Degradation of polymers by hyperthermal atomic oxygen

Simulated Low Earth Orbit Environment Interaction with Different Types of Polyethylene

The present study investigated the interactions of four different types of polyethylene (PE) surfaces with RF oxygen plasma components, simulating the low Earth orbit (LEO) environment. The samples were exposed in the plasma afterglow, with and without the plasma-generated vacuum UV (VUV) radiation. The polymers studied were low-density polyethylene (LDPE), ultra high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE), and a composite of highly-oriented fibres of ultra-high molecular weight polyethylene (PolyEitan) having crystalline-to-amorphous phases ratio of 65, 74, 87 and 92%, respectively. Erosion of the LDPE, UHMWPE, and HDPE samples under atomic oxygen (AO) irradiation showed an etching rate dependence on the degree of crystallinity - the higher the crystallinity level the lower the erosion rate. The addition of VUV radiation to the AO flux resulted in a significant increase of the etching rate. An increase of the crystallinity level resulted in a decrease of the total oxygen uptake and an increase of the surface roughness. The composite polyethylene, although highly crystalline, revealed similar erosion rate and chemical composition changes as highly amorphous LDPE. The erosion mechanism of the different PE samples under AO or AO+VUV is discussed in terms of the initial internal structure.

3D Graphene-Infused Polyimide with Enhanced Electrothermal Performance for Long-Term Flexible Space Applications

Polyimides (PIs) have been praised for their high thermal stability, high modulus of elasticity and tensile strength, ease of fabrication, and moldability. They are currently the standard choice for both substrates for flexible electronics and space shielding, as they render high temperature and UV stability and toughness. However, their poor thermal conductivity and completely electrically insulating characteristics have caused other limitations, such as thermal management challenges for flexible high-power electronics and spacecraft electrostatic charging. In order to target these issues, a hybrid of PI with 3D-graphene (3D-C), 3D-C/PI, is developed here. This composite renders extraordinary enhancements of thermal conductivity (one order of magnitude) and electrical conductivity (10 orders of magnitude). It withstands and keeps a stable performance throughout various bending and thermal cycles, as well as the oxidative and aggressive environment of ground-based, simulated space environments. This makes this new hybrid film a suitable material for flexible space applications.

Ultraviolet or atomic irradiation effect on the polyimide composite lubricating coating

The molybdenum sulfide/lanthanum fluoride/polyimide (MoS2/LaF3/PI) coating was synthesized and irradiated by ultraviolet (UV) or atomic oxygen (AO). The friction and wear behavior were evaluated sliding against GCr15 steel balls (ASTM A295:1998) using a ball-on-disk tribology test rig. This paper aims to discuss these issues. The chemical structure of the irradiated surface was examined by Fourier transform infrared (FTIR) spectroscopy. The worn morphologies of the coatings and counterpart were observed by scanning electron microscopy (SEM) to reveal the wear mechanism. Experimental results indicated that the height and position of the characteristic functional groups and the coefficient of friction of the PI coating changed and the wear rate value decreased after UV or AO irradiation. The effect of LaF3 combined with MoS2 on the tribological properties of the coating and wear mechanism under different irradiated conditions is discussed.