As the main load-bearing part of fiber reinforced composites, reinforced fibers must have excellent mechanical properties, and PBO fibers have the highest tensile strength ( up to 5.8 GPa ) and modulus among polymer fibers, and have broad application prospects in the field of high-performance composites. However, PBO fiber exhibits strong surface inertia and lacks polar functional groups, which leads to the accumulation of defects at the interface when it is compounded with the resin matrix and damages the comprehensive properties of the composites.

Therefore, PBO fibers need to be pretreated to a certain extent before preparing composite materials to improve their surface state and enhance the bonding with the resin matrix, so as to prepare composite materials with high performance. However, the surface modification of PBO fiber is extremely difficult. Its special rigid rod-like molecular structure brings strong corrosion resistance, and the smooth surface is not conducive to the adhesion of active substances on the fiber surface. The oxygen plasma treatment can bombard the surface of the PBO fiber by high-energy oxygen plasma to form a rough structure and produce reactive oxygen species and dangling bonds. The active surface after oxygen plasma treatment is more conducive to the adhesion of the coating and increases the designability of the surface structure of the PBO fiber.

Effect of oxygen plasma treatment on the properties of PBO fiber

The working principle of PBO fiber treated by oxygen plasma is shown in Figure 1-1. When the sample is placed in the cavity, when the oxygen molecules in the vacuum chamber are subjected to sufficient external electric field, the electrons will obtain sufficient energy to enter a higher energy level, and then ionization occurs to produce charged particles or metastable particles. The energy of this ionization atmosphere is higher than the chemical bond energy of PBO, so that it is partially oxidized and decomposed under the bombardment of charged particles to produce gaseous oxides ( such as CO2, NO2 ). At the same time, this process can also form oxygen-containing functional groups and surface microstructures on the surface of PBO fibers, thereby improving the surface state and surface activity of the fibers, and improving the interfacial compatibility between PBO fibers and resin matrix.

Fig.1-1 Schematic diagram of PBO fiber treated by oxygen plasma

Surface morphology analysis of PBO fiber

The surface morphology of PBO fibers was observed in detail by SEM, and the results are shown in Fig.1-2. It can be seen that the surface of the untreated PBO fiber is smooth and flat, which is mainly due to its unique spinning process and chemical structure. After 5 min of oxygen plasma treatment at 300 W power, circular oxide particles with uniform distribution and nanoscale size appeared on the surface of PBO fibers. The formation of these particles is due to the bombardment of high-energy oxygen plasma, which triggers the oxidation reaction on the fiber surface.

Fig.1-2 SEM of PBO fiber surface with different oxygen plasma treatment time

With the extension of treatment time, after 10 min oxygen plasma treatment at 300 W power, the oxide particles on the surface of PBO fiber gradually increased and showed a trend of aggregation. Compared with 5min treatment, the oxide particles showed a more scattered and sharp edge on the surface. When the treatment time was extended to 20 min, the surface oxide particles further aggregated and an obvious island-like convex structure appeared. This phenomenon can be attributed to the dissolution and expansion effect of some areas on the surface of PBO fibers caused by long-term oxygen plasma bombardment, thus forming a more significant microstructure change.

Oxygen plasma treatment can significantly change the surface morphology of PBO fibers in a short time due to its high energy density and fast reaction characteristics, forming a microstructure with fine control.

Analysis of surface chemical state of PBO fiber

As shown in Figure 1-3, the FTIR spectra of PBO fibers after oxygen plasma treatment showed a significant C = N bond stretching vibration peak at 1615 cm  1, corresponding to the oxazole ring in the molecular structure of PBO. In addition, in the range of 1619 cm  1 to 1415 cm  1, the stretching vibration peak of the benzene ring can be observed, which further confirms the molecular characteristics of the PBO fiber.

Fig. 1-3 FTIR spectra of PBO fiber after oxygen plasma treatment

In addition, an obvious hydroxyl ( -OH ) absorption peak was detected at 3480 cm  1, indicating that the oxidation of oxygen plasma introduced polar groups on the surface of the fiber, thereby improving the surface activity of the fiber. In addition, a carbonyl ( -C = O ) stretching vibration peak appeared at 1730 cm Ω 1, indicating that some PBO structures underwent an oxidation process, resulting in the formation of carboxyl ( -COOH ) or ester ( -COOR ) groups. The introduction of these oxidizing groups significantly enhanced the hydrophilicity and chemical reactivity of the fiber surface.

The oxygen plasma treatment not only increases the surface roughness of PBO fiber, but also introduces oxidation groups on the fiber surface through chemical modification, thereby improving its surface activity. This modification effect provides more active sites for the subsequent binding of ion-chelating copolymer coatings.

Plasma treatment is an important means of PBO fiber surface modification, and its mechanism of action is derived from the physical etching and chemical synergistic effect of high-energy active particles on the fiber surface. In the plasma field, the excited particles ( such as electrons, ions and free radicals ) bombard the fiber surface through kinetic energy transfer. On the one hand, the nanoscale gully structure is formed by physical sputtering to increase the surface roughness. On the other hand, oxygen-containing polar groups ( such as hydroxyl and carboxyl groups ) are introduced on the inert surface through chemical bond breaking and recombination, thereby improving the interfacial compatibility between the fiber and the matrix. The advantages of this technology are reflected in high process efficiency, environmental friendliness, and strong parameter controllability. By adjusting the discharge power, treatment time, and working gas composition ( such as N2, Ar / O2 mixing ratio ), the surface topography and chemical activity can be directionally regulated.