Influence Of Radio-Frequency Inductively Coupled Plasma Treatment On The Interfacial Adhesion Of PBO Fiber-reinforced Bismaleimide Composite

Poly(p-phenylene benzobisoxazole)(PBO) fiber has increasingly attracted attentions, due to its superior high specific strength, modulus and resistance to high temperature, flame and chemicals. It is expected that composites made from the PBO fibers and high performance bismaleimide (BMI) resins will find great applications as structural or structural-functional materials in the fields of aerospace and national defense. But the potential is restricted by the poor interfacial performance of the PBO/BMI composite, owing to the smooth and inert fiber surface. Thus much work is needed to modify the PBO fiber surface without lowering its excellent mechanical properties in order to get the high performance PBO/BMI composite.In this work, the PBO fibers were treated by inductively coupled gas plasmas and three different gases were chosen:reactive oxygen, inert argon and oxygen/argon mixture. The influences of plasma discharge power, treatment time and pressure (gas composition) on the interlaminar shear strength (ILSS) of the composite were systematically investigated. The fiber surface chemical compositions, topography, roughness and surface free energy were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and dynamic contact angle analysis (DCA), respectively. The interlaminar shear fracture morphology of the composite was observed using scanning electron microscopy (SEM). The mechanisms of both the fiber surface modification and the improved interfacial adhesion were discussed.After treatment at10Pa,50W in the O2plasma for10min, the composite ILSS was increased by28.9%. The conditions of large powers, high pressure and long or short treatment time were detrimental to the ILSS. The O2plasma destroyed the phenyl ring and oxazole ring in the PBO molecules and introduced a large amount of O atoms and a small number of N atoms onto the fiber surface, generating many polar and active groups, such as-COO,-NCO (-C=O) and-N-O-groups. The plasma etching and sputtering effects obviously changed the fiber surface topography, increasing the roughness. After modification, the surface free energy was increased and the wettability was improved, which favored the resin wetting and spreading on the fiber surface.The composite ILSS was improved by39.7%after exposure to the Ar plasma at80Pa,200W for7min. The treatment power and time had much influence on the ILSS, but the gas pressure did not. Comparing to the O2plasma, the Ar plasma broke the oxazole ring of the PBO molecules and introduced a very small amount of O atoms onto the fiber surface, creating some amide groups. The functionality of the Ar plasma was less than the O2plasma. The plasma sputtering also destroyed the surface crystallizing layers and increased the roughness. The wetting between the treated fiber and resin matrix was improved, which contributed to the enhanced adhesion.In the O2/Ar plasma case, the composite ILSS was increased by38.1%when the optimal treatment parameters were:40Pa,60%O2,150W and7min. The process feasibility was better than both the O2and Ar plasma. The ILSS first increased and then decreased with increasing the treatment power or time, while the gas compositions had more complicated effects. Synergy effect existed in the mixture plasma and made much more O atoms incorporated into the fiber, as a result, the categories and contents of the newly-created polar groups also became more. After treatment the fiber surface physical structures also became complex and the roughness increased.After the plasmas treatment, the interlaminar shear fracture occurred in the matrix instead of the interface, and the failure mode changed from interface debonding to partly cohesive failure. The interface was considerably strengthened, but meanwhile the composite brittleness was enhanced as a result of lacking of flexible interface layers. The formation of covalent bonds and hydrogen bonding contributed to the improved interface. The roughened fiber had larger contact areas with the resin, which was beneficial to form more covalent bonds and stronger mechanical interlocking. But interface defects might increase when the fiber surface roughness was too high and thus reduced the composite mechanical performance. The PBO/BMI composite had a very low water absorption, but its mechanical properties reduced due to interface swelling stress and hydrolyzation at hydrothermal conditions. The plasma treatment could not improve the hot and humidity resistance of the composite, but the composite ILSS still retained by90%or higher after being boiled for72h.Aging behavior was not observed in the O2plasma case, which might result from crosslinking with incorporation of oxygen atoms or oxygen-containing groups. But the composite ILSS decreased rapidly in the first several days after treatment by the Ar and O2/Ar plasmas. It was considered that no crosslinking layers were formed and thus the active structures disappeared from the fiber surface, so the interfacial adhesion reduced. Consequently, the Ar and O2/Ar plasma treated PBO fibers must be made into composites immediately, while the O2plasma treated fibers could be stored for a long time.