Synthesis、Structure And Properties Of Modified Poly(P-phenylene Benzobisoxazole) 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 superior tensile strength,modulus,excellent thermal stability and flame retardant properties.Thus PBO fiber provides great potential applications as reinforcement fiber for advanced composites.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.PBO also has poor resistance to ultraviolet radiation.So,further improvements in ultraviolet radiation resistance are expected in the research of PBO fibers.Moreover, incorporation of nano material will be the main method to modify PBO fiber.In this study,chemical and physical methods were adopted to prepare modified PBO fibers respectively.The main results were as follows:modified PBO（p-SPBO） containing ionic groups（-KSO₃） in macromolecular chains was obtained by copolymerization from 1,3-diamino-4,6-dihydroxybenzene dihydrochloride（DAR） and terephthalic acid（TA）,with addition of certain amounts of 2-sulfoterephthalic acid monopotassium salt（STPP） in replace of TA in polyphosphoric acid（PPA）.The p-SPBO fibers were spun successfully by dry-jet wet spinning technique.The surface properties of the fibers were analyzed and quantified by contact angle measurement.It was found that the contact angle of p-SPBO to water,alcohol and acetone got smaller compared to PBO fiber.For p-SPBO1.5 containing 1.5%ionic group and p-SPBO5 containing 5%ionic group fiber the contact angle to water was 66.2°and 61.2°respectively,while the value for PBO fiber was 71.4°.The interfacial shear strength （IFSS） was studied by microdroplet pull-out test.The results showed that the IFSS of p-SPBO1.5 and p-SPBO5/epoxy were 12.9 MPa and 17.4 MPa respectively,which was increased by approximately 34%and 80%compared to PBO fiber/epoxy system （IFSS=9.69MPa）.The effect of ionic group on the aggregation structure of PBO was analyzed by X-ray diffraction analysis and density test.It was indicated that the crystallization degree,crystal size and fiber axial orientation of PBO decreased with the incorporation of STPP,which resulted in the reduction of mechanical properties of PBO ultimately.However,appropriate content can be controlled to achieve a balance between the surface and mechanical properties of PBO fiber.For example,about 1.5mol%ionic group could minimize the loss of mechanical strength at the same time effectively improve the surface properties of the PBO fiber.With in-situ bending multi-wall carbon nanotubes（MWNTs） were introduced into the solution polymerization system of PBO and the blends were spun into high performance fibers via liquid crystal spinning.Under the observation with polarized optical microscope and result of intrinsic viscosity test,it was found that strong acid treated MWNTs,lower amount,pre-diffusion in poly（phosphoric acid）（PPA） instead of direct adding MWNTs would promise better dispersal and high molecular weight MWNTs/PBO polymer.The results of the mechanical and thermal properties of PBO and MWNTs/PBO fibers showed that the tensile strength and modulus of MWNTs/PBO fiber containing 2wt%MWNTs were 4.7GPa and 128.8GPa respectively,which are approximately 20%higher than that of the pure PBO fibers. The decomposition temperature of the blend fiber was also improved by 30°.As the content increased to a high level,MWNTs aggregated seriouly,giving a negative influence on the spinnability of MWNTs/PBO blend and the mechanical properties of blend fibers.XRD analysis indicated that the introduction of MWNTs result in the reduction of the fiber axial orientation degree and destruction of the regular molecular arrangement of PBO.Therefore,the reinforcement of MWNTs in PBO matrix may depend on the high mechanical properties of MWNTs itself.According to the analysis on the reinforce mechanism of MWNTs,it could be expected that well-dispersed MWNTs,improve orientation degree of MWNTs as high as possible in polymer matrix and relatively shorter length could greatly realize the reinforcement of carbon nanotube in PBO matrix.With in-situ bending mixed（rutile and anatase coexist） nano-sized titanium dioxide（TiO₂） were introduced into the solution polymerization system of PBO/PPA, and high performance m-TiO₂/PBO blend fiber was firstly obtained.The use of appropriate technology could achieve good dispersal of nano TiO₂ in PBO matrix. The tensile strength and modulus of the m-TiO₂/PBO fiber containing 1.5 wt%TiO₂ were 4.6GPa and 110GPa respectively,which were approximately 36%and 18% higher than that of the pure PBO fiber under the same process conditions.XRD analysis showed that the fiber axial orientation degree of PBO was improved from 93.88%to 96.52%by the addition of TiO₂.It could be concluded that PBO macromolecular chains were easier to orientate by the presence of TiO₂ particles when the PBO/PPA solution under tension in spinning process.UV absorbent 4,4’-bis（2-benzoxazolyl）stilbene and rutile nano-TiO₂ were introduced into PBO system respectively and the blend fibers were tested under aging condition both of outdoor weathering and ultraviolet accelerated radiation companied with MWNTs/PBO and m-TiO₂/PBO blend fibers.The effects of additives as well as the aging effect of the different light sources on the mechanical properties,molecular weight and surface morphology of PBO fiber were analyzed.The results showed that the pure PBO showed a large deterioration in mechanical properties and different additives showed varying degrees of inhibition on the UV aging of PBO.Rutile nano-TiO₂ as well as UV absorber 4,4’-bis（2-benzoxazolyl）stilbene signficately improve the ultraviolet stability of PBO.Low content of mixed nano-TiO₂ showed certain inhibiting effect in PBO aging.However,high content of mixed TiO₂ would accelerate the photodegradation of PBO after a certain period of ultraviolet radiation. An aging mechanism of photodegradation was suggested.The mechanical properties of PBO dropped sharply at the initial period of ultraviolet radiation maybe due to the destruction of PBO surface morphology,which would form stress concentration point. The experiments under different aging conditions basically reflected the ultraviolet stability of PBO fiber.Considering a variety change of climate,the outdoor weathering result was more obvious and credible than ultraviolet accelerated radiation.