Investigation of degradation mechanisms in mechanical properties of polyimide films exposed to a low earth orbit environment

Effects of Electron Irradiation and Temperature on Mechanical Properties of Polyimide Film

Polyimide (PI) is widely deployed in space missions due to its good radiation resistance and durability. The influences from radiation and harsh temperatures should be carefully evaluated during the long-term service life. In the current work, the coupled thermal and radiation effects on the mechanical properties of PI samples were quantitatively investigated via experiments. At first, various PI specimens were prepared, and electron irradiation tests were conducted with different fluences. Then, both uniaxial tensile tests at room temperature and the dynamic mechanical analysis at varied temperatures of PI specimens with and without electron irradiation were performed. After that, uniaxial tensile tests at low and high temperatures were performed. The fracture surface of the PI film was observed using a scanning electron microscope, and its surface topography was measured using atomic force microscopy. In the meantime, the Fourier-transform infrared spectrum tests were conducted to check for chemical changes. In conclusion, the tensile tests showed that electron irradiation has a negligible effect during the linear stretching period but significantly impacts the hardening stage and elongation at break. Moreover, electron irradiation slightly influences the thermal properties of PI according to the differential scanning calorimetry results. However, both high and low temperatures dramatically affect the elastic modulus and elongation at break of PI.

Formation of Nanoscale Protrusions on Polymer Films after Atomic Oxygen Exposure: Observations with Positron Annihilation Lifetime Spectroscopy

Atomic oxygen (AO) is one of the dominant components of the residual atmosphere in low Earth orbit. AO collides with spacecraft with a translational energy of 5 eV, forming nanoscale protrusions on polymeric materials. To clarify the influences of a polymer's chemical structure on the formation of AO-induced microstructures, this study investigated the size of free-volume holes and the layer thickness that interacted with AO for polyethylene (PE), polypropylene (PP), and polystyrene (PS) by positron annihilation lifetime spectroscopy. The injection energies of positrons varied from 1.3 to 10 keV to adjust the injection depth (range) into the polymers (40 nm-1.6 μm). For the pristine films, the lifetime of ortho-positronium (o-Ps, τ3) was longer in the order of PS, PP, and PE regardless of the injection energy of positrons, showing the different sizes of free-volume holes with radii of 0.29, 0.31, and 0.32 nm, respectively. The fraction of the decay component corresponding to o-Ps in all decay components (relative intensity of o-Ps, I3) was used to investigate the chemical change induced by AO exposure. The I3 values for the three polymers were decreased by AO exposure of (2-5) × 1018 atoms/cm2 or more at a depth of 40-48 nm, obtained by 1.3 keV positrons. This indicates that AO formed polar groups (i.e., an oxidized layer) on the polymer surfaces. The maximum depths of such chemical change for PE and PP were deeper than that for PS. The different sizes of free-volume holes would affect the diffusion or ballistic penetration of AO, resulting in the difference in the oxidized layers' thicknesses and surface morphologies.

Dependence of Incidence Angle and Flux Density in the Damage Effect of Atomic Oxygen on Kapton Film

Kapton film is a polymeric material widely used on low-Earth-orbit (LEO) spacecraft surfaces. In the LEO environment, atomic oxygen (AO) is spaceflight materials' most destructive environmental factor. The erosion mechanism of AO on Kapton films has long been an important issue, where the parameter dependence of the AO effect has received increasing attention. Studies of AO energy and cumulative flux have been extensively carried out, while the influence mechanism of the incidence angle and flux density is not fully understood. The AO incidence angle and flux density in space are diverse, which may cause different damage effects on aerospace materials. In this paper, the dependence of the incidence angle and flux density in the damaging effect of AO on Kapton films was investigated using ground-based AO test technology and the reactive molecular dynamics (ReaxFF MD) simulation technique. Firstly, the ground-based experiment obtained the mass loss data of Kapton films under the action of AO with a variable incidence angle and flux density. Then, the mass loss, temperature rise, product, and erosion yield of Kapton during AO impact with different incidence angles and dose rates were calculated using the ReaxFF MD method. The influences of the incidence angle and flux density on the damage mechanism of the AO effect were discussed by comparing the simulation and test results. The results show that the AO effect in the lower incidence angle range (0-60°) is independent of the incidence angle and depends only on the amount of impacted atomic oxygen. AO in the higher incidence angle range (60-90°) has a surface stripping effect, which causes more significant mass loss and a temperature rise while stripping raised macromolecules from rough surfaces, and the erosion effect increases with the increasing incidence angle and amount of impacted atomic oxygen. There is a critical value for the influence of flux density on the AO effect. Above this critical value, AO has a reduced erosive capacity due to a lower chance of participating in the reaction. The amount of each main product from the AO effect varies with the incidence angle and flux density. Nonetheless, the total content of the main products is essentially constant, around 70%. This work will contribute to our understanding of the incidence angle and flux density dependence of the AO effect and provide valuable information for the development of standards for ground simulation tests.

Formation of Nanoscale Protrusions on Polymer Films after Atomic Oxygen Irradiation: Changes in Morphologies, Masses, and FT-IR Spectra

Atomic oxygen (AO) is the main component of the residual atmosphere in a low Earth orbit. AO with a translational energy of 5 eV colliding with artificial satellites forms nano- and microscale protrusions on polymeric materials. This study investigated the influences of AO (fluence and velocity distribution) and a polymer's chemical structure on such surface morphologies. The correlations between samples' mass losses and positions in the irradiation field of an AO beam were analyzed with polyimide (Kapton) films, a standard reference material for AO fluence measurements. The characterizations of polyethylene (PE), polypropylene (PP), and polystyrene (PS) films were studied using gel permeation chromatography and X-ray diffraction. The sample surfaces were observed using a field emission scanning electron microscope. Nanoscale protrusions were formed on all the samples and were larger but fewer with increasing AO fluence. The numerical density of protrusions formed on PE and PP was lower than that on PS. However, the erosion yields and functional groups of PE, PP, and PS were similar per FT-IR spectra.

Asynchronous Synergistic Damage Effect of Atomic Oxygen and Space Micro Debris on Kapton Film

In the low earth orbit environment, many environmental factors lead to the degradation of material properties. The synergistic effect of long-term atomic oxygen (AO) irradiation and instantaneous impact of micro debris (MD) on long-term and transient space environmental factors has attracted more and more attention. In this paper, the performance evolution of Kapton films under the conditions of MD, AO single factor load spectrum and MD + AO, AO + MD asynchronous synergistic load spectrum were studied by laser driven flyer and microwave atomic oxygen technology. The macro morphology, optical properties and quality changes of Kapton films before and after each load spectrum were compared, and the mechanism of micro morphology and structure changes was explored. The results show that compared with MD + AO loading spectrum, the surface holes of Kapton films are larger under AO + MD load spectrum condition, the residual aluminum particles formed by reverse sputtering of Al particles during impact are less, the average transmittance of the film decreases slightly, and the weight loss of Kapton film is slightly more under the same atomic oxygen exposure time. Under the condition of MD + AO load spectrum, plastic tearing cracks, craters and holes are formed on the surface of Kapton film; the edge of the hole formed under the condition of AO + MD load spectrum is straight, without obvious depression and tear characteristics. Under the condition of MD + AO load spectrum, due to the adhesion of Al after the impact of micro debris, the subsequent atomic oxygen erosion of the film is reduced, so the C-C bond is not seriously damaged, and a considerable part of the residual aluminum flyer is oxidized to alumina by atomic oxygen; The AO + MD loading spectrum test makes the film first eroded by atomic oxygen, resulting in the reduction in C–O bond and C–C bond. The fracture of C–N bond is caused by the hypervelocity impact of micro debris. Hypervelocity impact leads to the thermal decomposition of the material, destroys the C–N bond in the imide ring and generates an N–H bond. This study will provide a method reference and a reference for the multi-factor ground collaborative simulation of space environment of spacecraft materials.

Effect of 172-nm UV irradiation on polyimide and its application in surface modification by grafting

Polyimides (PIs) have a wide range of industrial and scientific applications due to their excellent thermal and mechanical stability and chemical resistance. Their response to ultraviolet (UV) irradiation is of further interest in high-value applications such as spacecraft technology and electronics packaging. In this work, we investigated the effect of 172-nm UV xenon excimer lamp irradiation on samples of pyromellitimido-oxydianiline (PMDA-ODA) commercial films in the absence of oxygen. The average irradiance received at the sample position was 90 mW/cm2, and the total radiation dosage varied from 0 to 64 J/cm2. X-Ray photoelectron spectroscopy, time-of-flight-secondary ion mass spectrometry, atomic force microscopy, and contact angle measurements were used to characterize the effect. Calculated UV-visible spectroscopy absorption spectra were obtained using the ZINDO//B3LYP/3-21G method to give an indication of which orbitals are involved in the transitions near 172 nm. The reactivity of the different UV-treated PI samples toward nitrogen-borne heptafluorodecene vapor was then investigated using the above techniques. Grafting reactions occurred on the surface of the photochemically activated polymer. This study explored the potential for modification of PI surfaces using UV-light-assisted grafting to impart valuable functionalities.

Compostable Polylactide and Cellulose Based Packaging for Fresh-Cut Cherry Tomatoes: Performance Evaluation and Influence of Sterilization Treatment

For food packaging, plastic materials display large appeal, mostly due to their versatility, mechanical, optical and barrier properties. However, they play an important role in environmental concerns and waste management issue. Compostable bioplastics represent alternative materials designed for a lower environmental impact. In this work, a biobased compostable packaging, constituted by polylactide (PLA) trays and NatureFlex™ film, was evaluated for fresh-cut cherry tomato. A comparative analysis was accomplished using traditional packaging materials, that is, polyethylene terephtalate (PET) trays and polypropylene (PP Coex) film. Structural stability under food contact conditions, mechanical and physical-chemical properties were investigated. Tensile mechanical properties, puncture resistance, contact angle (CA) and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR), before and after UV or radiofrequency (RF) sterilization treatment, were evaluated. UV irradiation method resulted the less invasive one. Therefore, oxygen and water vapor transmission rate (OTR and WVTR), overall chemical migration test, biodegradation assessment by biochemical oxygen demand (BOD) according to ISO 14851 and disintegration test by ISO 20200 were carried out to establish the further influence of UV sterilization on the packaging. Overall, data showed that the biobased compostable packaging for a prolonged shelf-life of fresh-cut cherry tomato has better properties that were surprisingly enhanced by the UV treatment.

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.

Failure behavior of nylon products for red phosphorus flame retardant electrical connectors

The failure behavior of red phosphorus flame retardant electrical connectors was investigated by their thermal degradation, changes of surface morphology and elements under an accelerating environmental experiment. The results showed that the nylon electrical connector degraded and cracks were generated on the surface. The temperature of maximum weight loss rate was advanced in the thermogravimetric analysis and the third weight loss peaks appeared after 28 days. The relative atomic content of O and P increased from 0 day to 28 days. The chemical environment of C changed. Partially, C–C bonds broke and became C–O bonds. The release of phosphine increased and this further oxidized to an acid with P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O and P–O groups in a warm and humid environment.

The failure mechanism of red phosphorus retardant PA66 electrical connectors is investigated.

Surface characterization and degradation behavior of polyimide films induced by coupling irradiation treatment

Schematic of irradiation-load-heating coupling treatment and degradation process of polyimide film.

Electron migration behavior of Au/Cu multilayer films on Si substrates under UV radiation

The interfacial broadening was caused by an increase of defects in Au/Cu films which was induced by UV radiation.