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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.

Engine oils are of great significance in the automotive industry as they provide lubrication of the moving mechanical components of the engine, thus decreasing friction, and preventing wear and corrosion. There is little understanding of the underlying molecular-scale mechanisms that contribute to the low-friction environment, and how the additives in lubricant oils behave under engine-like conditions. Previous research is based mainly on experimental work and empirical evidence, although in the past decade, computational techniques have been used successfully to simulate the engine environment and verify experimental results. This work is mainly focused on the computational investigation of certain surface active additives under a range of relevant physical conditions, such as high pressure and high shear rate, in order to provide an insight into the behaviour of these compounds at the atomistic level. The simulations are performed using molecular dynamics (MD) and they explore important microscopic processes such as adsorption and self assembly in bulk and under confined conditions. In the first part of the thesis, the focus is on lubricant additives in oil. Surfactant adsorption helps protect the surface from contact with other engine parts, whereas aggregation plays a major role in preventing impurities and byproducts from depositing onto the surfaces. The frictional and structural properties are studied over production runs. The additives studied are polyisobutylsuccinimide-polyamine (PIBSA-PAM), glycerol monooleate (GMO), synovene, and glyceryl monolinoleate (GML), and squalane is used as the lubricating oil. The additives tend to adsorb onto the surfaces, or form micelles, which break under high shear conditions. Interesting trends are identified, linking self assembly with friction and shear rate, and the co-operative interactions between the additives are highlighted. In the second part of the thesis, MD simulations are used to study self-assembly and surface adsorption of ionic surfactants in water. An investigation on the self assembly properties of the surfactant compounds hexadecyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) in water is performed, and the results are linked to experimental findings. The molecules are confined between muscovite mica surfaces, and are found to adsorb and form lamellar structures on mica. The morphology of the mica surface is deemed crucial for the structure of the adsorbed formations, and the simulation results are compared with recent experimental measurements by calculating the scattering-length density profiles and the reflectivity. This study provides a detailed analysis of a range of surface active molecules, either in bulk or confined conditions, and emphasises the importance of the co-operative interactions between two or more additives in bulk and confined solutions.

Glycerol monooleate (GMO) is a common engine oil lubricant additive used to reduce friction and wear in engines. The aim of this thesis is to investigate the properties of GMO in bulk and under confined conditions with shear using molecular dynamics simulations. The self-assembly of GMO into reverse micelles (RMs) in toluene and n-heptane solvents is studied at a range of concentrations and subsequently with several common engine impurities over simulation timescales of 5-30 ns. The dimensional properties of the RMs are found to correspond well with experimentally studied SANS/SAXS measurements. Secondly, the properties of GMO confined between mica surfaces are studied under quiescent and shear conditions. Under shear, the performance of GMO as a friction modifier is studied and the structural and frictional properties are examined. In particular, the mass density and velocity profiles of the fluid are used to gain insights into the structure and dynamics of the confined GMO fluid films, under a variety of shear rates and surface separation. The data is found to fit excellently to the universal friction curve. Following this the effect of hydrolysing the GMO to oleic acid is studied in bulk and under shear, it is found that increasing oleic acid concentration typically reduces the propensity to self-assemble and under confinement increases the friction coefficient. And finally, a study on a range of other similar surfactants is conducted to investigate the effect of unsaturation, head group and chain length on the calculated friction coefficient and the structure of the surfactant films.

Lubricants oils play an important role in a wide range of industrial and mechanical processes, where they are used to reduce both the friction and wear between interacting moving surfaces. The current understanding of lubrication is mainly based on empirical evidence, obtained from experiment. In this work, computer simulations are used to gain insight into the microscopic processes that lead to the modification of friction and wear by additive molecules adsorbed on sheared surfaces lubricated by thin liquid films. The specific area of application under consideration is the lubrication of automotive engine parts. The interactions between additive molecules are first determined using density-functional theory calculations. The interactions are then validated against available experimental data, and incorporated in to large-scale molecular-dynamics (MD) simulations, which are used to explore the structure and frictional properties of lubricated surfaces. The surfaces considered are alumina and iron oxide. The lubricating oils are squalane and hexadecane, which are representative of automotive lubricants, and the additive molecules are stearic acid, oleic acid and various oleamides. MD simulations are performed over wide ranges of the relevant physical conditions, namely pressure, temperature, and shear rate. The additives adsorb on to the surfaces and provide a physical connection between the surfaces and the lubricating liquid. The structures of adsorbed films are analysed in microscopic detail using functions of atomic positions and molecular geometry. Several important trends are identified, linking molecular isomerism and architecture with the structure and stability of the adsorbed film. In addition, the simulation results are used to gain insight on recent experimental measurements of film structure. The friction coefficients in various situations are computed and analysed with reference to the structures of the adsorbed films. The synthesis of these data and observed trends yields new insights on the intimate link between the molecular properties of lubricants, and the macroscopic frictional properties of macroscopic lubricated engine parts.