A molecular dynamics study of the structure of functionalised copolymers

Polymers can be decorated with functional groups (FGs) to confer specific and tuneable properties to the molecules. By varying the chemical architecture, concentration, and distribution of the FGs on the polymer, it is possible to control the chemical and physical properties of the polymers. Such functionalised polymers are applied broadly, in chemical synthesis, biological science, chemical engineering, medicine, agriculture, food science, optical science, and the energy sector. This thesis is devoted to the study of functionalised polyethylene/polypropylene (PE/PP) copolymers used to control friction and wear in combustion engines. The aim is to investigate how the distribution of FGs on linear PE/PP copolymers controls the structural, dynamical, and tribological properties of the molecules when dissolved in n-heptane (representing an oil), either in bulk, or confined and sheared between solid iron oxide surfaces (representing the moving parts of engines). Large-scale atomistic molecular dynamics simulations are used to gain detailed and unique insights on the links between molecular-scale and tribological properties. Simulations of isolated copolymers in solution are carried out to explore how the radius of gyration, end-toend distance, and scattering form factor are dictated by the distribution of a proprietary FG. It is shown that the size of a copolymer can vary by as much as 38% by varying the FG distribution, due to the association of the FGs in solution. It is found that solvophobic interactions are responsible for this association, and the results are supported by simulations of different FGs without the opportunity to stack. Predicted form factors will enable comparisons to be made with future X-ray and neutron scattering experiments. The shear viscosity is found to correlate well with the FG distribution and copolymer size at low concentration, while at high concentration the viscosity does not seem to depend with FG distribution, in agreement with the lack of variation in static structure. Next, the properties of copolymers solutions confined between iron oxide surfaces are explored, in static conditions and under shear. It is found that the FG distribution strongly affects the nature of the adsorption of the copolymers, and the response of the adsorbed film to shear, as measured by mass-density and velocity profiles across the liquid layer.