Influence of the Composition of the Hybrid Filler on the Atomic Oxygen Erosion Resistance of Polyimide Nanocomposites

The structure and properties of nanocomposites based on organosoluble polyimide (PI) and branched functional metallosiloxane oligomers with different types of central metal atoms (Al, Cr, Fe, Zr, Hf and Nb) were investigated. Under the same weight content of the filler, the geometric parameters of the nanoparticles and thermal properties of the nanocomposites did not exhibit a direct relationship with the ability of the materials to withstand the incident flow of oxygen plasma. The atomic oxygenerosion resistance of the filled PI films was influenced by the composition of the hybrid fillerand the type of metal atom in the hybrid filler in the base metallosiloxane oligomer. To determine the effectiveness of the nanoparticles as protective elements of the polymer surface, the nanocomposite erosion yields pertaining to the concentration of the crosslinked organo–inorganic polymer forming the dispersed phase were determined and expressed in mmol per gram PI. The filler concentration in the polymer, expressed in these units, allows for comparison of the efficiency of different nanosize fillers for use in fabricating space survivable coatings. This can be important in the pursuit of new precursors, fillers for fabricating space survivable polymer composites.

. Chemical structure of polyimide. Polymer with Мw=10.7* 10 5 was synthesized by one-step high-temperature polycondensation of the relevant monomers in m-cresol. A reduced viscosity of 0.97 dL/g was measured for the polymer solution in N-methyl-2-pyrrolidone at 25 °С.

Preparation of filled films
A precursor solution in chloroform was added to 0.10 g PI solution in 4 ml of chloroform with stirring in an argon stream. The detailed formulations are listed in Table S1. The precursor concentration was 3 and 14 wt%. The resulting solution was poured into a teflon form and dried at room temperature for 3 days. Afterwards, the polymer film was heat treated in a drying oven with a gradual increase in temperature from 50 to 200 °C for 6 hours.
The thickness of the obtained films was 110-120 μm. The filler contents, according to calculations carried out under the assumption of 100% conversion of the ethoxy groups, are presented in Table  S1.
For analyses of filled films, the polymer film and the film of 100% precursor, obtained under identical conditions, served as samples for comparison.

AO Beam Exposures
The facility consists of an evacuated vessel in which a plasma accelerator is placed. The vessel has a specimen holder and beam diagnostic equipment. Using vacuum pumping by cryogenic pumps with a rate of 5 m 3 /s, the vessel maintains a pressure of (0.5-2)×10 -2 Pa with a plasma-supporting gasoxygen requirements of 0.5 L Pa/s. The beam components are atoms, molecules and oxygen ions with a predominance of atomic ions. The power flux density absorbed by the specimen was 15 mW/cm 2 , which approximately corresponds to the heating of the specimen by solar radiation in space.
Film samples 20×20 mm in size were used. The specimens were degassed beforehand and held for 24 hours at a temperature of 20 °C in a vacuum of 10 -4 Pa.
Specimens were irradiated by an oxygen plasma beam from a plasma accelerator, simulating low-earth orbit conditions. To ensure the same exposure, the analysed specimens and the reference specimen were mounted on a rotating disk, placed normally to the plasma flow. The specimens' masses were measured outside the evacuated vessel on an analytical microbalance HR-202i (AND, Japan) with a scale multiplier of 10 -5 g, before and after each irradiation cycle with plasma flow.
In the experiment, the effective fluence method was used to determine the intensity of AO exposure [1]. The equivalent AO fluence, F (O atoms cm -2 ), was determined by the change in weight of a reference sample (Kapton H polyimide film, DuPont) with an erosion yield of EK = 3 × 10 -24 cm 3 / atom: where Δ mK is the reference sample weight loss (g) during AO exposure, S is the exposed surface area (cm 2 ) and ρK is the density of the reference sample (1.42 g/cm 3 ). The effective AK flux density in polyimide equivalents was (3-4)×10 16 аtom/cm 2 s.
The erosion yield coefficient (Ey, cm 3 atom −1 ) of research samples is defined as the volume loss caused by one AO attack, calculated by Eq. (2) where Δm is the sample weight loss (g) during AO exposure, S is the exposed surface area (cm 2 ), and ρ is the density of sample (1.38 g/cm 3 ).