Control of contact charging on polymers with organic charge-transfer complexes and quantum dots

Abstract

Contact charging, also known as triboelectrification, occurs when two insulator materials are contacted and separated. The mechanism of polymer triboelectrification and the following charge dissipation are still ‘mysteries’ in the current scientific research. However, generation and transfer of contact charges on surfaces could be harmful to many industries. It causes substantial economic losses in, e.g., space industry where satellites are damaged, in pharmaceuticals where charging of powder drugs is a severe problem for process and manufacturing, and in microelectronics where MEMs devices suffer from triboelectric charging. So far, the common approach to this problem is to render surfaces conductive with high loading of conductive additives, which is not ideal for most of the industrial applications. This thesis presents some efforts to open and explore new light-controlled discharging mechanisms for contact-charges on polymers, without an increase in surface conductivity of the polymer surface. The antistatic behavior of the polymer surfaces are achieved by doping of organic charge-transfer complexes (CTC)s or quantum dots (CdSe, CdSe/ZnSe). In the first part of the study, the CTCs formed from pyrene and its derivatives, and TCNQ doped into the polydimethylsiloxane (PDMS) were found to affect a faster discharge compared to the discharging on undoped polymers, as monitored by Faraday cup measurements. In a second setup, solutions of CTCs dropped into the vials of contact charged polymer beads in hexane affected a similar faster discharge of the beads, which was monitored by the fall time of the beads. In both solid samples and with the beads in hexane, the time required for the polymer discharge mediated by each CTC was related to the CTCs degree of charge transfer. Additionally, when the samples were excited by a UV source, the charge decays were faster in comparison to the non-illuminated samples. Theoretical calculations confirmed that HOMO-LUMO gap decreases upon excitation, which enhances the dissipation of tribocharges. It was also found that hydrogen bonds between donor and acceptor moieties alter the CTC morphology in PDMS, which yields differences in charging and discharging behavior. The formed CTC-composites were characterized by UV-Vis, AFM, XRD, and SEM. In the second part of this thesis, CdSe, CdSe/ZnSe quantum dots (QD) were doped in the polymers to test the polymer discharging performance of these materials. The formed QD-composites were characterized by UV-Vis, TRF, XRD, SEM, and TEM. This new set of materials bring in their unique properties to the polymer composites, i.e., bandgaps which are tunable, and quantum confinement effects in the nanocrystals. These properties unlatch new doors to the charge dissipation, one of which was explored by changing spatial delocalization of holes and electrons in semiconductors with band-gap engineering. In this study, the faster discharging on polymers doped with CTCs and QDs affected by a control of their properties, like degree of charge transfer (for CTC) and hole-electron localizations (QD) were displayed. It was shown that light-controlled remote discharging maybe expanded to other types of materials, and it can be fine-tuned by materials’ properties. The results point towards other possible but yet unexplored discharging mechanisms with unconventional additives, which do not simply increase the polymers’ surface conductivity. Finally, we believe that these findings can be useful where materials are needed to be antistatic, but not conductive, such as in the electronic coatings.