Theoretical nanoscale design of self-cleaning coatings
Creators: name: Yiapanis, George
Publisher: name: RMIT University
Year: 2024

description: This project involves atomistic level modelling of the interactions of carbonaceous contaminants with polyester based surfaces in order to study the mechanism by which airborne and rain-carried carbon particles adhere to polyester based paints, and identify key surface properties that may aid in the development of dirt repellent paint coatings. A detailed description of the motivation for this project, and a literature review on important experimental discoveries in the areas of polymer surface modification and computational modelling of polymer surface and polymer/carbon interfaces are presented in Section 1. Also, additional literature reviews are included in the introductory sections of Chapters 4 to 7 pertinent to the subject matter in question. <br><br>Classical molecular mechanics (force-field methods) is employed to describe the physical interactions between the polymer surfaces and carbon species, and molecular dynamics is used to obtain time and temperature-dependent properties of these polymer/carbon systems. A detailed description of the computational techniques is included in Section 2. <br><br>A fully-atomistic polyester surface model is used to simulate the paint surface, and a range of carbon particles (graphite, amorphous carbon and fullerene C60) are used to simulate the airborne contaminants. The strength of adhesion between polymer and contaminant is characterised by a procedure that simulates the instantaneous separation of the polymer/carbon interface, allowing determination of the work of separation. This property is related to the ideal thermodynamic work of adhesion. In this project we explore nanoscale modifications (hydrophobic and hydrophilic treatments) of the polyester surface that potentially reduce the strength of adhesion (work of separation) between polymer and carbon adding value to polymer coatings where contamination protection is desirable. We have included a detailed description of the polymer surface models, organic contaminants, nanoscale modifications and the procedure used to evaluate interfacial adhesion in Section 3. <br><br>An important property utilized by natural stay-clean plants, such as the Lotus leaf, is nanoscale surface roughness. In Section 4 of this study, we investigate the effects of atomic-scale roughness combined with chemical surface modifications on adhesion between polymer and amorphous carbon (an industrial char derived contaminant). x x Modifications to the polyester surfaces include the addition of hydroxyl (OH), carboxyl (COOH), and fluorine (F) functional groups at varying levels of surface coverage. Also, in Section 4, we employ a procedure that approximates relaxation (energy-minimisation) and achieves restructuring of the modified surfaces. As we will show, surface reorganisation has an important impact on the effectiveness of our surface modifications. <br><br>Aging is an important phenomenon in polymer science, whereby treated coatings tend to recover to their natural state after some time. To investigate the effects of aging on our functionalised polyester surfaces, in Section 5, we undertake molecular dynamic (MD) simulations of carbon/polyester interfaces at 400 K (just above the glass transition temperature of polyester). We show that physico-chemical changes of the polymer surfaces that occur during aging, significantly impact on the strength of adhesion between polymer and carbon contaminant. <br><br>We extend our investigations of contaminant adhesion to include an aqueous environment by modelling the interaction of fullerene (an ideal model of a soot derived particulate) with polyester surfaces in water (Section 6). A series of MD simulations are undertaken with the fullerene positioned at distinct locations above the hydrated polymer surfaces. In this section, we show that a specific combination of physical and chemical properties can prevent fullerene from adhering to polyester in water. <br><br>Finally, we develop a computational methodology to chemically crosslink the surface of fully atomistic polymer models (Section 7). Surface crosslinked polyesters are constructed using hexamethylene and isophorone di-isocyanate crosslinkers. In this section, we also introduce a methodology to simulate nanoindentation of the crosslinked films, whereby a fullerene is used to probe the hardness of different regions of the polymer surface. Through this in-silico nanoindentation experiment, we not only determine the surface hardness of the polymer film but also calculate the work of adhesion between polymer and fullerene. This enables us to explore the effects of surface hardness on adhesion between polymer and carbon. From the knowledge gained, we have developed design principles which we believe will lead to the development of novel dirt-repelling paint surfaces. These principles are summarized in the Section 8 of this study. Our future work is presented in the final section of this thesis.<br>