Influence of POSS Type on the Space Environment Durability of Epoxy-POSS Nanocomposites

In order to use polymers at low Earth orbit (LEO) environment, they must be protected against atomic oxygen (AO) erosion. A promising protection strategy is to incorporate polyhedral oligomeric silsesquioxane (POSS) molecules into the polymer backbone. In this study, the space durability of epoxy-POSS (EPOSS) nanocomposites was investigated. Two types of POSS molecules were incorporated separately—amine-based and epoxy-based. The outgassing properties of the EPOSS, in terms of total mass loss, collected volatile condensable material, and water vapor regain were measured as a function of POSS type and content. The AO durability was studied using a ground-based AO simulation system. Surface compositions of EPOSS were studied using high-resolution scanning electron microscopy and X-ray photoelectron spectroscopy. It was found that with respect to the outgassing properties, only some of the EPOSS compositions were suitable for the ultrahigh vacuum space environment, and that the POSS type and content had a strong effect on their outgassing properties. Regardless of the POSS type being used, the AO durability improved significantly. This improvement is attributed to the formation of a self-passivated AO durable SiO2 layer, and demonstrates the potential use of EPOSS as a qualified nanocomposite for space applications

Concentric dopant segregation in CVD-grown N-doped graphene single crystals

Heteroatom doping in graphene leads to bandgap opening and tunable electronic, magnetic and optical properties, which are important for graphene-based electronics applications. In recent years, scalable growth of nitrogen-doped graphene (NG) by chemical vapor deposition (CVD) has been extensively studied because of its potential for practical applications. A phenomenon that occurs exclusively for CVD-grown NG films is the segregation of doping concentration. However, most studies to date are conducted using highly polycrystalline NG films comprising small grain sizes. It is still unknown whether dopant segregation occurs in single crystalline NG domains. Here, we used hexamethylenetetramine ((CH2)6N4) as a single-source solid precursor to grow hexagonal-shaped monolayer NG single crystals of ∼20 µm on Cu substrates. The NG single crystals exhibit discrete concentric hexagonal rings comprising N depleted regions as determined by Raman spectroscopy. Supported by scanning tunneling microscopy experiments, we propose that the segregation of N dopants is caused by a competing N attachment mechanism to either zigzag or Klein edges during growth; where the former should result in higher N concentration and the latter with lower N concentration. This work provides critical insights into the growth mechanism of CVD-grown NG and enables new opportunities to engineer the properties of graphene by fabrication of lateral heterostructures.ASTAR (Agency for Sci., Tech. and Research, S’pore)Accepted versio

Novel timed and self-resistive heating shape memory polymer hybrid for large area and energy efficient application

Shape memory polymers (SMPs) are a polymeric smart material that can register two or more temporary shapes and transform to one another through an external stimulus. Despite their compactness and customizability, SMPs haven't been able to be adopted for mainstream applications. Since the majority of SMPs are triggered by heat, and SMPs have a very poor thermal conductivity, large thermal gradients within the polymer appear which cause slow response, inhomogeneous heat distribution and thus non-uniform transformation of shapes and cracks. Many have attempted to improve their thermal performance through the incorporation of filler-based nanomaterials. However, the outcome is ineffective as the spatial dispersion of fillers within the SMP is inhomogeneous and leads to performance loss. Contrastingly, the herein presented new class of nanocomposite-SMP, composed by 3D-foam fillers, showcase a much more efficient SMP adaptable to larger area with faster transformation speed and without any performance loss. Furthermore, the improved thermal properties lead to a decrease in required input energy, as well as render the SMP a self-heating capability which can be further designed into timed multi-step SMP behavior

POSS enhanced 3D graphene - polyimide film for atomic oxygen endurance in low earth orbit space environment

Recently, 3D-graphene infused polyimide (3DC/PI) films have shown to be an effective protection coating for electrostatic discharge in spacecraft. However, these films are not suitable for Low Earth Orbit (LEO) due to atomic oxygen (AO) erosion. Here, we used Polyhedral Oligomeric Silsesquioxane (POSS) to enhance the AO durability of 3D-C/PI films. Three different ways of adding POSS to the composite films were studied with ground-based AO exposure. For all infusion approaches, their electrical conductivity behaviour is well preserved and the presence of POSS results in reduced AO erosion yield. Of all the methods studied here, incorporating POSS directly into PI results in the lowest erosion yield of 4.67 × 10−25 cm3/O-atom (one order of magnitude lower than that of Kapton). Adding POSS to PI, extends the durability of the composite film beyond 10 years, making it an ideal protective material for long term mission in LEO.Accepted versio