analysis indicated the compressive roperty of DHPBO fiber was also improved.The anti UV properties of PBO and DHPBO fibers were investigated under UV accelerated aging condition.The strength retention of DHPBO was 74%after exposing to UV for 412 hours,while for PBO fiber the retention was only 35.9%.The UV stability of PBO fiber was improved after hydroxyl groups were introduced.