Effects Of Oxygen Plasma Modification On Surface Properties Of PBO Fibers And Interface Properties Of PBO/PPESK Composites

Due to the exceptionally high specific strength and modulus, excellent thermal and oxidative stability, good wave-absorbing and stealthy performance, poly(p-phenylene benzobisoxazole) (PBO) fiber reinforced poly(phthalazinone ether sulfone ketone) (PPESK) composites provide great potential applications as outer-structural and inner-carrier materials in aeronautical and astronautical applications, national defense and military applications. However, the interfacial adhesion of PBO/PPESK composite is poor. Interface is the key structure of composites, it plays important roles in linking reinforcements and the matrix, which influences the mechanical properties, heat-resistance and even life-spans of the composites significantly. Good fiber-matrix interfacial adhesion results in high quality of the composites. The interfacial adhesion of PBO fiber reinforced composites is poor because of the relatively smooth and chemically inactive fiber surfaces which prevent efficient chemical and physical bonding in the interface. Therefore, surface modification of PBO fibers is of great importance in the field of composites application.PBO fibers were surface-modified by inductively coupled oxygen plasma treatment in this paper. The effects of oxygen plasma treatment time, discharge pressure and plasma power on surface chemical composition, surface morphologies, surface roughness and surface free energy of PBO fibers were investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and dynamic contact angle analysis (DCA), respectively. The results indicated the oxygen plasma treatment introduced some polar groups (such as O-C=O) to PBO fiber surfaces, enhanced surface roughness and changed surface morphologies of PBO fibers by plasma etching and oxidative reactions. Surface wettability of PBO fibers was significantly improved by increasing surface free energy of the fibers via oxygen plasma treatment. Plasma treatment conditions should be appropriate during surface modification of PBO fibers by oxygen plasma.The effects of oxygen plasma treatment time, discharge pressure and plasma power on interfacial adhesion of PBO/PPESK composite were characterized by interlaminar shear strength (ILSS) and water absorption measurements. Fractured surface morphologies of the composites were still observed by SEM. The results showed that the ILSS of PBO/PPESK composite increased while the water absorption degraded after oxygen plasma treatment, the interfacial adhesion of the composite was significantly improved consequently. The primary failure mode of PBO/PPESK composite shifted from interface failure to matrix fracture after oxygen plasma treatment. Plasma treatment conditions should be appropriate during interface modification of PBO/PPESK composite by oxygen plasma treatment. Too long plasma treatment time, too low discharge pressure and too high plasma power would damage the inherent-performance of PBO fibers, which then led to the degradation of interfacial adhesion of PBO/PPESK composite. The interfacial adhesion of PBO/PPESK composite was improved by oxygen plasma treatment due to the chemical linkage and mechanical bonding between PBO fibers and PPESK matrices, and the chemical linkage effect do more contribution than the mechanical bonding on improving the interfacial adhesion of PBO/PPESK composite. The interfacial adhesion of PBO/PPESK composites reached to plateau when the plasma treatment time is 15min, the discharge pressure is 30Pa and the plasma power is 200W.Aging behaviors of oxygen plasma treatment were investigated by DCA, XPS and AFM analysis. The results suggested that when the oxygen-plasma-treated PBO fibers were exposed in air for some days, the active particles and polar groups in fiber surfaces would took some reorientation and post-reactions, which results in a degradation of surface free energy of PBO fibers and a reduction of polar groups in the fiber surfaces. Surface roughness of the fibers increased firstly and then decreased. The ILSS and water absorption measurements indicated that the interfacial adhesion of PBO/PPESK composite degraded because of the aging behaviors of oxygen plasma treatment. Aging behaviors of PBO fibers and PBO/PPESK composites appeared in the first several days after oxygen plasma treatment, minor aging effects were observed with the duration of the aging time because the surface and interface states became stable with aging time increasing.Oxygen-plasma-grafting-epoxy treatment was further used modifying the interfacial adhesion of PBO/PPESK composites. Optimal treatment condition was obtained by orthogonal experiments, which can be stated as: the plasma power is 50W, the plasma treatment time is 10min, the discharge pressure is 30Pa and the concentration of grafting solution (Epoxy/Acetone solution) is 3%. The ILSS, water absorption, SEM and thermogravimetry analysis (TGA) results showed that the oxygen-plasma-grafting-epoxy treatment improved the interfacial adhesion of PBO/PPESK composites significantly but without influencing heat-resistance properties of the composites. The composites were hard to destroy by interface shear load after interface modification. FT-IR and DCA analysis indicated the epoxy groups polymerized in PBO fiber surfaces, and the reaction extent reached to 60.8%. Surface free energy of PBO fibers increased after the treatment, surface wettability of the fibers was improved consequently.