Thesis

A study of monomer liquid mixtures and polymer network formation at a solid surface using molecular dynamics

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Awarding institution
  • University of Strathclyde
Date of award
  • 2023
Thesis identifier
  • T16564
Person Identifier (Local)
  • 201877579
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Qualification Name
Department, School or Faculty
Abstract
  • Fibre-reinforced composites are used in a wide variety of industries for their light weight, high strength and durability. However, the structure of the polymer matrix near the fibre interface is not well understood and difficult to characterise experimentally. The aim of this project is to use molecular dynamics (MD) simulations to elucidate the effect of a fibre on polymer network structure. The DREIDING force field has been widely used, in molecular simulations, to predict thermomechanical properties in reinforced epoxy composites, such as the well-known EPON 862/DETDA matrix. This work compares four variations of this force field with OPLS and presents the structural and dynamical properties of the liquids. The OPLS force field is deemed appropriate to study such systems and presents several advantages over DREIDING force field variations, such as a ease of use. Results show that there is a distinct interface region adjacent to the carbon fibre-like surface that differs from the centre region. Additionally, density, molecular orientation, and molecular conformation away from the surface match values found in bulk. To study the effects of crosslinking, we have developed a generic bead-spring model to investigate polymerisation near a surface. The model is based on a Lennard-Jones (LJ) liquid in conjunction with the Kremer-Grest model, at constant pressure, and with a probabilistic approach towards achieving bond formation between individual monomer particles. This model was easily implemented, in LAMMPS, and was also used to study the properties of liquids and liquid mixtures near a surface. The same distinct interface region adjacent to the surface was also observed. The polymerisation mechanism, by which crosslinking occurred, was representative of a step-growth mechanism, with and without the surface present. T is consistent with the epoxy curing process, represented by the aforementioned EPON-862/DETDA system. Additionally, it was verifiable that when a particular chemical species adsorbs to the surface more strongly, other molecular species are displaced from the interface region and move towards the centre film region. The resulting sterochemical imbalance can cause the resulting crosslinked network to contain higher amounts of non-bonded monomers. Often, carbon fibre surfaces dipped into epoxy resins (e.g. EPON-862) prior to inserting them in an epoxy/hardener mixture, in an attempt to improve surface adhesion. The stoichimetric imbalance shown to exist near the surface is further aggravated by this procedure and leads to a cured material of lower quality. This work sets the precedent for review of such fibre sizing procedures. In addition, the developed beadspring crosslinking model allows for quick and efficient similar studies.
Advisor / supervisor
  • Johnstone, Karen
  • Mulheran, Paul
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