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Molecular dynamics studies of the transfer of ions in multi-walled carbon nanotube poly(2,5-benzimidazole) composites

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North-West University

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When energetic protons bombard satellites in the context of Low Earth Orbit (LEO), they are shielded by materials such as aluminium and polyethylene [1]. At a certain depth, depending on the energy of the proton and the radiation shielding material, the movement mechanism is no longer energy dominated. Three of the main mechanisms used to describe proton transport in polymers are Grotthuss, diffusion, and direct transport via polymer chain segmental motions[2]. Poly(2,5-benzimidazole) (ABPBI) and two multi-walled carbon nanotube (MWCNT) composites (1wt% and 5wt%) were simulated, using Molecular Dynamics, at typical satellite operating temperatures to investigate the ion movement mechanism and influence of MWCNT loading and temperature. The ions considered in this dissertation are hydrogen ions, i.e. protons. The simulated X-ray diffraction (XRD) patterns give insight into the crystallinity of a structure. The XRD for ABPBI was indicative of a semi-crystalline nature for which direct transport via segmental motions of the polymer chains, according to Gao and Lian [2], is not possible. The simulated temperatures showed little to no effect on the mean square displacement (MSD) of the proton and their affinity to be within hydrogen bonding range of the nitrogens of ABPBI and accompanying composites. The MSD results also ruled out the diffusion mechanism, leaving the Grotthuss mechanism as the most likely vehicle for proton transport in ABPBI and associated composites. This is supported (and corroborated by other studies [3, 4]) by the radial distribution function (RDF) results indicating strong interactions, across all composites and temperatures, between the proton and ABPBI nitrogens possibly leading to hydrogen bonding which is central to the Grotthuss mechanism. ABPBI is not yet a recognised shielding material for space application. The work from this dissertation will, hopefully, aid in the understanding of how ABPBI reacts to proton radiation and add to the merits for using it, and its composites, for space application in future.

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The North-West University, Master of Science in Astrophysical Sciences, Potchefstroom Campus

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